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[
"How is the result \"1+2+3+4+...=-1/12\" used in string theory if it's based on a faulty proof?"
] |
[
false
] |
[deleted]
|
[
"My apologies but this is a hard question to answer without some analysis, I hope it's understandable, I've tried my best but it goes pretty deep.",
"This question rests deeply on what kind of limit you are using to evaluate the sequence. They are using an unusual sort of summation so it seems weird.",
"So what is usually done is to look at the subsequences, x(1) = 1, x(2) = 1 + 2, x(3) = 1 + 2 + 3, x(4) = 1 + 2 + 3 + 4 etc",
"and say \"for any e > 0 is there an N such that |x(n) - L| < e for all n > \nN\". (see wikipedia on Limits)",
"Because if there is then L is the limit of the sequence in a classical sense.",
"What this really means is \"is there a number N so after that number the partial sequences are always close to L?\"",
"Now obviously for this sequence there is no limit in this sense (the sequence is said to diverge to infinity). And this corresponds to what most people think.",
"The video you reference is full of cheating and is quite unhelpful (and I think elitist, it is condescending) but they do have a reasonable argument.",
"There is a thing called the Riemann Zeta Function and it's the sum of n to the power of -s (have a look on wikipedia).",
"Now we know there are some reasonable sums of the zeta function, that is for some values of s we can say Zeta(s) = L.",
"Then we can use a thing called analytic continuation, which extends the function.",
"The best analogy for this is if I give you two points on a line you can plot a straight line between them.",
"Well if I give you some values of Zeta(s) then there is a way to extend the function to all other values of s.",
"But when you do this extension the values of Zeta(s) you get are counter-intuitive and not like a normal limit as described above.",
"So what the video is talking about (and what is used in string theory very resonably) is the extension of the Zeta function to all values of s, which is legitimate.",
"But of course they don't go into any of that, because it's very complicated, and they just smugly produce something from nowhere.",
"I hope this is helpful, my apologies if I haven't explained it well."
] |
[
"There is a commonality between the way the \"faulty proof\" works and how the field of physics that uses it works. ",
"In the faulty proof, we are dealing with infinite series that either diverge by going off to infinity, or just don't rest at a single value (1-1+1-1+... just alternates between 1 and 0, never gets big at all). What we do in this faulty proof is ask: \"What would happen if these series ",
" converge?\" We pretend that they converge and see what happens. If it did sum to something, the alternating 1-1+1-1+... would have to sum to 1/2. If it did converge, 1-2+3-4+5-..., it would have to converge to 1/4. And if 1+2+3+4+... did converge, it would converge to -1/12. When we ignore the fact that the sums don't converge, we get these results. ",
"I do just want to say that these results do have rigorous proofs in a more advanced framework, as others have mentioned, but this cheating method will mimic what physicists do.",
"In Quantum Field Theory (the field of physics where this is used), the goal is to see what happens when two particles interact. Since Quantum Physics is weird, it turns out that anything that can possible happen, does happen. For instance, when two electrons interact, they could just repulse each other, or they could spontaneously spawn a photon that gets destroyed before they repulse each other. Quantum Field Theory says all of these possibilities happen, some happen stronger than others and to understand the interaction of two electrons coming together you need to add up the contributions from every possible interaction. Now, there are an infinite number of possible ways they can interact and generally when we add them together, we get infinity. They sum together too much. What physicists do is then see what happens when they pretend that this sum doesn't diverge. When this happens, they end up with a finite answer that they can use to make surprisingly accurate predictions. This is the context in which 1+2+3+4+... =-1/12 comes up, so since the physicists are pretending that their sum doesn't diverge, then it seems okay to use the proof where mathematicians pretend their sum doesn't diverge. ",
"This pretending is actually a huge problem in modern physics called the ",
"Renormalization Problem",
". Even though it works extremely well (it's made some of the most accurate predictions that we've been able to test in all of physics), it is artificial without any real physical interpretation so there's a big hole there. It's not a problem for mathematicians, because we have a framework to deal with this stuff so it works out, but this framework doesn't have a physical interpretation so physicists are still on the look for a way to explain renormalization."
] |
[
"Basically, the video is arguing for the right thing with the wrong arguments, in particular due to oversimplification. The proof is bad, and it should feel bad.",
"In particular it's worth noting that the right-hand side (\"-1/12\") and the left-hand side (\"1 + 2 + 3 + ...\") are not equal, but both of them represent (different) extensions of the same mathematical object, namely the Riemann Zeta function, which is usually denoted by ζ (this is the Greek letter zeta.).",
"The Zeta function ζ is a rather involved function, but for a natural number larger than one, it looks like a harmonic series, ζ(n)= 1",
" + (1/2)",
" + (1/3)",
" + ...., which sums up to a finite value.",
"If you were to insert n=-1, you'd end up with the left-hand side term, which is actually an unbouned series. But this shows that the operation to just plug in n=-1 isn't well-defined: you don't get a finite real value of this extension of the Zeta function to the point -1.",
"On the other hand, there are more advanced extension techniques (which actually step away from representation as a harmonic series) which allow you assign a value to ζ(-1), which is -1/12. This value is actually not uniquely determined, but depends on the extension technique used.",
"If you want to read more on this, you can have a look at ",
"https://en.wikipedia.org/wiki/Riemann_zeta_function"
] |
[
"Why haven't suits been created to withstand a crazy amount of radiation?"
] |
[
false
] | null |
[
"Because walking around with hundreds of pounds of lead shielding on you is not practical.",
"A Faraday cage only stops electromagnetic radiation with wavelengths at least on the same order of magnitude as the grid spacing. You can't make a Faraday cage for gamma rays."
] |
[
"So granted im going with the dentist chair idea here. Is that how much it would take? Hundreds of pounds of lead?"
] |
[
"The thing you wear at the density is for x-rays, but you’ll need even more for gamma rays.",
"How much you need depends on how safe you want to be. But there’s a reason why we don’t do this: because people have thought about it, and it’s not practical."
] |
[
"Need help identifying a rock I found when I was very young, which glowed when I struck it"
] |
[
false
] |
[deleted]
|
[
"Wouldn't have been quartz, would it?",
"http://www.youtube.com/watch?v=eLkIoB5Iv5o"
] |
[
"I'm puzzled by this rock - you do not appear to be describing a simple sparking process, but more of a kind of glow. Sounds like a mineral property called triboluminescence. Several minerals can display this behaviour and it is not sufficient onto itself for identification. Any chance this could have been in the general area of New Jersey? There is a zinc deposit over there with several types of strangely luminescent minerals."
] |
[
"Does this happen because quartz has piezoelectric properties? If not, how would you explain why it produces sparks?"
] |
[
"If by chance we could make it to the Moon and build structures, would we be able to build larger building on the Moon then here on Earth because of the Gravity?"
] |
[
false
] |
just talking to my genius friend who is a med professor at wash u. talking about Cosmos series done by BBC. this question came up
|
[
"Yes, in fact there is an idea to build giant radio telescopes etc on the moon because you could build them at a much larger scale than on earth, and on the dark side of the moon to block out signals from the earth."
] |
[
"It's called the Lunar Array for Radio Cosmology (LARC). Here's an ",
"article",
" about it."
] |
[
"Taking gravity alone into effect, you could easily build mile-high structures, but I suspect things like solar and cosmic radiation and asteroid impacts would make underground mega-structures far more appealing. Also, tall buildings here tend to be pretty narrow, and you'd probably want better surface to volume ratio out of whatever you built up there, for both material availability and vacuum-hazard surface exposure reasons."
] |
[
"Why can we measure the diameter of a proton but not an electron? Do we have any proof that the electron has size rather than being a point particle?"
] |
[
false
] | null |
[
"Electrons are elementary particles, so the Standard Model treats them as pointlike. We have no reason to believe that electrons have any spatial extent. However protons are composite particles, so they have a meaningful radius, which can be measured."
] |
[
"At one point we did think the proton was pointlike, but experiments at Stanford’s Linear Accelerator showed otherwise. We can make predictions for electron scattering from a pointlike proton but the SLAC data showed deviations from this. As Robus says, since the proton is now known to consist of smaller (pointlike) particles, it therefore has a radius. On the other hand, there is no such evidence for electrons. Theoretical predictions in quantum electrodynamics with pointlike electrons are remarkably accurate. ",
"As a digression, there is an ongoing puzzle about the correct size of the proton. Basically there’s a disagreement of the proton radius when scattering electrons from normal hydrogen atoms instead of muonic hydrogen (proton-muon bound system). I suggest searching for “proton radius puzzle” if you want to read more!"
] |
[
"Thanks for the help!"
] |
[
"After a vasectomy, where does the stem go after an ejaculation?"
] |
[
false
] |
[deleted]
|
[
"Sperm only constitute 2 to 5% of total semen volume",
". The remainder of the seminal fluid is still expelled from the body during ejaculation.",
"Your body will recycle unused sperm cells that reside in the testes. As new sperm are generated, old sperm will be broken down and their component nutrients recycled. Thus there is no build-up."
] |
[
"The vas deferens tubes aren't just left open after the procedure, the ends are cauterised for two reasons, firstly so that the sperm can't leak out, and secondly so that the ends don't come together at a later point and heal, reversing the vasectomy by itself. "
] |
[
"The tubes are closed off. So, the ejaculate doesn't end up in the scrotal sack.",
"You are wondering what happens to the buildup? The same thing that happens to a person who doesn't masturbate or ejaculate for a while, it just gets absorbed back into the system. "
] |
[
"How fast does the small metal ball at the end of an average pen spin when you're writing?"
] |
[
false
] |
When I was writing notes for classes, I became curious as to just how fast that little ball is rotating in the tip of the pen I was using. I have no idea how to calculate it and I haven't found an answer, so I was wondering if anyone knew. Just for example, say the pen is an average .5 mm rollerball and you're writing about 30 words per minute. Or maybe an easier example would be 1 centimeter per second in a straight line.
|
[
"Assuming perfect grip, so 1cm on paper is 1cm on the ball surface:",
"\n.5mm (assuming diameter) gives a circumference of .5(pi)mm or around 1.6mm.",
"\nFor 1cm travel this translates to 10mm/1.6mm rotations per second which is: 6.25 rotations per second, although I think 1cm travel per second is an underestimate the maths still works for any speed.",
"\nADDED: This is equivalent to 375RPM."
] |
[
"Whilst I can't make a reasonable estimate of the grip (although I imagine it would be close to 'perfect' since the flow of ink is uninterrupted), The ball would be rotating even on a wavy line. The angle of the pen is irrelevant since the ball is 'free floating' effectively. Just imagine a giant exercise ball rolling around on the floor, the path length will equal the rotation irrespective of the path shape.",
"\nEdit: with to will, typo"
] |
[
"I think they meant fast, not fat"
] |
[
"The impact if our moon was an icy body."
] |
[
false
] |
With the surface and albedo differences between our earth and the moon, I calculated that, if our moon was an icy body like Europa, 6 to 7 times more light would be reflected to earth then our moon does during the perfect full moon, which would come down to about 10-13% of the light that arrives from the sun directly, compared to about 1.6% our moon reflects. Taking into account that the earth will reflect another 37% of that light back into space, that not only light contributes to the energy delivery, the moon goes trough its cycle and the numerous other factors, in terms of additional energy arriving on earth, how much of an impact would that have on our climate? Is it negligible, will it be substantial during the full moon? Is it even noticeable compared to the impact something like the moons gravitational tug has?
|
[
"Not 100% sure what you're asking, but maybe I can help clear some things up:",
"The sun is about 400,000 times brighter than the moon. If Earth's moon were as bright as Saturn's moon, Enceladus, which has an albedo of .99, ",
"This would have no impact on our climate, but it would make nights much brighter, which could certainly have effects on wildlife.",
"Comparing the energy that the moon reflects to Earth as light to the gravity of the Moon doesn't really give any meaningful insight, AFAIK.",
"Edit: Just saw the downvotes, and I'm guessing it's from my claim that this would not effect the climate. Just to provide some further evidence:",
"The average amount of solar irradiance reaching Earth is 1361 W/m². This varies throughout a year, due to Earth's varying distance from the sun, by about 7%. There is also a slight variation through an 11-year cycle of sunspots. This variation is about .1%.",
"Increasing the moon's albedo would only change the irradiance at Earth by .02% at full moon, so that amount is well within the noise of solar output variation and cyclic flux from solar distance. Even if the moon could somehow always be full, the effective temperature of Earth would only be about .013 °C warmer with this additional radiation. Because of lunar cycles, the actual difference in effective temperature, averaged over time, would be closer to .006 °C.",
"There are just too many other variables that have a more pronounced effect for this slight addition to really make any measurable difference."
] |
[
"Using the word \"impact\" threw me off for a while.",
"This is a neat question, I'm interested in seeing what the night time sky would look like with a moon roughly 6-8 times brighter than ours.",
"If we assume the mass and orbit of the moon is the same, the effects on nature would mostly involve the amount of light throughout the day. Predators would have a harder time sneaking up on food while the prey has a harder time finding hiding places.",
"One cool thing to think about though is how different the moon would look, with the added light you'll see smaller details more easily, the moon would most definitely be a lot prettier than it is now. The brightness of the moon would also effect the color of clouds a lot more than it does now."
] |
[
"wikipeadia",
" says the moon is albedo of 0.12 is the reflected light so abolutely diffuse that it would litterally be the difference between a full moon with overcast and a brighter full moon? would there (mabey theoretically) be a focusing point, where there would be something like an actual reflection, and not just a diffuse \"global\" reflection... mabey if the icy body were somehow convex/cave in a point that earth passed the congruence point?"
] |
[
"Question about electricity."
] |
[
false
] |
Why is electricity so volatile and dangerous? EDIT: What about it causing fires and explosions?
|
[
"Wow. great answer. I was just clicking through videos on youtube, and on some occasions I noticed that the fire and resulting explosion was orange with black smoke..."
] |
[
"Wow. great answer. I was just clicking through videos on youtube, and on some occasions I noticed that the fire and resulting explosion was orange with black smoke..."
] |
[
"Duh... makes sense. How about the electrical arc that sometimes happens for a few seconds after opening a circuit? (I hope that the right term)"
] |
[
"Can anyone explain the chemical difference between \"runny\" and \"set\" honey?"
] |
[
false
] |
I visited myparents house recently and they only had "set" honey, microwaving it for a few seconds made it runny enough to spread easily, but can anyone explain the difference between the two states of honey from a scientific perspective?
|
[
"Over time, honey tends to crystallize. This is because honey at room temperature is a supersaturated solution, so glucose will precipitate out of the unstable solution.",
"Rock candy is prepared similarly, by making a supersaturated sugar solution. Carbonated water and soda are more examples of supersaturated solutions.",
"Also, it's recommended to redissolve the glucose by putting the honey container in warm water and waiting (and slowly adding heat if needed). Heating in a microwave, or using other means that might induce localized overheating, will destroy many of the beneficial chemicals in honey."
] |
[
"Honey is ~80% sugars, mostly a blend of fructose and glucose. Runny honey generally has a higher level of fructose. In set honey, the sugars have crystallized and heating them causes some dissolving into the ~15% water content."
] |
[
"Fine, added ellipsis to keep my quote fair.",
"It's beside the point though, which is that just because honey isn't crystallized doesn't imply that it has more fructose."
] |
[
"Why the exact moment I turn on my desk lamp (that has lost the protection glass) in a room my speakers, in another room, make a sudden noise?"
] |
[
false
] |
When I kept them close I figured not having the little protection glass my lamp would "disturb" my speakers (even though I couldn't give a proper scientific explanation), but I was surprised when the same thing happened even when I moved the lamp to another room. When I turn the lamp on I can hear in the exact same moment my speakers making something like a dull thud. Why exactely does this happen? How is it possible than the noise comes up in the same moment I turn the lamp on even with a wall in between? And I guess this is bad for the speakers, isn't it? Sorry if my english wasn't perfect, thanks in advance! Edit: Thank you everyone, I appreciate you taking time in answering my question
|
[
"You sound like an early investigator of electricity.",
"When searching for a relationship between electricity and magnetism, someone (Faraday or Ampere or Volta, one of those dudes) tried placing a needle next to a current-carrying wire. Nothing happened.",
"However, when he watched the setup carefully, ",
" and the wire carried current, the compass needle deflected for a brief instant. He then surmised that a ",
" electric field/current produces a magnetic field, and a ",
" magnetic field produces an electrical current. This is one of the reasons everyone uses Alternating Current power: transformers couldn't transform voltages without a changing current producing a magnetic field, and that changing magnetic field producing a current.",
"Now, is this phenomenon directly responsible? I don't know, but you reminded me of that anecdote of the guy flipping a switch and seeing a momentary magnetic phenomenon."
] |
[
"sounds like you have a bad ground wire"
] |
[
"This sounds right to me, I supposed a way to test this would be to find a plug that is not connected to the same breaker. That way they're completely isolated and you can run a quick experiment to test this."
] |
[
"What happens when a muscle cramps, and what happens if it cramps for too long? Does the muscle get damaged?"
] |
[
false
] |
[deleted]
|
[
"C is for cookie, that’s good enough for me.",
"But also compression, which is not as fun as cookie."
] |
[
"A muscle cramp is caused by dehydration, muscle strain, or overuse of a muscle, among other factors. When it occurs, a sudden and involuntary contraction of the affected muscle takes place. To answer your main question, most muscle cramps are harmless, and last between 30 seconds to 15 minutes. A muscle cramp will not permanently damage the muscle tissue, but it will cause fatigue as the muscle is constantly contracted during the cramp. If you are experiencing frequent cramps, the suggested course of action to remember is RICE, which stands for Rest, Ice, and Elevation."
] |
[
"What does C stand for?"
] |
[
"How much did the invention of the nuclear bomb affect Carbon 14 dating?"
] |
[
false
] | null |
[
"Oooooh. So much. Eventually anyway. (Also, be sure to see the below comments, people have some interesting corrections/nuance below).",
"The bomb spike began along with large-scale atmospheric nuclear testing. In earnest, this means it peaks ~mid 1960s, (below I've not NOAA's link.) ",
"In short, as one might expect every bomb represented a huge flux of C14 into the atmosphere. C14 has a natural generation rate; its the result of cosmic ray bombardment of the upper atmosphere. Sometimes after the cosmic ray hits the atmosphere, yadda yadda yadda (i.e. various reactions), a free neutron is born. When this neutron hits an N14 atom, it may knock off a proton, creating C14. IF the neutron hits a C12, you have a C13 waiting and ready.",
"Atomic bursts represent huge spikes of neutron radiation. (An aside. Uranium-235 may spontaneously decay by neutron decay, and if this neutron hits another U-235 atom, it too will break apart. Usually it releases 2 neutrons. Get enough of these close together, you achieve criticality and boom!. Additionally, many daughter particles of nuclear detonations release neutrons as well. While fusion detonations don't themselves release neutrons, the Teller-Ulam design (believed to be the US thermonuclear design as well as the Soviet/Russian) relies on an atomic devices as the detonator. Yes I just used nested parenthesis, deal.) ",
"When all these neutrons went out into the atmosphere they created a large amount of C14. Since this occurred in the upper atmosphere, the global wind patterns help achieve rather quick and equal distribution. The result is a huge influx of C14 into the terrestrial biosphere, well above the natural range. As with natural C14, plants take up C14 as part of heavy-CO2 in the course of their normal photosynthesis.",
"The result is it is really easy to date something as coming before or after the ~60s, based on the amount of C14. Anything which died before the \"bomb spike\" as it's called has a much lower percentage of anything coming after. ",
"The bomb spike is really useful, especially in my field (ag-science), in archaeology, geology etc. This is especially true given the amount of land disturbance which has occurred in the last 50 years. Places without bomb-spike material have obviously not been heavily modified since the 60s.",
"The bomb spike is ending. For the most part, the additional heavy CO2 has partitioned into the oceans. We're also diluting it by increasing the CO2 levels with all our fossil fuels (the C14 has a half life ~5000 years. After a few million there's nothing left).",
"TL, DR: The proliferation of large-scale atmospheric nuclear testing created a huge spike of C14 in the atmosphere. It is really easy to tell if something lived before or after the 1960s. ",
"NOAAs stuff: ",
"http://www.esrl.noaa.gov/gmd/outreach/isotopes/bombspike.html",
"Edited for accuracy."
] |
[
"14C from nuclear testing doesn't have any effect on objects that predate the tests. Carbon is only added to wood or other organic material when it is living. Once something dies, it absorbs no more carbon.",
"The bigger issue for 14C dating used to be atmospheric variation - the amount of carbon 14 in the atmosphere varies naturally over time, which changes the carbon ratios in an object depending on when it was alive. However, we've been able to use tree rings and other easily correlated things (lake varves, coral, etc) to construct calibration curves that can correct for atmospheric variation."
] |
[
"Okay, so what I'm hearing is you can be absolutely confident something came before or after the 60s, but did it interfere with being able to identify how old something pre-1960s something is? Are we still able to be certain that objects like prehistoric tools are actually 10,000 years old?",
"If not, it sounds like you can basically tell if an artifact has been fabricated or not, that's about it."
