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[
"A question about SETI: Assuming that there were actual signals out there - How could we possibly hope to detect them?"
] |
[
false
] |
I mean, think about how electromagnetic radiation gets exponentially weaker as it propagates away from the source. Now think about nebula, gravitational bodies, interstellar dust, asteroids, and every other obstruction between here and the next hundred light years. There's plenty to mix up and scatter whatever original signals might be out there. By the time the waves reach earth, I don't see how any signals could be much higher than zero. I mean, unless you have the ability to lase a star, or modulate a quasar, or vibrate a black hole... ...logically, why would we even expect to find a discernible signal?
|
[
"It doesn't get exponentially weaker, it goes as 1/r",
" until you start looking at absorption from gas and dust.",
"We can detect radio signals from space, for instance from pulsars or from the CMB.",
"The thinking is that if there are civilizations that are communicating across vast distances, they are using frequencies that are not absorbed by interstellar gas and dust. Thus, it would be wisest to search in those frequency bands."
] |
[
"I think that the answer might be that we don't necessarily expect to find a signal - but if we did, it would be quite something! So we look. Chances are we'll eventually get better at it if we keep trying.",
"Note also that the intensity only decreases as the square of the distance if you're emitting in all directions. Directional radiation is much less lossy."
] |
[
"Note also that the intensity only decreases as the square of the distance if you're emitting in all directions. Directional radiation is much less lossy.",
"Not true. ",
"Free space path loss",
" is a fixed value for a given frequency over a given distance, regardless of antenna gain. If your antenna has gain, then signals radiated from the antenna will be stronger in one direction than another, but over a given distance will experience exactly the same loss as if they were radiated by an isotropic antenna.",
"A look at the ",
"Friis transmission equation",
" verifies this. Transmit and receive antenna gains (Gt and Gr) are independent from path loss (lambda/(4",
"R))",
" when computing the transmit/receive power ratio. Gt and Gr are gains, and do not change whether path loss is squared or not."
] |
[
"Are human beings still selectively evolving to get rid of vestigial or unwanted traits/diseases? Or has modern society and modern medicine ensured that those traits & diseases will continue to get passed down through our progeny?"
] |
[
false
] |
[deleted]
|
[
"The genetic code keeps changing, mixing up, taking on mutations, but features only get knocked out if they prevent individuals from having offspring. There's no reason to not have male nipples because they take little energy to produce and they don't prevent mating. Also, if a man didn't have nipples, a woman might question his ability to produce female offspring having nipples, so he might not get any action.",
"On the other hand, all kinds of people with apparent genetic \"problems\" are able to find mates and have offspring, so it seems unlikely that oily skin, unwanted female hair, propensity for cancer, and the rest will be selected against.",
"However, I'm pretty sure the human jaw bone is smaller than it used to be due to neotenization of the human face by sexual selection. "
] |
[
"There are also a lot of things that we generally think of as 'undesirable traits' that can actually be useful which is why they persist: ADHD (seeing/hearing everything), nearsightedness (social behaviors and reading emotions), even sickle-cell anemia (malarial resistance). It's not as simple as 'if we don't have a use now, will it die out'. "
] |
[
"unless there's a strong selection pressure happening (selective breeding), traits generally randomize. If a certain trait (e.g. reproductive tract malformation) results in you not reproducing that's a pretty strong selection pressure, and generally is reduced in the population. ",
"This is not true of all traits, though, because of the way that we as a society define the term. (insert standard-issue \"gay gene\" discussion here)."
] |
[
"Why is it that during kidney transplants the recipients kidney is often left in place if it's not causing any issues such as high blood pressure or infection. Would it do any harm to remove it?"
] |
[
false
] | null |
[
"Wow, so you just add in another connection to whatever the kidney connects to, wild"
] |
[
"Wow, so you just add in another connection to whatever the kidney connects to, wild"
] |
[
"How does attaching another kidney not crowd the other kidney out?"
] |
[
"Why are most planetary bodies/the asteroid belt/kuiper belt aligned very similar orbital planes?"
] |
[
false
] |
Am I mistaken or is there a reason?
|
[
"The most accepted idea for the formation of a solar system begins with a big cloud of gas/dust that has some angular momentum, and as it contracts due to mutual gravity it starts to spin faster, and the spinning spreads it out into a disk, ",
"like this NASA animation",
". I always imagine it like a ball of pizza dough being spun out into a disk. The center of the cloud coalesces into a star and the rest of the disk condenses into planets/asteroids/whatever else junk makes up the solar system. So when everything forms out of the disk it will lie in the same plane as that disk. There are of course variations based on multiple stars, or gravitational interactions or captured objects that happened to be passing through from somewhere else, but that's the basic idea."
] |
[
"Thanks, that clears it up nicely."
] |
[
"Well, the solar system is hypothesized to have formed from a ",
"nebula",
". Nebulas are just irregularly-shaped clouds of gas, and the gases (mostly hydrogen with some helium) are under the effects of gravity so they slowly move towards the center of mass. As they fall towards the center, the individual clumps bump into each other and push each other out of the way of a straight line towards the center.",
"Imagine if you are pouring water down a sink, starting from the most sloped part of the bowl. If there's nothing in the way, the water will just tumble down the bowl straight into the drain. But what if there was something in the way? The water will bounce of it and go around it. But when it does so, it picks up some angular momentum and no longer goes straight towards the drain anymore, but rather spirals towards it. And the water starts out slow but slowly picks up speed, with the fastest movement right at the drain.",
"Gravity works in a similar way, in that it's pulling all the matter towards the center of mass. The difference is that gravity works in three dimensions, instead of just the two-dimensional surface of the sink. So what you have are spirals (made of gas) approaching the center of mass from all directions. And eventually what happens is that these spirals bump into each other and they add their velocities together, making them into bigger spirals. These bigger spirals spin much faster, and the more that gets added to them, the faster they get. Eventually the spirals get so fast that they miss the center of mass entirely, and instead simply orbit it. The biggest spirals now influence the center of mass as well, and all the matter that is already at the center of mass begins to spin ",
" the biggest spirals, that now contain a significant amount of the matter. These biggest spirals now dominate, and all the other spirals that are orientated differently collide with the bigger spiral and as a result they get absorbed into it. The plane with the fastest spinning spirals becomes the only plane left that actually has any gas in it, because the other planes are left unobstructed and matter falls right into the center of mass.",
"And what we see today is that most of the non-sun matter is on a plane, and this is the same plane with the biggest and fastest spinning spirals that prevented matter from falling straight into the center of mass the most. All of the rest of the matter was not as obstructed and as a result fell right into the center of mass of the Solar System. ",
"The Sun is 99.86% of the total mass of the Solar System",
", so most of the matter of the Solar System did in fact just fall right into the center of mass. And these spirals themselves, while they went around the center of mass, also slowly fell into their ",
" centers of mass. One of them ended up being Jupiter, which has ",
"2.5x as much mass as all of the other planets in the Solar System combined",
".",
"For more technical information, please check out ",
"Centripetal force",
", ",
"angular momentum",
", ",
"orbital mechanics",
", and ",
"gravity well",
"."
] |
[
"How to detect cancer"
] |
[
false
] |
If cancer cells have to "recruit" blood vessels to feed them, couldn't we do a whole map of the blood system regularly and just check for differences in the blood vessels to find areas where those vessels have be recruited to feed cancer cells?
|
[
"I see where you are going with this but there are several problems with your idea. First, it isn't cancer cells that do this \"recruitment\" but rather tumors that do it. A cell here or there has sufficient nutrients to where it does not need a new blood supply to meet it high metabolic demands. It is when you have too many cells aka a tumor that this neovasculature occurs. Some argue, that at the point at which a tumor creates it's own blood supply, there is already a good chance that a metastatic cells has been formed. There exist drugs called VEGF inhibitors (VEGF is a signalling protein which cells release when they want new vessels to form) and this gets to the 2nd part of your question or something that you could easily ask. Why don't we all just take VEGF inhibitors all the time. Well the problem with this idea is the problem with your whole mapping idea, the production and destruction of vessels is an every changing process in the body as is. Based on supply demands (let's say you get a cut or an injury) or in order to combat things like blood pressure fluctuation, your body is always changing blood vessels. Hope this helps!",
"TL;DR: Too much variability in the production of blood vessels of the body to map the system",
"Source: Cancer Biology Ph.D. student"
] |
[
"a whole map of the blood system",
"sounds incredibly expensive... maybe someone with some insight into medical imaging could expound on this? "
] |
[
"So is there a way to screen for cancer?",
"I ask because my neighbor went to the doctor 2 weeks ago for headaches and was just diagnosed with a brain tumor, so the question that's obviously been on my mind is was there a way he couldn't known before this?"
] |
[
"How do we have pictures of our own galaxy?"
] |
[
false
] | null |
[
"I believe the most accurate representation was published during the 212th American Astronomical Society meeting in St. Louis, Mo. NASA's Spitzer Space Telescope was used to obtain Infra-red images of the galaxy, and scientists determined that The Milky Way is a 'Barred Spiral Galaxy' with 2 major arms and 2 minor arms. I can't find the paper right now but maybe someone can find it for more detail?\n",
"Artists concept from their findings"
] |
[
"I believe the most accurate representation was published during the 212th American Astronomical Society meeting in St. Louis, Mo. NASA's Spitzer Space Telescope was used to obtain Infra-red images of the galaxy, and scientists determined that The Milky Way is a 'Barred Spiral Galaxy' with 2 major arms and 2 minor arms. I can't find the paper right now but maybe someone can find it for more detail?\n",
"Artists concept from their findings"
] |
[
"A nice animated gif",
" created by the UCLA Galactic Center Group showing the stellar orbits of the stars at the center of our Galaxy revolving ",
" fast around some unseen object. A simple application of Kepler's Laws reveals the object at the center to be of the order 4 million solar masses. The best evidence yet for a supermassive black hole.\n",
"Gif of the actual footage",
" focusing on the especially important S-02, which was observed for more than 1 full orbital period."
] |
[
"Is Adaptation a direct result of Intelligence, or is the relationship more complicated than that?"
] |
[
false
] | null |
[
"Again, I don't know what she means by that. How are you measuring and defining intelligence? What does it mean to adapt? I'm not sure I understand what the picture you are describing looks like or what someone is supposed to do with it. Do you mean that you are still able to read a sentence if you replaced all the e's with 3's? That would presuppose that someone knows how to read. Does that mean that animals or illiterate humans are not intelligent? ",
"In any case, this question is too vague to answer properly. Ask your friend what they mean and ask them to provide some examples."
] |
[
"What do you mean by adaptation? Adaptation to what?"
] |
[
"They were pretty general about it. It's why I think they oversimplified."
] |
[
"Are there any quadrinary star systems? (4 stars in an orbit)"
] |
[
false
] | null |
[
"Yes! I think the record is currently ",
"seven stars in one system",
".",
"The way it works is that only binary-like orbits are stable, ",
" you can nest those binary-like orbits. So if you have two stars in a close binary, you can have another star orbiting at a large distance from those two, and have a ternary system. Or you have have two pairs of close binaries both orbiting each other, and then have another pair orbiting at a larger distance around those four. If you look under the \"multiplicity\" section in the wikipedia entry above, it has a nice couple of plots of how these orbits are nested.",
"So yeah, there definitely are 4 star systems. You can have 3 stars all orbiting around one star that's near the centre of mass. You can have a two stars in a binary, and two stars orbiting at different distances around that binary. Or you can have two pairs of binaries orbiting around a common centre of mass."
] |
[
"Additionally, the brightness of a star (during the main sequence) is very sensitive to its mass. A star that's twice as massive is much more than twice as bright. So a more distant massive star in your star system could be as bright as or even brighter than the Sun you primarily orbit around. There really are a lot of possible skies that should quite realistically exist."
] |
[
"When you talk about these various orbits, what are we looking at in terms of distance from the other stars? In other words, if you were on a planet in one of these systems, would you see multiple suns in the sky ala' Tattoine, or would the other stars be so far away, that they would just show as really bright stars at night?"
] |
[
"If lightning tries to find the quickest and shortest way from the skies to the ground, why does it squiggle so much?"
] |
[
false
] | null |
[
"Lightning does not follow a pre-existing path of least resistance. Air is incredibly resistive, and cannot carry any significant amount of current. The only way to make it conductive is for it to be ionized, which lightning does by itself in the early parts of the strike. So it's not following a path of least resistance, it's creating one, but its method of doing so is imperfect and not totally predictable--hence the squiggliness.",
"For reference, here's a high-speed video recording of a lightning strike:\n",
"https://www.youtube.com/watch?v=JVXy-ZqqZ-g",
"In the first part of the strike, local accumulation of charges called leaders work their way down from the cloud in zig-zaggy branching paths. The leaders advance by ionizing the air in small jumps, which is somewhat unpredictable. So, in general they approach the ground, but the exact path they take is indirect because of that unpredictability. ",
"Note that this is very different from the usual rule of \"electricity follows the path of least resistance\". Air is extremely resistive. As a result, lightning doesn't follow the path of least resistance--it creates the path of least resistance by ionizing the air.",
"Leaders don't carry very much current--their role in the strike is to create an ionized channel--basically, a wire--that the main current pulse (the return stroke) can follow. But, they are still luminous and visible even without a huge amount of current because the high electric fields and the corona surrounding them. I just checked two flashes in my data, and the leader propagation phase lasted a tenth of a second in each. This is very fast to human perception, but far from instantaneous.",
"When a leader gets close to the ground, a streamer from the ground takes off to meet it, and when they meet, there's an electrical connection between ground and cloud. If you've ever played irresponsibly with wires by joining two things of different voltage--like car battery or outlet terminals (if you haven't, don't)--then you know that a huge amount of current will flow through it and it will get very hot. So that's the return stroke--a very brief current pulse that heats the air to about 20,000 K, making it glow from the heat. (This rapid heating also produces thunder, and how the thunder sounds is dependent on the shape of the strike. It's a fascinating subject that I wrote my masters thesis on.) Most of this current flows along the single channel that connects cloud to ground--the other leaders basically shut down because there's no longer a voltage difference driving them.",
"In the video, the leaders propagate downward from about 0-4 seconds, and the return stroke happens at 4 seconds. The channel keeps glowing after that for a while after that, which is only a small fraction of a second in real time. Subsequent return strokes follow the first and can make the channel light up again."
] |
[
"It doesn't find the shortest or the quickest, it 'finds' the path with least resistance. Because electric currents in ionized gases are complicated that path won't necessarily be the physically shortest route.",
"In reality the current takes all paths, with the vast majority traveling along the first ionized path left by the cloud-to-ground strike. You can get funky shapes and forks if more than one low-resistance path exists nearby."
] |
[
"This is wrong. The return stroke comes from the ground up, as you said. But the large zig-zagging branching structures that come down from the cloud and make up the strike are totally visible before the return stroke happens. And, lightning doesn't follow the path of least resistance: it creates it. Air is so resistive that the only way any significant current can flow through it is for lightning itself to ionize it. See my other comment."
] |
[
"Do Alzheimer patients realize they have Alzheimer?"
] |
[
false
] |
I don't know if there are multiple stages of Alzheimer, but I'm talking about the more serious cases here. I've known an old lady who often wouldn't even recognize close relatives anymore. What is the thought process of someone in this situation? Have there been any studies on this? Do they think they're meeting complete strangers? In that case, do they wonder where the strangers came from, who they could be and why they're acting like they knew the patient? Or do they realize that their illness could be the reason for not recognizing the people? I've also heard that Alzheimer patients sometimes have "good" moments, when they are able to remember things and think clearly. During this time, do they realize what's happening to them and that they could go back to the "bad" state anytime?
|
[
"You're asking a bunch of questions, so I'll try to answer them in sequence.",
"1) Patients realize they have Alzheimer's until late stage. At that point, they may or may not realize that something's wrong with them. They tend to recognize that they're in a hospital or not in their own home, but they don't know why. They're known to still \"glimpses\" of reality at that point, in fact, the guy who the disease is named after first realized it was a separate disease when his patient suddenly pulled out of psychotic dementia (a relatively young woman that he saw from her late 40's-early 50's) and told him with great clarity, \"I'm losing myself.\" He performed an autopsy on her brain, finding amyloid plaques in the temporal region, and subsequently was able to find many more asylum patients with the same symptom cluster that showed the same feature at autopsy. By end stage, they have little to no consciousness of the world around them. When they can talk, they show no orientation to time, often thinking it's still the era of their childhood. They forget to eat, rarely sleep, and many develop ancillary illnesses like dysphagia that causes attacks of choking. ",
"2) The location of damage in this disease begins in the hippocampus, located in the medial temporal lobes of the brain. This structure is needed for all conscious learning and memory, and as it degrades in Alzheimer's, patients are left with progressing degrees of amnesia. When it's totally destroyed, they're only able to access memories by accident that have become so entrenched that they're part of what's called a cortico-cortical connected network, in other words, they no longer need the hippocampus present to be recalled. In the early stages of cognitive decline, and for a while thereafter, patients generally recognize that it's the disease preventing them from remembering things, and it often causes distress, frustration, and shame in them. Formation of cortico-cortical connections happens as a function of how frequently something gets recalled, and how many things are connected to that thing, so early learning, like how to read, do math, and recognize their siblings in photos gets retained, and memories of spouses get retained for a long time, too, sometimes up until death. Memories that happened throughout their adult life are usually lost in reverse chronological order, most recent go first, and trivial memories are lost before more profound memories.",
"3) Another thing that commonly happens in this disease, as the damage spreads, is development of psychosis, and loss of sleep/wandering at night. Patients become paranoid and sometimes violent. When meeting family, they may believe that their children are the product of their spouse's infidelity (infidelity is a frequent accusation by late-stage patient's, if their spouse is still alive), or they may greet them apologetically but warmly; either way, they're confused by the experience. Only on rare occasion will they suddenly recognize their kids. I'm describing late-stage here, though. When psychosis is a big feature, you can't even explain to them why they don't recognize their kids, as the psychosis will supply faulty information to them, and they feel like they \"know\" that information. ",
"4) The \"good\" moments are really fleeting towards the end, so there isn't a lot of time for them to be aware that the moment will pass. That's more early to middle stage stuff.",
"Lastly, although I've mentioned the beta-amyloid plaques that are seen in the disease, and lots of money and time have been devoted to researching them, they're not a great correlate as they're neither necessary or sufficient to cause Alzheimer's. People can die of AD without them, and many people with them don't have AD, but it happens to be a feature that can be easily visualized by scans. Another feature, build up of tau-phosphorylated protein in something called neurofibrillary tangles, correlates better with sequelae, but they're very tough to scan for. ",
"There are a lot of good resources out there, if you want more info, but here are a few: ",
"http://www.alzheimers.gov/",
", ",
"http://www.ninds.nih.gov/disorders/alzheimersdisease/alzheimersdisease.htm",
". "
] |
[
":( Because there's not a lot to be done in this disease, regular scans aren't really indicated. Mostly, people complain to their doctors in the pre-AD stages, something called \"mild cognitive impairment\". Frank AD can't be diagnosed from that, though. Neurologists will try and rule out other diseases and medicational side-effects first, as things like beriberi and some heart meds can cause similar complaints; diagnosis is usually coupled to scans. ",
"Longitudinal testing with AD drugs is being done on people with early cognitive decline, though, so maybe they can stave off the disease yet, but I haven't heard or seen any big breakthroughs with these. ",
"If you had the chance to get regular MRIs, and they were specifically targeting this disease, they'd look for early changes in hippocampal volume, as that's a predictor of AD development. Really, though, no doctor would likely send you for these because they're very expensive, unless they felt concerned that you had serious problems already."
] |
[
"Wow.. thanks, this was really informative.",
"What are ways to detect onset of AD? Can regular MRI scans spot any degeneration? "
] |
[
"When it’s said that the mantle is fluid, how fluid are we talking here? What kind of texture is it?"
] |
[
false
] | null |
[
"The mantle is best visualised as a crystalline rock, which is hot - and therefore plastic - enough to deform at the rate of a few mm per year.",
"To give it numbers, the upper mantle has a viscosity of about 2.8 × 10",
" ",
"https://earth-planets-space.springeropen.com/articles/10.1186/s40623-019-1014-x",
" for comparison, peanut butter has a viscosity of about 10",
" Pa s. Water has a viscosity of about 0.0009 Pa s. Ice has a viscosity of around 10",
" Pa s.",
"Some geological processes are able to bring chunks of mantle to the surface. These peridotite rocks are then able to be analysed (we've never filed anywhere close to deep enough to directly sample the mantle). There's some nice examples shown here, with more information. ",
"http://www.alexstrekeisen.it/english/pluto/peridotites.php"
] |
[
"It's all about timescale. If you tried to hit it with a hammer you'd be happy to call it rock. ",
"It's a fluid on geological timescales. It's a solid at short timescales."
] |
[
"Correct me if I am wrong here but isn't the convective timescale of the mantle around 100 thousand years? The convective turnover timescale essentially gives you a timescale in which you can treat the mantle as fluid."
] |
[
"Nitromethane Model Fuel Evaporation"
] |
[
false
] |
I use nitromethane fuel for RC models. The fuel is a mix of methanol, nitromethane and oil. I understand from reading that this fuel evaporates over time. However when the bottle of fuel is left in storage it begins to create negative pressure in the container, pulling the walls in (plastic bottle). If the fuel is evaporating, wouldn't the gas take more space than the liquid, increasing the pressure in the bottle? *Edit. I should add, the temperature where it is stored is generally constant. Not varying more than maybe 10 degrees +-.
|
[
"Yes, if the fuel was evaporating inside the bottle then the pressure would increase. If you put any liquid into a closed container some of it will evaporate until the partial pressure of that component in the gas phase matches the vapor pressure of the compound at the given temperature. The only cases where there should be negative pressure in the container (do to vapor/liquid equilibria) would be if the container cooled at all from when it was capped, as depending on the components it could drastically reduce the vapor pressures and cause a noticeable pressure drop as the molecules condense. 10 degrees (F or C) is certainly enough to have a noticeable effect on the internal container volume due to ",
"Charle's law",
" as well.",
"It also could be possible that some material is able to escape the storage bottle through diffusion through the plastic. If some component could \"leak\" out without letting air in then the volume would certainly decrease inside the bottle. If you are using a bottle designed for storing this fuel then that should not be a problem.",
"It is also possible if you mix this fuel yourself that the original mixing releases some energy and slightly warms the fuel. If you subsequently put it in a bottle and cap it the temperature could be significantly higher than ambient and when it cools down it would result in a lower pressure/volume inside. If you don't mix the fuel yourself this wouldn't be an issue, and it would only occur after the initial mixing."
] |
[
"Interesting. I don't mix the fuel myself, it was purchased this way. Also, the bottle is just a standard gallon jug, i suppose its possible that some materials could diffuse through.\nNow that the jug has collapsed (just slightly, but visibly) if I were to move it to a place that was consistently 10 degrees F warmer, should I expect it to build pressure and push the walls of the jug back out?"
] |
[
"Likely the 10 degree increase would push the walls back out, but that depends on a few things. The more headspace (gas) in the bottle, the larger the volume change will be from Charle's law. If the bottle is mostly full of liquid then the thermal expansion will be minimal.",
"It is likely the vapor pressure of the components will increase with a 10 degree change, but unless there is a significant jump for any of the compounds there shouldn't be a huge change in gas volume from liquid evaporating. I'm sure there will be some change, but your vessel shouldn't explode or anything exciting."
] |
[
"How does the internet work and how replaceable is it?"
] |
[
false
] |
[deleted]
|
[
"The Internet is just an idea, the idea is to connect computers so that they can share data in real time. How it is done now is via packets of data that are switched and routed to their destination. It is part optical and part electrical. The idea will not be replaced and the implementation is being constantly improved so I doubt there will ever be a complete replacement of either."
