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[
"I have a problem understanding a certain aspect of radiation dating"
] |
[
false
] |
I understand the concept that over time the isotopes in various substances decay over time, but my question is why our bodies, and also igneous rocks, have always got 'fresh' isotopes in them. Why don't all these substances contain the same amount of depleted isotopes? I apologise if this has a really obvious answer I'm overlooking, but it occurred to me that I couldn't explain this to another person to my own satisfaction, which, I think, means I have an incomplete understanding.
|
[
"When rock melts and is in lava form, any argon it contains is released. That resets the clock. Once the lava solidifies into igneous rock, any argon resulting from potassium decay remains trapped in the rock. You can then date the time the rock was formed by the ratio of K-40 to Ar-40 it contains."
] |
[
"Argon dating is a complicated matter (This is my research field). Potassium 40 decays to Argon 40 and then you measure the potassium and argon and calculate an age. There is a complication to this which is that usually we irradiate samples to turn 39K into 39Ar and then measure a 40Ar/39Ar ratio (it's just a better measurement). Now in a really nice system (one that cools quickly and contains no initial argon) the ratio of 40Ar/39Ar is constant through out the entire gas release (you release the gas in steps usually 10 or more by changing the temperature to which you heat the sample). This then is the age at which this sample started retaining argon (it is usually younger than when the rock started forming). Now in some samples you trap some initial argon when cooling the rock and normally this is gas of atmospheric argon concentration (40Ar/36Ar ~ 270). To correct for this you also measure 36Ar and multiply it by 270 and subtract that from the measured 40Ar peak. If the argon is of another composition (besides air) then you can make an isochron to figure out what the initial composition is but this is an exceedingly rare case (though when it first happened it lead to a bit of confusion but now we can recognize it). ",
"The more interesting thing with Argon dating is that the interesting samples cooled slowly so you can actually reconstruct their temperature - time path by doing diffusion modeling (because that is how the argon leaves the sample). The way we recognize this is that the 40Ar/39Ar ratio changes as the sample is heated (generally it comes up slowly to a maximum).",
"The reason I gave you such an elaborate explanation for what we consider when dating is that really the popular explanation is not entirely correct. We are aware of a lot of these effects and correct for them."
] |
[
"Every time you ",
"eat a banana",
", you consume radioactive isotopes. Also, new radioisotopes can and are produced every day in nature, such as ",
"carbon-14",
", which can end up in plants. Which we eat (such as bananas) or are eaten by animals, which we then consume. "
] |
[
"What causes the Mushroom shape of the cloud during a large explosion?"
] |
[
false
] | null |
[
"The ",
"Rayleigh-Taylor instability",
" does a pretty good job of explaining the formation of a mushroom cloud in terms of fluid dynamics. Also there is a good wiki article on mushroom clouds ",
"here",
"."
] |
[
"The air that is blasted out by the explosion rushes bak in pushing the soot created by the explosion in from all sides-- this pushes up the smoke/other stuff into a mushroom cloud. "
] |
[
"Thanks this was exactly what i was looking for"
] |
[
"If Earth has a close call with an asteroid, could it affect our orbit enough to change the way we calculate time?"
] |
[
false
] |
The referenced article about the New Horizons spacecraft states, "To balance the books, Jupiter lost as much kinetic energy as New Horizons gained, causing its orbit around the sun to slow by a small amount. A year on Jupiter today is slightly longer than it was before--all because humans wanted to [get] a good look at Pluto." The New Horizons spacecraft weighs a little over half a ton, including fuel. So what would happen if Earth had a very close fly-by with a gigantic asteroid? Would it have enough of an effect to change the way we calculate years or months or minutes or seconds or anything? I realize that New Horizons' effect on Jupiter's orbit was minuscule, but then again, the leap second that we recently experienced, temporarily-caused some pretty significant issues (such as T-Mobile experiencing 5 minutes of hard downtime as a result). If the same thing that we did to Jupiter happened to us on a much larger scale, would could the possible repercussions be, if any?
|
[
"One second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. This exact amount was chosen as it was the number of transitions that occured in the the previous standard second called Ephemeris Time. This won't change no matter what happns to the Earth's rotation or length of a year.",
"Also, the length of a day has been and will continue to ",
" get longer by about 2 miliseconds every century due to tidal interactions with the Moon. These tidal interactions have already slowed the Moon's day until it became tidally locked with the Earth, and over the life of the solar system the Earth's day will continue to slow down.",
"Additionally, events like major earthquakes and volcanic erruptions can cause slight shifts in the Earth's mass distribution changing the length of a day. The 8.9 magnitude quake that struck Japan a few years ago shortened the day by 1.8 microseconds.",
"So changes happen naturally whether something big flies by or not.",
"To now answer your main question, yes an asteroid that has a close flyby of the Earth will add or subtract a little orbital energy from the Earth depending on how close it passes and whether it passes in front of or behind the Earth's orbit.",
"Of course, the Earth is quite massive compared to any asteroid that would make a close pass. The Earth is a little shy of 6 x 10",
" kgs and is travelling around the sun at 107,000 km/h. It will take something really big coming really close to impart a change.",
"The largest known near Earth asteroid is 1036 Ganymed is estimated to be 3.3 x 10",
" kgs (about 30 trillion tons). If it came much closer than normal (in 2024 it will come around 150 earth-moon radii from the Earth) it could slow or speed up the Earth's orbit a small amount.",
"Someone with better math skills than me might be able to calculate the impact of a 30 trillion ton asteroid slipping past us by maybe 1000 km either in front of or behind our orbit."
] |
[
"So, ",
"/u/rubynorails",
", we can put some numbers to this for context.",
"The average orbital speed of Earth is 30 km/s, and that of 1036 Ganymed is 17 km/s. If we just took these speeds, and assumed they were traveling towards one another with the precise trajectory required for a gravity assist, we can determine the change in orbital speed of Earth.",
"Note that that assumption is completely invalid, but it will give an approximate upper bound for the problem. In reality, the asteroid moves in roughly the same direction as Earth's orbit, and it would be traveling faster when it's near Earth since that's close to perihelion for the asteroid, but anyway, it's just a rough order of magnitude calculation since this will never happen.",
"So anyway, an ideal gravity assist will add twice the planet's speed to the asteroid's speed, so it would increase from 17 km/s to 77 km/s. Applying conservation of momentum, this would decrease Earth's momentum by .0000011%, which would cause the length of a year to elongate by about a third of a second.",
"Ultimately, it comes down to the fact that the asteroid is only one hundred millionth the mass of Earth. Even though that's a big asteroid, it is 8 orders of magnitude smaller than Earth, so it can't change Earth's velocity by much."
] |
[
"which would cause the length of a year to elongate by about a third of a second.",
"A normal person would be afraid of the chances of 1036 Ganymed hitting the earth. As a sysadmin I'm afraid of the many leap seconds that would cause…"
] |
[
"Is damage of radiation linearly dependent of radiation exposure?"
] |
[
false
] |
Two questions: Is staying out in the sun for 10 hours straight the same in terms of risk of cancer as ten hours spread out over a larger timeframe? A simalir is wether a single, large dose of radiation has the same effect as several smaller ones?
|
[
"It's a difficult question to answer, but for the purposes of radiation protection, cancer risk is considered proportional to dose and is not time dependent, in accordance with the ",
"Linear No-Threshold model",
". Now that is certainly not true with biological damage in general. For instance, the spreading out of the UV dose will prevent you from getting sunburned. Radiation-based cancer treatments are usually spread out over the course of multiple days or weeks to allow for normal (non-tumor) tissue recovery. But for ",
", yes, with caveats you can look up in the Wikipedia entry.",
"Interestingly, some folks think that very small doses of radiation may actually REDUCE your cancer risk (hormesis). But there's not enough evidence to support that at this point, so we still use the linear no-threshold to predict excess cancer risk from radiation. And excessively high doses, well, that will kill you, so I guess at that point it's not really linear anymore..."
] |
[
"No, these exposures are not at all the same. ",
"The key is that cells have a tremendous capacity to repair DNA in the hours just after exposure. If it has a chance to repair damage after a small exposure before more piles on, the success rate will be higher."
] |
[
"DNA can be repaired, and in fact most radiation damage is successfully repaired. By giving cells time to repair between exposures the final result is less permanent damage.",
"That's why radiation therapy is usually spread over multiple sessions: to give normal cells a chance to repair and recover."
] |
[
"If breaking the sound barrier causes a sonic boom, what would breaking the light speed barrier do?"
] |
[
false
] | null |
[
"A charged particle moving faster than the phase velocity of light in a medium emits ",
"Cherenkov radiation",
"."
] |
[
"Sound travels faster in water than in air. Light works the same way, it moves faster through some materials than other.",
"For example, the speed of light through water is lower than c (speed of light in vacuum). When you have a nuclear reactor immersed in liquid coolant, the charged particles released by the reactor are moving faster than the speed of light through water, which causes cherenkov radiation.",
"Check out the Wikipedia page on cherenkov radiation, it includes some neat pictures that I'm too lazy to link. Cherenkov radiation looks like a bluey-green glow coming from nowhere in particular."
] |
[
"As Cherenkov radiation has already been mentioned, the problem with any object going faster than the speed of light (in a vacuum) is that the math breaks down. The Lorentz factor (gamma) shows up in a lot of the equations of special relativity, and it's equal to:",
"1 / sqrt( 1 - v",
" / c",
" )",
"When velocity v is equal to the speed of light c, v",
" / c",
" becomes 1, the denominator becomes zero, and the Lorentz factor ends up with a divide by zero error. One of the implications of this is that it takes infinite energy just to accelerate something to the speed of light, meaning it's an insurmountable barrier.",
"Of course, even if you found some way to circumvent that barrier and instantaneously skip acceleration through the speed of light, when v is greater than c, v",
" / c",
" is greater than 1, so you're taking the square root of a negative number. Plug that into an equation and it's going to start spitting out results that don't have any physical meaning that we know of, like a particle with imaginary mass."
] |
[
"If all the objects and respective distances in our solar system were shrunk to 1/10 their current measure, would the orbital relationships stay the same? More specifically, does gravity function in the same way at different scales?"
] |
[
false
] |
This would assume no effect from forces outside our solar system.
|
[
"Newton's Law of Gravitation relates the force between two objects by F=GMm/r",
" If both the sun and earth had their masses cut but 90% and the same for the distance between them, the gravitational force of attraction would remain the same. ",
"The earth (basically) orbits in a circle. The force required to move an object in a circle is given by F=mv",
" Now there are two cases to consider:",
"1) If the earth's orbital velocity, v, is unchanged, the new required centripetal force would match the new gravitational force, and the earth would happily continue in its new orbit.",
"2) if the earth's orbital velocity is also reduced by 90%, the earth would be moving too slow to maintain its new orbit and would fall into an orbit closer to the sun. ",
"For other objects with more elliptical orbits like comets, the analysis would be more involved, but likely have results along the same lines.",
"Another interesting result to consider would be the effect on the surface gravity of earth. Depending on what got changed (radius of earth and/ or mass of earth) you might end up feeling much lighter or heavier than before!",
"Did my best, hopefully that was helpful!"
] |
[
"Gravitationally, there shouldn't be many differences.",
"In other aspects, however, things would change. The problem here is that the size of objects is not so important in orbital mechanics; their mass is more so. And if you'd scale the mass of everything down by 10, some of the objects would probably be of a different nature; the Sun for example would be around the lowest boundaries of being an actual star (actually a 0.1M⊙ object should still be able to undergo nuclear fusion according to Wikipedia - it seems like the dimmest star that has been observed so far has 0.083M⊙)."
] |
[
"That was super helpful and fascinating. Thanks for taking my question on! Would this continue to be the case no matter how much smaller things get? "
] |
[
"If we were to lose 90% of bees over the next 10 years, how would this impact human life? And what technologies and ways will we use to cope with this loss of pollinators?"
] |
[
false
] | null |
[
"I have heard that in China they are already employing people to go out into the fields with paint brushes to pass pollen from flower to flower. Seems like a bit of a dyer situation if it comes to that... but hey if your unemployed theres some hope for you"
] |
[
"Our crops are clones in that they are genetically identical, but they still have to be pollinated."
] |
[
"I dont know about that, many crops still have to be harvested by hand. Im sure that pollination would have to be quite delicate so as not to harm the plant too. So I'm not sure there would be a plausible way to automate it. ",
"Also with respect to bees dying, you would not only have to think of all the crops we would lose but all the wild plants too that may dissappear as they rely on bees for pollination. But still you may never know with nature, there could be other organisms which bees dying out could affect too, populations of a competing species may boom. ",
"There was an article I will have to find - but it said something about the decline of bees may cause the decline of mankind"
] |
[
"Claim: shampoo is harmful to you hair, and the 'no poo' movement is the response. Would someone take a scientific look at this?"
] |
[
false
] |
The basic contention is chemicals in shampoo strip too much natural oil out of your hair, leaving it dry and unhealthy. The alternative is no cleanser, or commonly baking soda solution cleaner and apple cider vinegar conditioner. Could someone explain the science behind shampoo, and see if the 'no poo' movement has legitimacy? Bonus points for a scientific source of best hair health practices. No poo articles: "If you wash your hair every day, you're removing the sebum," explains Michelle Hanjani, a dermatologist at Columbia University. "Then the oil glands compensate by producing more oil," she says.
|
[
"Ok, everybody, let's try this again, but with citations this time!"
] |
[
"When a thread gets out of control like this, the general AskScience way is to wipe the speculation clean. Speculation and guesses aren't what we do here.",
"And I didn't delete ",
" comment. There's somebody with links to previous discussions we've had, and a couple followup questions."
] |
[
"Your dermatologist claims (as does, as far as I can find on Google, the \"no poo\" activists) that the removal of sebum will drive the sebaceous glands to compensate for the loss and produce more sebum — so-called ",
" — but this seems to be a myth that is not supported by evidence. According to ",
"this article",
" from the ",
":",
"Many studies have shown that the early sebum flow after the skin is cleaned comes mainly from a reservoir in the pilosebaceous follicle, and not from any feedback stimulation of the sebaceous gland ... most of the antidandruff shampoos showed an absence of significant SER change over time ... Such a finding confirms the unreality of the so-called reactive seborrhea, in which the sebaceous excretion increases with the frequency of most hair washes",
"The study shows a link between excessive oil production and anti-dandruff shampoos containing selenium sulfide, but not ordinary shampoos. ",
"Shampoos are essentially a combination of ",
"surfactants",
" which serve to dissolve oils as well as trap dirt particles. In addition to cleaning away dirt, the shampoo also reduces the natural oils in your hair and scalp. This ",
" make the scalp drier than usual, but the scalp is constantly supplying little droplets of sebum to the surface, and the scalp will replenish itself quickly, with no harm done. The exception is if you're particularly prone to dry scalp issues (or psoriasis, seborrhea, etc.), in which case you will have to take that into consideration.",
"If you wash too frequently, you will be constantly removing the sebum, and logically your scalp will be chronically dry. I don't think you can scientifically say it's ",
" unless, again, you have scalp problems. Also, some people (anecdotally, me) need some oil in the hair to keep from being too bristly. I wash my hair once a week, and add a tiny bit of oil afterwards, works pretty well.",
"Note that if you use hair-styling products such as pomades, creams or waxes, you will eventually need to wash your hair with some kind of oil-dispersing surfactant, since they typically include small amounts of water-insoluble compounds such as petroleum jelly or paraffin wax. (As anyone who uses such products will tell you, they will build up in your hair over time, and eventually you have to start \"fresh\".)",
"Edit: Just to be clear, the above-mentioned study suggests that your scalp will ",
" compensate for dryness if you overshampoo, ie. that this is one of those myths that sound reasonable, so therefore it must be true. Edited for clarity."
] |
[
"What is the mechanism causing separate atom nuclei to repel?"
] |
[
false
] | null |
[
"Nuclei have positive change, so they repel each other due to electrostatic forces."
] |
[
"Are the electrostatic forces you're referring to also considered the electromagnetic forces? How do these electrostatic forces interact with gravity, electromagnetic forces, the strong, and weak force?"
] |
[
"Yes, electrostatic forces are electromagnetic."
] |
[
"I recently learned COVID has a lipid membrane but I was taught in school that viruses are DNA or RNA packed inside a protein shell, so how many different virus physiologies are there and what exactly is COVID made out of?"
] |
[
false
] |
As the title said, I was taught that viruses are a protein shell with a DNA packed in that inject their DNA (or RNA) into the host cell. Now I learn that some viruses actually have membranes and multiple membrane proteins, which is much more complex than the image I have in my mind. Now I'm wondering about how many different phenotypes of viruses there are, is it a binary thing, some are a protein shell and some are a membrane, or more of a spectrum? Do we know?
|
[
"The estimate I remember seeing, but don't ask me to cite it, is that 60% of viruses are \"icosahedral\" ",
" (EDIT: vague, see below). So most viruses you hear about are like this, which is what you see in school textbooks, eg the classic bacteriophages, the common cold, polio, hpv, rotavirus.",
"Some viruses have both a shell and a membrane, eg herpes and HIV. The purpose of the membrane is to fuse with the surface of your cell, that gets the protein shell and other stuff inside.",
"Measles and flu are in the same category as coronavirus, they've got a membrane and no shell.",
"Then there are helical viruses like ebola and tobacco mosaic virus.",
"There are some weird ones out there which have protein shells that aren't icosahedral, like these ones that look just like wine bottles that affect archae. I don't think any of those are known to infect humans. Nobody knows why they're that shape.",
"You could make a very fun, if very small, zoo out of viruses :) it's estimated that 90% of all species ",
" (EDIT: philosophically dubious, see below) are viruses, so you should expect to see variation!",
"ADDENDUM: Lots of people are liking this comment soooo, here's a documentary I made about virus structure :) ",
"https://viruspatterns.com/"
] |
[
"Viruses fall (to my knowledge) into 3 key morphology categories: Icosahedral (or isometric, such as herpes/rhino viruses etc), helical (Influenza, Ebola, measles), and head and tail virus (such as many bacteriophage T7,T4, T4like etc) (There are also outliers like poxviridae (as highlighted in a comment) that dont fit the description perfectly but these are the exception not the rule. There is a 4th type which are called enveloped viruses, which as you have picked up, have a segment of the host lipid membrane wrapped around them, aiding in infection of new hosts, and protecting the virus from detection (in some cases). All of the 3 morphologies i mentioned can be enveloped or non-enveloped (only one family of phages have envelopes).",
"Im not sure by what you mean by how many phenotypes of viruses there are? there are billions of different viruses out there, and each that had different mutations will be a different phenotype, assuming the mutation has an active effect on a protein it is translated into.",
"Hopefully this helps"
] |
[
"The estimate I remember seeing, but don't ask me to cite it, is that 60% of viruses are \"icosahedral\", meaning they have a protein shell.",
"All viruses have a \"protein shell\", the capsid. Its primary function is protecting the genome until it can be released into the host cell. There's different types of these capsid in terms of structure roughly divided into the terms \"helical\", consisting of a polymer of few structures with highly variable length, \"icosahedral\", consisting of 20 equilateral triangles and \"complex\" ones, that are, well, a bit more complex...",
"They ",
" optionally have an \"envelope\", a lipid membrane derived from the host cell, on top of that, altering the strategy used for infection of the given virus compared to viruses that don't have such a membrane.",
"Strictly speaking the only obligatory features of a virus is this capsid and the genome.",
"(I know that you are most likely aware of this, but I thought that that part of your comment was a bit unclear.)"
] |
[
"Why do patches of still water persist behind the wake of a boat?"
] |
[
false
] |
[deleted]
|
[
"I believe it relates back to wave motion and how it interacts with one another. \nWave nature by force is in the constructive or destructive. ",
"What you are seeing is the destructive wave motion. The boat pushes through sending off wake and destroying the waves that are sent from surrounding shorelines or other movements in the waters. ",
"Source) I was on a rowing team and encountered this effect almost daily for about 4 years straight. \nOn choppy days this flat water was a god send.",
"If anyone has better sources, which i'm sure they are out there, feel free to correct me. I love to learn new things :D"
] |
[
"You get the same effect from a kayak or sailboat, the oil may be part of it, but not a compete explanation. It has something to do with turbulence, but I can't say how. If you look closely at that flat water section in the wake there are lots of small eddies."
] |
[
"It could be turbulence, I'm not sure of the exact indepth mechanics of fluid dynamics anymore. ",
"Bossier330 has a sound idea, to build off it a bit, the motion of the propeller churning the water up could result in denser water rising up to the surface for a brief amount of time as well"
] |
[
"Is it actually bad to consume too much soybeans or soy milk because of the estrogen?"
] |
[
false
] | null |
[
"Nope. read this, and then read the citations.... ",
"http://www.livestrong.com/article/519107-soy-estrogen-in-men/",
"then read this, particularly the last paragraph or two: \n",
"http://examine.com/faq/is-soy-good-or-bad-for-me.html"
] |
[
"This",
" came from the same site. I think there really hasn't been a long enough time period to truly understand what the effects could be. "
] |
[
"Soy milk cartons I come across in the US has a fine print stating that it is 'Not to be used as infant formula'."
] |
[
"Could dark energy be used to harvest electricity using temperature differentials?"
] |
[
false
] |
In a video I watched recently, it was theorized that empty space is not empty, but has a dark energy that is the reason for the expanding universe. Also mentioned was how that energy also gives empty space a radiant temperature. Sterling Engines run off temperature differentials, so if dark energy is a force that also creates heat, could it be harnessed to generate electricity? If not temperature differentials, are there better competing theories? (I'm intentionally ignoring the size such a system would need to be, and efficiency of the system, just asking if it's even feasible)
|
[
"Conceivably, yes. But there's the added problem that in order to get an energy differential, you'd need a space with even less energy. I.e., \"turn off\" dark energy in a given volume.",
"However, even given all that...",
"https://en.wikipedia.org/wiki/Dark_energy",
"The density of dark energy (~7×10",
" g/cm3)",
"As per E=mc",
" this works out to about 9E",
" Joules per cubic meter. The only reason it's powerful enough to inflate the universe is because space is crazy-stupid big, and that much volume adds up."
] |
[
"Dark energy is EVERYWHERE. All work occurs in disparity-- and in doing so you even out that disparity. If the energy is the same amount in all locations, you can't really do anything with it."
] |
[
"Do you know of other options for harvesting this radiant energy for electricity? "
] |
[
"Why do different blood types exist?"
] |
[
false
] | null |
[
"Blood type is an example of a polymorphic trait. This means that it is a heritable difference in phenotype determined by the genes at a single locus. As all polymorphic traits, blood types arose after certain selective processes caused certain (mutant) blood types to be more fit than others. For example, blood type is closely related to a person's susceptibility to cholera or ",
".",
"Diversity as an evolutionary adaptation is key as well.",
" Links:",
"Individuals with blood group O are more susceptible to cholera",
"Individuals with blood group AB are more likely to have asymptomatic response to cholera",
"Individuals with blood group O are more susceptible to placental malaria",
"Individuals with blood group A are less susceptible to influenza Type-II, but more susceptible to adenoviruses"
] |
[
"Clarification: The gene for sickle cell is a mutation in hemoglobin, the protein that carries oxygen in your blood, and has nothing to do with ABO blood groups from the OP's questions.",
"Also, carrying a single copy of the sickle cell gene is enough to protect against malaria, but these individuals do not have sickle-cell disease. "
] |
[
"Clarification: The gene for sickle cell is a mutation in hemoglobin, the protein that carries oxygen in your blood, and has nothing to do with ABO blood groups from the OP's questions.",
"Also, carrying a single copy of the sickle cell gene is enough to protect against malaria, but these individuals do not have sickle-cell disease. "
] |
[
"what would happen if two supermassive black holes (like those in the center of 2 large galaxies) collided?"
] |
[
false
] | null |
[
"This simulation was of rotating (Kerr) black holes, which most black holes are theorized to be. While black holes rotate, they induce \"frame drag\" which is basically space-time moving in the same direction as the rotation. Think of swirling your finger in a pool - your finger is the black hole, the water space-time, and the little vortex is creates is frame drag. There is an area around a rotating black hole called the ergosphere, where spacetime itself is rotating faster than the speed of light. ",
"Now, you'll notice that in the first video, the spins are the same. As the holes get close, they add, and it's very smooth. In the rest, the spheres are parallel to one-another, so at a certain radius, the ergospheres intersect and cancel each other out. This is what allows them to \"snap\" like they do."