] |
[
"Why are all the planets in our solar system orbiting the same “height”?"
] |
[
false
] | null |
[
"It is unclear exactly what you are asking here, since the planets don't all orbit at the same distance from the Sun (a literal definition of \"height\"). Are you asking why they are all on roughly the same plane? If so, that is answered in the FAQ. If something else, you will need to rephrase your question to clarify and resubmit it."
] |
[
"Yes, that’s what I was asking thank you"
] |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):",
"You question is either commonly occurring or has been recently posted on ",
"/r/AskScience",
". It may also be answerable using a Google or Wikipedia search.",
"To check for previous similar posts, please use the subreddit search on the right, or Google site:reddit.com",
"/r/askscience",
" ",
"Also consider looking at ",
"our FAQ",
".",
"For more information regarding this and similar issues, please see our ",
"guidelines",
".",
"If you disagree with this decision, please send a ",
"message to the moderators."
] |
[
"'Geo' Do seismic shockwaves and vibrations heat up the ground?"
] |
[
false
] |
I had a dream last night where an earthquake happened and the ground all around me started steaming with heat. I wonder if a similar thing can/does happen in reality? Can the ground feel warmer after a severe earthquake (at the surface)?
|
[
"Seismic waves heat up the rock they travel through. This is called attenuation. As the waves travel, some of their energy is absorbed into the rock as heat. If this didn't happen, the Earth would never stop ringing after an earthquake. However, the additional heat from the passage of seismic waves is miniscule, so you wouldn't notice anything. "
] |
[
"It shouldn't, no. It would just require too much movement to create such a temperature change."
] |
[
"yes it does heat up because work is being done. no it isn't very noticeable because rocks are poor conductors"
] |
[
"Earth Sciences How much does the carbonated beverage industry contribute to greenhouse gas effects?"
] |
[
false
] | null |
[
"Are you asking about the co2 released from when you (and everyone else) open up your cans and bottles of carbonated drinks? ",
"Because that contribution to greenhouse gases is 0%. They get the CO2 as a waste product from fossil fuel burning energy suppliers. So all that CO2 would be going into the atmosphere anyway. They are just delaying that, and putting it to use first. ",
"Now if instead you are asking how much contribution to greenhouse gases the industry is making due to things like production of the bottles, the shipping of product and that sort of thing, that is a much more difficult thing to answer. "
] |
[
"When it comes to fermentation, the chemical reaction by itself is carbon neutral. The CO2 comes from the sugars which are made by from CO2 from Photosynthesis. Now as for all the processes to do that on an industrial scale, that's another story."
] |
[
"I am definitely asking about the CO2 from the fermentation process (brewed beverages) and added CO2 for soft drinks. It would blow my mind to find a source that a majority of beverage companies use waste CO2."
] |
[
"Why does the Osprey use propellors instead of jets?"
] |
[
false
] | null |
[
"They do use jets. It's a turboprop powered aircraft - jet engines are driving the propellers.",
"They use (very large) props because it's much easier and more stable to hang a heavy transport aircraft's mass from below a rotor/prop disc than it is to balance it on top of jet blast.",
"It's also much more energy efficient, which is the same reason why commercial aircraft now use extremely high bypass turbofan engines instead of plain old jet engines like they did at the start of commercial jet service. The vast majority of the thrust for a turbofan comes from the fan blades being powered by the turbine, not the turbine itself. It's also quieter, since you can use a smaller jet engine with a larger fan disc and most of the exhaust thrust gas is slower.",
"For something like the Osprey to use only jet engines the engines would have to be massive and would reduce range/payload. It would also burn more fuel per minute so you'd have to carry more fuel, which would also reduce both range and payload.",
"The you have landing field requirements. With a jet you likely couldn't land on dirt or unimproved landing sites like you can with a helicopter or Osprey.",
"Harriers and other true VTOL aircraft can't just land anywhere. They generally only take off and land on concrete strips or aircraft carriers. If you tried to land a Harrier on a dirt/grass/sand field it would tear it up so bad and throw debris and dirt so far into the air that it would likely destroy the engines due to debris ingestion or otherwise make it unsafe or impossible to take off again."
] |
[
"Awesome thanks, that makes great sense"
] |
[
"Since you've already been thoroughly answered I'm going to hazard a pointless little anecdote. I live in Amarillo, and they manufacture the Osprey just east of town on Tiltrotor Drive. Between the wind turbines cropping up all over the panhandle and the regular sightings of Osprey overhead it really feels like I'm living in some rotor-centric science fiction setting. Pretty damn cool."
] |
[
"How strong are GPS signals? What would happen if I \"stood\" in front a functioning GPS satellite in orbit?"
] |
[
false
] | null |
[
"They are about ",
"25",
" to ",
"50",
" Watts depending on the satellite, equivalent to a old style domestic light bulb. They broadcast at 1.6 and 1.2 GHz which is in the ",
"/microwave range.",
"\"What would happen if I \"stood\" in front a functioning GPS satellite in orbit? \"",
"Let's disregard the problem of the fact you would either be in a vacuum which would kill you or in a space suit that would protect you, and consider the situation where a GPS satellite would be right next to you on earth. Would you be safe? More or less. A ",
"World Health Organisation report",
" states that the maximum human exposure to microwaves should be around 50 Watts per metre square so you would be at the top end of a safe limit of standing right next to a GPS satellite if it was on and on earth."
] |
[
"Yes, L-band would never be considered \"far infrared\". The post is otherwise accurate though.\nSource: GPS engineer"
] |
[
"Yes, L-band would never be considered \"far infrared\". The post is otherwise accurate though.\nSource: GPS engineer"
] |
[
"[Physics] If matter can't move faster than light, how did the inflationary epoch of the big bang make the universe much bigger than one lightsecond in radius in one second?"
] |
[
false
] | null |
[
"Matter can't move through the coordinates of spacetime with a velocity of c, but that speed limit does not apply to the changing geometry of spacetime itself. Inflation is not the spreading of matter through space, rather it's the early expansion of spacetime itself. "
] |
[
"Up to one doubling per planck time in the extreme limit, but no faster,",
"I haven't heard this before. Have you got a source?"
] |
[
"Up to one doubling per planck time in the extreme limit, but no faster,",
"I haven't heard this before. Have you got a source?"
] |
[
"What direction/navigational system is used in space? Obviously things like up and down, or east and west don't really apply."
] |
[
false
] |
How does one navigate around space? You can't really use the Earth as a point of reference unless you're orbiting it, so how does a probe report back on where it is? Is it in reference to the sun? If so, is the sun actually stationary? Is there some 3D version of latitude and longitude? How are any space navigational systems affected by space itself expanding/collapsing? Does regular geometry really apply if a space latitude line happens go near a black hole?
|
[
"There are several navigation systems used in spacecraft. Besides ground tracking, inertial and star tracking are particular favorites. With 6 degree of freedom accelerometers and knowledge of a spacecraft's initial position and velocity, location can be determined. These systems have a tendency to drift over time.",
"Star trackers, which use the angular separation and distances between known stars are able to precisely determine a spacecraft's position. Essentially 4 stars are required required to determine position and most star trackers follow 57 so-called blue light stars, so error in reading the position of a few does not hinder the craft's ability to locate itself. A good explanation can be found ",
"here",
"These navigation systems are useful for navigation within the solar system, but obviously the visible star catalog changes as you venture into deep space. As the universe is expanding and the solar system's position in the Milky Way changes, the starcharts that the spacecraft uses obviously become less accurate over time, unless this movement and the movement of the catalog stars is anticipated. ",
"I do not know of a proposed deep space navigation system outside of the one proposed by ",
"nodonut",
"."
] |
[
"A spaceship in the future might use pulsars to map the galaxy and orientate itself.",
"http://en.wikipedia.org/wiki/Pulsar#Applications",
"Pulsar maps have been included on the two Pioneer Plaques as well as the Voyager Golden Record. They show the position of the Sun, relative to 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so that our position both in space and in time can be calculated by potential extraterrestrial intelligences.[22][23] Because pulsars are emitting very regular pulses of radio waves, its radio transmissions do not require daily corrections. Moreover, pulsar positioning could create a spacecraft navigation system independently, or be an auxiliary device to GPS instruments.[24][25]"
] |
[
"Six axis from a known point. If you can't define a known point, you can't navigate, period (you can't figure out where you are going unless you are able to determine a guide for your direction). The earth, with proper mathematics, can be defined in the larger framework of the solar system, our galaxy, and even the (relatively) adjacent galaxies. As the mapping of the movements of adjacent solar systems, star clusters, and galaxies becomes more and more detailed and their movements calculated, we can theoretically navigate further and further afield.",
"It is possible to navigate within what you know, even if it is moving through the unknown--just as it is possible to navigate the various rooms of a moving train, even if we don't know the country through which the train is passing, we can navigate in space amongst the bodies we have cataloged. ",
"What we are unable to do is navigate beyond what we know with any hope of returning."
] |
[
"How can nuclear reactors work without steam?"
] |
[
false
] |
There are spacecraft in space right now that are powered by onboard nuclear reactors. Surely they don't use steam to spin a turbine like a normal land-based nuclear power plant. That's a lot of extra weight to carry into space. Turbines are heavy. So how do these reactors work? Have we found a way to convert heat into electricity?
|
[
"Well there are no spacecraft in space ",
" that is powered by a nuclear reactor. All the nuclear powered spacecraft operational now are powered by RTGs (radioisotope thermoelectric generator). Other can explain the difference better but the basic idea is that a RTG is a purely passive device that produce heat from the natural decay of some radioactive material. In a nuclear reactor the fission is controlled and can be shut off completely. About 35 real nuclear reactors where ever launched in space between the 60 and late 80's (nearly all of them Soviet, 1 American). ",
"RTGs and nuclear reactors all use thermometric converters relying on the Seebeck effect. It's the same thing used in thermocouples (or Peltier devices). The idea is that two different metal at different temperature put in contact will produce a little bit of electricity. It is super convenient for space applications because it has no moving parts. The issue is that it is super inefficient, you usually get less than 5% of the thermal energy converted into electricity. It is also fairly expensive which is why it's not used much on earth."
] |
[
"SNAP-10A",
" and Soviet TOPAZ were the only spaceborne nuclear reactors. The American version used the thermoelectric effect as ",
"/u/electric_ionland",
" described. The Soviet version used the thermionic effect, which is basically getting something hot enough that electrons start boiling off your anode and moving through vacuum to a nearby metal plate.",
"Neither are very efficient, although there's some hope for thermionic conversion.",
"Nobody has put a proper Brayton cycle (steam) nuke into space yet. If they did it would look ",
"something like this.",
" Note the vast radiator \"wings\" needed to dump heat and recycle coolant. "
] |
[
"A well-deserved upvote. ",
"So we ",
" able to convert heat directly into electricity, but it's just really inefficient?"
] |
[
"How feasible is the 'Aclubierre drive'?"
] |
[
false
] | null |
[
"It isn't. Almost completely impossible. General Relativity is a set of equations on the one side, the arrangement of energy and mass, on the other side, the curvature of space. To do physics, we usually start on the energy and mass side, put in some known energy configuration, like a spherically symmetric mass, and out pops the space time curvature.",
"But it's an equation, right? So we can start with a desired space-time curvature, say an alcubierre drive, or a wormhole. We put in those space-time configurations and look at what kind of energy is needed. And the answer turns out that the energy configuration necessary for alcubierre drive or wormholes is not possible with any kind of energy we're aware of. Could we find new types? maybe. But these are things like negative mass, things we really really don't expect to find in reality."
] |
[
"I see, I had not thought of the maths being that way. Thank you for explaining it to me. From what I can gather from your post it seems to me that you are saying that general relativity essentially dictates the possibility/impossibility of the Alcubierre drive. Would you mind clarifying what you mean by 'energy configuration'? Is that simply the amount of energy required or something more complex?"
] |
[
"ah, well just a few points to clarify, general relativity, the notion that energy and space-time curvature are related ",
" allow alcubierre. The rest of reality is what prevents it ;-). The details are the ",
"Einstein Field Equation",
". The energy configuration, in this case, is the \"",
"Stress-energy tensor",
".\" That's why I was trying to be a bit... vague about things. There's a lot of things that factor in to space-time curvature."
] |
[
"Cross post from r/environment - Which is worse on the environment - Aluminum cans or glass bottles?"
] |
[
false
] |
From what I understand, there are two major drawbacks of each - with aluminum, you have bauxite strip mining which destroys large swaths of land. With glass production, you have large amounts of energy being used to produce the bottles. There are other issues such as the weight of a glass bottle, the energy needed to recycle, and the recycleability of aluminum and glass. So is there anyway to calculate the impact of using glass vs aluminum for beverage storage?
|
[
"You seem to be asking about embodied energy - the amount of energy, logically enough, embodied in that physical product - or how much process energy went in to transport, fabrication, recycling, etc over the entire lifecycle of the product. It turns out that:",
"The energy cost of aluminum is highly dependent upon the amount of recycled material used to make it, while the energy cost of glass is only slightly dependent.",
"At 100% primary feedstock (zero recycled material), aluminum is sightly energy-cheaper on a per unit basis than glass. At 100% secondary (recycled) feedstock, aluminum is ",
" cheaper than glass.",
"Some numbers:",
"Aluminum\nGlass",
"100% Primary (MJ/kg)\n194.7\n13.0",
"100% Secondary (MJ/kg)\n47.5\n11.3",
"Present Rates (MJ/kg)\n94.9\n12.6",
"100% Primary (MJ/unit)\n2.59\n3.57",
"100% Secondary (MJ/unit)\n0.63\n3.10",
"Present Rates (MJ/unit)\n1.26\n3.46",
"Present rates are 67.8% secondary for aluminum and 23% secondary for glass. All of this analysis is for the continental US. Of course, these numbers are the result of lots and lots of other caclculations, which could be provided if desired. It's not as straightforward of an analysis as one might expect."
] |
[
"While glass certainly requires energy to produce, it pales in comparison to the energy required to produce aluminum.",
"Additionally, glass has actually begun to be re-purposed as landfill cover, after being crushed, because of the extremely low environmental impact it has in a landfill environment.",
"The rub is, in many areas of the country, glass is beginning to be considered a non-recyclable due to cost. Ultimately, if your region is able to recycle glass, glass containers are going to be better for the environment than their aluminum counter-parts. If not... well that then depends on your philosophy on landfilling and human waste.",
"BTW, I'm an environmental planner in a rural part of the country, this is actually something we talk about quite a bit."
] |
[
"The problem with aluminum is separating it from the ore in initial refinement. Ancillary issues with storage of the sulfuric acid and separation byproducts cause immense problems - see the flood in Hungary for example.",
"Aluminum can be reused indefinitely, once it's out of the ground and refined. The problem is that it is a bitch to separate."
] |
[
"Has the power of telescopes increased at a predictable rate? If so, when will it be likely to get a good image of a planet in another solar system?"
] |
[
false
] |
By "power" I mean the detail that can be resolved. By "good image" I mean being able to see whether it has clouds. Just curious...
|
[
"Not exactly; telescopes (and any optical device, in fact) faces a fundamental limitation of physics that equates eye/lens size with maximum resolution. Bear with me, my memory on this is a few years old. Basically one way to quantify it is by imagining you're looking at a car's headlights in the distance- at some distance, you won't be able to tell it's two different lights, only one. The wider your eye, the father away you can distinguish two lights- and this is also affected by the wavelength of whatever EM energy you're picking up- I'm assuming your question is omitting radio telescopes, which could be considered \"more powerful,\" though perhaps being less satisfying than resolving a visual image through analog means. ",
"This isn't to say there haven't been any advancements at all, it's just there's a certain bottleneck presented by physics that we already run up against. ",
"The real advancements have been in signal- and data-processing capabilities, and our ability to squeeze more information out of the data. "
] |
[
"Good question. It's not so predictable but like many things in nature it can be roughly parametrised by a power law. Now this power law may not hold for long periods of time, engineering limits may be reached but let's put that to the side.",
"To image a planet at say 5000km resolution at 5 parsecs (at 600nm) you require a telescope diameter of approximately 18500 meters. Now let's say from this ",
"paper on the growth rate of telescopes",
" that the aperture doubling time is 20 or 45 years, and lets say the current diameter is about 18m (because we're in between 30m class and 10m class). It will take 10 doubling to reach 1000 times larger proximately (very approximate) it will take 200 to 450 years to get a telescope that could resolve an exoplanet.",
"The paper however also suggests the aperture doubling time is changing with time with an e folding time of 70 years. This along with the assumption that the current rate is 20 years gives a number of 71 years.",
"I don't much believe these numbers but that's what they are."
] |
[
"3D doesn't come from the width of our eyes, but their distance apart from each other (allowing us to see something from two different angles) ",
"As for telescopes on opposite sides of the earth, we've done that with opposite sides of earth's orbit, 6 months apart"
] |
[
"Quantum computing explanation... non-layman but non-expert version"
] |
[
false
] | null |
[
"An amplitude is never complex valued."
] |
[
"Scott Aaronson: ",
"Quantum Computing and the Limits of the Efficiently Computable - 2011 Buhl Lecture ",
". It's a very interesting lecture but intented non-layman but non-quantum-computation-expert."
] |
[
"Well, you've taken quantum mechanics right? So I assume you're familiar with basic QM concepts.",
"To understand QC it's best to not think of any particular realization but to think of an abstract quantum computer. Consider a ",
" probabilistic computer. It is a computer where the input is not a single well-defined input but is a probability distribution, and thus it will not have a specific output but a probability distribution as an output. Like say you have a computer that is the XOR operation, and the input distribution is P(01)=P(10)=0.5, P(00)=P(11)=0. The output would be the probability distribution where P(1)=1 and P(0)=0, as you can verify for yourself.",
"A quantum computer is an extension of this idea, except the probabilities are complex-valued probability amplitudes (and of course the probability of measuring a specific output is the square of the ",
" absolute value of the amplitude). The gates are simply unitary operators. To perform a quantum computation, we start with an input state (state = complex-valued probability distribution), and apply unitary operators to it until we reach an output state, then we do a measurement. The measurement is the output of the computation.",
"To simplify the maths, the notion of a quantum gate is developed. This page offers a nice layman-readable introduction to the notion of quantum gates: ",
"http://michaelnielsen.org/blog/quantum-computing-for-everyone/",
"EDIT: thanks hikaruzero for pointing out my somewhat loose usage of terminology."
] |
[
"Is there such a thing is DNA reprogramming?"
] |
[
false
] |
A couple of people I know have posted this on facebook- To me it seems like nonsense, particularly as the writers seem to make huge jumps between apparent 'findings' and the conclusions they make.Can anyone explain if this kind of thing is actually possible? Edit: Oops, sorry about the typo in the title
|
[
"Nonsense?",
"I think you are showing a great deal of restraint to describe ",
" as nonsense.",
"I'm trying to describe just how insane that article is but words are failing me."
] |
[
"Oh wow. It's complete nonsense. Let's see:",
"Only 10% of our DNA is being used for building proteins. It is this subset of DNA that is of interest to western researchers and is being examined and categorized. The other 90% are considered “junk DNA.”",
"That view went out of fashion years ago. It's well understood by basically everyone who works with DNA that the non-coding regions have important functions, mostly relating to the control of under what circumstances a given protein is made.",
"They found that the alkalines of our DNA follow a regular grammar and do have set rules just like our languages.",
"That's true, but it's not groundbreaking in any respect. DNA has fairly universal ",
"vocabulary",
" and grammer because it has to be \"read\" by enzymes that make proteins. It doesn't have any connection to human language other than that it's also an ordered way of communicating information.",
"\"Living chromosomes function just like solitonic/holographic computers using the endogenous DNA laser radiation\"",
"This is literally a bunch of words strung together to sound fancy. No way it could possibly be construed as anything that actually happens inside a cell.",
"Living DNA substance (in living tissue, not in vitro) will always react to language-modulated laser rays and even to radio waves, if the proper frequencies are being used.",
"Radio waves and lasers can interact with DNA, but being \"language modulated\" makes absolutely no difference. Also, we have no reason to think that if the interaction happens in vivo it shouldn't happen in vitro.",
"This finally and scientifically explains why affirmations, autogenous training, hypnosis and the like can have such strong effects on humans and their bodies.",
"Nope. That's mediated mostly through neural and endocrine pathways. ",
"The Russian scientists also found out that our DNA can cause disturbing patterns in the vacuum, thus producing magnetized wormholes!",
"The fuck? I'm pretty sure they don't actually know what DNA is at this point",
"These are tunnel connections between entirely different areas in the universe through which information can be transmitted outside of space and time. The DNA attracts these bits of information and passes them on to our consciousness.",
"OK, I'm done."
] |
[
"It's one of those pages that is basically impossible to even refute because you would have to spend 5 minutes on each sentence to fully explain how wrong it is."