] |
[
"It's very possible - what we know now as the \"Internet\" was originally called \"Arpanet\". Both are basically a switched network that interconnects locations for the exchange of information. ",
"Arpanet",
" was originally created for use by Government Agencies and Learning Institutes. ",
"The latest change that I know is the introduction and implementation of IPv6 addresses' to extend the Internet to handle more addressable locations compared to IPv4. Right now many providers dual stack, to support both types of address - but it is still a TCP/IP network. ",
"There is a replacement to the internet called ",
"\"the grid\"",
" but iirc it is still in its infancy - I would expect it to take some time for this to grow globally. CERN originally created the new network, to transmit the huge volume of research data from the LHC. "
] |
[
"Even if the grid is the future, it still follows the basic principles of the internet. At least at the idea level. It's not about how the packets are passed, it's more about the ability to share information. ",
"The fundamental principle of what the internet is today (ignoring all the suppression) is the ability to freely share information. ",
"Whatever comes next will have to embrace that principle."
] |
[
"How do computers simulate probability distributions?"
] |
[
false
] | null |
[
"So the probability density function (PDF) for your random variable X is P(x) = (1/σ√(2𝜋)) exp(-(x - µ)",
"/2σ",
"), where P(x) is the probability density for X = x (mean = µ; standard deviation = σ). The associated cumulative distribution function (CDF) is",
"F(x) = 1/2 [1 + erf(x - µ)/σ√2] ,",
"where erf is the ",
"\"error function\"",
". F(x) is the probability that X lies between -∞ and x (so F(x) always lies between 0 and 1).",
"You want the inverse of this function, which is:",
"x(F) = µ + σ√2 erf",
"(2F - 1) ,",
"where erf",
" is the ",
"\"inverse error function\"",
", which can be approximated by a unique Maclaurin series.",
"Now use a random number generator (which gives a number between 0 and 1 according to a rule which, for most practical purposes, approximates a flat probability distribution over the [0,1] interval) to generate a value for F. Plug that value for F into the equation above and Bob's your uncle."
] |
[
"And when an analytic transformation is not available, you can use the acceptance-rejection method to draw values from an arbitrary probability distribution."
] |
[
"Upvote. Generically: find a random number between 0 and 1, and then find the inverse CDF of your distribution at that number."
] |
[
"How do chromosomes identify their partner chromosome after mating?"
] |
[
false
] |
When a sperm cell fertilizes an egg. How does the male chromosome 7 know to attach to the female chromosome 7 to make a chromosome pair and not chromosome 8, 9, 10 etc. How does Y know to attach to X as well, since they aren't even similar in length?
|
[
"We are still learning the details of the cellular and molecular machineries involved in both somatic (matching chromosomes pairing in almost every cell of the body) and meiotic (matching chromosomes pairing during the development of germ cells) homolog pairing. Interestingly, aside from genes involved in pairing (more than 40 identified to date), there are also a fairly sizable number of genes involved in anti-pairing.",
"Bosco (2012)",
" might be a good starting point for further reading."
] |
[
"Chromosomes don't attach in the traditional sense, it's local sequence homology that drives transient interactions. There are a lot of structural proteins that stabilize this interaction (",
"synaptonemal complexes",
" being well-known), and more are found every day. "
] |
[
"microtubules keep track of which female chromosome is which. this is why when you suck out the female chromosomes and put another 13 in, the fertilized egg doesn't develop right, because the microtubules get broken when you just swap chromosomes like that."
] |
[
"Gravity is described as bending space, but how does that bent space pull stuff into it?"
] |
[
false
] |
I was watching a Nova program about how gravity works because it's bending space and the objects are attracted not because of an invisible force, but because of the new shape that space is taking. To demonstrate, they had you envision a pool table with very stretchy fabric. They then placed a bowling ball on that fabric. The bowling ball created a depression around it. They then shot a pool ball at it and the pool ball (supposedly) started to orbit the bowling ball. In the context of this demonstration happening on Earth, it makes sense. The pool ball begins to circle the bowling ball because and the bowling ball makes it so that the stretchy fabric of the table is no longer holding the pool ball further away from the Earth. The pool ball wants to descend because Earth's gravity is down there, not because the stretchy fabric is bent. It's almost a circular argument. It's using the implied gravity underneath the fabric to explain gravity. You couldn't give this demonstration on the space station (or somewhere way out in space, as the space station is actually still subject to 90% the Earth's gravity, it just happens to also be in free-fall at the same time). The gravitational visualization only makes sense when it's done in the presence of another gravitational force, is what I'm saying. So I don't understand how this works in the greater context of the universe. How do gravity wells actually draw things in? Here's a picture I found online that's roughly similar to the visualization:
|
[
"I found ",
"this",
" video to really help explain how gravity changes the paths of objects, I think it's particularly effective because he demonstrates it as a bending of space and time, not just space, and is able to do so by reducing it down to only 1 spatial dimension. "
] |
[
"You can visualize what this guy is talking about by considering straight lines on the surface of a sphere. Remember the surface of the sphere is the space you have to work with, so a \"straight line\" means the line you'd follow if you were an ant on that sphere that's walking straight forward without turning. In the specific case of a sphere, it's also the line formed when you stretch a string between two points in exactly the shortest distance the string will travel, so you can test yourself using a large ball (Pilates ball works great), a marker and some string.",
"So, you take your sphere and draw a triangle on it using your string and marker to make lines that are straight as far as the surface of the sphere is concerned. Then measure the three angles in your triangle. You'll find the angles in your triangle add up to more than 180°. You'll even find it's possible to make a polygon that has surface area but only two sides. (Run your straight lines between opposite sides of the sphere, and pick two directions.)",
"You'll also notice that straight lines made from one point will 'curve back' on each other and intersect. (In 'flat' Cartesian space, this doesn't happen. They go their separate ways.) In the opposite curvature, hyperbolic space, it gets even weirder. If you make a triangle, the sum of its angles is less than 180°, and if you mark down two parallel lines they start veering away from each other and end up infinitely far apart at the horizon. So if you were to put on roller blades that follow those lines, you'd end up doing the splits and fall off. Parallel lines are an impossible concept in ",
" spherical space! ",
"(Edit:)",
"Caught myself in an error. Sticking with 2D space for simplicity, given two points A and B and a straight line through A: In spherical space, there are zero straight lines through B that are parallel to the line through A. (But there are circles parallel to it!) In 'flat' Cartesian space, there is exactly one line through B that is parallel. In hyperbolic space, there are infinite lines through B that are parallel to the line through A.",
"(/Edit)",
"So what do you do if you want to make train tracks in hyperbolic space? Turns out, your rails have to constantly curve toward each other as they run off into the distance. This also means that if you are a sizable object and not an infinitely small point, as you move along those rails you'll feel like you have to work to keep your arms in. Your arms and legs will want to fly away from your body, and if you go fast enough you'll get ripped apart by the tidal force of your body trying to accelerate its outer parts back together as the curvature of space tries to send them in \"straight lines\" in all directions.",
"The difficult part is taking that understanding up a dimension. You can easily play with it in two dimensions (hyperbolic is harder than spherical but possible), but getting to a point where you can understand what it means in 3D is a bit of a mental challenge.",
"Edit: ",
"Thanks everyone! I'm glad this helped some people understand spacial curvatures!",
"The class to take is Non-Euclidean Geometry. Check your University's math department. Mine involved lots of cutting up and taping strips of paper together, making models of different spaces that we could play with, draw lines on and measure angles. Lots of \"whoa, dude\" moments. Also talked about how to make a map of something round like the Earth on something flat like a piece of paper, the different kinds of distortions you'd see, etc. Fun class! (Disclaimer: Yes you'll have to do proofs.)"
] |
[
"XKCD brings up the same objection. ",
"http://xkcd.com/895/",
"The real answer is that gravity doesn't just bend space. It bends spacetime. Spacetime is really tough to wrap your mind around as the time dimension acts qualitatively differently than the space dimensions. However, objects in free fall move in a path that is as straight as you can define it in spacetime, that is a geodesic. This just happens not to be a straight line in just plain old space, without time added to the mix.",
"http://en.wikipedia.org/wiki/Introduction_to_general_relativity#Probing_the_gravitational_field",
"http://en.wikipedia.org/wiki/Geodesics_in_general_relativity"
] |
[
"How is a virus \"born\"? I am under the assumption that viruses are not alive, so do they reproduce? How does one come into our world?"
] |
[
false
] |
See title. Thanks for the answers!
|
[
"The replication point has been made here already, but that does not explain how viruses \"come to be\". ",
"For that I give you this",
"http://www.biology-direct.com/content/1/1/29",
"TL;DR Ancestral viruses evolved in a pre-cellular environment as selfish genetic elements and adapted to prokaryotic and eukaryotic life as they emerged",
"EDIT: Replication, not reproduction"
] |
[
"Not all viruses integrate into the host's DNA, and some viruses carry their own transcription machinery (such as reverse transcriptase) but most do use the cell's. They also use host cell energy and nutrients to reproduce."
] |
[
"This is the most on-point answer. Further recommendations: Vincent Racaniello's ",
"blog",
". For the emergence of viruses see ",
"this",
" page, specifically the evolution and emergence section. ",
"This",
" page talks about viruses in the context of the tree of life. I also highly recommend Vincent's podcast \"This Week in Virology\" for discussion of these issues. "
] |
[
"Why is TESS exciting?"
] |
[
false
] |
TESS the transisting exoplanet survey satellite is scheduled to launch soon, what are the benefits of TESS compared to other planet hunting hardware and when should we expect to hear about it's first discoveries?
|
[
"Scientist at <famous CS research institute> here,",
"The Transiting Exoplanet Survey Satellite (TESS) will discover thousands of exoplanets in orbit around the brightest stars in the sky. In a two-year survey of the solar neighborhood, TESS will monitor more than 200,000 stars for temporary drops in brightness caused by planetary transits. This is the same method used by Kepler in its planet hunting mission, the \"Transit Method\". This first-ever spaceborne all-sky transit survey will identify planets ranging from Earth-sized to gas giants, around a wide range of stellar types and orbital distances. No ground-based survey can achieve this feat.",
"but what about Kepler? isn't that space-based too?",
"TESS is an ",
" survey. Kepler only looks at 1/400 of what TESS will observe in the sky. TESS is also designed to look at stars that are nearer and brighter than the ones Kepler has studied. Because of this, exoplanets that are identified by TESS can be potentially followed up and researched further by ground based telescopes. Many Kepler exoplanets cannot be followed up by ground based telescopes for mass measurements since they are too far away. With both the size and the mass of an exoplanet, you can learn the density, and TESS opens that door because it looks at the nearest and brightest stars, where we can follow up with ground-based telescopes."
] |
[
"It will miss some, the only way to avoid that would be to launch enough copies of TESS that the entire sky could be simultaneously monitored. Given a long enough time though it shouldn't really matter, unless the planet's orbital period is exactly the amount of time it takes you to come back to that patch of the sky. Assuming it's not, each time you come back to observe that star the planet will be in a slightly different place, so eventually you will catch every transiting planet, it just might take a while."
] |
[
"Followup question: How will TESS avoid missing transits when it's looking the other way?"
] |
[
"A couple questions about the big bang..."
] |
[
false
] |
So.. I have been wondering about the age of the universe. I often hear that the universe is around 14 billion years old. If I understand correctly, we were able to calculate the age of the universe once we determined it was expanding. Although many navel gazers still seem to think the big bang came from a singularity, it appears as though the primordial energy that made up the universe is better described as significantly more dense than it is now and considerably smaller but still infinite and certainly not confined to a singularity. Is there any reason to believe this early composition was stable for any length of time? If so, the universe could be significantly older. So, is 14 billion years the age of the universe or the amount of time it has been expanding? Or both? I have other questions, but I will save them depending upon the answers I receive. Thanks
|
[
"More than that, by all accounts nothing that hypothetically \"existed\" or \"occurred\" before the start of the Big Bang can have had ",
" effect on anything that exists now, ever has existed, or ever will exist."
] |
[
"I … didn't actually understand that.",
"I did see the word \"intuition\" go by, though. Stop that, right now. Your intuition has been finely honed over decades of falling out of bed, peeing and dropping things. It is ",
" when you're studying things that are completely outside the realm of human experience."
] |
[
"I … didn't actually understand that.",
"I did see the word \"intuition\" go by, though. Stop that, right now. Your intuition has been finely honed over decades of falling out of bed, peeing and dropping things. It is ",
" when you're studying things that are completely outside the realm of human experience."
] |
[
"It is possible to slow a photon down to the point where it can be easily observed?"
] |
[
false
] |
Maybe freeze it?
|
[
"How are you planning on \"observing\" it?"
] |
[
"Light travels slower, not individual photons. In a dense material, the photons that make up the light we see are constantly hitting atoms and being absorbed/emitted and changing directions because of that. But they all go at c between collisions. And not in a straight line. Therefore the light is slowed, but not the photons."
] |
[
"It is absolutely 100% not possible to slow a photon down. There is no situation in which a single photon will not be traveling at c."
] |
[
"Symmetry breaking and forces"
] |
[
false
] |
[deleted]
|
[
"So if it weren't for the spontaneous symmetry breaking of the Higgs field, the W and Z wouldn't have mass and there wouldn't really be any difference between the EM and weak interactions.",
"The W and Z wouldn't even exist if not for the Higgs field; nor would the EM and weak interactions. It's more like what the OP described as \"electroweak force\", whose nature became obscured when the Higgs field turned on soon after the Big Bang.",
"Instead of \"spontaneous symmetry breaking\" you can read \"symmetry hiding\" to help with understanding. The underlying symmetry is there, but we never get to see it because the Higgs field is interfering with every single interaction involving the underlying fields.",
"Link with detailed explanation and pictures",
"Another link, with fish"
] |
[
"So if it weren't for the spontaneous symmetry breaking of the Higgs field, the W and Z wouldn't have mass and there wouldn't really be any difference between the EM and weak interactions.",
"The W and Z wouldn't even exist if not for the Higgs field; nor would the EM and weak interactions. It's more like what the OP described as \"electroweak force\", whose nature became obscured when the Higgs field turned on soon after the Big Bang.",
"Instead of \"spontaneous symmetry breaking\" you can read \"symmetry hiding\" to help with understanding. The underlying symmetry is there, but we never get to see it because the Higgs field is interfering with every single interaction involving the underlying fields.",
"Link with detailed explanation and pictures",
"Another link, with fish"
] |
[
"This was very helpful, thank you!"
] |
[
"What happened to me?"
] |
[
false
] | null |
[
"That's not a normal reaction to caffeine. It should have just made you more energetic, but energy drinks are crap. They're a ton of things that are bad for you in a can. You could have reacted to any number of things in it. ",
"Have you had caffeine (like in strong tea, coffee, or a regular soda), or any sort of energy drink since?",
"If all of your panic attacks have only come when drinking caffeine, I'd say it's a bad reaction to the stuff, and to just avoid it. If they come at other times, too, but caffeine is known to set them off, yeah, just avoid it. ",
"If this hasn't happened in a year, and it never happened before, I'd personally just go on like it didn't happen. If it happens again, I'd go to the doctor. ",
"And yea, on the safe side, always go to the doctor if something freaky is happening, but \"oh, hey, I had this problem a year ago that hasn't happened again...\" is probably not really something a dr. would be able to do anything with. "
] |
[
"Have you had caffeine (like in strong tea, coffee, or a regular soda), or any sort of energy drink since?",
"Yes, and I have had no ill effects.",
"If all of your panic attacks have only come when drinking caffeine, I'd say it's a bad reaction to the stuff, and to just avoid it. If they come at other times, too, but caffeine is known to set them off, yeah, just avoid it.",
"My panic attacks did not all come from consuming caffeine, I think caffeine would sometimes make me anxious as it increases heart rate.",
"The main thing I'm worried about is the head rushes I got, that is the only time I ever experienced them, I'd like to know if there is a link to them with panic attacks or if it is a separate phenomenon."
] |
[
"My son drank way too many energy drinks at an outdoor concert once - they were giving them out free, and I didn't notice. ",
"He ended up in a panic attack he described like that. We chalked it up to too many people, brand new situation, and too many energy drinks. It hasn't repeated since. He even mentioned what I think you mean by head rushes. He said he kept feeling like his head was light and tingly and the world was kinda grey. "
] |
[
"Can SpaceX's rockets travel faster than the speed Earth orbits the Sun?"
] |
[
false
] |
[deleted]
|
[
"If Earth moves 30km/sec, everything else on Earth moves also 30km/sec. If you launch a rocket from Earth it will have this 30km/s velocity by default + anything you give it by burning propellants. ",
"The velocity needed to escape Earth's gravity is called: Escape Velocity! ",
"https://en.wikipedia.org/wiki/Escape_velocity",
" ",
"In space travel you always think in deltaV's, meaning change in velocity. If you want to leave Earth you need a certain amount of dV to do so, and also some more dV once you reach Mars to slow down. There are dV maps of the Solar System: ",
"http://i.imgur.com/AAGJvD1.png",
" ",
"When a spacecraft goes from one planet to another they usually use a Hohmann transfer, which means they wait for the moment when the less dV is needed. ",
"https://en.wikipedia.org/wiki/Hohmann_transfer_orbit",
" ",
"A gif can explain it the best: ",
"http://ccar.colorado.edu/asen5050/projects/projects_2014/VonHendy_Michael/images/Hohmann.gif",
" ",
"And as always: look for Kerbal Space Program videos on YT with these keywords, or even better play the game yourself! You will learn a lot!"
] |
[
"I feel like everyone should be required to player Kerbal Space Program for a few hours.",
"Can SpaceX's rockets travel faster than the speed Earth orbits the Sun? ",
"The short answer is yes. ",
"You have to remember that we are all traveling the same speed as the Earth, and when we launch we do so to the East, and as close to the Equator as feasibly and economically as possible. This gives us the most fuel efficient head start. ",
"You also have to remember that essentially any acceleration from Earth's orbit will have you orbiting the Sun more quickly. However, we apply a little bit of thrust away from the Earth, and do so until the rocket's new orbit intersects with Mars' orbit at the same time as the red planet itself. It's not as simple as just pointing at Mars and hitting the gas.",
"So that got me thinking, if the rocket were to take off when the Earth isn't at it's closest point, will Earth overtake the rocket (assuming it's orbit takes it towards Mars at first)?",
"This leads me to believe that you think we launch when the two planets are at closest together linearly, sorta like if you drew a straight line from Mars to the Sun, that we launch when Earth is on that line. We don't necessarily do this. We simply launch when what I said above is true, and this is what its called the launch window.",
"I guess that means the answer to your second question is probably yes, we'd still have to slow our orbital velocity to match Mars'. We'd also traverse the distance between both planet's orbits more slowly, to wait for Mars to come around again. This would take much longer and is why we don't do it. ",
"I'll let someone else answer your other questions.",
"Edit: Fixed answer to second question after thinking about it again."
] |
[
"Obviously I'm not used to thinking about mechanics in space, so for some reason I didn't realize we would still maintain the 30km/sec even after the velocity needed to escape Earths gravity. ",
"If you want to get a feeling for this, i suggest you watch ",
"this video",
" explaining the basics on orbital mechanics. If this still leaves you with questions about interplanetary transfer, then you can ",
"watch this",
" right after. Kerbal Space Program is a wonderful game to learn orbital mechanics, i can only recommend to get it and try all this for yourself, you will love it. Also, the community at ",
"/r/KerbalSpaceProgram",
" is one of the kindest and funniest i have ever experienced."
] |
[
"In solids, is heat conductivity related to the speed of pressure (sound) wave propagation?"
] |
[
false
] |
From what I understand, sound waves typically propagate faster in a solid when compared to a gas. I was curious if there is a relation between wave-propagation and heat-conductivity in solids. What got me thinking about this was how some of the best insulators are extremely low density solids, and how sound waves usually move faster in dense solids rather than air. A good example could be: Would the tile used cause a sound wave to propagate slowly, because of the same physical properties that allow it to be a good insulator?
|
[
"It's a complicated question that may has different answers for different materials (and indeed for different processing methods for a given material), but in general the speed of sound is not inherently related ",
" to thermal conductivity (though both of them increase with increasing elastic modulus).",
"But you don't really need to go in depth to answer your question. Low-k materials like the Space Shuttle tiles really are not standard materials. Their low thermal conductivity isn't really a function of ",
" but rather ",
". They aren't really solids in the conventional sense - instead, they're basically a very rigid sponge. They're filled with air. That's why their bulk conductivity is so low. The thermal conductivity of the actual ",
" (the ceramic that makes up the matrix in which the air is embedded) is not remarkably low."
] |
[
"Pretty much, yeah. The vast majority of the bulk volume of an aerogel or similar insulating material is - you guessed it - air. That's why their lower limit on thermal conductivity is the thermal conductivity of air."
] |
[
"Interesting! Thank you for the reply. ",
"So if I'm picturing this right, the structure is more of a network \n",
"(sort of like this)",
"\nof solid mass conducting the heat, with fewer volumetric pathways for the heat to flow. Is that the gist of it?",
"I'm realizing I was more interested in the nature of heat transfer in solids rather than the sound propagation. I've yet to take a class on heat transfer but this gives me hope that I'll enjoy it when I do."
] |
[
"If someone has sleep apnea and they get knocked unconscious, will they die in their sleep?"
] |
[
false
] | null |
[
"That's why I'm asking. If you're unconscious, can your body wake you up?"
] |
[
"First, I think you might have a Hollywood-esque picture of what it means to get knocked unconscious. Getting knocked out so hard, that you are unconscious for several hours doesn't really ever happen, except for a large trauma (f.e. car accident) which will almost always lead to significant brain damage in any case. But - regarding your question - let's just say this happened and you are lying unconscious in the hospital.\nYou don't always wake up from apneas during sleep. You don't have to. And since breathing is largely unconsciously controlled by the brainstem, I don't know why unconsciousness should affect it. "
] |
[
"Sleep apnea does not kill you. It merely wakes you up lots during the night as you struggle to breath. You choke a bit, wake up a bit and start breathing again and then go back to sleep. ",
"The issue is not death, just a really shitty sleep. The breathing machines merely help the sufferer breathe better so his or her sleep is not interrupted. "
] |
[
"How does Titan maintain its thick atmosphere?"
] |
[
false
] |
Most planets or moons with dense atmospheres (Earth, Venus ect) have strong magnetospheres which prevent the atmospheric gasses from being stripped away by solar winds. However Saturn's moon Titan has no real magnetic field of its own Yet it maintains a rich atmosphere. So how does the moon maintain this thick atmosphere with the absence of a magnetosphere?
|
[
"Solar wind and insolation are much less strong at Saturn's distance from the sun, and temperatures are relatively low there.",
"For a much more detailed explanation, just read the wikipedia entry at ",
"http://en.wikipedia.org/wiki/Atmosphere_of_Titan#evolution"
] |
[
"Just a minor point: Venus does not have a strong magnetic field at all and there are no other moons with significant atmospheres. Only the earth matches the description you are putting forth of a body with a magnetic field which protects its atmosphere."
] |
[
"couldn't one say that the magnetospheres of the gas giants protect their atmospheric gasses from being stripped by solar winds? "
] |
[
"What exactly will happen when you swallow a piece of chewing gum?"
] |
[
false
] |
What if i become lazy, so instead of finding a piece of paper to throw away my chewing gum, I just swallow it? Is there any consequences?
|
[
"It will not be broken down slowly over time... it will pass, just like most other indigestible materials (like fiber or coins), in stool- mostly unchanged."
] |
[
"It will not be broken down slowly over time... it will pass, just like most other indigestible materials (like fiber or coins), in stool- mostly unchanged."
] |
[
"You can't digest it. It passes through he digestive system and gets thrown out normally.",
"It is not recommended for very young kids to chew gum, as swallowing a lot of chewing gum can block a small intestine and create problems."
] |
[
"Are hydrates mixtures or compounds?"