] |
[
"Nothing overly special. They fall into each other, creating a new black hole of a mass of the two combined. Lots of gravitational waves would be generated, which could spur star formation near-by, but that's about all. Check out ",
"this",
" article for some details and a video.",
"The cool thing is what happens to the rest of the galaxy. The two colliding galaxies swirl and spiral around each other, and massive amounts of stars get formed, while others get flung out into intergalactic space. I highly recommend looking up videos of these simulations."
] |
[
"I've looked up ",
"this",
" video, but found that even in the slow motion segments there comes, in all but the first simulation, a dramatic and discontinuous change in the last moments of merger. Why such a discontinuity?",
"And a ",
"link",
" for those wanting to know what Ringdown means."
] |
[
"What medical tools / drugs does NASA outfit space shuttels/ISS and what kind of medical dificulties have been encountered by astronauts and dealt with them?"
] |
[
false
] |
[deleted]
|
[
"This page seems to give a pretty good overview of what medical emergencies have occured.",
"And this page",
" says that shuttles carried medical kits that could be used by 2 trained crew members. "
] |
[
"Related question: What is the shortest amount of time between going from a regular working \"day\" in the ISS to shaking hands with someone on the ground? Say a serious medical emergency occurs that needs immediate attention by a trained and equipped hospital staff, how fast can enough people needed to deorbit and land get everything ready to depart?"
] |
[
"People seem to forget the I in ISS stands for \"international\". Why does everyone assume only NASA does space stuff? Every NASA heavy launch now goes up on Russian engines."
] |
[
"Through a narrow slit in my blinds a beautiful colour image of the street below is projected onto my ceiling. How is this possible?"
] |
[
false
] |
This might be difficult to accurately describe. The blinds are polished paint and the slit through which the light passes is about 50cm in horizontal length. Here's the tricky point: the gap in the (venetian) blinds is over 1 cm wide but is covered by a lip from the outer blind. The light isn't passing directly through the slit but being reflected off the exterior left hand blind and then the interior surface of the lip of the right hand blind. How is it that this can create clearly defined, undistorted colour images? Many thanks.
|
[
"pics?"
] |
[
"Can you take a picture of the blinds so we know the exact setup? "
] |
[
"Yes. If you can get the right lens in from of the slit, you can bring the image into focus too."
] |
[
"When an ejaculate is thicker, does that mean it has a higher sperm count?"
] |
[
false
] |
[deleted]
|
[
"Short answer: no, it is likely to have ",
" sperm count.",
"In a healthy male, abstinence time is strongly correlated to seminal viscosity. After ejaculating, seminal viscosity decreases in a linear fashion with respect to time. ",
"Source",
"In a healthy male, abstinence time is the primary determinant of total ejaculant sperm count. After ejaculating, sperm count increases in a linear fashion until a maximum level is reached after about 5 days of abstinence. ",
"Source",
"Both of them are strongly influenced by abstinence time, so in general, a thicker ejaculate will have had a recent ejaculation, and will therefore have a lower sperm count.",
"Now if you'll excuse me, I have to clear my browsing history..."
] |
[
"Your efforts are both a testament to the availability of information on the internet and how little you value your free time. Kudos."
] |
[
"I have other, much more important things I should be writing, so obviously I'm writing stuff here."
] |
[
"Are trees immortal?"
] |
[
false
] |
[deleted]
|
[
"In theory, many plants could be essentially immortal. For instance, many cultivated plants that are propagated via cuttings or tissue culture are effectively the same organism over thousands of years. Plants do not die of cancer and they do not have organs in the sense that animals do.",
"Many trees die when they do purely due to environmental stressors, such as disease or insect infestations. I don't have a link with me, but a while back I read a research paper on the mortality causes of red alder in tree stands. Basically, the alders do not die of 'old age'. By the time that they reach roughly 100 years old, their heartwood has grown to the point that the tree cannot really defend it from invaders, and it generally gets infested by a disease or parasite that kills the tree from the inside.",
"Theoretically, a red alder tree with depressed growth or one that is kept from disease (in a sterile environment, say) may be able to live much longer than a hundred years."
] |
[
"Here",
" is a more complete explanation, but in a nutshell:",
"Plants have cell walls. If the DNA of a specific cell causes a mutation that inspires abnormal growth, it's kind of stuck in place. This doesn't mean it won't form a tumor (plants form galls, and ",
" may be used to produce tumors in plants that have foreign DNA- this is one way transgenic plants are made).",
"As best as we know, plants don't allow cells to move within the plant. (Perhaps someone will discover some obscure form of cellular transport in plants, so I won't say it doesn't happen.) Unlike humans (with our bloodstream and lymphatic system), they do not allow for cells to break off and metastasize elsewhere (i.e., \"throwing mets\"). With this, there is no formation of remote tumors."
] |
[
"Plants do not die of cancer ",
"Why don't plants die of cancer?"
] |
[
"If all the rivers leading to oceans slowly wash dirt into the sea, will there eventually be no dry land?"
] |
[
false
] |
Water evaporates, forms clouds, and then rains. Some of that rain evaporates again, but some of it will go into tributaries, streams, and rivers. Some of those will go into the sea. The water going into the sea washes dirt away with it. Will this process on a large enough time scale level out all the dry mass till the entire planet is encased in unbroken water?
|
[
"Erosion is only half of the equation. Mountains are also being slowly formed by the action of plate tectonics, driven by the heat of the earth's core. Today's earth is in a near steady-state balance between erosion washing the continents away and plate tectonics pushing up new ground. In the long term, when earth's core cools and plate tectonics cease, than it would be possible for erosion to gradually remove all the land masses."
] |
[
"New crust is created at divergent plate boundaries, such as the mid-ocean ridges between the North American and Eurasian Plates, the boundary between on the eastern edge of the Pacific plate, and along the edge of the Antarctic plate. In all of these places, new crust is created as two opposing plates move away. The Pacific/South American plate boundary you're referring to is likely the Nazca Plate/South American plate convergent boundary, where the Nazca plate is being subducted underneath the South American Plate. In this location on the eastern side of the Nazca plate, crust is being destroyed (The Nazca plate is being subducted). On the western side of the Nazca plate, new crust is being created at the divergent boundary with the Pacific Plate. Of course, this is oceanic crust.\nThe Indian/Eurasian plate collision is a convergent continent-continent boundary, where the Indian and Eurasian plates are effectively stacking on top of each other, forming some of the thickest crust on the planet. This gives rise to the Himalayan plateau and the Himalayan mountain range. However, new crust isn't being created here - existing crust is deforming, stacking, thrusting and faulting, but not being created at that particular boundary.\nAs sverdrupian says, new crust is created and destroyed at effectively the same rate - if it were not, the Earth would either grow or shrink, and that's not thought to be happening. Now, as to the original point - new continental crust can be created by partial melting of oceanic crust and other processes, like volcanism, accretion and high-grade metamorphism. There is a lot of continental crust, and while erosion is constantly wearing it down, uplift is also happening. This uplift can raise up continental basement rock, overlying sedimentary rock, even oceanic crust. When those rocks get exposed, they begin to weather. The products of weathering - sediments- get deposited in the ocean or on land. The sediments that make it to the ocean are often deposited rather close to shore, still on the continental shelf. Any of these sediments can lithify and become sedimentary rocks, which, in time, can be uplifted because of convergent plate collisions or faulting, at which point these become the areas subject to erosion and weathering. Some of the sediments can get subducted if the plate they are on is being subducted at a convergent zone, and these sediments and sedimentary rocks will melt (fully or partially) and can then rise up into the overlying rock and form new masses of continental crust.\nThis whole thing I'm (poorly) describing is the rock cycle, and it's a process of constantly recycling the materials on the Earth's surface. It is driven by plate tectonics, which in turn is driven by the heat from the Earth's core as well as the heat of radioactive decay in the mantle.",
"\nSo, will this all end? Someday. When there's no longer enough heat to drive plate tectonics, the dynamics of the surface will change (number of plates, motions, amount of volcanism, etc). But as most of the heat flow coming from the interior is provided by long-lived radiogenic isotopes decaying (eg U235, K40), that time a long way off - likely billions of years."
] |
[
"In some areas, like the Great Lakes, the elevation is actually rising since the ",
"land is still bouncing back from the weight of the glaciers being lifted."
] |
[
"how does voltage get distributed in a weird circuit"
] |
[
false
] |
I came across this weird circuit that looks like this So I'm not sure how will the voltage be distributed when the two imput wires connect to the object. I understand that parallel circuits have the same voltage as they both have the same potential difference in the sense of Pd=p(final)-p(initial). But in this case since there are two imputed wires I am unsure of the voltage of the object. Also, how do I calculate the total effective resistance and current? Thanks for the help.(please ignore the other stuff on the right if the picture)
|
[
"You're not really using standard schematic symbols, so I'm having a hard time understanding what you've drawn. What is the \"logic box\"?",
"BTW: this might be better over in ",
"/r/AskElectronics",
" "
] |
[
"It can we assumed to be just another appliance I'm just unsure about the voltage across it how to determine resistance and current when um presented with a circuit similar to this where two wires from different branches are brig connected to an electrical load. Thanks for the help."
] |
[
"No, I mean ",
" on the diagram is the \"logic box\"."
] |
[
"Has our Sun ever \"ate\" or engulf a planet?"
] |
[
false
] | null |
[
"After the creation of our solar system the sun was surrounded by a disk of debris, consisting mostly of dust and small chunks of rock. At this point the sun must have been eating anything that lost too much speed to stay in orbit, but nothing of considerable size existed yet. When these formed their orbital speeds were high enouch to keep them away from the sun. \nFun fact though, comets and asteroids are frequently still passing the sun very closely, if not crashing on it."
] |
[
"Early in the solar system it definitely did but I don't think you could call the stuff it engulfed 'planets'. They were mostly rocks and debris. In the future however at the end of it's life approximately 4-5 billion years from now the Sun will expand and swallow up the inner planets. "
] |
[
"It probably did in the early Solar System days, but there's no way to prove that it happened. We do know the Sun eats comets fairly often though, and have ",
"good observations of that",
".",
"The Sun will eventually swallow Mercury and Venus in a few billion years as it swells at the end of its life. The jury is out on if the Earth will be swallowed as well or merely left as a charred rock- I think it depends on what stellar evolution model you use for the answer there."
] |
[
"If you run around a track twice, the first time slowly, the second time much faster so that the average for the two laps is twice the speed of the first lap. People are getting infinite speed for the second lap. Why?"
] |
[
false
] |
This question pops up in Veritasium's new . People are getting infinite speed for the answer. If you run the first lap at 6 km/h and then the second lap at 18 km/h you get an average of 12 km/h. That average is 2v . How is this not correct? You can also check people's answers and the third answer to a Youtube comment . There are also multiple answer videos that say the same thing. Help me not be confused.
|
[
"If you run the first lap at 6 km/h and then the second lap at 18 km/h you get an average of 12 km/h.",
"You can't average velocities in this way. You can only do this if the time is equal for the two velocities. In this case, that's not true, since it's the distance that is equal.",
"To illustrate, assume that the track is 18 km long. The first lap takes 3 hours to complete at 6 km/h. The second lap takes 1 hour to complete at 18 km/h. The total distance covered is 36 km and this took 4 hours. That means that average speed is 9 km/h, only 50% more than the speed of the first lap.",
"If you want to have the average speed by the double of the speed of the first lap, the average speed should be 12 km/h over the total distance of 36 km. But that means that the total time spent should be 3 hours, which is already how much time was spent on just the first lap. That's why people are saying you'd need an infinite speed on the second lap in order to achieve the desired average."
] |
[
"The unstated principle here is speeds should be weighted by time -- the runner spent three times as long running slow, so 6 km/h gets weighted three times as much:",
"(3*(6 km/h) + 1*(18 km/h))/(3 + 1) = 9 km/h.",
"Or equivalently, we should take ",
"harmonic mean",
" of the velocities rather than arithmetic mean.",
"A priori it's not clear why one mean is more appropriate than the other for the generic situation as stated. If someone on the sidelines is watching the runner pass by every so often (think of many runners in a marathon) it's the arithmetic mean that's more perceptible and relevant."
] |
[
"It's not about weighting, it's the definition of the term. Speed is the ratio of distance to time. Instantaneous speed is defined as the derivative of distance with respect to time, and average speed is defined as just total distance divided by total time. Since that's your definition, that's how you calculate it. The problem is when someone wants to calculate average speed between two pieces of travel using a formula for the mean, which is a different object that is not average speed.",
"Edit: Okay folks, let's do an example. You're driving a distance of 100 miles. It takes you 4 hours. What is your average speed? 100 miles divided by 4 hours = 25 mph. This is always the answer. It doesn't matter if you go 80 mph per hour for 1 hour, then stop for 2 hours then 20 mph for 1 hour, or if you go 25 mph for 4 hours, or if you sit still for 3 hours and 50 minutes and then go 600 mph for the last 10 minutes. If you think calculating average velocity works by some sort of (A + B + C + ... )/N type average, you'll get different average speeds, and you'd be wrong."
] |
[
"How good is a raptor's hearing? Does it go beyond the human range?"
] |
[
false
] |
Hi there asksciencers! I have a question out of mere curiousity. Hawks and other raptors are well renowned for their sense of sight. But so far all google searches have turned up that raptors (excluding owls) have 'good' hearing with no comment to the definition of 'good'. How good is a raptor's sense of hearing? Does it go above the human hearing range for infrasonic frequencies? Any papers or sources I could read?
|
[
"Birds generally have a more limited range of hearing, and inferior thresholds, to humans. Humans are restricted to 20 Hz -> 20 kHz. For certain ultrasonic hearing in birds has been investigated and has not been found (over 20 kHz), and also low infrasonic frequency hearing has not been found, but is less well investigated. ",
"Mammals have a lot of specialization for sound reception that birds lack. "
] |
[
"I want to thank you for using the term 'raptor' correctly. The idiots in Toronto named their basketball team \"The Raptors\" when they really meant \"Velociraptors\". I keep hoping they'll fix the logo to show a bird, but we are talking Toronto sports teams here ..."
] |
[
"While going a bit further off-topic, it's fun to note that Toronto's mascot bears little resemblance to a velociraptor, which is narrower and about the size of an elongated chicken. Largely because of Jurassic Park, the animal people think is a velociraptor is in fact a ",
"Deinonychus",
". Even its name, \"terrible claw\", is a reference to the enormous claw on its back foot that is the creature's most remaked-on physical feature in the film."
] |
[
"What causes the body to produce dental tartar, and how does tartar control toothpaste reduce the amount of tartar buildup on the teeth?"
] |
[
false
] | null |
[
"Tarter is caused by a bacteria that resides in your mouth, plaque. When plaque builds up it creates a biofilm. If you leave this biofilm on your teeth it will eventually harden which is commonly known as tartar. Toothpaste uses a combination of “cleaning” chemicals that interact with the biofilm to dispel it, however, biofilm are hard to get rid of which is why we use toothbrushes to physically remove it."
] |
[
"Your saliva has minerals, calcium, sodium, etc, that are used to build a mini-coral-like reef in your mouth."
] |
[
"Toothpaste also has various abrasives that help to physically remove the films when combined with the brushing action"
] |
[
"If I were to leave my fully charged laptop turned off and unplugged for five years, could I still start it up on battery only?"
] |
[
false
] |
Average run-of-the-mill HP laptop, fully charged. I unplug it, turn it off, leave it at room temperature, and then I wait five years. Will I still be able to turn it on using battery power, or is the battery somehow discharged?
|
[
"Most likely not. All batteries suffer from a phenomenon called \"self discharge\", which is the slow reduction of capacity when a battery is not in use. Rechargeable batteries have higher self discharge rates than non-rechargeable batteries.",
"Lithium ion batteries that are used in laptops and many other mobile devices, have a self discharge rate of about 2-3% per month. The rate depends on the temperature, with higher temperatures resulting in a higher self discharge rate."
] |
[
"This is likely because it's an old laptop. The older laptops would charge the battery to 100% then continuously give it a small constant charge to keep it there. This was bad for the life of the battery. Newer laptops have more intelligent charging algorithms and sensors so they know when and how much to charge the battery to maximize it's lifetime."
] |
[
"Exactly right. Plus if their voltage is too low, they will never hold a charge again. "
] |
[
"Hypothetically, what would happen if a neuron synapses onto it's own dendrites?"
] |
[
false
] |
If you could somehow get a neuron's dendrites to connect to it's synapses what would happen if an action potential was triggered?
|
[
"So this does happen, and it's called an \"Autapse\" as in an auto-synapse And I've written a couple papers on it even. A specific class of inhibitory interneurons does do this, and it does it very strongly, in fact, it's one of the strongest synapses in the brain, and certainly in the cortex. So what this means is that after that the cell fires an action potential it strongly inhibits itself.",
"Why does it do this? Well it's not entirely obvious. You see, directly after an action potential, a neuron is already strongly inhibited (because of all the the potassium channels that are open after an action potential) so adding extra inhibition doesn't do as much as it normally would. It definitely slows down the firing of the cell to some degree.",
"What I found, using a computational model, was that these inhibitory autapses make the cells \"oscillate\" is a slightly more reliable fashion. You see, these kinds of cell seem to take part in group activity, where all the cells like it in a local area fire at about the same time, and then they fire again about 15 milliseconds later, i.e. they oscillate, like a swinging pendulum. This oscillation takes place because all the cells a) inhibit each other through synaptic connections, and b) are directly electrically coupled to each other through \"gap junctions\". What happens is that when a cell fires, some of that depolarizing activity spreads to it neighbours through gap junctions. If those neighbours were close to firing anyway, then that depolarization brings them to threshold and they spike. After that, because these cells are all inhibitory, and all connected to each other synaptically, they all inhibit each other, meaning any cell that hasn't fired yet gets silenced, and then the cycle repeats. Adding inhibitory autapses basically increases that action, meaning that it is even less likely for a cell to fire out of turn.",
"Recently, new evidence showed that some excitatory cells in the cortex also have autapses, though they are rarer and smaller than in inhibitory cells. They seem to encourage the neuron to \"burst fire\", that is, fire a couple action potentials very close together.",
"Despite the fact that I spent a couple years studying these type of synapses, I believe they largely form by accident. I believe this because the interneurons I studied typically form synapses on other interneurons of the same type, hence when there axons grow, the seek out the same type of neuron as themselves. Hence it is no surprise that they accidentally find themselve. When people looked at excitatory autapses, they found that neurons in layer 2/3 of the cortex has less autapses than neurons in layer 5. And it turns out that layer 2/3 cells are less likely to connect to other layer 2/3 cells than layer 5 cells are to connected to other layer 5 cells. i.e. the is a correlation between the frequency of autapse formation and the liklihood that a cell will connect to another cell of the same type."
] |
[
"I don't have any conclusive proof that they form by accident. An experiment that would prove me wrong is finding a cell type that has lots of autapses, but doesn't ever connect to other cells of the same type. We really only have three cell types to build up our correlation, so it's shakey.",
"I mean, the \"by accident\" starts to get a bit... semantic? Biological stuff follows the blueprint laid down by the actions of molecules. Nothing is more or less by accident that anything else. So should say that I believe it happens as a result of the same mechanism that causes these neurons to synapses to other neurons of the same type. Obviously their axonal growth cones are attracts to neurons of the same type, and so sometimes that attractions causes them to synapse on themself. Other evidence that supports this is that the autaptic synapses are physiologically identical to the synapses between neurons of the same type: they have the same short term plasticity, and same receptors, same anatomy and are modulated by the same neurotransmitters. Also, we know that if you trap a neuron on an microscopic island as it grows, so it's axon can't go anywhere, it forms autapses on itself.",
"So my papers on the subject can be found here\n",
"https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0089995",
"and here:\n",
"https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.2009.185199",
"This is the one on the excitatory autapse:\n",
"https://www.nature.com/articles/s41467-018-07317-4"
] |
[
"This is so damn cool. Thanks for taking the time to explain it! :D"
] |
[
"Are Photons everywhere?"
] |
[
false
] |
If were looking at light from a galaxy a million light years away, are there photons everywhere in between the telescope and origin point? If so does this mean that photons are potentially everywhere?
|
[
"Technically yes. However, obviously an object can intersect between the viewer and the light source cutting off that particular source of photons for some time.",
"So, for example, if you are observing light from 1 million light years away, and an object intersects at 500,000 light years, you won't see the light disappear for 500,000 years. ",
"That doesn't mean that photons are still not there, from other light sources - but simply not from the one that was blocked.",
"Another possibility to think of is the object emitting the light is no longer there. In that case you will still receive photons up to the point where the object stopped creating them (delayed, of course, by the distance to the object). So... if a star 1 million light years away burns out today, you will still see photons for 1 million years - even though the star no longer exists."
] |
[
"Could you clarify what you mean by \"photons everywhere in between the telescope and origin point?\" Do you mean a continuous stream of photons that lie along a line between the origin point and the telescope?"
] |
[
"OP's context:",
"http://www.reddit.com/r/science/comments/se8py/by_taking_advantage_of_a_rare_cosmic_zoom_lens_a/c4db8k4"
] |
[
"Is there a way to convert black and white photographs into accurate full-color photographs?"
] |
[
false
] |
[deleted]
|
[
"Is there a reason why multiple colours can translate into a single black and white \"colour\"? Why doesn't each individual colour appear as a slightly different shade in the black and white photo?"
] |
[
"Is there a reason why multiple colours can translate into a single black and white \"colour\"? Why doesn't each individual colour appear as a slightly different shade in the black and white photo?"
] |
[
"As others have noted, it is not possible to produce an accurate colour image from a single black and white photo because you lose the hue and chroma of the original image.",
"It is, however, possible to reconstruct a colour image from three black and white images using red, green, and blue filters separately. This was used at the beginning of the 20th century by ",
"Sergey Prokudin-Gorsky",
" to produce colour images by later combining the images through a projector using the proper filters.",
"In television, NTSC uses this same sort of principal to produce colour images. 3 signals are transmitted, the luminance, which is roughly analogous to the black and white image, and the chroma which consists of two signals, one transmitting the difference between the blue channel and the luminance and the other transmitting the difference between the red channel and the luminance. Each signal is a monochrome image and could be viewed as black and white video separately, but together, the television can derive the red, green, and blue channels and produce a colour image. The reason they selected this method was for backward compatibility with b&w tv. They also were able to save on bandwidth by cutting the bandwidth to the chroma signals."
] |
[
"How does one select for specific traits when breeding animals? And what traits can you select for?"
] |
[
false
] |
I am curious about animal husbandry and how to create hybrids through the selection of specific traits. what I want to know is how this is done, and what traits you can select for and which ones are at random. for example can you select for speed and for athleticism? or for intelligence? or for strength? what traits are not hereditary?
|
[
"You can theoretically select for any trait, and you do so by picking the most outstanding members of a generation with respect to that trait and pairing them to breed the next generation. The members of the next generation will likely vary from somewhat worse than their parents to somewhat better than their parents, so you repeat the process with the cream of the new crop. You'll usually start to see something of a plateau at some point as your population becomes increasingly genetically homogenous, or even a decline if some harmful recessive genes have snuck in and started to be expressed at high rates. Or, as is the case with squished-face dog breeds, you may have simply been unconcerned with the negative side effects of the trait you were trying to promote.",
"The only traits that you can't select for are ones that are either learned or caused by external factors such as temperature during gestation (the embryos of some species, such as sea turtles, will develop as different sexes depending on the temperature of the egg's environment). However, you can always teach the new generation the same things as the old or control the external factors that influence development."
