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[
"How does Quantum Tunneling not violate the Law of Conservation of Energy?"
] |
[
false
] |
How can a particle escape a potential well without "creating energy out of nothing"?
|
[
"If the system before and after tunneling has the same energy, the law is not violated. That's the whole point of tunneling - it doesn't need to acquire more energy and climb that potential well to escape."
] |
[
"This is what tunnelling of an electron through a potential barrier looks like in terms of the electron's wavefunction:",
"https://en.wikipedia.org/wiki/File:Quantum_Tunnelling_animation.gif",
"If you add up all the probability (i.e. add up the area of the curve over all the space) it will always add up to 100% at all times. Even when the reflected portion and the transmitted (i.e. tunnelled through) portion separate.",
"Why do you have an intuition that such a thing violates conservation of energy?"
] |
[
"In the so called \"classically forbidden region\" you point to, the kinetic energy is indeed negative as the momentum is an imaginary number. This is because the wave is not propagating in that area but rather exponentially dying out. However, total energy is still conserved in that area. Perhaps the conceptual issue is that what the kinetic energy is in quantum mechanics is not what it is in classical mechanics but in so many scenarios the two are very similar, however, in this scenario they are quite different (a classical particle with a classical kinetic energy would indeed violate energy conservation). However, QM is the more correct theory so particles do tunnel and there's not a problem with that (because the quantum kinetic energy is the more correct quantity)."
] |
[
"Why does the Trinity site still have high background radiation, but Hiroshima and Nagasaki do not?"
] |
[
false
] |
So I was reading a Wikipedia article about the Trinity site where they tested the first atomic bomb ( ) and it says "More than seventy years after the test, residual radiation at the site is about ten times higher than normal background radiation in the area. The amount of radioactive exposure received during a one-hour visit to the site is about half of the total radiation exposure which a U.S. adult receives on an average day from natural and medical sources." Meanwhile, Hiroshima and Nagasaki are considered to have normal levels of background radiation. Some articles I read attribute the lack of background radiation levels in these places to the bombs being detonated in the air versus ground level. So why would they be different? I'm guessing perhaps the Trinity bomb was detonated closer to the ground? And why does ground versus air detonation affect the background radiation? Thanks in advance!
|
[
"had parachutes attached",
"This is a pedantic point, but they did not have parachutes of any sort. They had metal tails that stabilized them, but nothing that retarded their speed of dropping to any degree. ",
"The Trinity tests where ground bursts and therefore irradiated the surrounding area and particularly the sand. Fun fact that same sand killed John Wayne and other actors as they used it in the movie the Great Conquerer. They filmed close to the tests and shipped some of it to Hollywood ",
"There was only one Trinity test, in New Mexico. The test that harmed John Wayne was much later (Upshot-Knothole Harry) and was in Nevada. So different sands."
] |
[
"had parachutes attached",
"This is a pedantic point, but they did not have parachutes of any sort. They had metal tails that stabilized them, but nothing that retarded their speed of dropping to any degree. ",
"The Trinity tests where ground bursts and therefore irradiated the surrounding area and particularly the sand. Fun fact that same sand killed John Wayne and other actors as they used it in the movie the Great Conquerer. They filmed close to the tests and shipped some of it to Hollywood ",
"There was only one Trinity test, in New Mexico. The test that harmed John Wayne was much later (Upshot-Knothole Harry) and was in Nevada. So different sands."
] |
[
"There are really two factors here:",
"1) How long does it take the radiological risk to human health to go away?",
"2) When will people willingly move back given the risk that is present?",
"Take Pripyat for example, since you bring it up. The actual radiation levels in most places in Pripyat are not very significant today. ",
"http://chernobylgallery.com/chernobyl-disaster/radiation-levels/",
"When looking at that table, the units are in microsieverts, or uSv. The average annual exposure to natural sources in the USA is about 3100 uSv. That breaks out to about 8.2 uSv per day or 0.35 uSv per hour. There are definitely \"hot\" spots in Pripyat, but nothing is radioactive enough to cause an immediate threat to human life.",
"But how \"hot\" does a place need to be before it's unsafe? We actually don't really know. We know that large doses of radiation are unsafe, but we don't have a really thorough understanding of the human health effects of long-term low-dose exposures. It's hard to say, for example, if you were exposed to double or triple the natural background radiation in the USA if you'd suffer any measurable health effects.",
"The most work in this field has come from the nuclear industry, where lifetime radiation exposures are closely tracked. The Health Physics Society has taken the position that we should not attempt to quantify health risks from yearly exposures under 50,000 uSv per year. In the link below, they suppose a worker who recieves a 30,000 uSv yearly dose between the ages of 18 and 65. Under the current risk model that worker would have a 5% chance of dying from a radiation-induced cancer and is associated with a 6-month shortening of life versus the regular population. ",
"https://hps.org/publicinformation/ate/q10031.html",
"More telling is that ",
" exposures can be extremely high relative to natural sources. For example, a single chest CT scan is associated with a dose of 7,000 uSv on top of your regular exposure. Clinicians always have to worry whether the diagnostic/theraputic benefit of x-rays and CT scans are worth the potential radiological risk, and our long experience with these technologies suggests that artificially elevated but still low radiation exposures do not pose a significant threat to human health.",
"So the answer to the first question is that, according to our current understanding, people could probably live in significantly more radioactive environment than is natural without too many ill effects. A community with consistent monitoring efforts and diligence about minimizing time spent in particularly radioactive areas would probably be feasible today.",
"One major area of concern for human health would be small children and pregnant (or trying to be pregnant) women. Radiological effects are significantly riskier for these people since small radiation-induced DNA damage can have serious cumulative effects as fetuses and infants grow. I'm not sure how such a society would deal with these problems, but again it's not outside the realm of possibility. ",
"This all brings us to the second question, which is at what point is it worth it to move into and live in an irradiated area? Right now there isn't much. Most countries have plenty of open land and space so there is a strong disincentive to go into an area you perceive as being radioactive. Pripyat was originally built to service the Chernobyl power plant, and as a town it's been abandoned for 30 years. It doesn't really control or contribute any significant resources to the region, so there's not much point of going to live there.",
"A city like Denver or Baltimore would face similar problems. They both have their advantages in location, but there's nothing there that isn't replicated by other cities in the US. My guess is that most people would prefer to avoid an irradiated city, no matter how slight the risk, unless there was a strong incentive to go back.",
"Ask yourself what kinds of benefits are worth an estimated six months off your life, and then you'll have your answer as to when those cities might be reclaimed. "
] |
[
"Can hallucinations occur in a normal situation without outside influence such as drugs or mental illness?"
] |
[
false
] |
More specifically, can a normal person for example,who has no history of mental illness and who is not on drugs just taking a walk and have extremely vivid hallucinations while on that walk for no apparent reason?
|
[
"Nearly everyone hallucinates at some point in their life. It's quite common and most cases are not associated with drug intake or mental illness. And a variety of physical or mental stimuli and practices can reliably induce hallucinatory states in most individuals. Some kinds of hallucinations in very young children, for example, can be a normal part of development.",
"However, just because drugs or mental illness are not the cause of most hallucinations it does ",
" mean that they're medically irrelevant. Spontaneously hallucinating for no discernible reason is not unheard of, but rare compared to cases where a cause can be determined.",
"For a scientific overview of the matter I suggest Oliver Sacks book \"Hallucinations.\" He's a great writer and famed for his work in the treatment of a variety of neurological disorders (I think Robin Williams played his character in \"Awakenings.\") ",
"http://www.amazon.com/Hallucinations-Oliver-Sacks-ebook/dp/B0082XLY6G/ref=sr_1_1?s=digital-text&ie=UTF8&qid=1396330412&sr=1-1&keywords=hallucinations"
] |
[
"I'd like to add, extended sleep deprivation will invariably produce hallucinations unless you are taking antipsychotics.",
"Some perceptual anomalies will usually crop up after as little as 24h awake, and full blown hallucinations are pretty common by 72h."
] |
[
"Sensory deprivation chamber. It's completely dark, zero light, and also insulated from all outside sound. After a while, around an hour or so, your brain will be so starved of stimuli that it makes it up. Usually auditory, but stay in there long enough and the visual will start."
] |
[
"How do welding magnets with switches work?"
] |
[
false
] |
They aren't noticeably powered, as in they aren't electromagnets as they have very long lives without any noticeable batteries. They are seemingly just permanent magnets with a switch on them. Are there other magnets that cancel each other's fields? That's the only possibility I have come up with. Hope you can all help. Here is a link for people who don't know what I'm talking about, here's a link to some. their see how it works page was useless
|
[
"I've disassembled one of these before, but this isn't my field. Somebody else may be able to describe it better, but here is how you could make one:",
"Take a couple of pieces of iron and sandwich something non-magnetic in between. Drill a hole through the edges of the pieces and insert a permanent magnet. If the north-south poles are parallel to the iron pieces, that is, pointing towards the non-magnetic material then the magnetic field will be sort of trapped in the iron. If you rotate the magnet so the poles are pointing towards the iron, the field now moves along the outside as the iron becomes part of the magnet.",
"The switch just rotates the permanent magnet 90º within the device."
] |
[
"So a related question - Turning the \"switch\" in these magnets is effortless (not literally but very easy to turn) but the magnetic force generated when the switch is \"on\" is huge. How come the magnetic field doesn't resist your trying to turn the switch when rotating the internal magnet?"
] |
[
"Ohh, that's a neat trick! Reminds me of how when you take the buckyball magnets and make a cube or other solid shape with them, their magnetism towards other objects is almost nill, but the bond inside the structure is really strong. "
] |
[
"Why don't/can't astronomers use a method similar of that used to discover exo-planets to determine if there is or is not a ninth planet lurking around our solar system?"
] |
[
false
] |
After reading on the Washington Post about some theory of the existence of a ninth planet (planet x, nibiru) I have to wonder if it is still all a hoax or if there is enough credence to the theory to actually search for planet x. I know astronomers are able to determine the number of exo-planets around a distant star based upon the "jiggle" the see the star do - is a similar method not able to be performed on our own sun to determine the true number of orbiting bodies, or does it require a perspective that we are not presently capable of achieving?
|
[
"The acceleration of the star is inversely proportional to the square of the distance to the planet. If you have a large planet right next to the sun, it works great. If you have a tiny planet way out, it's not going to be very helpful."
] |
[
"Correct me if I'm wrong, but I believe that it is because the \"Jiggle\" is the light being blocked, reflected etc by the planet with that light then hitting earth. Since the supposed 9th planet is not between the sun and the earth the jiggle method wouldn't work. Take this with a grain of salt."
] |
[
"Also it's orbital period is believed to be on the order of 10,000-20,000 years. So even if we could detect the doppler jiggle in the sun, the frequency is so low that we'd need hundreds if not thousands of years of observations to even detect a change."
] |
[
"[Astronomy] Science in fantasy, Game of Thrones to be precise"
] |
[
false
] |
So some of you may or may not have read the fantastic series by George R. R. Martin. If you enjoy fantasy I definitely recommend it. One of the important characteristics of this fantasy world is that seasons last years (e.g a character asks another character who is a grown man "How many winters have you seen?" and the response is 4) and they're not always the same length. At the point in time which the books cover, the world is experiencing one of the longest summers it has ever known ( 9 years if I remember correctly ) and the "wise men" constantly say "a long summer means a long winter". Now, I recently learned that seasons are not based on the distance of the earth from the sun but the way the earth tilts. Our tilt seems to be in sync with our revolution around the sun. So my question is, are there planets whose axial tilt is not in sync with their rotation? Is Earth the exception in this case? Second, are there planets that have irregular "periods" of tilt? Is it actually possible to have seasons of vastly different lengths?
|
[
"Anything caused by orbital mechanics such as distance, axial tilt, or procession would be very regular and predictable.",
"The seasonal variations you describe would probably be better explained by a long period semi-regular or irregular variable star, that actually gets brighter and dimmer over time."
] |
[
"I assumed that the presence of \"winter\" had nothing to do with normal seasons or the planet tilt, but was more like ice ages. I'd figured the climate of the planet was natural unstable, and frequently oscillated between two extremes called \"summer\" and \"winter\". ",
"I guess we'll have to keep reading and see what happens when winter comes (I haven't finished the most recent book yet)."
] |
[
"As a fan of science and Martin's series, kudos to you for asking a very cool question! I've given a ",
"heads up to /r/gameofthrones",
", I'm sure they'll find this as fascinating as I do. Thumbs up for using fantasy to encourage interest in science!"
] |
[
"Does listening to audio books have the same beneficial effects akin to reading an actual book? Does it stimulate the brain the same way reading does?"
] |
[
false
] | null |
[
"First of all, you should stop thinking about things in terms of how they are 'stimulating the brain', because that is a vague and pseudosciencey qualifying criterion. It doesn't mean anything. If you're asking whether the two mediums, mainly listening vs reading, are equally effective for comprehension and recall, it appears they are similarly effective for most people (",
"1",
"). Obviously, there are important differences between the two modalities (",
"2",
"), but it seems that in the end the more important predictor of comprehension and recall is language proficiency of the listener/reader (",
"3",
"), given that processing of both auditory and visual language information are connected (",
"4",
"). All language comprehension (regardless of modality) is processed within ",
"Wernicke's area",
" of the brain."
] |
[
"I take some objection against the latter statement: \" All language comprehension (regardless of modality) is processed within Wernicke's area of the brain.\" There are many, many other areas involved in language processing. To mention just a few: the left and right IFG, the basal ganglia, and the right STS.",
"Back to OP. First, you've got to ask yourself what are the benefits of reading? For studying, the goal is comprehension of the text, but there could be other goals, such as relaxation or stimulating thought.",
"Second, it's not really a matter of \"brain\". It's much harder to skim an audiobook and to page back and forth, which is what many people do when studying. It's also easier to lose track of spoken word without noticing and there is no an easy way to find the last paragraph you actually understood. In that way, audiobooks are probably worse than printed or e-books."
] |
[
"It is definitely not correct to say that Wernicke's area is ",
" for language comprehension, as many other regions are involved in sensing and processing language (although Wernicke's area is ",
" for speech comprehension, see ",
"Wernicke's Aphasia",
"). Sorry if that was misleading. My point here was to say that the critical brain region involved in language comprehension is not directly tied to a language ",
" - it does not respond specifically to either written or spoken language but simply language in general.",
"Also, the IFG contains Broca's area, which is responsible for motor control of speech (but is not necessary for speech comprehension), see ",
"Broca's Aphasia",
"."
] |
[
"Discharging Capacitors as a weapon"
] |
[
false
] | null |
[
"Not sure what you mean, but if the idea is to charge in parallel and then discharge in series, that is pretty easy to do with spark gaps. It is called 'Marx generator'",
"http://en.wikipedia.org/wiki/Marx_generator",
"You can also look for it on youtube. "
] |
[
"Just a note here. This is pretty fucking dangerous. Don't hurt yourself.\nEdit: I don't necessarily discourage this though, I think Marx Generators are pretty cool. Just be aware of how dangerous it is. I think this was posted on reddit a while back (",
"http://lucidscience.com/gal-rock%20disaggregator-1.aspx",
"). It is a HUGE Marx generator that some guy built in his basement to blow rocks apart to get at small emeralds that would otherwise be lost with conventional processing."
] |
[
"you can pretty much do this by attaching wires to the solder points for the flash on a disposable camera. It is a pretty nasty shock that could potentially kill someone. So i would advise against doing it."
] |
[
"How do archaeologists ensure that they do not damage any surrounding fossils when digging?"