] |
[
false
] |
I hope it doesn't sound like an idiotic question because I am truly confused. I realised I could argue that it can be a mixture, and a compound. For example, an anhydrate can absorb water from the atmosphere to form a hydrous salt. Water that is absorbed does not form chemical bonds (I am not particularly sure about this fact. My Googling skills still needs further practice as I could not find a proper answer). Therefore, it is a mixture. However, a hydrate, when placed under heat, decomposes to form an anhydrate and vapour. This is under thermal decomposition, and as my chemistry notes states: Pure substances can be classified under two can be further classified as elements or compounds depending of whether they can be decomposed further by a chemical process. therefore, it can also be argued that a hydrate is a compound. Can someone please enlighten me as to whether they're compounds or mixtures? Also, if there is any fallacies in my statements, please do point them out and correct me. Thank you.
|
[
"If we use anhydrous copper (II) sulfate, and add water, it'll form copper (II) sulfate pentahydrate. H2O forms a dative covalent bond with copper. Doesn't heat break the dative bond? "
] |
[
"A \"dative covalent bond\" is what's more commonly known simply as \"coordination\", and really a \"covalent bond\" in the usual sense of the word. It's in no way fundamentally different how water molecules coordinate in solutions.",
"But isn't it a chemical bond? Compounds are formed when they undergo chemical reactions to form bonds, right? Or is it a form of non-covalent bonding? Also, if you could, can you elaborate further on \"no way fundamentally different how water molecules coordinate in solutions.\" I don't think I quite understand that bit. ",
"And yes, I do know that solutions are mixtures, but that's one of the reason why I am confused. When an anhydrate absorbs water, it's almost akin to placing them in a beaker filled with water, which sort of makes them a solution."
] |
[
"In inorganic chemistry, the water becomes part of the crystal.",
"Source: ",
"http://en.wikipedia.org/wiki/Hydrate"
] |
[
"Can a laser be white"
] |
[
false
] |
I know that lasers are of a specific wavelength, and white light is all colours combined. But I believe there is a way to combine multiple beams. If a white laser was created, would it be any stronger than a single laser of the same power as the combined?
|
[
"It cannot, for the reasons you mentioned. You could focus multiple lasers onto a single spot, but it wouldn't be a white laser, it would be several laser beams."
] |
[
"It looks like those were actually separate lasers that focused together.",
"http://www.iqsdirectory.com/copro/66111-the-cooke/"
] |
[
"To my knowledge, you can't build a laser cavity to be simultaneously resonant at all the frequencies associated with \"white light\". Do you have a paper or citation describing the construction of the HeCd laser you're referencing?"
] |
[
"Why do my actions or others actions in my dreams/imagination favor doing things facing left rather than right?"
] |
[
false
] | null |
[
"We can't really comment on anecdotes / isolated incidents without resorting to speculation which we try to avoid."
] |
[
"It’s every thought I have involving an action, not really isolated. Unless you mean isolated as in this is the first time you’ve heard of something like this"
] |
[
"We can't answer questions about why something specific happens to you in particular."
] |
[
"AskScience AMA Series: I'm Dr. John Troyer, Director of the Centre for Death and Society at the University of Bath and I'm here to talk about death, dying, dead bodies, grief & bereavement, and the future of human mortality. Ask Me Anything!"
] |
[
false
] |
Hello Reddit, my name is Dr John Troyer and I am the Director of the at the University of Bath. I co-founded the website (@DeathRef), the (@FutureCemetery) and I'm a frequent commentator for the BBC on things death and dying. My upcoming book is Technologies of the Human Corpse (published by the MIT Press in 2020). I'll be online from 5-6pm (GMT+1; 12-1pm ET) on Friday 27th September to answer your questions as part of .
|
[
"I’m a librarian with no medical and minimal scientific training. My community is yearning for a chance to talk about Death, so I’ve been hosting monthly Death Cafes so that the adults in my area have a safe place to come talk about all topics death. As a medical professional, is there anything you think laypeople like myself and the people I serve should know about death and dying? Are there any common myths or misconceptions you wish were dispelled? ",
"Thanks!"
] |
[
"As someone who has devoted their professional life to the subject, how do you, personally, feel about your own aging and eventual death?"
] |
[
"How do I most effectively donate my body to “science”? I don’t want to be in a body brokers warehouse waiting for a knee to get lopped off for a conference. That’s valid, but that has become a pretty shady business, right?"
] |
[
"The universe appears to still be expanding at an accelerated rate, but is it only because we are seeing the universe in a past state? (MIC)"
] |
[
false
] |
This seems like a simple explanation, but I'm wondering, if just maybe, that this mayhave been overlooked by scientists. To clarify my question in the title, I'm asking that since light takes time to travel, is it possible that not enough time has passed since the big bang to be able to observe the universe in it's current state - possibly in a state of deceleration? Essentially we are looking into the past. And in the past the universe had to be in a state of acceleration at some point (using the big bang model). Is there a chance it is decelerating and we have no way to observe this? Surely this is considered in all the calculations right? Or did I just unlock the universe? I don't know anything about physics, so I guess my official question for the post is, "why is my theory wrong"? EDIT: I said decelerate but I wasn't thinking about inertia in space. In stead of decelerate, I mean, no longer to continue accelerating.
|
[
"http://www.livescience.com/16367-nobel-physics-universe-expansion-accelerating.html",
"Bascially, they are expecting even the furthest objects to be closer than they are. They are already specifically looking for the furthest objects so they have included your concern.",
"THere is no known mechanism that would have suddenly reversed the motions of these distant objects or made them decelerate."
] |
[
"I'm asking that since light takes time to travel, is it possible that not enough time has passed since the big bang to be able to observe the universe in it's current state - possibly in a state of deceleration?",
"No. It's not the age of the object emitting the light that is the question -- it's the light itself.",
"What we see is that light emitted from distant objects has increased its wavelength through a process known as redshift. We know the frequency spectrum of many of these objects, and when we filter out the redshift we get the correct spectrum, but the fact is that the more distant the object is, the more it is redshifted. The only reasonable way to model this increase in wavelength is where the space which the photon is travelling is expanding as the photon travels. The longer the photon has been travelling, the greater the wavelength becomes. If space were not expanding, no redshift would be measured.",
"So don't think about it as \"we're seeing the very distant and very old object,\" think about it as \"the light we're seeing ",
" that object is changed in a way that makes no sense unless space is expanding.\" It's about the light's wavelengths and the distances involved, not about the object that emitted the light.",
"And in the past the universe had to be in a state of acceleration at some point (using the big bang model).",
"This isn't really correct. You're assuming that the expansion is ",
", as if there were a center of an explosion where the Big Bang occurred and from which everything else has been thrown away by a local force, but there isn't. The expansion is ",
" (meaning the notion of distance itself is getting larger), and the Big Bang did not have a center.",
"Is there a chance it is decelerating and we have no way to observe this?",
"If it were decelerating merely due to the effects of gravity, and space were not expanding, we would not be measuring any redshift. Heck even if it were ",
" due to some strange anti-gravity effect, we would still not be measuring redshift. The fact that we measure the redshift tells us space is expanding.",
"Surely this is considered in all the calculations right?",
"Yes.",
"I said decelerate but I wasn't thinking about inertia in space. In stead of decelerate, I mean, no longer to continue accelerating.",
"The redshift point still applies ... if space were not expanding then the measured redshift would be the same for all distances. That the redshift is proportional to the distance tells us that ",
" distances are expanding (a metric expansion, as opposed to an inertial expansion).",
"You can read more about the phenomenon of metric expansion here:",
"Wikipedia: Metric expansion of space"
] |
[
"Oh........",
"Thank you for taking the time to thoroughly disappoint me. I knew I most likely wasn't right but I had no idea how hilariously wrong I was. I guess if I knew there were different types of expansion I wouldn't even have asked. ",
"Is there any chance that something like redshift should be revisited in science or are we pretty damn sure that's exactly how it works in all situations?\nThat was very informative, thank you."
] |
[
"If virtual particles are appearing and then annihilating themselves constantly where is the energy from the matter antimatter collision?"
] |
[
false
] | null |
[
"Virtual particles don't literally exist. There are no physical particles being created out of the vacuum and annihilating."
] |
[
"OK but something is happening. There is a disturbance in space right? Like the Casmire effect is a real thing and people think that black holes will evaporate as a result of virtual particles right?"
] |
[
"OK but something is happening. There is a disturbance in space right?",
"No, there is not dynamical process happening.",
"Like the Casmire effect is a real thing and people think that black holes will evaporate as a result of virtual particles right?",
"The Casimir effect and Hawking radiation are effects of QFT, but they don't imply that virtual particles literally exist.",
"There is an entry in our ",
"FAQ",
" about virtual particles, and ",
"here",
" is a thread from yesterday about them."
] |
[
"I know that perpetual motion shouldn't exist, but super-fluid fountains?"
] |
[
false
] |
is an example of one. I do know understand where the energy is lost.
|
[
"Wikipedia",
" says:",
"If a capillary tube is placed into a bath of superfluid helium and then heated, even by shining a light on it, the superfluid helium will flow up through the tube and out the top as a result of the Clausius-Clapeyron relation.",
"So it's very likely that the energy required to do this comes from the heat source."
] |
[
"The only truly closed system is the entire Universe."
] |
[
"Ah, didn't see that. Hard to imagine that it gets enough heat to offset the energy given off though."
] |
[
"What's in old gold tooth fillings/crowns besides gold? Why aren't they pure?"
] |
[
false
] |
A relative gave me some old gold tooth fillings they had lying around and asked if I would be willing to melt them down and purify them at the assay lab I work at. When I got them it turned out to be three gold fillings, two crowns and a gold bridge. I wrapped the whole mess in lead foil and cupelled in a 1700F furnace for an hour. What came out was about a 10.8g bead of what looks like very impure gold. I re-wrapped this and cupelled it again, for longer this time to make sure all the lead had boiled off. Still ended up with a very dirty-looking gold bead with what seems to have a dull grey graphite-like sheen to it. Basically I'd just like to know what else might be in old (1930s/40s I believe) crowns, fillings, and bridges, and why they aren't pure gold. Was it too expensive, or did adding other metals make it easier to use somehow? I thought that pure gold was used specifically because it's a non-reactive substance that won't break down in the mouth, so why add impurities? Thanks.
|
[
"Gold by itself is pretty malleable. Alloying it with other metals can increase it's hardness, which would be valuable to keep the crown from deforming while chewing. Poking around, it actually looks like standard practice. Many metal fillings today are made by alloying some noble metal (Gold or Platinum which have similar mechanical and biological properties), and other metals (copper, silver, tin). It doesn't appear that the alloying effects the stability of the crown all that much. ",
"In fact, there are so many options that there's apparently a certificate system to certify your crown/filling as having different compositions. ",
"http://www.identalloy.org/identalloy",
" "
] |
[
"Adding to this is the old cliche of biting gold coins to see if they're real. If real, they dent. Thus why you wouldn't want pure gold filling or replacing things you use for biting.",
"A question I'd like to add onto this now is why use gold at all in the first place? Certainly there was a more common and more practical metal to use?"
] |
[
"Pure gold is extremely brittle and has low tensile strength. Jewelry and fillings are usually made of a gold alloy to increase the strength and durability of the piece.",
"The other metals are probably silver, platinum, palladium, tin, and maybe copper."
] |
[
"Why do so many things follow Normal Distribution?"
] |
[
false
] |
Many things in nature and economics seem to follow a Normal/Gaussian distribution. Is there a reason for this?
|
[
"This is a good question so I would be interested to hear how other people answer.",
"As I understand it, one of the reasons it is so common is because of the central limit theorem, this tells us that any set of measurements of independent, identical distribution will converge to a normal distribution if you have a large enough sample. What we mean by independent and identical is that each measurement does not depend on any other and that each measurement is taken from the same probability distribution.",
"There are a lot of things that meat these criteria, dice rolls, coin flips, roulette spins but not all. That is why normal distributions aren't always there. You specifically mention economics where things like log-normal distributions are common too.",
"Once you know that a large sample will converge to a normal then you can exploit this by assuming a single measurement is plucked from the unseen normal distribution and this allows things like least squares fitting to become common tools.",
"One example of distributions following a Gaussian is the velocities of atoms in a gas. The interesting thing is that this distribution minimizes entropy, a fact that will be the case anytime there is a quadratic conserved quantity (energy is proportional to v",
" So you would expect them to frequently spring up in nature.",
"That said, there are also reasons why they are used as approximations even where the data does not necessarily fit one exactly."
] |
[
"Yes, it is the Central Limit Theorem. Your explanation is almost correct. You do not need lots of independent measures of the distribution. The measure you are taking needs to be the sum (or mean) of a lot of independent factors.The variance of each factor needs to be under some sort of control (there are formal limits used for the proof). If the measure meets these criteria, it will follow a Gaussian distribution. ",
"Log-normal is kinda similar - except the small independent contributions are multiplicative, and not additive. ",
"When I analyze data and find a distribution that is not Normal or Log-Normal, I get excited. The lack of normality suggests there is a dominant contributor to the variance. So, I look for it. "
] |
[
"As ",
"u/Robo-Connery",
" mentioned, the primary answer is the central limit theorem. To expand on that a bit: When your result comes from a single random variable, it can have all sorts of probability distributions: a die has a flat distribution for any integer between 1 and 6, a single pixel on your camera has a poisson probability of sending N bits over a small time window, or a radioactive isotope has a binomial probability of decaying over a certain time window. For each of these processes, we can assign a mean µ and standard deviation sigma. ",
"Now let's consider a variable that is the sum of N independent variables. It is pretty easy to see that the mean of the sum is just the sum of the means:",
"µ",
" = ∑ (µ",
")",
"Perhaps less obviously, it is also true that the total variance (i.e. sigma",
") sums in the same way:",
"sigma",
" = ∑ (sigma",
")",
"This is true for the first three ",
"central moments about the mean",
". ",
"If all the values of µ",
" and sigma",
" are identical, this means that:",
"µ",
" = µ",
"*N",
"And the standard deviation normalized by the mean scales as:",
"sigma",
"/µ",
" = N",
" * sigma",
" / µ",
"Since the normalized standard deviation scales as N",
", this demonstrates the ",
"law of large numbers",
": as N increases the standard deviation shrinks relative to the mean. Therefore the simplest approximation to the distribution as N becomes large would be a delta function centered at µ",
". ",
"We can do better than the delta function though. You can define a series of ",
"standardized moments",
" that are normalized by the variance, such as skewness or kurtosis. ",
" Because the third central moment is additive, we can show that the skewness (i.e. the third standardized moment), will scale as:",
"N/N",
" = N",
"Therefore it tends to zero as N becomes large. More generally, the odd standardized moments will tend to zero while the kth even standardized moments, µ",
", will approach the values:",
"µ",
"=(k-1)!!",
"Here the !! symbol is the double factorial. The formula comes from ",
"eq. (10) of this paper",
". The only continuous distribution that has the appropriate higher order standardized moments is the ",
"normal distribution",
". (Note that as N gets really large, the normal distribution starts to look like a delta function, so this is consistent with our first approximation). ",
". ",
"You can see where the central limit theorem breaks down as well. If N isn't big enough, it will not hold. If the skewness of individual distributions is very large, then you need a larger N before the sum converges. And if the individual distributions are not very uniform, it won't converge. For instance, if you rolled a normal die labelled 1-6 50 times in a row, then added that summed total to a roll of a die with 100, 200, 300, 400, 500, and 600 on each face, the total sum won't look very Gaussian."
] |
[
"Can organ recipients donate organs?"
] |
[
false
] |
If a recipient chooses to become a donor, can the organs they received be reused when they die? Edit: If organs can be donated on, how long can it be donated. Is there a specific time after which the organ will fail?
|
[
"Usually they only redonate an organ if the first recipient died shortly after receiving it - I don't know exact specifics, but it would be a scenario in which the organ was transplanted in a last ditch effort to keep someone alive that failed, which I think is at most a few weeks. I looked at some papers real quick and they were all 10 days or less. Most of the time the organ is damaged too much in the process of transplantation, spending time in the body of someone with a disease that damages said organ, and just general organ damage from death."
] |
[
"sometimes diseases aren't OF an organ, but still affect that organ. so replacing won't help stop the disease, but will prolong life/health."
] |
[
"I didn't know that new organs could become damaged from the disease, too. I would assume that the reason organs were replaced would be to stop the disease. Is this not always true?"
] |
[
"In the vacuum of space, would I be able to turn around?"
] |
[
false
] |
Say a human is in the vacuum of space and is somehow alive and all is good. They're stationary and just floating there. If that person is facing a certain direction, would they be able to move to face another direction? Like twist their body to face behind them?
|
[
"Yes. ",
"Check out this video of it being done"
] |
[
"You should be able to swing your arm around, and the rest of your body will spin in the opposite direction, until you stop moving your arm. ",
"This is basically how satellites control pointing (attitude). They have momentum wheels that spin. Spin it in one direction and the rest of the satellite spins in the opposite direction."
] |
[
"Aha! \"He can't change his angular momentum but he can change his body position\" is a lightbulb moment for me!"
] |
[
"How do drugs get into the brain?"
] |
[
false
] |
[deleted]
|
[
"Most drugs have structures that are incredibly similar to natural neurotransmitters! Not to get too into the organic chemistry but serotonin and LSD share the same indole rings. In essence, because of their similar structure, LSD can pretend to be serotonin and affect the same pathways and receptors. To get into the brain, the drugs have to pass the blood brain barrier (BBB). Since the structure is so similar to the natural neurotransmitters that are allowed through that barrier, LSD sort of tricks the membrane into letting it pass."
] |
[
"this is a great answer. I would also just like to add that the way the drugs get into the receptors via the bloodstream which is why all routes of administration (the way to take the drug) can affect how long it takes for a drugs effects to kick in. Which is why snorting things like cocaine and smoking drugs like meth can have such rapid onsets of action whereas things like LSD or MDMA or prescriptions meds can take up to an hour or two to take effect. this is b/c of how the drug gets into the blood stream. When take orally drugs are subject to first pass metabolism meaning they are actually converted into other substances in the liver which then enter the blood stream and get into the brain via the BBB. Where as snorting drugs or injecting drugs or smoking drugs avoids this which also increases the speed of onset and can even effect HOW the drug affects the brain. certain drugs like MDMA require first pass metabolism to work \"properly\" and will have differing effects when sorted than when ingested orally."
] |
[
"Good catch! After some research, I found that the exact mechanism isn’t quite known (or at least there are no publications). LSD and other illicit substances are especially hard to research as they require special permits. However, it’s hypothesized that LSD travels through the choroid plexus epithelium. ",
"https://scholar.google.com/scholar_lookup?hl=en&volume=173&publication_year=1971&pages=1022-1024&journal=Science&author=IM+Diab&author=DX+Freedman&author=LJ+Roth&title=%5B3H%5D+lysergic+acid+diethylamide%3A+Cellular+autoradiographic+localization+in+rat+brain#d=gs_qabs&u=%23p%3D4QDR6gqpEO4J"
] |
[
"Are there underwater waves?"
] |
[
false
] |
Sound waves take place through out their medium as compression waves, but the waves in the ocean, which seem to also be dealing with a physical medium are only readily visible at the top layer. Is there a manifestation of the same physical force that generates tidal waves, but under water? And if they exist, what different characteristics do these underwater waves take as opposed to surface waves? EDIT: Thank you everybody for your answers, they really collectively hit the mark on the type of info I was after, which is rare. I'm very gracious you guys took the time to assuage my curiosity.
|
[
"Sure, they are called ",
"internal waves",
", although that is a broad term and they have many other names and sub-categories as well. There is a section of that article that nicely discusses how they differ from normal waves. They can be somewhat hazardous too - for example, the sinking of the submarine USS Thresher is attributed to one of these internal waves, and sparked a lot of research into them.",
"Surface waves on the ocean are primarily generated from wind, and only penetrate a short distance downward. Sound waves are ",
"compressional waves",
", but ocean waves are really ",
"surface waves",
", so they are really two somewhat different phenomena. Without a change in density of some sort, you won't find notable waves within a body of water."
] |
[
"Internal waves are a key component of mixing across the pycnocline as well, since they break when they interact with bottom topography (much like surface waves) and their energy is dissipated through the water column. The mixing rate often goes up substantially in the vicinity of seamounts, ridges, and other bathymetric features as a result of breaking internal waves.",
"E: ",
"This paper",
" shows the amplitude of some internal waves around Hawaii at up to 300 m! It also points out how the energy dissipation and diffusivity/mixing increase around bathymetric features."
] |
[
"Yeah, everyone has their own (usually strong) opinions about it. I absolutely agree it's speculative, which is why I said 'attributed to' and not 'caused by'. It at least shouldn't be hard to imagine how a submarine operating at max depth could run into trouble if an internal wave dropped its depth further, which is absolutely within the realm of physical possibility at least. It also shouldn't be hard to imagine something else going wrong at that depth. ",
"It's also not very compelling to point out that other submarines haven't been damaged by internal waves - from what I've heard from friends in the NAVY, the sinking of the USS Thresher resulting in additional precautions for subs traveling at significant depths.",
"But you're right, there are other explanations too that could easily be correct."
] |
[
"If SARS-CoV (2002) and SARS-CoV-19 (aka COVID-19) are so similar (same family of virus, genetically similar, etc.), why did SARS infect around 8,000 while COVID-19 has already reached 1,000,000?"
] |
[
false
] |
So, they’re both from the same family, and are similar enough that early cases of COVID-19 were assumed to be SARS-CoV instead. Why, then, despite huge criticisms in the way China handled it, SARS-CoV was limited to around 8,000 cases while COVID-19 has reached 1 million cases and shows no sign of stopping? Is it the virus itself, the way it has been dealt with, a combination of the two, or something else entirely? EDIT! I’m an idiot. I meant SARS-CoV-2, not SARS-CoV-19. Don’t worry, there haven’t been 17 of the things that have slipped by unnoticed.
|
[
"SARS-CoV-2 is worse than SARS-CoV because, paradoxically, it’s not as bad. SARS tended to have a faster disease onset and be more severe, so you had far fewer infectious people with mild or no symptoms walking around spreading the disease. In fact much of SARS spread was in hospitals, rather than on the street. That made it relatively simple to identify and isolate potential spreaders. SARS-CoV-2, on the other hand, has many people spreading it who are not sick and who don’t isolate. ",
"Even so, SARS was just barely controlled. People are complacent today, but SARS came much closer to being a pandemic than most people realize."
] |
[
"This is also why Ebola isn't and can't really become a huge problem in an educated populous with access to health care.",
"By the time people with Ebola are infectious, they are vomiting, having diarrhoea, and bleeding. Not likely to be around people. You also have to come in contact with those body fluids in order to get infected yourself. ",
"The only reason it became an outbreak where it did, was that first the locals opted to wash their dead, coming into contact with the virus themselves. And second when some groups refused to listen too, or even report cases to the medical personnel. Leading to more outbreaks that could easily be contained. ",
"This all makes Ebola only a real threat in areas with poor health care, uneducated/refuse to believe science population, and lack of worker rights that allow you to take sick days. So the developing world, and maybe that one developed nation."
] |
[
"Angiotensin-converting enzyme 2 (ACE2) is the receptor that both SARS-CoV and SARS-CoV-2 bind to. The (S) spike glycoprotein that binds to ACE2 is slightly different in both viruses and this results in different binding affinities.",
"\"Recent studies have found that the modified S protein of SARS-CoV-2 has a significantly higher affinity for ACE2 and is 10- to 20-fold more likely to bind to ACE2 in human cells than the S protein of the previous SARS-CoV. This increase in affinity may enable easier person-to-person spread of the virus and thus contribute to a higher estimated R0 for SARS-CoV-2 than the previous SARS virus.\"",
"Source: ",
"https://www.mdpi.com/2077-0383/9/3/841/htm#B16-jcm-09-00841"
] |
[
"What happens at the microscopic level when you cook food?"
] |
[
false
] |
Would it be accurate to say that cooking breaks down complex chemicals in food so that our bodies can use it better? Like an external first stage of digestion, of sorts.
|
[
"cooking, in the general sense, is the application of heat to a mixture. basically what you're doing is increasing the rates of some reactions that begin when your different ingredients come into contact with each other and providing the activation energy necessary for other non-spontaneous reactions to begin. the exact reactions that occur depend on what ingredients you've combined. that's a highly simplified explanation, but i hope it at least provides you with a general idea of what's going on "
] |
[
"Interesting. So cooking doesn't necessarily have the purpose of changing the molecular complexity of food so our bodies can better utilize its nutrients? I guess baking wouldn't make breads more simple, but what about roasting vegetables or browning meat? "
] |
[
"Browning foods is the ",
"Maillard reaction",
" - it's very complex and all the reactions that occur is not elucidated yet.",
"It doesn't necessarily make it better to utilize nutrients."