] |
[
"Most traits you are likely to think of, including all the ones you listed, can be inherited and thus selected for. The heritability of different traits varies though, you can try googling your trait of interest and 'heritability' (ie 'strength heritibility' or 'milk fat heritability') and you should find a number fairly easily. It will either be a decimal or a percentage. Closer to 100% or 1.0 means more heritable and thus more amenable to selection, closer to 0 means less heritable. ",
"As the other comment mentioned traits that aren't heritable at all tend to be learned traits. For example 'ability to speak English' is not heritable as it is something learned, and whether you are taught it or not determines ability, not genetics. However something like general language proficiency, or how fast one learns a language, is likely to have a heritable component. Other traits that are not heritable are those that no one has, for example for humans 'ability to survive having a thousand Megatonne weight dropped on top of them', would have a heritability of 0 since.",
"In general when selecting on traits three things are important",
"The heritability of the trait (that number I mentioned earlier)",
"The strength of selection (do you pick the tallest 1% to breed with of the top 10%?). The smaller that percentage the stronger the selection is",
"The number of generations to which you apply the selection. "
] |
[
"So, very importantly, you can only truly know if traits are hereditary by breeding individuals that have the trait for several generations then looking to see if the trait is still present in the population. Biology is complicated, so sometimes you can get two individuals with traits that look identical on the surface, but the trait is heritable in one individual and not the other. You can't classify a trait itself as heritable or not heritable, unless you are talking about a specific lineage or gene.",
"Let's assume for the sake of a thought experiment that anything is heritable though. Generally speaking, more complex traits will be harder to breed for. E.g., intelligence is probably caused by many genes that control things like neuron patterning, memory, neurotransmitter expression, etc, whereas something like fur colour could be simple enough to be caused by a single gene for some type of pigment. Additionally, complex traits like intelligence are more likely to be influenced by non-genetic environmental factors.",
"To understand why, let's look at how you would breed for a single gene.",
"Unless you are directly mutagenizing genes of interest, which is difficult, ethically questionable, and not common practice in animal husbandry, the only way you can get a new trait to breed for is if it randomly appears in a population. Usually, this would happen because of a random mutation. Say the mutation is in a pigment gene, and suddenly a red puppy is born in a population of dogs that has always been black. If you wanted to select for that red gene, you would breed the red dog with any other dog. All the puppies born from this match will get one red pigment gene and one black pigment gene. If you breed these puppies with each other, 1/4 will have two red pigment genes, 1/4 will have two black pigment genes, and 1/2 will have one of each pigment gene. You can keep breeding dogs and selectively overbreed puppies born with two red pigment genes. Ta-da, you have a new kind of dog that's red.",
"The more genes you are trying to select for, the more difficult this process becomes, because the odds of any one puppy being born with ALL the genes are way lower. Let's say a dog is born with red stripes. There's a gene for redness and a gene for stripes -- if you breed this dog with a regular black dog, now only 1/16 of the grand-puppies would be born with the red stripe trait, because the probability of inheriting only one of the genes is 1/4, so 1/4 * 1/4 = 1/16. Now, if you have a trait like intelligence that is caused by, say, 20 genes (random guess and probably a low ball), the % of grand-puppies born with the same intelligence trait you're trying to breed for would be 1/(4",
" which is something crazy like 1 in a trillion puppies. Notably, having only a few of the intelligence genes would still make the puppies more intelligent, but the key issue is really that more complicated traits are harder to control and predict. "
] |
[
"Can an electromagnetic an wave exist only within certain frames of reference?"
] |
[
false
] |
I was once pointed out this paradox by a physics teacher: My teacher already told us that this is still an unsolved paradox, and that Maxwell Equations and relativistic electrodynamics still hasn't a clear answer. I had been believing him for a decade, but the question poped-up in my head recently. Was my teacher right?
|
[
"There is a ",
"-paradox-",
" surrounding what you say, it is not however (at least to my knowledge) an \"open question\". Just a subtle consideration of electrodynamics in the context of general relativity. The starting place for the resolution is that falling charges do indeed emit EM radiation, like for example when mass falls towards a neutron star, something I discuss ",
"here",
". The \"exercise\" or \"paradox\" is then about making that observed fact okay with general relativity, which can be done (from the wikipedia above):",
"This section follows the analysis of Fritz Rohrlich (1965),[1] who shows that a charged particle and a neutral particle fall equally fast in a gravitational field, despite the fact that the charged one loses energy by radiation. Likewise, a charged particle at rest in a gravitational field does not radiate in its rest frame. The equivalence principle is preserved for charged particles.",
"When this coordinate transformation is applied to the electric and magnetic fields of the charge in the rest frame, it is found to be radiating, as expected for a charge falling away from a supported observer. Rohrlich emphasizes that this charge remains at rest in its free-fall frame, just as a neutral particle would. Furthermore, the radiation rate for this situation is Lorentz-invariant, but it is not invariant under the coordinate transformation above because it is not a Lorentz transformation.",
"In other words, when comparing the observed EM field for an observer on Earth (a so-called \"supported\" observer as the gravitational force downwards is being cancelled by a normal force from the floor upwards) with the frame of a resting charged particle in free-fall, transformations are needed that go beyond that of simply switching inertial frames in Maxwell's equations. It's strange, which is why it's a paradox, but it all works out.",
"For the sake of curiosity, there actually is an open question that is somewhat related, which is the ",
"Unruh effect",
" which states:",
"The Unruh effect (or sometimes Fulling–Davies–Unruh effect) is the prediction that an accelerating observer will observe blackbody radiation where an inertial observer would observe none. ",
"This effect is controversial and the reality of it is unknown."
] |
[
"a charged particle and a neutral particle fall equally fast in a gravitational field, despite the fact that the charged one loses energy by radiation",
"Where does the radiation energy come from? The gravitational and kinetic energy should be the same in either case.",
""
] |
[
"Sorry for the much-delayed response. Apologies. And a double apology because I fully must admit that general relativity (GR) is basically the opposite end of physics from my specialty and I'd rather be vague than give you wrong information. So take everything I'm saying with a beach of salt. ",
"However, the heart of the issue lies in some of the vagueness associated with the problem when talking about GR. For example, in GR, the comparison of local quantities, like energy, is something of a dirty no-no. When you have different observers resting at different places experiencing different space-time curvatures it is often a very tenuous, or even strictly mathematically forbidden, exercise to compare certain quantities between frames. For example, as I mentioned with the Unruh effect above, it would potentially seem to be a prediction of GR that different non-inertial frames shouldn't agree on the particle content or temperature of the vacuum of space around them. ",
"An aspect of this comes up, for example, if you've ever read about Big Bang Cosmology (BBC) and whether it is or isn't valid to say that there is global conservation of energy in BBC - which given redshift would ",
" not to conserve energy. However, on can recover conservation ",
" one assigns an energy to the \"curvature\" of space itself. The issue in that debate is that in GR one ",
" assign or construct a notion of a \"local\" energy density of curvature like one can when talking about local energy density of mass, light and dark energy. So one can have globally a balance of energy lost to redshift due to curvature and negative energy \"gained\" by curvature itself but no way to make that work at any specific ",
".",
"When it comes to the particular paradox here the issue seems to largely lie on what is or isn't \"radiation\". EM radiation is generally defined as having a component of an EM field whose strength diminishes AT INFINITY slower than 1/r",
" (like a static E or M field). I talk about it a bit ",
"here",
". Basically it's a component of an EM field that persists, distance-for-distance, more than a static field. So what separates the \"radiative\" part of an EM field from its \"near\" part is how it falls off in strength at infinity.",
"The issue then becomes, WHOSE infinity? Based on my, again, super limited GR knowledge, the resolution seems to be that a \"supported\" observer and an unsupported or \"co-free-falling\" observer don't have comparable infinities and it's okay for the supported observer to observer a radiative component at their infinity where the co-free-falling observer doesn't in their own because it's possible for the radiative component observed by the supported observed to be outside the horizon of what is even theoretically observable for the co-free-falling observer."
] |
[
"Dark Matter - Why not neutrons?"
] |
[
false
] |
[deleted]
|
[
"Free neutrons (those not bound in nuclei) will decay with a half life of about 11 minutes. The decay products are a proton, electron, and and (anti electron) neutrino. The proton and electron are charged, and so couldn't really be ",
" matter. The neutrinos aren't a separate hypothesis. Even if the universe was at one time full of neutrons, today all we'd see would be hydrogen and neutrinos. "
] |
[
"He's talking about the weak nuclear force, not that it interacts weakly with gravity."
] |
[
"It's not new at all. Neutrons are made of quarks, and quarks have electric charge. This gives rise to a magnetic dipole moment for the neutron that will interact with magnetic fields. This is well measured."
] |
[
"@Earthquake Engineering, in simple terms, what's the precise connection between response spectrum and time history for a building"
] |
[
false
] |
[deleted]
|
[
"You seem to be trying to create a model for the building, which is not exactly what the original question asked.",
"There are several methods to find the components of a spectrum by parametric analysis, it's an ongoing research. The most common methods are based on the ",
"Pisarenko's method",
", like the ",
"MUSIC",
" and ",
"ESPRIT",
" algorithms. This is rather advanced digital signal processing here, it probably takes a master's degree in EE to understand the details.",
"I haven't worked with earthquake engineering, so I don't know about the details of that field, but I do have experience in vibration analysis in aerospace engineering."
] |
[
"You transform a time series into a spectrum by using a ",
"Fourier transform",
". Any math library will have a version of it."
] |
[
"Is that answer correct or wrong? Don't you do something like the following?",
"--> Output is the earthquake response spectrum"
] |
[
"Are there forests of fruit trees in the wild? Or do they naturally only form isolated clumps?"
] |
[
false
] |
[deleted]
|
[
"OK, first of all, every tree produces fruits. Fruits are (essentially) just the parts of a plant that bear seeds. Even gymnosperms (things like pine trees) produce structures (such as cones) that can be considered fruit using this simple definition. ",
"Second of all, when you say \"forest\" you seem to be imagining a homogenous, dense population of the same tree vs. \"groves\" - smaller patches of the same kind of tree. It's not too often that you find forests entirely of a single kind of tree unless it's been planted by humans (pine, spruce plantations, etc) or if there's been some human selection for a particular species (here in Ontario we have forests of just sugar maple or just walnut because humans have selected for those species for their economic value). ",
"So the answer really is \"yes and no\". There are forests that contain many different species of trees, all of which produce fruit in some form or another. Some forests are dominated by nut trees (like hickories) or drupe trees (cherries, serviceberries, hackberries, etc). I suppose, in a sense, this might fit your definition of a \"fruit tree forest\". But in other ways, no, because you would never find a forest of just peaches or just apples unless its been planted by humans.",
"Source: I am a botanist.",
"Edit: I should say that I realize the cones and other seed-bearing structures of gymnosperms are not fruits. But for a simple layman's definition of fruit, it makes sense to count them. ",
"Edit 2: Some examples of naturally-occurring forests of just a single species of tree do exist. An example are the quaking aspen forests of western North America, which are often entire colonies of the same organism, connected underground by roots. There are also forests of just black spruce or just white spruce in Canada's far north. "
] |
[
"/u/ilexmucronata",
" is correct, but I can add a little more. The fruit that we get at the store, which is what OP is probably imagining, is largely the result of careful cultivation by humans. Many of the varieties that we eat could not grow in the wild (or at least not as we envision them). Apples, for example, grow wild throughout Central Asia (where they originate), but they are largely scattered, small, and mostly inedible."
] |
[
"Your first question made me think of food forests, not fruit forests. I'm not sure if you've ever heard of them, but I thought you would enjoy what information I could offer.",
"I learned about food forests in my Permaculture course and it was my favourite section of the course. It was understood that food forests were always designed by man, or to say it another way, to the extent that food is egregiously layered in a food forest doesn't occur naturally.",
"I know, off topic, but thought it might help answer the essence of your question, though not the specifics.",
"300 Year Old Food Forest - ",
"http://www.youtube.com/watch?v=-5ZgzwoQ-ao",
"2,000 Year Old Food Forest - ",
"http://www.youtube.com/watch?v=hftgWcD-1Nw"
] |
[
"Where is the North Pole of Uranus?"
] |
[
false
] |
How did it get sideways? Some kind of impact?
|
[
"Uranus has a rotational inclination of ",
"82 degrees",
", putting its pole closer to the solar system's plane than earth's tropics. ",
"I haven't read specifically about Uranus, but I'm familiar with planetary accretion. The young solar system was more spherical, less uniform in the orbits of the planetoids. Collisions have the effect of evening out the orbital planes of planets to produce the flat plane the solar system is in today. These collisions can cause shifts in orbital characteristics. ",
"It is notable that Uranus has a low orbital inclination- it orbits within the plane of the solar system. If a planetary collision occured with a planetoid with highly inclined orbit, I would think it would shift the orbital inclination of the planet."
] |
[
"I think the question should be \"where are the poles of Uranus\"",
"Since the usual convention used by most people seems to be Up is North, then Uranus doesn't really have a pole that's North. It's closer to an East and West Pole.",
"P.S. However, the definition currently endorsed by the International Astronomical Union is that the north pole of a planet or satellite is the pole that points above the invariable plane of the Solar System, regardless of the direction the planet is spinning",
"P.P.S. And yes, the current theory is that Uranus was impacted by another Earth or larger-sized planet early in it's formation, causing it's orbital tilt"
] |
[
"causing it's orbital tilt",
"That'd be ",
" tilit; Uranus orbits almost exactly within the solar plane."
] |
[
"What is the difference among CT scans, PET scans, and MRIs? More specifically, what is each one used for?"
] |
[
false
] |
What might a CT scan find that a PET scan or MRI might not? How does a doctor decide which one to order?
|
[
"Within each of the studies you mentioned there are subcategories and a lot of technical differences, but I will give you broad function/purpose in general terms:",
"CT (Computed tomography) is basically really high resolution X-ray that is obtained in 2-D and then can be reconstructed in 3D. Helical CT is pretty much exactly what it sounds like. The imager basically spins around the patient in helix at a pre-determined slice thickness. This can create very high resolution images of the body. Contrast (IV or enteral) can further delineate anatomy by highlighting vessels or bowel, respectively. CT is good for looking at bone, soft tissue (not as good as MRI though), and vascular anatomy. It is quick, relatively cheap, and accessible, so they are done very commonly. The down side is the radiation and the contrast (can cause allergic reactions or kidney problems).",
"PET scans are a nuclear scan where a radio-labeled tracer is given through an IV and a detector outside the body localizes the tracer. This is metabolic imaging, so it is looking for areas with increased energy consumption, typically. This is why it is often done in cancer imaging, because cancers are often very catabolic. The downside to PET is that they lack resolution, so the images can be quite fuzzy. More commonly now, PET scans can be combined with CT scans for more high-resolution 3D imaging (amazingly called PET CT). ",
"MRI is based on magnetic fields, not radiation. Basically every molecule in your body is aligned (precessing) in a very powerful magnet to the same orientation. A radiofrequency pulse is then applied and that changes the orientation of the molecules. The molecules then recover to the original state. Each tissue (eg bone, fluid, muscle) recovers differently, so the signal they generate is different. MRI is very good at looking at soft tissue, however the scan times are long. Accordingly, looking at moving things with MRI can be very difficult (blood, heart). This can be overcome by doing very specialized scans with timed pulses to create 4D images, but in general most MRIs are looking at static images in very high 3D resolution. MRI is typically less accessible and expensive, but doesn't involve ionizing radiation. "
] |
[
"Not sure how they specifically decide what to use what for, but a PET scan starts with a radioactive chemical being injected, and it’s absorbed into the organs and tissues being studied. The scan measures blood flow, oxygen use, and glucose metabolism which helps doctors identify the abnormal amongst the normal. ",
"Commonly used for cancer detection, heart problems (coronary artery disease, damage following heart attack, etc.), brain disorders (tumors, seizures). "
] |
[
"Also for detecting stress fractures, using radio-labelled bone growth cells, as they are invisible in X-rays."
] |
[
"What stops black holes from imploding on themselves?"
] |
[
false
] |
I'm familiar with theories and what we know. My background is in BioChem, MolecularBio, and Computer Science (I was bored in college) and I can't get enough of space talk. I was looking at the new equations for determining the densities of new planets based on their orbitals between each other when I though "Can we then determine the "weight" of a black hole"? If so, we can get the density? Then I thought, can it be dense enough where it would collapse in on itself? Then what? When it comes to astrophysics, I'm still a noob and will be for a very very long time. Oh great reddit, please help fuel another one of my infatuations with space.
|
[
"They are imploded on themselves."
] |
[
"Infinite density doesn't require infinite mass, only a finite mass in an infinitely small volume."
] |
[
"Yes. A black hole exists as a point with infinite density. While it is used to characterize the size of black holes, the Schwarzcshild radius says nothing about mass distribution, only the amount of mass at the singularity."
] |
[
"Why are certain saurians believed to have sails versus humps?"
] |
[
false
] |
[deleted]
|
[
"Consensus these days is that (outside of theropods), large dinosaurs were ",
"homeothermic ectotherms",
". They did not produce heat for the sake of producing heat like a mammal does, but rather given their size and the inability to dispel heat allowed to them be homeothermic while maintaing ectothermal metabolism. ",
"Well, at least that's what one of my professors said the consensus was last semester, but he could be wrong. Man's a herpetologist not a paleontologist. "
] |
[
"I don’t think the consensus is that all non-avian dinosaurs were cold blooded. I’d list references, but you can find that information in anything written after the early 90’s. Also Dimetrodon wasn’t a dinosaur."
] |
[
"If Spinosaurus' elongated spines, like those of a bison, were needed for anchoring muscle tissue, other similarly-built dinosaurs (e.g. Tyrannosaurus rex) should have had comparable features.",
"edit: phrasing"
] |
[
"What happens during storms on other planets? Does it rain some other liquids besides water, is there lightning, etc."
] |
[
false
] |
After reading about how difficult it was to calculate the length of a day on Neptune because it has no visible surface due to stormy weather and various gases, I am curious to learn what actually happens during a storm. I figured it would be too cold for any liquid or gas to exist on Neptune since it is so far from the sun, but apparently it does, so what can one expect?
|
[
"I'm not sure exactly what you mean by \"what can one expect?\", but....",
"On Venus for example, it rains sulfuric acid, but the rain never reaches the surface. Instead, it evaporates well above the surface, while still in the atmosphere. The Soviet probes that landed on Venus also detected quite a bit of lightning (with at least one probe also detecting thunder).",
"Moving further away from the Sun:",
"Titan has methane clouds from which liquid methane rains down upon the surface.",
"There's a lot more to the whole gas/liquid/solid thing when it comes to the states of matter on other planets than just distance from the Sun. This may sound obvious, but the various compounds found on the various planets/moons/etc out there can have very different melting, evaporation, and freezing properties than the ones we're familiar with on Earth. Pressure also plays a big role, so something that may be a liquid on Earth, might be a gas or solid somewhere else, even at the same temperature. But essentially, from what I understand, weather on other planets acts very similar to what we experience on Earth, just with different chemical makeups."
] |
[
"H20 as a liquid is fairly rare because H2O is only liquid in a ",
"small window of pressure and temperature",
". Even though Earth is really \"wet,\" water only makes up a small part of Earth as a whole. Deserts, icy tundras, and inner/out core are all nearly void of liquid water. ",
"That being said, H20 is a pretty common molecule in the universe. All ",
"comets",
" are mostly H20. Some planetoid sized objects such as ",
"Pluto are about half water",
". Looking at it statistically, water ",
" be one of the most common molecules in the universe. ",
"The universe is mostly Hydrogen",
". Second is Helium, and third is \nOxygen. Helium is inert so does not react with anything. That leaves Hydrogen and Oxygen at the most likely pair after H2. One of the most stable molecules that combos of Hydrogen and Oxygen form is water. So just the odds of atoms bumping into each other, H2O should be one of the most common molecules."
] |
[
"It would be awesome, but Venus is probably the worst environment in the Solar System. Try to build a probe able with its electronics and gears fully functional after having been in space for months, then suddenly exposed to 460 C temperatures, 93 atmospheres of pressures and sulphuric acid vapour all around. ",
"The Russians tried it a few times: some of the landers lasted a bit less than half an hour, and it was an amazing achievement. Nobody ever tried again."
] |
[
"How can we tell that the universe is flat and infinite if we can't be affected by anything outside the observable universe?"
] |
[
false
] |
I understand that our observations tell us that the universe is flat, and that this implies that the universe is either infinite, or that it extends very, very far beyond the observable universe. But if nothing outside the observable universe can affect anything inside, how can matter outside the observable universe have an effect on the curvature inside? Isn't the flatness of the universe information from outside the part we can observe?
|
[
"We assume that the universe we can't see is the same as the universe we can see. We can make this assumption because the universe we can see is the same everywhere, and there's no reason to think it's different anywhere else. So we can measure what we can see, and extrapolate to what we can't."
] |
[
"we can't know for sure. But when we extrapolate the measurements of our observable universe, it seems very likely to be the case."
] |
[
"Thanks a lot. I had my thinking a little backwards, but you and shavera cleared it up very nicely."
] |
[
"Visible Light"
] |
[
false
] |
[deleted]
|
[
"Goodness. Inquisitive minds want to know! There's more questions than I have fingers! :P Let me reformat this in a list, and reply to the list, since I don't have all the answers.",
"1a. What's the deal with magenta then, which seems to be weirdly absent from the spectrum? ",
"1b. What frequency is it [magenta], higher than violet or lower than red? ",
"1c. If it[magenta]'s between the two, why isn't it green? "
] |
[
"Why is one way of representing hues linear, and the other cyclic?",
"Perhaps the more fundamental question is not why are these representations somewhat different, but why are they so similar? Why are they both one-dimensional? The answer is that it's a complete mathematical accident. We have three kinds of cone cells. Three points determine a triangle, and the edge of that triangle is one-dimensional. It's basically a circle.",
"If we had four kinds of cone cells, our hue space would be the two-dimensional surface of a tetrahedron. Essentially, a sphere. See the upper-right part of this diagram: ",
"http://books.google.com/books?id=r70IEreOL-wC&pg=PA379"
] |
[
"Yes. The liquid crystals in a LCD monitor simply acts as a \"light valve\" on the polarized backlight, and UV light can be polarized (e.g., the setup in circular dichroism).",
"Not that I know of. The primary reason is that non-XMen cannot see UV light. The secondary reason that the backlight has no UV component is that the high-energy UV would damage the LC molecules much faster than visible light, decreasing the life-time of your monitor.",
"I'm not sure what you're talking about.",
"No, because UV light is physically blocked out by your lens. You ",
" surgically remove the lens (",
") and be able to see UV-light; the blue receptors in your eyes are actually sensitive to UV light if it could pass through."
] |
[
"Are they searching for a single particle in the search for black matter or is it believed the answer to the unobservable majority of mass within the known universe has multiple solutions?"
] |
[
false
] |
Everytime I hear about dark matter it sounds like they are talking about a single particle. I don't know the gist of it, but in my head it sounds more probable that the great unobservable mass consists out of a lot of different things we still can't observe. If we can only observe about 10% of the universe and this consists out of millions of things, it kind of would be crazy the other 90% would consist out of a single thing right? I understand dark matter could be the stepping stone to finding out more, but I have no clue what the exact theories are on this. So that's why I'm asking here! Edit: I can't change the title which is why it still says black matter instead of dark matter. In my defense, I am from Holland and just asked the question as it popped up in my mind.
|
[
"It's unknown whether the missing mass will turn out to mostly be one particle or several or... something else. There are a lot of candidates that are being explored, though.",
"Particle physicists have come up with a lot of dark matter candidates. ",
"This",
" paper reviews some of them, and they are a strange sounding bunch: axions, axinos, neutralinos, sterile neutrinos. I'm not going to pretend I know what all these things are; it's not my area. They are hypothetical particles. You could also make a long list of candidates that have been considered in the past but have been since ruled out. Arguably, we've got a better idea what dark matter is NOT than what it actually is.",
"Then you have dark matter candidates that aren't particles. For example, ",
"MACHOs",
" include things like black holes, brown dwarfs, and neutron stars. They are essentially a grab-bag of hard to detect objects that may nevertheless be abundant in the universe.",
"And then you've got other researchers that think the reason nobody has detected dark matter after all these years is because our understanding of gravity needs to be adjusted. These new gravity theories typically fall under the heading of ",
"MOND",
". They manage to solve some of the problems of dark matter, but they don't explain everything. Also, general relativity has proved extremely precise in other ways, and observations of the ",
"bullet cluster",
" argue compellingly that dark matter is real and it is a particle.",
"I hesitated to add this tidbit because another user took me to task for this the other day, but here goes. I think arguably there has been at least one dark matter particle discovered: the neutrino. They are 'dark' in the sense that they don't participate in electromagnetism, and they have enough mass to contribute non-trivially to the total mass-energy of the universe. Stars make huge numbers of neutrinos as a by-product of nuclear reactions in their core. Stellar neutrinos fill the universe as much as star light does. Neutrinos are passing through your body right now! They go straight through the whole Earth like a 'ghost' particle. They are obviously extremely hard to detect but are nevertheless detected as a regular matter at observatories like ",
"IceCube",
". Maybe one day we'll make a neutrino telescope, and a whole new branch of astronomy will open up.",
"All that said, we have known for a long time that neutrinos can't account for the large amount of dark matter in the universe. One of the things that dark matter does is shape the way that large scale formation of structure appears in cosmological models. It was discovered in the 1980s (I think) that the dark matter particle had to be 'cold' (meaning that it moves at non-relativistic speeds) and neutrinos always travel near the speed of light due to their vanishingly small mass. Thus, neutrinos are a type of 'hot' dark matter. This is the reason why dark matter participle candidates have large masses; since they have a large mass, they move at non-relativistic speeds."