] |
[
"In the event of a nuclear war (with humanity's current arsenal being used) How long would it take for the bioshpere to regenerate? (if it would at all)"
] |
[
false
] |
I guess I should define regenerate. Let's say, pre-war levels of habitability for humans, and animals. Though knowing chernobyl, animals couldn't give a bother about radition.
|
[
"One of the things to consider in order to address your question is how ecologies have rebounded from previous mass-extinction events. There is quite a bit of litterature on this topic, and the observed recovery times for biodiversity levels vary considerably form a few million years to about 30. I suggest you use the Permo-Triassic extinction as your benchmark. It has often been singled out as the singlemost severe extinction event in our history, which obliterated about 90%-plus of all extant species ... this gives you an upper limit, and an order of magnitude for the time scales involved.",
"In the case of the Permo-Triassic, the recovery of biodiversity to pre-extinction levels took 30 million years (",
"Sahney, S., & Benton, M. J. (2008). Recovery from the most profound mass extinction of all time. Proceedings of the Royal Society of London B: Biological Sciences, 275(1636), 759-765.",
"). The causes and mechanisms responsible for the severity of that event are multiple and complex, without singling out a specific cause, it involved massive climate change and aridification, ocean acidification, loss of marine platform habitat (gee ... sounds familiar ...).",
"The Cretaceous-Tertiary event, which kicked Dinosaurs and Ammonites out and opened the door to the expansion of mammals, was followed by a relatively quicker 3 million years of recovery time (",
"D'hondt, S., Donaghay, P., Zachos, J. C., Luttenberg, D., & Lindinger, M. (1998). Organic carbon fluxes and ecological recovery from the Cretaceous-Tertiary mass extinction. Science, 282(5387), 276-279.",
"). ",
"Overall and in a general sense, recovery from mass extinctions is believed to operate in the order of the 10 million year range (",
"Kirchner, J. W., & Weil, A. (2000). Delayed biological recovery from extinctions throughout the fossil record. Nature, 404(6774), 177-180.",
"). The big five have involved a certain variety of causes ... climate change, meteorite impacts, habitat loss, glaciation, etc. However, none has ever involved the mechanism you propose: nuclear fallout, so we have to be conservative in assessing how this would affect hypothetical recovery times, hence my suggestion of using the Permo-Triassic as a benchmark. And you have to keep in mind that this nuclear event would piggyback of current environmental stresses resulting from climate change. Together, they might pack quite a wallop... So, that raises the question of whether recovery would be possible at all, at least in terms of metazoans. In each mass extinction, post-recovery communities were quite different from the pre-extinction ones. There is no rule saying anywhere that all and any extinction event must necessarily be followed by a recovery. It is quite conceivable that some level of hardship is just terminal ... at least for some kinds of life. And therein might lie the difference between a world populated by metazoans, and a world of bacteria and archeans for instance.",
"One last word, to put things in perspective. The average life span for a species is in the order of 1-5 million years, and most of these species are marine invertebrates. Complex critters such as primates average shorter life-spans in general. The implication is that if we should go through an extinction level event within our lifetime, whether a nuclear holocaust, through climate change or some other mechanism, it is unlikely in the extreme that our species will make it to the other side and witness any hypothetical recovery which might follow.",
"other usefull references:",
"Solé, R. V., Montoya, J. M., & Erwin, D. H. (2002). Recovery after mass extinction: evolutionary assembly in large-scale biosphere dynamics. Philosophical Transactions of the Royal Society B: Biological Sciences, 357(1421), 697.",
"Chen, Z. Q., & Benton, M. J. (2012). The timing and pattern of biotic recovery following the end-Permian mass extinction. Nature Geoscience, 5(6), 375-383.",
"Payne, J. L., Lehrmann, D. J., Wei, J., Orchard, M. J., Schrag, D. P., & Knoll, A. H. (2004). Large perturbations of the carbon cycle during recovery from the end-Permian extinction. Science, 305(5683), 506-509.",
"EDIT: I can't spell..."
] |
[
"Just to be absolutely clear on the extinction level characteristics of a massive nuclear exchange. The main extinction mechanisms are not the actual explosions themselves, which as you correctly point out are local and of little consequence, or even the radiation, but:",
"1 - the mobilisation of particulate matter to the stratosphere and the resulting nuclear winter;",
"2 - the release of sizeable amounts of fallout and its effects of the biosphere.",
"The nuclear winter is deemed analogous to some of the effects of the K-T impactor, which resulted in one of the \"Big Five\" extinction events (see comment elsewhere in the thread & references by Vellekoop below).",
"The effect of the fallout is harder to assess, and of course will depend on the actual nature of the fallout itself. Nuclear exchange scenarios with cobalt-jacketed weapons would realistically be far more severe than the others.",
"And, of course, other factors such as ozone layer destruction and \"unforeseen consequences\"...",
"You may want to consider the following additionnal references:",
"Mills, M. J., Toon, O. B., Lee‐Taylor, J., & Robock, A. (2014). Multidecadal global cooling and unprecedented ozone loss following a regional nuclear conflict. Earth's Future, 2(4), 161-176.",
"Baum, S. D. (2015). Confronting the threat of nuclear winter. Futures, 72, 69-79.",
"Jacob, T. J. T. (2015). The impact of the preparation and outbreak of a nuclear war on man and his ecosystem. Journal of the Medical Sciences (Berkala ilmu Kedokteran), 19(03).",
"Vellekoop, J., Esmeray-Senlet, S., Miller, K. G., Browning, J. V., Sluijs, A., van de Schootbrugge, B., ... & Brinkhuis, H. (2016). Evidence for Cretaceous-Paleogene boundary bolide “impact winter” conditions from New Jersey, USA. Geology, 44(8), 619-622.",
"Vellekoop, J., Sluijs, A., Smit, J., Schouten, S., Weijers, J. W., Damsté, J. S. S., & Brinkhuis, H. (2014). Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary. Proceedings of the National Academy of Sciences, 111(21), 7537-7541.",
"Prosh, E. C., & McCracken, A. D. (1985). Postapocalypse stratigraphy: Some considerations and proposals. Geology, 13(1), 4-5.",
"Vaughan, G. L., Pimm, S., & Fields, D. E. (1987). Extinction cascade: biological consequence of nuclear war/winter. Trans. Am. Nucl. Soc.;(United States), 55(CONF-871101-)."
] |
[
"People way overestimate the power of nuclear weapons and underestimate the vastness of the earth.",
"The current total US nuclear arsenal is about 547 Megatons.",
"The largest nuclear weapon ever tested was the ",
"Tsar Bomba",
", which was about 50 Megatons. This bomb was huge and annihilated everything within a 100 km radius. It exploded in a remote arctic region of Russia.",
"So, with all the nuclear weapons in the world you could probably kill everything within about a few dozen circles of 100 km, something like a few hundred thousand square kilometers. With the proper aim, you could definitely kill millions, maybe billions, of humans. ",
"However that's only 1/10,000 of the earth's surface area.",
"The biosphere would barely notice. There'd be vast areas of the land and ocean where nothing would change at all."
] |
[
"What are the theoretical limits to flywheel energy storage?"
] |
[
false
] | null |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):",
"A good home for this question is our sister subreddit ",
"/r/AskScienceDiscussion",
". It might be too open-ended or speculative for ",
"/r/askscience",
". ",
"/r/AskScienceDiscussion",
" is also a better place for advice on education, book suggestions or general questions about working in STEM. Please feel free to repost there!",
"Please see our ",
"guidelines",
"."
] |
[
"That is unfortune. May I rephrase the question and post again? Possibly \"What prevents us from spinning flywheels faster and faster to store even more energy?\". To be honest, I assumed my question was already specific enough."
] |
[
"That's still an open-ended question. It should go to ",
"/r/AskScienceDiscussion",
"."
] |
[
"If/As the universe is expanding, what is it expanding into?"
] |
[
false
] | null |
[
"From the ",
"wiki on expansion",
":",
"The metric expansion of space is the increase of distance with time between distant parts of the universe. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion \"outward\" into preexisting space. ",
" A frequently used analogy is the expansion of the surface of an expanding rubber balloon. In this analogy the universe has two spatial dimensions (the surface of the balloon) rather than three. As the balloon expands, any two points on its surface get farther and farther apart. Another common analogy is a rising loaf of raisin bread—as the loaf expands, the raisins inside it move farther and farther apart from each other."
] |
[
"Correct. Reading the balloon analogy carefully... it says this:",
"A frequently used analogy is the expansion of the ",
" of an expanding rubber balloon.",
"The analogy only wants you to consider the surface itself, not the area below or above the surface (the analogy doesn't work there) - it is actually why the balloon analogy isn't liked very well among astronomers (because it can lead to misunderstanding).",
"Don't think of the universe being in a room (as an example) - when you think of it that way, you think the balloon is getting bigger in reference to the room it is in.",
"Instead... think that the universe itself IS the room, and the room is simply getting bigger. There is no room (universe) outside the room (universe). The ",
" of the balloon is adding more material between any two distant points.",
"You ",
" think of that room as the Tardis in Dr. Who - the inside of which seems to be infinite - regardless of the apparent outside phone-booth size.",
"When you want to think about what may be outside the universe, you have to step into speculations such as M-Theory, special black hole theory, Penrose cyclic universe, etc. - but those remain unproven for the time being."
] |
[
"Thank you! Wish I could upvote twice...once for the explanation and once for referencing Dr. Who."
] |
[
"Why did the solar powered Christmas lights on the tree outside my house light up on the day of the eclipse even though they hadn’t worked in over a year ?"
] |
[
false
] | null |
[
"We can't really comment on anecdotes / isolated incidents without resorting to speculation which we try to avoid."
] |
[
"Okay thank you. Who should I ask to find out more? "
] |
[
"You could try ",
"/r/askengineers",
". They have less strict rules for posts"
] |
[
"Is there a temperature where the water vapor in air (causing humidity) turns to ice?"
] |
[
false
] |
When it drops below freezing, there is still water vapor in the air. Some google-ing explained why this happens, and I am satisfied with this explanation. I'm just curious as to under what conditions may cause water vapor to freeze out of the air.
|
[
"Here's the phase diagram of water: ",
"http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg",
"So if pressure is below 0.006 atmospheres and the temperature drops past the line, it'll re-sublimate."
] |
[
"Crossing the red line",
" from bottom-right to top-left is turning water vapour into ice. The temperature needs to be below the triple point temperature (0.01C), and the partial pressure needs to be below the triple point pressure (0.0060373 atm). So if you have water vapour down there, and either reduce the temperature or put more water in, it will turn to ice."
] |
[
"Others have posted phase diagrams and those are helpful, but I'm not sure that is what you were asking for. Water vapor commonly freezes in the atmosphere, many clouds are composed of ice crystals. The spontaneous nucleation point is ~-42C, a temperature which can be reached in the upper troposphere. More commonly ice crystals form at higher temperatures, around some kind of condensation nuclei, pollen, dust, salt etc. So, for ice crystals to form in the air, the air needs to be wet, cold, and dirty; and it often is. "
] |
[
"If I have two perfectly insulated boilers, one tall and thin and the other short and wide, and I apply the same power to both, does the rate of vaporisation differ?"
] |
[
false
] |
I'm a home distiller, and I often hear that you will get more boil-off if there is greater surface area of the liquid in the boiler. This sounds weird. Assuming that the boilers are perfectly insulated, have equal masses of identical fluids inside them, and have equal power applied to them as heat, can the rate of vaporisation differ? And if so, where is that extra energy going in the tall thin boiler? Question originally raised . Edit to clarify - I'm thinking with an internal heat source (element), so actually equal heat, utilisation of external heatsources isn't an issue. Cheers fellas. Love your work, btw.
|
[
"Why?"
] |
[
"Same amount of heat. Same amount of water. The only variable is the area where heat may dissipate into the atmosphere. If they're insulated all around, you're just heating up the same amount of stuff in a different shape. The shape may influence how much the heat spreads through, but I don't think it is possible that one would gain more heat than the other. ",
"You've said rate of vaporisation, so we could assume a shape that makes for minimal spreading of heat, concentrating all the heat at one point, so that it will boil some liquid whereas the rest of it remains cool. But you seem to be speaking from a practical viewpoint, so I would say the answer to your question is no."
] |
[
"I'm assuming that you're talking about basically an insulated copper pot with a resistive heater of some kind on the inside. I don't think this changes the details of of my answer, but I'm going to ask some follow-up questions to make sure I know whats going on.",
"My understanding is that your goal is to maintain the beer at a constant, elevated temperature for some extended period of time while minimizing the amount of water you lose to boiling?",
"Also: Is the claim made that you want the surface area in contact with water of the pot to be as small as possible? Or is the claim that you want as little of the surface area of the water exposed as possible?"
] |
[
"Why does the maximum number of electrons per quantum level follow the pattern 2, 8, 18?"
] |
[
false
] |
Quantum level 1 has a max of 2 electrons. Quantum level 2 has a max of 8 electrons. Quantum level 3 has a max of 18 electrons. The pattern of maximum number of electrons per quantum level is: 2 * (quantum level ^ 2) 2 * (1 ^ 2) = 2 2 * (2 ^ 2) = 8 2 * (3 ^ 2) = 18 This pattern continues for quantum level 4: 2 * (4 ^ 2) = 32 Is there any explanation for this?
|
[
"Yes. Electron states are described by four \"quantum numbers\" n,l,m,s. These are subject to the constraints that s=+1/2, -1/2. m is an integer from -l to l, and l is an integer < n. ",
"So for n=1 you have l=0, m=0, and only the 2 possible s values. For n=2, you have l=0, m=0 plus l=1, m=-1,0,+1, times two, so 2*(1+3) states. So 2*1, 2*(1 + 3), 2*(1 + 3 + 5), 2*(1 + 3 + 5 + 7). The total number of states with a 'principal' quantum number n is twice the sum of the first ",
" odd numbers. That sum is n",
", so 2n",
". ",
"What these numbers represent is momentum (n), angular momentum (l), the spatial orientation of the angular momentum, if any (m), and the electron 'spin' (s). "
] |
[
"The state of an electron in an atom can be described by four numbers:",
"the principal quantum number, or energy level n",
"the angular quantum number l",
"the magnetic quantum number m",
"the spin s",
"But there are also constraints on these numbers. It turns out that l has to be less than or equal to n, m has to be between -l and l, and s has to be either 1/2 or -1/2. Doing a bit of math, you get that there are 2 * n",
" distinct states for any particular n. Two electrons can't share the same state, because they're fermions, so... there's your result."
] |
[
"The electron orbit numbers can arise from a very pure mathematical approach to electron orbits. You might be familiar with the \"wave function,\" commonly represented as capital ",
". ",
" is derived from a ",
"certain partial differential equation",
" (PDE) that has the following assumptions (initially assumed by the ",
"Bohr model of the atom",
", but has since changed):",
"; that is, kinetic energy and electrical potential energy of the electrons around the positive nucleus can't change unless acted on by external force.",
"; E = h * ",
" where h is Planck's constant and nu is the inherent wavelength of all particles, here an electron.",
"; where an electron can have both a sinusoidal and transverse component",
"Taking these assumptions into equation form, and then combining the equations ",
"in a clever way, you get the partial differential equation known as the Schrodinger Equation",
", where ",
" is the phase of the particles both-longitudinal-and-transverse wave (the interpretation of this is related to the wave density function).",
"OK, so we have an equation. All we have to do is solve it.",
"One way to solve PDE's is by ",
"Fourier series",
". The general idea for Fourier series is that ",
"any curve can be represented as an (infinite) sum of periodic equations",
". The wave function in 3 dimensions is no exception.",
"As you might expect, each periodic \"term\" represents something. In a very simplified explanation, you can think of each periodic \"term\" in the eventual equation as representing one electron. More electrons in the system requires more \"terms\" to make the wave fit the actual PDE and explain where the electrons are, probabilistically. I should note that these \"terms\" are actually complex summation equations themselves, and get more complicated the more electrons there are and the higher energy they occupy. ",
", and sub-coefficients in these terms correspond to the traditional n, l, m, s that is taught so matter-of-factly in introductory college chemistry.",
":",
"Since the wave function gives numbers with complex variables, we take the absolute value (to give it a real value) and square (since negative distance doesn't make much sense).",
"The Pauli Exclusion Principle that states no two electrons can occupy the same orbit (e.g. have the same n, l, m, and s) basically comes from the strength of the assumptions. Since the orbits and equations stem from the pure mathematical logic given the assumptions, they don't break down under normal circumstances.",
"When extreme forces like a neutron star's gravity are in action, these assumptions no longer hold true and ",
" break down. This is when two electrons overcome the electron degeneracy pressure."
] |
[
"Are there any moments where a severe allergic reaction or an auto-immune response is benefic to save someones life? From an evolutionary standpoint, what's the use of these reactions?"
] |
[
false
] |
I mean have you ever heard someone say:" luckily this guy went in an anaphylactic shock otherwise he would be dead!!!". Is there any point to be allergic or is it just the system going crazy? What about lesser allergic reaction, rash, swelling, does it have any purpose does it help in any way to something??
|
[
"Extreme immune responses are generally harmful, whether they are allergic, autoimmune, septic, or superantigen based. Since the immune system relies heavily on inflammation to recruit cells to an area, having a rapid or prolonged high-level response means that there will be a deficiency of circulating blood for your organs and will likely kill you.",
"Mild to moderate allergic and autoimmune responses are hypothesized to be helpful in different ways:",
"Most people think the reason we have allergies is a byproduct of a specialized antibody used to deal with infections of helminths (worms). ",
"Helminth infections are mediated by IgE",
", which has unique properties to induce white blood cells to respond primarily by releasing chemical toxins around the infection site instead of trying to \"eat\" the parasites as they would in bacterial/viral infections. The theory goes that since we evolved eating lots of raw and unwashed foods (especially meats here) the rate of infection with worms was historically much higher, promoting a modified antibody specialized for immune responses to macro-pathogens. Since we no longer have frequent worm infections the response has been \"hijacked\" by pollen, cat dander, etc.",
"There have also been hypotheses put forward that allergic responses ",
"helped early homonids choose environments which were more healthy to live in",
". This is a much newer idea, and I'm honestly skeptical: there's another theory among immunologists that severe allergies develop due to lack of exposure to potential antigens at critical periods in immune development - suggesting that people who live in \"dirty\" environments have more tolerant immune systems (the classic snooty lady walks into a dirty restaurant and sneezes problem).",
"The autoimmune response is an overproduction of normal immune defenses against cancer (this link won't format: ",
"http://www.journals.elsevierhealth.com/periodicals/cdp/article/S0361-090X(05)00052-8/abstract",
"). Since cancer cells are technically pathogens disguised as part of you, it's easy to misdirect a natural killer cell or cytotoxic T cell onto healthy cells under the right conditions - they ",
" to be able to kill cells that have self characteristics to be successful against cancer. It's even possible for ",
"cancers to manipulate the immune response",
" by downplaying all white blood cell activity in attempt to avoid detection.",
"Edited to fix link"
] |
[
"I wouldn't go so far as to say that \"the autoimmune response is an overproduction of normal immune defenses against cancer\". There are two problems with this statement. First, autoimmune responses can be caused via a wide variety of mechanisms that may or may not be related to each other. Many of these mechanisms have no apparent connection to cancer. Second, although the current consensus is that the immune system may play a role in cancer prevention, the mechanisms by which they achieve that is highly debatable and is an active area of research. Whether or not immunosurveillance is related to autoimmune diseases is again debatable. We simply do not know enough at this point."
] |
[
"Yes, not all autoimmune responses are related to cancer-prevention mechanisms - I was mainly thinking of NK and T cells here. ",
"Lupus",
" and ",
"rheumatoid arthritis",
" are heavily B cell mediated. However, these are cases of misregulation of the development of the cells or their activity, not an intentional adaptive response (in the evolutionary, not immunological, context), so I didn't really consider them relevant for the original question. ",
"My understanding of cytotoxic responses is that they evolved to deal with viral and tumor threats - hence the ",
"increase in cancer once reproductive age is passed",
". Do you have some good examples of the debate going on in this area?"
] |
[
"When we have nightmares or stressful dreams, does our brain release the same chemicals (cortisol, norepinephrine, adrenaline, etc.) that it would if the scenarios of these dreams were actually happening in real life?"