] |
[
"What would gravity be like in an infinite space, which held effectively infinite matter?"
] |
[
false
] |
For context, discussion on my DnD group about the elemental planes and what the physical properties of those planes would actually be like, assuming they were plausible. The lore in-game says that these planes extend infinitely with the primary matter that consitute that plane (air, earth, water, etc). We debated whether all that matter (and therefore mass) would essentially collapse to create an infinite black hole, or since the gravitational pull would effectively be equal in all directions if it would have a state of gravitational equilibrium, and therefore no observable gravity at all. So, for those more knowledgeable of physics than us, please tell us what the actual math says would happen if you had an infinite space filled with some form of essentially infinite matter? Extra kudos if anyone can also talk about any other odd qualities that physics says a universe composed almost entirely of one type of matter would have that aren't immediately obvious.
|
[
"One thing to point out is that \"infinity\" in physics is almost always shorthand for a particular limit towards infinity. Many physical paradoxes can be solved by reducing a problem to a finite version and then taking a limit, but the result will often depend on how exactly that limit is taken. It often occurs when multiple values are infinite, but the issue arises of how infinite relative to one another. This is because infinity \"crushes\" together every big number similarly to how zero \"crushes\" everything to a single point. ",
"Anyhow, the question of gravity in a universe of an infinite, homogeneous substance does make sense in both Newtonian physics and general relativity.",
"Under Newtonian gravity, the way you would approach the problem is to think about the force balance on each little piece of matter. ",
"* So this universe will be unchanging, static, forever.",
"However, this is an unstable configuration. Small deviations from a completely homogeneous gas cause it to start clumping up. This is called the ",
"Jeans instability",
", and it's why the primordial gas of the early Universe coalesced into stars rather than staying homogenous, and subsequently why the resulting gas of stars itself coalesced to galaxies.",
"At scales much larger than the Jeans radius, though, we can average over stars, galaxies, etc and the Universe remains effectively homogenous. This is the domain of cosmological models of the Universe and of general relativity.",
"In such a Universe, space and time themselves become dynamical variables, and if you solve Einstein's field equations for a homogenous matter distribution you get the ",
"FLRW solution",
". This solution can describe either an expanding, contracting, or static Universe depending on the parameters. (You might have heard of terms like Big Rip, Big Crunch, or Big Freeze to describe such solutions.)",
"So in short: the condition of homogeneity isn't sufficient to fully describe the fate of this universe. Crucial remaining parameters are the global curvature of the universe (closed, open, or flat) and the cosmological constant. If this universe is flat and the cosmological constant is ",
"fine-tuned",
" to exactly offset the contracting effect of the matter, then you can end up with a ",
"static universe",
". This doesn't describe the one we live in, however.",
"*E: Subject to certain boundary condition considerations -- see ",
"/u/UsernameStageFright",
"'s comment below and my response to it"
] |
[
"This answer is exactly what we were looking for, thank you!"
] |
[
"This is a good answer, but I disagree with the part where you say:",
"And as you have pointed out, the gravitational force on each such piece of matter is balanced out in all directions so as to create no net force.",
"This is implicitly invoking an additional assumption that most physicists assume isn't true for our own universe-- namely, that there is a distinguished point and thus that the laws of physics aren't translation invariant. You almost hit upon this with the first part of your answer when you pointed out that different limits usually give different answers, but the part I quoted makes it seem like you might not have realized this assumption, so I think it's worth pointing out.",
"As you said in the first paragraph, whenever infinities crop up in math, the only way to deal with them is a suitable limiting procedure. Likewise, when figuring out the force on a small volume of matter in a homogenous universe, the answer is going to depend on how you take the limit. The first limit you have to take is in dealing with the continuous nature of space and your matter distribution for this problem-- this is just framing everything in terms of calculus. The second one here is what matters, and that's the limiting procedure used to make the domain of your integrals extend to the entirety of space.",
"When you say \"the gravitational force on each such piece of matter is balanced out in all directions,\" what you're doing is defining a limiting procedure consisting of looking at the contributions of spheres of increasingly big radius ",
". But obviously this isn't the only limiting procedure-- say instead we take the limit of increasingly big spheres centered on a point a distance R away from the piece of matter we're calculating the force on. Mucking about with the shell theorem, you can verify that the force on our piece of matter is G*((4/3)*pi*R^3*rho)*(rho*dV)/R^2 pointed from the piece of matter to the point defining the center of our spheres, where G is the gravitational constant, rho is the homogenous mass density of our universe, and dV is the infinitesimal volume of the piece of matter we're calculating the force on.",
"So, we have a bit of a pickle-- for each point in space, the force on it is ill defined and depends on the limiting procedure we use. Since we like physics to tell us what the results of experiments will be before we do them, the answer \"the force at any point could be anything\" is useless, and we need to specify some additional structure, like there being a distinguished point that is the center of our limiting spheres for every point in space that we calculate the force on. But without that additional info (which as I said isn't true in our own universe as best we can tell) the problem doesn't have a unique, useful solution.",
"The math-y reason for this is that the theorem guaranteeing existence and uniqueness of the gradient to the solution for the Poisson equation requires a certain boundary term at infinity vanish, which can't happen if your mass distribution doesn't decay quickly enough, and a homogenous distribution obviously doesn't. This extra assumption is in effect another initial condition utilized by physicists all the time when doing calculations whether they realize it or not, but when it can't be satisfied due to the particular distribution under consideration, it can lead to misconceptions like the one you stated. Basically, what you're asserting is a 3D analog to the statement that \"the solution to the ODE df/dx = rho is f(x) = 0.\" This is clearly wrong, the solution is f(x)=rho*x+c, where c is a constant that can only be determined by utilizing additional information. ",
"All of this discussion was just for Newtonian gravity-- the structure of the GR equations is quite different and the FLRW metric is a valid, unique solution for a homogenous universe as pointed out by Movpasd. Just wanted to correct a misconception I often see propagated about how Newtonian gravity would behave for an infinite universe :)"
] |
[
"Would the \"naked body scanners\" be useful for veterinary medicine?"
] |
[
false
] |
One of my friends just lost one of their pets because a Vet visit missed a laceration that later became septic/abscessed. Would something like the "naked body scanners" the TSA likes so much be useful for vets? If it goes through clothes, would it not let them get a good view of the skin and any hidden wounds that may not be visible through the fur? Are these scanners something that would be beyond the purchasing of vets?
|
[
"The airport scanners use mm-wave (sub-THz) radiation. This radiation is non-ionizing, so it is completely safe. Therefore, you don't have to worry about using lead shielding of any kind. When used for medical imaging, it is capable of differentiating tissue types from one another, rather than just bone-not bone. This is especially useful in detecting tumors in people and animals.",
"These scanners are still very bulky and expensive, so it will probably be at least a decade before they make it into vet offices."
] |
[
"They already have \"better\" tech for actual soft tissue injuries like sonic imaging or even MRI.",
"I was thinking something like this would allow an easier external search for injuries.",
"Last time I had to take my cat to the vet for a bite the vet had a hard time finding the exact injury because his fur is so dark and thick. It involved actually parting the hair in order to examine the skin patch by patch.",
"Even knowing the approximate location of the bite, it still took a couple of minutes. In the case of my friend's pet, they actually missed the injury entirely and assumed it was a sprained shoulder. ",
"By the time they realized they were wrong the cat was already ill to the point of death because of liver damage from a blood infection.",
"My questions is would a scanner like this be cheap enough to allow them to use it to avoid that kind of thing? "
] |
[
"I very much doubt it. They were designed to be used en masse without much discrimination. Because of that they aren't very accurate, and are prone to incorrectly imaging the subject. Medical scanners capable of seeing soft tissue details need to be much more precise (higher radiation levels) and carefully operated. ",
"If airport scanners were functional in the same way as medical scanners, there would be allot more cancer being given to people."
] |
[
"How do electromagnetic antennas work?"
] |
[
false
] |
I understand how basic electric circuits work, but I don't understand when and how conductors would radiate energy via EM waves, versus simply radiating it away as heat (i.e. acting as an ohmic resistance). Does it have to do with the length of the conductors in the circuit? The frequency of the signal? Furthermore, what explains why antenna shapes and designs vary so widely? If you could go into detail on this, I'd appreciate it. I have a layman's understanding of Maxwell's equations, although I'm not on a strong mathematical footing.
|
[
"The electrical signal passes through an oscillator, which produces a wavelength. The wavelength is passed onto a conductor (i.e. Coax cable, which carries the wavelength to the antenna. The antenna is tuned for the systems wavelength, and passes the signal onto the atmosphere. The reverse is true of the receiver. The concept of electromagnetic radiation is difficult to grasp because we think of it as not being able to be observed, but this is not true. You can observe this in the energy of the sun, visible light, audible sound, and microwave ovens, for example. Everything in nature oscillates, or vibrates, when exposed to certain wavelengths. Human hearing is possible because the eardrum oscillates when exposed to, say, 2000 Hz and below. The cones of the eye oscillate when exposed to the visible light spectrum. The molecules in food oscillate when exposed to 2.4 Ghz, at high power, in a closed environment. The vibrating of the molecules results in friction, and therefore heat. Current radios primarily use synthetic transmitters and receivers that are capable of producing the oscillation necessary. But back in the day, they used crystals. These crystals oscillated at a certain frequency, and this is how we were able to \"tune\" to certain frequencies, without hearing interference from others.",
"Antennas are different shapes for multiple reasons, but primarily due to wavelength and pattern. A higher frequency will have a shorter wavelength, and vice-versa. So, a UHF antenna at 400 MHz, will typically be shorter than a VHF antenna tuned for 150 MHz, to compensate for the shorter wavelength. If it's an Omni directional pattern, meaning an even amount of energy in all directions, the antenna may resemble a pole (di-pole), or resemble a series of loops (folded di-poles). If an antenna is directional, it may resemble a dish pointed toward the sky (satellite), a dish in-line with the horizon (line of sight microwave), or look like a series of poles decreasing in size (Yagi). There are also antennas that make sort of a cone shape (corner-reflected)."
] |
[
"Imagine electromagnetic energy radiating from the base of an antenna as a wave. If the antenna is of a complementary length (relative to it's physical wave length) there is limited reflection from the tip of the antenna (the power of the wave doesn't reflect back into the base of the antenna). This causes positive interference, where the frequency of the wave increases the amplitude of the outgoing signal every occilation. ",
"You can calculate the voltage standing wave ratio using digital tools, or mathematically given the equation: 234/FREQ = 1/4 wavelength\n468/FREQ = 1/2 wavelength\n1005/FREQ = Full electrical wavelength ",
"Using this equation you can calculate the length of a go en antenna for it to be resonant at a specific frequency."
] |
[
"I understand how basic electric circuits work, but I don't understand when and how conductors would radiate energy via EM waves, versus simply radiating it away as heat (i.e. acting as an ohmic resistance).",
"Think of the magnetic part of the Electromagnetic (EM). Magnetism does not need a conductor. So all that's needed is to set up a wave in the air and it propagates away from the antenna. That's not the whole story, but it may help with visualization. It's actually a coupling device between the cable and the air. ",
"https://en.wikipedia.org/wiki/Impedance_matching",
"Does it have to do with the length of the conductors in the circuit? The frequency of the signal?",
"Yes. The elements of the antenna are tuned to the specific wavelength of interest. In fact one of the biggest challenges is to make an antenna work well across a broad range of wavelengths (frequencies). The most basic version is the 1/4 wave dipole. Those many elements in a yagi antenna like a TV antenna are each tuned to a certain range and combined for broad coverage. ",
"https://www.arrl.org/files/file/Technology/tis/info/pdf/8108032.pdf",
"Furthermore, what explains why antenna shapes and designs vary so widely?",
"The different shapes have to do with different properties and some shapes are better suited for certain applications. Like directionality. A TV antenna works because it's directional and you get a stronger signal. A mobile antenna would be a pain in the ass if it needed to be pointed. Cell towers have arrays of antennas to handle multiple signals simultaneously. Also there's polarity. Satellites use horizontal and vertical polarity to increase the signal capacity. Imagine a wave like those exercise ropes. You can go vertical or horizontal. And there's also Circular which is basically a corkscrew pattern. Linear vs Circular uses completely different shapes of antenna elements. "
] |
[
"Is there a specific range before we loose contact with the voyager?"
] |
[
false
] |
Also, is plasma the only thing we're reading off of it? How would that help us understand more about the universe? Sorry for any awful grammar.
|
[
"I'm not gonna do the math, but give you some useful terms to learn more about the effects involved. Both Voyager probes are similarly constructed and much of this is generic anyway, so it applies to more than just the Voyager I probe which has been in the news lately",
"Radio waves will keep traveling forever, but they get weaker, obviously. Whether you can actually communicate depends on a few factors:",
"In the case of Earth-based transmissions ",
" the Voyager spacecraft, we can control things like power and gain (within reasonable limits), and the receiver is in a fairly-quiet environment, far from noise sources except its own systems. However, we're limited in the types of transmission we can use, because the remote end only understands certain modulations and data rates. ",
"Coming from one of the Voyagers back to Earth, it's pretty much the same story in reverse -- the limiting factors are the spacecraft's transmit power, the modulations it knows, and terrestrial noise near the receive site.",
"As others have pointed out, it seems like the RTG power source on the probe itself will be the first limit we hit; the fuel within is decaying, and will eventually fall below the minimum required to run the basic systems onboard. When it can no longer transmit, demodulating data becomes moot. "
] |
[
"It comes down to an information theory question. At some point, the craft will get far enough away that we can no longer practically demodulate the signal due to physical limitations in terrestrial radio hardware. In theory we could keep building larger and larger dish antennas, and then cryogenically cooled dish arrays, and then orbital antennas to boost the range from our end, but like I said, there is a practical limit to how far we can take it (like, we probably won't hollow out the moon in order to use it as a parabolic reflector.) ",
"I don't know much about the comms link used by voyager, other than I think it uses FSK of some kind with forward error correction. If anyone wants to chime in with specifications, I could calculate link budgets for various scenarios (including the one where we use the whole moon as an antenna.) ",
"From an information theory perspective, though - the pedantic answer is \"when the SINR (signal to interference/noise ratio) becomes less than -1.6dB on the current hardware.\""
] |
[
"Yeap, both Voyager probes will hit the power limit long before we have to worry about distance and how to overcome it in terms of communication. As far as I know, by 2025, Voyager will be unable to power any of its scientific instruments. I certainly hope that it reaches the Oort cloud, although it would take the probe another 300 years just to reach it and mby 2000-3000 years for it to come at the other side."
] |
[
"Theoretically if there were 6 black holes making a cube shape that blocked off all entrances and you went into the empty space between the black holes (without getting sucked in) what would happen to space and time around you and outside of the theoretical black hole cube?"
] |
[
false
] |
For a better understanding of the question let's say it's a hollow sphere composed of black holes. This sphere is completely encapsulated and blocking off the outside universe. Otherwise great answers! I'm humbled for everyone who has taken interest and time to provide insight.
|
[
"If the black holes were close enough to block off all exits, they would have overlapping event horizons and immediately merge into one huge black hole, with you inside that black hole."
] |
[
"So im a little late. All these answers here are not very satisfactory. You can in fact have a region of space that is blocked off from the rest of the universe due to a lattice of black holes surrounding the region. One of my friends phd thesis was on this exact topic. And heres one of the papers that describes this. The phenomenon is called \"piecewise silence\" ",
"https://arxiv.org/abs/1402.3201",
"Basically what you get is a region of space that is distinct from the outside universe, and evolves completely independently. Its as if there were a smaller isolated universe inside the lattice of black holes"
] |
[
"How would the 2 singularities communicate to each other that their event horizons are touching? ",
"And why would the horizons touching cause the singularity itself to merge?",
"edit. The wiki seems to indicate the event horizon radically deforms into an X shape shared among the 2 black holes, but they continue to orbit each other until the singularity itself is 'within' the others horizon. ",
"https://en.wikipedia.org/wiki/Binary_black_hole#Shape"
] |
[
"What specific type of continuum is the concept of Space-Time?"
] |
[
false
] |
I know I'm out of my depth, but what label would be correct? For example, is it a homogeneous decomposable continuum?
|
[
"The word \"continuum\" classically refers to the set of real numbers (the real number line). If you pick any two real numbers ",
" and ",
", there is always a third number ",
" between ",
" and ",
". But there's more to it than that! (That property would also describe the rational numbers, which are not a continuum.) If you divide the line into two disjoint intervals ",
" and ",
", with all elements of ",
" less than than all elements of ",
", then either ",
" has a greatest element or ",
" has a least element, but not both. (",
": the intervals (-∞,π) and [π,∞) consist of such a division, and we see that the second interval has a least element. Note that this would not be true with the rational numbers because π is irrational. The second interval would not have a least element if this were done in the rationals.)",
"There are a few other properties that characterize what is called the order type of the real numbers, but the idea of it being a continuum is that all the \"gaps\" are filled in. That's just for the real ",
", which is one-dimensional. We can generalize this idea of continuum to higher-dimensional analogues of the real line (e.g., plane, space, etc.). They are given the label R",
", where ",
" is the dimension. We also sometimes call it \"Euclidean space\" since our notion of how to order the real numbers also gives us a notion of ",
", which turns out to be the same topology induced by the ",
", the usual way of measuring distances.",
"Now when we use \"continuum\" to refer to spacetime we mean that we model spacetime as the set of points of a so-called ",
". A manifold is a mathematical object which \"looks like\" Euclidean space in small sections (we call that the local structure). But those sections can be glued together in a way that makes the manifold look ",
" different from Euclidean space. An example of a manifold is the 2-sphere, i.e., the surface of Earth. Small pieces look like a flat plane, but globally, the entire surface curves around and eventually meets back with itself. In particular, the 2-sphere is ",
", which essentially means it's bounded, unlike a flat plane which stretches out to infinity.",
"It's probably also useful to have an example of something that we would ",
" call a continuum. The infinite plane is a continuum. But the points with coordinates (",
", ",
") where ",
" and ",
" are both integers is ",
" continuum. (See the ",
"left part of this picture",
".) This set of points is called a ",
". Note that each point has a little ",
" around it in which ",
" of the lattice appear. The lattice is said to be ",
", and so this would not be a manifold. (Strictly speaking, it's not a ",
" manifold, which is what you think it would have to be. Technically, the lattice is a disconnected, zero-dimensional manifold. Each point of the lattice itself is a continuum, but the lattice as a whole is not a continuum.)",
"In general relativity, all spacetimes are 4-dimensional manifolds. So spacetime always looks like 4-dimensional Euclidean space (in the topological sense) locally. We then give the manifold extra structure:",
"All of that extra structure gives all of the rich physics of spacetimes. For instance, the metric and the connection tells you all about the ",
" of spacetime, e.g., which regions of spacetime can communicate with each other and in which direction.",
"But fundamentally spacetime is a ",
", which is just something that looks like the ",
" of real numbers (in this case, a 4-dimensional space) up close, but which can have non-trivial global topology."
] |
[
"All of science is about using models to describe the natural world. So in the context of science, there is no difference between the statements \"spacetime is a manifold\" and \"spacetime is conventionally modeled as a manifold\". Perhaps in the context of philosophy we can ask whether our models imply anything about what something really is or whether different mathematical structures that describe the same physics are fundamentally different things (e.g., Hamiltonian vs. Lagrangian structure of classical mechanics). But as far as science is concerned, the model and the reality are the same thing.",
"Models can change when we gain new information. Our current model of spacetime is that of a 4-dimensional Lorentzian manifold. This continuity has been experimentally established (via pion scattering) down to scales of about 10",
" m AFAIK. The singularity theorems of Hawking and Penrose show that our model of GR must suffer from certain breakdowns. The ultimate cause of the breakdown could be one of several aspects of our model. The field equations could be wrong, for instance. Or maybe the manifold structure is what breaks down and spacetime is better modeled as something that is not a manifold.",
" I think you just interpreted my statement \"but fundamentally spacetime is a manifold\" incorrectly. I was not making any philosophical statement about the true nature of spacetime or even whether that is a well-defined concept outside of science anyway. All I meant was that our model is fundamentally a manifold. That is, spacetime itself is a ",
" manifold, onto which we add additional structure such as a smooth structure, a Lorentzian metric, and a connection. The word \"continuum\" really only refers to the topology of the manifold, which is its most basic structure."
] |
[
"We don't know the \"true\" nature of the world. That is why we use mathematical models to study and understand it. Their value is defined only by whether they are useful, and this model of spacetime is VERY useful.",
"The \"true\" nature of the world is a subject for philosophy, not science."
] |
[
"How would you determine Chlorine ion concentrations instead of Hydrogen ions?"
] |
[
false
] |
I've only learned how to do H+ concentrations in class so far, and I was wondering how I would determine the ion concentration of a mixture (with known molar concentrations) with Chlorine ions instead of Hydrogen? Am I missing something simple? x.x
|
[
"Well, you didn't give much detail, but based on that, I'm assuming this is for a gen.chem course. And based on that, I'm assuming you're adding HCl to water. In that case, for every H+ ion you're adding to the water, you're also adding Cl-, so they'd be equal (since this is a strong acid and you'd have complete dissociation)."
] |
[
"Start from the basics. What is concentration? It is mols of solute over liters of solution. The two mixtures you are given both contain chloride ion. You are given the volume of each, as well as the means to find the number of mols of chloride in each.",
"That should be vague enough to just point you in the right direction."
] |
[
"Sorry, I didn't want to give the exact question because I want to learn how to do it and not have it done for me. It's actually from my sister's college chem class: \"what's the concentrations of chloride ions in a solution made by mixing 115.0 ml of 2.00 M barium chloride and 250.0 ml of .450 M sodium chloride.\""
] |
[
"What exactly happens when a computer \"freezes\"?"
] |
[
false
] | null |
[
"Any one of a number of things could be happening, but they all come down to one particular general thing: The computer has become stuck in a state from which it cannot escape. I would say \"an infinite loop\" but sometimes \"a very, very long loop where nothing much changes\" also counts.",
"For example, something may go wrong with the hardware, and the BIOS may try to work around the issue, but in so doing encounters the same problem, so the BIOS tries to work around the issue and in so doing encounters the same problem... Not good.",
"There's also the concept of 'deadlock' in software: Say program A is running and is using resource B and program C is using resource D (these resources might be files or peripherals like the hard disk or video card). Program A then decides it needs to use resource D... and at the same time Program C decides to use resource B. This sounds fine, but what happens if they don't let go of one resource before picking up the other? They both sit there each waiting for the resource they want and never get it. Both programs are effectively dead.",
"If this happens in your operating system, your computer stops working.",
"There are plenty of methods to avoid situations like the latter, because that's all software, but it doesn't stop it happening occasionally, usually between unrelated software.",
"The first case, where there's something wrong with the hardware, is much harder to avoid."