] |
[
"You're entirely correct to say that neutrinos are a form of dark matter. For a time they were even thought to be ",
" dark matter. So whoever it was that took umbrage is just misinformed.",
"But as you say, in the 80s pioneering computer simulations of how cosmic structures form (in particular by the DEFW or \"Gang of Four\" group) proved that neutrino dark matter would not result in structure formation consistent with observations from galaxy surveys, which were themselves first coming to prominence around the same time."
] |
[
"Check out the ",
"wikipedia article",
"; I've linked directly to a table of hypotheses, some of which are already 'known' components of dark matter (but none of which is known to account for all of it, so far), and some of which are theoretical or hypothetical."
] |
[
"How can the corona of the sun be hotter than the surface?"
] |
[
false
] | null |
[
"The top comment is not correct. Nobody knows how the corona is heated. It is an excellent question to ask but no one can answer you for certain.",
"The comment suggest flares but they have been ruled out. They simply do not provide the energy that is needed where it is needed; they are too occasional and they are limited in number during solar minima yet the corona does not cool when the Sun has no flares, even for extended 100 year minimums. This would be very surprising if flares were responsible as the corona is radiating strongly so requires a constant supply of heat.",
"One of the interesting things about the coronal heating problem is that it is not a question of there not being enough energy. Despite the corona being over a million degrees K, due to it's low density, there is far more energy stored down below, in the chromosphere and photosphere. We only need about 1kW per square metre of the Sun's surface to heat the corona to it's several million degree temperature. The problem is instead one of dissipation and transport, we can not deposit the energy in the right places in the right amounts to get this power there.",
"The two favorite candidates right now are reconnection (but again not flares) and waves. For reconnection we are talking extremely small scale reconnection events, sometimes called nano-flares, that could occur all over the transition region (the area between the chromosphere and the corona) or the chromosphere. They would not be delivering power by energetic particles but by MHD modes or perhaps Ohmic heating. They have never been observed but that maybe is not a surprise due to their small size and our instrument resolution - spatial, spectral and in intensity.",
"The other strong candidate right now is waves, particularly Alfven Waves (although there is no easier way to start an argument than to discuss categorizing waves at a soolar physics session), these could start off as motion of the plasma in the photosphere causing the embedded magnetic field lines to wiggle. This resulting wave can propagate out to the corona. The problem with these are they are difficult to dissipate in the right place.",
"Modeling problems are present for both of them, it is not possible to model flares that we do observe let alone ones that we have never seen and it is challenging to get waves to deposit their energy in the right place in the right amount. There are also always new ideas being suggested and studied.",
"Either way, this is definitely a problem worth solving and people will absolutely keep trying."
] |
[
"If one is in a hot gas it's possible to cool off below its temperature if it has a low enough density, because it will be heating you more slowly than you're losing heat via thermal radiation.",
"However if you were in the Sun's corona you would get extremely hot (thousands of degrees) because of the intense sunlight there."
] |
[
"It's an aura of plasma which surrounds the Sun and other celestial bodies and can be seen during a ",
"solar eclipse",
". The sun's corona is much hotter (by a factor from 150 to 450) than the surface of the sun, something which is sometimes termed as an apparent paradox."
] |
[
"I understand how we distinguish whether sounds are coming from the left or right (difference in when sound hits the ears)... but how does my body tell that a particular sound is in front of or behind me?"
] |
[
false
] | null |
[
"This is one of the great examples that shows how complex some parts of the brain's signal processing truly are. It boils down to the ",
"Head-related transfer function",
". As sound travels through your head/body, there are subtle changes made to the signal. Your brain learns how to tell the difference between these changes, and you perceive this as your sense of where a sound comes from."
] |
[
"Followup question: would this mean that a sound in a medium in which it acts differently, say water, would be more difficult to pinpoint?"
] |
[
"Based on ",
"this paper",
", then yes, localization is more difficult underwater."
] |
[
"How do Significant Digits and Standard of Error work in other Bases?"
] |
[
false
] |
I have a pretty solid understanding of significant digits in base 10. What are the repercussions of using significant digits in a small base (eg base 2) or a large base (eg base 60)? Bonus: how to keep the same significant digits (in meaning) when switching bases.
|
[
"Significant figures are a quick way of displaying uncertainty, which is more accurately stated using the ± sign:",
"5.4 (2 significant figures) means 5.4 ±0.1 or 5.4 ± 0.05. ",
"In base 2 (approximately):",
"5.3= 101.0100",
"5.4=101.0110",
"5.5=101.1000",
"So your number is about\n101.011 ± .001, or just 101.011 (6 significant figures)",
"You can't translate the number of significant digits directly between bases, but you can do a rough estimate (which will often be off by ±1). If s_a and s_b are the number of significant figures in base a and b:",
"s_a=s_b (log b/ log a)",
"For a=2 and b=10, (log b/log a)= 3.33, while for our number 5.4:\ns_a/s_b=3",
"So this formula is roughly but not exactly correct."
] |
[
"I've always found it helpful to think of it as writing the uncertainty in terms of magnitude. In this case, the base determines the range of the magnitude. Base ten has 10 units per magnitude, while base 2 would have 2 units per magnitude. Smaller base has less range of uncertainty in your magnitude."
] |
[
"Just to clarify one bit. Sorry for being a stickler. ",
"5.4 (2 significant figures) means 5.4 ±0.1 or 5.4 ± 0.05.",
"Shouldn't that be 5.40 ± 0.05"
] |
[
"Why hasn't the \"Big Bang\" or any similar event occurred again?"
] |
[
false
] | null |
[
"Hmm, let's see what I can remember and how I can explain this.",
"1) we don't really know ",
" caused the Big Bang, because the cause of the Big Bang came before the start of the Universe, and we don't have any info on that time period... because the Universe wasn't around.",
"2) we do observe, however, that the universe has an average temperature (2.75K)--this is called the Cosmic Microwave Background. This is theorized to be caused by the Universe initially being both hot and dense (see footnote 1), and then expanding and eventually cooling off to the temperature we see today.",
"3) From the above, we can assume that we need a very hot dense universe to be near to a Big Bang. So I'd suspect that we haven't had our universe go through another Big Bang because we've expanded and the energy has been spread out. It's a bit like asking why an explosion doesn't explode again--because the explosive material is now a cloud of dust moving away from the place where everything exploded. (insert gif of Mythbuster blowing up that cement truck, if you like). ",
"4) ",
" but of course we can't observe this happening.",
"Footnote 1:\nnngh, it's a bit hard to break down without being able to draw you pictures on a blackboard. Basically, if we conserve mass (which is a principle that we assume to be true), and the universe is expanding (which is an observation we have made), then if we rewind time the universe would condense, and because mass is conserved it would get very packed in, very dense. Then if energy is conserved too, we end up with hot and dense material packed into a much smaller universe. eEEeeee well that's not very lay-person friendly.",
"Footnote 2:\nTry reading:\nBarbara Ryden's Introduction to Cosmology\nHawley and Holcomb's Foundations of Modern Cosmology"
] |
[
"Not technically right. It happened before the start of time, but that doesn't mean that there was no universe before the Bang, matter was just very dense but it still existed in the universe",
"Not really right right either. What we can say is that if you 'run time backwards' you eventually reach an extremely hot and dense state ",
" what happened before that because our current models of physics break down under those conditions.",
"Edit: Spelling"
] |
[
"the cause of the Big Bang came before the start of the Universe, and we don't have any info on that time period... because the Universe wasn't around.",
"Not technically right. It happened ",
" at the start of ",
", but that doesn't mean that there was no universe before the Bang, matter was just very dense but it still existed in the universe."
] |
[
"How long did the transformation from a hunter gatherer society to an agrarian one take?"
] |
[
false
] |
Did it happen all at once, or in certain parts of the world first? What role did language play in this? What was an average number of people in a group in a hunter gatherer society? How about agrarian?
|
[
"Exactly how long is really up for debate, but this is a process that takes millennia. Hunter-gatherers don't simply start farming one day (not the first ones, anyway), but experiment with crop cultivation, manage herds, etc. Over time, in places where agriculture developed, the plants and animals began to become domesticated and what we would call incipient agriculture began. Basically people grew some things, hunted and gathered others. If populations grew and people became more reliant on this incipient agriculture (because it can provide more food per square kilometre than is provided naturally) then agriculture was expanded, to the point where diets depended on staple crops and little or no hunting/fishing (though really, hunting and fishing often supplement agriculture, even today in some cases). It is hard to really see this process as one of agency, where someone said \"I am going to triple the amount of food I can produce in a day!\" or anything like that, but rather a gradual, evolutionary change. ",
"In the Near East and Middle East (Mesopotamia, the Zagros Moundains, the Levant; an area known as the Fertile Crescent), where the earliest evidence of agriculture is found, there was something that we call the Broad Spectrum Revolution that began around 20,000 years ago. Basically people really expanded the range of foods they ate. You can consider this the start of cultivation and the process that led to agriculture, I would argue, as people began to really experiment with their food sources rather than simply eat what's there. Then full-scale agriculture was happening by 10-12,000 years ago in that region.",
"The Near East is the earliest in the world, but it happened independently at least 8 times (The Near East, Egypt, northern Sub-Sahara Africa, two separate places in China, the U.S. Southeast, Highland Mexico, Peru and Ecuador) and spread from these places. These all happened at different times (though within a few thousand years of each other) and each region grew different crops, so that's why we think that they were independent. So it happened in some parts of the world first, definitely not all at once, although on a grand timescale you could say that they essentially all happened at once, since modern humans were around for 190,000 years before any agriculture, then it popped up pretty quickly all over the world. And agriculture did eventually spread to nearly every human group, but this didn't happen until European colonialism in some cases, so definitely not all at once.",
"There are probably 20-50 people in a typical hunter-gatherer community, although this varies widely. In some places, like Eastern North America, people would have come together into larger groups (a few hundred) in the winters, at least during some time periods. But agrarian societies are all over the place, and even pre-modern agrarian societies could number in the millions. ",
"I'm not sure what you are asking about language. What role did language play in what? "
] |
[
"Did moving to agrarian methods help the development of language significantly? I could imagine language would be more important in a larger society than one with just 25 people. How about writing? ",
"If it didn't have that much do to with language, did it at least potentially help unify it? If each 'tribe' of 25-50 people speaks only the same language there would be some difficulty in communication when they got into larger groups, no? "
] |
[
"Writing systems only develop in places with agriculture, but the writing systems themselves are related to administration and control by the state, essentially (record-keeping). States can't develop without agriculture, but agriculture itself does not lead to writing. And there are many societies that were fully agricultural and had advanced agricultural technology (like large irrigation works) but never developed writing.",
"But I'm still confused by your questions about language. Language as we know it likely developed around 70,000 years ago and all hunter-gatherers have languages that are just as fully-formed and complex as any around today. And a hunter-gatherer community might only have 20-50 people that live together but they would not be isolated, they would have regular contact with other communities who were culturally very similar and spoke the same languages. So each individual community might not have more than 100 people, but they would trade, interact, intermarry, etc. within their society and the society could have tens or hundreds of thousands of people speaking the same language or close dialects. "
] |
[
"Zika virus has been known about for 70 years, why is it only now a big issues? Has it always caused microcephaly, or is this new?"
] |
[
false
] | null |
[
"We don't know.",
"Could be a recent mutation in ZV.",
"Could be that there's always been a link to microcephaly, but that ZV didn't infect enough people for anyone to notice — there is some debate as to whether the sheer number of people infected with ZV is increased in the last few years.",
"(Which could be due to ZV mutating to spread more easily, or it could be due to changes in human-to-human contact patterns (for example, World Cup), or it could be due to changes in human-to-mosquito contact patters (for example, due to global climate change), to name just a few possibilities.)"
] |
[
"Alternatively, it could be due to host related factors. For instance, the people in Brazil might be more predisposed to complications during infection with Zika virus than people in Africa or Asia due to a different genetic makeup of the immune system. However, the Brazilian population has a pretty heterogenous genetic profile. So, it might indeed also be due to recently acquired mutations or it could be a combination of both.",
"In any case I don't think there currently is any definite answer to this question since Zika virus has only very recently caught the attention of biomedical research. It might take years to find an explanation but genomic analysis of different Zika virus isolates around the world could provide an answer. ",
"Edit: Also, the topic I referred to is called immunogenetics. For those interested in these factors this ",
"cell paper",
" might provide a good overview of the genetic variability of our immune system. "
] |
[
"On the BBC Science Hour they gave the following hypothesis. Zika is so common in Africa that everyone gets infected fairly early in childhood and then develops immunity, so that Zika disease in women during pregnancy almost never happens and microcephaly rarely shows up. When the virus was recently imported to Brazil, the population there was not yet immunized, and infection of pregnant women caused microcephaly much more often."
] |
[
"What's the practical use of complex numbers?"
] |
[
false
] |
I know it's used in physics (if I remember correctly it's related to quantum mechanics, and electricity among other things), but I don't understand how we use an unknown number (the square root of -1) to describe things in nature? Where does it appear and in what way?
|
[
"The complex numbers are just the algebraic completion of the real numbers, so if you have an algebraic equation involving real numbers, then it's perfectly possible that you might need to use complex numbers to write down the solution - even though everything you measure is \"real\".",
"It's very unfortunate that \"imaginary\" numbers got that name; they're not really much more imaginary than negative numbers or irrational numbers, and they're very useful. Just as you can't have -5 apples, but you ",
" find meaningful equations which you need negative numbers to solve; similarly just because you can't have ",
" bananas it doesn't preclude the fact that you might need to use ",
" to solve physically meaningful equations. "
] |
[
"i isn't an \"unknown number\". It is a symbol that represents something. Mathematically we can treat that symbol as the square root of -1 and the math follows the rules of what we want to happen. ",
"For example, in some disciplines, i is used to represent the y-axis (x-axis has the real component, y-axis uses the imaginary component). So instead of representing a value as something like (2,4) you represent it like 2+4i . This technique can be used with many 2D vectors. (There are also many where it makes no sense at all.)",
"This works out well mathematically. If you have a vector that is 4+0i, you can rotate it 90 degrees CCW by multiplying it by i. i(4+0i)=4i. Or, to use a more complicated example, start with the vector 2+3i. Multiply it by i to get i(2+3i) = 2i-3=-3+2i . ",
"In electrical engineering, we use \"phasors\" to represent the magnitude (real) and phase (imaginary) portions of an AC signal. "
] |
[
"Complex numbers are wonderful to represent sinusoidal signals. Let's say that you have the signals a = A*sin(wt) and b = B * sin (wt+c), what's the sum of those signals? You can use a trigonometric formula to solve it. ",
"Or you could represent each of those functions with a complex number. If you add those complex number, you get the same result. And better yet, time disappears in this representation, making things simpler. And you get a nice graphical representation too.",
"This kind of problem is very common in the analysis of electrical circuits, among others."
] |
[
"How often does oxygen concentration change in the atmosphere in a given area on a day-to-day basis, taking into consideration weather conditions such as rain, windiness, or humidity?"
] |
[
false
] | null |
[
"The atmosphere is generally well mixed. A fundamental law of geophysical fluid dynamics is the conservation of mass. So if mass is in motion through advection, divergence, etc, it is being replaced. The old saying “warm air holds more water vapor” is not correct. What happens is that temperature shifts the saturation point to a higher vapor pressure. Oxygen and nitrogen concentrations depended largely on their respective life cycles that have been ongoing for eons. Oxygen for example can be introduced via evaporation or transpiration. At the same time, rising motion is associated with air that cools adiabatically, and will eventually sink; this air is being mixed. A lot of what I am saying here can also be expressed with the second law of thermodynamics in terms of entropy, which the adiabatic cycle I mentioned being an example.",
"Additionally, gases like water vapor, CO2, etc, while increasing in concentration over time, still makes up a tiny fraction of the atmosphere. In fact, I think there is at least 100 times as much argon in the atmosphere than CO2. However their concentrations greatly affect the absorption of incoming radiation at a given wavelength (infrared particularly for CO2, water vapor basically holds everything that isn’t visible).",
"The ocean is similar: there is a constancy in the global average concentrations of halides, for example (salinity of ~35 psu).",
"One interesting example is ozone. It exists primarily in the stratosphere, but can be advected into the troposphere in instances when the potential vorticity gradient becomes large (tropopause folds). It is detected at a certain wavelength as well (I want to say far IR but not certain) and is often an indicator of what is dynamically occurring at the tropopause. But it does get mixed out eventually."
] |
[
"Outside of 'extreme circumstances', the concentration of oxygen in the atmosphere would typically vary on the order of 0.02 to 0.03%. The reason for this is that the relative concentration of oxygen in the atmosphere is immense compared to every other gas (outside of nitrogen).",
"To increase the oxygen concentration quickly enough, so that any regional variation isn't simply blown away and intermixed with the outside environment, would be nearly impossible. Decreasing the oxygen concentration would take an immense combustion reaction to consume the locally available oxygen, but again, in an open environment, more oxygen would be brought in a short order."
] |
[
"Not very much. Here's some data from the Scripps Institute of Oceanography, which has monitored oxygen and CO2 levels at their pier for 30 years now.",
"http://scrippso2.ucsd.edu/assets/pdfs/plots/daily_avg_plots/ljo.pdf",
"It's not daily data, unfortunately, it's roughly weekly, but it gives you a good idea. There's not a lot of day-to-day scatter. Over a year, the seasonal cycle of plant growth and decay causes oxygen levels to rise and fall by about 100 parts per million -- ",
" over the course of a year.",
"The long-term burning of fossil fuels has caused a rise in CO2 and a tiny but measurable loss of oxygen, about 600 ppm -- ",
" -- in the last 30 years. So it's not like we're going to run out of breathable air anytime soon.",
"The increase in CO2 is much more important than the loss of oxygen: there's so little CO2 in the atmosphere to start with that the change we see is proportionally greater."
] |
[
"Is it possible that our universe is part of a much larger system?"
] |
[
false
] | null |
[
"Yes. There are many theories concerning what's outside of our universe though the evidence for any one of them is extremely scarce.",
"One of the most popular theories is called ",
"eternal inflation",
" or sometimes bubble universe theory. It says that there is ever expanding (->eternal inflation) multiverse in which small little bubbles are forming. Our universe is one of these bubbles and there are many other universes out there that are completely different. In fact these other universe may even have completely different physical constants.",
"Pretty much the only testable prediction of this theory would be that the different universes may collide or split. In particular it has been suggested that what we call the big bang may have been the moment in which our universe split from a different universe. Such an event should have left noticeable traces in the cosmic background radiation. Proposers of the theory hope that we might find such traces when our resolution of the CMB increases.",
"This is a bit of a nonsensical question - your own speed relative to something else doesn't have any impact on you (this is one of the core premises of relativity).",
"If however you ask what effect our speed relative to them would have on their perception of us then I can only say that it would probably be minor compared to the other effects. We are talking about seriously exotic systems with possibly entirely different physical laws and constants.",
"Heck, even our own universe has undergone periods where measly speeds of a few million miles per hour didn't mean anything. During the inflationary epoch I would have moved away from you at something like 10",
" meters per second (the speed of light is about 3 * 10",
" meters per second).",
"This is also a somewhat popular theory which is belongs to the class of what I call recursion multiverse theories. Whether every universe is an elementary particle or every universe is a black hole or what ever, the general idea is the same: there are infinitely many smaller and larger universes and we are somewhere in the middle.",
"Do these theories make any testable predictions? Certainly. If our universe is a black hole in a different universe for example then we might expect our universe to radiate away over time (see ",
"Hawking radiation",
"). If on the other hand it is some elementary particle then it might rapidly decay any moment or change all its physical properties within the blink of an eye. Neither seems to be the case. In fact there isn't that much extraordinary stuff going on in our universe that would require a multiverse apart from our existence perhaps (-> ",
"anthropic principle",
").",
"The problem with these kinds of theories is that they either lack a consistent mathematical foundation or require an extreme amount of fine-tuning which is never a good sign.",
"I wan't to clarify something concerning my answer to the third question: certainly these theories don't necessarily imply that weird stuff must be going on like spontaneous changes in the physical constants or something. However, most of them need to be further corrected and fine tuned in order not to do so. In science violating the ",
"principle of parsimony (also called Occam's razor)",
" is usually considered a bad thing to do when there are other theories at hand making \"simpler\" claims (such as eternal inflation)."
] |
[
"Pardon the ignorance but if we found that the universe is part of a larger system, wouldn't we have to define that whole system as \"the universe\" and then come up with another name for our local systems? Much like until the 19th century astronomers thought that out galaxy was the only one and therefore the whole universe?."
] |
[
"This is more of a linguistic issue than a scientific one. One one hand, the work \"universe\" and its Latin origin have always been used for the entirety of existence, from the solar system in classical times to the billions of galaxies in the observed universe in the 20th century. On the other hand, the word \"atom\" (\"indivisible\") now describes something that is divisible, so \"universe\" isn't the first misnomer in physics."
] |
[
"Has there been any attempt to create one of the elements thought to be in the island of stability?"
] |
[
false
] |
Is it even possible to create such large nuclei? How does one go about to create one?
|
[
"The principle behind creating heavy elements is bombarding particles together to supply sufficient neutrons/protons. The island of stability is centered around very heavy isotopes, which require far more neutrons than protons. There is not enough access to the stable, heavy isotopes needed for synthesis to even come close to the number of neutrons required."
] |
[
"In part because the heavy isotopes you'd need have half lives which are of the order of the tiniest fractions of seconds. So it's not (yet) feasible to manipulate them for further experiments/reactions "
] |
[
"If you don't mind, why don't we have access to the heavy isotopes?"
] |
[
"In terms of evolution, why is vitamin D synthesis dependent on exposure to sunlight?"
] |
[
false
] | null |
[
"I'm not sure which direction you are thinking about this from, so I will answer both questions simplistically. Feel free to ask for clarification.",
"All organisms which use the chemical (as far as I'm aware) from phytoplankton to mushrooms to mammals synthesize vitamin D by the use of sunlight, (or by eating something that did) Biology simply hasn't come up with a better way to do it. Most species get enough sun exposure that they really don't need a \"better\" way, but there are enough species that would probably benefit that it's probably a safe assumption that there isn't a simple way to synthesize it without UV. (Keep in mind, natural selection works in tiny increments, where no increment can be too detrimental to survival)",
"Ultimately sunlight is so important to most organisms (either for their own photosynthesis or for their food's photosynthesis) that the benefits of D outweigh reliance on getting a few minutes of exposure every day. After all, they're going to be in the sun anyway.",
"Organisms which don't have reliable access to sunlight or nutritional D just don't use the chemical, or don't use it for essential functions."
] |
[
"Plus whatever's abundant in nature we will evolve to rely on , it's free and saves the body energy to make it itself , which is why things are essential and non essential for human nutrition. essential nutrients are abundant compared to non essential which are scarce and that's why we make the non essential nutrients ourselves."
] |
[
"Vitamin D is a fat soluble vitamin found in nature as one of three types Vitamin D1, Vitamin D2, and you guessed it, Vitamin D3. As with all other substances on earth there is a finite amount of each. The absorption of these vitamin components is dependent on an intact brush border in the proximal small intestine. Some is absorbed without, but best absorbed with fat in the diet. An intact liver and kidneys are needed for best conversion and absorption. UV radiation converts D2 to D3. This occurs in the subepithelial dermis and requires adequate levels of D1/D2. There is a feedback mechanism which regulates need for Vitamin D which is calcium bound and co regulated by PTH(parathyroid hormone). This is a guess but my bet is survival of the fittest when it comes to sunlight needs. Those who are in sunlight(hunters) probably need more Vitamin D than those not in sunlight(cavedwellers) to support good solid bone and muscle growth to compete with other predators."