] |
[
false
] |
I have always wondered what sort of preparation and processes archaeologists perform to ensure that the fossil they had initially discovered and intended to excavate did not damage any fossils that are potentially surrounding it. Does anyone have any experience in that area?
|
[
"When you mention fossils, you are actually talking about Paleontology, however I think the method is probably the same. I am not an official archaeologist, but when I was in high school I volunteered for The Archaeological Society in Oklahoma. My Mom was a cartographer for The Archaeological Survey, so I was always invited to come along on digs. Here's what we did. First they would do a Survey in which a group, or several groups, of people would walk together in a line through a territory, looking for signs of interest, such as arrowheads,clay pot fragments, bone tools, etc. Once a spot was tagged for excavation, they would plot a grid with strings and little wooden stakes in the area. I don't recall the size of the squares, but it was around 3ish feet for each square plot in the grid. Everyone was supplied with a standard kit of paint brushes in various sizes, gardening trowels (small ones), line levels, and a few smaller sculpting like tools. After removing the top layer of grass carefully, the surface of each grid was smoothed and perfectly leveled. That would start us at level 1. Very carefully with the paintbrush and trowel (and most often knee pads) we would slowly remove dirt bit by bit, kinda like slicing just the very top of the ground, with the trowels, and brushing the little bits into dustpans, and then dump them into buckets. We would do this until we were down an exact depth, such as 4cm down from level 1, and were precise about it, which is why we all had line levels. This would go on for however many levels would could complete with the a lotted time for the grant. Now the buckets of dirty were bagged and labeled for both the square it was found in the grid, and at what level depth it was from, and sent to the lab for further testing. I did a bit of this tedious work and found tiny things like baby sea shells, and bits of animal and flint scrapings from tool making. It was a really great experience, and I am sorry I don't do it anymore. So to answer your question, we used mostly paint brushes and lots and lots and lots of patience with trowels to make sure nothing was damaged. To pass the time, we once played archaeology bingo. You got to mark certain artifacts off a bingo card and the winner one a coveted \"golden trowel.\" It was a lot of fun really. \nFeel free to ask me anything else about it, I'm suppose to be studying right now so I'm kinda blazing through this lengthy description."
] |
[
"Archaeology is the study of humanity via remains. Excavating fossils can still be considered archaeology. Just a little nit-pick..."
] |
[
"It's very, very tricky sometimes. The reality is that unless the site has some remarkable preservation or some sort of intact structure, most ceramics won't be intact. The weight of the soil over hundreds or thousands of years pretty much crushes them. That's when you get your ceramics experts to piece them back together.",
"How do they keep from destroying the tomb lawn ornaments and whatnot, the location of which is unknown prior to finding? Or is there a museum somewhere dedicated to all of the statues and pots that have fallen in archeological pursuits?",
"In short, archaeologists can't always keep from destroying artifacts. It happens, sometimes, and if it does, they may or may not be reconstructed in a lab. It depends on where it's going to end up. If it's an amazingly decorated piece that is set to become a display, then it'll be fixed up. Otherwise, it will likely just be bagged and tagged and put in a bin in an archaeology lab at a university.",
"That said, there are many techniques that can be used to get a general idea of what might be going on under the surface. Archaeologists generally don't just rent a backhoe and dig deep into a random spot. Now, dig sites are wildly different. I've been part of teams in Pompeii (with ",
"this",
" group), Alaska, and across the American Midwest, and each dig came with its own challenges. ",
"Digging in a place like Pompeii is particularly challenging, as you generally have to move very slowly, or risk completely obliterating evidence. In some spots, we uncovered a new floor surface every centimeter or so. Each of these floor surfaces told of a new event in the life of the building we were investigating, so we really had to go out of our way to carefully uncover everything. ",
"In Alaska (and the Midwest, to an extent), we were working at sites from the late Pleistocene to early Holocene, which meant that we didn't even really begin to find meaningful artifacts until about a meter down into the soil. In sites like this, many archaeologists just take a meter square and drop it down some arbitrary amount (in centimeters, usually) and plot everything they find on a map. The artifacts you'd find in this type of site are usually lithics (stone), and aren't really as susceptible to damage as ceramics. However, in these sites, any burnt organic material can be incredibly important, and it's really easy to destroy.",
"Now, backhoes are actually great archaeological tools when you have a really, really experienced operator. I was doing some CRM (",
"Cultural Resource Management",
") work in Indiana and Ohio that required the use of backhoes, and it was pretty awesome. Most of the sites we were working with were probably about two feet below the surface (just working on memory, here), so we needed to quickly get through that two feet over huge areas, which just isn't really doable with small teams in any meaningful timeframe. We'd get these backhoe guys in there that could scrape that two feet away and have it perfectly level in about a half hour. More than that, if you asked them to take it down another centimeter, they could do it. I guess some of these guys compete in backhoe challenges that involve picking up and transporting eggs and such. So, backhoes aren't always super destructive.",
"I guess I'll stop there to avoid wall-of-text.",
"tl;dr archaeologists try to be really, really careful but they break stuff often. "
] |
[
"[Mathematics]Can anyone explain the Riemann Hypothesis in the simplest way possible?"
] |
[
false
] | null |
[
"At what points does the function y= x",
" - 9 equal zero? The answer is x=+3 and x=-3. What about y=x",
" + 9? There is no real answer, but the imaginary numbers 3i and -3i solve this, where i",
" =-1.",
"The Riemann zeta function is a much more complicated function, where Zeta(x)=1/1",
" + 1/2",
" + 1/3",
" etc. Zeta(2) is pi",
" /6, for example. Generally though, x can be a complex number (real and imaginary parts) and its output Zeta(x) can also be complex. There is a different form of it (an analytic continuation), if the real part of x is less than one.",
"Anyway, for negative even integers -2, -4, etc, Zeta(x) is zero. There are also complex numbers for which Zeta is zero, for example Zeta(0.5+i*14.134...) is zero.",
"The Riemann hypothesis says that besides the negative even integers, every value for which Zeta(x) is zero has a real part of 1/2."
] |
[
"It's not a simple theorem to properly interpret. If you know about complex numbers, you can understand the statement, but it will be meaningless. Understanding of the ideas behind it and why it is important is a little more subtle.",
"I don't have the time to write up something at the moment, but check out ",
"This Blog Post",
" which walks you through the reasoning, interpretations and main ideas of the Riemann Hypothesis with relatively few prerequisites."
] |
[
"The statement of the Riemann Hypothesis is useless without discussing [...].",
"I disagree, it is interesting and satisfying to gain an initial concept of the statement, even without delving deeper into the context. "
] |
[
"How small would a planetary body need to be in order for the curvature to be drastically obvious to a human on the surface?"
] |
[
false
] |
I realize an exact answer cannot be given since everyone will have a different definition for "drastically obvious", but I'm just looking for a general range. For instance, we can see evidence of Earth's curvature when ships sail far out to sea and disappear below the horizon. Did humans on the moon (or perhaps Mars) notice that the body on which they were standing was significantly smaller than Earth from where they stood, or was the moon so big that to the naked eye on the surface, the difference was negligible?
|
[
"Your question has a slight problem: what is \"drastically obvious\"? This very subjective.",
"To answer your question: it depends. The curvature of a planetary body will be discernible when the angle where you see the horizon exceeds a certain angle. When you are very near to the surface (near to the surface in this context means with respect to the diameter. For earth this could mean up to several hundred meters, for a pool ball this could mean less than a mm) of a sphere, the horizontal will be straight ahead (angle= 0°). When you rise above the surface, the horizon will in turn go lower. This will show you the curve. When you start seeing this and when you start to think it is obvious heavily depends on your personal sensitivity.",
"On earth you can see the curvature from high-flying planes.",
"If you take a certain threshold (say 5° dip of the horizon makes the curvature visible) then your vantage point needs to be at least at 1/130th of the diameter of the sphere or planetary body. ",
"If you want to see it while standing on the surface (your eyes are at roughly 2m ;) this would correspond to a body of roughly 1km in diameter (by comparison, earths diameter is almost 13.000 km)",
"Edit: Here are some formulae if you want to do the math on horizon dip yourself (",
"http://mintaka.sdsu.edu/GF/explain/atmos_refr/dip.html",
"). I did the calculations not taking into account refraction.",
"edit: corrected earth diameter, thanks to t52nd"
] |
[
", vs . depending where you live and what language is used by the software is a constant source of annoyance. I should just go with spaces...."
] |
[
"Let me rephrase it for you: Only bodies larger than approx. 400km tend to automatically become spherical."
] |
[
"When my candle is lit, why does dust caught in the wax gather around the base of the wick?"
] |
[
false
] |
I have a candle that has three wicks in it and is seated in a circular tin. I've used the candle enough such that all of the wax is liquid when it's lit. I lost the lid for the candle so it just sits on my desk collecting dust. I try to wipe off the dust before I light it but obviously I can't get everything. Anyways, last night after I had been using the candle for an hour (and all the wax was liquid) I noticed that the remaining dust still caught in the wax had accumulated beneath each of the three wicks. Why is that? *EDIT TO ADD ANSWER: When the candle is burning, the capillary forces in the wick are drawing in the molten wax. So the dust gets carried there by the flow. (Thanks Platypuskeeper and everybody else who answered)
|
[
"When the candle is burning, the capillary forces in the wick are drawing in the molten wax. So the dust gets carried there by the flow. That's my guess at least, I haven't researched it."
] |
[
"The wick is pulling up wax, the dust is caught in the flow"
] |
[
"This is exactly what happens. In fact, under the right conditions, you can actually see a toroidal convection current around the wick, circulating pretty fast (several times a second). I don't know exactly what's required to make it visible but I've seen it happen with soot particles."
] |
[
"Rehabilitated forests that see an increase in wildlife. Where do they come from?"
] |
[
false
] |
Just read an article about a forest planting project and after 20 years various mammals, reptiles and land based creatures returned. Where did they come from? How did they get there again after they were driven out previously?
|
[
"When a man deer and a mommy deer love each other very much...",
"Tongue in cheek, but true. The vast majority of repopulating takes place at the, erm, extremely local level. It takes a bit of time. ",
"Very rarely are populations completely destroyed in any one area. If that occurs - like, say, with beavers in parts of the US - the options are 1. wait for beaver populations elsewhere to spill into your newly habitable area 2. artificially introduce them. ",
"A bit off of the original spirit of the post, but let's say you have completely virgin land. Let's say primeval Hawaii, newly spewed up from beneath the earth. Over time, wind, waves, and fauna will populate the island. Will blow seeds and allow birds to colonise. These birds will themselves deposit seeds in their droppings and perhaps carry fish and invertebrate eggs on their feet or in their down. Waves will bring coconuts to shore, and periodic storms/flood will handle some additional dispersal."
] |
[
"The answer to this would be pretty context specific. The range, dispersal, and habitat requirements could all impact what animals could reestablish in a restored forest. The first step, however, is making sure that these animals have what they need (ie. creating a suitable ecosystem through ecological restoration). There needs to be good abiotic conditions for the plants. There needs to be the right kind of plants for the invertebrates. There needs to be enough invertebrates for the insectivorous tetrapods to eat. Then, usually chance leads recolonization. ",
"In a more broad sense, animals disperse into previously uncolonized land through various mechanisms. The classic examples revolve around how animals get to islands that just formed. It might be interesting to look into Island Biogeography Theory which describes the relative amount of species that will colonize an \"island\" (which could mean a lot of things besides land surrounded by water) and how fast that island will achieve its own equilibrium species richness."
] |
[
"Thanx!"
] |
[
"Why is NASA's James Webb telescope built out of hexagonal panels for its mirror? Why is the mirror not a curved surface, and why were hexagons chosen over triangles or octagons etc.? Why does the Hubble telescope (seem to) not require this geometry?"
] |
[
false
] | null |
[
"The mirrors are much larger than the hubbles and we didn't have the manufacturing technology to make them in one piece. The hexagons were chosen as they are the simplest shape you can make, requiring the least amount of precise cuts that will not create a continuous line while retaining symmetry in two axises.",
"If they used a square you'd have noticeable lines in the image being produced, if they used any odd sided shape you would have an image that is brighter or darker on one of it's four sides. The hexagon is the simplest shape that does not create optical flaws."
] |
[
"Sometimes it's a working title and it gets its cool official name when it comes out. Sometimes it's just a big international collaboration and it's good to choose an obvious and simple name rather than dealing with the politics of naming it after someone or something. It's also nice to have a somewhat descriptive name so that it's a bit less cryptic."
] |
[
"We also use this for large Earth-based telescopes as well. You can check out the list of biggest optical telescopes ",
"here",
" - the top five are all \"segmented\", which means they're made of a number of mirrors tiled together like this. Wikipedia has an illustration of these big segmented mirrors ",
"here",
", which shows that the primary mirror of the EELT is planned to have almost 800 segments!"
] |
[
"Is there a second complex plane?"
] |
[
false
] |
Does there exist another "complex" plane? Two analogies to explain what I mean: If the real numbers can be seen as the "x" axis, then imaginary numbers can be seen as the "y". Does there exist a number system which also introduces a "z" axis? The question "what is the square root of -1?" is a question about a real number that can only be answered by use of complex numbers. Does there exist a question about complex numbers that can only be answered by the use of a second complex plane?
|
[
"If the real numbers can be seen as the \"x\" axis, then the complex plane can be seen as the \"y\". Does there exist a \"z\"?",
"The complex plane is not the \"y\", the complex plane is all the x+iy, for x,y real. The \"y\" is the imaginary axis, the complex numbers that are purely imaginary, no x in them. We also denote x+iy generally by \"z\", so yes there's a z, but this isn't the one you're talk about.",
"Does there exist a question about complex numbers that can only be answered by the use of a second complex plane?",
"No. This is what the Fundamental Theorem of Algebra says. The \"question\" that the complex numbers answer is \"Can we find a root to the polynomial p(x)=x",
"+1?\". The answer is \"no\", unless you introduce the complex numbers, when it becomes \"yes\". The Fundamental Theorem of Algebra says that if p(z) is ",
" polynomials, then the answer to \"Can we find a root to the polynomial p(z) in the complex plane?\" is always \"yes\". So the complex plane is as big as we need to get.",
"Generally, polynomials and number systems go hand-in-hand. If you have any number system (eg the rationals or something), then if you find a polynomial with no root, then you can get a new number system by inventing a root for it. For instance, p(x)=x",
"-2 has no roots in the rationals, but if we include sqrt(2) into the rationals, just like we include sqrt(-1) into the reals, then we can get the number system of all numbers of the form x+sqrt(2)y, where x and y are ",
". In general, all number systems are created in this way. For something like the rationals, this goes up and up and up, you can find a number system of any dimension over the rationals. For the reals, you can only make the complex numbers.",
"Well, there is another way to create number systems. If you take the rational numbers, and give them a \"meaningful geometry\", then this geometry will have holes and filling them in gives a new number system. For instance, if you arrange the rationals onto a line, then filling in the holes gives the real numbers. The only other \"meaningful geometries\" you can give the rationals are much more abstract than a line, and each corresponds to a prime number, and so for each prime number p, you can arrange the rational numbers according to this prime, fill in the holes and get a new number system that is a cousin of the reals called the \"p-adic numbers\". Just like you can make the complex numbers from the reals, you can extend the p-adics in their own ways too. But you can go up as high as you want for the p-adics, you don't have to stop at 2-dimensions. But, for every p-adic, there is a gigantic, huge number system that has no holes, and contains all the other number systems you could make from the p-adics, and this is called the \"p-adic Complex Numbers\". This is much, much, much larger than the p-adics. But the funny thing is that even though the p-adic Complex Numbers are obtained in a way unrelated to the ordinary complex numbers, it turns out that, as number systems, ever set of p-adic Complex Numbers is identical to the original complex numbers. They are arithmetically the same, but their geometries are wildly different. Not just a plane.",
"So, polynomials and filling in holes are really the only ways to make number systems, and both of them say that the complex numbers are the top.",
"You can make different arithmetic systems more ad hoc-ly though. For instance, you could, for shits and giggles, include a couple extra numbers into the complex plane that work like i, but are different.. These are the ",
"Quaternions",
" and they are an arithmetic system that is more closely related to geometry than numbers, so they come from geometric questions and not number questions, and so it is not a \"number system\". But the Quaternions do live \"above\" the complex numbers and are a 4D arithmetic object. But the Quaternions are actually as high as you can go, if you want to keep associativity, and you can go one step higher to ",
"Octonians",
" if you don't mind losing that. Generally, arithmetic objects over the reals are highly restricted."
] |
[
"I think what you're getting at is Quaternions. The bane of computer graphics artists and video game developers everywhere:",
"https://en.wikipedia.org/wiki/Quaternion"
] |
[
"Worth noting quaternions are 4-D. There's no 3-D system in between complex and quaternions either."
] |
[
"Can a non-defective calculator randomly make an isolated mistake?"