] |
[
"This is a very good explanation.",
"Couple of extra points in my opinion/experience:",
"Another hardware example (rather common) - Hard disk defects (bad sectors, controller failures, etc). Say you have a defective external hard disk thanks to bad sectors (on the physical hard disk surface), which have affected a video file \"Rickroll.avi\" located on the bad areas. You try to display a folder/directory listing with a file manager (say Ubuntu's Nautilus file manager/Windows' Explorer), so the file manager gets the OS to scan the file table for a list of files, and in turn access the files themselves in order to produce thumbnails. What happens when the file manager tries to preview \"Rickroll.avi\" (thanks to bad sectors)? The underlying OS tries its utmost best to read the file and salvage each bit of data to other good areas of the disk, which ties up resources; this takes considerable effort if the damage is severe. Explorer or Nautilus might appear to freeze then. (Windows might pop a dialog saying that it Explorer is unresponsive). What ",
" says in his second paragraph applies here - the OS tries to salvage bits to copy to good sectors; in the process of finding good sectors, it stumbles upon more bad sectors that need remapping... etc etc.",
"Another example - Windows freezing during bootup (happened to me just yesterday). The main cause was that my Registry files became corrupted due to power failure (hence an unclean shutdown). However, when Windows starts, it tries to load data from the Registry (represented as a few files on disk). Due to corrupt data, Windows is stuck in an endless cycle trying to read data from the Registry during the Windows startup process... which does not stop even after 10 minutes and counting. (Side Anecdote: restoring the registry from backup worked for me).",
"Buggy/poorly written software... lets say the author of a simple antivirus program designed the program to scan files as they are accessed. If the code is poorly written (e.g. slow bloaty code, no optimizations, inefficient use of memory), a lot of resources will be spent on scanning one file, which doesn't matter if you're say opening a 1kB text file. However, if you are trying to play a 4GB video, your movie player might appear to 'freeze' but in reality most of your system's resources are tied up by the antivirus scanner trying to scan all 4GBs of the file. [my simplistic explanation ignores stuff like scanning optimizations etc, and assumes all files are scanned regardless of type.]",
"Hope it helps.",
"Also, ",
" has provided an excellent example of deadlock. To illustrate a rather humorous example in real-life (that I once taught in a tutorial years ago) - two people trying to cross a one-lane suspension rope bridge."
] |
[
"It's the operating system kernel that will try to work around the issue in the first example. When a problem like that appears and the kernel can not fix it it will crash the computer on purpose to protect the data in the computer as it can no longer guarantee its correctness.",
"On Windows this will manifest itself as a \"blue screen\", on Linux you get a \"kernel panic\". Please note that there are many reasons for a \"blue screen\" and hardware error is just one.\nThe most common is a driver that gets stuck in a loop and times out. Recent versions of Windows moves a lot of the drivers from \"kernel mode\" where they run as part of the operating system and will crash/hang/freeze the computer in many cases if an error occurs, into \"user mode\" where they run more like an ordinary application that can be restared without affecting the rest of the computer.",
"One good example of this is the graphics driver. On older versions of Windows an uncorrectable error would be fatal and the kernel would halt the computer with a \"blue screen\". On newer versions the kernel will detect that error and restart the driver to a known good state.",
"EDIT: Spelling"
] |
[
"Can meat get cancer? Could it still be eaten? Would the meat taste different?"
] |
[
false
] |
I don't mean does meat cause cancer in humans I mean can say cow or pig meat get cancer and is that still edible with proper cooking
|
[
"Yes and yes, and maybe. Cancer can happen with any kind of cell. Skin cancer, bone cancer, organ cancer. You name it. Muscles can get cancer and animals aren't immune. There are many different types of cancer and the results are different. ",
"Eating meat that is cancerous shouldn't taste much different than normal meat and it won't hurt you if the appearance is the same, but certain variations of cancer might result in different textures or fluids that are not very appetizing.",
"Let's just say that a pork chop will look better than a pork tumor on your plate."
] |
[
"There is really no reason to.",
"We can stimulate healthy muscle cells to grow as well and people won't be lining up to eat cancerous meat."
] |
[
"Follow up question, then since cancer meat is edible could we cultivate it and live off the cultivar? Similar to how some cancer cell lines get cultivated by biotech companies?"
] |
[
"Considering the importance of the moon to earth would it not be equally important to look for planets with moons to effect tidal conditions as it is to look for planets with water when searching for habitat planets?"
] |
[
false
] |
Edit: Habitable. Sorry
|
[
"We don't start by saying \"let's search for planets with water\", we find all the planets we can, and later we hope to be able to look at these planets in more detail and determine what their properties are.",
"There's also the question of how difficult it is to determine if a planet has water vs. if it has a moon. Maybe an astronomer can tell us how hard each of those is."
] |
[
"Maybe an astronomer [in training] can tell us how hard each of those is.",
"Happily!",
"Water - the way you'd detect this is through infrared absorption in the planet's atmosphere (or possibly emission). Whilst the main part of the planet blocks all light, its atmosphere will block only certain parts of it, depending on what molecules are present - if we can get suitably accurate spectroscopic measurements, we should be able to see the effect of the planet's atmosphere absorbing some of the star's light but not all. However, this is pretty difficult as the star already has plenty of things creating absorption lines!",
"The planet itself also emits some infrared radiation (as ",
"black body radiation",
", which all objects emit). Some of this will be absorbed by the planets atmosphere as it passes through it, again giving a tell-tale absorption spectra. The difficulty with this is that the IR emission from the planet is very weak, and as we can't resolve the planet and the star (i.e. see them separately - they're too close to each other and too far from us), we still see it mixed in with the light from the star. There has been a lot of work on this, but the results have generally been quite poor (due to it being so difficult, rather than any fault of the scientists).",
"Moons - I can think of a few ways to do this. Firstly, you might see a variation in eclipse depth from the planet depending on where the moon is - if the moon is directly in front or behind the planet as we see it, it won't block any extra light from the star. However, as it appears off to one side, it would block slightly more light. However, that would require the moon's orbit being on the same plane as the planets orbit (i.e. it passes in front and behind the planet) - otherwise it would never go behind the planet.",
"If the moon isn't in this plane, you might still see the effect on the transit light curve - the moon will pass in front of the star slightly before/after the planet starts doing so, which may be detectable.",
"The final method, which is the one I've seen talked about, is slight variations in the timing of the transits (called ",
"Transit Timing Variations",
") due to the gravitational pull of the moon. This will change the planets orbital velocity slightly (when the moon is behind it in its orbit, it will move slightly slower, and vice versa when it's in front), and its transits in front of the star will vary, being slightly early/late.",
"(There's a couple more methods on the ",
"Wikipedia Exomoon page",
")",
"Edit: Oops, apologies - that turned into an essay!"
] |
[
"I suppose we do find all of the planets we can but the tip of sword in the search tends to be \"is there water there?\" because that would seem to indicate the appropriate temperatures. But depending on where you think the origin of life was on earth, the tide of that water could be equally important and no one seems to mention it. "
] |
[
"Why do so many species have a rare chance of producing an albino “version”?"
] |
[
false
] |
What I meant was why is it so common that so many different species are able to produce a rare albino version. Like do they all share a specific gene that changes?
|
[
"Well, in a way yes. Albinism is caused by a malfunctioning gene that produces melanin, and since most species have melanin in their system, they do have the gene that codes for the synthesis of it. Since apart from pigmentation, this gene doesn't exactly affect anything else, it's one of the few \"bad\" mutations, that actually allows the animal to survive and procreate with. Sure it inhibits camouflage ability, it makes protection from the sun basically nonexistant, but apart from that everything else functions fine. Therefore, especially animals that live furter from the equator, where extreme sunlight isn't as big of a problem, these animals have a chance to survive and have offspring, which also enhances the probability of seeing an animal with albinism.",
"It's in a way similar in polydactily, in that animals that have it can survive, just that it's way easier to spot a completelly white animal, than an individual that has 6 fingers on each limb."
] |
[
"To add to this, we often use albinism in research when we want to test our ability to modify different parts of a genome as it's pretty easy to spot a success. Additionally, some of the most common lab strains of model organisms (BALB/c mice, Xenopus, etc) are albino, so it might appear like the incidence is higher then it actually is from the outside."
] |
[
"thank you! just what i was wondering."
] |
[
"Given current technology and information, what will ultimately be the death of our solar system?"
] |
[
false
] |
Bonus questions: How long would this method take?
|
[
"The Solar System will die when the Sun becomes a red giant star whose outer envelope reaches as far out as Venus, or perhaps even the Earth. Even if the envelope doesn't reach the Earth, the immense radiation given off by the Sun in this phase will eliminate all life on the planet. The Sun will then transition to a white dwarf star and slowly cool for the rest of time. Interactions with the material given off by the Sun during the red giant to white dwarf transformation may disrupt the outer planets as well."
] |
[
"Whoops! I forgot to answer your bonus question!",
"As far as we can tell, the age of the Sun is about 4.6 billion years. This is about half-way through its total \"main-sequence\" lifetime of about 10 billion years. After that, the Sun will enter the red giant phase and be a red giant star for about 2 billion years. The Sun will then go through a brief period called the \"Helium fusion\" phase where the Sun is fusing helium in its core (unlike hydrogen which is what it fuses whilst on the main sequence). This phase will last about 500 million years, then the Sun will expand once more, evacuating the remnants of its outer layers, and leaving behind its core - the thing we call a white dwarf. That white dwarf will last a long, long time. It won't do anything but slowly cool down for billions of billions of billions of years. Eventually the Universe will be a dark place, full only of black holes and old, barely luminous white dwarfs."
] |
[
"These cold white dwarves have a name - ",
"black dwarves",
". The universe isn't old enough for any to exist yet. "
] |
[
"How does water dissolve an ionic compound?"
] |
[
false
] |
Crash Course is saying that the ionic compounds polarity breaks apart hydrogen bonds in the water, which causes them somehow to dissolve the ionic compound? (The explanation given makes no sense). On most other websites I hear that waters polar bonds attracts ionic compound atoms towards it, breaking those bonds apart and causing the solvent to dissolve. Here is Crash Course's explanation: The fact that water is a polar molecule also makes it really good at dissolving things, which we call, it's a good solvent. Scratch that. Water isn't a good solvent, it's an amazing solvent. There are more substances that can be dissolved in water than in any other liquid on Earth. And yes, that includes the strongest acid that we have ever created. These substances that dissolve in water—sugar or salt being ones that we're familiar with—are called hydrophilic, and they are hydrophilic because they are polar, and their polarity is stronger than the cohesive forces of the water. So when you get one of these polar substances in water, it's strong enough that it breaks all the little cohesive forces, all those little hydrogen bonds, and instead of hydrogen bonding to each other, the water will hydrogen bond around these polar substances. Table salt is ionic, and right now it's being separated into ions as the poles of our water molecules interact with it. Can someone explain to me why what they're saying is different than what everything else is saying? It makes no sense. Thanks!
|
[
"If you just have a glass of water, the water molecules are continuously hydrogen bonding with one another. (This is the reason why water has such a high specific heat - additional thermal energy is needed to break the hydrogen bonds.)",
"If you add salt (NaCl) to the water, then instead of hydrogen bonding with each other, some of the water molecules will begin to electrostastically interact with the ionic substance. The more positively charged hydrogens will be attracted to the Cl- , and the more negatively charged oxygen atoms will be attracted to the Na+. The water molecules will begin to form hydration shells around each ion. See picture: ",
"https://ka-perseus-images.s3.amazonaws.com/6686e9247fca1f0841033cc99465511c1043ab2d.png",
"If the energy released to due the formation of a hydration shell (hydration enthalpy) is greater than or equal to the lattice energy (the amount of energy required to form the crystal from separated ions) , then the substance dissolves.",
"A less technical way to see this is that the hydration shells formed around each ion cause the ionic crystal structure to break apart. "
] |
[
"Random question: Why does salt water and actual salt taste the same? If salt water is simply Na+ and Cl- Ions, and salt is NaCl, why do they taste identical? Or is it that NaCl is ionized by your saliva as well?"
] |
[
"Saltiness is a taste produced primarily by the presence of sodium ions. Other ions of the alkali metals group also taste salty, but the further from sodium, the less salty the sensation is. A sodium channel in the taste cell wall allows sodium cations to enter the cell. ",
"So, yes, it's the ionized Na+ ion that is recognized by sodium channels in your cells. ",
"https://en.wikipedia.org/wiki/Taste#Saltiness"
] |
[
"Blizzard in the NE (Nemo): Would anyone mind discussing its potential impact in a manner free of media sensationalism?"
] |
[
false
] |
I live in the NH and they are basically saying we are going to get hit very hard. The media is perpetuating this as well. What are the facts?
|
[
"The national weather service puts out official forecasts, on their ",
"site",
". You can drill down and you are looking for the forecast discussion like ",
"this one",
" for the ME/NH area.",
"They are generally hype free. With ",
"this",
" message about Katrina being the closest they get to weather channel hype."
] |
[
"Depending on where exactly you are in New Hampshire, you could easily see between one and two feet of snow. At the same time, it will be very windy during the height of the storm, so there will be a great deal of snow blowing around causing whiteout conditions, and making travel dangerous if not impossible. Depending on how much snow falls, there could be infrastructure damage which could cause power outages, or could make it difficult to clear and re-open roads after the storm."
] |
[
"I know I'm late to the conversation, but essentially what happened was a combination of two storms. Low pressure systems mean bad weather to meteorologists. One low pressure system developed to the west, being fed by cold air coming down from Canada. This system strengthened as it passed over the Great Lakes area. At the same time, another low pressure system started to develop in the south, and came up the coast from the Carolinas. The two storms met and combined in the New England and Long Island region. The combination of the two low pressure systems is why everyone is saying your area is going to get hit wicked hard. I highly recommend the National Weather Service website (weather.gov) over weather.com. In NH, your NWS office will either be in Taunton, MA or Grey, ME.",
"Hope this helps! Stay safe!"
] |
[
"Could You Blow a Small Speaker (e.g. an iPhone Speaker, Computer Speaker) By Covering It Up Air Tight while Playing It at Full Volume?"
] |
[
false
] |
I was out golfing with my friend the other day and I was playing some music on full volume on his iPhone 5 when we started to come close to the guys ahead of us. So to be courteous I plugged the speaker with my thumb to cut the music off and then I turned the volume down and took my thumb off. He then got all pissed at me and told me never to do that again because he thinks it can somehow blow the speaker by trapping the sound waves? I don't know, his explanation didn't make any sense to me, and his phone speaker was alright, so that's why I came here to ask science. Can you blow a speaker by covering it air tight while it's playing at full volume?
|
[
"I think he believes that the increase in air pressure generated by the speaker would be enough to rupture the membrane that makes the sound.",
"When in reality, the maximum change in air pressure associated with a sound wave is very small. A jack hammer at 1 m from the source, which is about 100 dB, has a pressure rise of only 2 Pa. That's on the order of 1/50,000th of atmospheric pressure.",
"An iPod speaker will produce a much smaller amplitude sound wave.",
"Putting your finger over the speaker won't hurt it. Speakers normally blow out because they're driven to displace the cone farther than it was designed to. This won't happen with an iPod since the maximum volume should have been set such that the speaker can handle it."
] |
[
"there are really two ways to blow a speaker,",
"Undersized amplifiers or a amplifier being pushed too hard will produce \"clipping\" Clipping is when the AC waveform gets its peaks cut off (or clipped)and the amp starts producing DC voltage instead of a AC sine wave. you now have a prolonged burst of DC voltage that the speaker really can't use so it turns it into heat and melts your coils.",
"massively overpowered speakers can essentially melt the copper wire in the cones windings.",
"the end result is the same, your copper coils are now either melted and no longer capable of carrying the voltage to move the coil or are so distorted in shape that they will bind in the air gap."
] |
[
"i've heard a common reason speakers blow isn't because the speakers can't handle the amplifier's size, but because the amplifier is too small and thus you think it can be turned all the way up and it will clip, creating a large square wave that the speaker tries to replicate but can't. any truth to this?"
] |
[
"Could enough of something with a low pH dissolve anything?"
] |
[
false
] |
[deleted]
|
[
"Not really. Acids just don't eat stuff indiscriminately. It's always a specific reaction, and there's always something resisting that reaction. Think of it this way - even for the most aggressive substances we handle, we have vessels that can contain them. Pure acidity isn't that destructive in the first place. It gets more interesting when something is acidic and oxidating."
] |
[
"Not necessarily. pH doesn't tell the whole story when it comes to the reactivity of acids with other materials. There are many materials that simply don't react with acids at all."
] |
[
"Probably not. From my understanding for something to dissolve it is dependent on wether it can form intermolecular forces with the other molecules surrounding it. So for example sugar, it is able to form intermolecular forces with surrounding water molecules which allows it to dissolve in water However, oils cannot do this, which explains why they don’t dissolve in water and sit on top of it. So to answer your question, it depends on wether said acid is able to form intermolecular forces with surrounding molecules :)"
] |
[
"How did our universe become so organized?"
] |
[
false
] |
this question pertains to entropy in the early universe. as i understand it, entropy is the habit of particles to move from organized to disorganized. from order to chaos. we know that entropy always increases, and can never be reversed. knowing this, when our universe was young how did any form of organized structure form in the face of entropy? wouldn't entropy have kept everything random and chaotic? how did the first atoms form, then the stars that forged them into the elements with this force of chaos trying to drive them apart?
|
[
"Or gravity along with other natural functions of physics."
] |
[
"A \"random and chaotic\" gas is only a maximal entropy state in the absence of gravity. If you include gravity, then you'd have to include that in the measure of entropy. I saw a talk by Roger Penrose in which he suggested that black holes are the maximal entropy state of a given volume of space and thus tell us the eventual endpoint of the universe. There are ",
"additional issues",
" to do with the universe also expanding, and so the maximum possible entropy increasing.",
"So, probably once you account for gravity, the \"organisation\" that you see is in fact higher entropy than the more uniform state it was in before."
] |
[
"The error in your question is that you're taking entropy to mean \"disorder\" which is not entirely correct. If that's all it was, then you'd say \"of course a planet, a solid spherical object, has more order than a big cloud of gas, so it must have less entropy\". But entropy is better described as an inverse measure of how far something is from energy equilibrium, or how much work a system can do.",
"With most terrestrial systems, disorder/order works well, but on a large scale where gravity comes into play, that defintition breaks down. We talk of objects/systems that have potential energy as ones that are high up in a gravitational field and can fall, thereby performing work. Core principle of hydroelectric dams. Therefore, a giant spread-out cloud of gas has a lot of potential energy - it will eventually collapse under gravity and form a star/planet, radiating heat out as it does so. A planet is in a much lower 'energy-state' than the gas, and so has a much higher entropy."
] |
[
"Are there any domesticated species whose wild precursor still remains a mystery?"
] |
[
false
] |
I’ve read about how many of our domesticated crops can be traced back to the wild plant they were most likely first bred from. Are there any that scientists have been unable to identify thus far? Perhaps because the precursor is now extinct or lost or been overtaken in all its habitats by the domesticated breeds? I imagine it would be considerably less likely for domesticated animals, although I have heard conjecture at least over which exact wild cat was first bred by humans, since there has been much cross breeding over the millenia.
|
[
"Can you elaborate on citrus fruits? I was under the impression that we know all our cultivars are variously descended from pomelo, citron, mandarins (not the kind in stores), and papedas."
] |
[
"It is also unclear which of the wild tomatoes the domesticated variety is descended from. And the answer maybe be most of them as wild and domesticated tomatoes readily hybridise"
] |
[
"Just looked this up, you're correct and the information I was working from was just very out of date. "
] |
[
"What did our evolutionary ancestors do for hygiene, such as showers, brushing teeth, and cutting hair and nails?"
] |
[
false
] | null |
[
"Look to our evolutionary closest relatives, like chimps and orangutans.\nA huge difference between humans and our evolutionary ancestors is the food we eat, particularly cooked food and sugars. Our ancestors didn't eat that stuff, and therefore didn't have the same issues with oral hygiene.\nBathing? Live near water, or don't bathe.\nEver hear of people who can't kick a habit of chewing their fingernails? There could be a reason for that!"
] |
[
"Our ancestors didn't eat that stuff, and therefore didn't have the same issues with oral hygiene.",
"Been watching a lot of old episodes of \"Time Team\" (the old British archeology show) and it is interesting to see that some of those old skeletons really did have very nice teeth. ",
"One of the comments made was that you can tell the remains are not roman if they have good teeth, because the Romans had lots of rich foods that rotted their teeth out."
] |
[
"I'm surprised that nobody has mentioned scraping!",
"Okay, we think of the ancient Greeks as being beauty-conscious and pretty clean, right? Well, when they weren't bathing in water they would ",
" themselves clean, using a tool called a strigil. It's basically a knife. They kinda kept themselves oiled up most of the time, anyway, so it was a well-lubricated process. They must've loved doing it, too, because they made statues (now called apoxyomenos) of dudes ",
" themselves.",
"It doesn't have to be a metal knife, of course. Even a sea shell will do the trick. Sea shells are common in all sorts of archeological dig sites - even inland ones - with signs that they were used as scrapers.",
"There's also the practice of covering yourself in ash, animal fat, herbal oils, mineral-rich mud, or any combination of those. This keeps biting insects away, and can look or smell as interesting as you like. There are still tribes that do that, like the Himba people of Namibia.",
"If you don't have modern clothes or furnishings then you don't have to worry about getting your earthy, oily body-paint all over your modern clothes & furnishings. And if it's nice & greasy then you can scrape it away easily and apply some fresh stuff whenever you need to.",
"So there's another way to keep yourself entertained during lockdown!"
] |
[
"Does gender matter in organ transplants?"
] |
[
false
] |
Say someone gets a liver transplant, and that person is a woman. Are the chances of her body rejecting the organ higher if the donor was male rather than a female?
|
[
"Gender has little to nothing to do with organ transplants, far more important is the similarity of the donor immune system antigens to the patients. It's important that the patient has as little a response to the donor organ as possible. This is determined by HLA type matching, this is a genetic test that essentially says how close a 'match' the donor immune system is to the patient. Although minimising immune response in important (to reduce rejection), it is also important that the patient will need to be on immuno-suppressant drugs for the rest of their lives and a good match will reduce the complications in the long term.",
"http://en.wikipedia.org/wiki/Human_leukocyte_antigen"
] |
[
"When talking about bone marrow transplants, yes.",
"Chances of a GvH (graft versus host) reaction are higher if the bone marrow donor is female and has been pregnant a sufficient number of times. In the instances where we can choose, we pick a male over a female donor.",
"The reason this happens is because we transplant the immune system, so we get the situation of the \"organ\" rejecting the body. Women after some pregnancies have just a more lively immune system."
] |
[
"Women who have been repeatedly pregnant have been exposed to more different HLA antigens, when compared to a transplant/transfusion naive man. It happens that HLA class I is the main determinant of cellular rejection (which is becoming rare thanks to modern immunosuppressive regimes). Therefore, multiparous women may have it slightly more difficult finding an organ match. ",
"Source: Pathology and Laboratory Medicine education"
] |
[
"Is there the same amount of water on Earth today as there was when the Earth was created?"
] |
[
false
] |
[deleted]
|
[
"Far, far more important than water molecules leaving the planet, or addition of water through asteroids and comets, is that water is incorporated into the rocks of oceanic crust in massive volumes over time (metasomatism). This is why you get volcanic arcs above subduction zones, which account for a very sizable percentage of continental volcanism on this planet. ",
"Water which has been incorporated into rocks (such as the modification of Olivine to Serpentine, for example, which occurs in fantastic quantities) is removed from these rocks at moderate depths and temperatures, and water lowers the melting point of rocks so you can get the production of magma, which ascends to the surface and forms a volcano (which can, through eruption, return water to the atmosphere).",
"I haven't verified this, but a quite brilliant professor I know who works on petrology said that a volume of water equal to all the world's oceans has been cycled through metasomatism many times in the history of the planet. Hopefully someone can come along here and verify that, but every time I hear a surprising fact that like from a reputable professor, it's turned out to be true.",
"Regardless, it is at least very feasible that the volume of water on Earth has changed throughout the history of the planet. There is no way I know of to even come close to accurately estimating the volume of the oceans past maybe a billion years ago (I would also be interested if someone could shed light on this).",
"Again, this isn't my area of expertise - I just want to point out that answering this question may not be as simple as it first appears, and deserves a response from someone who has done significant work on this subject."