] |
[
false
] |
Do nightmares or stressful dreams have just as much impact on us chemically as actually experiencing the things we're dreaming about?
|
[
"The acute hormonal changes associated with nightmares are not particularly well studied. There have been some studies looking at changes in autonomic tone, including heart rate and respiration, showing that there are occasionally measurable effects of nightmares. Quoting from ",
"this review",
".",
"Most healthy individuals experiencing a nightmare (that is, individuals from a non-psychiatric sample), exhibit a moderate level of heart rate acceleration for the last three minutes prior to awakening during a nightmare REM sleep episode, while an equal proportion of healthy individuals usually manifest heart rate deceleration during non-nightmare REM sleep (Nielsen & Zadra, 2000). The EEG samples of these subjects also show cortical activation during the last two minutes of nightmare, exhibiting an amplification of the posterior alpha power observed during normal REM sleep, but also an atypical higher\nbeta power over frontotemporal regions (Cantero, Atienza & Gomez, 1999).",
"Another review",
" has a couple of passages relevant to your question:",
"Physiologic evidence for emotional activity during rapid eye movement (REM) sleep is substantial. Autonomic system variability increases markedly in conjunction with central phasic activation, as seen especially in measures of cardiac function, respiration, and skin and muscle sympathetic nerve activity. Brain imaging, too, demonstrates increased metabolic activity in limbic and paralimbic regions during REM sleep activity similar to that seen during strong emotion in the waking state. These dramatic autonomic fluctuations globally parallel dreamed emotional activity, which is detectable throughout most dreaming when appropriate probes are employed. Some studies indicate that most dreamed emotion is negative, primarily fearful, and may conform to a surgelike structure within REM episodes. Many theorists interpret the various forms of phasic activity occurring during sleep as indicating dream-related affective activity.",
"and",
"One laboratory study of nightmares indicates moderate arousal—increased heart and respiration rates—during some nightmare episodes, but unexpectedly low arousal in most others. These early findings constitute the principal empirical basis for diagnostic guidelines such as the ICSD and DSM-IV, but there are serious problems with the work, such as the inclusion of psychiatric patients and patients with posttraumatic stress disorder (PTSD) in the study sample.",
"Recordings of heart rate and respiration rate during nightmare and non nightmare REM sleep episodes confirmed a moderate level of sympathetic arousal during nightmares. Mean heart rate for nightmare sleep was elevated (by about 6 bpm) for the 3 minutes before awakening. Mean respiration rate was only marginally higher at this time. We have recorded higher absolute and relative alpha power over primarily right posterior sites in the last 2 minutes of nightmare sleep. However, these changes might reflect processes of awakening."
] |
[
"The method used is to ask the participants after they awaken from REM sleep whether they were having a nightmare. Some studies have allowed participants to awaken spontaneously, others have induced awakenings with stimuli (e.g., loud noises). There isn't yet any way of knowing what someone is dreaming about just from observing them sleeping."
] |
[
"Interesting. Somewhat related: How do they study nightmares, or know that the subject is having one?"
] |
[
"How come if I smack a fly in mid-air (not smashing him against a wall) it seems to be completely ineffective? If I were flying around, and a proportionally larger creature smacked me, I imagine I'd be reduced to liquid."
] |
[
false
] |
[deleted]
|
[
"This, combined with some of xaltug's explanation gives the whole picture. The force isn't the concern when it comes the fly going splat. It's the tensile stress (force/area). A fly can withstand much larger accelerations and decelerations than we can because as you increase in size, mass grows in a cubic manner whereas surface area grows quadratically. Thus, if you're relatively big, you have a lot more mass per unit area, and if you're relatively small, you have a lot more area per unit mass.",
"Now, as xaltug points out, because the mass of the fly is very small relative to the mass of the hand, the fly is essentially going to be brought up to the hand's speed without changing the hand's momentum. This is going to lead a change of momentum for the fly of its mass times the hand's velocity. Now, regardless of the speed of the swat, the fly is at a distinct advantage vs. us getting splatted by a giant. This is where the \"a lot more area per unit mass\" thing comes in. Because of this, the whole force/area number is a lot smaller, meaning the fly experiences much less stress than a human would over its entire body when getting \"splatted.\""
] |
[
"This, combined with some of xaltug's explanation gives the whole picture. The force isn't the concern when it comes the fly going splat. It's the tensile stress (force/area). A fly can withstand much larger accelerations and decelerations than we can because as you increase in size, mass grows in a cubic manner whereas surface area grows quadratically. Thus, if you're relatively big, you have a lot more mass per unit area, and if you're relatively small, you have a lot more area per unit mass.",
"Now, as xaltug points out, because the mass of the fly is very small relative to the mass of the hand, the fly is essentially going to be brought up to the hand's speed without changing the hand's momentum. This is going to lead a change of momentum for the fly of its mass times the hand's velocity. Now, regardless of the speed of the swat, the fly is at a distinct advantage vs. us getting splatted by a giant. This is where the \"a lot more area per unit mass\" thing comes in. Because of this, the whole force/area number is a lot smaller, meaning the fly experiences much less stress than a human would over its entire body when getting \"splatted.\""
] |
[
"Sorry to nag, but tensile stress is stress in tension. I believe you're looking for compressive stress."
] |
[
"If Pioneer can accelerate itself back towards the solar system by hitting its own dish with photons emitted from itself, doesn't that contradict the law of conservation of linear momentum?"
] |
[
false
] |
[deleted]
|
[
"Here's a different way to think of it: ignore everything that happens until the photos leave the dish and go sailing off into interstellar space (since the forces in the previous steps do cancel), and then think of this as an emission event. Each photon has a tiny bit of momentum which it takes from the probe, slowing it down.",
"To run with your metaphor, it's not slowing your car down by pushing yourself into the seat, it's slowing it down by exhaling in the direction your car is going."
] |
[
"Have you ever tried to hold a firehose under full pressure? It is an amazing amount of energy that tries to force you back, and the analogy is lightly equivalent for Pioneer.",
"Think of a fireengine on a frozen lake, it has a full tank of water and starts to spray that water in one direction at high pressure. The law of action and reaction states that as much pressure as the water receives from the engine, going in one direction, must act back on the engine in the opposite direction, which would make the engine move in the opposite direction of the the spraying water if the lake is sufficiently slippery.",
"In the same way, the photons and energy from the heat emission work against Pioneer, trying to push it back.",
"This is not the same, you may say, because, in Pioneer's case the heat is emitted towards the Earth and reflected off the back of the antenna, just like a sailor trying to speed himself on by blowing on the sail.",
"But it does work, ",
"!",
"The sailor would probably not see much action, as the reaction from his blowing is negligible in comparison to the drag of even the smallest of boats, but as you know, in space there is virtually no drag so all the energy emitted has a direct effect on the space craft. In this case, though, it is a bit unfortunate that the emission is directed in a way that retards its travel.",
"Oh, if you sit on the fireengine and spray on, say, an open door that reflect the water in the opposite direction, it is, from an external observer's point of view, the same as if you were spraying in the opposite direction."
] |
[
"The trick is isolating the system. You have a certain amount of mass (photons) being expelled from one side, so therefore conservation of linear momentum tells us the system will be accelerated in the other direction."
] |
[
"Why are silicon and oxygen so abundant on Earth? Are they more likely to be formed by dying stars?"
] |
[
false
] | null |
[
"Oxygen and silicon are abundant everywhere because they can be formed by the normal process of nuclear fusion in a star. Elements significantly heavier than iron are much more rare as they are not produced in normal fusion, but probably in supernovae.",
"A chart of the relative abundance of chemical elements in the solar system is ",
"here",
".",
"Oxygen and Silicon are particularly abundant in the Earth's crust as shown in the chart ",
"here",
". This is because they are 'lithophile' elements, which form relatively light compounds and tend to float up to Earth's crust.",
"Things like platinum are rare in the Earth's crust as they are 'siderophile' elements which bond with metallic molten iron and tend to concentrate in the Earth's core.",
"More information in the ",
"Goldshmidt Classification",
" system."
] |
[
"Iron and lighter elements -- including silicon and oxygen -- are primarily made in stars doing their regular business. Things heavier than iron are mostly made in supernovae.",
"See ",
"http://en.wikipedia.org/wiki/Stellar_nucleosynthesis",
" and ",
"http://en.wikipedia.org/wiki/Supernova_nucleosynthesis"
] |
[
"Cool chart. I was under the impression that as elements get heavier they were less abundant -- this logically makes sense. But how about things like Li, Be, B,and F? I know Li and F are exceptionally reactive, does this translate into them having less relative abundance, or is some other mechanism at work?"
] |
[
"Can a particle have negative mass or exert repulsive gravity?"
] |
[
false
] |
I was watching the awesome PHD Comics video, and they mentioned that particles cannot have negative mass, or exert repulsive gravity on other particles. Is it is a a necessary consequence of the laws of physics (as we undertsand them) that a particle with negative mass cannot exist, or is it simply something that hasn't been observed or predicted to exist?
|
[
"I'll only comment on the experimental side because theorists can come up with almost anything.",
"No such particles have ever been seen. However, it has not yet been experimentally verified that antimatter attracts regular matter gravitationally. This has been hard to do because most antimatter we create is coming out of some kind of high-energy reaction. The groups that have managed to trap neutral anti-hydrogen are planing to do a test though, where they try to \"drop\" the anti-hydrogen and see what direction it goes. This should happen in a few years (they have to design and build a whole new trap)."
] |
[
"I'm pretty sure this is not true. It is true that we think that dark energy is causing the expansion of the universe, but it is not a repulsive gravity effect. It is a pressure effect."
] |
[
"This is not necessarily true; again the mechanism of dark energy is completely unknown, but it has been suggested that is a type of repulsive gravity:",
"http://news.nationalgeographic.com/news/2012/02/120215-dark-energy-antimatter-physics-alternate-space-science/"
] |
[
"Since time moves relatively slower where gravity is stronger, if you have two twins the work in the same sky scraper their whole life, would the one who works on the bottom floor age slower than the one who works on the top floor?"
] |
[
false
] |
I know the difference if any would be minute, but what if it was a planet with an even stronger gravitational pull, say Jupiter?
|
[
"Yes, by a very small amount. This was shown by raising an atomic clock by a foot relative to another nearby atomic clock, and seeing that it ticked slightly faster. I saw the lead scientist give a talk and he mentioned jokingly that he was kind of sad that after all this development of the most accurate clocks possible, he had essentially created a fancy altimeter. ",
"For your skyscraper scenario it amounts to a few microseconds over an entire lifespan. There wouldn't be an appreciable difference unless you were near a black hole or neutron star."
] |
[
"GPS satellites experience exactly what's being referred to here in a way that must be quantified. Time dilation due to increased speed causes their clocks to fall behind 7 microseconds per day compared to earthbound clocks. The lessened gravity causes their clocks to outpace clocks on the ground by 45 microseconds per day. I'm not sure if anyone's done the calculations for a clock in a skyscraper, but you can see that the two sources of time dilation are by no means equal and opposite."
] |
[
"So would these effects always cancel each other out or would there be a point where one force is greater than the other?"
] |
[
"If you have a 64bit processor but only 4 GB of RAM, could you theoretically have the CPU just split the bus width in two and have it use the two sets of 32 bits at the same time to operate twice as fast?"
] |
[
false
] | null |
[
"It might theoretically be possible, but I don't believe it would give the speedup that you think it would.",
"Let's say that we have a 64 bit bus that is capable of fetching N bytes of data in M units of time. Now compare that with two 32 bit buses, each of which can fetch N/2 bytes of data in M units of time. If you combine the two buses, you're still getting N data in M time - no speedup."
] |
[
"No, RAM is built to fetch one data line at a time, so being able to address more than 2",
" locations is not at all beneficial for memory accesses.",
"If you are talking about speeding up operations by adding 2 pairs of 32 bit numbers together instead of one pair of 64 bit numbers (assuming your processor supports such an operation), that is also impossible as overflow from the low bits would modify the information in the high bits, resulting on unexpected behavior."
] |
[
"The example you give of adding two pairs of 32-bit integers is actually part of ",
"MMX",
", the earliest SIMD instruction set for x86 CPUs (introduced in 1997)."
] |
[
"How much toilet water is lost due to evaporation"
] |
[
false
] |
So weird question, Water evaporates and toilets are designed to keep the water level at a certain point within the bowl. I would have to believe then that as the water evaporates the toilet levels the water. So I'm curious our house has no A/C with a small guest bathroom we hardly ever use so how much water would be wasted every week we don't use it. (I live in New Jersey the temperature has been in the 80's to 90's). TL;DR~If a toilet isn't flushed how much water is wasted through evaporation in a week. I know its a random question but I'm really curious and Google isn't helping me lol.
|
[
"Well, it depends on a lot of different factors; Air currents, humidity, temperature, exposure to light, what kind of toilet it is, and how high the water level is.",
"The easiest way to do it, is probably to measure it practically by taking the toilet apart, removing the supply of water (Turning off the pipe), and then marking where the water level is in the cistern before leaving. When you return, you'll see with your own eyes how much has evaporated."
] |
[
"toilets are designed to keep the water level at a certain point ",
"From my experience, all toilets use the U-trap design to keep water in the bowl. ",
"http://en.wikipedia.org/wiki/Trap_(plumbing)",
"Every time you flush, the U-trap retains a portion of water in the bowl. If you do not use the toilet for a long time, evaporation will deplete this collected water. This water will NOT be refilled (until the next flush). No wastage happens, but losing water in this trap can allow nasty ordours in the sewer system to enter your washroom.",
"To proof that this water in the bowl is not refilled, you can try for yourself to empty the pool. Fill up a bucket with water, and quickly pour the large amount of water into the toilet. The very fast discharge of water will draw out some of the water in the trap, and you will find that the water level is lower than normal. If you do not flush the toilet or otherwise add more water, the water level does not increase."
] |
[
"Also might be appropriate for ",
"/r/estimation"
] |
[
"How does a telescope in orbit perform a ten day exposure and not end up with a blurry image?"
] |
[
false
] |
When the hubble telescope took its deep field exposures it collected light for ten-eleven days. If the telescope is orbiting earth and the stars are also moving around each other and away from us, how do we end up with a clean exposure? Is it because the intensity of the light is so low? After the fact image processing? Positioning correction on the spacecraft?
|
[
"No, it is because the things it is imaging are so far away.",
"The angular resolution of Hubble is 1/10 of an arcsecond or 1/2,000,000 or a radian.",
" Even if it moves to the other side of the Sun, that's about 1000 light-seconds away. Nothing farther than 2,000,000,000 light-seconds or about 60 light-years will move to a different pixel.",
"(this is the third time in a day I am giving this link)"
] |
[
"Right but Hubble orbited at an elevation of 560km with a 96 minute period. So over a ten day period the satellite rotates around the earth 145 times and its end position in space is 25 million km from its start position due to the movement of earth around the sun.",
"So it seems to me that the telescope would have to constantly correct for its target in order to avoid smearing of some sort."
] |
[
"Right but Hubble orbited at an elevation of 560km with a 96 minute period. So over a ten day period the satellite rotates around the earth 145 times and its end position in space is 25 million km from its start position due to the movement of earth around the sun.",
"So it seems to me that the telescope would have to constantly correct for its target in order to avoid smearing of some sort."
] |
[
"So if you got shot and weren’t hit in any vital organs (heart, lungs, arteries etc), do you just need to stop the bleeding to survive?"
] |
[
false
] |
[deleted]
|
[
"Well, if by \"just stopping the bleeding\" you include surgery required to stop possible internal bleeding, I guess. Gut wounds are almost always a life-threatening situation—you really don't want to introduce the contents of your intestines to your insides. It can be a slow and agonizing way to go. If the bullet hit one or more bones, you'll probably also have to do something about that sooner or later. ",
"You also have to remove any dead or dying tissue to prevent the decay products from poisoning you, and you have to ensure that blood continues to get to the living tissue surrounding the traumatized areas in order to prevent further damage. After all, a tourniquet is a very effective method for stopping heavy bleeding from a limb, but unless you receive expert medical attention quickly, said limb is going to kill you too as it dies from oxygen deprivation."
] |
[
"Typically it is advised to limit the use of a tourniquet to a maximum of two hours to avoid compications; probably longer if the trauma is severe enough that it is clear that the limb or extremity could not be saved anyway.",
"Your body tolerates your gut bacteria well, and indeed depends on them, as long as they mostly stay ",
" you (which the surface of your gastrointestinal tract is). A perforation of the gut can let them and their metabolic products, as well as toxic waste products from your own metabolism, into your tissues and bloodstream. This can easily lead to a lethal infection, especially in the absence of antibiotics."
] |
[
"why is it dangerous to get shot in the intestines? What's the main souce of danger from that?",
"Infection. Your intestines are full of bacteria. ",
"The membranes of your intestines are a good barrier against the bacteria that live inside, and also have specialized immune cells to keep them under control. But as soon as the intestine is breached, the bacteria are released into the abdominal cavity (what we call the ",
"). ",
"That cavity is full of other organs and tissue that don't have the same defenses. The bacteria will take hold and cause infection and inflammation (called ",
"). If untreated, that is practically always fatal."
] |
[
"How does the International Space Station get continuous supply of oxygen for the crew?"
] |
[
false
] |
Do they get oxygen cylinders from earth continuously?
|
[
"Former ISS flight controller here:",
"The ISS has at least 4 pieces of hardware capable of generating Oxygen via electrolysis (3 Elektron assemblies in the Russian side, and 1 Oxygen Generating Assembly in the US side). As other people have mentioned, there are also Oxygen candles that can be used in emergencies.",
"However, I believe most of the Oxygen that the crew breathes is still brought up on the resupply vehicles (Progresses, ATVs, HTVs, COTS). It's not a perfectly closed system up there, especially with all of the maintenance issues associated with the WRS, OGA, and MCAs, so constant resupply of both water and compressed O2 is still needed.",
"I would encourage you to read the ",
"Wikipedia entry for the ECLSS system",
". It actually has a surprising amount of detail about the life support systems on the ISS. ",
": More ECLSS information available ",
"here",
", but this also includes hardware that isn't 100% active yet."
] |
[
"According to ",
"this",
" article from NASA they strip the oxygen from water using electrolysis. along with standard tanks of just O",
" (fixed thanks to evilhom3r) "
] |
[
"Like this: ",
"*_subscript_*",
"So: ",
"O*_2_*",
"Turns into: O",
"Note that this is ",
"custom CSS",
" in ",
"/r/askscience",
", and will not work on all of reddit (so if you're looking at this comment in your inbox, you won't see it)."
] |
[
"I'm pregnant- if I moved at a significant speed (hypothetically, I'm a jet fighter pilot) are the g-forces exerted on my fetus the same as on me?"
] |
[
false
] |
On one hand, my instant answer is that they would be, since we share a body space- but on the other, my baby is in a fluid-filled sac. Is there significant cushioning that happens that change the way the forces are exerted on baby? Not to worry, I'm not going up in any F-15s anytime soon, I was just mulling as I drove up a big hill this morning.
|
[
"Your fetus would be protected from some shocks, depending on the density of the amniotic fluid and the flexibility of your belly. But there wouldn't be any protection from a ",
"9g turn",
", for example. In that case, your fetus would be smashed down against your lower spine and pelvis."
] |
[
"Not if it's neutrally buoyant.",
"If the fetus is denser than the amniotic fluid, then as you pull Gs, the fetus would sink.",
"If the fetus is less dense than the amniotic fluid, then as you pull Ga, the fetus would float to the top (although the amniotic sac as a whole would smash down onto your pelvis... the fetus at the top of the sac).",
"So if the density is juuuust right, the fetus would stay in the middle.",
"However, while supported extremely evenly by the fluid (basically by strong fluid pressure on the undersides of the fetus's arms, legs, torso, and head), all the organs inside the fetus would experience G forces: heavy stuff would sink, blood rushing towards the bottom, etc."
] |
[
"Fascinating! Once it hits the extremity of the body cavity- it stil isn't undergoing the same amount of pressure? I'm having a difficult time visualizing. ",
"And if that's the case- in the case of a typical, nonpregnant, pilot, why do your organs not get crushed the same way a uterus+fetus would? Or do they? Is organ crushing a common thing amongst pilots that experience high g-forces frequently?"
] |
[
"If there were oceans of liquid water on the Moon, would there also be tides on the Moon?"
] |
[
false
] | null |
[
"The moon ",
"wobbles",
"Not a lot, but might be enough for some small variation."
] |
[
"but the water table (assuming uniform water spread across the moon) would be higher on the side that is facing earth than the side that isn't.",
"No. ",
"See this picture",
" - ",
" The water table is higher than average in both locations. It's lower on the \"belt\" that neither faces to or away from the Earth."
] |
[
"The same face of the moon is always facing the earth. This is because the moon's revolution around the earth and the rotation on it's axis is \"coupled\" in that they are going at relatively the same speed. Because the earth would always be facing one side of the moon, you would not have tides that go in and out, but the water table (assuming uniform water spread across the moon) would be higher on the side that is facing earth than the side that isn't."
] |
[
"When an electron emits a photon, what determines the direction it is emitted in space?"
] |
[
false
] | null |
[
"It's random, according to some probability distribution given by the specific nature of the transition.",
"If you have a macroscopic sample of unpolarized atoms emitting photons, the distribution will appear uniform."
] |
[
"It's truly random as far as we can tell."
] |
[
"Is it truly random, or do we say it's random because the mechanism for how direction is determined hasn't been discovered yet? "
] |
[
"Will the first Quantum computer disrupt worldwide encryption and security?"
] |
[
false
] |
[deleted]
|
[
"TL;DR: ",
"No",
"."