] |
[
false
] |
As a kid, I used to do large calculations on handheld calculators repeatedly, hoping to catch them making a mistake. Now, as a college student, I feel a compulsion to do every calculation on my exams twice in case the calculator just messes up. Is there any rational basis to the idea that a non-defective calculator can make one-off, non-replicable calculation mistakes by chance? What about supercomputers running trillions of calculations? And if so, what is the expected frequency of such mistakes?
|
[
"These are called ",
"soft errors / soft faults",
" and they absolutely do happen. For a single calculator the chances of actually seeing one are astronomically low, they only really crop up in large systems like supercomputers and large network deployments. There's a ",
"great Defcon talk",
" about exploiting this kind of error in web infrastructure caused by random bitflips in memory made worse by the hot environment they run in so it's not some academic issue without real-world implications.",
"By virtue of their size supercomputers see these events many times a day:",
"When [LLNL] researchers analyzed BlueGene/L, the Laboratory’s 108,000-node supercomputer, they found that one data-cache bit flip occurred \nevery four hours.",
" (pdf warning)",
"and advanced techniques are used to detect this kind of event and fix it transparently.",
"Servers use a special type of RAM named ",
"\"ECC memory\"",
" (error checking and correction) to detect single-bit changes in RAM. Left unchecked these could corrupt vast amounts of data without ever being noticed until the server just crashes without apparent cause."
] |
[
"One of the most interesting bits of info in that wikipedia article, in my opinion, is that cosmic rays are a main source of soft errors."
] |
[
"At least in aerospace, these are called ",
"single event upsets",
" and need to be accounted for when you are determining the reliability of your design.",
"You are to assume that during every flight your CPU (or FPGA/CPLD) will be bombarded by one highly energetic particle that will flip a bit, and you have to make sure that there are no bits whose random flipping will cause a catastrophic failure."
] |
[
"Is such a thing as \"the smallest particle\" possible?"
] |
[
false
] | null |
[
"Leptions are just a name for the family of electrons and neutrinos and their relatives (muons, mu neutrinos, tau, and their antiparticles) - things that interact by the weak force, but not the strong force. As far as we know, electrons are as small as anything can be: they have zero \"volume\". What that means fundamentally maybe isn't so clear. It's quite possible that the quarks are the same way, and we've reached the limit of sub-divisible things. Why that would be occur where it does, though, would probably require that we answer the question What Is Everything, Anyway? Which we haven't done."
] |
[
"Here is a great answer :",
"http://www.quora.com/Will-physics-ever-find-anything-smaller-than-the-quark",
"?"
] |
[
"M theory (modern string theory) posits that everything is made of tiny vibrating strings of energy far far smaller than the smallest particle. ",
"Note that string theory is not yet proven",
"http://en.wikipedia.org/wiki/M-theory"
] |
[
"If the top of every man made structure on Earth were painted white, would this effectively raise the Earth's albedo enough to impact weather or climate?"
] |
[
false
] |
topic Also consider shiny or mirrored surfaces as potential alternatives to white paint please.
|
[
"If 80 percent of the roofs in urban areas in the tropical and temperate climate zones were painted white (or some other \"cool\" color, which reflects non-visible light), it would offset 24 billion metric tons worth of carbon dioxide missions. This is the equivalent of our 300 million cars, or of 500 medium-sized coal power plants.",
"-",
"Source"
] |
[
"Very relevant, thank you. It doesn't quite provide me enough information from which to calculate the answer to my question (including 100% of all building tops in all zones) but it does help me roughly estimate a guess. Nice sourcing, too.",
"Also, I don't know how to convert billions of metric tons of CO2 into either ppm (a la 350.org style thinking) or into average temperature changes (relative to the expected temperatures as far as we know it now)."
] |
[
"Except OP was asking about albedo, and that article only talks about reduced cooling bills.",
"I suspect the answer to OPs question is, \"no, there's far too little surface area covered by buildings, and Earth's albedo already varies quite a bit based on cloud cover and snow cover alone,\" but I'd also guess there aren't many people in the world with all of the specific knowledge required to answer this question. Namely, how much of the Earth is covered by structures?",
"Edit: spelling"
] |
[
"How would they triangulate the pinger on the black boxes of the Malasian flight?"
] |
[
false
] |
[deleted]
|
[
"Okay, I probably have some real insight in this because I had a conversation over lunch with one of the researchers trying to use acoustics to triangulate the sound. We were discussing methods for this exact problem.",
"Here's the essential idea, if I have one sensor by itself, I can try to figure out the distance using the level shift. Basically, the further away the sound, the quieter the sound I receive. This gives you a circle of some radius that the source could be at.",
"Unfortunately, there's a lot more going on than distance between the black box and your hydrophone. There are eddy currents, sound speed variations, fish, currents, etc. All of these will have some impact on the level you receive. This means that instead of a narrow circle along which the source could lie, you have a big fat ring, where the source could be anywhere inside of it.",
"If, however, you have multiple receivers, you can use the phase of the sound at each hydrophone to get an angle. In this respect, any two hydrophones can give you an angle to the source (basically a line that stretches out from the pair towards the source). However, because of the same reasons outline above, this straight line becomes a small arc, basically a triangle extending out from between the two receivers.",
"To find the source's exact location, you need multiple sensors. You need to either make multiple rings using the level or (more commonly) multiple arcs using pairs of sensors. It then becomes a process of elimination. I know my source is within the arc of pair one and the arc of pair two, so I only need to look in the place where those arcs intersect. The more pairs of sensors I have, the fewer places that intersect, and the better my estimation becomes.",
"I hope this answers your question, but if you have anything else you're wondering about, please just ask!"
] |
[
"Here's the thing, they aren't looking for MH370 using the sound of the crash. They are using the sound from the blackbox to locate it, which is a completely different problem.",
"The K-129 event is a loud explosion, which is very impulsive and has a really good signal-to-noise ratio and distinct arrival time. MH370's black box is putting out a constant frequency signal (or at least CW pulses), which will be considerably weaker than the explosion. That means that highly directional multibeam sonar systems are the best chance to pick the signal out of the noise, and the result will have an ambiguous arrival time but a distinct direction of arrival.",
"As for why they don't use the crash on the surface, the reason is simply that sound that originates deep in the water column carries further because it's trapped in the deep sound channel.",
"Ultimately: different problem, different solution."
] |
[
"The use of hydrophones is a tried and true technique. The start of ",
"Project Azorian",
" was largely based on sound data taken from 5 hydrophone stations.",
"This figure",
" shows how the data from 3 stations could be used to pinpoint the location.",
"The difference between the K-129 events and MH370 are largely what happened. It was assumed that K-129 imploded at depth, which produced a single, loud, independent sound (",
"Here's an example",
"). It's also a sound which technicians would have been familiar with and thus had a good idea what to search for.",
"Without knowing what actually happened to MH370, it's hard to tell what to check for."
] |
[
"Space expands more far from large masses than close to it, so how was there expansion during the early period of the universe?"
] |
[
false
] |
During the early inflationary period, space was pretty dense, but it was expanding incredibly fast compared to today. How could that happen, given the density at that time? It seems like if mass slows expansion, that period should have had enormously slowed expansion. Was the pressure of the contents of the universe high enough to override the effect gravity had?
|
[
"I provided a source. You could click on the link and then follow their sources."
] |
[
"I'm not sure if that's true. Anyway, the expansion of space is so small that the expansion within a galaxy is much too tiny to see.",
"Why doesn't the Solar System expand if the whole Universe is expanding?\nThis question is best answered in the coordinate system where the galaxies change their positions. The galaxies are receding from us because they started out receding from us, and the force of gravity just causes an acceleration that causes them to slow down, or speed up in the case of an accelerating expansion. Planets are going around the Sun in fixed size orbits because they are bound to the Sun. Everything is just moving under the influence of Newton's laws (with very slight modifications due to relativity). [Illustration] For the technically minded, Cooperstock et al. computes that the influence of the cosmological expansion on the Earth's orbit around the Sun amounts to a growth by only one part in a septillion over the age of the Solar System. This effect is caused by the cosmological background density within the Solar System going down as the Universe expands, which may or may not happen depending on the nature of the dark matter. The mass loss of the Sun due to its luminosity and the Solar wind leads to a much larger [but still tiny] growth of the Earth's orbit which has nothing to do with the expansion of the Universe. Even on the much larger (million light year) scale of clusters of galaxies, the effect of the expansion of the Universe is 10 million times smaller than the gravitational binding of the cluster.\nAlso see the Relativity FAQ answer to this question.",
"UCLA",
", and actually, I'm applying to their astronomy graduate program for next year. "
] |
[
"There is one thing I'm still not clear on regarding the issue of expansion, and perhaps it's because I've either been misinformed or simply not been informed enough. So, if any of this incorrect, please let me know...",
"Objects moving away faster than the speed of light do not violate relativity because the intervening space is expanding. This implies that space is somehow carrying matter with it as it expands. For space to be capable of carrying matter, it seems that space and matter would have to resist motion through each other. Otherwise, why wouldn't the space just expand through the matter without moving it? ",
"So, do space and time resist motion through each other in some way, or are the common (non-academic) explanations for the expanding universe flawed?"
] |
[
"What is a phase 3 clinical trial when we talk about vaccines?"
] |
[
false
] |
My understanding of the phases of clinical trials goes like this; Phase 1: test safety in small number of healthy people. Phase 2: test safety in small number of sick people and obtain early data whether it works. Phase 3: test efficacy in large number of sick people. Phase 4: use new treatment in the clinic and study it. My question is this. Vaccines are given to healthy people to prevent the sickness. So who gets studied in Phase 3? Do you just wait to see how many of those people end up sick after they get the vaccine?
|
[
"Ooh, I love this question. That is a great summary that you have there.",
"Just for clarity and completeness, the goals of a phase 3 clinical trial are the following:",
"Source: ",
"https://www.cdc.gov/vaccines/parents/infographics/journey-of-child-vaccine.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fvaccines%2Fparents%2Finfographics%2Fjourney-of-child-vaccine-text.html",
"To answer your question, of \"who receives the vaccine in a Phase 3\" stage, these participants will typically consist of healthy volunteers for vaccines (for other types of therapies, such as for cancer, they will consist of cancer patients). Generally, volunteers will be split into 2 groups (vaccine or placebo) and then will be monitored in an observational study (participants will be observed and followed up) over a long-period of time.",
"Since we have passed two safety phases, we can now administer this vaccine to a much larger group of volunteers. Why is it important to give to a larger group of volunteers? Well, we cannot infect the participants, that would be unethical! Instead, we have to wait to see who becomes sick and who remains healthy (hence, observational study), ",
" if the disease you vaccinated against is uncommon/rare, then you will need many people to even see an effect. For example, if the virus has an incidence of infecting 1 person out of a 100, then you would want at least several thousand participants to make sure you see enough infections and determine whether your treatment/vaccine even has a significant effect. This is why clinical trials are very costly because of the efforts and personnel involved to monitor these trials.",
"If you want an example in detail, I linked below the abstract to a phase III clinical trial for a vaccine against rotavirus, where they carry out an observation study.",
"Link: ",
"https://pubmed.ncbi.nlm.nih.gov/18395579/"
] |
[
"Thanks for the answer!"
] |
[
"As a research biologist finishing up his PhD in the drug discovery field, particularly looking at potential medical utility of snake venom proteins I isolate from extractions, I feel like there are some important things to be said about clinical trials that makes them so tricky and time consuming (whether for vaccines... or seeing whether a venom component significantly shrinks tumors). Specifically, the constraints of working with humans. Clinical trials are never going to be true experiments in terms of experimental design because we can’t take a random sample of the population, people have to volunteer. And then, with vaccines, we can’t purposefully expose people (the reason we can’t do this is especially clear when you think about the placebo group being purposefully exposed to a potentially lethal virus that they were never even vaccinated for). These constrictions make clinical trials very different from animal trials. And can be a potential reason that drugs fail in human trials after succeeding well even in primate studies. But a lot also fail because humans are different. ",
"Placebo effect is also very important as is double blinding (trying to make sure even the doctors administering the drug don’t know who is placebo and who isn’t). But some drugs smell... some vaccines hurt more when they are injected... and sometimes it’s just impossible to keep researchers blind. Humans also lie, sometimes to seem more manly, to seem better at withstanding pain, and sometimes for no reason we will ever understand. So reporting of symptoms verbally (like headache) can be less useful than observing variables controlled by the autonomic nervous system that (most people) can’t effect too much consciously, like HR, BP, vomiting, diarrhea... stuff like that. Placebo effect is often statistically significant for all sorts of things, including autonomic variables, even things like immune response levels because our state of mind does effect our stress levels for instance, and cortisol, releases when we are stressed, is an immunosuppressive. So if someone thinks they received a vaccine, they may relax (especially if they were so scared of the virus they volunteered for a clinical trial) and this may cause a drop in cortisol and present as what seems like an elevated immune response caused by a vaccine when it is actually pure placebo effect. This is why things like pain medications (which can really only be measured by self reporting and a few factors like bp and hr... but some people literally have opposite physical reactions to pain... so a promising pain drug may only be a few percent different than placebo in terms of total pain reduction.",
"Luckily, for vaccines, we have a physiological product that the brain can’t trick us into making following placebo administration, and is only going to be present if we have been exposed to a virus or a vaccine that works... and that’s antibodies specific for our virus. So clinical trials for vaccines are a little easier in one respect than clinical trials for other drug types. But harder to gauge in others. First off, people want a vaccine and they want it now, the only way to know if there are long term side effects or if the level of antibodies produced by the vaccine is only maintained at a level that provides a more rapid immune response for a month... or 11 is by watching the people in that trial for as long as we can. We won’t have any real idea if there are 10 year side effects of this vaccine until 10 years from now... nor whether it needs a booster every 6 months, nor whether boosters are actually dangerous or could cause a reaction not seen in original exposure until we do that. This is why it takes so long to get something approved, and also why it’s never 100% risk free even after approval, because everyone’s physiology is different and there’s a chance one millionth of the population has a violently adverse reaction, which would never be picked up in trials unless we had a one in a million person in the trial. And that would actually potentially be unfortunate because it could make a useful drug fail just because the trial was “unlucky” statistically... but also explains how drugs that killed someone in a clinical trial might still be approved, if the money, time, and need for a treatment where a 1 in X adverse reaction is tolerable. ",
"So, for everyone following the progress of vaccines and drug trials for the first time, don’t lose faith when something weird happens, if there is a long list of potential side effects, and don’t expect to ever be able to be told that there is absolutely no chance that there are negative long term effects to the vaccine. We know there are very negative short term consequences of this virus, and there are going to be many people with long term negative effects from the disease. The vaccine trials aren’t meant to make sure its “Absolutely safe for everyone forever” that’s impossible, and that’s never, ever, been a reasonable conclusion for any drug... hell, we can’t say that about water.",
"What the vaccine trials will show, is whether or not there is strong evidence that this will make it less likely for you to have a severe case of Covid-19 if you are exposed. We know how nasty this disease is and it’s been around 6 months. It would be weird if a vaccine was cleared without 6 months of human tests. And for the record... adverse effects 10 years after any drug or vaccine is just about unheard of, so just because we don’t have evidence for or against something definitely doesn’t mean 50/50 chances... it just means we have nothing to base it on. We can base it on similar vaccines that have been around for 10 years, but that’s not really evidence of this vaccine. ",
"TLDR: weird results that seem to conflict with logic happen all the time in clinical trials, trust experts who have been conducting and reviewing clinical trials for a lot longer than anyone just taking an interest this year because many can be explained logically... and many will remain mysteries, but in the end outliers (and I can already feel it... the media is going to sensationalize outliers, just as they are for covid-19 patients... that article about men with covid having testicular pain... is not that weird for those with high fevers and a rough cough, and the fact that ACE2 enzyme is present in the testicles is the last thing people should be super worried about... but if it gets you to wear a mask... then yes... covid eats your balls but it took months to notice) are outliers and no one should ever be surprised that science isn’t about proof or guarantees, it’s about what the evidence shows and what we are best able to conclude from it at the present time, which will never include a guaranteed prediction of 10-20-30 year safety. But we do have lots of evidence that covid-19 infection without an immune system boosted by a vaccine has lots and lots of potential health concerns both immediate and chronic."