] |
[
"Are nebulae and gas clouds in space dense enough that sound could travel through them?"
] |
[
false
] |
I'm basically wondering if in a nebulae you could hear stars being created
|
[
"Not any that you could hear if you were hanging out there in a spacesuit, but even in sparser regions of the interstellar medium especially energetic events like supernovae can create shockwaves that propagate for many light-years. In theory an immensely large and sensitive receiver could record these sounds, and this would be easier to do in a nebula."
] |
[
"Once propagating, a sonic boom is not any different than a normal sound wave. "
] |
[
"On a human scale, they are extremely diffuse. For example: the Cat's Eye nebula has an estimated average density of about 5x10",
" particles per cubic centimeter. Our atmosphere at sea level has about 2.7x10",
" molecules per cubic centimeter - 16 orders of magnitude difference. A human floating in a nebula would feel exactly like floating in a vacuum. ",
"You can still get \"sound\" propagation through such a medium, but not in any way that a human ear could pick up."
] |
[
"How were the most recently discovered Elements (ie. 113, 117, 119 ect.) documented, when their half lives are so short?"
] |
[
false
] | null |
[
"Depending on the lifetime of the nuclide and your experimental setup, you may or may not have time to directly detect it before it decays. If not, you can just detect its decay products and infer what the original nucleus was.",
"The shortest nuclear lifetimes are in the order of 10",
" seconds, and those ones can’t be detected directly, but we still study them and learn a lot of interesting things about them."
] |
[
"119 hasn't been discovered yet.",
"The elements were discovered via their decays: The short half life is an advantage. If one of these would be stable we wouldn't have found it.",
"If you see a nucleus undergo a series of alpha decays then the element number decreases by 2 in each step. Eventually you'll reach decays you know already, and then you can calculate back what the first element was."
] |
[
"Yes, exactly. This is how exotic isotopes of lighter elements are routinely discovered, like the recent discovery of calcium-60 that got some media attention.",
"All you need is particle identification; we have many techniques for achieving that."
] |
[
"Can light be turned to microwave through the Doppler effect?"
] |
[
false
] |
I know that light can change color due to shift in wavelength due to the Doppler effect. If the source/observer moved at the right speed, can a visible light change its wavelength to that of a microwave? and if so does it act the same way as a microwave? To anyone that isn't familiar with the topic.
|
[
"Absolutely. This is why the Cosmic Microwave Background is microwaves, and not visible light.",
"The opposite is true as well. If you travel extremely fast towards the sun, its light would appear to you as x-rays or gamma rays."
] |
[
"so therefore you would get radiated with gamma rays? So you'd be dead. Great."
] |
[
"Yeah remember this as we discuss interstellar spaceships that travel at large fractions of the speed of light. That motion blue-shifts the light of stars along the direction of travel to increasingly higher energy. "
] |
[
"How were the number of atoms in a mole determined?"
] |
[
false
] |
I'm trying to do a bit of reading on this and what I've read so far doesn't seem to get at it. I get the definition of there being 6.02x10 atoms in 12 grams of carbon 12, but how did they 'count' the atoms in those 12 grams?
|
[
"First, a bit if history. Avogadro had made the idea of a mole as you described (12 grams of carbon in carbon 12), but he didn't actually create the constant of 6.02x10",
" What's important is that moles have been around.",
"Michael Faraday worked in the 1830s on electrolysis, and he had been able to calculate the electric charge per mole of electron. The number of elections was still not known, though.",
"In 1910, Robert Millikan was able to measure the charge of an election. Taking something like hydrogen, for example, he had to get a mole of hydrogen which was a mass he already knew, calculate the charge, and divide that by the charge of one electron. That formula gave him the number of electrons, and the constant we use today."
] |
[
"Knowing the mass is the easy part... it was first DEFINED to be a specific mass in terms of carbon, so that was piece of cake. ",
"Then, to go from one element to another, that is an easy task too because we know the atomic masses of elements quite precisely as well. (H=1; C=12, etc) "
] |
[
"I don't understand; how did they know how much mass a mole of hydrogen is? Does it have to do with the molar masses of elements or did they use known reactions to algebraically determine how much a mole is?"
] |
[
"What determines the colour of a flame?"
] |
[
false
] |
I've been told before that the blue/red-orange colour of a flame is due to the blackbody radiation from hot bits of soot etc. in the flame, rather than emission from heated air. If that's the case, why is the colour different when the flame has other elements, such as sodium or lithium. Is it a matter of soot being a relatively large object?
|
[
"It is true that the red/orange hues in a common flames (camfires, candles) is due to blackbody radiation of soot particles as result of incomplete combustion.",
"Other colors appear because the large energies released during combustion excite electrons in molecules causing them to jump/increase their energy level. These excited electrons aren't in a stable state and eventually jump back down to a lower energy level. As they jump down they emit the energy as photons of a particular wavelength. This is what gives the blue you might see near the base of a candle, gas stove, or bunsen burner. ",
"A Bunsen burner is probably the easiest way to see both the red/orange and blue flames because you can change the fuel/oxygen ratio and can get closer to ideal conditions for complete combustion. The blue part of a bunsen flame is usually there in some form, but the orange/red flame is much brighter and makes the blue much harder to see.",
"Another fuel, methanol, burns at much lower temperatures and gives off no smoke and hardly any visible light. This was used as a racing fuel in the past and in crashes if the car or driver was on fire it wasn't possible to see. The scene in Talladega Nights where Ricky Bobby jumps out of his car and runs around is a reference to this.",
"The wiki page on ",
"Flame",
" has a good section on flame color which might be able to provide more details."
] |
[
"The red/orange in flames is radiation from soot particles in incomplete combustion which is pretty common to find in things like burners, candles and campfires. The blue (and other colors in other flames) is from electrons jumping energy levels which you've described well."
] |
[
"That makes sense, thank you."
] |
[
"If a dog bites someone, at what point can you be sure that the dog was not rabid?"
] |
[
false
] | null |
[
"First off, when I say “dog bite” I’m referring to a dog biting a human, in case that wasn’t clear.",
"If by “transmission” you mean transmission to the dog, then yes, I understand that the dog may or may not have rabies at the time it bites a human. That is the premise of this whole question. But no, it is not impossible to know, not at all. There is a maximum amount of time observed between a dog’s contraction of rabies, the first time the dog is able to transmit rabies, and the first symptoms of rabies in the dog. There is also an ",
" time between those events (10 days perhaps?). What this timeline looks like is what I want to know. With that information one can know that a dog is likely to have, not likely to have, or definitely does not have rabies at a given time."
] |
[
"If the dog had rabies at the time of the bite but did not transmit to the person then the dog will develop symptoms within X days and die within Y days. Those numbers are relatively small (from what I understand less than a month, but my question, again, is what the average and maximum value of X?).",
"If the dog did not have rabies then the dog will not show symptoms up through X days have passed."
] |
[
"No, you can not test a dog for rabies because the only test is to kill the dog and test the brain. ",
"Sorry but I’ve stated my specific question a few times now, not sure how else I can ask it."
] |
[
"What would happen to my body in space (vacuum) if I had an endless supply of oxygen through a mask, water, food and the temperature was 20 degrees Celsius"
] |
[
false
] |
I ask specifically with oxygen/heat/cold/food not being an issue. I'm more curious how the absence of air pressure would affect my body. What would I end up dying of?
|
[
"Temperature is based upon the kinetic energy in the molecules/atoms in a region of space. A vacuum cannot have any temperature because it does not contain matter. (At a simplistic level, anyway--I'm ignoring all sorts of \"virtual pairs\" stuff.) ",
"Your skin is actually pretty porous, thus in a vacuum, the lighter molecules in your body would be constantly escaping without being replenished. As they escape, they take energy with them. Assuming the sorts of life-support additions you described, it seems like you would freeze to death.",
"If you want to do a 'what if' for your body somehow not losing energy, your skin would get very, very dry. Your body would desiccate, eventually."
] |
[
"You have many things happening in this situation (and it should be noted that cold isn't an immediate problem, you don't lose heat super fast through radiation): ",
"Fluids exposed to the vacuum would start boiling to vapor (fluid in mucus membranes, mouth, eyes, etc) and fluids in the muscles and body start to evaporate, and I'd guess you'd start seeing a pretty nasty case of the bends. ",
"If you have a lung full of air/oxygen/whatever, you either need to exhale or you will almost certainly rupture your lungs. They were never meant to take that much pressure between the interior air pressure and your exterior environment.",
"If you chose option one above (to exhale and not rupture your lungs) then after about 10 seconds you're going to start going to a bad place as your lungs start dumping O2 to the outside environment, due to lack of pressure (working in reverse if you will). This accelerates hypoxia and where rapid evaporation is occurring, those tissues will begin to reach freezing due to the heat carried away by evaporation, shortly after convulsions, cyanosis (bluing of the skin) and unconsciousness. ",
"If you're lucky enough to have made it back into a pressurized environment in about 90 seconds, you'll likely survive with minor injuries, though hypoxia-induced blindness may persist. After this point (I'm not sure on the timeline here, so if anyone knows better let me know!) your blood pressure will drop to a point where your blood starts to do funny things, like boiling. ",
"Ninjaedit: As per usual, please point out where I'm wrong if I am, those of you who know better. I'd love to know where I messed up and need to learn more. "
] |
[
"eventhorizon is not explaining it quite right - he got latched onto the wrong topic for your initial question.",
"To explain the issue with lungs: Imagine trying to breathe when an elephant is sitting on your chest - you could imagine it would be difficult, no? That would be because outside pressure is preventing you from inhaling - exhaling will not be a problem, but that ",
" breath would be impossible.",
"The inverse is true of a vacuum on the outside. Our bodies are capable of withstanding a certain range of pressures - massive deviations from that tend to result in dead humans. Now, imagine instead of an elephant sitting on your chest, you now have a massive pressure differential from the inside out. Your lungs can essentially \"pop\" from this differential, see ",
"http://en.wikipedia.org/wiki/Barotrauma",
" - interestingly this is true of both large pressure increases and decreases. You can escape this sort of trauma in a vacuum scenario by exhaling completely (but then see issue number 3 as posted by Uberbob)",
"So the issue here is that: If you pressurize the mask, your lungs pop. If you have oxygen flow at all, unless we are working with levels of oxygen far below blood concentrations (By this, I mean levels that would be useless), your lungs still could not withstand the pressure differential required long-term for levels that would prevent brain death.",
"You are right that your blood will not boil inside of your blood vessels. Your body is capable of exerting significant pressure on these that would prevent any sort of boiling, at least until body integrity is compromised. What you ",
" see is boiling from surface membranes and rapid cooling due to the endothermic nature of evaporation - causing damage there. Or, if you had an arm lopped off and you were spewing blood everywhere (due to, say, something tearing your pressure suit apart and taking your arm with it), you would see blood boiling out of the wound."
] |
[
"Could entanglement be used for long-distance communication (and if not, why)?"
] |
[
false
] | null |
[
"Your question is answered in the ",
"FAQ",
"."
] |
[
"I have just noticed the reply, and quickly checked the FAQ, found the matching question, and looked at the first linked thread, which continues to leave me dissatisfied. The first answer reads: ",
"Short version: Because you're measuring a fundamental aspect of a particle (let's say spin in this case), and you then know that its entangled partner has the opposite spin, but you cannot cause your particle to have any particular spin, which is what would be necessary to actually send information.",
"This is absurd, since any communications theorist or cyberneticist would point out that system resolution is itself a message, where a message may be composed of any variety of features including its timing. A single bit state upon fulfillment of remote event is still a message, and the foundation of binary computing. The message here would be single bit. ",
"Then someone adds: ",
"You can't control the outcomes of the specific particles, but you can set the system into one of 4 Bell states (if it is a qubit), and transmit one of 4 messages by the system's state.",
"Giving us four discrete values. ",
"The second linked page then adds this: ",
"Entanglement certainly happens much faster than the speed of light, and as far as we've measured with current experiments its consistent with instantaneously.\nYou would think you could use it to send information, but try to create a communication device given the fact that you can't choose the state of your entangled electron. You'll see its impossible!",
"Leaving us with an entanglement able to exceed the information horizon. The assumption that quantum unpredictability denotes the absence of message value is erroneous, even if this argument is correct for other reasons. Further, while a single system may occupy any number of states, reducing its value to the event and a single bit, a system composed of a number of discrete systems and states would not. ",
"It may be that the error in assuming message value derives from assuming that system resolution for a local observer is system resolution for a remote observer. If it is impossible to define a given state as outcome, then remote observation has no way of correlating its outcome with the fact that local measurement has taken place at all; while a local observer may infer remote system state, a remote observer cannot infer that distant observation has taken place. If so, of course, communication using this method would be impossible. "
] |
[
"What is \"system resolution\"?",
"You may be using a different interpretation of communication as the one used when proving the ",
"no-communication theorem",
". The point is that there is no information you can communicate using entanglement that couldn't also be communicated classically without entanglement. For example, I could put a left shoe in one box and a right shoe in the other, and give you and me the two boxes without saying who has which. Now if we don't open the boxes until we are on opposite sides of the galaxy, we know instantaneously who has the left/right shoe, but it doesn't really help us communicate with each other.",
"Entanglement is more complicated than the shoe example, but for the purposes of thinking about how to send information, they're effectively equivalent."
] |
[
"Would it be possible to light a piece of wood on fire with water vapor?"
] |
[
false
] |
Would water vapor at an extremely high temperature be able to ignite wood or would it just heat it up until it just disintegrated?
|
[
"Yes, it's very much possible.",
"Heck, many years ago we demonstrated exactly that capability, only using the fire-retardant treated paper towels in the chemistry lab. To this day I have no idea how hot that steam was; the source was a boiling flask on top of a burner, with a copper coil attached to the stopper. There were two more (large) Bunsen burners heating that coil. You could see no vapor exiting the end of the copper tube, and where the copper was in the flame it glowed a dull red. Placing our best laboratory thermometer in the output pushed it past its maximum reading which I want to say was several hundred degrees C.",
"As long as we held that paper towel in that stream of superheated steam it burned vigorously. If you wanted to -- just to prove a point, I assume, as this would be quite impractical and probably hard on your chimney -- you could build a similar, larger scale device to light your wood stove!",
"(You'd just want to put enough heat into the system that you wouldn't be condensing water vapor in your combustion chamber... and that's a ",
" of heat.)",
"Further reading: ",
"Popular Science, Feb 1948, pg. 228",
" (Boy, they sure don't make magazines like they used to anymore... 306 pages!!)"
] |
[
"Yes. ",
"Here's",
" a simple demonstration on how it's possible. "
] |
[
"It depends on whether oxygen is present or not. If oxygen is present, and the steam has the capacity to raise the temperature of the wood above its ignition temperature, then yes. If the environment is starved of oxygen, the wood will be converted into a sort of mushy pulp.",
"This is how pulp is made for paper products. The wood chips are bombarded with very hot steam as well as mechanically broken down in, what they call in the industry, a thermomechanical pulping process, or TMP.",
"I worked in a paper mill for a few months on a co-op term."
] |
[
"Why does the water recede from the shore before a tsunami?"
] |
[
false
] |
[deleted]
|
[
"Yes and generation mechanisms depressing the water like landslides or underwater avalanches will cause the trough to arrive first. "
] |
[
"This will happen when the trough of the wave hits the land (rather than the crest). If the crest hits first, the water won’t recede, and the wave will be bigger than the normal waves.",
"Source (and a much more detailed explanation):\nInternational Tsunami Information Center FAQ\n",
"http://itic.ioc-unesco.org/index.php?option=com_content&view=article&id=1133&Itemid=2155"
] |
[
"It doesn't always do that. Waves go up and down - sometimes \"up\" comes first, sometimes \"down\" comes first. It depends on the cause of the tsunami."
] |
[
"How to identify a substance?"
] |
[
false
] |
In general if someone finds a solid/liquid/gas. Then what are steps taken to figure out what is the chemical composition of it?
|
[
"Infrared spectroscopy",
" is a common technique used to identify unknown samples. Whilst it won't tell you exactly what your sample is, the portions of the EM spectrum that are absorbed will tell you what bonds are present in your sample.",
"An O-H bond absorbs a different frequency to a C-H bond and so forth. It allows you to make some powerful deductions.",
"A further technique you might use to narrow it down is ",
"NMR spectroscopy",
" which makes use of the magnetic properties of nuclei. It allows you to determine the structure of the compound you're analysing by providing information on the 'coupling' of atoms.",
"Especially useful in organic compounds where different carbon atoms are bonded to different numbers of hydrogens."
] |
[
"If you want to determine the ",
" something is made out of, you use something like ",
"AES",
", where you basically blast apart the matter into a vapor of atoms, and measure the absorption/emission spectrum of the atoms. Each element has a unique set of absorption/emission frequencies (through which the element Helium was actually discovered in the sun's spectrum before it was found on Earth - hence it's named for the sun - helios). ",
"The Mars Curiosity rover uses ",
"PIXE",
", which is conceptually similar, except you're blasting the thing with charged particles and measuring the x-ray emission lines. (while AES typically measures the visible-UV range)",
"Both these methods are very sensitive and accurate, so you don't need a huge sample. "
] |
[
"Depends on your budget, the state of matter (solid, liquid, gas), and the reason for analysis (i.e. Hazmat). ",
"If it's a metal, portable XRF, if it's liquid ICP-MS, or GCMS, or even flame atomic absorption, there are literally hundreds of ways and I would need more info to tell you which would be the best for your application. "
] |
[
"Is it true that a human can outrun (endurance) nearly every other animal on the planet?"
] |
[
false
] |
I have just heard of this and was intrigued. A bit of Google-Fu turned up in discover, but it was the only thing I could find. I wondered whether the guys and gals in had heard of this before or knew the answer. edit: Thinking about it, I realise that there would be many variables (hydration, fitness, weather, weight etc...) which would effect the outcome.
|
[
"Not my field at all.",
"There's an old BBC episode [1] which tracked some African endurance hunters kill a few elk over the course of a day. They used superior tracking skills and the ability to carry water to help exhaust the animal. They were also obviously in peak physical shape.",
"[1] ",
"http://www.youtube.com/watch?v=fUpo_mA5RP8"
] |
[
"Funny, I just watched a TED talk on this subject: ",
"Are we born to run?"
] |
[
"As an avid distance runner with distance runner friends who occasionally drag their dogs out for a few miles, I doubt many other animals could outrun a human in peak physical condition.",
"I don't think there are many dogs, cats, gazelle, etc., who could cruise at 7 min/mile for 16-18 miles, which is what a fairly serious (but not outrageously talented) distance runner might call \"Sunday morning\".",
"To shift away from anecdotal/personal experience, and to a more scientific bent:",
"If we are to accept that prior to the advent of agriculture humans had to hunt for food, and one of the ways they did this was by the method the OP suggests, then it's extremely unlikely that that physiological ability would have disappeared. We are, physiologically speaking, essentially the same beings that hunted and gathered on the plains of Africa thousands of years ago (there have certainly been some changes, lactose tolerance being among the most well known, but generally, not much has changed). ",
"Most present day adult humans may not be capable of running down an adult gazelle if you asked them to right now. But if forced to get into good running shape I bet many would."
] |
[
"What percentage of the earth does the sun directly shine on?"
] |
[
false
] |
Thanks to my 12 year old son I'm on a mission to get an answer to this. This one might sound silly because '50%' could be the right answer but I suspect it might not be. If you have the Sun a large star, radiating light towards tiny earth - would the edges of the sun in reference to the earth actually 'encroach' so that the direct sunlight hits over 50% of the surface of the earth? The question probably involves some geometry and knowledge of the relative size differences and also the distance of the sun to the earth. Help?
|
[
"(supporting picture (very rough Paint drawing without legend): ",
"http://i.imgur.com/rUmZVKz.png",
" )",
"50% is an obvious starting guess, but you're correct in noting that the sun being larger than the earth, will shine just a bit \"over the edge\". To compute just how much is not straightforward.",
"Lets start by using ",
" for the radius of the sun, ",
" for the radius of the Earth and ",
" for the distance between the sun and the Earth. Note that this involves a few assumptions, namely that the sun is a sphere, the Earth is a sphere and the distance between the sun and the Earth is constant. All of these assumptions are false, but the difference is small enough for us to ignore it (just don't build any super-high precision gizmos based on this calculation).",
"We can draw the sun as a big circle, the Earth as a small circle with a line of length ",
" connecting the two centers and then draw a ray from the top of the sun to the top of the Earth. See the first part of the picture. The slope of this line, lets call it ",
" can be found by:",
"s = (Rs - Re) / D",
"Note that this is an approximation as the real point where the sun ray is tangent to the Earth is not at the top, but slightly behind (which is the whole point of this question), which would mean we should use a slightly smaller value for ",
" and a slightly larger value for ",
" in this case. Since the deviation is so small and the effect on those two variables partially cancel, we can safely use this approximation.",
"Now, we need to determine where on the circle that represents Earth the line touches. The answer is at the point where it is tangent to the curvature of the circle. Ok, so next up, we draw a nice little coordinate system through Earth, centered in the center (where else?). We're interested in the upper right quadrant. See the second part of the picture. In this quadrant, the circle can be described by the equation:",
"y = sqrt( Re",
" - x",
" )",
"(sqrt = square root)",
"Now, to determine where the sunray is tangent to this function, we need to compute its derivative:",
"y' = -x / sqrt( Re",
" - x",
" )",
"Next stop, find the value of ",
" for which the light ray is tangent to the circle:",
"(Rs - Re) / D = -x / sqrt( Re",
" - x",
" )",
"At this point, I summon the help of Wolfram Alpha. I used ",
" and ",
" for ",
" and ",
" respectively, since Wolfram Alpha was misinterpreting the original variable names. I'll spare you the full expression, but you can see it here:",
"http://www.wolframalpha.com/input/?i=Solve[%28A+-+B%29+%2F+D+%3D%3D+-x+%2F+sqrt%28B^2+-+x^2%29%2Cx]",
"Fortunately, we can stop being all algebraic and just put in the values that we know for ",
", ",
" and ",
". Wolfram Alpha saves us a trip to Wikipedia, since we can just plug in their names. Doing it all in one go is a bit too much for the poor website, so we go step by step. The left side of the equation:",
"(Rs - Re) / D = 0.004535\n(",
"http://www.wolframalpha.com/input/?i=%28sun+radius+-+earth+radius%29+%2F+distance+earth+to+sun",
")",
"Plugging in the right side: ",
"http://www.wolframalpha.com/input/?i=+Solve[0.004535+%3D%3D+-x+%2F+sqrt%28%28earth+radius%29^2+-+x^2%29%2Cx]",
" gives us",
"x = 0.00453495 Re = 28.9245 km",
"Good. That's how far over the edge the sun shines (on the equator, it will be ever so slightly more on the poles). Now how does this translate into a total surface area? Take a look at the third part of the picture. The grey area is what we want to know, but for a 3D sphere, not a disc.",
"The first bit is easy, the left half of the sphere is fully lit. For the remaining part, we need to perform an integration. Using the same coordinate system as before, at a distance ",
" from the origin, the surface of the Earth can be described by a circle of radius 2 * pi * sqrt( Re",
" - x",
" ). We integrate this expression from ",
" to ",
" km.",
"Again, Wolfram Alpha to the rescue (I filled in the value for the earth radius, it got confused otherwise):\n",
"http://www.wolframalpha.com/input/?i=2*pi*+%28Integrate+sqrt%286367.4447^2+-+x^2%29+from+x+%3D+0+to+x+%3D+28.9245%29",
"And the outcome is 1157200 km",
" .We bother Wolfram Alpha once more to determine the fraction of the total surface area of Earth: ",
"http://www.wolframalpha.com/input/?i=1157200+km^2+%2F+surface+of+earth",
"And we get 0.0023 or 0.23%. This brings the total up to 50.23%. Note that this fluctuates very slightly throughout the year (as the distance between Earth and sun is not constant) and it assumes a perfectly flat, spherical Earth, which we don't have.",
"An added twist is that our atmosphere scatters sunlight in all kinds of directions. Because of this, you can see sunlight over the horizon, without seeing the sun. This explains why the sky is already getting lighter before the sun actually rises. And this also means that a larger part of the surface of the Earth is illuminated by the sun than what we've just calculated (though this illumination is indirect).",
"edit: It's quite possible that the calculation is not free of errors. The result is believable (slightly more than 50%) and the dimensions are correct in the steps that I've done through Wolfram Alpha, so it seems OK, but I can't exclude errors.",
"edit2: Your assumption of the variables (sizes of both bodies and distance) and method (geometry) required was correct."