] |
[
"It's going to be a long slow process from first proof-of-concept prototypes (which we've seen), to prototypes that work reliably, to prototypes that have an actually useful capacity, to demonstrations of breaking weak crypto as another proof-of-concept, then finally something usable in a lab, then the NSA getting one and not able to use it all the time, to finally being commercially available someday.",
"The point is that there will be plenty of time for cryptographers to watch the technology develop and to respond. When big enough and reliable enough quantum computers become a reality, they will be able to instantly break today's asymmetric ciphers and significantly weaken (but not break) the symmetric ciphers. That may be a cause of concern for anyone worried about recorded communications, but by that time I imagine post-quantum cryptography research will have settled on some usable alternatives that can hold up.",
"It's an uncomfortable fact of most cryptography algorithms that they are not mathematically proven to be secure, it's just that after pounding on it for a long time they haven't found a way to break them. Probably that will happen again with post-quantum crypto. A cryptographer will say \"well I think this holds up against quantum computers, but I'm not sure, can you break it?\" And if no one does for a few years they'll say ok, that's good enough to use for now."
] |
[
"Man... that ending is depressing. Not in the scenario alone, but because it's so plausible."
] |
[
"Why is it that when I put in my correct password into the computer, it logs in almost immediately, but when I put in a wrong password, it takes significantly longer to reject me?"
] |
[
false
] | null |
[
"It's a deliberate pause that serves two purposes. The first is that it makes it take years for someone to try all possible passwords (called a brute force attack). The second is that if the computer always responded as quickly as possible a sophisticated attacker could measure the slight difference (nanoseconds) it takes the computer to reject a password to infer how many characters in the password are correct (called a timing attack); by making all rejections take the same amount of time this information leak is plugged."
] |
[
"It's not. But even in that trivial case:",
"Imagine if you had the computed hash and the reference hash in comp and ref. How do you compare them?\nmemcmp(comp, ref) ?\nIf you do that, memcmp will return faster if the hashes near the beginning and will return slower if the hashes diverge near the end. Now, hopefully your hash function is strong and you can't make anything out of this correlation but in other cases (where you have chains of blocks), you might be able to gain valuable information if you knew how far through the computation you got.",
"As an example, look up the how PKCS padding and CBC conspire to create vulnerabilities in a number of protocols including SSH and TLS.\n",
"http://www.thoughtcrime.org/blog/the-cryptographic-doom-principle/",
"In the post above, you can distinguish between two states (valid and invalid messages) based on the code returned. The attack can easily show up in scenarios where the server returns identical return codes but takes longer in one situation and shorter in another. It's still a signal and if you can reliably suppress the noise, you can use it as a side-channel to attack the encryption used."
] |
[
"Do you have more details on a timing attack? I was under the impression that most places hash your password when you sign up. Then when you try to log in, they check your hashed log-in password attempt against the hashed password they recorded in their database.",
"If a password is wrong, the hash will be different, no matter how many characters it's \"wrong by.\" For a password of \"password\", why would an attempt with \"passwort\" fail more/less quickly than \"applesauce\" ?",
"Is my understanding of passwords/log-ins fundamentally flawed? "
] |
[
"What would fire look like under a microscope?"
] |
[
false
] |
I'm in biology using microscopes, and this thought just hit me, so I googled the question and nothing came up. I was wondering if you guys could help me out.
|
[
"It would look like a floating aerosol of soot and dust, each particulate of which would be bright enough to be glowing. ",
"So you know how when you poke a log that's burning and a bunch of glowing embers are kicked up? Keep magnifying and it would look like that.",
"This is assuming a candle type fire. If you're talking about a gas type fire (like from a kitchen stove or Bunsen burner), it would look the same for all normal microscopes, because it's not glowing soot that you're seeing, but actually the atmospheric gases are so hot that ",
" glowing. So it would look roughly the same. "
] |
[
"While candle fires do contain a little bit of soot, most of the fire is wax turning to gas which then burs and so is not that unlike a natural gas flame. Even with wood fire, most of the flame is consists of hot gases, not hot soot."
] |
[
"It would look the same."
] |
[
"What two common objects could I use to accurately represent the Scale of the Earth to the Sun?"
] |
[
false
] |
Somewhat accurately would be a better way to ask it I suppose. Something like a basketball and a pea.
|
[
"http://www.synapticsystems.com/sky/solarsys/ssscale.html",
"Earth --> Sun",
"Hole Punch --> Beach Ball"
] |
[
"30.5cm radius",
"ftftfyfy"
] |
[
"30.5cm radius",
"ftftfyfy"
] |
[
"Why can I hear interference on headphones when someone gets a text message?"
] |
[
false
] |
Normally this happens, I get interference on my headphones, when my roommate gets a text message. I haven't run any tests but the typical situation is that I we will be in the same room and I will be listening to music on my headphones. If he gets a text I can hear a short distribution in the music. I don't think this happens when he gets a call or when I get any message or call. We both have different service providers, and like I said I should run some tests. Such as how far away does the effect work? Do types of headphones matter? Do different service providers or phones make a difference? What I would like to know, that I can't test for myself, is why does this happen? My guess is that it has to do with the EM waves, but then why don't I get this all the time since everyone is getting messages or calls? Second, is this all harmful to my health? My guess is that in such small amounts no but there is still some very minor damage.
|
[
"There is no health hazard at all.",
"What you are hearing is the effect of GSM timeslicing in the (audible) kilohertz range. In other terms it's a pulsed transmission, and you're hearing the pulses."
] |
[
"So I found out that putting ferrite choke cores on the cable leading up to my speakers would eliminate the buzz. Follow up question then, If I put ferrite choke cores on balanced audio cable, would it work? Where would be the best place to put them on?"
] |
[
"If I put ferrite choke cores on balanced audio cable, would it work? ",
"Probably",
"Where would be the best place to put them on? ",
"At the non-source end. ie. If you're connecting the output of your CD player to the input of your amplifier, you want the ferrite at the amplifier end."
] |
[
"Why does sensor size and not lens size matter for digital cameras?"
] |
[
false
] |
Media covering digital cameras and smartphones never stops mentioning sensor size. With the release of Nokia L1020, I've had enough. As a person holding a bachelors degree in physics I cannot understand why. After reading several articles on line and searching for it in this subreddit I can't shake this question: Isn't it the size of the lens, not the size of the sensor that matter for the amount of photons collected? The roll of the optics is to change the image size. With unlimited, perfect optics the same number of photons can be spread out or focused onto a sensor of any size. It's the size of the main lens that determines how many photons are collected. When discussing research telescopes no one talks about the size of the CCD (although the sensors are admittedly huge compared to consumer cameras) the most basic spec is the size of the main reflecting mirror which is equivalent to the outermost refracting lens. Please explain why I am wrong that a never mentioned spec, lens size, is less important than the much heralded sensor size. I feel like my question has been answered but in bits and pieces so I will summarize what I have gotten from all the posts. My original understanding of photon collection was correct. However: For CCDs (and I think CMOS sensors) the size of each pixel determines the amount of light that can be collected and thus the dynamic range. For well lit pictures this is important to prevent washing out the bright areas. A larger sensor size allows for less powerful and lower quality optics. This is especially important in compact smartphone cameras. Although lens technology does improve, for a given depth there is a close relationship between sensor size and useful lens size. For the same reason as the above point, for a give thickness of the optics stack, a larger sensor allows for a narrower depth of field. This is considered desirable for the artistic effect. Depth of field is traditionally attributed to the size of the aperture, much like total light captured, aperture is restricted by the thickness of the stack. That pretty much answers my question. My main fault lay in taking the physicist approach of assuming perfect optics. It turns out that what you learn about 4 meter telescopes might be a little different in a smartphone 8mm thick.
|
[
"Yes, the sensor ",
" does, in fact, matter. A larger sensor has a larger area to collect photons. As you mention above, a better lens could, theoretically, just focus that light down onto a smaller sensor. However, the more you bend light, the more artifacting you get. For example, you get a lot of chromatic aberration (when some colors are separated from the others, similar to a prism effect) from a zoom lens at the two bounds of its focal length, hence why professional photographers stick in their lens's sweet spots which are generally the middle of the zoom range. Lens's that bend light more (see, long and short focal length) are more expensive because they have to be built to a much higher standard than the mid focal length lenses do. With a small sensor, even a little chromatic aberration becomes extremely pronounced. The same goes for most adverse lens effects, meaning you need a much more expensive lens. ",
"You also have the issue of current sensor technology. A larger sensor is more sensitive to light than a tiny sensor. With that extra surface area, you can get much better low light performance. That tends to be why a 35mm DSLR has vastly superior performance to a cell phone camera with more megapixels. Even with the same number of photons hitting a smaller space, you couldn't get the same kind of sensitivity using current technology. Thats not likely to change, as you would increase the sensitivity of both small and large sensors. ",
"Also, there is the artistic aspects. A larger sensor has a much more narrow depth of field (for the same field of view) than a smaller sensor. That means a large sensor gives a much more pronounced extraction of the subject from the background than you can get from a small sensor. Again, its probably possible to compensate with a different lens, but it would be much more expensive and would suffer from the consequences of that. "
] |
[
"The pixels are ",
" in modern CCD/CMOS sensors that they fill up. Think of them as tiny buckets. If your bucket holds a thousand photons, you've got 1000 different brightness levels you can detect. If it's got room for ten, well, you get the idea. A larger sensor has both a wider dynamic range (a bigger difference between the lightest it can detect and the darkest it can detect) and a greater number of \"shades,\" simply because the buckets are bigger.",
"Small photosites are more sensitive to noise for the same reason. If you've only got room for ten photons before you max out, and you get one random one from heat, the noise is huge. But the same random IR noise is two orders of magnitude smaller if your bucket holds a thousand photons.",
"Those are the issues most people are concerned with. But there are also optical concerns-- a physically larger sensor produces a narrower \"focal plane\" from an equivalent lens. To get a similar focal plane on a smaller sensor, you would need a much larger aperture-- but if your hypothetical big-sensor camera is using a lens that's up against our engineering limitations for aperture (like, say f1.4) we can't make a lens with an aperture large enough for a smaller sensor to have the same resulting depth-of-field.",
"TL;DR: little sensors (usually) mean little pixels. little pixels mean more noise, less dynamic range, and fewer brightness levels. little sensor dimensions mean it's difficult to make a lens that gives a narrow focal plane."
] |
[
"The material that collects photons does not change the density of photons it can absorb just because you use smaller pixels nor just because you can focus more photons on it. You could put 20 pounds of glass in front of small sensor and get the same number of photons onto each pixel but you're going to bleach out the material from that much intensity and you're also descretizing the number of photons that each pixel can absorb in a given amount of exposure time by making them smaller which creates noise."
] |
[
"If I was standing on Phobos or Deimos, could I throw a ball fast enough to make it goes into orbit around the moon itself?"
] |
[
false
] |
Not fast enough that it leaves the gravity of the moon altogether but fast enough that it orbits.
|
[
"Phobos and Deimos have escape velocities of about 40 km/h and 20 km/h, respectively. The speed required for a circular orbit at surface level is (escape velocity)/sqrt(2), so 28 km/h and 14 km/h. That's well below the typical speed of a pitched baseball.",
"However, both Phobos and Deimos are so small and so close to Mars that tidal effects would tend to make any such orbit unstable. For both bodies, but especially for Phobos, the ",
"Hill sphere",
" isn't much bigger than the moon itself."
] |
[
"I read this and instantly laughed at the possibly of a far into the future youtube video hit. \"Punk kid knocks space station/satellite out of Phobos orbit with baseball thrown from the surface\""
] |
[
"With no atmosphere it might be hard breath on either one."
] |
[
"If we can make up a number for \"√-a\". Why can't we do the same for \"(-a)!\"?"
] |
[
false
] |
I googled about this and I can't really understand well the results. Could someone please explain this briefly?
|
[
"There's no reason you couldn't, but the question is whether it would be useful. A major impetus behind the definition of complex numbers is that it gives solutions to otherwise unsolvable polynomial equations (most famously x",
" +1=0), and it turns out to have lots of deep connections to other parts of mathematics, because you can do a lot of things with polynomials.",
"There are ways to define a factorial in such a way that (-n)! has a value (most famously with the gamma function), and I'm sure that there are uses for it that I'm not aware of. In the field that I've studied however, factorials are most often used to represent permutations of a set, and so there isn't really a reason to define a value for negative inputs.",
"I'm not sure if this answers your question at all, since it's late and I'm typing this on my phone. I'll do my best to address any follow-up questions."
] |
[
"That's a very good point. ",
"/u/oldhelixman",
" has a very good overview of the gamma function extension, and he also points out that it also doesn't \"work\" for complex numbers with a negative real part.",
"Now someone might ask why we can't just declare that that (-n)! is zero for all n>0, or ssomething like that, in order to get a function that doesn't have crazy things like poles at the negative integers, but it turns out that if you do this then your function won't have lots of nice properties like smoothness and being analytical."
] |
[
"Note that using the Gamma function doesn't work for (-n)! when n is an integer, because it has ",
"poles",
" at those points."
] |
[
"are there clocks on Curiosity and/or Voyager so we can measure gravitational time dilation?"
] |
[
false
] |
if so how much difference is there between time in space and time on Earth?
|
[
"We don't need to go as far as Curiosity or Voyager for that. The GPS satellites are affected by both special relativity (because they're moving quite fast relative to us) and general relativity (because they're farther away from a massive body than we are). The net result is that a clock on board the satellites would tick faster by 38 microseconds per day."
] |
[
"If there is any type of processor on board there is a clock."
] |
[
"It's accuracy probably isn't good enough to measure the effects of gravitational time dilation. You'd need atomic clocks for that."
] |
[
"Why can't you vent radioactive steam into a container or pressure vessel?"
] |
[
false
] |
Why do they vent it directly into the air? It seems plausible to me to vent it into a lead container of some type to prevent the radioactive material from entering the atmosphere. Obviously if this was possible, someone else would have thought of it.. so what am I missing?
|
[
"I think you're missing the fact that they already have one. It's called the drywell, and when the steam pressure builds up there, steam is forced into the torus or pressure suppression pool, located at the bottom of the reactor. The steam is forced through the water, condensing it and lowering the pressure.",
"But the torus can only handle so much pressure. So if that fails, they have to vent it to the outside, before which it's passed through filters to remove radioisotopes."
] |
[
"They weren't exactly prepared for this situation as is evidenced.",
"It's surely possible, but if they wanted to make the plant safer they would have just retrofitted it's backup power to withstand a 9.0 quake rather than built a steam venting holding tank.",
"It was, remarkably, only rated to withstand a 7.9",
"edit: To answer your question, it's cost, cost is the factor that kept the plant from being retrofitted to withstand a larger quake. If funds were unlimited the entire plant could have been made safer in several areas, although if funds were unlimited it would have just been replaced with a modern plant."
] |
[
"ok.. After all those things have taken place and the gas is finally set to escape to the atmosphere, why not pipe it into another lead container instead? Is it simply the cost and/or mechanical feasibility and risk that prevents it? Is so much steam vented as to create a situation where you would need thousands of lead tanks to contain it all?"
] |
[
"Meters are sufficient to measure distance in one, two, or three dimensions. What unit takes its place for the fourth (spacetime)?"
] |
[
false
] | null |
[
"Spacetime is just space and time. If you call the first three dimensions the dimensions of space, then the fourth is just time. So any units of time are appropriate to describe the coordinate in this dimension."
] |
[
"So, what, like a meter-second? I understand that if you were to use vectors to describe relative displacement, you'd have <x> for 1D, <x,y> for 2D, <x,y,z,t> for 4D. But in 1D-3D the magnitude can be expressed as a constant + unit, eg: \"It's 3 meters away.\" Is there not a single unit for describing similar with the addition of time? Or do we only add another coordinate, eg: \"It's 3 meters and 42 seconds away\"?"
] |
[
"So, what, like a meter-second?",
"No, just a second. Or a year, or any other unit of time.",
"If you want all coordinates of spacetime to share the same units, then just multiply the time coordinate by c."
] |
[
"I understand that certain elements are only theoretical or mostly can only exist for a extremely short time before degrading. Are there any elements like this that could be combined into a compound that would make them stable?"
] |
[
false
] |
Please keep in mind I'm talking about the extreme elements here that are so large that they're not even close to stable. Thanks ! Title edit: an* (sorry, forgot to proofread).
|
[
"Not really.",
"The instability of these elements is a property of their nuclei, and most possible mechanisms by which nuclei can decay are not dependent on their electronic environment - which is what you change when you incorporate an atom into a molecule.",
"Nuclei which decay by electron capture are an exception. Electron capture, as the name suggests, involves the capture of an outer-shell electron by the nucleus. Incorporating these nuclei into compounds (i.e. reducing the number of electrons in the outer shell) can slightly alter the half-life (",
"Wikipedia",
" cites an example where the half-life is altered by 1%). In order to make the nucleus 'stable', as in to eliminate this decay pathway entirely, you'd need to strip all of its electrons - which isn't typically compatible with the formation of stable compounds."
] |
[
"He didn't mention anything about how heavy the elements are, nor does that statement have anything to do with OP's question."
] |
[
"That's a good question. I'm not an expert on radiochemistry so hopefully someone more knowledgeable can give you a specific answer. Some general comments though:",
"The result is going to depend on a number of factors, including the specific decay process (so which nuclides are being converted, and what compounds they are part of), the chemical environment (what other atoms are nearby? Solvent? Solid substrate? etc), and the kind of decay (which particles are emitted? Do they interact with other atoms/bonds in the molecule?)",
"There are cases where the resultant compound is pretty stable - for example, ",
" Tc is a meta-stable isotope of technetium which is widely used in medical imaging. It decays to the corresponding ground-state isotope ",
" Tc by gamma emission (mostly). For medical imaging purposes, Tc is typically bound to a ligand; the ligand will survive the radioactive decay from ",
" Tc to ",
" Tc.",
"In other cases the initial and product nuclides will have very different chemistry. I don't know any good examples of this - hopefully someone else will - but you can easily look at an isotope table to find a hypothetical example. Which is what I just did: so consider the decay of ",
" K to ",
" Ar by beta emission, which has a half-life of minutes. While potassium forms stable ionic compounds - inorganic salts like KCl, or soaps of fatty acids, for example - Ar is a noble gas and thus pretty inert chemically. I don't know exactly what would happen if you (hypothetically) prepared ",
" KCl and allowed it to decay, but I imagine it'd be pretty reactive: presumably Ar and Cl2 gas would be liberated, and you'd probably get chloride radicals flying about - not even taking into account the beta particles being generated. I don't think it'd explode, but I wouldn't want to do that experiment.",
"I don't know about your specific example as I'm not terribly familiar with the chemistry of elements this heavy. Th can form hexa-fluoride salts, so it's at least plausible (without knowing any details) that the decay would be relatively controlled in this compound."
] |
[
"What, in your opinion, is the biggest ethical issue in science?"
] |
[
false
] | null |
[
"This is something that is absolutely subjective so there are no real answers. This is the sort of question we tend to avoid because it is open ended ",
" ethics come into play. These generally degenerate into a gigantic mess. ",
"However, this would be right at home in ",
"/r/askphilosophy",
" or ",
"/r/philosophyofscience",
"."
] |
[
"Sorry about that, I'm new here and thought this was a more general discussion subreddit rather than a Q and A type deal. You're right, although it'd be an ",
" mess!"
] |
[
"It's all good. We're Q and A here, which is a bit different than most reddits. Those two reddits I listed are actually pretty active so I think you'd get a good response. I know a lot of panelists frequent those reddits.",
"The main issue is that not just panelists weigh in but ",
" does whether they are qualified to talk about it or not. :D If you ever have any questions just send us some mod mail!"
] |
[
"Do certain languages have higher percentage of people who stutter?"
] |
[
false
] |
I believe (as layman) that some sounds 'trigger' a stutter. Different languages have different sounds, so maybe there are languages that trigger stuttering more than other languages. And if so, which languages has the most people who stutter?
|
[
"There's another aspect to this. I came across what I thought was a stutter in Zulu speakers when I visited South Africa. Zulu people, speaking English, would sometimes say the beginning of a word over and over several times before continuing. I assumed it was a stutter, but nearly everyone did it. Then I noticed they did the same thing in their native tongue. Eventually I figured out that it wasn't a stutter, per se, but the Zulu version of \"ummm\" or \"uhhh\", the common English sounds for \"I'm still formulating my next thought\". Zulu speakers never used those \"ummm\" or \"uhhh\" sounds, but instead would just repeat the first syllable of the next word they were saying until they were ready to proceed. Served the same function, but sounded quite different.",
"I learned later that different languages have very different means of expressing that they're still formulating the next thought. It's not a word, but it conveys meaning nonetheless."
] |
[
"We don't know for sure. There are some theories that certain languages have lower rates (and there's some epidemiological data showing that it might be different in certain countries) but there's a lack of good data to back these theories up.",
"See here:\n",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3687212/"
] |
[
"I’m kinda curious now. With different languages, you’ll have different diets, possibly different races, different regions on the globe, etc. I wonder if it’s a language thing, or more like a regional (lending itself to a dietary or some other odd detail) or some other odd minute detail. "
] |
[
"Does sitting posture have any affect on learning ability, problem solving, etc?"