] |
[
"More or less, water molecules can leave and arrive earth through the atmosphere, humans can do various chemical reactions turn some other material into water or visa versa, but overall the level has remained pretty constant.",
"Should also be noted when the earth was created it was a big ball of molten rock, so there was no water then. Once it cooled water started to accumulate into the oceans we have now, and since then has remained pretty much constant. "
] |
[
"Your professor is right. Due to ocean currents, upwelling in the ocean brings water up from the depths. This water, now on the surface, begins to play its part in evapotranspiration, which travels into the air, rises, condenses on cloud condensation nuclei (CCN) to form clouds, then travels to land where those clouds precipitate that excess moisture to the terrestrial environments. ",
"The water of our Earth has constantly been recycled, and this occurs by the evaporation, then forming clouds, then precipitating, then travelling down fluvial systems to the ocean, then through transitional and shallow marine environments, across the shelf through use of the longshore current and rip currents. Then, some of this water gets caught in more powerful ocean currents, such as the Gulf current, which brings up warm waters of the Gulf of Mexico through its deep conduction (and warm air with it) to heat Western Europe. As the water loses its heat, it sinks, and produces a shallow, cool current which moves south across the coasts of France, Spain, and West Africa. Some of the water sinks further, and over time makes its way into cracks underneath the ground, or soaks a subducting slab near an island arc, thereby lowering the melting temperature of the rock, producing magma, and when that magma is released, the water vapor gets spewed back out (or gets crystallized via fractional crystallization in hydrous minerals).",
"So again, the water on Earth has been cycled many times. This is from a more deep understanding of hydrology than anything, as it's a deep consideration of the entire water cycle, from where it evaporates (or gets released from volcanic outgassing), to condensation and precipitation (transpiration / baseflow / interflow / and surface runoff are also critical in terrestrial regions, as dictated by the water budget equation, P-R-G-E-T = ΔS)."
] |
[
"We hold our eyes closed, or hold our eyes open?"
] |
[
false
] |
How do we know?
|
[
"The orbicularis oculi muscles contract to close the lids. The palpebral portion acts on its own when you are closing your eyes to sleep. The muscles that contract to open the eyes are the levator and meuller's muscle along with relaxation of the orbicularis. So opening and closing of the eyes require both relaxation of some muscles and contraction of others.\nTLDR: Both. "
] |
[
"Not possible to relax both sets at the same time because of the anatomy, one set flexes as the other relaxes. Similar to when you flex the biceps brachii to bend your arm at the elbow, the triceps brachii relax and vice versa. "
] |
[
"Not 100% sure, I've seen open, closed, and in between upon death, but I believe the mechanism of death and other present pathological processes (increased intracranial pressure, neuropathies) play a role in which set of muscles loses the most tone and therefore will dictate what your eyes will do. ",
"Side note: Don't worry, blink 182 will never die. "
] |
[
"Weapons-grade uranium vs weapons-grade plutonium: what's the difference?"
] |
[
false
] |
What's the difference between WG Uranium and WG Plutonium? For example: how many kilograms per kiloton are needed? Availability? Warhead size difference? Which might be better suited for a "suitcase nuke"? Etc. (I'm going to use this for a story, for you curious folks.)
|
[
"Well first off, one is uranium and one is plutonium, they are different elements. WG uranium is typically called highly enriched uranium (HEU). Technically anything above around 20% U-235 is highly enriched and theoretically can make a fission weapon. In reality, when talking about weapons grade uranium it will have an enrichment of over 85% U-235. The rest mostly being U-238 and some U-234. ",
"WG plutonium is a bit different. It has to have a high percentage of Pu-239, the main fissile isotope of plutonium. What makes it weapons grade versus reactor grade is the concentration of other isotopes. Pu-238,Pu-2340, and Pu-242 have high spontaneous fission rates. This would cause a bomb to fizzle before maximum compression of the pit. So weapons grade would have small amounts of those even A isotopes of Pu. ",
"Kg per kiloton doesn't depend on the type of fuel. That is just E=mc",
" in action. What determines material amount is the critical mass. The minimum amount of mass needed to have a self sustaining chain reaction. This depends on mainly factors (enrichment, reflectors, temperature, shape...) For a 95% uranium sphere in a vacuum it is around 50 kg. For pure Pu-239 sphere in vacuum the amount is around 9 kg. ",
"Enriching uranium is a pain. It is expensive and takes lots of electricity. Pu is easier to make, just need a nuclear reactor, but the weapon is more complex (Implosion). Uranium you can dig up, plutonium needs to be made. There are a lot of factors that determine the availability. ",
"Implosion devices are smaller than gun type devices. However, uranium can be used in either device type. Plutonium will only work in an implosion design (although there is a possibility of an advanced gun type that can work). The size really depends on how much bang you want. Modern weapons are about the size of an adult and weight less than 1000 lbs. ",
"Suitcase nukes are a bunch of bs. If you really think they exist, it would have to be plutonium since the uranium mass is too large. Due to their size it would need an advanced implosion system. "
] |
[
"I'm curious about this advanced gun type, what exactly differentiates it from the original gun type? "
] |
[
"Modern advances. You need to fire the two halves of the pit very quickly and accurately. Modern machining is needed along with modern explosives to get the required velocity."
] |
[
"In special relativity, what causes length contraction?"
] |
[
false
] |
As I understand it, everything in the universe is constantly moving through spacetime at . Objects that are stationary in space (relative to the observer) are moving at full speed through time, and objects moving through space must necessarily move slower through time to maintain constant velocity in spacetime, hence time dilation. But I don't understand how this also causes length contraction. Why does relativity cause the axis an object is moving on to shrink?
|
[
"This animation might help",
"."
] |
[
"To maintain the balance that time dilation offsets. ",
"The muon decay experiment is the best way to understand how they go hand in hand. Muons have lifetime of a certain amount. Without SR they wouldn't reach or very few would reach earth from the sun. ",
"However people measured a lot of muons reaching earth from the sun. To make sense of this from the observer on earths perspective you say \"ah its rate of time was dilated\" so its lifetime is longer. But from the muons perspective its rate of time is fine, but the distance from the sun to the earth has contracted instead."
] |
[
"http://en.wikipedia.org/wiki/Lorentz_contraction",
"Everything you need to know to answer your question is right here",
"EDIT: After I posted this, I wanted to show you this: ",
"http://en.wikipedia.org/wiki/Ladder_paradox",
", which is really cool if you have never seen it."
] |
[
"Why is it so important to stay awake after sustaining a critical injury?"
] |
[
false
] |
I actually joined reddit a couple months back to ask this very question. User docmedic replied that "it greatly increases your chance of going into a coma, if not dying." Why? I am aware that the body's processes slow down during sleep, so wouldn't going to sleep help a seriously injured person by acting as a "hibernate" option if you will, to conserve energy? EDIT: I too have found many journals on the correlation between increased chance of coma/other exacerbated risks and falling asleep after suffering a head injury. I realize a conscious person is better serving to monitor for symptoms of a more serious condition than one who is knocked out, but keep in mind I am asking about critical injuries in general. This could be an arrow to the foot, multiple crushed organs, gunshot wound, a mauled face, a combination - just making it clear that I've seen enough material isolated to head injuries. A few interesting things I've stumbled across while researching: "....an episode of 'I Shouldn't Be Alive' I saw recently, in which a solo peakbagger gets badly injured in an avalanche and spends about 3 days crawling/limping his way out. After at least 2 days of no sleep, exhaustion and blood loss, he struggles to stay awake thinking he will die if he succumbs to sleep. He falls asleep anyway and wakes up the next morning under a pile of fresh snow, feeling surprisingly refreshed." -snow=insulation? "I once saw a show on Discovery Channel that described the final stage of hypothermia, which includes extreme sleepiness, before the person drifts off and dies." "I've heard that people who get frostbite more easily are less likely to die of hypothermia because the body is quicker to cut off bloodflow to the extremities in order to preserve core temperature. But that's just something I read from who knows where..." -Sample evidence seems to positively support cold weather for extending life force... or is it the rest? How much does body temperature matter? -What about ? "It is important to keep the person warm as well as adequately manage pain and anxiety as these can increase oxygen consumption." "....data showed that the climbers core temperature stayed up as long as they kept moving, slow as that pace might be at extreme altitude. If they stopped, some climbers were able to maintain a tolerable core temp, but in others, their core temps immediately started heading down to dangerous levels until they started moving again. ....when you're that close to the borderline, the ability to differentiate whether you can stop and rest or whether you are in danger of severe hypothermia is critical. But if you're already near that point of hypothermia, that's kind of like asking a person who's been drinking heavily to assess whether they are safe to drive. (A clear headed buddy is helpful in both the hypothermia and DUI scenarios!) ....physiologic difference observed probably partly explains why some people in dire straits can fall asleep and survive, while others succumb to the elements if they drift off to sleep. People come to differing conclusions based on such survival stories, but miss the possibility that the people were quite different in some critical way." I have a and a point to make but I'll brb, post-turkey dinner food poisoning... EDIT 2: Back! Thanks to everyone who took the time to reply. I read the whole thread and I'm currently compiling a conclusion, although the contributors pretty much explained everything. :) EDIT 3: Almost there. I've been busy but I'm currently bedridden so I'll have it up by tonight. EDIT 4: Horrible person. It will be here. I promise. EDIT 5 (One Year Later): I forgot about this. I'm not sure if I will end up finishing the conclusion as it's not fresh in my head anymore, but users D-discoideum and Neurokeen explained and elaborated quite well below. Sorry. Further Reading
|
[
"I'm a former EMT-B with wilderness medical and disaster response training. My last refresher course was about 3.5 years ago, and all of my certifications expired last June. This is a rather long response to the question, but I hope it's interesting.",
"Once upon a time, doctors noticed that head injury patients who went to sleep for the night sometimes wouldn't wake up in the morning, having either slipped in to a coma or died in the night. It was further realized that patients who were kept awake were more likely to make it through the night. Based on that, it was reasonable to conclude that there was some not-understood reason that the going to sleep was causing problems.",
"Although the conclusion was reasonable, it also turned out to be wrong. Head injuries have a rather unique progression. When the impact occurs, some changes in consciousness may occur immediately which the patient will recover from quickly. In some cases, that's the end of the story, and from there things get better. For some injuries however, there's something still happening: the patient's brain is bleeding. When most areas of your body bleed without breaking skin, the blood goes in to the interstitial space causing a dark color to become visible through the skin, and possibly some swelling as the area fills with blood. This phenomenon is commonly known as a bruise. Unlike most areas of your body, your brain is not surrounded by skin, it is surrounded by an extremely hard shell (your skull) protecting it from bumps and scrapes. There isn't a lot of extra space in there though, and so when you start bleeding in to your skull, it very quickly fills up, and since it's inside your hard, inflexible skull instead of your stretchy skin the pressure starts to build up and push on your brain, compressing it more and more.",
"This increased intercranial pressure causes some very obvious changes in the patient's level of consciousness, and they'll always happen in the same order.",
"First the patient will forget recent events, they won't be able to tell you what happened recently. Patients in this stage are somewhat confused, but still able to carry on a normal conversation, understand their current situation, even if they can't remember getting there. They're generally able to make reasonable decisions, and still seem pretty with it even though they can't remember certain things.",
"As the pressure increases, the patient will lose track of time. I'm not talking about thinking an hour has gone by when it's only been forty minutes here, I'm talking checked their watch 60 seconds ago and now they can't even tell you if it's day or night. They might think it's 1994, or they might tell you straight up they haven't a clue what year it is. At this point, they start to get weird. If you're having a conversation with them, there's no way you won't know there's something going wrong. Often times the conversations start to get cyclical, you may feel like you're living in a broken record. They may ask what time it is, be satisfied with the answer, ask you what happened, be satisfied with the answer, and now they're curious about what time it is. This level of awareness change also has specific legal consequences, because at this point they are no longer considered capable of making informed choices about their own medical care. From a legal standpoint, they might just as well be passed out from a medical consent standpoint.",
"As the pressure continues to build, the patient will lose track of where they are. This isn't about knowing their exact GIS coordinates, and this isn't about someone who's lost. A patient who got in a car accident and tells you they have no idea where they are because they were following some mapquest directions, but then they got on the wrong highway, got off in the wrong section of town, were freaking out a little and were futzing with their GPS to try to get unlost when they hit the lamp post and now they have no clue where they are is just lost. When you ask the lost patient if they know what city they're in, and they give you the right answer and then ask if you're stupid or something is not who we're talking about. We're talking about can't tell you what city, province, or maybe even what country they're in. Sometimes they'll be certain of where they are, they just happen to be quite wrong. \"This is obviously Los Angeles. Omaha? What the hell are you talking about? I've never even been to Nebraska!\"",
"As the patient's brain continues to bleed they may even lose track of who they are. They may forget their name, their occupations, their parents names, etc. They may become convinced they are someone else. They may not even understand the question, struggling with the concept of identity itself. By this point patients are super weird and cannot carry on anything resembling a marginally reasonable conversation.",
"As they're losing brain function, they may also become irritable and then even combative before passing out. They'll still respond to verbal stimulus, though it may not be quick. An emergency responder may get in their faces and bark orders at them to see if they're able to respond. If you can shout \"OPEN YOUR EYES!\" or \"WIGGLE YOUR FINGERS!\" and they do, that's a good sign (as much as there are good signs when a person has passed out due to intercranial pressure)",
"Continuing to fall down the level of consciousness scale, the patient will stop responding to verbal stimulus, but will still respond to pain. There are several techniques for causing a patient pain without actually hurting them in order to evaluate this. Many emergency responders will preform sternal rubs, which is basically giving the patient a noogie on their sternum. It hurts like hell, and looks nasty. I personally prefer to use one of several pressure points which are actually more painful (which is good from an assessment standpoint), but don't look nearly as awful from the outside (which is good from a dealing with the patients loved ones standpoint).",
"The last step before a patient dies of a head injury is that they'll be unresponsive to any stimulus, including pain. When a patient gets to this level, it's a very bad thing. At that point, I really don't fancy their chances of surviving the whole ordeal.",
"This is all relevant to the question in that all of these really obvious signs are only really obvious if the patient is awake. If the patient is asleep, the first time you notice anything is amiss might be when they don't get up in the morning, you go to wake them, and lo and behold, over the last 9 hours they've gone all the way through this process.",
"It turns out that sleep is actually a really important part of the healing process, and so keeping patients awake for 24 hours after a head injury is actually a pretty bad idea. This process really doesn't happen that quickly, and so these days rather than keeping patients awake, we tell them to go ahead and get lots of sleep, but that we're going to wake them up at least once every four hours to check on them. When we do wake them up, we'll ask them if they remember who they are, where they are, about what time it is, and if they know why I'm waking them up."
] |
[
"As a neurological intensive care nurse, I can tell you that we keep patients awake primarily to monitor neurological status. That is in the neuro ICU though. In other ICUs, they monitor your stability with frequent vital signs (as do we) but also by periodic checks on your level of consciousness, which is directly related to your cerebral perfusion and can suffer for a large number of other reasons besides low blood pressure/volume.",
"In the movies, when healthcare professionals are shown trying to keep a patient awake, it is because they are concerned that there is an acute change in the mechanisms that facilitate wakefulness. Anyone will wake up (and stay awake) if given enough painful stimuli (e.g., sternal rub, pinching, etc.). If the patient does not, it means that they are acutely decompensating and in need of emergency medical attention. This is why all life support (BLS, ACLS) begins with an attempt to arouse the patient."
] |
[
"English isn't my first language so I had to google noogie. I can now confirm that a sternum noogie does indeed hurt. Even when giving it to yourself.",
"Really informative answer, and I especially appreciated the explanation of how this belief came to be."
] |
[
"How did we acquire the knowledge / understanding of the \"structure\" of atoms or the behaviour of sub-atomic particles that we currently have?"
] |
[
false
] |
[deleted]
|
[
"Most observations are a form of \"bouncing stuff off crap and seeing how it bounces back.\" Even when you're looking at the world around you, you're bouncing visible-energy photons off of things and how those photons come back into your eyes lets you \"see\" the world around you.",
"Well for atomic and subatomic physics, we do a lot of the same. First, electrons... well they didn't exactly work this way. ",
"Thompson",
" found that \"cathode rays\" were some negatively charged particle about 1000 times less massive than hydrogen. So we get the old \"plum pudding\" model, where you have this blob of positive... stuff, and then little \"corpuscles\" of negative charge. ",
"Rutherford",
" begins our quest of bouncing things. We'd detected radioactivity and had classified them into various categories. Alpha rays were \"heavy\" particles that were positively charged, and didn't really pass through much matter. Beta rays were \"light\" particles that were negative and passed through some, but not a whole lot of matter. Gamma rays had no charge and would go through an awful lot of matter. So Rutherford (or more accurately, a team of students led by Rutherford) take an alpha-ray emitting source and make a little hole in a box so it comes out as a \"beam.\" (other alpha rays are absorbed by the box) Then they shoot the beam at a very thin gold foil. What they find is that every so often, the alpha particle \"rebounds\" flying back the direction it came (almost). But usually it passes through. Well if atoms were \"positive goop\" with \"negative corpuscles\", then alpha particles should always pass straight through (since on the whole, the atom is neutral). But these deflections meant that the positive charges must, in fact, be very tightly confined to one volume, a positive \"nucleus.\"",
"Now at the time, we knew Hydrogen was the lightest element, so Rutherford proposed its nucleus was a \"fundamental\" particle, a proton. But it was also known that if you have a bunch of protons... why on Earth aren't they flying apart all the time? A couple ideas existed at the time, one being \"nuclear electrons\"(electrons in the nucleus to balance charges. For instance, oxygen would have 16 protons and 8 nuclear electrons, to give a total +8 charge) and the other being an as-yet unfound neutral particle like the proton, the neutron. Well some Soviets showed the nuclear electron model wouldn't work, and eventually ",
"Chadwick finds the neutron",
".",
"Now to go further, we start colliding stuff and making more stuff out of it. Exotic particles we don't find in every-day matter. We start labelling each of them, building up this \"bestiary\" of particles. Which made people wonder... why so many particles that ",
" normal matter? Well Gell-Mann (and Zweig and some others) discover(s) if you list the particles in a certain kind of way, a new \"periodic table\" of these particles, that there are certain symmetric properties. This leads him/them to suggest that these particles are composed of",
" yet smaller particles",
", specifically the Baryons are 3 \"quarks\" and Mesons are 2 \"quarks\" (properly speaking a quark/anti-quark bound state). ",
"And while the quark model explained the results pretty well, the real (imo) verification came with ",
"Deep Inelastic Scattering",
" where we \"bounce\" very high energy electrons \"off of\" quarks in the proton. By measuring how the electrons leave, we're able to determine that a proton is indeed made up of quarks (and gluons)."
] |
[
"One should also mention the quite cool story about the Zeeman effect, after the discovery of which Lorentz was able to establish a formula for the the ratio of the mass of the hydrogen nucleus (proton as we know now) to electron mass, which was subsequently measured by Zeeman, all within one month!",
"This is depicted glass panels by the nephew of Kamerlingh Onnes that can still be seen in Leiden University:\n",
"http://www.lorentz.leidenuniv.nl/history/lorentz/lorentz.html",
"By the way, the ratio is 1600:1 to the precision then possible."
] |
[
"And how did you know that an atom contains electrons in the first place?",
"So this is a very interesting historical episode.",
"J.J. Thomson gets the credit for \"discovering the electron\" but it's more complex than that. Thomson was using a common physics tool of his day, the cathode ray tube, to create little rays. He determined (though a set of careful experiments) that the rays coming out of the cathode tube were 1. negatively charged particles (they could be deflected by a magnetic field toward a positive terminal); and 2. very light compared to even very light atoms (like hydrogen).",
"From these rather humble observations he made the following somewhat extreme extrapolations:",
"that the particles were subatomic particles stripped out of the atoms in the tube",
"that these subatomic particles were in fact ",
" subatomic particles that made up ",
" atoms",
"He dubbed these particles ",
" and thought they must be the ultimate particle (he was influenced in this by the philosopher Prout who called these hypothetical particles ",
"). He imagined that atoms were just clusters of ",
" of electrons revolving around each other in very complex vortices, held together by a diffuse positively charged force. These corpuscles were, in his mind, the ultimate connection between the diffuse electromagnetic aether and the world of matter. ",
"Which as you can appreciate is a pretty big extrapolation from the data and somewhat far-out in general. In fact it is now seen as quite incorrect except for the fact that these rays are subatomic particles and are stripped out of atoms. Even the name has changed to match other theories of negatively charged particles — the electron. ",
"(Which begets the question: Did Thomson really discover the electron as we understand it today? Or did he discover something else — the corpuscle — which didn't actually exist, but led eventually to a modern understanding of the electron?)",
"But it's just this kind of work that is how atomic theory developed. First you have some kind of rather straightforward and seemingly basic experiment. This rules out some possibilities but not others. The scientist in question then takes a giant leap, interpreting from the experiment an entire model of the subatomic world. Then other scientists say, \"well, that's interesting, but is it true?\" and start doing other experiments. Which starts the whole process again — to Rutherford, to Bohr, to Chadwick, to many others.",
"Because you cannot meaningfully \"see\" these things (you can visualize them with machinery, which is not the same thing at all), it is always a lot of groping in the dark, refining the view, inferring from data, trying to improve the model. It's always part experiment, part math, part logic. We take the parts of the past models that seem right, and throw the stuff that seems wrong out, and we call this \"progress.\""
] |
[
"Does the location of a pipe outlet in a full dam affect the water pressure in the pipe?"
] |
[
false
] |
We have a small dam which is used to feed a pipe running down the section to provide water for gardening and other uses. Intuitively, I feel that putting the outlet to this pipe at the bottom of the dam (i.e. as far underwater as possible) would create the highest amount of pressure due to the added weight of the water above. Another person believes that it shouldn't matter if the pipe reaches nearly the surface (as long as the pipe is fully submerged), since the head remains the same and so the pressure is constant. Who is correct?
|
[
"The amount of waterflow is dictated by the pressure difference between inlet at outlet",
"By placing the outlet at the bottom, you add more weight to the water column in the pipe. However, the water coloumn of water over the outlet also adds pressure, so the pressure difference is the same whether you choose an outlet just under the surface or at the bottom should not matter."
] |
[
"The pressure at the garden will remain the same with the pipe at the bottom or top of the dam. (the height of the water above the pipe does not change either way). Placing the pipe near the surface will just limit how much water is available."
] |
[
"https://en.m.wikipedia.org/wiki/Hydraulic_head"
] |
[
"How is the mass of a gas measured?"
] |
[
false
] |
[deleted]
|
[
"What do you know about this gas? There are constants used to figure this out, but we need more information. If it's lighter than air, you're really measuring how much lighter it is rather than how much heavier it is. Follow?"
] |
[
"The balloon experiment would work fine for a gas lighter than air since you (presumably) know the volume of the balloon and you can work backwards from the buoyant force (force pushing up) to find the mass. Alternatively if you know what gas you're dealing with you can just use pressure temperature and volume to go straight to the mols (more useful than mass and convertible to mass)",
"Here are some links to get you started with gases:\n",
"http://en.wikipedia.org/wiki/Ideal_gas_law",
"\n",
"http://en.wikipedia.org/wiki/Buoyant_force"
] |
[
"The \"air in a balloon\" experiment is only accurate if the gas has equal or greater density than the surrounding air.",
"Measuring the mass of a gas lighter than air is actually pretty simple. Basically, we have a number called the Gas Constant R (you can check it out ",
"here",
") that creates a relationship between the pressure, volume, temperature and number of molecules in a gas. ",
"So to calculate the mass of the gas, all you need is the pressure, volume and temperature of the gas to calculate the number of molecules it contains, then you multiply the number with the gas's Molar Mass to find mass. There are many other ways to do this, but this method is the easiest and most accurate. "
] |
[
"Why don't human \"polar bears\", who dive and swim in ice water, get hyperthermia?"