] |
[
"We can improve the approximation a bit without too much extra work. Particularly, we can improve the approximation of the slope of the red line.",
"Suppose the Sun is the circle",
"S(x) = sqrt(R",
"-x",
")",
"and the Earth is the circle",
"E(x) = sqrt(r",
"-(x-D)",
")",
"where R is radius of Sun, r is radius of Earth, and D is Sun-Earth distance. Consider the ray of light from the Sun which reaches the farthest point on Earth (largest value of ",
") and let ",
" denote this ray. What conditions must ",
" satisfy?",
"(1) ",
" is tangent to the Sun at the point (a, S(a)).",
"(2) ",
" intersects Earth at the point (b, S(b)).",
"(3) Given the value of ",
", the value of ",
" is unique.",
"Conditions (1) and (2) mean that ",
" and ",
" satisfy the equation",
"E(b) = S(a) + S'(a)(b-a)",
"which is the same as",
"sqrt(r",
"-(b-D)",
") = sqrt(R",
"-a",
") - a(b-a)/sqrt(R",
"-a",
")",
"This equation, with a bit of algebra, can be arranged into a quadratic equation in ",
". If that equation is to have a unique solution, its discriminant must vanish. The discriminant of the resulting equation is",
"d = (a",
"-R",
")(D",
"a",
"-(2aD+r",
")R",
"+R",
")",
"Since by construction we must have 0<a<R, the discriminant vanishes if and only if ",
" satisfies the equation",
"D",
"a",
"-(2aD+r",
")R",
"+R",
" = 0",
"Going back to the equation for ",
", we then find that with this value of ",
", we must have",
"b = (-Da",
"+(a+D)R",
")/R",
"It turns out that the discriminant equation for ",
" is rather trivial since it factors exactly. The two solutions are",
"a",
" = R(R+r)/D",
"a",
" = R(R-r)/D",
"Substituting these values of ",
" into the equation for ",
" gives the two solutions for ",
" as",
"b",
" = D - r(R+r)/D",
"b",
" = D + r(R+r)/D",
"The only solution pair that is meaningful is that which has 0<a<R and D<b<D+r. We can easily verify that both values of ",
" are okay, but since b",
"<D, only the second solution pair is okay. (We can also verify that b",
"<D+r.)",
" The considered light ray leaves the Sun at coordinate ",
" and strikes Earth at coordinate ",
", given by the respective equations",
" = ",
" = ",
" + ",
"In other words, the Sun shines a distance of ",
" = ",
" over the hemisphere that faces the Sun. Substituting the known astronomical values, we get ",
" = 29,410 meters, which is a very slight improvement over the original approximation of ",
" = 28,924.5 meters. Now what is the surface area of the Earth that actually has shining on it? Now to simplify things we can re-center the Earth at the origin and assume that",
"E(x) = sqrt(r",
"-x",
")",
"We want to take the portion of this graph from x=-r to x=r(R+r)/D, rotate it about the x-axis, and find the surface area of the solid of revolution. This is easily set up as the integral of",
"2pi * E(x) * sqrt(1+ E'(x)",
")",
"It turns out that algebra simplifies immensely, so that",
"E(x) * sqrt(1+E'(x)",
") = r",
"(Yes, it's just a constant.) Hence the entire surface is just",
"AREA = 2pi*r(c+r)",
"As a fraction of the total area 4pi*r",
", we have:",
"FRACTION = 1/2 + (R+r)/(2D)",
" The fraction of the surface area of the Earth that has sunlight directly shine onto is",
"1/2 + (R+r)/(2D)",
"The naive approximation would be exactly half of the Earth. So the amount of the Earth's surface that has direct sunlight, ",
", is",
"EXCESS OF EXACTLY HALF = (R+r)/(2D)",
"Phew. All done. Nice neat formula. For completion sake, substituting the astronomical values gives 0.2309%."
] |
[
"An added twist is that our atmosphere scatters sunlight in all kinds of directions.",
"Another important effect is atmospheric refraction (which is different from atmospheric scattering). Direct sunlight is bent over the horizon by the atmosphere. As a result, you see the sun on the horizon after it has geometrically slipped below the horizon."
] |
[
"Do insects ever interact with their larvae?"
] |
[
false
] |
[deleted]
|
[
"Honey bees ",
" (and related honey bee species) feed their larvae from hatching until they cap the cell and pupation starts ",
"up to 800 times per day",
". The larval stage is is a 6 day time period. They feed pollen and secretions from their own mandibular glands. They remove dead/dying/diseased larva and in some cases will even remove pupa that are infected with parasites in the middle of the pupation process. ",
"The related wasps of which there are thousands of species hunt other animals in order to feed their larva doing much the same that honey bees do just with meat instead of pollen as the protein."
] |
[
"Ants also interact with their larvae. They feed them, tend to them, some species use them for silk production.",
"Tending to larvae seems to be relatively common in the sphecoid wasps (stinging wasps) like social wasps, bees, and ants. This would make sense since they're mostly social and have colonies."
] |
[
"Post hatching parental care is known for insects...see this publication for a beetle species and take a look at the reference cited section for other examples:",
"https://www.nature.com/articles/srep29323"
] |
[
"How do the kalahari dirt circles form?"
] |
[
false
] |
I was watching BBC's Africa serie (stunningly beautiful documentary btw) and in the first episode they talk about Kalahari desert and they mention those mysterious dirt circles. What are the hypothesis for their formation. The second picture on this slide show is what I'm refering to
|
[
"The non mobile version of the article: ",
"http://en.wikipedia.org/wiki/Fairy_circle_(Africa)"
] |
[
"http://en.m.wikipedia.org/wiki/Fairy_circle_(Africa)",
"Carbon monoxide from an unknown source appears to be the culprit."
] |
[
"Not a full answer more a hypothesis"
] |
[
"How rapidly would a flamethrower deplete the oxygen in an enclosed space?"
] |
[
false
] |
Hypothetical: A person is in a bunker that is sealed. They have a flamethrower, and fire it. Is there a noticeable depletion of oxygen? How long would the fire have to be sustained in order to deplete oxygen in an area maybe 3000 cubic feet in volume? EDIT: I guess a better question is "How long until you suffocate in that enclosed space?"
|
[
"3000 cubic feet of air would contain about 1520 moles of oxygen. Wikipedia says that modern flamethrowers use propane.",
"Propane combusts as follows:",
"C3H8 + 5 O2 -> 3 CO2 + 4 H2O",
"So 1520 moles of oxygen could combust 304 moles of propane. 304 moles of propane has a mass of 13.4 kg.",
"Unfortunately, i'm unable to find out how much fuel a flamethower consumes, so anything beyond this point is speculation.",
"However, let's assume that the flamethrower uses 500 grams of propane per minute, you'd deplete all the oxygen in the room in about 27 minutes. However you'd die once the carbon dioxide levels reach 10%. Not to mention how hot it'd get from using a flame thrower in such a small space.",
"tl;dr you'd die long before you'd deplete all the oxygen."
] |
[
"I'm running a pen and paper RPG online via a forum with a bunch of buddies from undergrad. It's in a sci-fi setting, and one of them just decided to use a flamethrower in a bunker that is sealed shut. ",
"I want to have a good basis for forcing them to take suffocation checks and damage. "
] |
[
"Your analysis is good for civilian \"weed burners\". ",
"Military flamethrowers, however, use gelled gasoline (napalm) as fuel, with propane used as a constant-on ignition system. ",
"Wiki.",
"Suffocation is one of the effects flamethrowers were used for, along with the ability to flow into buried fortifications and to stick to skin.",
"Run your analysis again using gasoline (1 to 8 gallon shots would be reasonable), and the time required to cause suffocation (10% CO2 level) will probably be within a few minutes."
] |
[
"Is thrust exerted from a light emission source via the expulsion of light?"
] |
[
false
] | null |
[
"Yes, it's called radiation pressure and it's extremely weak. It's roughly equivalent to the power of the light emitted divided by the speed of light. So a 60 Watt lightbulb, if perfectly efficient and unidirectional, would give 200 nanonewtons of force."
] |
[
"If what you mean is, if you put a laser out in space and turned it on, would the laser accelerate due to the thrust of the laser light? The answer is yes, by Newton's third law of motion, or simply by conservation of momentum (light carries momentum). "
] |
[
"Yes, this is why we don't have satellites that run purely on solar power.",
"We do have objects in orbit that drive on ion thrust. We accelerate ions in an electric grid to high speeds and eject them from the end of the craft.",
"The main drawback of this is that this method does not produce enough thrust as conventional chemical rockets."
] |
[
"Why does minus attract plus and vice versa?"
] |
[
false
] |
I understand that plus attracts minus, and minus attracts plus. I also know that two similar charges (+ and + or - and -) repel eachother. The thing is, I've never learned , and I can't find any explanation.
|
[
"The reason is that the photon has spin 1. ",
"Forces are mediated by particles; electrical forces are mediated by photons. Now particles have intrinsic angular momentum, called spin. The way things work out, if the force mediating particle as an odd spin (such as the photon, with spin 1), like sign charges repel and opposite sign charges attract. If the force mediating particle has an even spin, then like sign charges attract and opposite sign charges repel. Since the photon has spin 1, we have the case that opposites attract and likes repel.",
"Unfortunately, I don't know of a way to explain ",
" things work out this way that's not technical, but if you want a technical explanation, I'd point you to ",
"this",
" post."
] |
[
"It doesn't always. You're thinking of ",
", which comes in two types +/-, like-kinds repelling, opposite attracting. There are other types of forces with other types of charges.",
"Gravity has only one kind of 'charge', mass, which is attractive (like 'charges' attract). [Yes I know gravity is better expressed geometrically. But it can be formulated as a field theory (with problems)].",
"The strong force has ",
" kinds (red, green, blue) of charge, with a more complicated set of rules for attraction.",
"There are truly beautiful mathematical theories, called gauge theories, that describe these forces. See wiki article and links within.",
"http://en.wikipedia.org/wiki/Strong_interaction"
] |
[
"So what causes photons to have spin in the first place?"
] |
[
"Does a photon have any space-filling properties?"
] |
[
false
] | null |
[
"A photon is a boson (integer spin). You can have as many bosons as you want in the same location.",
"Particles that comprise matter are fermions (half integer spin). They stay in their assigned seats and keep their distance."
] |
[
"Let's say that the math for that would give me nightmares. Because you can't just say \"oh there's some point with a lot of electromagnetic energy in it.\" You'd have to account for the flow of the energy in and out of that point too. Maybe someone can answer that question, I just can't."
] |
[
"Hang on a second. There are a number of errors there. You can't just substitute ",
"² into an equation like that, for starters. And second, you're ignoring the uncertainty principle and pretending that photons are cannonballs.",
"The bottom line is no, you can't make a black hole with laser beams."
] |
[
"If right now, and by now I mean southern hemisphere autumn and northern spring, I took a deciduous tree from the southern hemisphere and transplanted it to a very similar climate in the northern hemisphere, what would happen?"
] |
[
false
] |
Would the tree stop dropping its leaves and sprout new buds, would it shut down for a few seasons to readjust its internal clock, or just die?
|
[
"If the tree uses photoperiod to measure seasonal change, it will probably be fine, as it will only shed it's leaves when it detects a change in the length of day. If we're assuming it hasn't already dropped the leaves for the southern hemisphere winter, the tree will probably just interpret this as a super long summer. ",
"If the tree uses changes in temperature or precipitation, it would likely have a pretty bad time for a season but I'll leave that to someone else who does work with that branch of phenology instead. "
] |
[
"It would no doubt just assume it went through a ",
" short winter and start regrowing leaves again.",
"Same reason transplanting a human from a warm climate to a cold one won't cause that person to freeze, they'll just start wearing warmer clothes. Humans and trees decide what to do based on environmental conditions, not by a clock and a rigid timetable."
] |
[
"But what about if it is ALREADY in the process of shedding its leaves? Also, what if it has already completed this?"
] |
[
"What is a muscle knot?"
] |
[
false
] |
What cause "knots" in the muscles and why do they feel the way they do?
|
[
"The medical term is myofascial trigger points. There is some debate about what causes muscle knots but it seems to be connected to an abnormal build up of protein after a release of lactic acid.",
"Muscles that form knots are muscles that have gone into a muscle spasm either due to injury, overuse, or a sedentary lifestyle, and then have remained \"stuck\" in that tense state. A muscle knot is your muscle remaining flexed and refusing to relax, which is a big part of the reason they can be so painful. "
] |
[
"This seems like a good place to ask: Does foam rolling work to relieve this condition?"
] |
[
"As a swimmer I can say that with bihkram yoga, and a ice bath, the foam roller does an amazing job. I do it once a week to detox from all the lactic acid build up in my muscle (sweating a ton and stretching) as well as rolling my self after."
] |
[
"Why are cathodes known as negative electrodes and cations known as positively charged ions?"
] |
[
false
] |
This might be more of an english question if anything, but I always get these mixed up in experience with science courses. Sorry if it’s a silly question, but I have always been confused at the similarity of the words. This question also obviously applies to the opposite of these terms, anode and anion.
|
[
"The negatively charged cathode attracts positively charged cations, the positively charged anode attracts negatively charged anions. The prefixes cat- and an- on their own do not mean positive or negative."
] |
[
"I believe there's also an historical side of this as well, the names and polarities were decided sort of arbitrarily before we knew what the charge carriers were (aka electrons), and by the time we figured out we had assigned them incorrectly it was too late. I could be mistaken on this, but I ",
" to remember it being a thing. "
] |
[
"Cations move to the cathode, anions move to the anode.",
"It makes sense if you don't understand what positive and negative charges are. Same with the magnetic field. The north pole of a magnet will point north - which means Earth's magnetic field has the magnetic South Pole at the North Pole."
] |
[
"If you're in a plane that breaks the speed of sound, does the noise outside of the cabin suddenly fall completely silent, since you're moving away from the sound waves the plane is creating before they reach your ears?"
] |
[
false
] |
I've tried to find a video of a plane going supersonic filmed from inside the cabin to answer this, but so far I've found nothing of the sort.
|
[
"No. The cabin is still vibrating and transferring those vibrations to the air inside of the cabin.",
"EDIT: Maybe I misread your question, are you asking if someone was listening to the plane from outside the cabin while it was supersonic? Or if something outside of the cabin that wasn't the plane itself, made noise would you hear it inside?"
] |
[
"No. Despite the misconception that the sonic boom is the noise created the moment the plane breaks the sound barrier, the sonic boom is actually caused by the bow wave of sound waves created by the plane, and is actually one continuous stream of noise that onlookers will only hear for an instant as the wave reaches them shortly after the plane passes. It's essentially hearing all the noise the plane is making at one moment and people inside the plane wouldn't experience it as it goes out at an angle."
] |
[
"You hear noises inside the cabin (subsonic or not) through vibrations on the outside surface of the vehicle, which are transmitted to a locally subsonic medium (the cabin air) as pressure waves. I would say no, the average passenger wouldn't notice any difference. In fact, if anything it would probably get louder in the cabin, as the engines will need to work harder to maintain their speed."
] |
[
"Do light photons experience time dilation?"
] |
[
false
] | null |
[
"Photons do not have a valid reference frame and thus can not experience anything. ",
"If you ignore the reference frame part, you ",
" say that they experience maximum time dilation where time stops completely. I would be careful with that statement though. "
] |
[
"I'd love to give you an answer, but that's not even valid as a hypothetical question due to the invalidity as a reference frame. Kinda like saying \"if an orange was an apple, how good of an apple would it be?\""
] |
[
"If you ignore the reference frame part, you could say that they experience maximum time dilation where time stops completely. I would be careful with that statement though. ",
"Perhaps the best way to phrase it is to not speak from the viewpoint of the photon, but rather to simply say that the proper time distance between the photon's emission and absorption events along the path the photon takes through spacetime has a length of zero. Seems to me that as long as you are talking about spacetime intervals, and not measurements by things defined ",
" spacetime, you avoid a lot of the messy reference frame business. :)"
] |
[
"gravity - size, mass and the effects"
] |
[
false
] |
ok so i posted on softscience and a replier directed me here, heres the thread: so yeah, a really in-depth explanation is requested here, thanks in advance.
|
[
"Next time post the actual question:",
"hey quick question guys, 1 my intuition isnt enough to answer. does the size of an object matter when it comes to gravity? if you have a chunk of matter that maintains its mass at any size, is its gravitational field affected? eg: if the earth was crushed into the size of a basketball, maintaining its mass, would its g-field stay the same?",
"Basically the more massive the object the stronger its gravitational field. But the distance to the object also matters. If the Earth was the size of a peanut, the gravitational field would be as strong at the same distance: 6380 km from its center. If you were ten feet from it, it would be a lot stronger."
] |
[
"To be a bit more clear:",
"Far away, the size doesn't matter. When you start getting close, the finite size may be important. The size is important ",
" it is comparable to the \"gravitational radius\" of that mass, which is to convert mass into a length: R_s = 2 GM/c",
" (this is the \"Schwarzschild radius\").",
"Outside of a non-rotating spherical body, the metric will be exactly the Schwarzschild metric. Perhaps the most useful result for understanding the difference between Newtonian gravity and GR is that the orbital frequency for a circular orbit at radius r is given by\n \\omega",
" = M/( r",
" * (r - 3M) )\nin units where c=1=G. Note that when r >> M, this reproduces the Kepler frequency, \\omega",
" = M/r",
" . But if a body is more and more compact, then you can have orbits closer and closer to r on the scale of the gravitational radius. The frequency starts to deviate from the Kepler frequency the closer you get."
] |
[
"would you be able to put multiple super dense objects close together? wouldnt they just collide with each other?"
] |
[
"AskScience AMA Series: Cosmology experts are here to talk about our projects. Ask Us Anything!"
] |
[
false
] |
We are four of 's cosmology panelists here to talk about our projects. We'll be rotating in and out throughout the day (with more stable times in parentheses), so send us your questions and ask us anything! (8-11 EDT)- I'm a theoretical cosmologist interested in how we can explain the accelerated expansion of the Universe, in a way that's theoretically satisfying, by modifying the laws of gravity rather than invoking a mysterious dark energy. Most of my work over the last couple of years has been on a theory called massive gravity, in which gravitons are massive (in Einstein's theory of general relativity they're massless, like photons), and a closely-related theory called bigravity, in which there are two spacetime curvatures (or equivalently two gravitational fields). I've just finished my PhD and will be starting a postdoc in the fall. (10- EDT)- My research is primarily focused on constraining the cosmological parameters related to dark energy. Currently, I'm involved in a project focused on finding new galaxy clusters using CMB and galaxy survey data. (13-15 EDT) - I do research at a major US university. My primary focus is on large-scale redshift surveys (namely, SDSS and DESI), studying properties of dark energy (observational constraints, time-evolution, etc.) and galaxy/QSO clustering. (10-12 EDT) - I'm a graduate student studying computational physics. My research involves simulating compact bodies like neutron stars and white dwarfs to calculate their physical properties. For example, I'm interested in neutron star mergers as a site of heavy metal nucleosynthesis and as a source of gravitational waves.
|
[
"Honestly, it was probably watching a bunch of stupid videos on youtube when I was in high school. I always liked science, but having Mind Blowing Content",
" on demand about quantum mechanics and relativity and space and Neil deGrasse Tyson really made me think, \"Holy ",
", I need to do ",
" with my life.\" ",
"It's probably half the reason I contribute to askscience - I had questions ten years ago that I'm lucky enough to know the answer to now, so writing walls of text about black holes and nuking Jupiter is just my way of giving back. "
] |
[
"There are ",
"articles",
" for ",
"each",
" of these on Wikipedia, although they're aimed mostly at people with a working knowledge of theoretical physics. (Full disclosure: pretty big chunks of these articles are my work.)",
"Our best theory of gravity to date is Einstein's theory of general relativity, or GR. In GR, space and time are combined into a single entity - the aptly-named spacetime - and matter is able to ",
" spacetime. Since matter also lives in spacetime, matter moving on straight paths (or the closest thing to straight) through a curved spacetime will appear to move on curved paths, and these turn out to look exactly like they're moving in the presence of a gravitational field. Voila! Gravity. This is in sharp contrast to Newton's theory of gravity, taught in high school, where gravity is caused by a force acting at a distance between two masses, although Einstein's theory reduces to Newton's in a certain limit (as it should).",
"Einstein developed GR in 1915. In the 50s and 60s, people realized that it could equivalently be described in the same language used for particle physics. If you imagine that spacetime isn't curved, but there are massless particles with a high spin (twice as high as that of photons, or light particles, and four times as high as that of electrons and quarks), then demand that these particles interact with other particles and with each other in a way that is theoretically consistent - i.e., stable, conserving energy, etc. - then you uniquely get back GR! In this picture, the notion of matter curving spacetime emerges out of matter's interactions with these ",
" particles. The end result is the same as Einstein's.",
"So we have two equivalent descriptions for GR: one geometric (i.e., in terms of spacetime), and one in terms of particles called gravitons. As far as known physics is concerned, we can use these interchangeably.",
"Alright, so FINALLY onto massive gravity and bigravity! Remember that we had to assume gravitons were massless in order to get back GR. Massive gravity is what results when you instead let them have a mass. It's an alternative theory of gravity to GR, and so makes different predictions for cosmology, black holes, and so on. In particular, since gravitons can be thought of as mediating the gravitational force, it turns out that a massive force-carrying particle is (for the most part!) similar to a massless one over short distances, but leads to a much weaker force over large distances. This is, roughly speaking, because massless particles move at the speed of light, but massive ones travel more slowly. So in massive gravity, gravity is weaker at large distances than in GR.",
"Bigravity is a generalization of massive gravity. It's usually introduced to handle a couple of concerns with massive gravity. We constructed massive gravity by considering massive gravitons living in a flat spacetime background, and then finding a consistent theory to describe them. It turns out the theory you get is different if you instead consider a black hole spacetime background, or a cosmological one, etc. This dependence on the background is unusual, and doesn't happen in GR - if you started with ",
" gravitons on any of those backgrounds, then you'd get back GR in every case. So there are actually an infinite number of massive gravity theories, one for each choice of the background spacetime. In bigravity, you allow that background spacetime to itself be curved by matter, so that it's determined dynamically, rather than being put in by hand by you. The result is a theory with two notions of spacetime curvature (one from the background and one from the massive graviton, very roughly speaking), or equivalently, of two gravitons, one massive and one massless. My personal interest in this theory stems from the fact that it's much easier to obtain cosmological solutions - i.e., spacetimes describing our Universe on large scales - in bigravity than in massive gravity."
] |
[
"When you graduate from college, believe it or not, you're not finding a job. You're going back to school. ",
"After you've got a 4 year degree, you apply to grad school, and you basically retake all the same classes but in much more depth, and you start on research. The research is about 4-5 years of work that ultimately becomes a thesis - some collection of original scientific contributions. In general, if you do decently well in college, you won't have trouble getting into a PhD program, which should be able to financially support you for 4-7 years, depending on a bunch of factors. Basically, grad school is your job for 4-7 years. After getting the PhD there are fewer scientific opportunities in academia - there's far more people graduating with PhDs than there are \"post doctoral\" positions. But if you want to sell your soul and go work on Wall Street, having the kind of computer and numerical skills of a physicist will get you a good starting salary. Astrophysics and pure science is hard field to make a career in (what isn't these days?) but very few physicists are unemployed - employers really like the numerical and problem solving skills of physicists, but those jobs won't necessarily have you simulating supernova. ",
"As far as it being awesome - the average day is not big bangs and black holes. In fact, you rarely do that stuff - that's mostly reserved for shooting the shit over beers at conferences. The average day is drinking coffee, writing code, wondering why the code doesn't compile, reading papers, submitting abstracts to conferences, drinking more coffee, and going to a meet where you tell everyone your code still doesn't compile. Also, lots of conference travel - so that's nice. It's the best job in the world and I wouldn't trade it for all the money on Wall Street. "
] |
[
"Is yeast necessary for the production of alcohol during decomposition? Or is alcohol just a consistent byproduct of decomposition?"