] |
[
"How did different neurons evolve?"
] |
[
false
] |
I understand the basic properties of sensory, motor and interneurons. But how did their different shapes/complexity/function evolve and in what order? I've found a lot of info on the general evolution of the brain (reptilian>limbic>neocortex) but not much on the neurons themselves.
|
[
"i don't know about other types of neurons, but photoreceptors are very well studied in terms of their phylogeny - e.g. look at ",
"this illustration",
". (let me know if that's behind a paywall, I don't think it should be).",
"photoreceptor phylogeny is studied (generally) by looking at the genes that are used to produce photopigments - since this is the job of a photoreceptor, tracing these genes is basically tracing photoreceptor evolution. I bet there are, similarly, studies of how neurons in different species build their ion channels, or different types of receptors or neurotransmitters that they emit. i don't know anything about that area, though (i'm a vision guy)..."
] |
[
"How is not really the right question because we can't observe the pressures or chance events that brought them into play. We may be able to infer relatedness of groups of cells based on the tissue they develop from though, and the relatedness of groups of organisms (which are what evolve). Let me posit your question a bit better: where in our ancestral lineage of eukaryotes did different types of neurons appear? Of course different neurons arise from the same tissue developmentally (neural crest for example), but to adress the question about when glutamatergic vs gaba or glycinergic cells came about, it's about what organisms had mutations in neural development genes to confer such patterning differences and can we see if extant organisms have that ancestral state of fewer neuron types."
] |
[
"the function of the neurons themselves usually determine the shape and complexity, muscles in jellyfish were likely the origin of complex nerve nets, but it is likely that other sensory cells developed concurrently as stimuli for the muscle cells"
] |
[
"Why doesn't the distribution of elements in the universe follow the progression of the periodic table?"
] |
[
false
] |
Hydrogen and helium are the two most common elements in the universe, followed by... oxygen? I expected the next element to be lithium, followed by beryllium, boron, and so on. Lithium doesn't even make the top ten list. Why? If stars are fusing nuclei in their cores to form new elements, why isn't it a simple linear distribution by number of protons added?
|
[
"Let's start at (or near) the beginning. Shortly after the Big Bang, the universe was a quark-gluon plasma which then condensed into protons and neutrons (both of which are composed of 3 quarks). For a short time, the universe was still hot and dense enough for some fusion to occur, so about 24% of the mass got turned into helium, and a tiny amount was turned into lithium. These are what we call \"primordial abundances\". Heavier elements up through iron are created in stellar cores, and elements heavier than iron are created in supernovae.",
"Now, I'm not sure how familiar you are with nuclear physics, but when protons and neutrons (collectively known as nucleons) are bound together by the strong nuclear force, a portion of their mass is converted into binding energy. Different nuclei have a different ",
". For example, if you take two deuterium nuclei, they each have pretty low binding energy per nucleon. If you fuse them together into a helium nucleus, you can get energy out of the reaction. This is basically how stars are powered, although the actual reaction chains have more steps: the ",
"proton-proton chain",
" and the Carbon-Nitrogen-Oxygen or ",
"CNO cycle",
". ",
"You're generally right that elements with fewer protons should be easier to form, but there is another process at work. As you can see on the linked image, certain nuclei have higher binding energies per nucleon than others, and nuclear reactions \"prefer\" nuclei with higher binding energy per nucleon. Oxygen happens to have a higher binding energy per nucleon than any smaller nuclei do, which means it is easier for stellar cores to produce in large quantities.",
"Lithium, as it turns out, can easily be fused into heavier nuclei (it has very low binding energy per nucleon), so stellar cores will tend to fuse whatever lithium they have available. This is why it is not very abundant-- the nuclear furnaces which create it then turn around and eat their babies for fuel :)"
] |
[
"Great answer. Thank you."
] |
[
"I can't explain this in incredible detail, but to give you a preliminary answer:",
"Most of the matter in the universe is either heavy elements created in supernovas, or light elements that have yet to be turned into heavy elements. Hydrogen is #1 because pure energy eventually \"solidifies\" into it, more than any other element, at least. And because very little non-hydrogen was not formed in a star: your options (as matter) are A. be hydrogen, B. be some other element but be in the middle of a giant fusion \"machine.\" And because stars are \"gravity powered\" the heavier an atom is, the closer it is to the fusion area."
] |
[
"Why is it that gravity slingshots work? Don't the violate conservation of energy?"
] |
[
false
] |
[deleted]
|
[
"No scientist here, but I'm pretty sure that the conservation of energy is true with planets as well. The amount Neptune slowed down is probably like 0.0000000000000000001% but that was enough to propel voyager further. "
] |
[
"Close - the energy comes from the ",
" of the planet - the planet slows down in its orbit around the sun.",
"It's useful to look at this problem from a couple of different perspectives. First, our camera is following along with the planet. Here, the planet is stationary. The probe falls towards the planet, slingshots around it, and then flies away from the planet. From this perspective, the probe's speed has not changed, even though its direction has. The probe has the same speed relative to the planet before the interaction as it had afterwards.",
"Next we focus our camera on the Sun. In this perspective, the planet appears to be moving quite fast. The probe's speed also looks different from here. Suppose the probe was flying directly towards the planet at 10 km/s (relative to the Sun), in a \"retrograde\" orbit - i.e. they're moving in opposite directions and are going to hit head-on. The planet is moving at say, 30 km/s relative to the Sun. So from the planet's perspective, the probe is approaching at 40 km/s. It swings around the planet, and suppose it achieves a full 180 degree turn. It is still moving at 40 km/s relative to the planet, but it is no moving in the opposite direction. This means that relative to the Sun it is moving at 40+30=70 km/s - a huge increase in speed. To make this balance out, the planet must be slowed down a little in its orbit - while it was slingshotting around, it was pulling the planet in the opposite direction, slowing it down slightly.",
"p.s. I like your French spelling of \"Voyager\" :P"
] |
[
"Slingshots only work with planets that are in motion. For example, it would not work with a stationary planet (assume all the planets were stationary for simplicity). But if a planet is moving quickly, we can gain some of that energy with a slingshot maneuver.",
"http://en.wikipedia.org/wiki/Gravity_assist",
"\"The \"assist\" is provided by the motion of the gravitating body as it pulls on the spacecraft.\"",
"This line is also interesting and relevant:",
"\"Interplanetary slingshots using the Sun itself are not possible because the Sun is at rest relative to the Solar System as a whole. However, thrusting when near the Sun has the same effect as the powered slingshot described below\""
] |
[
"Is a 'randomly' generated real number practically guaranteed to be transcendental?"
] |
[
false
] |
I learnt in class a while back that if one were to generate a number by picking each digit of its decimal expansion randomly then there is effectively a 0% chance of that number being rational. So my question is 'will that number be transcendental or a serd?'
|
[
"When we talk about probability distributions on the real numbers, we are really ultimately talking about a ",
". A measure is a rather technical way of assigning a length to an interval or a volume to a region. We can actually define many measures on the real numbers, but we typically stick to so-called ",
". This is a measure that allows us to assign \"lengths\" to subsets of real numbers in such a way so that the length of the interval [a, b] is just what you expect: ",
"-",
".",
"We can forget about most of the technicalities. What's important for your question is that Lebesgue measure assigns measure 0 to single points. That should make sense: the length of a single point is 0, right? It's also important that any measure satisfy certain properties. One property that makes a lot of sense is that if ",
" and ",
" are two ",
" subsets (that means they don't overlap), then the length of their union is just the sum of their lengths. For example, the measure of [1, 3] together with [5, 6] should be 2+1 = 3, and it is. What's interesting about measures is that we extend this rule of ",
" additivity to ",
" additivity. So if we have ",
" many disjoint sets (and there could be infinitely many of them), then the measure of the union is the sum of the measures.",
"So what happens when we combine those rules to a countable set of points? For instance, what is the Lebesgue measure of the set N = {0, 1, 2, 3, 4, ....}? Well, each number in the set is a single point and has Lebesgue measure 0. There are countably many numbers in N, so the Lebesgue measure of N is just 0+0+0+... = 0. This applies to ",
" countable set. All countable sets of real numbers have Lebesgue measure 0. That should also make sense. Remember that the real numbers are uncountable. So a countable set, even though it may be infinite, is very small when compared to the uncountable set. Our sense of that smallness is reflected in how we construct Lebesgue measure.",
"Okay, so what does this have to do with transcendental numbers? Consider the set S of algebraic numbers, that is, all real numbers that are a root of some polynomial with rational coefficients. It turns out that S is countable! How can that be? (We need to use the fact that the rational numbers are countable.) We can actually just make a list of them. Forget about degree-0 polynomials since those are constants. What about degree-1 polynomials? Each degree-1 polynomial is determined by 1 rational number (we can always assume the leading coefficient of the polynomial is unity). There are countably many such polynomials, each of which has at most 1 real solution. So we get S",
" = set of real solutions to degree-1 polynomials with rational coefficients, and that is a countable set. Then we move on to degree-2 polynomials, and there are countably many of those since they are determined by 2 rational numbers. Each of those degree-2 polynomials has at most 2 solutions, so there are countably many solutions. (Two solutions times countably many polynomials = countably many solutions.) So S",
" = set of real solutions to degree-2 polynomials with rational coefficients, and that is a countable set.",
"We can generalize clearly. Let S",
" = set of real solutions to degree-n polynomials. That set is countable, being at most the size of finitely many countable sets. Finally, we see that S (the set of algebraic numbers) is the union of S",
", S",
", ..., S",
",... . Here we have to use the fact that the union of countably many countably sets is itself countable. The proof is not that difficult. If you have a list of the members of each set, you can form a list of the union by listing them like this:",
"S",
"(1)",
"S",
"(2)",
"S",
"(1)",
"S",
"(3)",
"S",
"(2)",
"S",
"(1)",
"...",
"The pattern is rather simple. List one element from the first set. Then the next unlisted elements from the first two sets. Then the next unlisted elements from the first three sets. Then the next unlisted elements from the first four sets, and so on. Eventually, every single element in the union will appear on the list.",
"So now once we know that the algebraic numbers are countable, we know that their Lebesgue measure is 0. We say that ",
" real numbers are transcendental. So, for instance, if you consider the uniform probability distribution on the interval [0,1], there is probability 1 that a randomly selected number is transcendental."
] |
[
"It is 0. This may seem counter-intuitive, but after all, they are an element of the set from which we pick, so any single number can be picked. This is unlike a dice roll, were a roll of 7 on a standard die is impossible.",
"The probability, however, is infinitesimal, so incredbly low, that any number greater than 0 is an overstatement. And no matter how often you pick, the estimated number of real numbers you pick remains 0."
] |
[
"Stepping back from the mathematics angle and looking at it computationally: the algorithm you specify - picking each digit of a number at random to build your random number - is guaranteed to be ",
" because you have to stop at some point to return the number. Your algorithm would require an infinite number of steps in order to 'arrive' at an irrational number."
] |
[
"How thick in atoms does carbon need to be to absorb light?"
] |
[
false
] |
When a atom absorbs, and transmissions light, I have been told that single atoms are transparent, whatever the element. So how many atoms thick in carbon does it take to completely absorb all wavelength, and frequency of light. Is it basically as you increase the amount of atoms in a crystalline, or non crystalline structure, there is just too many electron absorbing the amount of photons hitting, and getting absorbed by the electrons, and light just cannot get through. The light just gets absorbed weather the material is amorphous, or not. So its only the energy of electron that will let light pass, and transmission through like in glass, and carbon diamond Is this correct. Thank you for your help, anything helps, even a few words.
|
[
"1.",
"Graphene absorbs about 1% of incident light.",
"http://www.cz2dcarbon.com/en/uploadfile/201352315521996.jpg"
] |
[
"So how many atoms thick in carbon does it take to completely absorb all wavelength, and frequency of light.",
"The first layer of graphene absorbs 1% of the light, the second absorbs 1% of what's left over, etc. After n layers about 0.99",
" of the light remains. If you want to absorb all light, ten you have 0.99",
" = 0, so you take log",
" 0 which works out to infinity. It must be infinity atoms thick."
] |
[
"I'm guessing you are being facetious, but the result of this is rather interesting if you simply look at a practical limit (i.e. when does only 1% of incident light pass through the graphene layer?). It would take about 460 layers of graphene which is about 1500A, only a tenth of a micron. "
] |
[
"If a great dane and a chihuahua can't breed, how can they be considered part of the same species?"
] |
[
false
] |
Forgive me if I'm making any false assumptions here, but it is my understanding that: - a) fertile members of the same species are able to breed with one another, b) great danes and chihuahuas are both part of the species Dog (specifically, subspecies ), and c) great danes and chihuahuas are too far removed from each other to be able to breed. Which of these assumptions is false, and if none, what does it all mean?
|
[
"What makes you believe that Great Danes and Chihuahuas cannot breed? That is a false assumption. Just because you haven't seen any puppies of that mixed breed doesn't mean they ",
" exist. Although it may be geometrically difficult, it is possible."
] |
[
"\"Species\" is an artificial concept which is very useful in some subfields of biology, while not so with others. I think almost every question about evolution and ",
" puts the latter into the \"not useful\" bucket.",
"What if two breeds of dog can't or don't want to breed? It's not an effect of their belonging to one or two different ",
". What if two humans can't breed? Should we also classify them as different ",
"?",
" (much like ",
" (in the culinary sense) and ",
") is where we choose to draw the line in the ",
" of all possible animals.",
"There's a similar line in what \"",
"\" means as well. Consider your example:",
"Answer 'no' to any of these questions and you'll get a different definition for 'able to breed'."
] |
[
"Not true. Dogs have 78 chromosomes (same as wolves) and foxes have 34, there's no way they can possibly produce fertile offspring because they lack homologous sets of chromosomes. You may be confusing this with Sibfoxes (",
"http://www.sibfox.com/",
"), which are actually foxes bred to be more docile, which gives them a variety of morphological traits similar to dogs.",
"However, all members of the genus Canis (\"dingoes\", wolves, domestic dogs, coyotes, and golden jackals) can interbreed. Interestingly, the chromosome problem is not clear cut; golden jackals have two sets of chromosomes that are not homologous to dog chromosomes but nonetheless the two species can interbreed."
] |
[
"Is there an easy to remember formula for calculating the cooling time of a given amount of liquid?"
] |
[
false
] |
A couple of months ago, I tried to google a formula for calculating how long it would take to cool a can of beer to about 3 degrees Celcius (I'm Swedish, hence the non-Fahrenheit units). Based on a thread I found in some random forum, I gathered the following: 4,18 (some kind of constant?) x [Amount of liquid in liter] x [The temperature difference between the beer's current temperature and the compartment used for cooling] You would then take the result of that equation and divide it by the temperature in the compartment used for cooling. This is the weird part, because it didn't say anything about negative degrees. So, is this formula at all right, or just something that seemed to produce a somewhat correct answer? Does it even exist an easy to remember formula for this? (The follow-up question to this would be if there's an easy way to calculate how long it would take for a steak to reach 59 degrees Celcius in a 200 degree owen. Feel free to educate me if there is.)
|
[
"To answer your question about negative degrees, Heating and Cooling (in fact, every equation that uses temperature) in science require use of the Kelvin scale of temperature.",
"T(in K) := T(in C) + 273",
"So 0C is actually 273K. The reason Kelvin are used is because it extrapolates all the way down to where 0K corresponds to no molecular motion (so called Absolute Zero).",
"Also, your (some kind of constant?) is known as the thermal conductivity of the material. It's a material property that measures how well it can conduct heat. The conductivity changes depending on the object and material you are looking at heating or cooling."
] |
[
"In principle, yes. It's Newton's law of cooling. But in practice, no. On the scale found in a kitchen, the approximations necessary to use the formula are significant. For instance, Newton's law requires that the temperature of the object's interior be the same as its surface. This is clearly not the case when cooking.",
"If you wanted to model the situation more accurately you'd use Fourier's law … but that's a differential field equation of three variables, so it definitely doesn't help in the kitchen."
] |
[
"Thanks for the clarification! So I suppose the 4.18 is the thermal conductivity of aluminium (at least that was what the guy at the other forum pinned it at). But if that guy also used Kelvin, I must've read his other numbers wrong. "
] |
[
"What would happen physically to Earth if it was hit with a Gamma Ray Burst?"