] |
[
false
] |
I saw this youtube on the reddits of and sent it to my father, who is a Red Cross instructor. He asked how this man does not get hypothermia, especially if a person is wet and standing outside with no clothes in the cold, and than dives back into ice water. I don't have an answer for him. Is there some special regional human biology at work here? edit: hyper- to hypo
|
[
"probably you mean hypothermia, as in a low body temperature. Hyperthermia would be overheating.",
"That video was less than two minutes long, meaning this guy did not spend a long time under water or wet. Hypothermia isn't an immediate thing. You are talking about cooling down the core body temperature a large amount for that to be the case. The body will ramp up its calorie consupmtion to ward this off, and begine processes like shivering taht burn a huge amount of calories and create heat. ",
"I'd bet anything this guy ran for a blanket and a hot beverage instantly after filming this, further inhibiting hypothermia from setting in. It also looked like a decently nice day out there, so the air was probably relatively warm.",
"He also looked drunk and seemed to be drinking alcohol at the the end. Combined with the adrenaline from doing something like this, he probably was able to not notice the negative affects it was having on his body for a little while.",
"There can be a biological component as well. People swim in cold water all the time. If you have a certain body type, its easier to do (a layer of fat is helpful), but will never fully protect you. I saw something on TV (discovery channel I think) about a woman who swam with no protective clothing in bodies of water like the the english channel. They actually ahd her jump into the antartic ocean with nothign but a bathing suit on. She lasted a few minutes and then they took her out and warmed her up to try and prevent hypothermia from setting in. She didn't look as sucessful in that one, but she had no problem spending plenty of time in other cold bodies of water (just not antartic water and air temperature)"
] |
[
"Additionally, the ",
"mammalian diving reflex",
" basically transforms your body as you hit the water -- our heartbeat slows, capillaries restrict, etc. It's good for letting mammals stay underwater (and in deep pressure), but would also assist in not freezing to death as well."
] |
[
"Wow TIL.",
"However, wouldn't a reduced amount of blood flow be disadvantageous in cold weather? From what I understand, warm blood flow is a good thing."
] |
[
"What happens, phisically, to all the alpha/beta particles and neutrons that are stopped by a radiation shield?"
] |
[
false
] | null |
[
"These are all probabilistic, so not every event will happen in the same way. So I'll just describe the ",
" for each.",
"Alpha particles undergo many small-angle scatters off of ",
" atomic electrons, eventually slowing down, grabbing onto two electrons, and simply becoming a neutral helium atom. Alternatively, they can induce various nuclear reactions.",
"Beta-minus particles (electrons) scatter many times off of other electrons, until they slow down and simply become extra electrons in the material.",
"Beta-plus particles (positrons) also scatter off of many electrons, eventually slowing almost to a stop, and then annihilating with a nearby electron.",
"Neutrons scatter many times off of ",
" (not electrons) in the material until they're thermalized, and probably captured into a nucleus, transmuting it into a different isotope. However they can also induce various nuclear reactions besides capture."
] |
[
"Yes, the buildup of hydrogen and helium gas inside materials can cause all kinds of undesired changes to its material properties.",
"It’s an issue for things like structural steels used for reactor vessels, which are subjected to huge fluences of neutrons over their lifetime. (n,p) and (n,α) reactions are constantly producing H and He within the steel itself."
] |
[
"An alpha particle is two protons and two neutrons, thus it's a helium nucleus."
] |
[
"Is it possible to insert a drug into a virus and have it spread like influenza?"
] |
[
false
] |
Would people get the accompanying symptoms of the drug as well? Does the type of drug make a difference? If so, on what level?
|
[
"Yes. Viral vectors are used all the time in research. Basically what happens is that you cut away the \"bad\" part of the virus' DNA and then recombinantly add in whatever gene you want, without altering the virus' structural and replicative genes. By using different genes, or by inserting different DNA into your virus vector, you can target different parts of the body. ",
"I'm not positive that it's possible or practical to insert any kind of drug into a virus, but when the effect you're looking for can be achieved through genetics, it's a great way to go."
] |
[
"Yes. Viral vectors are used all the time in research. Basically what happens is that you cut away the \"bad\" part of the virus' DNA and then recombinantly add in whatever gene you want, without altering the virus' structural and replicative genes. By using different genes, or by inserting different DNA into your virus vector, you can target different parts of the body. ",
"I'm not positive that it's possible or practical to insert any kind of drug into a virus, but when the effect you're looking for can be achieved through genetics, it's a great way to go."
] |
[
"Thanks for the response and moderation, much appreciated. Regarding the side effects though, I know that many drugs come with some ridiculous ones, would they be retained as well? Would things like fatigue, nausea, blurred vision and other such effects be present?"
] |
[
"When doctors are drawing blood, is there an advantage or purpose to drawing from a vein instead of an artery?"
] |
[
false
] |
[deleted]
|
[
"1) You'd generally have to dig deeper into the skin to find an artery. Veins are very close to the surface of the skin, while arteries are more buried.",
"2) Blood in arteries has higher pressure, which makes messing with arteries much more dangerous to your health (higher risk of excessive blood loss)."
] |
[
"The major reason is that veins are usually closer to the surface than arteries and thus more accessible. Arterial draws are only used for obtaining arterial blood gas (ABG) measurements and for certain medication infusions. ",
"Edit: Forgot a word"
] |
[
"(Surface) Veins are also far larger than arteries, so they are a lot harder to hit."
] |
[
"Can't see the LCD screen of my synthesizer on stage with polarized sun glasses."
] |
[
false
] |
The screen of my synth is a one color screen ( ). I just bought these expensive RayBans that I'd like to wear while performing. The only way to describe what it looks like when I look at the screen is that it looks "cross-hatched"...almost like all of the pixels on the screen are turned on (not black). If I put a piece of polarized glass/plastic over the screen will the effect "cancel" out? If so, where would I get something like that....a piece of material to lay over the screen? Thanks a bunch, science ppl.
|
[
"Any additional filter will only block more light.",
"This is wrong. If you insert a 45 degree polarizer between two orthogonal polarizers, more light will pass through the system. It makes for a ",
"great optics demo",
" and it's actually strongly related to how our friend's LCD display works. "
] |
[
"This is an awesome real-life problem involving polarization. If I ever teach, I'll be sure to use it.",
"So, LCD screens work by altering the polarization of incoming light and reflecting it back out. When it's reflected back out, the light is horizontally polarized. I know this because sunglasses are vertically polarized. The vertical polarization is designed to reduce glare coming off of the ground/water because the reflected light is mainly horizontally polarized. ",
"So, as to your question, yes! If you put a polarizer over your display with an angle of 45 degrees to the display axes, the view should improve. There will still be loss, but it will definitely get better. ",
"edit: ",
"Here's a wonderfully British demo",
"edit 2: If you actually want to buy some polarizing plastic, ",
"here you go",
". Just cut out a screen-sized rectangle that is angled 45 degrees to the axes of the plastic sheets."
] |
[
"The same place you get anything these days",
"."
] |
[
"How can astronomers tell what a planet is made of when they are many light years away?"
] |
[
false
] |
I was looking at various pictures such as and and had two questions. How can scientists tell what a planet is made out of when they are millions of miles away? Also, how are scientists even able to see the planet? I understand telescopes are powerful enough, however in the vastness of space, don't other things get in the way? With all the other stars/planets/debris it would seem like looking at something millions of miles in the distance would be almost impossible due to how many things pass in the field of view. Just wondering.
|
[
"I remember learning about the atomic signature from cosmos, the tv series, how the spectrum of an object can be used to determine what elements it is made of.\nThe spectrum is the array of colors (the rainbow) that is presented when the light passes through a prism, when magnified, the spectrum includes dark lines along the color array, these dark lines are caused by the elements scattering a certain color. the dark lines in the spectrum sort of looks like bar codes. The spectrum of a sodium atom is different from that of a uranium atom, as well as a carbon atom, and so on.",
"i know that the spectrum coming off of a star can be used to determine what elements are inside it, which was one of the reasons we found out about the most abundant elements in our known universe. but I'm not exactly sure if this works with planets as well. ",
"of course, I just learned all this from a tv show, I'm sure there's someone around here who knows more about this than i do, but i hope this helps a little."
] |
[
"They are extrapolating from the fact that that 55 Cancri is a carbon-rich star system. (Ours has relatively more oxygen and silicon.) If the planet is mainly carbon, then, based on the measured density and temperature, they speculate that it is largely diamond. This is a bit more speculation than I'm comfortable with (but I'm not an exoplanet expert),",
"See ",
"this news article",
"."
] |
[
"Spectroscopy. If you can observe the absorption spectra of a planet or star, you can tell what makes up the planet."
] |
[
"Are there any animals that have showed signs of being ticklish?"
] |
[
false
] |
Or are humans the only species that are ticklish?
|
[
"Rats, apparently : ",
"http://neuroskeptic.blogspot.com/2009/08/tickling-rats-for-science.html?m=1"
] |
[
"Dog paws are ticklish! "
] |
[
"Seriously? Cats."
] |
[
"Do gravitational waves weaken over distance?"
] |
[
false
] |
And, in case they do, what are they interacting with that weakens them?
|
[
"Far away from the source they weaken with an inverse distance square law for their power and an inverse distance law for their amplitude, just like all other waves - as the wave spreads out over a larger volume the intensity goes down.",
"There is a non-zero interaction with matter that can reduce their intensity in theory but that is completely negligible."
] |
[
"Basically, gravity waves can induce tidal forces in matter. This can heat up the matter, transferring energy from the gravity wave, and into the matter."
] |
[
">There is a non-zero interaction with matter that can reduce their intensity",
"What form does that post-interaction energy take? "
] |
[
"Why does it matter when I eat my meals?"
] |
[
false
] |
It is common knowledge that you should eat larger meals earlier in the day and smaller, healthier meals later in the evening. says "Your metabolism slows down towards the end of the evening and is less efficient at digesting foods." My question is why does it matter when I eat my meals if I consume 2000 calories per day and burn 2000 calories per day? Thermodynamics is thermodynamics. Assume a balanced, healthy diet, ample exercise, and no bad habits or medical issues. I'm not looking for more common knowledge / nutritionist-speak. I'm seeking answers with a scientific/biological foundation.
|
[
"It may be common knowledge where you're from, but I know at home in Paraguay (and I'm pretty sure most of Latin America) people have pretty large meals late at night. Meal times vary from culture to culture, at home I can distinguish five meals a day that the average person has (at least in the city):",
"From my (entirely up for discussion and not expert-like at all) experience people seem healthier at home, and definitely look much less fat than in places I've been to that have the eating schedule you mention (namely the US and Canada). We also have similar life expectancies to developed countries (75 as of the 2006 census) even though we have terrible health care (ok, I guess it's at least better than in the US) and large portions of the country don't even have running water. So it's definitely not killing us.",
"Anyway, my point is I don't think it has any scientific/biological foundation and it's entirely a cultural thing.",
"Edit: I actually think it's better to have a large meal late at night because that way your body can devote all of its energies to digesting while you sleep. But I also know little to nothing about biology."
] |
[
"In terms of thermodynamics, sure it doesn't matter. But the process is not that simple. The machinery has quirks. My understanding is that the recommendation of a large meal in the morning is due chiefly to the fact that you haven't eaten all night. Consider: When you eat at noon for lunch, and your last meal of the day is at six PM, you've only had six hours between those two meals.",
"You go to bed, sleep and wake up at 7AM. It's been eleven hours since you ate. Almost twice as long as the time between lunch and supper, or between breakfast and lunch. The research I've seen presented the perspective that when one had a large breakfast, one would be inclined to eat less through out the remainder of the day, leading to a net reduction in calorie intake without negative affect. I believe this is in line with my own experiences.",
"As an aside, this is the origin of the word breakfast - one is breaking the fast."
] |
[
"Funny, I've noticed the exact opposite; if I eat a big breakfast, I've just consumed a large amount of calories, meaning i have less to \"spend\" for the rest of the day. This might be OK if it was offset by not wanting to eat from that point on, but I don't notice any particular appetite suppression that lasts more than a few hours either. If anything, larger meals usually make me want to eat more later, probably due to the stretching of the stomach.",
"There's so much pseudoscience and woo involved with the fitness industry, just look at the claim listed above on wikihow; I've read so many articles by trainers suggesting the exact same thing, that our bodies don't digest foods well at night. Even if this were true (highly doubtful) A) they don't have a real reason to believe it other than someone else told it to them and it's therefore \"common knowledge\" or \"common sense\" B) how big would the effect have to be to offset the fact that for many people, not eating at all in the evening means you're going to bed hungry and are therefore miserable all night and don't sleep well?",
"I'm generally pretty skeptical of almost all claims dealing with weight-loss, because even for those backed up by studies, the studies themselves are often horribly flawed; poor controls, reliance on self-reporting by participants, not tracking the participants gaining weight back after going off the diet, etc. etc."
] |
[
"Why is the range of the strong and weak nuclear interaction so small compared to the infinite ranges of the other two interactions?"
] |
[
false
] |
I've tried researching it a bit on my own, and it seems like it has something to do with the mass of the force carrying bosons, i just dont really understand how.
|
[
"The weak force has a range on the order of 10",
" meters, because the W and Z bosons have very large masses (around 90 MeV/c",
").",
"The strong force is different, because the gluons are massless. But it still ends up having a short range in practice, because of confinement. The energy that it would take to separate bound quarks is enough to produce more quarks and antiquarks, so you only ever see quarks and antiquarks in bound states, which have sizes on the order of 10",
" meters."
] |
[
"The potential energy of interaction which is mediated by a particle of mass m is generally proportional to a function of the form exp[-mr]/r, in units where c and ",
" are 1.",
"For a massless particle, you get a 1/r potential energy, like gravity and electrostatic interactions. But if m is not zero, then you have an exponentially decaying factor, with a “decay constant” proportional to m. So larger mass means shorter range, and the W/Z have huge masses."
] |
[
"Thanks for replying! I was however also wondering why the large masses of the W and Z bosons make the range of the weak force so tiny?"
] |
[
"Why do I get itchy when I take my preworkout drink?"
] |
[
false
] |
[deleted]
|
[
"do any of them have a high amount of niacin in them? old trick I learned long ago was to buy niacin from a health store a take a couple any time I wanted to appear \"sick\"... they make your skin feel itchy and look splotchy for a short while after taking them. perfect for getting out of school or church or whatever you don't want to go to."
] |
[
"It's called ",
"paresthesia",
". The beta alanine found in many workout drinks can cause it, as can high doses niacin."
] |
[
"Possibly Beta Alanine. Also, have you considered asking ",
"/r/Fitness",
"?"
] |
[
"Do people who look young for their age live longer?"
] |
[
false
] | null |
[
"Here's a news article discussing this: ",
"http://news.bbc.co.uk/2/hi/health/8411329.stm",
"Essentially, it appears that yes, younger appearing people do live longer, could be due to their telomeres (a part of your genes) replenishing better, or it could be that older looking people have lived a hard life. Keep in mind though, Correlation ≠ Causation"
] |
[
"An article I cited below essentially had a \"Guess the age\" contest with twins, and some would look younger than others, as judged by a jury of your peers, so to speak..."
] |
[
"I'm 35 and get asked what school (college) I go to all the time. Honestly the most awkward element is dating. Women my own age look so much older than me on the flip side women that look closer to me in age are so much younger I have a hard time relating to them. Full disclosure I have had some cosmetic surgery/treatments. I broke my nose a few times so had that fixed, my left eye and laser treatments over most of my face to remove some scarring. "
] |
[
"Could an HIV+ person receive organ donations without risk of rejection?"
] |
[
false
] |
My understanding is that organ rejection occurs when the body's immune system attacks the new organ thinking it's an invader. But people with HIV that has progressed enough don't have much of an immune system. So could you give an HIV+ person a new kidney or liver without having to worry about rejection? This is hypothetical, because I doubt someone with HIV or AIDS could get on the organ list.
|
[
"While there may be less of a chance of having an immune response, that doesn't mean there won't be one. Doubt you are going to find many people with no immune system left and even if you did, they would probably not live very long with one that doesn't work at all."
] |
[
"My guess is that there would be a much greater risk of infection due to the procedure than rejection of the organ itself. "
] |
[
"There is still an immune response with T-cell counts >200, anything below that and you are diagnosed with AIDS. It is highly unlikely that someone would receiving an organ transplant with a T-cell count <200, the risks of surgery would be immense and the first priority would probably be stabilizing the immune system or dealing with an OI.",
"For those on HAART, an immune system certainly exists. In such cases, there are ongoing studies about the viability of liver and kidney transplantation. Larry Kramer, a noted AIDS activist and founder of ACT UP, received a liver transplant.",
"See here for some mor einformation: ",
"http://www.uptodate.com/patients/content/topic.do?topicKey=~SSKOjZYzAxMYvA6"
] |
[
"Are venomous and poisonous animals immune to their own venom or poison?"
] |
[
false
] |
Like if a rattlesnake was injected with rattlesnake venom, would it undergo the same effects on it as it does to other creatures.
|
[
"Cobra envenomates itself, develops abcess",
" (Contains mild pictures of the snake's wounds)"
] |
[
"Interesting, Thank you"
] |
[
"I have seen rattlesnakes envenomate themselves by accident, and die hours later."
] |
[
"What defines the edge of the observable universe?"
] |
[
false
] |
Why is some of the universe observable and the rest not?
|
[
"Because the Big Bang happened a finite time ago and light has a finite speed. That means that a lot of faraway objects just haven't had time to send light to us.",
"The Universe is roughly 13.7 billion years old. That doesn't mean, however, that the observable Universe is 13.7 billion light years across. That would only be true if the Universe weren't expanding - but as we know, it is, so the distance to the furthest objects we can (theoretically) see has expanded to be a fair bit larger than 13.7 billion light years."
] |
[
"13.7 billion light years in radius. Sorry if I was unclear on that. Does that answer your question?"
] |
[
"It wouldn't, it would have radius 13.7 billion light years. I think this was just a typo."
] |
[
"If the Sun instantaneously disappeared, we would have 8 minutes of light on earth, speed of light, but would we have 8 minutes of the Sun's gravity?"
] |
[
false
] | null |
[
"Some previous threads about this:",
"1",
"2",
"3"
] |
[
"Yes."
] |
[
"And if you don't mind, to short circuit any of the debates that often follow, I'd like to clarify what I think you mean by this for others:",
"Gravity is the effect of matter traveling through a curved space. But in order to know how that space curves, we need to know the distribution of mass, energy, momentum, stress, and strain throughout the region of interest. If the sun was to leave by any physical means, then you've got to account for all the momentum and stress and strain terms in your stress-energy tensor to properly speak to what the effect on gravity will be. ",
"If the sun suddenly disappears for unphysical reasons.... what happened to its mass and energy anyway? Now from other analyses, we know that other ",
" in gravitation proceed at the speed of light, so if the sun disappeared, we think that the change in curvature would also proceed at the speed of light. "
] |
[
"What would the shadow of a black hole look like; a perfect sphere, blobby, or something different?"
] |
[
false
] | null |
[
"A non-rotating black hole with nothing around it appears as black disk, with some light distortion around that disk. For a rotating black holes (in practice they are all rotating) it gets more complicated, ",
"here is a simulation",
". If there is material near the black hole then things get even more complicated."
] |
[
"Again, my apologies.",
"I understood your answer to describe what a ",
", as opposed to what a ",
"."
] |
[
"Right. And the moon doesn’t emit any light itself, yet casts a shadow onto the Earth during an eclipse. (Yes, I know it reflects light, this isn’t a perfect example)",
"So dude is asking, what the shadow of a black hole would look like, say, if the moon passed in front of the sun, and the moon were a black hole, instead of, well, the moon."
] |
[
"Is the Irish Blood Transfusion Service lifetime ban on blood donations from gay men scientifically justified?"
] |
[
false
] |
This debate is going on in re the IBTS donation ban for any man who has had sex with a man. The guidelines have a 12 month ban for men who have sex with prostitutes etc vs lifetime ban (see page 12) --You must NOT give blood for at least 12 months after you last had: • Sex with anyone who has: - HIV - Hepatitis B - Hepatitis C • Sex with anyone who has ever been given money or drugs for sex • Sex with anyone who may ever have had sex in parts of the world where HIV is very common. This includes Africa and South East Asia • If you are female: Sex with a male who has ever had oral or anal sex with another male with or without a condom or other form of protection You must NEVER give blood if: • You are a male who has ever had oral or anal sex with another male, even if a condom or other form of protection was used.
|
[
"While this question is a \"hot button\" issue in the realm of politics, this, like all questions in ",
"/r/askscience",
", must be answered by expertise or sources. Political opinions are not acceptable answers. "
] |
[
"Quotations from the Canadian Medical Assn Journal (Wainberg et al., 2010: vol. 182 no. 12, pp 1321-1324):",
"\"With the development of more sensitive tests for detecting HIV, the potential occurrence of a false-negative result is now remote, since the system no longer relies exclusively on the enzyme-linked immunosorption assay introduced in 1985 and the more accurate confirmatory Western blot test, also introduced in 1985. 15 Now, highly sensitive nucleic acid testing is routinely used to screen blood. One participant in an FDA workshop held in March 2006 observed that “the probability that errors in routine screening will result in release of a unit [with hepatitis C virus or HIV] is so remote as to be inconsequential.\"",
"The article notes that the justification for such bans in the 1980s was based on the precautionary principle; that in the absence of firm scientific evidence in either direction, a policy of caution is justified. Part of the justification for such a ban is that risk factor-based exclusion was needed due to HIV tests that were not sensitive enough to sufficiently reduce risk of HIV transmission. That justification is no longer valid.",
"They also note that the statistics can cut both ways, you could say that Canadian MSM have a much higher risk of being HIV+ compared to other groups, yet at the same time 94% of Canadian MSM are HIV-. "
] |
[
"I can't speak to the validity, but it's not limited to the IBTS; ",
"the American Red Cross has a similar restriction",
". ",
"The section it's under in both places implies that it's due to HIV transmission risk. "
] |
[
"When I look at a word, why do I automatically read it?"
] |
[
false
] |
When I look at a word, I automatically read it. Why is it that I cannot not read a word if I look at it?
|
[
"Going a step further, I occasionally read words without knowing where they are. It happens most often when walking down a grocery store aisle, surrounded by words and packaging in my periphery. A word will pop in my head that I know I've \"read\" but it takes a couple of minutes of looking to actually find the word.",
"Feels like real life word search."
] |
[
"Happens to me too then I spend the next 30 seconds looking for the word. "
] |
[
"Does this apply to other objects? If I see an apple, I find it difficult to not see it as one."
] |
[
"Is there a chemical that produces heat when compressed?"
] |
[
false
] |
I don't mean those ones that they use in hot'n'colds, I mean one that just releases extra excess heat when put under pressure, but not extreme pressure? More like 100 ish lbs
|
[
"Ever use a pump to inflate a tyre? I've given myself small burns by underestimating how much heat was released by gas compression. When you release the pressure, the gas cools, like when using a spray can - it can also get very cold.",
"Different gasses will change temperatures at different rates when pressurised, this is given by the Joule Thompson coefficient Hydrogen is a notable one, as it has a negative coefficient (i.e. it does the opposite of most gasses) near room conditions."
] |
[
"Is this correct? My understanding is that when a gas exapands it draws in energy from the surroundings. Which is why its previous vessel cools down like how you mentioned the can getting cool, but the gas itself gained energy. Meanwhile, a gas compressor is used in AC units to force the gas flowing through the line to release its heat which is then dissipated across a radiator with air flowed over by a fan. Am I missing something?"
] |
[
"Anything with a negative jt coefficient will get colder when compressed, eg nitrogen and hydrogen over their relevant temperature ranges.",
"http://en.wikipedia.org/wiki/Joule-Thomson_coefficient"
] |
[
"How do we get Vitamin D from the sun? Is it like photosynthesis in any way?"
] |
[
false
] | null |
[
"Photosynthesis is a process where photon is captured and its energy transported across a chain of molecules for the purpose of storing the said energy. This process is well-controlled at various steps, and organisms have evolved over the years to use the best spectrum of light available.",
"In humans the majority of Vitamin D synthesis requires sunlight at one stage, but the UV light just provides the activation energy to convert one isomeric form of provitamin D to another (i.e. there is no active storage of energy). This is a spontaneous process not requiring cellular machinery, and the effect is limited to the substrate and product."
] |
[
"Yes, lack of sunlight and a source of vitamin D will certainly make you deficient. Rickets comes to mind as a result",
"To answer your original question, sunlight absorbed by the epidermis, vitamin D precursors produced and absorbed in blood stream eventually make their way to Liver, and on to your Kidneys where the active form of vitamin D is made. \nHowever active vitamin D production is much more complicated, involving the parathyroid gland and parathyroid hormone (PTH). PTH is also involved in regulating Calcium levels in the blood serum. When blood calcium levels are low PTH is secreted to increase blood calcium levels, and signaling the kidneys to continue to make active Vitamin D. Now vitamin D can signal increased absorption of calcium in your GI tract. Thus restoring equilibrium of blood calcium levels and stopping secretion of PTH and halting kidneys from producing active vitamin D. ",
"Side note. Calcium is a BIG importance in overall health, most people forget that calcium plays a major role in nervous system and muscular system functioning. Not just bone health. "
] |
[
"Just to stress Calcium's importance a little more: it's essential to the coagulation cascade and proper clotting when you bleed."