] |
[
false
] | null |
[
"Yeast metabolize sugars and replicate and in the process generate CO2 and ethyl alcohol, among other things. When alcohol levels reach a certain level it becomes toxic to the yeast and the production comes to a halt, unless they run out of available sugars first. Some yeasts are more tolerant of alcohol levels than other types."
] |
[
"Alcohol is not a consistent byproduct of decomposition in general, but it is a common byproduct of a particular ",
" of decomposition, namely fermentation. Fermentation is a nice old fall-back (The other common byproduct of fermentation is acid; for example, when your muscles run out of oxygen they begin to ferment sugar instead of oxidizing it, producing lactic acid).",
"Yeast is not the only organism that can ferment sugar into alcohol; various bacteria can do this as well, and contribute to the brewing of various kinds of alcoholic beverages. However, if you're trying to ferment a product, there needs to be ",
" living in there, whether bacteria, yeasts, or other fermentation-capable organisms. Fermentation is a complex process that organisms carry out to extract energy from sugars; it doesn't happen spontaneously outside of living cells."
] |
[
"Alcohol is the waste product of anaerobic catabolism of sugars in the glycolytic pathway (also called fermentation) - if there was enough oxygen in the solution, you'd instead have CO2 production from the yeast, which is what is produced by aerobic respiration via the TCA cycle."
] |
[
"Does the conjecture P = NP in computational complexity theory assert that encryption is unreliable?"
] |
[
false
] |
I'm a philosophy grad student and came across the PvNP question recently in some literature. I was wonder that since P=NP asserted that the class of problems with efficiently checkable solutions (finding an algorithm in polynomial-time) is equivalent to the class of problems with efficiently solvable problems, that it concomitantly asserts that encryption based on prime factorization could be solvable in polynomial-time. Not that anyone makes this conjecture it seems, but an interesting consequence to think about.
|
[
"I mean, the answer to your question is \"Yes\".",
"More specifically, if it turns out that P=NP, encryption will be much less secure than it currently is and that because we don't know if it's true then we don't know if encryption is actually secure.",
"It's possible that P=NP but the algorithm that we prove exists to solve an NP-complete problem has an unfathomably high constant exponent...such as O(n",
") in that case, practically speaking encryption would still be safe."
] |
[
"Let's start with some things about integer factorisation & discrete logarithm, two problems on which encryption schemes (such as RSA or Diffie-Hellman key exchange) are based:",
"quasi-polynomial",
"NP ∩ coNP",
"and those make us sufficiently certain these problems are ",
" to base an encryption scheme on. Put another way, we can leverage these problems to establish confidentiality, in a way that is not feasible to break. Feasible is a key word here: given infinite resources, one could find solutions to the above problems which would apply to an encrypted communication channel or encrypted file that they wanted to peek inside. However, they do not have infinite resources, and the amount of resources they would need to do so in a reasonable time frame is prohibitive.",
"The other side of the coin is, of course, that if we had a ",
" at our disposal, we might be able to break RSA, for instance, in a reasonable amount of time.",
"What, then, does it mean for integer factorisation if P = NP?",
"It means there exists an algorithm, call it A, which can factor all integers in polynomial time. By way of knowing ",
"P = NP",
", however, we do not know that algorithm. Our best choice would still be GNFS. Therefore, communications would be safe ",
", as we moved away from schemes like RSA and Diffie-Hellman key exchange. This does not change the fact that the theoretical proof of A's existence makes integer factorisation an ill choice to base a cryptosystem on, just like it is not advised to base a cryptosystem on other problems in P, such as the problem of determining if a number is prime, or the ",
"maximum matching",
".",
"The basis of all this lies in the fact that encryption systems like RSA are inherently based on the P-NP equivalence. Rendering a whole class of problems ",
" would have enormous implications. By knowing problems like the travelling salesman and problems in protein structure prediction ",
" efficiently solvable (by way of being in P), the benefits of attempting to find a polynomial-time solution far outweigh ",
" difficulty. You could make an analogy to this as a risk-based approach to problem solving, and as the risk disappears (",
"P = NP",
"), more researchers are given incentive to find efficient solutions, or as the risk is theoretically proven (",
"P ≠ NP",
"), we know for sure there are no \"fast\" solutions and stop searching.",
"Ironically, this whole situation matches the pre-Goedel era, when mathematicians (most prominently, David Hilbert, with his famous ",
"\"In mathematics there is no ignorabimus\"",
" ) were certain of the truth in Mathematics, not by way of proof, but intuition. It remains to be seen who will be the next Goedel. :)",
"PS. For those that are interested, only a few weeks ago, László Babai gave a talk in which he outlined a quasi-polynomial solution to the ",
"graph isomorphism",
" problem, another problem which shares certain properties with integer factorisation & discrete log and has made people optimistic about putting one of those problems in ",
"P",
"."
] |
[
"You're correct that we can't prove factor-based encryption schemes are impossible to break in polynomial time with a classical algorithm. It is theoretically possible that someone could publish an algorithm that busts our banking system wide open tomorrow, or that some government agency like the NSA is already sitting on such tools. This, however, is very unlikely. ",
"We also know that factor based encryption ",
" be broken in polynomial time with a quantum computer, and some specialized, highly limited forms of these (e.g. quantum annealing) are already in use, although none meet the requirements for cracking encryption ",
". ",
"Finally, consider the fact that the ciphertext of messages encoded with factor-based encryption and transmitted over classical networks can be archived, and probably are being archived by the NSA. They might be impractical to crack now, but are unlikely to remain so for more than a few decades, at most. You should therefore consider communications which use factor-based encryption as being private for now, but public after a while. This means credit card info, which changes fairly regularly, is safe to transmit, while documents with long-term sensitivity, such as medical records or state secrets, should be handled in a different way. ",
"This sheds some light on why world governments were so pissed off by wikileaks posting of their secrets in the ",
"form of an encrypted dump",
". This was not the threat of publication, but actual publication with delay. Sooner or later, those dumps will be decrypted. "
] |
[
"How do scientist take pictures and videos of incredibly microscopic things?"
] |
[
false
] |
I was going through the Nikon Small World competition and was baffled by how scientist were able to produce their videos. Thing first place video was of a zebrafish embryo growing its elaborate sensory nervous system
|
[
"The specific technique used in the zebrafish embryo video was Selective Plane Illumination Microscopy (SPIM) which is designed to image small organisms without the photons damaging the organism. Instead of lenses projecting an image onto a plane like a normal camera, this method scans the organism with a sheet of light, then rotates and repeats to get a 3 dimensional image. Very sensitive detectors and very precise tools are used to get these precise images, although scientists can't view everything with just light.",
"The wavelength of visible light ranges from 380 nanometers to 700 nanometers, this means we can't view anything smaller than 380 billionths of a meter with a typical microscope. Luckily electrons have a much smaller wavelength than photons do, so scientists have built electron microscopes to view these objects (think down to the size of atoms). This is done by creating an electron beam, which scans a very small area by focusing the beam from left to right, down a line, then left to right, then down a line, etc. like reading a page.",
"Another small imaging technique doesn't use beams or optics at all. This is called a Scanning Tunneling Microscope, which scans like an electron microscope, but uses a very small needle on a very flat surface to detect a quantum tunneling current from atoms. A famous example would be A Boy and his Atom, by IBM. ",
"https://www.youtube.com/watch?v=oSCX78-8-q0"
] |
[
"SPIM is also called Light-sheet Fluorescent Microscopy and it has only come to be a commonly used microscopy technique in the last ~5-6 years due to technical advances in cameras, computing power and data storage. The data sets used to create a single movie are huge! It has the advantage of being gentle to living tissue (cells don’t like being blasted with lasers, go figure) and it can be used with relatively large samples (like fish embryos) to study development of whole organisms. A light-sheet microscope uses an optical and detection scheme that allows scientists to take “optical slices.” Other types of light microscopy imaging cannot make these slices and cells that are below the surface become blurry and lose contrast due to the physics of light and optics. Some other amazing examples of light-sheet imaging can be found here by one of the pioneers in the field: ",
"https://www.janelia.org/lab/keller-lab/microscopes",
" "
] |
[
"We are actually capable of imaging things below the diffraction limit now (200-250 nm, roughly half the wavelength of visible light). This is done with super resolution microscopy, and we're able achieve resolution down to 20 nm, with some laboratories claiming they've gone even further down.",
"Since we can't separate two events of light (i.e. molecule with fluorescent probe attached to it) closer than 200ish nm in the spatial dimension, we separate them either temporally or by wavelength.",
"Explaining the exact mechanisms of the different types of super resolution imaging is probably out of scope for an ",
"/r/askscience",
" comment, but I encourage those interested to either look up PALM, fPALM, or dSTORM on wikipedia to read more on super resolution microscopy. Alternatively read ",
"this",
" paper by Markus Sauer and colleagues. Unfortunately the field is relatively new, so most papers are not that accessible unless you have experience in microscopy."
] |
[
"If two identical twin females marry a set of identical twin males and have children and had a DNA test on each others children would it say that Sister A's kids actually belonged to Sister B because of the identical DNA?"
] |
[
false
] |
I've had this question in my head for a long time. Like, genetically could it look like the other twins children are her own? I've always wondered about things like that. I mean how identical is their DNA?
|
[
"A test of all children of the two couples would show up as if all children had the same two parents. Strictly genetically speaking, since it was two pairs of identical twins reproducing, all of those children would be (genetic) siblings, not (genetic) cousins. ",
"Our modern paternity testing's sensitivity would show them as siblings, but technically, due to random mutation shortly after the zygote split and other environmental factors many sets of identical twins do have base-pair differences (although they would need much more in-depth analysis to catch than a simple paternity/maternity test would provide). ",
"Source"
] |
[
"To clarify, the type of genetic test commonly used to determine paternity would not be able to tell them apart. A much more thorough (and expensive) test would probably be able to tell them apart, but it might require a huge number of genes to be tested. "
] |
[
"I don't believe the OP is asking if the children are genetically identical to each other, the OP is asking if paternity/maternity can still be ascertained by DNA testing and the answer is no."
] |
[
"When Intel or AMD build a CPU, do they know in advance how high the CPU will be able to clock?"
] |
[
false
] |
I know they probably have targets but I was wondering if there was some method they use to simulate the physical properties of the CPU before production.
|
[
"CPU developers have to find the maximum clock speed at which their architecture will be stable. They should have a pretty good idea of where this stable speed is, since they developed the architecture and roughly know the limitations of the hardware design, but there's a lot of testing to find the true maximum.",
"If you set the clock speed too high, one instruction may still be processing while the next instruction tries to read its result. This is what makes an incorrectly overclocked computer unstable.",
"Once the engineers find a stable clock speed, they turn it down to have of a safe space between an unstable clock speed and the speed at which they sell the CPU. That buffer is why you can safely overclock by a certain amount.",
"Source: university course",
"Edited for clarity."
] |
[
"A great deal of really expensive or vendor-proprietary software involving graph theory to model circuits, propagation delays, etc. likely running on supercomputers of previous generations of CPUs. Basically, the longest propagation delay possible through a circuit dictates its maximum clock speed. I've personally used some tools to calculate this for FPGAs (programmable circuits), and though I have no experience with VLSI, I assume it's something similar, but on steroids, to determine what you're asking about. VLSI and its challenges are probably the real subject areas you'd want to study/research to answer your question."
] |
[
"you know already how fast the silicon gates can switch and the whole cpu is designed in VHDL so you can simulate the clock cycles on a computer, I think you have a fairly good idea before you build it, but obviously what the simulated cpu and the actual do might differ, however I imagine they debug it as much as possible in the computer to start with, reading voltages off the running cpu is probably a bit tricky."
] |
[
"Why would someone be prohibited from donating blood after a clinical trial?"
] |
[
false
] | null |
[
"Drug trial?",
"They don't know how long it takes the body to remove 100% of the drug and there may be some random contraindication with someone receiving your blood. Like a random freak allergy that just appears.",
"I wasn't allowed to have unprotected sex for like 6 months after mine for a similar reason"
] |
[
"It depends on the nature of the clinical trial and the treatment that the donor received. A general statement is that donors who receive a systemic experimental medication as part of a clinical trial should be deferred for 12 months unless stated otherwise by the sponsor of that trial. Source: ",
"https://www.uspharmacist.com/article/assessing-the-suitability-of-blood-donors-on-medication",
"More detailed information about deferrals for different types of drugs is available from the FDA (",
"https://www.fda.gov/vaccines-blood-biologics/biologics-guidances/blood-guidances",
")."
] |
[
"That depends on the trial. ",
"If it's a medication trial, it's still unclear how long the medication can be present in the blood, and how long after taking the medication the participant can still have negative effects. You don't want to put the recipient or donor at risk unnecessarily. ",
"If it's a non-medication trial, it might be due to infection risks and other unforeseen health risks for the recipient or donor. ",
"Lastly, blood donations might alter the outcome of the trials, rendering the participation worthless."
] |
[
"What is the biological purpose of rejecting blood types?"
] |
[
false
] |
How would this have evolved? Only with modern medicine could we get enough foreign blood in our bodies to have a reaction, and even that seems to serve no protective role.
|
[
"You're probably referring to the ABO system as well as the Rh system, but for a fun fact, there are ",
"many 'minor' blood type differences",
" on top of those. ",
"Blood antigen variation is thought to be a response to pathogens like viruses spreading from person to person and carrying some component of membrane protein with them, such that individuals who created an antibody response to foreign ABO markers had more success fighting off infections. ",
"wiki explanation here",
", ",
"journal article here"
] |
[
"From an evolutionary perspective, the different ABO types served as a mechanism to avoid disease. ",
"Depending on blood type, people are more or less susceptible to particular pathogens. Type O people, for example, are more susceptible to cholera and plague, while people with type A are more susceptible to smallpox. ",
"Source",
"Evidence supporting the view that blood group O provides a selective advantage against severe malaria has been recently reviewed.14–16 The argument is persuasive. Group O is presumed to have arisen in Africa before the migration of early humans. ",
"Source",
"If you then look at the population frequency of ABO types today, you can see this reflected, e.g. Europeans tends to have high number of A & O, with B type being significantly less common.",
"So to actually answer your question of \"rejecting blood types,\" this would come down to the fact that if the types could mix freely, it would defeat the purpose of being a mechanism against disease. In order for type A to be somewhat plague-resistant, those people need to be completely incompatible with B-type, which is susceptible to infection.",
"*edit for grammar"
] |
[
"Rejecting other blood types is a by-product of your immune system being able to identify foreign substances (antigens) in your body. It is really just an immune response. This response is generated like any other immune response, your body detects something and determines that it is not one of your cells. One of the main ways your immune system does this is by searching for a marker that is on the surface your cells that identifies them as \"self.\" This marker is called major histocompatibility complex I (MHC I.) This is why it is so hard to find a matching organ donor and doctors are concerned that they will be rejected. (The genes that make up MHC are the most diverse of the human genome. You have about a 25% chance of matching a biological sibling.) If the MHC I is different your immune system recognizes it as \"non-self\" and generates an immune response against it. Your immune system actually fights cancer and viral infection by detecting compromised MHC I (altered self). ",
"Red blood cells actually lack MHC I, so there is much greater flexibility in what your immune system sees as foreign (non-self.) That flexibility stops at the RBO blood typing system. The determinates for the RBO system are different enough that the different types are detected by your antibodies. As your body generates it pool of antibodies, it destroys any that react to self antigen, like the determinates of your own blood type. Be cause you don't have determinates for the other blood types, antibodies that match them make it into circulation. If you receive a transfusion of the wrong blood type, the antibodies binds to the foreign red blood cells, which leads to their destruction. "
] |
[
"Why are we so confident Leptons to be truly the most fundamental particles of Nature?"
] |
[
false
] |
In Science, shouldn't we always give the benefit of doubt to our lack of knowledge? How are we so confident the Leptons are truly point like particles. Do they have no dimensions? Also, how likely is the discovery of something even more fundamental?
|
[
"We are not so confident. There are many ",
"searches",
" for effects related to a hypothetical compositeness of our current fundamental particles and no one is claiming that what we have now is the last brick.",
"However what we know for sure is that a point particle model describes perfectly all data we have on leptons and quarks and that any compositeness introduces potential new effects that have for now never been observed.",
"Science is fundamentally skeptical ",
" conservative. We don't hold our current knowledge for absolute, unshakable truth, but the burden is on new ideas to show that they do better at describing nature than the current theory. This is not the case for lepton compositeness since this hypothesis does not in any way describe experiments better than the current theory of point-like leptons."
] |
[
"Why are we so confident Leptons to be truly the most fundamental particles of Nature?",
"Leptons aren't considered to be more fundamental than other elementary particles; they share the same \"fundamental\" status as quarks and the force-carrier bosons.",
"But we are reasonably confident (don't confuse reasonable confidence with assured certainty, now) that leptons are fundamental because (a) our models which treat them as fundamental have an outstanding track record when it comes to predicting the measured behavior of systems involving them; and, (b) we have the ability to crash them together with enormous force and have a keen understanding of how they interact and decay, and all our attempts to find any substructure have failed -- consider that we did the same thing for protons and neutrons and discovered (based on what they decayed into, among other things) that they do in fact have substructure, and that substructure was only explainable through the quark model.",
"At the end of the day, if it walks like a duck, quacks like a duck, and otherwise has all of the same behaviors and properties as a duck, then it's at ",
" an \"",
"effective",
" duck.\"",
"In Science, shouldn't we always give the benefit of doubt to our lack of knowledge?",
"Oh don't worry, we most certainly do. I linked the word \"effective\" above to the Wiki article for an \"effective theory.\" Effective theories are extremely common especially in solid-state physics, where the models are ",
" to not be fundamental models; instead, the physics that these models describe are ",
"emergent",
" physics.",
"We consider the standard model of physics to be fundamental in the sense that we have not found any other model which underlies it, from which standard model physics is emergent. That doesn't mean it ",
" emergent, and in fact the prevailing hypothesis among physicists these days is that the standard model ",
" the end-all-beat-all. These days we are using colliders to desperately search for ",
"physics beyond the standard model",
" in the hopes that we can find evidence to support an even more fundamental model. Many scientists believe that the Standard Model will eventually be replaced with a ",
"grand unified theory",
" which reduces to standard model physics in the low-energy limit, and still others think that it won't stop there, but that we will eventually resolve problems with models of quantum gravity and unite the description of all natural phenomena into a ",
"theory of everything",
".",
"What's of particular curiosity about this is that despite mankind's best efforts to date, the standard model's predictions have been borne out in experiments to outstandingly high accuracy, and we're actually ",
" to find evidence for physics beyond the standard model. Right now all we have to go off of is neutrino masses (which isn't terribly complicated to add to the standard model and doesn't change a whole lot), dark matter, and violations of ",
"lepton universality",
" (which suggests that different generations of leptons may interact differently under the known forces in at least some circumstances; and before you ask, no, this does not imply that leptons aren't fundamental or pointlike ;).",
"How are we so confident the Leptons are truly point like particles.",
"We've tested them extensively and found no evidence for substructure or minimum spatial extent, and our models which treat them as truly pointlike are extremely accurate; more accurate than any model which treats them as composite or extended. Again, that doesn't mean we assume that they ",
" are fundamental and pointlike, but in practice you have to use the model that does the best job, and the reality is that the model which treats them as fundamental and pointlike is vastly better at making predictions than alternative models. It's also ",
" that way, so there is an argument to be made in favor of using ",
"Ockham's razor",
" when investigating alternative possibilities.",
"Do they have no dimensions?",
"As far as we can tell, no -- but the truth is, saying they are \"pointlike\" is somewhat facetious because ",
" \"particles\" in the standard model are actually more like ",
"wave packets",
" and their physical location/size in space is best described via a probability distribution, which can be ",
" concentrated into a very small area ",
" extended in space, due to ",
"wave-particle duality",
". In other words, in the actual physical models, it's ",
" more complicated than simply saying they are \"pointlike.\" A better way of phrasing it might be that they have no minimum spatial extent.",
"Also, how likely is the discovery of something even more fundamental?",
"Given the fact that we've been building particle smasher after particle smasher and still come up empty ... it's not very likely. But to be clear: ",
" is ruling that out as a possibility. No serious scientists are, anyway. I fear you may have a misconception that scientists are united in support that the standard model is somehow absolute, and there just is no truth to that idea.",
"Hope that helps!"
] |
[
"The mistake is mine then ... but I definitely remember your username being filed in my \"crackpot\" bin, and a glance through your post history confirms why I remember it. Do you cite ",
" sources at all besides that one nutty WordPress blog?",
"Edit: Oh. I see the blog is yours. With lots of rambling writings in diverse fields that are not your field, which you seem to be fond of linking to as if it was authoritative ... despite there not being any actual research or data. By the way, the section you referred me to doesn't even say anything remotely relevant to this topic, it just dribbles nonsense on and on like a leaky faucet of misinformation."
] |
[
"What actually happens to electronics when they are damaged by water, why do they often not work when dried out again?"
] |
[
false
] | null |
[
"Semiconductor reliability engineer here. (Thanks for posting a question in my area of expertise!) ",
"Water has various bad effects on electronics: ",
"Causes a \"short circuit\" or electrical \"leakage\". Instead of following the wires it is supposed to, it travels along the water. This can cause malfunctions, and can even lead to some things seeing too much power and being destroyed. Pure water is mostly nonconductive, but by the time it gets anywhere that matters the water is rarely pure any more, having picked up ionic contaminants. ",
"Corrosion. Water can make things \"rust\" (oxidize). Copper is particularly susceptible, but other metals can be corroded as well. This might not cause immediate failure, but can eventually cause conductors to become \"open\" (don't conduct enough electricity) or make poor contact to other conductors. It can also cause physical failure of things like metal capacitor housings, although that's fairly rare. ICs used to have a major corrosion mechanism caused by leaching out phosphorus from the dielectric glasses, but P isn't used much any more. And Cl contamination would cause somewhat similar problems when water carried aqueous Cl onto the part, but Cl contamination levels are now so low that this risk is almost gone. Br is in somewhat the same boat as Cl. ",
"Integrated circuits nowadays have some porous layers inside of the IC that are used to make up insulators (\"low-K dielectrics\"). Moisture can cause these materials to swell and crack, causing opens or even shorts. This is a relatively recent failure mechanism that didn't exist 10 years or so ago. ",
"Many integrated circuits are not moisture-proof (they are \"non-hermetic\"). Water can fairly easily get inside of them. If this happens when they are turned off, it can lead to bad things when they are then powered up and the integrated circuit chip gets very hot very quickly. The trapped water can turn to steam and expand, causing the packaging to rupture, sometimes violently. This is called the \"popcorn effect\" in the industry. ",
"Dendritic growth. Moisture + metals + voltage + contaminants can cause metals to migrate in a pattern that looks like the ",
"roots of a tree",
". These dendrites can grow until they cause short circuits. ",
"There are a few other, but these are the biggies. ",
"(1) can happen right away. (3) tends to take a little time, (2) takes longer, and (5) can take a long time. (4) is variable; once the part dries out it is not a risk."
] |
[
"It was probably moderately purified DI (DeIonized) water. An aqueous rinse followed by a rinse in a drying agent is a common practice, to get rid of solder flux residues, which can be much more harmful than water. Although some manufacturers use \"no clean\" fluxes making this step unnecessary. ",
"I didn't mean to imply that electronics are immediately doomed for death upon contact with water. OP asked what it does, so I listed a bunch of bad things it ",
" do."