] |
[
false
] |
For example, purely anecdotaly, I think that I tend to learn better when I'm comfortable such as being slightly reclined or slouched. But I think I do things better, such as writing code, in a more upright position. That's probably purely personal preference, but I was wondering if there was any scientific corrilation between posture and performence at all.
|
[
"http://en.wikipedia.org/wiki/Context-dependent_memory#Multiple_learning_context_technique",
"This is the closest I could find. Hope it helps."
] |
[
"I've read of a study that discussed how various positions and locations we are in affects our ability to learn and retain information. For example, changing where you normally sit or recline to read and study every once in a while can have a positive effect on learning.",
"Some of it is also personal preference, like you say. "
] |
[
"Do you have any relevant links?"
] |
[
"What (if any) are the functions of the appendix?"
] |
[
false
] | null |
[
"Building on this, there is some evidence to suggest that the appendix still plays a role in gut immunology. If you look at histologic sections of the appendix and compare it to the small bowel (jejunum and ileum) you'll find a high concentration of Peyer's Patches throughout the appendix which are more spaced out in the small bowel. These are related to gut immune cell production and/or activation. Since the appendix has a very large concentration of these within it's wall there may be some role in gut immune responses."
] |
[
"Building on this, there is some evidence to suggest that the appendix still plays a role in gut immunology. If you look at histologic sections of the appendix and compare it to the small bowel (jejunum and ileum) you'll find a high concentration of Peyer's Patches throughout the appendix which are more spaced out in the small bowel. These are related to gut immune cell production and/or activation. Since the appendix has a very large concentration of these within it's wall there may be some role in gut immune responses."
] |
[
"The appendix is often thought of as unnecessary, however recent research suggests it can act as a refuge for bacteria to hide out in case there is a wide die off of normal gut bacteria (i.e. due to infection with a pathogenic bacteria that outcompetes the normal gut flora).",
"",
"Ref: Heather F. Smith, William Parker, Sanet H. Kotzé, Michel Laurin. ",
". ",
", 2017; 16 (1): 39 DOI: ",
"10.1016/j.crpv.2016.06.001"
] |
[
"Do mild infections yield mild immune 'memory'?"
] |
[
false
] |
TLDR: Is the probability of re-infection from a virus proportional to the severity of the initial infection? I have heard a lot speculate that viruses evolve to be less severe, but hadn't seen any mechanism posited for that to happen. Thinking about Omicron and the number of people who have already been infected with Covid, I thought that the bigger 'advantage' could be reinfection rather than pure transmissibility (not sure the correct term). Comparing two variants, one severe and highly transmissible but can't re-infect versus one mild, mildly transmissible (effective R slightly over 1), but CAN re-infect. Over a long evolutionary history, the second could be the one that survived. I'm naive (to the science, not immunologically), so a pointer to introductory or moderately specific texts would be appreciated.
|
[
"What's your definition of a mild infection? A vaccine with a weakened or attenuated version of a virus could be characterized as a \"mild infection\" and if it's successful it could lead to life-long immunity. Omicron has more mild symptoms but reproduces inside the body 70 times faster, strictly by the numbers of viruses produced Omicron is more severe infection. In terms of advantage, an infection with mild symptoms would be more advantageous and likely to be naturally selected for since people are less likely to stay home with a mild infection giving the virus more opportunity to spread. The virus doesn't need to kill you to be successful, it just needs to spread."
] |
[
"I was thinking about the rhinoviruses and non-SARS coronaviruses that cause common colds. Are there just so many rhino & coronaviruses that do not have cross-serotype antibodies that every cold is our initial exposure?",
"Great point about the weakened/attenuated versions acting as vaccines. Upon further review, I think my idea is off base."
] |
[
"That 70x number is not definitive and just coming out.",
"\"Dr. Michael Chan Chi-wai and a team of researchers found that the variant replicates much faster in the bronchus, which connects the windpipe to the lungs, 24 hours after infection. Yet it reproduces more than 10 times slower in the actual human lung tissue, they said.\"",
"\"Omicron’s rapid replication in the airway may explain why it transmits faster than previous variants of the virus, but lower infection in the lungs may indicate that it causes less severe disease, according to the study’s findings.\"",
"https://www.cnbc.com/2021/12/16/omicron-lung-infection-less-severe-replicates-faster-than-delta-in-human-airways-study.html"
] |
[
"Why aren't we already at maximum entropy?"
] |
[
false
] | null |
[
"We haven't had enough time since the Big Bang yet. The Big Bang was really low-entropy – like, REALLY low-entropy – so it takes a lot longer than 14 billion years for the heat death to come about.",
"In a sense, it's by chance that we exist now, as the time before the heat death of the universe is in theory a lot shorter than the eternity after. On the other hand, nothing sentient could exist after the heat death of the universe, so this would be the answer any time this question is asked."
] |
[
"No one knows. You can make the same sort of anthropic argument and say we couldn't be here if it hadn't been... but if you think that the universe was a one-time event then that's not quite the same argument. "
] |
[
"Why was the entropy so low?"
] |
[
"Is there any scientific evidence of the idea that harmful toxins build up in your body, that need to be removed by special diets, fasting, or other forms of cleansing?"
] |
[
false
] | null |
[
"From NPR",
"From the article: ",
"\"The body has its own amazing detoxification systems: the liver and the kidneys,\" says Ranit Mishori, a faculty member in family medicine at the Georgetown University School of Medicine who has reviewed the medical literature on colon cleanses. \"Unless there's a blockage in one of these organs that do it day and night, there's absolutely no need to help the body get rid of toxins.\""
] |
[
"First define toxin. Whilst there are actual toxins in existence, people talking about fasting and detox are often not being precise and scientific. Not only do they misuse the word 'toxin' they don't have examples of specific toxins or toxic substances that their diet will help remove or any explanation as to the actual process that is happening. ",
"I guess some people see it as being like scrubbing dirt from a floor - all vague and such. The chemistry inside your body isn't really like that.",
"To remove a substance from your body it generally needs to be broken down or physically go through your digestive/kidney system. I can't see how a 'cleansing' diet would make that happen if your kidneys and liver etc couldn't manage it. ",
"The other thing to consider is that the presence of a 'bad thing' inside your body isn't ",
" cause for alarm. Often the difference between 'harmful' or not is based on the amount in your body and over what time period. A tiny amount of mercury from a vaccination is not harmful and the body can deal with it.",
"Remember, too, that 'good' substances can be bad for you. Excessive doses of things like iron, calcium and fat soluble vitamins can be harmful."
] |
[
"On the other hand, none of the cleanse diets or preps will get them out either."
] |
[
"When did the various celestial bodies in our solar system form?"
] |
[
false
] |
While looking around I noticed that the given age for our sun is around 4.6 billion years, and for every other planet it's ~4.5bya (with Earth given a few more significant figures). I understand that it's nearly impossible to date other planets (and even our own), but is it actually accepted that most of the planets formed around the same time? There was a question posed here a while ago that asked what "event" is given for the formation of Earth (when it first began accreting mass, when it was "done," when we got the moon, etc) and the consensus was "it's pretty arbitrary," so I understand a lot of this stuff is speculation, but it just doesn't seem correct to say that all of our planets began forming at the same time.
|
[
"The planets took a few million years to form. The gas giants grew the fastest, and the terrestrial planets took a bit longer, but the difference is small compared to the 4600 million year age of the solar system."
] |
[
"The only thing that can be said is that it's after the ",
"protoplanetary disk",
" had formed. The estimations come from models and scientifically based guesses. The most probable thing that happened. ",
"We know what the different stages are in the birth of a solar system and a star. The timelapses are difficult to predict."
] |
[
"I almost went crazy for the title of this post but then I read your description and calmed down. But honestly you answered your own question there mate. We cant ever truly know something like this unless we invent a time machine. Any \"historical\" sciences are just professional guess work and the farther back they go, the more guessing they do. For some reason it's taught in schools like fact when really astronomers and cosmologists have no clue. If they can't see it or use instruments to test for something, its a theory."
] |
[
"Has a solution been discovered for the curvature of space-time due to the hydrogen atom?"
] |
[
false
] | null |
[
"Sure, you could create a stress-energy tensor for the hydrogen atom, plug it in to the Einstein field equations, and see what you get, but the question is \"are the Einstein field equations still valid on the quantum scale?\" We don't really know how gravity works on really small length scales."
] |
[
"If you want a more technical answer, let me give you.",
"The problem is not really Heisenberg. Well, in a sense it is, but it is derived from a more fundamental fact, which is: Observables are operators. In quantum physics, if I write momentum \"P\", I don't really mean that P is a number, but \"P\" is like a machine that acts on an object which I call state, and this state is a vector in a very special space called the Hilbert space. In order to get a number out of this guy, I need to compute \"expectation value\" of P, in which I kinda average out \"P\" over an arbitrary set of of states. That's why we have uncertainty, because whether I act on states first using \"P\" or using \"x\" matters. So determining the position and then determining the momentum is not the same as doing it the other way around.",
"So far, so good, but what does have to do with curvature?",
"Well, when you write einstein's equations, you have two sides of the equality. On the LHS you have the curvature tensor, and this guy is nothing more than a number - (well, a tensor field, an object that takes value on space-time coordinates and spits out a matrix, which is an appropriate combination of derivatives on the metric). ",
"On the right-hand side you have the energy-momentum tensor. This is a more complicated object, because if you believe that quantum mechanics has something to do with nature, you need to express it in terms of operators. For big objects, it's ok for us to disregard the fact that this guy is made of operators and just use the expectation value of the operators - because the effects of operators non-commuting is very suppressed (which is like saying that the Heinsenberg's uncertainty principle doesn't matter on big scales), but at small scales, we have to take 100% into account the fact that we have operators instead of numbers.",
"The solution to that is to promote the LHS to operators as well - which is how one creates a \"quantum field theory\", opposed to the \"classical field theory\" which is general relativity. The only problem is that this procedure only makes sense at very small energy scales, because at very high energies you get problems because the non-renormalizability of the theory (which is like saying that at the order of the Planck scale, we need a better theory). ",
"So yes, as you very well thought, the curvature at small scales is unknowable - simply because classical concepts such as curvature don't make sense in quantum mechanics. We have though something that is the \"curvature operator\", which can be in fact calculated for the hydrogen atom. But for all practical purposes, gravitational effects are suppressed by ~24 orders of magnitude compared to the electromagnetic interaction for the hydrogen atom, so we're safe!"
] |
[
"If you want a more technical answer, let me give you.",
"The problem is not really Heisenberg. Well, in a sense it is, but it is derived from a more fundamental fact, which is: Observables are operators. In quantum physics, if I write momentum \"P\", I don't really mean that P is a number, but \"P\" is like a machine that acts on an object which I call state, and this state is a vector in a very special space called the Hilbert space. In order to get a number out of this guy, I need to compute \"expectation value\" of P, in which I kinda average out \"P\" over an arbitrary set of of states. That's why we have uncertainty, because whether I act on states first using \"P\" or using \"x\" matters. So determining the position and then determining the momentum is not the same as doing it the other way around.",
"So far, so good, but what does have to do with curvature?",
"Well, when you write einstein's equations, you have two sides of the equality. On the LHS you have the curvature tensor, and this guy is nothing more than a number - (well, a tensor field, an object that takes value on space-time coordinates and spits out a matrix, which is an appropriate combination of derivatives on the metric). ",
"On the right-hand side you have the energy-momentum tensor. This is a more complicated object, because if you believe that quantum mechanics has something to do with nature, you need to express it in terms of operators. For big objects, it's ok for us to disregard the fact that this guy is made of operators and just use the expectation value of the operators - because the effects of operators non-commuting is very suppressed (which is like saying that the Heinsenberg's uncertainty principle doesn't matter on big scales), but at small scales, we have to take 100% into account the fact that we have operators instead of numbers.",
"The solution to that is to promote the LHS to operators as well - which is how one creates a \"quantum field theory\", opposed to the \"classical field theory\" which is general relativity. The only problem is that this procedure only makes sense at very small energy scales, because at very high energies you get problems because the non-renormalizability of the theory (which is like saying that at the order of the Planck scale, we need a better theory). ",
"So yes, as you very well thought, the curvature at small scales is unknowable - simply because classical concepts such as curvature don't make sense in quantum mechanics. We have though something that is the \"curvature operator\", which can be in fact calculated for the hydrogen atom. But for all practical purposes, gravitational effects are suppressed by ~24 orders of magnitude compared to the electromagnetic interaction for the hydrogen atom, so we're safe!"
] |
[
"How are electrons \"high energy\"?"
] |
[
false
] |
I read in this AP bio book that as an electron is passed though a series of increasingly electronegative protein pumps to pump H+ across a membrane. The electron is no longer high energy after doing so. Does this mean that an electrons potential energy is measured by it's association with an electronegative molecule/particle. such as in a molecule that has C and O sharing electrons, the closer the electrons are to the oxygen (Which is more electronegative than carbon) per the amount of valence electrons is the potential energy of that molecule? Is this redox in a nutshell? ~A sophomore in high school
|
[
"Sounds like you’re talking about oxidative phosphorylation. Your instinct is right, A free electron is hard to harness and control. ",
"Big fat analogy: metabolism can be though of as a slinky walking down stairs. Each step down is slightly more energetically favorable for the electron than the last, and so it steps downwards through different carrier proteins, and this motion can be used to create the proton gradient, energy carrying molecules, etc. in aerobic organisms, the floor for our biochemical slinky is oxygen, which acts as our terminal electron acceptor. "
] |
[
"Yes. First off, energy is always a relative quantity, so you're comparing the energy of compounds with and without the 'extra' electron, i.e their redox potentials. (on an aside here, the electron transport chain does not involve any truly free electrons; they're always bound to some molecule, even during intermediate steps)",
"Now, for most organisms except some bacteria and archaea and other weird and interesting thing use oxygen as the terminal electron acceptor, so the final reductive half-reaction (which occurs in cytochrome c oxidase) is:",
"½ O2 + 2H",
" + 2e",
" --> H2O",
"In that reaction the electron come from cytochrome C, so you also have the oxidative half-reaction:",
"CytC (red) --> CytC (ox) + e",
"Now the first half of the reaction here is exergonic (gives off energy) and the second half is endergonic (requires energy), and the total reaction has to be exergonic for it to happen - actually more than just a little exergonic, because some of the reaction energy is used by CcO to pump protons. (so Cytochrome c Oxidase isn't really a very good name, because it's the reduction of O2 and proton pumping that's really the interesting reaction, not the oxidation of Cytochrome C) ",
"So the electron bound to CytC here is 'high energy' relative to adding that electron to O2 and forming water. Meaning it requires less energy to remove it from CytC, and thus lowers the energy of the overall reaction. When an electron bound to a compound has high energy it means the compound is easily oxidized, the electron easily removed. (and vice-versa) Which is not the same thing as a 'high energy' electron in particle physics where they're talking about an ",
" electron. But that's not something you run across in chemistry much, where electrons move straight from atom to atom through quantum-mechanical tunneling. "
] |
[
"Pretty much this. Thinking about the electron itself is maybe a bit confusing to the picture-- the passing of the electron down these various intermediate molecules through the cytochrome system is, essentially, analogous to electricity in a circuit. The body exploits this movement of energy by capturing it in ATP which it can readily use to fuel cell processes. Furthering the circuit analogy, without oxygen aerobic respiration stops-- because the intermediate molecules build up till saturated because they can't be recycled. This is analogous to breaking an electrical circuit, stopping the current."
] |
[
"Why some stars in the night sky look like they are changing colours or flashing light rapidly?"
] |
[
false
] |
I just saw one today in the clear night sky and was wondering. Sorry if it’s a silly question. Thanks!
|
[
"Small fluctuations in our atmosphere cause the light to refract and can appear as focusing, diffusion, or slight colour changes. Observatories are built as high as possible on mountains to try to reduce the amount of atmosphere they have to look through, and the Hubble gets a very clear view because it is in space. Modern observatories have adaptive optics which counter the effects of the atmosphere and they use a reference 'star' created by a laser shining high in the atmosphere and calculating how the light is being affected."
] |
[
"Because planets are closer, they aren’t just a single “point source” and aren’t as affected by things like atmospheric pressure changes. With a regular telescope, you can see them as circles, whereas the stars will still just be the same pixel!"
] |
[
"Does this sketch help",
"?",
"The angular diameter of Mars is of the order of 10 arcseconds or 0.5 m over 10 km, the angular diameter of stars is of the order of 0.001 arcseconds or smaller, 0.05 mm over 10 km."
] |
[
"Is there a study on the average/ most common answers for the original Rorschach test?"
] |
[
false
] |
[deleted]
|
[
"It's a constructed (synthetic) test - the interpretion table begins with the most obvious patterns. An overall statistic is hard, because it's an open question without limitation what and how you say your inspection/impressions. That is also why Rorschach Test is critisized as non-normalised (maybe that's not an english term, i don't know) See it as Trend-o-meter ;-) for holomatic, phantastic, pendantic, minimalistic, nerdic, ... personality"
] |
[
"This is spot on. The question is basically asking for the the interpretation table.",
"I always describe the origins of test when I talk about it. Ink blot interpreting stated as a ",
"parlor game",
" - either for fun or to predict someone's future ",
" tarot cards and palm reading, or maybe both. And Hermann Rorschach, being the good scientist he was, started to notice some trends (perhaps his own projections) in how different people interpreted the same shape."
] |
[
"There are tables of content responses used as one part of the test in order to provide some objective determination of common vs. uncommon responses. They are included in the scoring manuals for the various scoring systems, including the R-PAS, the one most current/updated now in use."
] |
[
"How badly would i be hurt if a single atom of anti-hydrogen was shot onto my finger?"
] |
[
false
] | null |
[
"You wouldn't notice."
] |
[
"Aw man i was hoping for something a little more spectacular. Thanks for the answer!"
] |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):",
"megathread",
"If you disagree with this decision, please send a message to the moderators."
] |
[
"How is a number proved to be irrational?"
] |
[
false
] |
I was reading the pi day megathread and saw this reply: To phrase it a bit differently, how can you prove a number cannot be expressed as a ratio of two numbers?
|
[
"Usually you assume that it ",
" be expressed as a ratio of two integers, and then derive some kind of contradiction, proving your assumption false (i.e. it ",
" be expressed thus), although it doesn't have to happen that way. In general, there's no easy way to answer questions like 'how do you prove things' because then mathematicians would be out a job.",
"For example, let ",
" be a positive integer that is not a perfect square, and assume that there are two integers ",
" and ",
" such that sqrt(",
") = ",
"/",
".",
"Then ",
" = ",
"/",
", so ",
" = ",
".",
"Now, if you are familiar with the fundamental theorem of arithmetic, you may know that squares have an even multiplicity of each prime factor, but since ",
" is not a square, there must be a prime factor of ",
" with odd multiplicity. The multiplicity of this prime in the product ",
" must then be odd (since ",
" only has even multiplicities), so it cannot be a square as indicated. Contradiction.",
"So square roots of integers cannot be rational (unless the integer in question is a square, in which case the root is also an integer).",
"Proving that pi is irrational is ",
"a tad more involved",
" however."
] |
[
"Nobody in mathematics calls it that. \"Proof by contradiction\" is a vastly more common terminology."
] |
[
"This video",
" shows a simple proof to why squareroot 2 is irrational."
] |
[
"How would time be dilated for a white hole?"
] |
[
false
] |
I know that the closer you get to a black hole you are moving slower relative to everything else. but if anti gravity were possible would time be moving too fast for us to actually see matter of reverse gravity.
|
[
"You're multiplying, not adding. If it's less than one it slows and if it's greater than one it speeds up. If it's negative it would go in reverse. General relativity is time reversible, so it really is both the negative and positive value simultaneously. The metric tells you how fast you go through space and time, but not which direction you're going in."
] |
[
"From what I can gather, it's just extending the metric on a black hole. It sort of exists infinitely before the black hole. If you're outside the black hole, you're also outside the white hole. And I'm pretty sure the time dilation would be the same on the inside. As would the gravity.",
"If you mean a negative mass equivalent of the black hole, I have no idea if this works but you could try plugging a negative Schwarzchild radius into the ",
"Schwarzchild metric",
". Time dilation in a regular black hole has a factor of sqrt(1 - r",
"/r) where r",
" is the Schwarzchild radius and r is your distance. For a negative black hole with the same magnitude of mass, you'd get time dilation of sqrt(1 + r",
"/r). So if you're at the distance the event horizon would be, instead of time passing at a rate of zero and you being completely frozen for eternity, it passes at sqrt(2) times speed, so you experience 1.41421 seconds in the time it takes someone else to experience one. Also, there's nothing at all like an event horizon, so this isn't really comparable to a black hole at all. It's perfectly possible to both approach and leave the singularity."
] |
[
"Negative mass black holes are sometimes called 'repulsons' because they do actually exhibit repulsive gravity. They show up occasionally in semiclassical approximations of string theory, but they're thought to be unphysical and it's thought that in the full string theory description they get 'resolved' or basically capped off at a certain finite radius (although that radius is not anything like an 'event horizon')."