] |
[
"Why does the shadow of a bottle of water show up as a \"dark\" shadow even though water is clear, and the bottle is clear."
] |
[
false
] |
Context: I was holding a bottle of water and looked down at the shadow of it and it was showing up as a "shadow" and not like I expected. The shimmer was showing up on the ground, but the shadow was basically like if I had motor oil in the bottle. Weird question, but it doesn't make sense to me.
|
[
"A bottle of water is basically a sort of misshapen lens. If you take a proper lens and try the same thing, you'll see it casts a shadow except where it's focusing light. The light that would shine where the shadow is has been diverted to the bright spot. The effect is just less obvious for the bottle, because it's focusing light in all kinds of crazy, constantly-shifting places."
] |
[
"This is correct; the vast majority of the light is in fact making its way through the bottle without being absorbed, but it comes out going in lots of different directions.",
"As an experiment, take a pair of regular glasses (preferably low power, like reading glasses). Unlike a bottle of water, glasses deflect the light just a little bit, and in an ordered way. If you hold the glasses so they are almost touching a surface (say, a table) and shine light through it, you'll see that they almost do not cast a shadow - the light travels through the lens and hits the table on the other side almost exactly where it would have had there been no lens at all. As you slowly lift the lens off the table, you'll see the light in the shape of the lens either shrink or expand, and now it'll look like there's a shadow of your glasses. The lens ",
" block the light from landing where it normally would, which is why you see a shadow, but most of that light didn't get absorbed; most of it got refracted and bent to create the larger or smaller image of the lens somewhere else.",
"The water bottle is the same - the light from the sun ended up travelling through the bottle, but that light didn't end up where the shadow is; it ended up bending and bouncing around all over the place."
] |
[
"I didn't notice that before, but after your reply I see what you mean. That's pretty cool. "
] |
[
"With ionic radius, I know how to compare an ion to the neutral atom and how to compare isoelectric ions but how do you compare ions of different elements that are not isoelectric?"
] |
[
false
] | null |
[
"I saw a YouTube video comparing ",
"I- Br- Ga3+ In+",
"What rules did they follow to order them smallest to largest?"
] |
[
"Hi AtenderhistoryinrusT thank you for submitting to ",
"/r/Askscience",
".",
" Please add flair to your post. ",
"Your post will be removed permanently if flair is not added within one hour. You can flair this post by replying to this message with your flair choice. It must be an exact match to one of the following flair categories and contain no other text:",
"'Computing', 'Economics', 'Human Body', 'Engineering', 'Planetary Sci.', 'Archaeology', 'Neuroscience', 'Biology', 'Chemistry', 'Medicine', 'Linguistics', 'Mathematics', 'Astronomy', 'Psychology', 'Paleontology', 'Political Science', 'Social Science', 'Earth Sciences', 'Anthropology', 'Physics'",
"Your post is not yet visible on the forum and is awaiting review from the moderator team. Your question may be denied for the following reasons, ",
"/r/AskScienceDiscussion",
"There are more restrictions on what kind of questions are suitable for ",
"/r/AskScience",
", the above are just some of the most common. While you wait, check out the forum \n",
" on asking questions as well as our ",
". Please wait several hours before messaging us if there is an issue, moderator mail concerning recent submissions will be ignored.",
" ",
" "
] |
[
"‘Chemistry’"
] |
[
"If a photon's frequency determines its color, how does combining colors work?"
] |
[
false
] |
My understanding of light is that the frequency determines color. But if you shine a green light and a red light at a (white) wall, it appears to us as yellow. Are both green and red photons being reflected and processed by our eye somehow? Or, maybe if both green and red photons hit the electrons in the wall, then they emit a yellow one? If my first hypothesis is true (that the eye is actually doing the "merging") then how does that work chemically? It must mean that our eye treats green + red photons the same as yellow photons, which is pretty fascinating. If the second is true, then how does that work with respect to energy levels? The frequency determines energy, so if you get a green and red photon then maybe you emit 2 yellow photons? What about red + blue photons - magenta is a higher energy photon so would it emit fewer? Follow-up bonus question: Why are there 3 primary colors of light? I suspect this has more to do with the chemistry in our eye than any properties of EM waves, since AFAIK there is nothing special about the range of the EM spectrum that makes up visible light.
|
[
"Your eye has different ",
"cones",
" for different colors, so the actual 'merging' is done in your brain. ('color' is ultimately defined perceptually, since what we call a 'color' doesn't always correspond to specific wavelengths, as you seem aware)",
"Two-photon absorption (and emission) does exist. But it's an exceedingly unlikely thing in the best of circumstances. In practice it's only seen with certain materials where it's unusually likely to occur, and with high-intensity laser beams.",
"Why are there 3 primary colors of light? I suspect this has more to do with the chemistry in our eye than any properties of EM wave",
"More or less. ",
"Here's a CIE diagram",
", which is a 'map' of the colors we can perceive. The round edge corresponds to pure wavelengths of light, while the middle part corresponds to mixtures of them. So white is roughly in the middle. It's not a 'true' diagram of course, because your computer monitor can only actually display a subset of all the colors you can perceive. There's more green because our eyes are most sensitive to it, we can distinguish green hues better than reds or blues.",
"Anyway, so if you make a triangle with red-green-blue as the corners, you'll see that you'll fit most colors inside of it. That's why they work as primary colors. But you don't have to use those three. you could also pick cyan, magenta and yellow, too. (a triangle rotated about 60 degrees compared to R-G-B) You could choose any colors you like, but to work well, you want them to span as much area of the diagram as possible.",
"But R-G-B is in fact the set of colors our eyes work with. It is chemical, ultimately amounting to three different variants of the photoreceptor proteins ",
"photopsin",
".",
"There ",
" some properties of visible light that make it more-or-less best for seeing with. UV radiation doesn't penetrate so far, it's absorbed by lots of stuff; and it's chemically 'harder' to absorb with specificity because it's so energetic. X-rays, on the other hand, well there's no a lot of them. Radio waves simply don't interact with almost anything at the 'chemical' level, so it'd be hard to evolve. While IR radiation would be very 'noisy' due to heat, and therefore also difficult to detect with specificity. ",
"The visible range is a good one; it has a lot of energy, but not so much energy that it's easily absorbed or often breaks chemical bonds. Many things, most importantly perhaps, air and water, don't absorb much of it. "
] |
[
"Oh, I know this one =) There are some wavelengths (like for example some specific shade of yellow) that are indistinguishable from some combination of wavelengths (like, some specific combination of green and red wavelengths). They would be different light beams in the sense you could distinguish them with instruments, but not by looking at it.",
"That's because we don't perceive the light wavelengths themselves. We perceive the light excitation on cones. Our cones have response curves in a way that it's possible for a light beam with a given frequency to excite them exactly like two beans of different frequencies. See ",
"this",
". Notice the overlap and general spreading: the green cones actually absorbs a lot of wavelengths we wouldn't recognize as \"green\" - and the reason we don't recognize the yellow or orange wavelength as green is that the red cones will absorb it too",
"If you add more cones, you may be able to distinguish more colors. In fact, many combinations we see as being \"the same\", would be considered as different colors by other species, like birds. The inverse is true for people with color blindness.",
"A fascinating thing is that our imaging systems (computer displays, image compression techniques, etc) are geared towards ",
"color spaces",
" similar to the space of colors we can recognize. A computer graphics professor of mine once said that such images would seem unnatural to some other animals because there would be a bunch of colors missing.",
": while researching in order to answer this, I learned that ",
"some colors can only be represented by a mix of multiple wavelengths",
". For example, pink. Other colors, that can be a single wavelength, are called ",
"spectral colors",
"."
] |
[
"As some of the other comments have pointed out, color is a psychological phenomenon, not a physical property of light. So whenever you're talking about how color perception works, if you're going to do it properly, you need to be careful not to talk about colored photons or colored light waves. Light doesn't have a color, it has a frequency. (The frequency is related to the wavelength, in the sense that the product of the two is the speed of light, but that's not important here.)",
"You've of course heard that the energy of a light wave is related to its frequency. But it's a bit more subtle than that. The frequency is actually related to the ",
" of the light's energy; in other words, the energy carried by light is emitted and absorbed in discrete \"packets\" (quanta), and the frequency determines the size of the packet. You can kind of think of a light wave as containing various numbers of various sized packets; for example, you might have 100 packets of size 32.14, and 72 packets of size 41.104, and 16 packets of size 44.779, for a total energy of 100×32.14+72×41.104+16×44.779=6889.952.",
"These packets are important because, as I said, light can only be emitted or absorbed one packet at a time. This is where energy levels come in. Suppose you have an atom or molecule (or crystal or whatever) with energy levels at 10.225, 31.115, 64.706, 105.810, and so on. If the system (atom, molecule, etc.) is in one energy level, and if a wave comes along that happens to include a packet with energy ",
" equal to the difference between that energy level and a higher one, then the system can absorb the packet and \"jump\" up to the higher energy level. For example, suppose this system - let's say it's an atom, for concreteness - is in the third energy level, so that it has energy 64.706. If the light from the previous paragraph comes along, then the atom could absorb a packet with energy 41.104 because that is exactly the right amount of energy to \"promote\" it to the fourth energy level: 64.706 + 41.104 = 105.810. The atom will later \"drop\" down to the lower energy level, giving off the same amount of energy (41.104) in the process, but this time the energy could be turned into heat or an electrochemical potential or some other form, not necessarily light. This is basically how the photosensitive proteins (photopsins, thanks to Platypuskeeper) in your eye work: they have an enormous number of energy levels, perhaps even a continuous spectrum, so they can absorb many many different sizes of packets of light. As I understand it, the transition that takes place involves the protein restructuring itself in some manner, basically changing shape, and that change in shape triggers the sending of a signal along the optic nerve to the brain. The strength of the signal that gets sent this way depends on how exactly the protein restructures itself, which in turn depends on the size of the energy packet it absorbed.",
"Now, again as other comments have mentioned, there are three different types of cone cells on your retina, usually labeled red, green, and blue. But the labels are misleading. What really makes the difference between these three types of cones is that each one contains a different kind of photosensitive protein. The photopsin that exists in the \"blue\" cone cells is most sensitive to a particular frequency that we happen to perceive as blue, meaning that it sends the strongest signal when it absorbs an energy packet whose size corresponds to this particular frequency. But it can absorb energy packets of other sizes as well, and it will still send signals to the brain, just not as strong. The same goes for the other photopsins: the \"green\" one sends the strongest signal when exposed to a particular size of energy packet which we perceive as green light, and the \"red\" one sends the strongest signal when exposed to the energy packets which we perceive as ",
" ",
"yellow-green",
" light. (I told you the names were misleading! For this reason the cones are called \"long\" for red, \"medium\" for green, and \"short\" for blue, or simply L, M, S.) In each case, the protein will send signals for a whole range of frequencies, just not as intense ones as it will for the peak frequency. The function that describes how strong a signal a given protein sends for a given frequency of light is called a ",
", and Wikipedia has a ",
"graph of the response curves for each of the three photopsins",
".",
"Armed with those response curves, consider what happens when, say, monochromatic yellow light (and remember, by this I mean light of the particular frequency that we perceive as yellow, because light doesn't actually have color) falls on the eye. To be concrete, suppose I'm talking about light of wavelength 580 nm, which corresponds to a frequency of 5.2×10",
" Hz and an energy packet size of 3.4×10",
" J. According to the ",
"numeric data",
" for the response curves, the L cones will send a signal of intensity 0.969429, the M cones one of intensity 0.653274, and the S cones one of intensity 0.000109. The brain receives these three signals and processes them, and comes up with the psychological concept of \"yellow.\" (As usual, the story appears to be much more complicated than that, but hopefully this is the basic idea.)",
"But consider this: instead of sending light of wavelength 580 nm, you could send some combination of light at, say, 570 nm (the peak of the L response curve), 543 nm (peak of M), and 442 nm (peak of S) which would produce ",
" as the 580 nm light, so that the brain would receive the exact same signal. (caveat: this particular example doesn't seem to work, but hopefully the point is clear) So your brain would conclude \"aha! yellow\" even though there is no light that has a frequency we would actually perceive as yellow. This is how computer monitors and TVs are able to work: each pixel consists of a tight cluster of LEDs (or phosphors, in the old days), one of which emits red light (i.e. light that we would perceive as red on its own), one green light, and one blue light. By controlling the intensities with which each of these color elements radiate light, you can control the degree to which each of the cones responds and simulate a whole bunch of different colors. Naturally, this allows you to produce combinations of the signals from the three cones which don't correspond to any single frequency of light, which is how you get things like magenta or white that don't actually have a wavelength."
] |
[
"Why does gravity affect matter and light so differently?"
] |
[
false
] |
So gravity is not really a force. It is actually the bending of space around one mass, and another mass continuing to go in a straight-bent line-curve in this warped space. If you look at a black hole, all the stars behind it seem clumped together because the black hole bends the space around it, and at the event horizon it turns a straight line into a circle, so there's no way out at all. Right? But if a proton passes a black hole, it doesn't just get deflected a little, it gets sucked in. How does curvature of space explain that? I guess it has something to do with the fact that the proton bends space too, but I can't figure out what that amounts to.
|
[
"They get affected the same way. A proton passing by a black hole wouldn't get sucked in if it was passing by quickly enough. The only thing affecting the deflection is that light is travelling faster than those objects. "
] |
[
"But if a proton passes a black hole, it doesn't just get deflected a little, it gets sucked in.",
"No, generally not. Black holes are tiny things and they are not cosmic vacuum cleaners. Only protons (and photons) that are directed exactly at the hole will be absorbed. All others will have their trajectories deflected."
] |
[
"Light that is headed into the black hole will get absorbed just like a proton headed into the black hole will.",
"It is only photons and protons that are headed PAST a blackhole that get deflected."
] |
[
"How do genetic engineers fine tune gene expression?"
] |
[
false
] |
Hey , could you help me understand some about genetic engineering? I think I understand the basic processes of inserting genes into organism and getting them to produce new proteins and the like from the new code, but from what I've read that's really only the beginning. As I understand it, the modified organism is probably only going to produce a very low yield of the new proteins. I assume that the yield depends heavily on the base pairs that border the new code, but gene expression is dynamic right? It changes depending on the circumstances. Can anyone explain some of the factors in play here? What are some of the practices genetic engineers use to up regulate genes? Am I even on the right track? Could you direct me to some resources that explain this kind of stuff in depth?
|
[
"your title ask for fine tuning but your question asks for maximising protein production so for briefness I'll focus on that.",
"Gene expression is dynamic; however, there are a number of \"housekeeping genes\" which are constitutively expressed - ie. expressed in all/most circumstances. The promoter of a housekeeping gene can be used to drive the expression of the your gene of interest so that it is \"always\" expressed. Alternatively, you can pick an inducible promoter (a promoter responsive to specific cues - typically the level of a nutrient eg. the carbon source is often used for induction in micro organisms used to produce pharmaceuticals)."
] |
[
"This is the correct answer.",
"\n Can just add in that it is possible to knockdown expression of genes being expressed using systems such as siRNAs."
] |
[
"transcription factors, promoter sequences, can use some gene silencing techniques to downregulate any conflicting proteins "
] |
[
"How can particles collide, if they do not have a definite position when unobserved ?"
] |
[
false
] |
I thought particles are more like probability fields, how can they collide at a definite position then ?
|
[
"Yes, in quantum mechanics you only have probabilities of saying where a particle is, although that's not to say it's equally likely to be everywhere/anywhere.",
"The term 'scattering' is what's usually used rather than 'collision'. Because it's not like hard spheres colliding; elementary particles have no volume, and they interact at a distance through forces. Also, in QM there's nothing actually stopping two particles from going straight through each other (by which I mean, have a probability of having been in the exact same spot at the exact same time). ",
"The short answer is that it's probabilities all the way. You have incoming probability 'waves' describing the path(s) of incoming particles are, and outgoing ones describing where they're likely to have gone after the scattering. They don't collide at a single definite location, rather it behaves more like a wave or several waves scattering off each other, or a solid object. (",
"image",
")",
"As particles get heavier and thus have higher momentum, they have more and more definite locations and behave less and less wave-like. The result is that the probabilities become increasingly concentrated around the trajectory of classical point-particles. So things start behaving classically in the limit of high masses (or alternatively, as Planck's constant goes to zero)."
] |
[
"(by which I mean, have a probability of having been in the exact same spot at the exact same time)",
"I barely know a thing of Quantum Mechanics, but isn't that the exact opposite of what Pauli's exclusion principle states, in the case of two fermions scattering?"
] |
[
"In the strict sense yes they can't exist in the exact same eigenstate but they can still overlap very heavily."
] |
[
"Why can the gravity of black holes extend outside of their event horizon?"
] |
[
false
] |
Well, if nothing can escape a black hole, how come gravity can? Yes, I understand that mass bends space, and what we sense is the curvature of space created by a black hole. However, that curvature travels at the speed of light, not faster, right? So how can it exit the black hole then?
|
[
"Gravity isn't \"escaping\" a black hole; the spacetime produced by a black hole has a certain curvature, and the effects you're talking about, such as the formation of an event horizon, are effects of that curvature. So it's a bit silly to talk about those effects without there being any curvature. Gravity doesn't work by some sort of signals constantly moving about which would need to escape the event horizon. ",
" in gravity propagate at the speed of light, not gravity itself.",
"You might want to think of it as being due to conservation of mass-energy. As a ball of matter collapses to form a black hole, the gravitational influence of that matter can't suddenly vanish as soon as it enters its event horizon and forms a black hole, because that would violate conservation of mass. The spacetime effectually knows what went in, and behaves accordingly."
] |
[
"Ok, so changes in gravity, not the gravity itself propagates at the speed of light. That's fine, but it doesn't explain the problem. Because what if someone were to create a micro black hole, that just pops into existence? The change in gravity would have to propagate outwards, but at the same time it can't move faster than the speed of light. So from an outside point of view, there would be no change in gravity, no?"
] |
[
"To use an analogy, picture a marble gently placed upon a sheet of tissue paper. While it does not sink instantly, shortly thereafter it will indeed sink, causing the tissue paper nearby to be drawn down, sloping towards the weight. if you roll another marble along the tissue, the curve will affect its motion. Similarly, based upon how fast you're rolling the marble, there will be a distance where it will always sink to the center alongside the first, instead of continuing, even if its being constantly pushed.",
"In a very simplistic manner, gravity is the same thing in three dimensions; everything that has mass warps space-time towards itself. Just like with the tissue, changing the location of the marble(s) doesn't instantly change the surrounding landscape, but it's very rapid. Gravity works like that as well; the surrounding landscape alters at the speed of light.",
"The event horizon of a black hole is simply the distance at which point light cannot escape its gravitational pull, so if light crosses the threshold, it will always spiral towards the singularity. This has no effect on the gravity of the black hole, but is a result of said gravity. And as a note, the point at which a rocket or the like could not escape a black hole is quite a bit further out then the event horizon; depending on your reaction mass, you may not have the power to escape a much weaker fraction of the gravitational pull.",
"So, once again: the event horizon is a consequence of, not a limitation upon, the gravity of a singularity."
] |
[
"What exactly is the Navier-Stokes millennium problem trying to solve?"
] |
[
false
] |
I just took fluid mechanics 2 and I saw that there was a millennium problem regarding the Navier-Stokes eqn. Can someone elaborate on what it involves?
|
[
"It's a physics problem. Kinda. When you combine a bunch of known physical laws, typically expressed as differential equations, and apply them to fluid flow, you get the Navier-Stokes Equations. A solution to the Navier-Stokes Equations then represent a possible way that fluid can flow. These equations are applied all over the place to generate models of fluid flow. But these are all numeric approximations, or restricted in some way to provide a solution. None of these provide a general solution to these equations. ",
"If you look at the fluid without really zooming in that much, then you'll have a fairly coarse view of the fluid. This can allow you to approximate solutions to these equations and get good models of physical things because fluids are not, in reality, infinitely fine, they are made of things of discrete size. But if you take these equations and allow for infinite zoom, then you're treating these fluids as infinitely fine things. Since these equations allow for turbulence (because that is a thing that happens to fluids), this infinite fineness can produce some chaotic results. A small nonuniformity at one point in time can blow up into a hugely compounding distortion that breaks the smoothness of the fluid. You can get infinite peaks of density or velocity within the fluid and loose nice things like differentiability. ",
"So far, every attempt at solving these equations looses one of these nice things we typically associate with fluid. We either lose the smoothness or it can become unboundedly energetic. ",
"The Navier-Stokes Millennium Prize Problem is then the question of whether or not this differential equation can be solved without losing these nice things we associate with fluids. It could very well be that these equations admit no actual solution, so they don't actually describe how fluids behave. But we already know that they don't, because fluids are not infinitely fine. The question is whether or not this assumption is a thing that breaks the model. ",
"Details of the problem are ",
"here",
", and a good exposition about the troubles involved with solving it are ",
"here",
".",
"Overall, though, differential equations are really hard to solve, and we have no real general method to attack them. The reason why this problem is so important is because a solution to it is expected to provide us with new, more powerful and more interesting tools with which to study differential equations."
] |
[
"Just wanted to say what an excellent response."
] |
[
"With CFD you're still chopping the flow domain into very fine discrete chunks and solving for them, and even the highest resolution CFD methods (direct numerical simulation) the length scale of these chunks is appropriate for modelling the smallest length scales of turbulence but still far larger than the atomic scales at which the fluid ceases to be continuous. "
] |
[
"Could i survive without drinking?"
] |
[
false
] |
I know what you're thinking. You're thinking 'No, dumbass'. But aren't some foods so rich in water that you could eat them as a substitute? Here's a thought experiment: Imagine I'm escaping, on the run in a foreign land. There's nothing to drink, but cucumbers growing everywhere, cucumbers as far as the eye can see, and much further. Now it seems to me that cucumbers are pretty damn watery. How far could i get on just cucumbers, no liquid water?
|
[
"It would work.",
"Here's a big ol' database containing nutrient values, including water per serving: ",
"http://ndb.nal.usda.gov/",
"Don't see apples on there, unfortunately, but oranges have about 88gm of water per 47 calories, so if you ate only oranges and ate a 2000 calorie diet, you'd get about twice as much water as you needed. Bananas aren't as wet, but you'd get close to enough water if you only ate bananas.",
"Most people consume about 2-3L (=2-3kg) of water a day, in food or drinks. (For most people, the majority is in food, not drinks.) You could live on significantly less than 2L-- 1-1.5L would probably be sufficient for an otherwise healthy person.",
"In general, the limiting factor to how much water you can get out of food is how many calories you're willing to consume, but don't try to get all of your water intake by eating polar bear liver."
] |
[
"You would get full on cucumber or most other \"watery\" foods before you had adequately hydrated your body. But, theoretically it is possible if you ate those foods every 1-2 hours, continually keeping your stomach full.",
"EDIT: After some research, it seems as if the average human can comfortably hold about 33 oz. of food in his or her stomach. It takes about 30 minutes to empty the stomach of water-based foods. A cucumber is 95% water. So, if you completely filled the stomach with cucumber, about 31.35 oz. would be good old H2O. There isn't really a consensus on how much water we need to ",
", but the lowest estimate I could find was 1 Liter per day, or 33.8 oz. So, from all these standpoints it does seem as if the theory is plausible, but it is important to note that dehydration may \"feel\" more severe to the person if their mouth and throat become dry due to lack of pure liquid consumption. They would survive, but depending on climate may be incredibly uncomfortable, leading to effects on mental clarity that as of yet have not been described."
] |
[
"Polar bear liver is very high in vitamin A-- it's meant as an example that there are foods where the amount of water you could get out of them is limited by something other than too many calories. In the case of polar bear liver, it's too much vitamin A."
] |
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