] |
[
"The OP said nothing about temporary exposure to tap water. Certainly a brief immersion (such as a dunk in the toilet) minimizes the risk of any of those things. OP also did not specify a particular electronic device. ",
"So I'll address your bullet points one by one:",
"porous layers inside of the IC (um no, they are sealed) ",
"Yes, they are \"sealed\" in that there is a seal ring ",
" to reduce moisture ingress, but that seal ring is imperfect by design and can be damaged by the die scribing process. TSMC recently added a 2nd seal ring to their design kit, presumably because just 1 didn't do the trick. And some companies use a monitor ring around the edge of the chip so that they can try to reject parts with damaged seal rings. ",
"integrated circuits are not moisture-proof (um no, most things in your phone are sealed) ",
"No, most things are \"sealed\" in plastic material which is not hermetic. The use of hermetic packaging is fairly rare for ICs nowadays. While the plastic is probably fine for a brief immersion, it is ineffective at preventing moisture ingress over longer time periods. In fact, we know that parts simply sitting on the shelf can fail from internal corrosion mechanisms just through the effects of humidity (which is the main impetus for doing HAST testing on the component level at all...external corrosion is expected and normally ignored during component HAST). ",
"\"popcorn effect\" (at room temperature with consumer electronics under 30 watts!!!! come on..... GTFO) ",
"Room temperature is not the issue, die junction temperature is. A 30W device with minimal heatsinking can exceed 100C, although I agree that would be unusual. But there are consumer devices which dissipate much more than 30W, such as CPUs and GPUs. Fortunately, they are probably less likely to get dropped into the toilet. I didn't make assumptions about that which were unstated in OP's question.",
"\n(The biggest risk of popcorning is not during use, of course, it is during PCB assembly, which is a lot hotter than 100C, although the delta T rate isn't too bad.) ",
"Corrosion (really in the 30 seconds of water exposure causing immediate failure) ",
"Again, you specified 30 seconds of exposure, not OP. As I indicated in my response, corrosion typically takes a long time. Of course, if you get moisture trapped in a low power device (which doesn't get hot enough to drive it off) then you potentially have a long time. "
] |
[
"Einstein's Principal of Equivalence states that gravity and acceleration are the same thing. But wouldn't acceleration have a limit (speed of light) while gravity is infinite? Would the discovery of gravitons disprove this principal?"
] |
[
false
] | null |
[
"Acceleration isn't a speed. What do you mean?",
"The existence of gravitons has little to nothing to do with the equivalence principle, it could be true with or without them."
] |
[
"You can never accelerate to the speed of light but you aren't taking into account any relativistic principles here. From the point of view of an accelerating object it can accelerate forever."
] |
[
"You can never accelerate to the speed of light but you aren't taking into account any relativistic principles here. From the point of view of an accelerating object it can accelerate forever."
] |
[
"Why does Helium liquefy at a lower temperature than Hydrogen?"
] |
[
false
] |
I would have thought it was related to the molecular weight of the molecule, but that is not the case? Although "naturally occurring" hydrogen is diatomic, the total mol wt is still less than helium and yet it condenses at a much higher temperature. So why does helium condense at such a low temperature, relative to other gases? For reference (from Wikipedia), boiling points of various gases: Background - I'm working on a helium recovery project and I was curious about the history of helium and its physical properties and why there really is no substitute for it's use in the science and medical fields.
|
[
"Boiling points are a product of the overall strength of the intermolecular forces at play between the molecules in solution.",
"Since the species you mentioned have no dipoles, the only forces at play are London dispersion forces (or Induced Dipole - Induced Dipole forces). These forces occur when molecules spontaneously posses a dipole for a short period which in turn induces dipoles in other surrounding molecules, and provides an attractive force. The reason for the spontaneous dipole is the quantum probability of finding the electrons at any point within their respective orbitals. The reason for helium's extremely low boiling point is it is the hardest gas to induce a dipole in, or spontaneously possess a dipole.",
"Comparing helium and hydrogen, both species have 2 electrons and both within the 1s orbital. However in hydrogen this orbital is elongated to enclose both nuclei. This elongation makes the hydrogen orbital easier to induce a dipole in.",
"Now going to Neon, we have a much larger outer orbital with 2 electrons, this is easier to induce a dipole in than hydrogen, due to the relative size of the orbital and the distance from the nucleus.",
"The best way in my experience to think about these effects is to think about the relative statistical probabilities of finding one of the gases in a polarised state, the more room in the orbitals, and the more elongated their shape, the higher the probability of finding an induced dipole."
] |
[
"But even if it is elongated, how would a dipole form if the two nuclei are identical? Is it just that the molecular hydrogen is easier to polarize than an isolated atom is, so dipoles tend to form most of the time?"
] |
[
"As far as your main question goes, Helium is the lightest noble gas. Not really an explanation, but it at least makes sense that it doesn't strongly attract itself, and why it doesn't take a lot of energy to make the molecules scatter everywhere.",
"Concerning it's irreplaceability, medicine mostly just wants it for running MRI magnets. Someday those could be replaced by something like YBCO which operates above 77K. There's a lot of science which also just wants the superconducting magnet, so ditto there. But if you want to reach the coldest temperatures using a closed-cycle refrigeration system, there is no substitute. Basically, any refrigeration system is only as good as it's refrigerant. If the water or freon or helium in the pipes freezes, you can't run a cycle."
] |
[
"When we take footage of the ocean floor that isn't reached by sunlight, are the lights used for filming harmful to the ocean life?"
] |
[
false
] | null |
[
"Generally not. ",
"You might be drawing an analogy to 'if i sit in the dark for a while, a bright light is really bright - imagine if i lived in the dark!', but it isn't really like that.",
"Most of the visible spectrum doesn't penetrate too far, with only the blue end of the visible spectrum penetrating to the depths. As a result many animals that far down are insensitive to light, and those that are tend to be sensitive to only blue light - It's why so many deep sea creatures appear to be red or pink when we film them with white light (since red light doesn't penetrate that far down, a red coloured object is functionally black).",
"The only minor impact i can think of is in regards to a handful of species that have developed the ability to broadcast and sense red light. They are predators, and essentially have red 'headlights' under their eyes that cause the red/pink animals to light up like a christmas tree. Broadcasting white light down there would illuminate the subject to these predators...but it's a pretty niche thing, and not something to be concerned about."
] |
[
"Water can absorb a very large amount of energy without heating up by any significant degree. Heat will also diffuse fairly readily to surrounding water and wont remain localized."
] |
[
"Adding on to this, water has the highest specific heat capacity of any stable and abundant liquid, meaning it takes an IMMENSE amount of energy to heat it up. Specifically, it takes 4,184 joules to heat 1 kg of water by 1° Celsius (consider that an entire kilogram of copper takes only 385 joules to increase temp. by 1° Celsius). This is why it is used to cool things like power plant equipment/exhaust and why even a small temperature increase in a heat-sink as large as the ocean is terrifying.",
"Source",
"Edit: Changed wording"
] |
[
"Ambidexterity"
] |
[
false
] |
Is there any reason everyone cant be ambidextrous? I remember when I was little a teacher would come around and ask us which hand was easier to write with, and then told us to stick with it. I had no preference so I switched off when one hand got tired and I feel like other students would do the same thing. If you start early enough is there any scientific evidence why anyone cannot be ambidextrous?
|
[
"Just a reminder for commenters: Please don't post your personal anecdotes in AskScience."
] |
[
"The brain is divided into two hemispheres.",
"There are typically right-handers, and non-right-handers. They're classified this was because non-right-handers have a brain with relatively equal hemispheres.",
"Most people can become ambidextrous through practice, but it only comes easy to a small percent of the population."
] |
[
"I may be a little late to this, but I have recently been practicing my handwriting with both of my hands and I have reached a point where I can write legibly with both hands. My left doesn't quite have the speed my right hand, the dominant one, does, but it is still legible."
] |
[
"Is the universe 13.8 billion years old, or is the observable universe 13.8 billion years old?"
] |
[
false
] |
I've tried googling and researching this to no avail. If our best telescopes and other sensors can only see so far into the universe, how do we supposedly know the age of the whole thing? Edit: Didn't know whether to put Physics or Astronomy for the flair. It's Astrophysics, isn't it? A mixture of both?
|
[
"It's a good question! ",
"For the sake of argument, let's say that 'the universe' is infinite. Here, by 'the universe', i mean everything that exists.",
"We know from our observations of the cosmic microwave background (",
"CMB",
") that to a very good approximation, the universe is both homogeneous and isotropic on very large scales. More importantly, we know that we can define a frame of reference that is at rest with respect to the universe as a whole. This is a very important point, because it means that we can talk about the age of the universe ",
". ",
"This is important because we know from relativity that fast clocks run slower. That means that two observers might disagree about 'the' age of the universe, without either of them being wrong. It is only ",
" observers that will agree on positions and lengths and times and distances. But so, by defining a frame of reference that is comoving with the universe at large, we can establish a standard or common use measure for the age of the universe.",
"In this way, modern cosmology measures the age of the universe to be 13.7ish billion years. (As a fun aside, a year in this context is defined to be 365.25 x 24 x 60 x 60. That is, a year is defined to be the number of seconds equivalent to a standard Julian year. Strictly speaking, this has nothing to do with the number of times that the earth has gone around the sun -- not least because the earth has not be around for all that many of those 13 billion years!)",
"Next thing: light travels at the speed of light. This means that it takes time for light to travel from one place to another. Sometimes you will hear people say something to the effect that this allows you 'to look back in time', but i think that this is potentially misleading. A better way to think about this is that when you look across a great distance, you see ",
" than the here and now.",
"To see this, imagine that two points in space somehow had two identical clocks from the very beginning of the universe. A person standing right next to one clock would see that 13.7 Gyr had passed. But looking at the other clock some distance away, they would see significantly less time had passed, depending on the amount of time it took for light to travel from one place to the other. If they could measure the distance, and account for the light travel time, then of course everything would work out. But the point is that the age of that part of the universe that they are looking at is younger than where they sit.",
"And of course, both of them see the same thing -- both see that their observable universes appear younger and younger as they look out across greater and greater distances.",
"And the same is true for us: the age of the observable universe varies as a function of distance.",
"This is actually what limits the size of our (or anyone else's) observable universe: as we look back to the CMB, we are in a sense seeing the big bang ",
". ",
"Thus is our observable universe finite, even ",
" the universe in its entirety is infinite, or if it's just ",
" infinite, or if it's just big.",
"Great question!",
"*edit: words."
] |
[
"This is also a very excellent question.",
"It is true that space is mostly a vacuum. But it is not ",
" empty. Between the galaxies, the average density is somewhere on the order of one proton (and one electron) per cubic metre.",
"The thing that puzzles me, though, is that i don't know whether or not space (and/or time) can be said to ",
".",
"An analogy might be the electric field. There are at least some people would be happy enough to consider the field as an abstract fiction, which is introduced to describe the dynamics of charged particles. It is a construct; it is disembodied; it does not really 'exist', in the same way as a electron or a photon. ",
"I do not know whether spacetime should be thought of in the same way: as a useful construct for describing the relations between things. I am genuinely confounded by the very question! What would it mean if space ",
" compared to if it did not? I just don't know."
] |
[
"...by 'the universe', i mean everything that exists.",
"I honestly don't know what I'm talking about, so bear with me when I ask: Isn't there a whole lot of ",
" in space? Does that exist?"
] |
[
"Why did the luner module get cold in Apollo 13? Why didn't the vacuum of space act as a thermos and keep the astronauts toasty warm?"
] |
[
false
] | null |
[
"All objects radiate away energy as a function of their temperature. So while it's not an instantaneous process, over time the module loses heat into space."
] |
[
"Especially when secondary systems are disabled to save power, which would normally generate more heat."
] |
[
"Yes. This is also true. Now over short times, the vacuum of space ",
" make a good insulator. If for instance the astronauts happened to find their module deep in the ocean (ignoring the pressure which would crush them), the heat drain from the water would kill them much faster than the equivalent heat lost to vacuum. "
] |
[
"Given the right circumstances could flora become sentient?"
] |
[
false
] | null |
[
"define sentient."
] |
[
"I would think this very fact would make it extremely unlikely, bordering on impossible, for a plant to evolve sentience. Theres nothing plants could do to actually use this intelligence, they'd just sit there and think. And think some more. And think some more. All that energy used in thinking and building and maintaining the thinking structures would be at the expense of growing taller and wider. It would be shorter and less fit than its stupid unthinking cousins, and unable to do anything about it."
] |
[
"Given the right circumstances, I suppose just about any organism could eventually evolve sentience, however, plants developing sentience is highly unlikely and here's why: Plants have no nervous system, they don't even have the evolutionary beginnings of higher thought. Also, sentience would have no evolutionary benefit for plants. Higher cognitive ability is selected for when that cognitive ability causes the organism to be able to survive when others of its kind would not, however, plants are limited in their range of possible behaviors i.e. they cannot move to avoid prey or seek better conditions, all they can do is adjust themselves to maximize their efficiency. Plants already adjust themselves to maximize their efficiency, for example, they open and close their stomata as circumstances change around them, turn their leaves towards the sun to capture light the best they can and selectively uptake water and minerals from the soil, they do all of this without relying on a complex nervous system ,in fact, having a complex nervous system capable of sentience would be detrimental to plants because such a system would require high amounts of energy, energy the plant cannot afford to expend.",
"That being said, the concept of plant intelligence is a fascinating one and for more information I'd recommend some of the things Anthony Trewavas has written on the topic or this article:",
"http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2676749/"
] |
[
"Book list for science subreddits?"
] |
[
false
] |
Could we try to compile a list of well-regarded books in the various fields represented here? Here are some examples similar to what I had in mind: It would be nice to have labels distinguishing how in depth a book is. It might be ideal to have a format similar to 's list or to have a subreddit dedicated to this subject in the vein of . Features to consider: 1) Individual suggestions can be upvoted or downvoted. 2) Individual suggestions should allow people to describe/discuss the pro's and con's of a title. 3) If in , perhaps panelists' suggestions should take priority. I'll try to xpost this to . (Would've posted there originally but they only accept links.)
|
[
"Feynman Lectures on Physics",
"Any else by him is also wonderful."
] |
[
"Introductory reading for biology, molecular biology and biochemistry:",
"Campbell biology",
"Molecular biology of the cell",
"Lehninger principles of biochemistry"
] |
[
"This'd give you a good start if you wanted to get a good (and easy-to-understand) familiarization with what archaeology is all about. There are many other well-regarded books, but I don't think most people want \nto invest the kind of time needed to read them all. ",
"Rathje's ",
"Deetz's ",
"Trigger's ",
"Johnson's "
] |
[
"Did membranes evolve before cells?"
] |
[
false
] |
I was told this recently, for the first time, so went online to try and find out if this was the case, but couldn't find any evidence toward it (fossils, etc.), how it happened, or even just an article relating to it. If anyone could explain, or even just link me to some relevant information, it would be much appreciated!
|
[
"One theory for the origin of life is that bubbles of phospholipids (lipids with a charged polar end that are attracted to water, and a nonpolar end that are repelled by water) formed bilayers (as in extant cells), which allowed for the evolution of nucleic acids and proteins that we see in life today. So yes, cellular membranes may predate cells themselves!"
] |
[
"One neat observation is that certain types of lipids can spontaneously form lipid bilayer vesicles (i.e. liposomes). In 2003, ",
"a study",
" was published that showed how the same clay environment that is capable of catalyzing the polymerization of RNA nucleotides is also capable of catalyzing the formation of liposomes that can grow and divide, a primitive form of self-replication."
] |
[
"The current theory is that cellular membrane was present before or at least at the same time of the first cells.",
"the fat that make up our cell's membrane is very easy to have in nature and naturally will form into a ball."
] |
[
"Will a black hole created by compressing a pencil (hypothetically) and a black hole created by a collapsed star have the same gravitational strength?"
] |
[
false
] | null |
[
"Gravitational influence is directly proportional to its mass."
] |
[
"But, of course, a black hole's density is what gives it it's commonly-portrayed properties. "
] |
[
"Not exactly. At say one kilometer from each source the gravitational force is much greater with the collapsed star. In fact the pen black hole would have same gravity as a normal pen. However once you reach the event horizon,where the escape velocity is greater than the speed of light, of each case the gravity would be the same. The difference is that the event horizon of a pen black hole is incredibly tiny. "
] |
[
"Why do mutations occur at all during DNA replication? How/why do the proteins that replicate DNA mess up?"
] |
[
false
] | null |
[
"It's because the process is not as organized as we have learned it. We tend to think of intracellular reactions as they're supposed to work. Only thymine is supposed to bond to adenine. Only a specific type of ligand bonds to a certain receptor etc. this type of thinking leaves it hard to imagine what's actually going on. Imagine complete and utter chaos. Enzyme being hit by all the wrong and right molecules many many times per second. Molecules flying off in every which direction and hitting against anything and everything. When we learn about these reactions we tend to think it's ordered based on how specific things are. Not only is an enzyme/protein site made to fit only a specific molecule, it has to be the correct orientation. So even if the right molecules are in the right place hitting against the right enzyme does not mean it will attach, assuming it doesn't approach at the right angle. ",
"Now you might be thinking that it's impossible that reactions happen in the first place. How can there be order out of all the chaos? Two things help: one is the incredibly large number of molecules and two is the speeds at which they travel. As I mentioned, an enzyme can be hit by different molecules many times a second. Let's figure out the speed of adenine. The molecular mass is 135 amu. Let's say the temperature is 37 deg C so about 310 K. Between collisions kinetic energy should be about equal to thermal energy. So we end up with 240 m/s. It may be lower considering the density and makeup of cells so let us assume 100 m/s. That is equal to 100 million microns per second. Typical human cells are 20 microns long. But let's say we're looking specifically at the interaction involved in DNA replication so we're just in the nucleus. That's about 6 microns long. So, an adenine molecule can traverse the nucleus around 16 million times per second. That's very fast.",
"The whole point of that was to show the relative craziness in a cell, specifically in a nucleus. So how does an error occur? Base pairs are moving and hitting the DNA polymerase involved millions of times per second. Perhaps it needs an thymine next to match with an adenine. Perhaps an adenine hits it in such an orientation that it fits the groove enough to be stable. With that much chaos, chance is bound to happen. In fact we should have more errors on our DNA than we do now. The reason we don't is because we have mechanisms for checking our DNA and correcting mistakes. But still, on all the chaos some mutations will persist. ",
"So I guess my answer is that mutations and errors happen because they just do. It is unavoidable.",
"Edit: Sorry for the typos. I wrote this on my phone."
] |
[
"/u/jimmyth3xplod3r",
" spoke about the bigger \"why?\" question in terms of evolution and the necessity of mutations to generate diverse progeny, but I will come at your question from the angle of biochemistry / protein chemistry regarding the replication machinery.",
"Disclaimer: I will be speaking about the replisome (the collection of proteins that act in DNA replication) from the most well-understood system of ",
" bacteria. The general principles will be relatable to humans since we're talking about the general error-prone nature of the replisome.",
"In bacteria, the complex of proteins responsible for DNA replication is known as the replisome",
" The main protein that catalyzes the reaction of adding new nucleotide bases to a growing daughter DNA strand is known as DNA polymerase III holoenzyme (meaning it's an enzyme comprised of multiple protein subunits)",
" DNA polymerase III serves as a great polymerase for replicating the majority of our genomic DNA because it has an extremely high fidelity, which means it can add an incredible amount of new nucleotide bases that are of the correct identity (based upon the parental DNA stand the polymerase is using as a template, and the Watson-Crick base-pairing interactions",
" before it finally makes a mistake. ",
"Now, why do polymerases make mistakes, and therefore insert an incorrect base into the daughter strand of DNA - and therefore why do mutations occur at all during DNA replication? There are many, many answers to this question and we certainly haven't uncovered every answer yet. ",
"One huge factor in incorporating an incorrect base on the daughter strand of DNA is that the base on the parental strand has been modified in such a way that the polymerase thinks it must incorporate a certain base when in reality it shouldn't. An example: Sometimes nucleotide bases, during cell division, can undergo a process known as tautomerization",
" that changes the chemical structure of that nucleotide base. Since template reading by DNA polymerase III relies on hydrogen-bonding patterns between a new nucleotide base and a parental one, if the parental base has been modified chemically to have a different hydrogen bonding pattern, a different base may be inserted on the daughter strand in error (See link #4 for specific examples of base tautomerization that lead to nucleotide misincorporation). ",
"Other examples of base modifications include a slew of DNA damaging activities, including reactive oxygen species (that may react with nucleotide bases, altering their chemical structure and therefore their hydrogen bonding capability, thus changing the inserted base on the daughter strand), UV light, etc. DNA damage creates a multitude of DNA lesions",
" that make it difficult for the DNA polyermase III holoenzyme alone to effectively replicate DNA in an accurate manner. Double-stranded breaks, thymine dimers, and a multitude of other lesions can cause problems with nucleotide incorporation by DNA Pol III holoenzyme. Thankfully, there are several pathways that can effectively deal with the damage, and fix it via homologous recombination",
" trans-lesion synthesis",
" or nucleotide-excision repair",
" among other mechanisms. ",
"Third, the rate of catalysis of nucleotide base addition by DNA pol III holoenzyme is roughly 1000 per second",
" making it difficult to be entirely accurate with each and every catalyzed reaction. ",
"In summary, the reasons mutations occur during DNA replication include: the modification of parental nucleotide bases, resulting in a misincorporation of a new base on the daughter strand; DNA damage occurring before or after replication resulting in DNA lesions that make effective replication difficult by generating unknown DNA structures; and the high rate of chemical action of the polymerase holoenzyme in DNA replication.",
"But do not fret, while the DNA polymerase III holoenzyme makes a mistake 1/1000 bases, the multiple interdependent mechanisms (in part thanks to proofreading activity of Pol III, Pol I, and other polymerases in bacteria), the estimated error rate of nucleotide misincorporation is 1/1,000,000,000 to 1/100,000,000,000",
" Which is not bad, considering the size of the bacterial genome in ",
" is about 4 million base pairs.",
"Sources:",
"http://en.wikipedia.org/wiki/Replisome",
"http://en.wikipedia.org/wiki/DNA_polymerase_III_holoenzyme",
"http://www.annualreviews.org/doi/pdf/10.1146/annurev-biochem-061208-091655",
"http://blc.arizona.edu/courses/181Lab/MoBiByMe/Tautomers.html",
"http://en.wikipedia.org/wiki/DNA_repair#Types_of_damage",
"http://en.wikipedia.org/wiki/Homologous_recombination",
"http://en.wikipedia.org/wiki/DNA_repair#Translesion_synthesis",
"http://en.wikipedia.org/wiki/Nucleotide_excision_repair",
"http://www.ncbi.nlm.nih.gov/pubmed/22404288"
] |
[
"All of the answers here are brilliant. I'll just add that DNA can also take mutation-inducing damage from outside sources such as ionising radiation and toxic chemicals like free radicals, which are generated by various cellular mechanisms. On a molecular level, a DNA base's structure can be altered by such reactions, which can lead to mismatching of bases. The replication proteins may also read this mutated base as another, for example an A as a C, and add the wrong complementary base (creating an A-G pair). Upon subsequent replication of the new strand, the G is read and a C added, thus creating a GC pair where there was once an AT pair.",
"This occurs on a much smaller scale, though, and there are many repair mechanisms, but naturally some mistakes will always slip through."
] |
[
"Are there any materials that become softer when cooled and harder when heated?"
] |
[
false
] |
Materials made of iron for example become very soft and malleable when heated but extremely brittle and hard when cooled -- are there any materials that have the opposite effect?
|
[
"Plastic deformation (ie. permanent deformation) generally occurs by the passage of dislocations through a material - you can think of them as defects in the crystal that let it move a bit like a caterpillar - (",
"https://www.nature.com/nmat/journal/v9/n4/images/nmat2737-f1.jpg",
"). Heating things up lets them move more easily (and overcome any obstacles they might encounter), so materials get softer and more malleable. ",
"There are a couple of exceptions to this though - Ni superalloys actually get stronger at higher temperatures (up to a point - see ",
"https://www.researchgate.net/profile/Stan_David/publication/277354413/figure/fig21/AS:268052629553171@1440920135525/Figure-8-8-Yield-strength-anomaly-yield-strength-of-Ni-base-superalloys-increases-with.png",
") through a phenomenon called Kear-Wilsdorf locking, which is quite a complicated dislocation / crystallographic effect. The really unparalleled high temperature strength of Ni alloys is why they're used in jet engine turbine blades - the temperature needs to be as high as possible to maximise the engine's efficiency. "
] |
[
"Not all nickel super alloys. Inconel while exceptionally strong does not get strong with temperature"
] |
[
"Yeah all Ni superalloys exhibit this effect - in certain inconels it might just offset the drop in strength due to temperature though, so you wouldn't see an explicit strength increase."
] |
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