] |
[
false
] |
What would happen physically to Earth if it was hit with a Gamma Ray Burst such as the 1997 ones on ?
|
[
"Here's the Gamma ray burst section of a show called ",
"Last Days on Earth",
". The consensus is that we're pretty much screwed. Also, Neil DeGrasse Tyson FTW!"
] |
[
"I hereby summon the Bad Astronomer to answer this question! Let's hope he's still redditing actively."
] |
[
"I thought it would be interesting to work out how close a GRB would have to be to give humans radiation sickness.",
"So let's say the GRB is composed of ",
"150 keV photons",
". From wikipedia, the density thickness of the atmosphere can be worked out to be about 1 kg/cm",
" , so using the energy absorption coef. of air at 150 keV (.025 cm",
" /g) the atmosphere would absorb 99.999999999158% of the incoming energy. ",
"Now let's say that it takes a radiation dose of 4 J/kg to destroy the bone marrow and induce radiation sickness. Let's assume the GRB is isotropic, that the cross sectional area of a person is 0.5 m",
" and that they weigh 70 kg. The GRB is 10",
" J, and dissipates as 1/(4",
"d",
" ), d being the distance from us. This works out to a lethal distance of 200 light-years.",
"Also note that this distance would increase if the GRB were more \"directed\" (or less isotropic)."
] |
[
"[META] F-O-O-D Food Food!"
] |
[
false
] |
Dear AskScience, Starting this week we are introducing a new regular META series: theme weeks. They won't happen every week, just once in a while, but we think having themes every so often would be a lot of fun. As a brief intro to our first ever theme, there are 2 aspects to how the theme weeks will work: Theme week will kick off with a mass AMA. That is, panelists and experts leave top-level responses to this submission describing how their expertise is related to the topic and We'll have special flair, when appropriate. : panelists and experts leave a top level comment to this thread, and conduct an AMA from there. Don't ask questions on the top-level because I have no idea! This week we begin with an important topic: FOOD! This week we hope to spur questions ( ) on the following topics (and more!): Taste perception Chemistry of gastronomy Biophysics of consumption Physics of cooking Food disorders & addiction Economic factors of food production/consumption Historical and prospective aspects of food production/consumption Nutrition Why the moon is made of so much damn cheese? ( ) Growing food in space Expiration, food safety, pathogens, oh my! What are the genomic & genetic differences between meat and milk cows that make them so tasty and ice creamy, respectively? Or, anything else you wanted to know about food , such as physics, neuroscience, or anthropology! Submissions/Questions on anything food related can be tagged with special flair (like you see here!). As for the AMA, here are the basics: The AMA will operate in a similar way to . Panelists and experts make top level comments about their specialties in this thread, and then indicate how they use their domain knowledge to understand food, eating, etc... above and beyond most others Even though this is a bit different, we're going to stick to our normal routine of "ain't no speculatin' in these parts". All questions and responses should be scientifically sound and accurate, just like any other submission and discussion in . Finally, this theme is also a cross-subreddit excursion. We've recruited some experts from (and beyond!). The experts from (and beyond!) will be tagged with special flair, too. This makes it easy to find them, and bother them with all sorts of questions! Cheers! PS: If you have any feedback or suggestions about theme weeks, feel free to share them with the moderators via .
|
[
"Expertise: Chief Creative Officer at Serious Eats and author of ",
"The Food Lab",
", a weekly column that explores the science of home cooking. My education is in science and engineering (biology and architecture), my work experience is in restaurants. Prior to Serious Eats, I was Senior Editor and in-house science adviser at Cook's Illustrated magazine. I've also worked as an adviser for that Harvard Food Science class, though not currently active in that capacity.",
"My book, The Food Lab: Better Home Cooking Through Science is currently in design/layout phase, and will be a two-volume, 1,200 page box set that covers all the basics of home food science in a fun, thorough way, with tons of photos, experiments, and of course recipes.",
"Specific Expertise:"
] |
[
"Expertise: PhD in Microbiology - Infectious disease, with a research focus on foodborne pathogens.",
"I'm at work right now but promise to get back to you at some point today. "
] |
[
"My rule of thumb for dealing with raw meat (and trust me, this is always on my mind since my research was on ",
" O157:H7 and shiga-like toxins) is this:",
"Handle the meat as you need to, but any time you touch it, the next thing you touch should be the sink with warm water and soap- not the salt and pepper, not a dish, not a spatula. If you are taking a steak out of the package and seasoning it, you should then go to the sink and wash, THEN go get the salt and pepper to season it. It's basically the same in you kitchen as good aseptic technique in the lab. Is it overkill? Sometimes probably yes. Do you know when it's overkill and when you are saving yourself from illness? No, so you just do it all the time anyway. It may require a little thought at first but eventually becomes second nature.",
"So in your example, I would say: Don't touch the handle of your roasting pan after touching the raw chicken in the first place. Put the chicken in the pan, then wash your hands, then move the pan (with or without potholders) to the oven.",
"If you do think that you have contaminated your potholder, if you want to be safe, then yes, wash the potholder. The chance of a cross contamination from chicken to your hand to the pan to the pothold back to your hand is slim but is a possibility, so why risk it?",
"Another way to be safe (which would prevent sickness even if your potholder IS contaminated) is to be extremely cautious in preparation of items that will not be being cooked (ie: salads). I always use a fresh cutting board for prepping salads and wash my hands thoroughly before hand. I also move them \"out of the line of fire\" so that if I'm opening a package of chicken and some liquid goes flying it doesn't land in my salad (blech).",
"Again, the chance of actually becoming sick from a small ingestion of ",
" or ",
" (most common on chicken) is low. These guys require large doses to cause infection in people with healthy immune systems, but a little bit of caution and learning proper techniques goes a long way, even if its just toward your own peace of mind. "
] |
[
"Assuming you could create a hole in the middle of the earth without it collapsing, if you threw an object down would it just float in the middle?"
] |
[
false
] |
I've always heard that gravity is caused because somehow matter distorts the surrounding space-time. If that's the case, wouldn't that mean that if a hole existed in the middle of the earth, then objects thrown into it would just be pulled towards the hole's wall rather than just floating in the middle?
|
[
"This depends on what you ask. Since you speak of throwing an object in, one would assume you mean a tunnel through the Earth's core, out the other end. Let's take a look at what would happen. There's a ",
"youtube video",
" and an ",
"article on howstuffworks.com",
".",
"First off all, the Coriolis effect would have the object slam the side of the hole rather than falling down the shaft. This is because of the Earth's rotation, and the fact that the surface moves faster than the rock miles below it. This has been tested with the Kola borehole in Russia. If your hypothethical tunnel would be on the poles, this effect wouldn't apply anymore, so that's one problem out of the way.",
"But there's still air friction. This limits your object's top speed, so the effect of gravity, though increasing, would not accelerate you to maximum levels, and your object wouldn't have enough momentum to escape the other end before the gravity of the Earth now below your object would slow you down until you start falling down the other way. This will repeat, friction continuously taking away speed, until you hover in the centre of mass on Earth.",
"Were friction to not apply, the momentum of the gravitational attraction would pull you right through until you arrive at the other end with about the same speed you entered. Obviously, this is a slightly unrealistic idea. Also, it requires your tunnel to cross the centre of mass, or the centre of gravitational attraction, so your object wouldn't be pulled to the side of your tunnel and receive friction that way.",
"If you however imagined a cavity in the Earth around the centre of mass, and an object would magically be formed in said cavity then things change. I am not sure of what would happen in that scenario, but a speculation would be that it basically hovers in space, because all of the Earth would pull at it from all angles, cancelling out.",
"However, none of these scenarios were ever experimentally tested, mainly because it is slightly difficult to dig through our crust, mantle, outer core and inner core, and there isn't a lot of funding for such experiments. Therefore, this hypothesis mainly flows from our current understanding of Newtonian physics. The theory of space-time distortion by matter is merely an alternative explanation of the phenomenon of gravity, intended to explain light bending, whose lack of mass seems to make the classical equations not make sense. Nonetheless, assuming your object is a regular rock or of the sort, the hypothetical effects remain the same. Please correct my mistakes and inaccuracies if found. Either way, HTH!"
] |
[
"There is a ",
"FAQ",
" on this topic."
] |
[
"Nice summary! Just two things: one, even without air resistance, an object will oscillate around the center of the tunnel, assuming it coincides with the earth's center of mass--it will just never come to rest, but will keep bouncing back and forth between the poles like a ball on the end of a slinky.",
"Regarding forming a cavity in the earth, this scenario is no different from a tunnel, as long as we're not assuming the tunnel has zero radius. As a matter of fact, if I am located somewhere inside the Earth, only the part of the Earth which I am \"outside of\"--i.e., all parts of the Earth which are closer to the earth's center of mass than I am--produces a net gravitational force on me. A massive spherical shell outside of me produces no net gravitational force on me, because it attracts me equally from all angles, as you said, and this can be shown with math. It might be true for non-spherical shells as well, but I haven't done that proof so I'm not sure. An interesting side effect of this is that the ",
" of the gravitational force I experience inside the Earth actually decreases linearly the closer I am to the Earth's center.",
"Anyway, whether I am in a tunnel or a spherical cavity, as soon as I am in a place where there is no mass closer to the Earth's center of mass than I am, I experience zero net gravitational force. If I am dropped into a tunnel, I am probably moving with some speed once I enter this region (which would be a sphere with radius equal to the tunnel's radius), so I will move through it at constant speed and begin decelerating (very gradually, since the force of gravity is quite small down here) once I exit the other side. Then I will continue to oscillate around this region as normal, eventually coming to rest somewhere inside it if air friction robs me of my kinetic energy.",
"In the spherical cavity the same thing happens, but if I am not initially moving at a speed I will simply remain stationary wherever I am in the cavity, even if I am not at the center. If I am initially moving at some speed, I will continue to move in a straight line at that constant speed and I will bounce of the walls of the spherical chamber like a billiard ball. The only difference between this and the tunnel is that now I can't exit the region of zero gravitational attraction, so I will probably die of thirst or starvation."
] |
[
"Can domestic and/or wild animals recognize reflections of themselves in mirrors?"
] |
[
false
] |
I realize that they do stop freaking out (for lack of a better term) over it after a while, but is this due to them "getting used to it" or actually recognizing it as them? And, to extend the question, is it possible for an average dog to do this?
|
[
"Dolphins and some apes can, the vast majority cannot."
] |
[
"Magpies can, too.",
" Knowing the intelligence of crows and ravens--especially compared to magpies and \"less intelligent\" corvids--they should be able to, as well."
] |
[
"Do you have any citation for this? I appreciate the answer, but I'd like to have a starting point for any further inquiry I have into the matter."
] |
[
"Do radiologists (doctors) have higher incidences of cancer or dangerous radiation exposure?"
] |
[
false
] |
I was wondering of medical students take this into account when deciding on their futures (provided they have the scores to compete).
|
[
"Before 1950 or so, the risks of radiation were not very well known. In fact, it was early increases in skin cancer among radiologists that helped link radiation to cancer. ",
"Anyone that works with radiation in their day-to-day job is classified as a \"radiation worker\" - they wear badges that measure their exposure to radiation over time, and these are closely measured and monitored. So these types of people (imaging technicians, radiologists, nuclear power plant workers, nuclear medicine techs) are actually some of the most well-tracked populations exposed to radiation.",
"For radiologists specifically, you can read about some of the results ",
"here",
". A statistically significant increase in leukemia was noted in radiologists before 1950 (when radiation protection standards were put in place). After that, there is no evidence of an increase. ",
"tl;dr: radiation protection standards prevent radiation workers from experiencing a measurable increase in cancer rates."
] |
[
"Rewording it, I think he meant to say that \"the idea that for any increase in radiation dose there will automatically be a higher incidence of cancer is still up for debate\"",
"While we know pretty well what the effects of large exposure to radiation are, we're not really certain what the effects of much smaller doses are.",
"\nFor obvious reasons, lab trials on human test subjects are not permitted."
] |
[
"There's some interesting discussion of the effects of increased daily radiation exposure in this article on Wikipedia: ",
"http://en.wikipedia.org/wiki/Ramsar,_Mazandaran",
"In short, the thought that any increase in radiation dose will lead to higher incidences of cancer is an open question."
] |
[
"How does scientists create higher nr of isotopes of uranium and why haven't we gotten any further already?"
] |
[
false
] |
I read the askscience thread about the island of stability and it got me curious. We're able to create new isotopes of uranium for example, but how do we do this? Do we insert neutrons or something? If so, how does one come about carrying neutrons? Since we know how to create higher isotope numbers of this element, how come we haven't been able to come any further than we already are? can't we just continue with the same method as the previous ones that's been created?
|
[
"I'm not exactly an expert, but to answer your direct questions, neutron bombardment is one way to create heavier nuclei. However it has inherent rate limitations. If the new nuclei decay faster than you can create them, you can no longer continue building up larger nuclei with the same method."
] |
[
"You are right. You are essentially describing the r and s process that occurs in stars. Neutrons either slowly or rapidly making isotopes. "
] |
[
"To add to the previous answer, not only can we make new isotopes by neutron bombardment. We also do fragmentation reactions where we smash large isotopes apart and measure the fragments. If you do this with something like uranium, the fragments are really neutron rich. We can also do knockout reactions where we shoot high energy particles and hope to knock out a bunch of neutrons. "
] |
[
"What is the highest altitude at which a mosquito can sustain flight? Why? What about other insects?"
] |
[
false
] | null |
[
"fully laden?"
] |
[
"I don't know the exact answer to your question.",
"I am, however in East Africa and am on Malaria pills. The info sheet I have for Tanzania says that Malaria risk exists at altitudes below 1,800m (5,906 ft). So take that for whatever it's worth."
] |
[
"If it's any consolation, the number of downvotes will probably be the minimum number of people who got the reference.",
"In the case that you were not attempting humor, however, you bring up an interesting consideration. How different would the absolute ceilings be of empty and blood/nectar/juice-full mosquitoes be?"
] |
[
"How does the eye actually turn photons into signals in our brain? What's the physical process that allows that to happen?"
] |
[
false
] | null |
[
"Chromophores capture light photons and are located in the visual pigments of the retina. Known as retinol, which is derived from vitamin A (precursor is beta-carotene, eat your carrots!).",
"When we receive light from our favorite star, photons are absorbed by rhodopsin (visual pigment in rods) and transfer energy to chromophores. This process is called ",
", and causes a bunch of events that eventually close channels on the cell membrane, making the cell negatively charged. Glutamate concentration is then reduced at synaptic terminals and this tells bipolar cells (kind of like the intermediates) that the rod has captured a photon.",
"Bipolar cells then pass information to ganglion cells, and subsequently will be relayed to axons in the back of the eyes to carry the information to the optic nerve."
] |
[
"u/Ha2ha3ha4",
" did a good job of answering it, but I like simpler answers, so I'll try. Pigments (rhodopsin) change shape when a photon hits them. Photons = energy and energy => shape change. The pigment is bound into a protein so that when it changes shape, the protein does too. The shape change closes channels in the membrane to prevent ions from passing through. ",
"This stops the otherwise constant inhibition of the neighbouring cell and allows it to fire. ",
"Idk if you know how nerves work, but this process kicks off a wave of gate openings that travel down the nerve. ",
"TLDR: Photons -> shape change -> channel closes -> inhibition removed-> next cell fires -> local membrane charge change -> neighbouring charge sensitive channels open -> Repeat previous step -> nerve impulse travels to brain. ",
"Shorter TLDR: Pigments change shape when photons hit them + neurology."
] |
[
"what you want is ",
"Phototransduction",
"I'll give you a quick summary: You know how light is an electromagnetic wave? Bonds in molecules are also electromagnetic, so light affects those bonds. In your Retina you have molecules which change their structure when hit by light waves. This change in structure is being detected by the cells and transmitted as a nerve signal to your brain. The cells in your Retina then change the structure of the molecule back to its original state, so that it will be ready to detect the next light wave."
] |
[
"What is the purpose of interpreted languages?"