] |
[
"What technologies actually use our understanding of general or special relativity?"
] |
[
false
] |
I've got this friend who keep saying that all this new astrophysics research is totally useless, and that Einstein's theories don't actually help anyone. But I know that's not true. Can you guys help me out and give some examples of of technologies that wouldn't be possible without understanding relativity, and how? Thank you so much!
|
[
"That might be the only one that uses the general theory, but the special theory is used in many places.",
"Off the top of my head: Radio communications, police speed radar/laser guns, particle accelerators for medical use, nuclear power, and radiation dosimetry all invoke it.",
"Edit for more: Mössbauer spectroscopy for geology and biochemistry, MRI, PET, atomic clocks, heavy metal chemistry, Doppler radar.",
"Edit: brief explanations",
"The Relativistic Doppler effect (the change in frequency from the reflection or emission of an EM wave of a moving body) is used to measure the speeds of objects with Doppler radar and police radar/laser guns, and probably other applications. It needs to be accounted for in radio communications with high-speed spacecraft so that the transmitter is sending a frequency and data rate that the receiver is expecting. ",
"Particle accelerators like those used in radiation therapy move electrons at near the speed of light such that special relativistic effects dominate. Because they are pegged against the speed of light, ",
"the cavities in linear accelerator waveguides",
" can have equal spacing and use a single frequency of microwave to push electrons through each cavity.",
"The momentum of photons is also exploited in ",
"Mössbauer spectroscopy",
", which measures the recoil of gamma-emitting nuclei to probe the crystal structure of materials.",
"Nuclear power converts mass to energy (E=mc",
"), and experiments on fission products to understand the reactions taking place in advanced fuel cycles use mass-energy equivalence.",
"Radiation deposits energy in matter via fast charged particles like electrons zipping through and ionizing atoms in the wake of their electric fields. The strength of that interaction depends on the speed of the electrons, which is dictated by special relativity. This is described in the ",
"Bethe formula",
", and the details of the effect are necessary to be able to figure out how much radiation dose an object receives based on measurements with detectors that respond differently to this effect.",
"PET works via the conversion of electron-positron mass energy into photon energy (E=mc",
" again), and the conservation of momentum in photons gives the back-to-back equal-energy photons which are necessary for making an image.",
"MRI: The frequency used for probing nuclei is based on the gyromagnetic ratio of the proton, which is based on the proton's magnetic moment, which is based on the internal behavior of quarks in the proton, which is a relativistic environment. Most of the mass of protons and neutrons are in fact the kinetic energy of their constituent particles.",
"Atomic clocks: the Doppler shift of atoms even from thermal motion is enough to broaden the frequency signal and reduce clock accuracy. The most precise clocks have to be cooled to avoid this.",
"Heavy metal chemistry: The core electrons in heavy atoms are moving at significant fractions of the speed of light, which changes the shell structure and alters chemical behavior.",
"Probably not the best-selected list, but hopefully it will do on short notice."
] |
[
"Basically the only technology that does this is GPS, where the clocks on the satellites have to operate at sliiiightly different frequency than the ones on Earth to account for gravitational time dilation."
] |
[
"(note: ",
"/u/iorgfeflkd",
" already gave an example of the usage of GR and SR, I just wanted to expand on the bit of \"why is this useful?\")",
"Ah yes, \"why is this science exactly useful?\" question. I always tell them this story:",
"In 1888, Heinrich Hertz discovered the so called \"radio waves\", after they were mathematically predicted by James Clerk Maxwell in 1864. ",
"When he discovered these radio waves, they were labelled as useless and something that would have no use at all in our society. Little did they know, because look at us now!. The usage of radio waves have become such an important communication technology in our modern society. ",
"It takes time for new science to develop and for us to find practical uses for it, and this is especially true for gravitational waves. As many people describe it: it's as if we could only \"look\" at the universe, but now, with the discovery of gravitational waves we can see and ",
"!",
"Nothing is really useless. Ask him: what's the point of music, books and movies? Will they cure cancer? Will they bring us to Mars? No, but that does not make them useless. ",
"Your friend should realize that no science is really \"useless\", and you should be especially careful when calling new science \"useless\". It takes time to develop, but that's the beauty of it. Who knows what pratical uses we'll find for gravitational waves in 100 years?"
] |
[
"Can dark matter just be black holes?"
] |
[
false
] |
Pretty simple concept here. I'd find it hard to believe nobody's brought it up before, but I've not heard it so I'll ask. First things first, my starting concepts (in case those need correcting) The question: why can't it simply be that we just don't realize there are vast amounts of black holes providing all the gravity we're attributing to dark matter? I'm assuming it's possible that a black hole not have an accretion disk making them totally invisible.
|
[
"That's an idea that has been considered in the past, yes, but it doesn't fit with other observations. It's still technically possible for a fairly narrow range of black hole masses to make up some part of dark matter, but to make up all of it, it is not a realistic mass distribution that all the primordial black holes around just happen to fall in the very narrow range of masses of black hole we haven't been able to rule out yet from observations.",
"Black holes as dark matter as an idea are generally referred to as Massive Compact Halo Objets - MaCHOs for short, because of course it is. (A more favored candidate for dark matter today is Weakly Interacting Massive Particles - ",
"WIMPs",
". Astrophysicists love jokes like that.) While MaCHOs are not ",
" dead as a dark matter candidate, the search for gravitational microlensing in experiments like the Optical Gravitational Lensing Experiment (",
"OGLE",
") and ",
"EROS",
" - I know, I know, EROS OGLE-ing at MaCHOs...I promise ",
"these are real",
" - have failed to find enough free-floating black holes to explain dark matter and have further put limits on what the mass distribution of black holes responsible for a dark matter halo could be. There are other constraints on that mass distribution from studying the Cosmic Microwave Background and tiny dwarf galaxies near the Milky Way as well. ",
"At any rate, while there may be some contribution to dark matter from black holes, ",
"it seems clear at this point",
" ",
"that it can't be all of it or even most of it.",
" We couldn't say that 30 years ago."
] |
[
"I think it's worth noting that recent micro lensing survey such as the ",
"Subaru HSC",
" still leave an interesting parameter space: ",
"Black holes with ~30 solar masses can still constitute around 70% of dark matter.",
" Interestingly, this is exactly the mass range in which LIGO is detecting black hole mergers frequently."
] |
[
"The figure you linked to shows that there's a completely open window in the 10",
" - 10",
" solar masses range. Why is this this mass window usually ignored in favour of the now mostly closed 10-100 solar mass window? Is it just based on interest in that mass range black holes since LIGO sees black holes in this range?"
] |
[
"Why there is a need to convert time domain signals to frequency domain signals?"
] |
[
false
] | null |
[
"Such conversion from time to frequency domain, done using ",
"Fourier transform",
" has a very wide variety of uses. Here are some of those I am familiar with:",
"A lot of devices, including transistors, amplifiers, detectors, our ears and others respond differently, depending on the frequency. Some frequencies can be detected or retransmitted, while other will be blocked. If you have some signal in time domain, you need to transform it to frequency domain to calculate how some device will change it and afterwards you can use an inverse transform to see how your time domain will look after passing through device. ",
"Many audio compression algorithms delete the frequencies that we can't hear from the recording to reduce the file size. Sometimes the sources in your audio signal consist of different frequencies, so you can emphasise one of them by deleting other, like in ",
"this Vuvuzela example",
".",
"In spectroscopy, the use of Fourier transform allows to make spectrometers that don't sacrifice light intensity to get higher resolution (",
"FTIR",
"). Using interferometer, the spectrum is recorded in length domain, which is related to time domain. Applying the FT to that signal gives the spectrum.",
"In laser optics and electronics, FT allows to calculate what will happen with your signal as it passes elements with different spectral responses. "
] |
[
"You're not actually converting the signal to a different one, you're just transforming it into a different domain to look at it differently. For instance, you can't look at a complex waveform in the time domain and be able to analyze it's frequency content. In order to see how a signal or system behaves in the frequency domain, you need to transform it using the Fourier Transform. "
] |
[
"Awesome explanation. I also heard a lot about Laplace transform that convert the time domain into s-domain (also known as frequency domain?). ",
"Why is there a need to convert the time domain to s-domain? and what is the difference between the frequency domain obtained by Fourier transform and the frequency domain obtained by Laplace transform?"
] |
[
"When certain ethnic groups are stated as having a higher risk for certain medical conditions, is this based purely on genetics or general lifestyle for the specific ethnic population?"
] |
[
false
] |
For example, when I look up Type-2 diabetes, almost all health related websites state that essentially every "non-white race" is at a higher risk. Is this just based on the general dietary and exercise habits of the population, or would a healthy non-white individual still be at a greater risk than an equally healthy white individual? Secondly, what is considered Asian American in these studies? From what I looked up they group Asian Americans with Pacific Islanders. Does this include people from South Asia and the Middle East as well? Despite them being from a different ethnic group. I apologize if my terminology is not scientific enough when dealing with race. As I am aware the difference between it being used as a social construct compared to it being used in a scientific discussion.
|
[
"The short answer is that both genetics and lifestyle as well as cultural differences are factors contributing to higher/lower risk. Genetics can significantly contribute to risk or response to treatment, diet/cooking and other cultural elements like smoking and alcohol consumption can also directly contribute. Indirectly, things like lower rates of screening associated with certain ethic groups can contribute to overall increased risk as can things like prevalence of virus infections in specific regions.",
"I cannot speak specifically for diabetes, as I am not too familiar with the research and statistics.",
"Asian American is usually considered the far east and India, so South Asia is included but not the Middle East. "
] |
[
"Thanks for the response. So I guess my next question would be why would South Asians have the same likelihood as East Asians for a disease if the types of food that they eat, culture, and etc are different. Wouldn't it make more sense for them to be in their own category, similar to people from the middle east, since I'm assuming genetically (and correct me if I am wrong) there are more similarities because of the geographic locations being closer."
] |
[
"It's a false assumption that the same likelihood of diseases is shared between these regions. Actually, individual (separate) risks for ethic populations and countries are documented when possible, but are not always available. ",
"For example, this study compares cancer risk factors and incidence between Chinese, Filipino, Vietnamese, Korean and Japanese Asian Americans.",
". ",
"Significant differences are found between groups (even living in the same region/country) and we we certainly want to take these differences into consideration when making health care decisions."
] |
[
"What is it called when you are awake, blink, and all of a sudden it is morning?"
] |
[
false
] | null |
[
"Ohhh! I know what you're talking about. It is similar to the feeling you get when you wake up after really blacking out. Like the 8 hours you just spent asleep transpired in half a second. I have no idea how that happens, but sometimes i wish it would happen more often"
] |
[
"No. I mean the instant sleep. Where you don't dream. You LITERALLY blink. "
] |
[
"Yes! I know. I wish I could control it."
] |
[
"If we received an alien signal, is it even possible that we could decode it?"
] |
[
false
] | null |
[
"hmm, you could try ",
"/r/AskScienceDiscussion",
", of course."
] |
[
"This question has been removed because highly speculative in nature. Exceedingly imaginary hypotheticals often invite non-scientific speculations.",
"For more information regarding this and similar issues, please see the ",
"FAQ."
] |
[
"Where would be an appropriate place to submit it?"
] |
[
"FAQ Friday: Why are most people right handed? Ask your questions about \"handedness\" here!"
] |
[
false
] |
This week on we're discussing how and why people show a preference for using one side of their bodies. While we often refer to this as "handedness", it's technically called . Have you ever wondered why most people are right handed? , or ask your questions here! We remove comments containing anecdotes or asking for explanations about individual situations. More information is available in our . .
|
[
"Is there any prevalence of handedness in other creatures? For example, do birds have a dominant talon, or do we see handedness in other apes? I'm not even sure how this would translate to non-bipedal animals, but are there similar phenomenon?"
] |
[
"Yes! There has been research to suggest that parrots tend to be ",
"left handed",
". in the study 20 parrots were fed a piece of fruit daily and it was recorded which foot they picked up the piece of food with. \"In captivity, a parrot, if fed on the floor of the cage, will descend from the perch, grasp the bit of food desired... with one foot, and then climb to the perch...\". Not all birds feed by picking up their food; hummingbirds are a good example. I am not sure how you would test for handedness in creatures like that. As for a reason as to the left handed tendency in parrots, no one is for sure. there has been some speculation that their ability to make incredibly diverse range of vocalizations has made one side of their brain better at complex muscle control. The side of the brain that practices finely controlling the syrinx (birds vocal chords) is better at delicately picking up food. "
] |
[
"Yes, handedness is present in at least some capacity from birth and some research even suggests its present in utero. ",
"This study",
" (sorry pay wall) showed a strong preference for 15-week fetus to suck on their right hand. ",
"Here is a book that reviews newborn handedness",
". Unfortunately my newborn is crying so i will have to finish this answer later.",
"EDIT: One test they tend to do that correlates highly with handedness in early childhood is the head position test. If you set a newborn down on a flat surface with their head facing forward, they will usually tilt it to one side with a head right-side bias. This has been correlated with hand preference at 60 and 74 weeks of age where right handed children tend to tilt their head to the right."
] |
[
"Is there a size limit for terrestrial planets?"
] |
[
false
] |
Pretty straightforward question, but I'd like to add a wrinkle. Can such a planet form beyond the frost line?
|
[
"There is, but we don't exactly know what it is. Above a certain mass (",
"approximately ten earth masses",
") the planet is heavy enough to maintain a thick hydrogen and helium atmosphere, so it is believe the cutoff is around there."
] |
[
"Because that would make them by definition gas giants, and not terrestrial."
] |
[
"Essentially, it is a guarantee. When the mass is great enough, hydrogen and helium cannot be stripped away in the same way that solid material on Earth cannot be stripped away."
] |
[
"How was cancer treated before modern techniques were invented?"
] |
[
false
] |
I was reading about Adolphe Sax, who's birthday is honored in today's google doodle, and saw this near the end of his biography on Wikipedia: Sax suffered from lip cancer between 1853 and 1858 but made a full recovery. I was always under the impression that cancer was kind of a death sentence back then. Did they just keep cutting out cancerous material until it didn't return?
|
[
"During the 19th century after the invention of aseptic technique, surgical removal of the tumor was the primary method of treatment and the results varied largely depending on the skill of the surgeon and the nature of the tumor. Some doctors would inject bacteria directly into the tumor but it didn't work very well."
] |
[
"Adding on to ",
"/u/Wave_Existence",
", we've been carving cancerous tissues out of people long before aseptic technique, until quite recently that was the only real method of treatment. ",
"Mr. Sax would likely have had a surgery (or several) until it stopped coming back, as we couldn't identify where exactly the cancerous tissue began or ended in most cases. We still can't in every case, that's why most cases end with someone going 'into remission', even when it isn't detectable. There's no ",
" tumour, but that doesn't mean you don't have cancerous cell clusters hiding out in there.",
"Here's a nice little PMC article",
" that gives a reasonable history of cancer detection and treatment."
] |
[
"Here's ",
"another",
" a source on the history of cancer, people have indeed been cutting it out for a while, but even the egyptians have ackgnowledged that the disease is not really treatable that way.",
"A ",
"side mention",
" of a considerably more modern (late 19th century) but way less bloody method are viruses. Pretty much as long as we know of their existence we've known some of them somehow help against cancer, although the recoveries associated with spontanous infections could hardly be called \"treatments\". You could do that ",
"now",
", a hundred years later though :)"
] |
[
"Why can a complex number be used in Physics?"
] |
[
false
] |
I'm currently trying to understand how to use complex numbers and with that how to calculate with i. I think I now understand the basic math behind it, but I'm still confused how a number, that in some kind of way doesn't exist, is able to describe something like Oscillation in Physics. Or to better describe my question: Why do we need something imaginary to describe a progress that happens in the "real" world? Edit: Thanks everyone for helping and answering. Special thanks to :)
|
[
"\"i\" is just as real as pi or e or sqrt(2) or -4 or 7 or 1 or 0 etc. We've invented every single number because the way they combine helps us keep track of patterns we perceive. Math is not of the universe, we originally invented math to try to communicate patterns that we see around us, the more intricate patterns we perceive, the more intricate our math needs to be to communicate it well. ",
" numbers are imaginary!",
"What makes, for instance, sqrt(2) more \"real\" than sqrt(-1)? I invent sqrt(2) in order to have a solution to the equation x",
"-2=0. I invent sqrt(-1) in order to have a solution to the equation x",
"+1=0. Why do we look back at the Pythagoreans for not accepting sqrt(2) as a number with humor, yet some people today have an issue with accepting sqrt(-1)? ",
"Real numbers keep track of quantity, it's the properties of addition and multiplication that make this valuable. We don't really care that the real numbers are on a line, we care that we can add, subtract, multiply and divide them in a continuous fashion. Complex numbers keep track of quantity and rotation. They follow the same rules of addition, subtraction, multiplication and division, they just have more room to move, and it's these rules that are important to their use. We can use complex numbers to keep track of more quantities than with just real numbers, so why artificially restrict our imaginary number systems to just the real line when the sky's the limit? (Actually, the Fundamental Theorem of Algebra says that the complex numbers are the limit)"
] |
[
"It's a bad name, but used for historical reasons. Technically, all numbers are imaginary so it's not really any different. Math itself exists just in our thinking, but not in reality."
] |
[
"Math is not of the universe, we originally invented math to try to communicate patterns that we see around us, the more intricate patterns we perceive, the more intricate our math needs to be to communicate it well. All numbers are imaginary!",
"/u/functor7",
" I love what you've done on this forum, and you've been one of the best expositors of mathematics around. That being said, to state the above as if it were fact is slightly absurd.",
"The vast majority of mathematicians (along with huge subsets of physicists and philosophers) subscribe to some form of mathematical realism. Sure, notation is invented by us so we can define and manipulate mathematical objects. But when the progression of the most fundamental areas of science starts to showcase things like atoms being almost entirely empty space and subatomic particles having no known substructure (they literally seem to behave like zero dimensional mathematical points), then what is \"physical\" in this picture of reality? Couple that with needing to introduce things like infinite-dimensional Hilbert spaces when talking about quantum mechanics and it really does begin to look like physical things (no one has ever really given a decent explanation of what ",
" even means) give way to mathematical relationships at bottom. ",
"I get that people have a hard time with that, and it might require a rethink of materialism/naturalism, but it in no way implies anything mystical or supernatural. You can be a mathematical realist and still be a naturalist as well (I sure am). It just means that things we thought had an ontological existence start to give way to something a little stranger and more ephemeral."
] |
[
"If PCR tests look for specific DNA sequences, how can they be false positives?"
] |
[
false
] |
[deleted]
|
[
"According to ",
"this paper",
" the causes of false positives include:",
"They cite contamination and cut-off issues with the two most common sources of error."
] |
[
"As an example of how it can occur, PCR uses an enzyme called polymerase to duplicate the RNA/DNA present. Every time it cycles it doubles the amount, until the machine is capable of detecting it, so the more DNA at the start, the quicker it reaches the detection threshold.",
"To bind to the DNA/RNA polymerase needs a primer that also binds to the DNA. There is a small chance of these primers sticking together and eventually being detected. Generally this takes a long time to occur, longer than it would take for positive cases to be detected, but it can be a source of false positives or indeterminate results. If you have a negative control with no DNA, you can tell when its occurring more easily."
] |
[
"Tl:dr..pcr data interpretation is..complicated.",
"I've noticed an interesting issue I've been had with a PCR test..maybe 1 reaction out of every 100. Instead of the usual logarithmic increase in fluorescence, I see a small increase that creeps up and above the threshold I've set (essentially resulting in what could be considered a 'false positive')\nHowever, after noting something was \"off\" I looked at the raw data and noticed something interesting. These increases in fluorescence are no more notable than that in any of the negative samples overall..and something is causing the software to select an early baseline. This means that any increase at all in fluorescence is determined by the software to be more drastic than it actual is. So, my conclusion is that there is an odd random increase in fluorescence that happens sometimes with this assay, which tricks the software into thinking it's a meaningful increase. It then essentially \"hits record\" and starts blowing subsequent increases in fluorescence out of proportion and giving you a \"false positive.\" Now imagine this happening on a test that just spits out a \"positive\" or \"negative\" result with no human intervention meticulously going over the data and saying \"hmm, this one isnt actually positive..\" \nIt's not easy to design software that can account for every random thing that may be observed in real-time PCR data. This whole post is only accounting for software issues though. There are equally as lengthy posts that could be made about the other potential false positive causes"
] |
[
"My daughter was born completely blind in her right eye due to an under-developed optic nerve. Does anyone that follows the sciences closely know of any information or progress in stem-cell work with the optic nerve?"
] |
[
false
] | null |
[
"I am very sorry but she will, at best, (re)gain very blurry vision in her right eye because the necessary nerual connections in the brain that interpret the signals coming from the eyes mature in the first few months. If in that time the nerves are not stimulated they degenerate and are effectively useless, together with the eye.",
"Caveat is, as I said, the blurry vision. She could gain some very blurry vision in her right eye but good vision as you are probably used to will need some really advanced therapy such as gene therapy as the maturation is directed by genetic triggers.",
"My qualifications? I am a physics student, I love all sciences but please take my writings with a grain of salt and wait for a person in the field of medicine or similarly relevant qualification."
] |
[
"I have \"lazy eye\" in one eye which, as I understand it, is a result of a poorly developed optic nerve. I'm hazy on the details, but I believe my eye was actually removed from the socket and operated on when I was weeks old. ",
"Early on in life, I was made to wear a patch one day a week over the working eye in order to force the poorly developed nerve (and relevant neural pathway too, I think) to develop. I hated it. It was the bane of my existence. On top of that, no-one ever explained to me why I had to wear this patch beyond \"it helps\". I really think if someone had of explained it to me in a little more details, I may have at least been able to deal with the patch days a little easier.",
"I stopped wearing the patch after approximately 3 or 4 years and never regained anything more than strongly blurred vision in my right eye. At close range, I can make out some things eyes are useful for (facial features, writing) but anything smaller than a few inches at more than about 3 feet from my face loses detail very quickly, and it gets harder to see larger things as they get further away.",
"I can still function daily without glasses thanks to a healthy remaining eye, but I live in fear that I'll lose my remaining vision one day and be left heavily affected for quite some time while I learn a new way to see.",
"Moral of the story? Even if they cant do anything with stem cells, OP's daughter may still regain some vision and, whilst it wont be anything near perfect, it might still be usable and allow her to live normally. All that said, I see little reason why having only one eye from birth would ever be a problem for a person as they'll learn to live with it/work around it as they age and mature.",
"Good luck, OP."
] |
[
"A company called Advanced Cell Technology Inc. is currently working on clinical trials in the US and Europe for macular degeneration. ",
"Scroll down a few times to check out their bio. ACTC.",
"http://seekingalpha.com/article/620561-hope-springs-eternal-in-biotech"
] |
[
"Calling all Material Scientists!"
] |
[
false
] |
Sorry if this isn't the right place, but I would love it if you could tell me why you're in the material science field and what you're researching (or have done research in previously). I had always planned to study physics in University, but I'm at a university where I didn't get to do straight physics until my 3rd year (this might be common in America, but not in the UK). So I took physics, materials and chem in year one, and am now taking physics and materials. I've decided to concentrate solely on material science for my final two years. I just find the course a lot more interesting. Although some people have criticized my choice, they think that I'm taking the "easy option" or that it'll be harder to get a job with a material science degree than with a physics one. It's getting a bit disheartening to be honest, so I'd like to hear your stories from the other side :) Edit - Just wanted to say thanks for the replies guys. Really interesting stuff.
|
[
"Hi! Materials Scientist here. I'm doing steel research now for my PhD, but I used to work with thin films of magnetic oxides, which are slightly more physics-y. The stuff I do now is looking at the effect of nitrogen on steels containing aluminum.",
"I'm hoping to go into consulting/failure analysis \"when I grow up,\" and the few firms I talked to while doing my undergrad were looking for PhDs, so I don't have much insight into the job market. ",
"FWIW, I love my major. I love the fact that as an undergrad you have the flexibility to study any of a wide variety of materials. I went into undergrad thinking I might be interested in doing electronic materials, quickly realized that solid state physics weren't my thing, and then discovered how fascinating fracture is. But the fact that the defects that lead to ductility in metals are the same defects that physicists are looking at in ",
"supersolid helium",
" is really cool."
] |
[
"Polymer chemist. I'm researching stimuli-responsive copolymers. In solution, they undergo some change in physical properties based on some external stimuli like pH, temperature, or ionic strength."
] |
[
"I got into materials by a long a circuitous road. After high-school I studied enrolled in an applied physics program at a large university and promptly failed out twice. I then worked as an industrial mechanic while going to technical school to be an aircraft mechanic (a&p). I worked on airplanes for several years and was always bothered by questions related to material response that didn't make much sense to my mechanic brain. For example why do fatigue cracks in landing gear components make a right angle and not cross a weld or why do certain types of aluminum rivets need to be kept in the freezer to keep them from becoming too hard to buck?",
"So at about 30 I went back to school and received my BS in materials science and engineering. Then stuck around for my masters and Ph.D. I worked as an adjunct professor for a bit teaching mechanics of materials, strength of materials (typical mech e course) and intro to materials science.",
"I am currently employed as a researcher at a large national laboratory in the atomic southwestern United States. My current research interests include polycrystalline plasticity in hexagonal metals, stochastic modeling of microstructure, in-situ stress measurement via electron backscatter diffraction and uncertainty quantification in microstructure evolution modeling.",
"As far as materials being the easy choice I guess it is all a matter of perspective. Materials has historically had a lack of people with good solid math skills. Often the people on the mathematical side of things are mechanical engineering or physics converts. If you find the classes more interesting then take them for pete's sake. No sense in studying things you don't like because someone called you a wimp.",
"On the job side of things, I know a whole lot of pure or theoretical physics people now working in materials. Look through the job adds. There are a lot more job postings for assistant professors in materials departments than in physics departments. A majority of the post docs in government labs in the US are in materials or materials related fields. Oh and chances are your advisor will have money to pay you, and you can probably find a tenure track or professional job with only 1 (or maybe even zero) stint as a post doc.",
"Please PM me if you want to talk more about it. "
] |
[
"Why do we measure decay in half-lives rather than just lives?"
] |
[
false
] | null |
[
"To explain in simpler terms. I've run into a misconception occasionally that OP might be falling prey to. If a certain material has a half life of 100 years that doesn't mean it'll be fully decayed in 200 years, it means about half will decay in 100 years, then half of that half in another 100, then half of that half in another 100, etc."
] |
[
"Basically, each atom has an x% chance of decaying during some given time interval. Let's say that an atom of some radioactive element, let's call it Examplium, has a 10% chance of decaying in any given hour. This means if you start with 100kg block of it, and wait an hour, then you can expect that 10% of it will have decayed. You now have 90kg of Examplium. ",
"But atoms don't have a memory, and don't know how many of them are in the block. I could have easily said that there was only 20g of it, and that 2g decayed. This means that in the next hour, you can expect that 10% of your Examplium will decay. Since you have 90kg of it, then 9kg will decay, leaving you with 81kg, then 72.9kg, then 65.61kg, and so on. ",
"You can see that this is just a=b*0.9",
" , which is exponential. With a little algebra, we can figure out that Examplium has a half life of about 6.57 hours."
] |
[
"Basically, each atom has an x% chance of decaying during some given time interval. Let's say that an atom of some radioactive element, let's call it Examplium, has a 10% chance of decaying in any given hour. This means if you start with 100kg block of it, and wait an hour, then you can expect that 10% of it will have decayed. You now have 90kg of Examplium. ",
"But atoms don't have a memory, and don't know how many of them are in the block. I could have easily said that there was only 20g of it, and that 2g decayed. This means that in the next hour, you can expect that 10% of your Examplium will decay. Since you have 90kg of it, then 9kg will decay, leaving you with 81kg, then 72.9kg, then 65.61kg, and so on. ",
"You can see that this is just a=b*0.9",
" , which is exponential. With a little algebra, we can figure out that Examplium has a half life of about 6.57 hours."
] |
[
"Can astronauts knock a spacecraft off course my moving around inside it?"
] |
[
false
] |
Watched Apollo 13 for the nth time and wondered this. They were coming back from rounding the moon, heading for Earth in a (relatively) straight line. They had to do critical course corrections so as to reenter the atmosphere at the right spot. When they were moving around inside the LM and capsule, they were pushing off pretty hard sometimes. If they did this on the side of the spacecraft, could it put the whole ship into a slow spin, or is the inertia of the craft enough to negate this?
|
[
"By changing the distribution of mass about the center of mass of the object they can affect its trajectory. Pushing off would not as the sum of all forces would equal zero. It is like trying to pick yourself up to fly."
] |
[
"No. I'm going to confuse you by using the term center of mass again, but the center of mass of the system is still the same and it is still moving with the same velocity. This is defined as momentum.",
"For example say you and I are hugging and flying through space. Our center of mass as a system is right around our touching belly buttons. Me pushing off of you will result in the exact same effect as you pushing off of me. We will both move away from one another however our center of mass will still be located in the same place (about halfway between our - now distant - belly buttons) and will be traveling at the same speed and in the same direction."
] |
[
"Ah, I see what you are saying.",
"I was thinking when propulsion is on. Let say your craft is a tube and the center of mass is in the middle and the engine is at the end. If you are near the engine and you push off, you would temporarily swing the engine to point at a different direction. During that time, the engine would be pushing at a different trajectory."
] |
[
"What happens if a diamond is hyper-pressurized? What about other precious gems?"
] |
[
false
] | null |
[
"Diamond is quite resistant to pressure - in fact, diamond surfaces are used in pressure anvils for just that reason. If you Google \"carbon phase diagram\", you can see a plot of what happens at various temperatures and pressures. Diamond is what you get a \"normal\" temperatures as you start cranking up the pressure, but for a given pressure, it appears that you can eventually melt the diamond phase to a pressurized liquid. But we're talking temperatures of over 5000K, not far from the temperature of the Sun's surface. ",
"There may be a metallic carbon phase beyond these conditions, at ultrahigh pressures, but the conditions to investigate this are beyond human capabilities at present. What the limits of stability are for that one is a matter for theoreticians."
] |
[
"Diamonds can burn like any other form of carbon, not sure what happens when pressure is applied or no oxygen is available.",
"Diamond has a very high melting point, so high that it has never experimentally been reached. So without oxygen and with heat only, nothing much happens :)"
] |
[
"Heat treatment of precious gemstones has been practiced for a long, long time. It's a terrible idea with diamonds, the carbon would react with any available oxygen, literally disappearing into thin air, but garnets, sapphires, and topaz can change color and brilliance after heat treatment. Sometimes the value goes up, other times down, other times the stone is irreparably damaged. The heat treatment kind of does what naturally occurs in the Earth, it'll change the structure a bit. Inclusions (cracks in the crystal) may form, giving it a brilliant sparkle, or the new structure may have a completely different color, among other affects. Honestly, without heat treatment the price of stones would likely be much higher, as most raw untreated gems don't have the amazing color and shine we now appreciate. Raw gems are usually kind of dull. I don't know the exact pressure and heat required and don't recommend you try it at home, as you may just destroy your gems. "
] |
[
"Is there a biological basis for being a \"night owl,\" or am I just horribly indisciplined?"
] |
[
false
] |
I've always been a night owl like my dad. For some reason, my brain just seems much sharper and alert between 1:30 am and 4 am. Conversely, many others wake up naturally with the sun, and can't keep their eyes open past the early hours of the night. It seems that no matter how many times I try to alter my sleeping habits, I can never become a mornig person. Is there a biological basis for this, or are night owls just making excuses for their reddit addiction? Edit: Wow, thanks so much for the response guys! I'm a bit late for the obligatory "holy shit front page!" but still, it seriously made my day. A lot of interesting articles and suggestions have landed here. Well-done, Ask Science!
|
[
"There is a biological basis. Medically, the term is \"night preference.\" A Japanese study showed that a polymorphism of the CLOCK gene is strongly associated with being a night owl. Later studies have seen relationships with ADHD prevalence as well.",
"Source"
] |
[
"I did a brief paper on the subject a while ago, I doubt I'll find it, but perhaps I can dig for some of the old references later. To put it simply; having variances in the time at which individuals of a particular group sleep provides a protective advantage. If in your group of 12, let's say, everyone slept from exactly sundown to sunrise, you'd be at increased risk from night time predators during that entire time. If you had a small number, perhaps two, that naturally tended to stay up a few hours past sundown (the \"night owls\"), as well as maybe two or so who woke up a few hours before sunrise (the \"early risers\"), you'd limit the amount of time that at least someone wasn't awake and looking out for the safety of the group.",
"EDIT: ",
"I found the paper",
", but it doesn't address the OP's questions as directly as I thought. If you do decide to read it, I hope you enjoy, but this was during my master's work and I had to pump one of these out on a weekly basis on various topics. In other words, don't expect a literary masterpiece."
] |
[
"The influence of the 3111T/C polymorphism on ME preferences in Caucasian populations remains controversial.",
"/Edit:",
"It has even been ruled out as cause of sleep disturbances for untreated depressed patients",
"3111T/C clock gene polymorphism is not associated with sleep disturbances in untreated depressed patients.",
"http://www.ncbi.nlm.nih.gov/pubmed/20370469"
] |
[
"What exactly triggers a salty taste? Will a solution containing only Na+ or Cl- ions taste salty?"
] |
[
false
] | null |
[
"On each taste bud, called a papillae, there's a few bundles of cells that actually act as a mediator between your mouth and the gustatory nerves that actually send pulses to your brain. On those taste receptor cells, there's 4 different proteins that can bind to different things.",
"TAS1R1 and TAS1R3 bind together and are activated by Left chiral amino acids L-glutamine especially. (This is why monosodium glutamate elicits the flavor it does).",
"TAS1R2 is the type 2 receptor, it binds to sugar types. Triggered by Glucose, and Aspertame.",
"TAS1R3 is the third one, it can detect sugars and other chemicals on it's own, but it tens to bind with TAS1R1 and TAS1R2 for different effects.",
"TAS1R4 type 4 taste receptor protein binds to sour chemicals, and some 'sugar substitutes'",
"None of them interact directly with a nerve. They have protein mediated cascades through those taste receptor cells. There's one specifically for NaCL which is a sodium channel, but not exactly the same as the nerve sodium channel. The protein is called ENaC. ",
"There's better, and more concise information here.",
"https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/tas1r1"
] |
[
"You can't have a solution containing ions of only one charge, there has to be a counter ion to make it electrically neutral. ",
"Neither hydrogen chloride (hydrochloric acid) nor things like sodium hydroxide or other sodium salts taste salty. In (very!) dilute solution HCl tastes acidic/sour, NaOH tastes bitter.",
"Looking at elements in nearby positions in the periodic table, sodium bromide does apparently taste salty, but so does lithium chloride. Potassium chloride also tastes salty but potassium bromide has a variety of sweet/bitter/salt tastes depending on concentration. So it's the basic overall similarity rather than any specific ion.",
"I strongly don't recommend tasting chemicals, the above comes from internet data, except for the taste of highly dilute acids."
] |
[
"Interesting source! It definitely suprises me that there would be a receptor that requires the presence of both constituents of NaCl to trigger a response rather than the mixed response caused by simultaneous Na+ and Cl- receptor triggers, since receptors within my knowledge only have a single signal molecule site for triggering a response. Even more surprising is the dual action response of TAS1R1 and 3 to trigger a taste from a single signal molecule (glutamate) , which is something quite novel for me. Thank you for your detailed response."
] |
[
"Why is it that I can stand on a balance ball and look everywhere and be steady, but when I close my eyes, I lose balance quickly?"
] |
[
false
] | null |
[
"Because vision is important to balance, particularly so on an unsteady surface where constant weight redistribution is necessary. 20% of our optic nerve's output goes to our vestibular system.",
"Some ",
"articles",
" to back this up."
] |
[
"Really? The links say something slightly different, ie. that good vision is necessary for calibrating the vestibular system and that people with poor vision often have poor balance with their eyes closed."
] |
[
"The relationship between vision and balance is discussed in ",
"this",
" classic, educational, and somewhat hilarious video (the video relates to ",
"Lishman & Lee, 1973",
" )"
] |
[
"Could a combination of different types of particle accelerators make hydrogen?"
] |
[
false
] | null |
[
"Sure, you could split alpha particles to make protons. But that would not be a good way of doing it."
] |
[
"On a planet where there's naturally large amounts of water, you can electrolyze water to make hydrogen and oxygen gas.",
"Particle accelerators are not an efficient way to make macroscopic amounts of material. We only use them to produce relatively small amounts of material, and usually only radioactive materials that can't be found in nature."
] |
[
"It just would be nice if we only used nuclear power where no life exists. I'm sure it could still be useful for terraforming in certain circumstances. Like once the raw materials are on the planet. I am pretty sure you are 100% right the more I think on this. Thank you for taking the time to explain it to me."
] |
[
"At what distance from us does the apparent recession of the galaxies due to the expansion of the universe reach FTL speeds?"
] |
[
false
] |
I am pretty sure it lies beyond the CMB (≈ 14 billion light years away).
|
[
"Hubble's constant is 70 km/s/megaparsec. Divide the speed of light by this to get the distance at which the recession is that speed. This is about 14 billion lightyears. "
] |
[
"This is about 14 billion lightyears.",
"This looks strangely familiar ... Is this a complete coincidence or is this an obvious consequence of something ?"
] |
[
"Is this fair, considering Hubbles constant isn't an actual constant. Wouldn't it actually be a given that the FTL point from our POV would occur somewhere during the insanely fast early expansion of the universe? "
] |
[
"What would happen if you applied a lot of pressure but no heat to a piece of graphite?"
] |
[
false
] | null |
[
"It should turn into diamond, although you might need a lot of pressure and/or a lot of time. See the temperature/pressure phase diagram for solid carbon referenced in ",
"this page",
" or ",
"this page",
". "
] |
[
"History of a material can absolutely have an effect on its structure, although often it's more related to kinetic effects. As an example, at room temperature graphite is slightly more thermodynamically stable than diamond, so why do diamonds exist at room temperature? It's because there is no easy way to change from one phase to the other- strong covalent bonds must be broken and rearranged to accomplish the phase transformation, and at room temperature the average thermal energy is far below the level required to do this. At high pressures and high temperatures diamond is more stable, and there is enough energy available for the transformation to occur. Upon cooling to room temperature the diamond remains diamond; by the time graphite becomes the more favorable state there is no longer enough thermal energy available to break the bonds and accomplish the phase transformation. In theory I think this means that for diamond synthesis you would want to cool the press down first and then depressurize it, but I believe they mostly just change both the pressure and temperature simultaneously because it still works and it requires less finicky control of the process. "
] |
[
"That has more to do with the nature of the covalent bonds in diamond (or ionic/covalent bonds in other ceramics) versus, say, metallic bonding in metals. The ability to deform without breaking requires that atoms be able to slip past one another without destroying the structural integrity of the material, which is possible with metallic bonding but very difficult to impossible with ionic or covalent bonding. Almost all hard materials tend to be brittle as well- the fact that the atoms cannot easily slip past one another is what makes them both hard and brittle. "
] |
[
"What kind of digital memory is used in high radiation areas (e.g., in orbit around Jupiter)? What is the expected lifetime of data integrity of these devices?"
] |
[
false
] |
Also, besides masked or fused prom, is there any digital memory technology that will last the hundreds or thousands of years that could be used in deep space probes? Can you even buy fused prom anywhere?
|
[
"http://en.wikipedia.org/wiki/Radiation_hardening"
] |
[
"3-5 semi conductors with a larger band gap are becoming more popular. It turns out that AlGaAs for example is quite hard to UV, p, n, and e-. ",
"Also, larger chip formats are better because you are less likely to get a cascade event, or a multiple point failure."
] |
[
"http://imgur.com/jvyFg.jpg",
"Specs for radiation hardened Sram from BAESystems. Proms are available as well as fused gate arrays. Check em out ",
"http://www.baesystems.com/ProductsServices/bae_prod_eis_rad_hrd_electrnic.html"
] |
[
"How would a Google Password Ring work?"
] |
[
false
] |
Regarding , explain password rings to me like I'm a STEM graduate student. Where exact implementation is unknown or multiple implementations exists, simply choose any method that would 'work'. I simply want to understand in my head what happens in the background during the interaction, and why the ring is so robust to identity theft (sans stealing the ring).
|
[
"Have you ever used google authenticator? Perhaps implementation would be similar. "
] |
[
"Google Authenticator generates a second (time-sensitive) password for extra confirmation. This is not the same thing. Thanks though."
] |
[
"I think he means you would need both a text password and the ring to log in."
] |
[
"Let's say an asteroid the size of a small town is heading toward Earth. Where would be the best place for it to make impact?"
] |
[
false
] | null |
[
"The best spot would probably be in the center of a large continent. Any impact in the ocean would cause immediate tsunamis worldwide followed by long term sea level changes that would cause major damage to coastlines. ",
"Obviously the least populated area would probably be the most ideal. At first I though somewhere in northern Asia but the surround populations would probably be in extreme danger from dust flung into the atmosphere. ",
"I propose central Australia or central northern Canada as the best point of impact. They are both old stable cratons with a small population relative to it's size and most of the population is concentrated far away from those areas.",
"EDIT: Someone suggested Antarctica based on my criteria. I have to agree with them that Antarctica would be a great location based on this criteria. "
] |
[
"I was not referring to the volume of the object entering the ocean as the cause of sea level change. I was considering the massive alterations to coastlines and the destabilization of continental ice sheets by tsunami. Do you see that as a possibility?"
] |
[
"no real global issues.",
"Only in terms of the Earth's orbit. There would be some very serious global climate issues from an impact this large."
] |
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