] |
[
false
] |
Maybe I'm just not understanding virtual machines like JVM or CLR correctly, but I just don't understand what the purpose of interpreted languages are. I'm thinking of it like this (a horribly generalized analogy, but please catch my drift). Say I wanted to translate French (high level programming language) to Chinese, Japanese, and Korean (machine code for different processor families), wouldn't I just write 3 dictionaries? But instead, what I'm understanding is people want to translate French to Latin (e.g. Java bytecode or CIL), then translate that into Chinese, Japanese, and Korean. I'd have to write 4 dictionaries, which is one more dictionary than I would like to write! I'm just imagining a poor sod at Microsoft (more like a team I suppose, or several, or a whole department) developing a version of .NET for each instruction set they can think of. Why not, like C++, just write the same number of compilers to achieve the same thing? But then again, I guess C++ is very old and has compilers for everything. You can get technical in your answer, but I'm less interested in the nitty-gritty and more interested in the path that lead us here. Is it because of technological constraints of which we are now free? Is it because development teams have become so large that how we view workflow has changed (I'm not sure how an interpreted language would help that, but I'm thinking something-something portability)? What is the human factor in all this? Please Reddit, I'm here instead of Stack Overflow because Redditors are nice and don't try to be intentionally esoteric. P.S. Understanding compiled vs. interpreted just as a general public consumer of software is enlightening, but it only explains . How did I end up having a million versions of the .NET framework installed on my computer? How does Java have the balls to bug me every week about updating it? But it doesn't answer ?
|
[
"Say I wanted to translate French (high level programming language) to Chinese, Japanese, and Korean (machine code for different processor families), wouldn't I just write 3 dictionaries? But instead, what I'm understanding is people want to translate French to Latin (e.g. Java bytecode or CIL), then translate that into Chinese, Japanese, and Korean. I'd have to write 4 dictionaries, which is one more dictionary than I would like to write!",
"What if you also want to translate Chinese, Japanese and Korean back to French? Then you'll need three more dictionaries. And what if you want to translate Japanese to Korean and Korean to Chinese? That's two more. If you want to translate between each pair of your four languages, you'll need 12 dictionaries (for both directions).",
"But if you could translate each language to Esperanto as an intermediary language, you'd only need 8 dictionaries.",
"Plus Java bytecode and CIL can be optimised on the fly. It carries a bit more context with it than machine code, so the interpreter can optimise out some things that it knows it won't need this time around."
] |
[
"An additional advantage of the bytecode virtual machine model is that you can separate the steps of the traditional compiler chain, in either space or time.",
"For a traditional compiled language, there are a number of stages to the process of converting program text to machine code. This basically involves:\n1. Parsing the text (to figure out what the programmer wrote)\n2. Converting it into an easily-manipulated intermediate form\n3. Optimizing the code (removing redundancy, etc)\n4. Converting the optimized intermediate form into machine instructions",
"With a bytecode virtual machine, you are essentially \"freezing\" the state of the compilation around step 3, then shipping it somewhere else to perform step 4.",
"This means that the folks who write the Java compiler don't have to do code generation for every possible processor and operating system. Other developers (possibly employees of the processor manufacturer) can do that part of the work.",
"It also means that you as a developer can compile your Java program to bytecode, and then when each individual user downloads it, the JVM on their machine can produce the best possible machine code for their particular computer. If you did all the compilation up-front, you'd have to decide not just which processor families to support, but even which varieties. Different Intel processors have (slightly) different instruction sets, so if you're doing a single compilation for \"all Intel processors\", you either need to use the lowest common denominator instructions, or build multiple versions, only one of which any particular processor will use."
] |
[
"As for your post script, there are different versions of .NET and Java on your machine because people are still working on them. Bugs are constantly found and fixed. New features are added and existing ones are changed. Version X might break a program people use, so it's easier to keep version W around than than it is to get all developers and end users to upgrade."
] |
[
"Does a fish's mouth heal after it's released from being caught on a hook?"
] |
[
false
] |
The "skin" there near the mouth sort of resembles cartilage. Does this sort of tissue, whatever it is, heal properly after a fish is hooked?
|
[
"It does heal. I've seen though with experience that larger holes caused by large barbs or a hook in a particularly thin tissue (think mouth of a crappie), that it may take some time for those.",
"I don't have a source off hand but I've been an avid fisherman most of my life."
] |
[
"You can often see scar tissue in larger fish that have been caught before. If the hook was removed the right way it leaves a relatively small hole which is very difficult to spot, but if the hook was torn it often leaves holes that look like old piercings."
] |
[
"How can you tell it heals?"
] |
[
"What is happening to the Gulf Stream and the Great Conveyor now and what is predicted if temperatures continue rise and the Arctic ice melts?"
] |
[
false
] | null |
[
"There's an excellent section on thermohaline circulation in the most recent ",
"IPCC report",
" (Chapters 11 and 12). Basically, climate models all pretty much project the thermohaline circulation in the North Atlantic (AMOC) to become weaker over the next hundred years in response to greenhouse warming. How much? How fast? We really don't know. But it's worth noting that the models seem to project the reduction as related to changes in surface heat flux rather than freshwater flux. "
] |
[
"Thank you. Just what I was looking for."
] |
[
"The salty water that's being moved by the gulf stream warms up and moves north. When it gets up around the arctic circle, the water cools down, sinks and returns. \nAs the ice caps melt, they introduce more fresh (not salty) water to the ocean which can interfere with its current. As far as I'm concerned (and I may be wrong) the dilution of the water won't have that big of an effect on the current. You can draw two conclusions from the same set of facts and they make sense. What I heard a while ago is that the current will basically stop reaching as far up and won't warm up the northern most countries like Norway, but it's hard to stop such a huge current, and on the other hand I've also heard scientists that claim that the dilution won't have much effect since it's still the temperatures mostly driving the current. \nI hope this helps you, I know it's probably not the full anwser you are looking for, but still :)"
] |
[
"Cetaceans evolved to live in the sea, most of them lost the need to sleep. Why sleep is uncommon and even not needed in a lot of sea animals?"
] |
[
false
] | null |
[
"Cetaceans do in fact sleep, they just do so in a strange alternating way to avoid drowning. They can sort of turn off parts of the brain and swim slowly to rest while still having the motor function necessary for sleep. Many fish also sleep as well, and since they have gills breathing is largely not a problem for them. You may be asking “don’t sharks need to keep swimming to breathe? What about them” and you’d be partially correct as some sharks must keep swimming to breathe as they are obligate ram ventilators. However even these sharks enter rest states with their mouths open and swimming slowly or facing into currents to keep water running over their gills.",
"As for other sea creatures such as jellyfish and anemones, they are very simple and one of the most important parts of sleep is resting the brain, something these creatures lack and many other simple sea creatures have minimal nervous systems. ",
"TL;DR: They do sleep, just differently from us to ensure they don’t drown."
] |
[
"Amazing"
] |
[
"As for other sea creatures such as jellyfish and anemones, they are very simple and one of the most important parts of sleep is resting the brain, something these creatures lack and many other simple sea creatures have minimal nervous systems. ",
"But even such simple creatures still exhibit signs of sleep. It appears sleep evolved before brains.",
"https://pubmed.ncbi.nlm.nih.gov/33028524/"
] |
[
"General Science Blog Suggestions?"
] |
[
false
] |
[deleted]
|
[
"I've entertained the idea of writing a weekly or monthly recap of the recent articles that don't make the news, whether they be important but not sexy, or just silly or weird."
] |
[
"I thought about that as well, mostly because it doesn't seem like the actual \"news\" can ever get it right. I have seen the most absurd assumptions made of research projects made in the news."
] |
[
"Then we would need a ",
" for posting content or links to these articles. assuming there's other panelists who'd be interested..."
] |
[
"Can sleeping on a part of your body (i.e. your arm) actually cause tissue damage by cutting off circulation?"
] |
[
false
] |
Holding a limb in an odd position or sleeping on it often causes pins and needles. If maintained long enough, would gravity/your own weight be enough to cut off circulation and cause local hypoxia/necrosis/any damage besides pain when circulation is restored? And if so, how long would it take?
|
[
"The pins and needles feeling is actually not caused by cutting off blood supply, but by compressing the nerves. You can damage your nerves to some degree this way, I know two people who had their arm go numb for an entire day or two from sleeping strangely, but it should heal and return to normal. Constant, excessive strain can cause permanent damage, like tennis elbow, that can need surgery, though some people are more or less genetically prone to this. ",
"As far as circulation, no, you're not going to cut it off to a dangerous degree from sleeping or sitting weirdly, there's lots of blood vessels in your limbs and it's hard to accidentally cut off all of the major ones, and you'd have to stay asleep or sitting weirdly for many hours straight or even days to form a blood clot in the compressed vessels, it would become painful very quickly. Individuals who are in comas, sedated or otherwise immobile need to be rotated around to prevent bedsores, but not really for circulation issues, though their circulation and O2 saturation likely isn't going to be great anyways from laying down and being immobile so long. Immobility does cause blood clots in the legs, partly from the pressure of sitting, partly from gravity. They are very dangerous, and it is why its recommended to do leg exercises if you're immobile, and there are these need contraptions attached to some SICU hospital beds that squeeze the legs rhythmically every 10 minutes or so to break up clots that feel very cool lol.",
"If something is constricting the limb, like a rope, you can accidentally tourniquet yourself which can cause a blood clot or eventual death of the tissue. I tell people, if you're ever practicing bondage, if any part of your body becomes cold feeling, tingly, or numb, take off the ropes, you're causing nerve damage. Extremely tight clothing, like the chest binders men with breasts wear can sometimes cause nerve damage to the arms if they're the wrong size as well. ",
"Source: Mom's a cardiac nursing director, and I spent a lot of time visiting the SICU earlier this year"
] |
[
"Over time if you don’t move around enough when in bed, you could wind up with bed sores. That’s damage to your skin, but eventually can become non-healing wounds that go all the way to bone, and actually become fatal (not to mention quite painful)."
] |
[
"Former ER RN here. This is a big issue with alcoholics and drug abusers. You can cut off circulation, feel paresthesias (pins and needles sensation), and will consciously it unconsciously move. People who are under the influence may not. It's called \"compartment syndrome\" when they cut off circulation to a part like that. Then we may cut off the part."
] |
[
"I've never seen a adequate explanation about the speed of light vs. the speed of electricity. Is light only faster because of the medium it moves through? How fast does electricity travel in optimal conditions?"
] |
[
false
] |
Title says it all.
|
[
"There's a vast difference between the speed of an electric ",
" through a conductor, and the speed of ",
" through a conductor.",
"The first one is relativistically fast. Generally between 0.6c and 0.99c for various modern cable designs. This is a measure of how fast ",
", in the form of an electromagnetic wave, can be conducted through the sea of free electrons in the conductor.",
"The second one is called free electron ",
", and measures how fast actual individual electrons will move through the conductor under the influence of an electric field. This speed is mind-bogglingly much slower, on the order of 1 mile per hour for a conductor like copper.",
"Edit: forgot to answer the other part of your question. The speed of electric signals is related to the speed of light (they're both EM phenomena, after all), but depends on the dielectric constant of whatever material you're talking about.",
"See also: ",
"Speed of Electricity",
" on Wikipedia, and its linked articles."
] |
[
"My old electronics teacher taught us, \"imagine you have a mile long pvc pipe filled with ping pong balls. This is your copper wire. Now when you connect this copper wire to a power source, it's like adding a new ping pong ball to one end. You see immediate effects at the other end of the pipe, but each ball only moved a single balls width.\""
] |
[
"Another way to look at the problem is water flowing through a hose. If you turn on the tap with the hose full of water, the information that the tap is open travels at the speed of sound in the water, meaning that the water starts flowing out of the end quite quickly. However, it takes much longer for water from the tap to reach the end of the hose. ",
"The key difference is that one bit of water (bunch of electrons) communicating with their neighbors versus actually moving themselves. The communication happens at the speed of sound (light) whereas the actual movement of the water (electrons) generally happens much more slowly. ",
"Edit: It occurs to me that I should mention that both water and electronic distributions are relatively in-compressible. A hose full of marshmallow would be a less awesome analogy. "
] |
[
"Is there a nutritional difference between fruit ripened on the plant and fruit that ripens on the shelf at the grocer?"
] |
[
false
] |
A lot of fruit is picked from the plant in a "raw" state, such as bananas, tomatoes and avocado's. Does the fruit receive a lot of nutritional value if it ripens while still attached to the plant? Would an unripe fruit that ripens on the shelf in your local grocer have just as much nutrients as one that ripens while still attached to it's plant?
|
[
"For most fruits, the ripening process is controlled by the ",
"plant hormone ethylene",
". This is a gas that is produced in the fruit to induce ripening, but is also a diffusible signal that can act over distances. For instance in some species, if you put ripe fruit in with unripe fruit, the ripe fruit will produce ethylene that induces nearby fruit to ripen.",
"The first commercial GMO was the ",
"flavr-savr tomato",
", which is an antisense RNA construct that interferes with ethylene production. By preventing the tomatoes from making their own ethylene, you can control the ripening process by adding ethylene when you want to. The flavr-savr failed as it was developed in a variety unsuited for commercial production. Reduced ethylene production has since been bred into most commercial varieties.",
"Nutritional changes upon ripening are very complex and depend on a number of factors, including light and temperature.",
" It's important to realize that this is occuring in the mature fruit tissue, and very little phloem activity occurs in a mature fruits that can ripen off the plant. While the mature tissue may not be growing, it is still functioning biochemically. ",
"I wasn't able to find anything that directly addressed the issue of on vs. off-plant ripening, but my inclination is that this difference is minimal compared to other factors. Age of the fruit, and the conditions it was kept in (light, temp.), and most of all the growing conditions and variety of the plant, are going to play the biggest roles. If I wanted to look for nutritional differences, I'd focus my search on looking at different cultivars, not the ripening regime."
] |
[
"*I would like to apologize for any misinformation that I may have given. I wanted to retract my post until I have some time to write a thorough, concise answer that explains my previous posts' links and statements. I encourage you to read some of the other posts below that all have relevant information but I don't believe have fully answered the question. I have a tendency to not fully explain what I have provided. So, I encourage each of you to downvote my response until explained in detail.",
"I have left one resource that does, in fact, show there is a statistically significant difference between a tomato ripened on the plant vs that off the vine when looking at a few specific criteria. (Please note the level of Ascorbic Acid in Table 1).",
"http://lib3.dss.go.th/fulltext/Journal/Journal%20of%20food%20science/2000%20v.65/no.3/19414jfsv65n3p0545-0548ms19990846%5B1%5D.pdf",
"In addition I have left another source that shows similar findings that illustrate a difference. (Please note the level of Ascorbic Acid) in Table 6. In addition to pointing out in the conclusions that more research needs to be done to see what, if any, difference there may actually be.",
"http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CC4QFjAA&url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F222549470_Preharvest_and_postharvest_factors_influencing_vitamin_C_content_of_horticultural_crops%2Ffile%2F9fcfd506c55a4a352a.pdf&ei=urhNUt6kDZLm8QSFlYG4Bw&usg=AFQjCNHU2FUOTIdlbTBlSAZsGzU8EB4cCA&sig2=MPmJuXP9qoOZPGKEoS9YZg&bvm=bv.53537100,d.eWU",
"However, these two studies show different findings of one of the same tested variables (levels of ascorbic acid). The first has a no significant change in Ascorbic Acid while the second does. There are method differences between the two studies which may account for that. The first study also points out the differences (as some have mentioned) up to 30% comparing the vine ripened vs. not with regards to 4 of their research points.",
"In the basic essence of this. Yes, there is a measurable nutritional difference between the two when looking at THIS ONE item, a tomato and those specific nutritional elements tested. The question that is at the heart of my original statement of (no, there isn't much)...is that the nutritional difference would be so small that it would not make a difference in the scheme of an individuals nutritional well being.",
"This is a matter of how one would interpret the significance of the phrase \"nutritional difference\" and to what it is referring. So, for that I apologize. However, I will address those concerns brought forward. I would like to say that because the tomato findings are different this does not necessarily mean this would apply to all fruits. "
] |
[
"Using tomatoes, fruit ripened on the plant accumulates nearly all the sugars and many of the flavonoids in the last 3 days on the vine. So when someone asks if the nutrition is there, I ask if the flavor is there because when the flavor is present, the nutrition will be optimal.",
"Caveat that a lot more goes into producing an outstandingly good flavored tomato than just the ripening process. There is a huge genetic component, soil nutrient status, and pest/disease pressure to consider. Commercial tomatoes were developed to be bowling ball hard, uniform size, highly productive, and a bunch of other traits associated with shipping them 2000 miles before putting them in an ethylene chamber to ripen. Where was flavor in any of this?",
"Background, I produce vegetable seedlings for a living and am a tomato critic for a pastime. ",
"http://www.selectedplants.com/culture.htm",
" and read the last few items.",
"Edit: Bananas do not suffer so many flavor and nutrient losses from ripening off the plant. Tomatoes on the other hand when force ripened lose a huge amount of carotenoids, flavonoids, and volatiles that significantly reduce the nutrient content and the flavor. If you want to spend an afternoon reading some interesting research, look up \"Klee tomato flavor\". Harry is at least trying to breed a better tomato, though I'm not sure he can succeed given the bowling ball genetics of most commercial tomatoes."
] |
[
"How does NASA detect what chemical elements/substances are on the surface (or in the composition) of certain planets?"
] |
[
false
] |
This is in regard to he recent discovery of water at the poles of Mercury but not necessarily specific. I'm not exactly interested in the apparatus that does that but HOW it does that? How do we know it's water and not something else?
|
[
"They use ",
"spectrographs",
", a device which measures the light that comes off that planet. This works because most substances have their own spectral fingerprint, which is very recognizable. ",
"For example, if you hold salt in a candleflame and look at the spectrum, it will have a bright orange emission line, which stands for sodium. ",
"pic"
] |
[
"The answers so far, while they give ways of identifying materials, do not actually cover the way ice was determined to be on Mercury. That process is described ",
"here",
" and, in more detail with some excellent images, ",
"here",
".",
"The work was multiple-step. First, a while ago, radar from the Earth was bounced off Mercury. It showed high reflectivity, characteristic of ice, but not only ice, at spots hearth the North Pole of Mercury. Then, more recently, using data from the Mercury Messenger orbiter's altimeter (which is laser based, but uses simple reflection, not spectroscopy or interferometry), scientists modeled where on Mercury would be permanently in shadows, and found this agreed with where they saw high reflectivity, making it likely that these spots had ice.",
"But the laser altimeter data also seemed to show that the amount of light it saw reflected from the poles was dimmer, not brighter; this seemed to argue against ice. But Messenger's neutron spectrometer also gave low neutron counts near the pole, which was consistent with ice. The working hypothesis, based on the data and what has been seen elsewhere, was that there was ice, but with a layer of organic compounds on it, lowering the reflectivity the altimeter saw.",
"To settle things, they adjusted the trajectory of Messenger so that it had a better angle for the laser altimeter to reflect light off the deepest and darkest spots on Mercury, and when they did this, they found the kind of bright reflection you'd expect from ice.",
"And this is how it was determined that there is ice on Mercury."
] |
[
"Think of the electrons in an atom like a set of bookshelves. All of the lower shelves (energy levels) are filled with electrons, but there are also additional higher shelves which are unfilled. If an electron gets just the right amount of energy (in the form of a photon) it can pop up to a energy level (shelf), and an electron in a higher energy level (shelf) can fall back down to its old (now empty) position while releasing exactly the amount of energy equal to the difference in those two energy levels. As it turns out, the particular different energy levels in an atom or a molecule are fairly unique.",
"Broad spectrum light, like that from a star, that is passing through a particular material will have the photons of the specific energies that can excite electrons to those higher energy levels in the molecules of the material absorbed (and then re-radiated randomly in all directions). So if you looked at, say, an incandescent lamp through a glass container filled with hydrogen gas it would be just a little bit dimmer than without the gas present. And if you broke apart the light you could see into a spectrum (using a spectrograph) with a high enough resolution then you could see little gaps in the spectrum where the hydrogen gas absorbed those specific wavelengths (energy levels) of light. And if you didn't know what sort of gas was in the glass container you could look at the spectrum and find out. Similarly, a hot sample of material will tend to create excited electrons which will decay and release light, and that will have a similar characteristic set of spectral lines only in this case they will be bright in the spectrum rather than dark.",
"In these ways you can look at distant stars and planets and determine what sorts of molecules are present on them.",
"Now, in the case of the recent discovery of ice on Mercury an entirely different and equally fascinating technique was used. The surfaces of all of the planets in our Solar System are continuously bathed in high-energy \"cosmic rays\" which are various sorts of very high-energy sub-atomic particles (mostly protons). Sometimes when these collide with atoms they will cause neutrons to be generated. As it turns out the overall rate of neutron generation from this process is fairly consistent and predictable. And for the most part the neutrons tend to just pass through most matter or ricochet off other nuclei. However, the nucleus of a Hydrogen atom (a proton) weighs almost exactly the same as a neutron, so when a neutron hits it it just stops moving (like a cue ball hitting a pool ball). The MESSENGER spacecraft in orbit around Mercury can detect these neutrons, and it found that there was a consistent reduction in detected neutrons above these specific regions of Mercury. This indicated that there was Hydrogen on Mercury at those spots. Combined with earlier radar observations of those parts of Mercury it adds up to fairly strong evidence of H2O ice."
] |
[
"There is a bird inside a large cargo plane. Does the plane weigh less if the bird flies inside the plane?"
] |
[
false
] |
This question was asked and the popular answer is that the downward pressure from the bird's wing would push against the floor of the plane and have the same effect as if the bird were standing on the floor. I argue that this pressure would mostly dissipate before reaching the floor and have minimal effect, and that the lift created by the bird's wing would nullify any effect it had on the weight of the plane. Any thoughts on this? Edit: I believe could test this problem, unfortunately I don't have the resources to do this myself. You can build the enclosure out of a lightweight but airtight material with one transparent face. Pilot the drone as high as possible within the enclosure to try to eliminate any ground effect. I'm almost 100% certain that the counterweight will drop as soon as the drone is near the top, if not before.
|
[
"The original answer to this question is of course correct.",
"You are wrong to assume that the \"downward pressure\" (or better said downward momentum of air) ",
" before reaching the floor. In fact it ",
". That means the momentum transferred by the wings onto few air molecules is then spread by molecular collisions to many more molecules. Each molecule carries a much smaller downward momentum, but the much larger number of molecules carrying some imposed downward momentum is much larger. Therefore the net downward momentum is equal and preserved when being transferred to the floor.",
"Conservation of momentum is one of the most fundamental laws of classical mechanics, therefore its sum always has to cancel in a closed system.",
"The situation is slightly different when the airplane has an open cargo latch. In this some of the downward momentum can \"escape\" the plane before being transferred to the floor. Therefore, the plane would weigh somewhere in between the empty plane and the plane with sitting bird, depending on the size/geometry of the opening.",
"\nI see that you have edited your question to include your experiment after I wrote my comment. Again, your expectation is wrong and the counterweight will certainly keep the balance leveled because the weight of the airtight can will stay the same. You would see a quick fluctuation when the drone accelerated upwards, but it will return to balance once the \"cruising altitude\" is reached and the drone maintains a level flight.",
"\nI wrote an explanation further down ",
"here",
" which might explain the problem of your expectation. Since it might get buried let me quote myself:",
"Also, air does not have to show a net flow in order to carry momentum from one place to another. This is the reason why waves can transport momentum without net transport of matter over large distances.\nIf you compare a hovercraft (floating on its ground effect cushion) and a helicopter (hovering at its ceiling altitude) the difference should be more clear. Both can hold their altitude by imposing downward momentum onto the air. In both cases the momentum has to be finally transferred to the ground. The difference, however, lies in the \"fate\" of kinetic energy (i.e. a net downward flux of air). While the hovercraft will generate this flux, leading to compression of air on the ground and local force acting on the ground (resulting in increased air pressure directly below its cushion) the air flow generated by the helicopter is dispersed and the kinetic energy of the air has dissipated before it reaches the ground. Nevertheless, the momentum is still transferred onto the ground, but now in a near infinite area. Therefore no net pressure increase could be measured on the ground below the helicopter."
] |
[
"It would have to be exactly the same as the gravitational force on the birds weight to keep it in the air, any more and the bird would climb and hit the roof, any less and it would hit the floor."
] |
[
"The energy could be converted into heat, but conservation of momentum says the plane ",
" become heavier due to the flapping. A force is keeping the bird up; an equal and opposite force is pushing the plane down."
] |
[
"How does Molecular Beam Epitaxy work?"
] |
[
false
] |
Also why is the electron gun necessary? Thanks for your attention.
|
[
"What electron gun? The RHEED (Reflection High-Energy Electron Diffraction)? It has nothing to do with the growth. It's just a way of measuring the current thickness of the film as you grow it. ",
"As for how it works I don't think it's conceptually that different from any other growth technique. A very dilute gas of material is made to condense onto the surface of a substrate. The substrate is kept heated so its surface is quite hot and thus when the gas condenses on its surface it is incorporated into a solid thin film. You can therefore grow thin-films of one material ontop of another with a level of atomic precision based on having extreme knowledge and control over this very slow growth rate."
] |
[
"Thank you. Our teacher said electron gun instead of RHEED and it confused me."
] |
[
"Well the \"molecular-beams\" are really effusors that \"shoot\" a stream of gas particles (just look up effusion,). Now in terms of HOW one produces these gas particles, I believe, though I'm not too knowledgeable on this stuff, one strategy involves using an electron gun to sublimate particles off. A more conventional way, I believe, is to use one of these:",
"https://en.m.wikipedia.org/wiki/Knudsen_cell",
"Regardless, MBE as a technique is separate from how you actually go about creating molecular effusors,. For the purposes of understanding MBE itself, the effusors are just \"black boxes\" that can be assumed to shoot out gas particles. However, your teacher may have been taking an aside and talking about how effusors themselves work and describing an electron gun approach perhaps. I can't find a good google reference atm, but using an e-beam to evaporate off gas particles to make an effusor sounds familiar."
] |
[
"Does the ozone layer affect the color of the sky?"
] |
[
false
] |
I know that the color of the sky is due to Rayleigh scattering so would the color change if the ozone layer were depleted? If so, what color would it be?
|
[
"You are right that the different atomic structure of oxygen (O2) and ozone (O3) affect their absorption properties.",
"What if the ozone layer was composed of a different molecule than O3?",
"What would happen if there was no ozone in the ozone layer? It would be called something else and there would be no ozone. This means no ultraviolet absorption and all of that radiation would reach the surface and be very bad for life. ",
"The type of molecule doesn't affect their rayleigh scattering properties very much as both molecules are very small, as long as there is atmosphere there will be rayleigh scattering."
] |
[
"The ozone layer doesn't greatly influence the color of the day sky. However, you can see the ozone layer at two incidents, both of which taking place at sunrise and -set: ",
"Earth's shadow",
"blue hour",
"In both cases, the blue hue comes from Chappuis absorption, not from Rayleigh scattering. "
] |
[
"That makes sense. So as oxygen levels in the ozone go down, the color of the sky would become lighter and lighter blue until white?",
"So what if there was an abundance of oxygen? Would it become bluer?",
"What if the ozone layer was composed of a different molecule than O3? I imagine a different atomic structure influences the type of light absorbed, right?"
] |
[
"How is a mirror both silver AND able to reflect everything at the same time?"
] |
[
false
] |
[deleted]
|
[
"I believe your question is asking how can a mirror be colored silver, and at the same time show a reflection.",
"The way vision works is that light hits an object, bounces off, and then hits our eyes. When this happens some of the colors of light get absorbed, and what is left gets reflected. A green leaf will absorb every color except for green, which then bounces off for us to see.",
"A mirror is a very very smoothly polished surface, so that light bouncing off does not go in random directions. Instead all of the light rays bounce in the same way, so the light we see is almost exactly the same as what originally bounced off the object. A mirror also does not absorb very much light, so most of it bounces off unchanged.",
"If the silver part of a mirror was roughed up it would start to appear more \"silver\" because the light would be bouncing around randomly instead of all reflecting the same way.",
"TLDR Mirrors don't absorb colors of light, so their color is whatever hits them. "
] |
[
"Mirrors are colorless in the visible spectrum. Or you could say they're whatever color they're reflecting at the time. However, mirrors are definitely not \"the color silver.\" In fact, you could say that the metal called silver isn't even \"the color silver\" as in meaning \"shiny grey.\" Silver reflects basically all light in the visible spectrum, and it does so via specular reflection (where light reflects away at the same angle it came in). Mirrors are just glass with a thin silver backing, so what you get is reflection of all incident light.",
"When we say something has a color, we ",
" mean that it ",
" reflects light of that color. Diffuse reflection is when light penetrates below the surface of a material, is not absorbed, bounces around a bit, and is then directed back out at a random angle. An apple is red in white light because it absorbs all colors of visible light that aren't red and diffusely reflects the red light.",
"As it turns out, metals do not allow light to penetrate their surface, and, thus, cannot truly diffusely reflect. When roughed up a bit, metals can seem like they're diffusely reflecting, but if you zoom in far enough, the light is really just specularly reflecting off at different angles from of all sorts of micro facets. This is an important distinction, because diffuse reflection is often explained as being due to roughness. This is not the whole story. For instance, marble can be polished nearly as flat as a mirror, but it will never lose its white color. ",
"That said, not being able to diffusely reflect doesn't mean that something can't appear to have color. Gold and copper certainly seem to have color, right? It turns out that gold and copper start to absorb the higher frequencies of light in the blue spectrum. So, they tint their specularly reflected light by shaving off some blue. Remember, though, this is a completely different process from how virtually every other opaque material has color (via diffuse reflection).",
"edit: It's worth mentioning that \"the color of an object\" isn't really well defined, so the initial question is a little weird to answer. It's light which has colors, objects just interact with light in interesting ways. In my discussion, I've taken \"color of an object\" to mean \"light which that object diffusely reflects.\""
] |
[
"Not in the same way that an apple has a color. Those silver bars ",
" gray, but they're not gray in the same way that a gray Lego block is gray. If you read my whole post, you'll understand what I'm saying.",
"Another way to put this is that \"silver\" is not a color, it is a description of specular reflection."
] |
[
"What would happen if you melted 2 pieces of uranium that are in different times of their half life together? Would it still decay at their original times?"
] |
[
false
] | null |
[
"The total activity would just be the sum of the activities of the two individual pieces."
] |
[
"So even though they are mixed they would still decay at their original times?"
] |
[
"Yes. Each atom has no \"memory\" of how long it has existed. It still has a probability of decaying in any instant of time which decays exponentially on a timescale set by the half-life."
] |
[
"Is our technological progress accidental?"
] |
[
false
] |
Just wondered if we went back in time and randomly removed from history many of great inventors ( marconi' edison, bill gates, tesla, etc), would we have world going in same direction? Would someone eventually invent all these (maybe a bit later) things or we were just lucky that had these people born?
|
[
"Well with those examples it's pretty easy. Bill Gates is an entrepreneur, not an inventor. As for the others, I don't think we'd have been set back much at all without them. They all had their most important inventions independently invented at the same time or very nearly the same time by other people. They were all embroiled in patent disputes and legal battles over who was first with what. ",
"Some inventors were of course quite ahead of their time and pushed the boundaries a great deal. But most of the late-19th century big names were more-or-less in lockstep with their age, in my opinion. It's certainly not an accident those particular things were invented at that particular time. To take an example of something that was ahead of its time, it'd be Babbage's computers. But that also illustrates why you can't be too ahead of your time: Manufacturing techniques of the day probably couldn't have finished his Analytical Engine. Similarly, Da Vinci had no engines for his flying machines. Etc.",
"I think that in general, we're never short of ideas. It's the ability to put them into practice that's holding us back most of the time."
] |
[
"There is a chapter in Jared Diamond's ",
" that deals with this. His argument is that all of the great inventors we think of generally built their inventions on the backs of the progress made by those before. So he claims that, had an inventor such as Edison never been born, the discovery would have been made eventually.",
"I like to quote Issac Newton about this as well. \"If I have seen further it is only by standing on the shoulders of giants.\""
] |
[
"Here",
" is an interesting discussion on just this.",
"Also note that it is common for separate groups or individuals to simultaneously discover the same thing, which seems to favor a social view of discoveries."
] |
[
"Were \"cavemen\" \"white\"?"
] |
[
false
] |
I know that the Homo genus is thought to have emerged in Africa, and that Homo erectus left Africa and developed in Asia and Europe while Homo sapiens were still developing in Africa. When I see depictions of Neanderthals in the media, they often seem to have more Caucasian than African features, with lighter skin and hair. Do we know anything about the skin color of early hominids, or is it all just speculation?
|
[
"TIL Northern Canadian and Siberian populations didn't become light skinned, even though there was less sun, because they were mainly fishing cultures, and so gained enough vitamin D from their diet."
] |
[
"Neanderthals did evolve light skin. ",
"Some of them even had the gene for red hair.",
" (Edit: ",
"better link for that",
" now I'm not on my phone.)",
"The earliest Homo sapiens sapiens were dark-skinned, though, and ",
"didn't evolve white skin till much later",
" . We didn't inherit from the Neanderthals.",
"So, when Neanderthals and Cro-Magnons coexisted, the Neanderthals were white(ish) and the Cro-Magnons black(ish)."
] |
[
"Yes, the region of where Neanderthals have been living has a large impact on the color of their skin for all the reason you listed. Also, you'd be surprised how much information DNA these days reveals, even though many exterior features can only be speculated. I'm not sure if this is exactly answering your question but I remember reading this article a while ago and it's somewhat relevant: ",
"link"
] |
[
"What is the difference between shock- and soundwave?"
] |
[
false
] |
As the title says, what is the difference? Isn't it basically the same thing, soundwave being a weaker version of shockwave? Like an explosion - you hear the pop due to pressure "front" reaching your ears, be you close enough - you'll get hit, far away - just a soundwave with no real damage potential. Or am I mixing something?
|
[
"I'm a PhD candidate writing a dissertation about this topic now (applied to volcanic explosions).",
"Sound and shock waves are both pressure waves. You can hear a shock, though it may blow your eardrums out if it's strong.",
"Sound waves are infinitesimal in amplitude, meaning that the pressure change of a sound wave is way, way smaller than the air's absolute pressure. For example, at sea level the absolute pressure is 100,000 Pascals (Pa), a quiet sound could be 0.001 Pa, and a painfully loud sound could be around 100 Pa. ",
"The fact that sound waves have low amplitude simplifies the physics a LOT. When amplitudes are not super-tiny, two weird things start to happen. First, the sound speed of the wave starts to vary signficantly, being higher for the peaks in pressure and lower for the troughs in pressure. This means that different parts of the wave move at different speeds: the peaks ultimately overrun everything else and end up at the front of the wave. (This is analogous to an ocean wave breaking as it approaches the beach.) When the peak pressure is at the front of the wave, we say that a shock (discontinuous pressure rise) has formed.",
"The second weird thing is that entropy increases when there's a shock. This contrasts with sound waves, which can be treated as conserving entropy (one of the things that makes sound waves much easier to study). Since a shock increases entropy, it leaves the air behind it hotter than its original temperature. That heat had to come from somewhere--that \"somewhere\" being the energy of the shock wave itself. Consequently, the wave is continuously losing energy to the air as it passes, becoming weaker and weaker. (Sound gets weaker mainly by spreading its energy out over a larger area as it gets farther from the source, though it loses some high-frequency energy to attenuation as well. Shock waves spread out but also deposit energy from heating at the shock.)",
"And yes, a shock wave does move faster than the normal sound speed in the air. ",
"In my models, these effects appear around a few thousand Pa wave pressure. It's a gradual transition between shock and sound though, and it's not easy to draw a line where one side is shock-like and the other side is sound-like. Ultimately, a shock wave will decay until its amplitude is tiny, and at that point it can be treated as a sound wave.",
"And yes, sound waves can turn into shocks as well. For example, if you have a sound wave at low elevation in the atmosphere, as it rises the ambient pressure will drop. Eventually, the ambient pressure will drop low enough that the sound pressure is no longer tiny compared to ambient, causing the wave to break and form shocks. Similarly, if you could focus a loud sound (for example, by funneling it down a tube with constantly decreasing radius), it could turn into a shock wave too."
] |
[
"Great info! I think I get it know!",
"Still a little confused about one thing - does the shock wave traveling faster than the speed of sound mean that the sound will ultimately travel faster as well or you will feel a punch first and hear a sound later? (If so, how can there be no sound from shockwave pressure?)"
] |
[
"It's not really accurate to say that a shock is sound. By definition, sound cannot propagate faster than sound. You could say that they are both pressure waves. ",
"Sound is really a special case of pressure waves where the wave pressure is small enough that the physics become easy."
] |
[
"AskScience AMA Series: I'm Marina Picciotto, the Editor in Chief for the Journal of Neuroscience. Ask Me Anything!"
] |
[
false
] |
I'm the Professor of Psychiatry and Deputy Chair for Basic Science at Yale. I am also Professor in the departments of Neuroscience, Pharmacology and the Child Study Center. My research focuses on defining molecular mechanisms underlying behaviors related to psychiatric illness, with a particular focus on the function of acetylcholine and its receptors in the brain. I am also Editor in Chief of the Journal of Neuroscience, a fellow of the American Association for the Advancement of Science, and a member of the National Academy of Medicine. I'll be here to answer questions around 2 PM EST (18 UT). Ask me anything!
|
[
"Hi! My fist ever AMA answer. I think systems neuroscience is exploding right now because of a lot of new tools. If you have been following the BRAIN initiative news coming out of the White House, the goal has been to develop new ways to study the nervous system, and many of them have enabled us to study neurons as large ensembles, rather than just one at a time."
] |
[
"As a student looking to specialise into neuroscience, what in your opinion are the most exciting new areas?"
] |
[
"How comes accessing one Journal of Neuroscience article for one day from one computer costs US$30.00? Neither the authors nor the reviewers receive any of that money, and taxpayers fund most of the research. Universities waste ",
"millions of USD every year in journal subscription fees",
" (e.g., Harvard's expenditures for library resources in 2008 included $9,248,115 for serial subscriptions. In 2012, this number was up to $16,391,638; French universities paid 172M EUR/5years to Elsevier; etc.)."
] |
[
"Is anorexia naturally occuring, or has it been \"created\" by modern society?"
] |
[
false
] |
What I mean is, has western culture and the ideas of having to be thin to be beautiful created this disorder, or are there cases and accounts of anorexia pre-popular culture (early 20th century) Any ideas, thoughts or anything relating to bulimia and other eating disorders would be interesting too. Thanks.
|
[
"Anorexia was first recorded in 1684 and has appeared across cultures since. This suggests that there is an underlying cause other than just societal impact in the form of a genetic predisposition, such as people who have a lessened pleasure response or even a painful response to the consumption of food, this can cause people to want to eat less, or not at all. Of course certain aspects of society can encourage this kind of behaviour, but it is not the only cause."
] |
[
"To be clear, you're probably thinking of the eating disorder ",
"anorexia nervosa",
". Anorexia is simply the symptom of lack of appetite, which may be caused by any number of non-psychiatric conditions."
] |
[
"Anthropology student here.",
"There are a number of culture-specific 'disorders', meaning certain 'abnormal' behavioural traits have much higher instances of occurrence in some cultures rather than others. Anorexia appears to be one of them, though it's not limited to only Western cultures. It seems to be a result of cultures which stress very slim bodies as being attractive, particularly for women."
] |
[
"Why does a lit candle make a sound when you blow on it a little?"
] |
[
false
] |
[deleted]
|
[
"Sound is practically (just like wind) is a difference in air pressure. You are making sound using your flute/sax because of this (",
"http://www.thefluteplayer.net/basics-flute-harmonics-explained.html",
" or, if you want to be scared: ",
"http://en.wikipedia.org/wiki/Bernoulli%27s_principle",
" ). Blowing a candle is like a the wind blowing a flag but in a milder way; pressure difference between warm and cold air can make a ",
" sound if disturbed by your blow."
] |
[
"A hot object can behave as a sound amplifier. This effect drives the oscillation in the ",
"Rijke tube",
" physics demo: varying the velocity of air adjacent to a heated wire will vary the air's thermal expansion rate, which produces pressure changes in the air (sound waves.) Probably this also causes the loud \"pinging\" sounds when the nichrome wires in an electric heater are first turned on. Speculation: perhaps turbulent air is expanding/contracting as it alters the amount of cooling in the flame, and this produces sound: \"audible turbulence.\"",
"I've always wanted to build an electric \"hell guitar\" with no amp required. Red-hot strings which, when plucked, should generate loud \"Rijke sound\" at twice the string frequency as the hot metal invades the cooler outside air during peaks of its transverse excursions. (or sound at 1f if there's some constant wind parallel to the motion of the strings.) Or, the \"Dante ice-hell\" version, with strings cooled with LN2 which produce sound by sudden air contractions."
] |
[
"Wait. So you're saying that blowing on a candle will create more noise than blowing on a homogenous air space? (I mean, I know this is true... I've been alive for a few decades). Is this essentially the same principle behind what happens when we hear thunder vs. wind? Is blowing on the candle creating a mini weather experiment? Sorry, I never took earth science so I don't know these things and that made me excited."
] |
[
"Why is water in oceans/seas salty?"
] |
[
false
] | null |
[
"All natural water has some salt content, but in fresh water it's just too little to notice. Lakes have rivers/streams that take water out to keep salt from building up. In the ocean, there's no flow of water out, just evaporation, so the any salt dumped in there from the rivers stays there and doesn't go it. It just keeps building up. Some lakes that have no water flow out are also salty, (Great Salt Lake, Dead Sea, etc.)"
] |
[
"To expound on this, the salt originally comes from the ground that rivers carve through."
] |
[
"It is a matter of accumulation. Inland water bodies are constantly \"flushed\" with freshwater from precipitation and all geogenic salinity is transported into the oceans. The latter are always the final receiving waters, thus salt accumulates as it will not evaporate (to form clouds, and rain aso.).\nFor inland salt seas, it is not that they don't have any outflow, but rather that their inflow is too little. Therefore, all water input is lost to evaporation leaving back all solutes (also salt) within the lakes. Practically speaking, this is the same thing that happens in a smaller scale as with the global water cycle. "
] |
[
"How would splitting an atom in a nuclear bomb affect the universe - using our understanding of string theory?"
] |
[
false
] | null |
[
"What reason do you have to think that this might occur?"
] |
[
"What reason do you have to think that this might occur?"
] |
[
"Got to thinking about the dangers of nuclear weapons and their effects in the macro sense. You can't just be messing with particles like that without considering quantum entanglement. ",
"Now the rest of my thought process for this one unravels into science-fiction (as we see it now). ie UFO's, aliens, blah blah"
] |
[
"Are there any Earth sized object in space that behave like comets?"
] |
[
false
] |
Are there any really huge objects out there that are just flying around through space the way comets do?
|
[
"What do you mean flying around through space the way comets do?",
"Earth does fly through space the way comets do, with the exception that comets are more highly elliptical and sometimes off the plane of elliptic. ",
"But there are no Earth-sized comets in our solar system, at least not that we've discovered. "
] |
[
"I mean are there any planets that don't have their own star? Flying through interstellar space.",
"Yes, there are. But those don't inherently behave like comets - they're not similar really. They're called ",
"Rogue Planets",
".",
"Are all object in our solar system from only our solar system? Do they ever come in from outside it?",
"We don't know, conclusively, if anything in our solar system originated in another system. We could, theoretically, verify this by measuring the chemical composition of every object in the solar system, but to date we haven't had the chance to do so. "
] |
[
"I mean are there any planets that don't have their own star? Flying through interstellar space.",
"Are all object in our solar system from only our solar system? Do they ever come in from outside it?"
] |
[
"What is observation?"
] |
[
false
] |
So obviously with all the Higgs discussion going on surrounding CERN's exciting announcement today, I've been reading (and re-reading) a lot about quantum physics in general. Very fascinating stuff, even for an uninitiated layman such as myself. I feel like I can grasp some of the major concepts and discoveries involving this branch of science, but it so nuances, and in some cases rocks, the very fundamental assumptions we make about the physical universe that some of these topics are endlessly elucidated by continuous and repeated explanation. One such topic, for me at least, is the role of observation in quantum science. I've been lucky enough to have some very intelligent people, including quantum physicists, explain in straightforward but rigorous terms certain implications of concepts like quantum entanglement, the double-slit experiment, superpositions, etc. One of the revelations, as I understand (please brutally correct me if I'm wrong), is that the observation of a system is endogenous to that system; i.e., in some cases the act of observing an experiment affects the outcome. So is this simply the result of the methods of quantum physics? Is it possible to observe these phenomena adding energy to or interacting with the systems in which they occur? And if so, what would we see? If not, what exactly is observation, and why is it that way? Does quantum science affect the philosophy of science and how? I know this kind of question could quickly devolve into mystical sorts of speculation very easily. I have to be clear that's not the kind of discussion I'm looking for. I'm hoping can (as usual) smack down my misunderstanding and give me some solid grounding on the fundamentals of science from experts who know what they're talking about.
|
[
"Searched",
"Relevant ",
"discussion",
"Original comment by ",
"Burdybot",
"My friend and I are currently arguing over this concept. He says that an observer requires consciousness to determine the state of a system according to quantum superposition. I say that an observer does not have to be a living, conscious entity, but it could also be an apparatus.",
"He also cites the idea that God is the only being with infinite observation capacity, and when God came into existence, that observation is what caused the Big Bang (he's agnostic, not religious; just said it made sense to him). I also disagree with this.",
"Top comment courtesy ",
"ABlackSwan",
"The idea of \"observation\" in quantum mechanics is one of the most misunderstood concepts in physics.",
"Observation in the case of QM can reduce to \"interaction\". Anything that will collapse the wavefunction of a particle can be classified as an interaction. (Let's ignore weak measurements...they are interesting but not my expertise and are a complicating factor)",
"An example for your friend: If we think about the double slit experiment. Say we have a beam of photons that get sent at the slit one at a time and behind the double slit is a film badge that can record the hits of individual photons (after you develop it perhaps) If we fire a photons individually (or an electron, or whatever) at a double slit we get a diffraction pattern visible on the film (this is because the photon, travelling as a wave, will go through both slits and interfere with itself before hitting the film).",
"When we try to \"observe\" which slit the photon/electron/whatever went through, this pattern disappears. This is because to \"observe\" the photon we need to put some sort of instrument in front of one of the slits that detects photons. Let's say that when a photon hits this instrument it sends a file to a physicist's computer and says \"AHA! The photon went through the right/left slit!\". This of course, via my and your friend's argument would constitute a measurement. The photon both interacted with the instrument (my def'n) and a being with consciousness saw the result (your buddy's def'n). So, we are in agreement, a measurement has been made, the diffraction pattern on the film disappears.",
"Let's say the physicist wants to get LOTS of data, but is rather tired. So, he sets up the experiment and once it starts he leaves the apparatus alone and let's the computer keeps track of which slit the photon goes through. Now, I say this is still a measurement and the diffraction pattern will not be on the film, but your friend says no measurement was done, and so the diffraction pattern will be visible when the physicist comes back the next day to develop the film.",
"This type of experiment has been done many times, and never ever in the literature does it say \"When the grad student was around, we got no diffraction pattern, but when he left to get a cut of coffee, it reappeared\".",
"This idea of an observation having anything to do with sentience is completely refutable.",
"EDIT: spelling...stupid english"
] |
[
"I understand that \"sentience\" and \"consciousness\" have nothing to do with this topic, but I'm still wondering what observation actually is. ",
"The top comment of this thread",
" begins to get at what I'm asking, I think. But is it theoretically possible to observe quantum particles without interacting with them and forcing them to behave classically?",
"Also, this is a pretty awesome bot."
] |
[
"The act of observation collapses the wavefunction, because it destroys the superposition of states. In fact, ",
". \"Observing\" them without interacting with them is giving the trivial solution - that is, in the case of the double slit experiment - the particle goes through both slits.",
"Imagine you're playing the slot machine. Each window is in a superposition of state of all the possible symbols it can land on. In order to observe a window, you ",
" to stop it such that it lands on one particular symbol. A continuously spinning slot machine offers no information - so all you can say is \"this has x chance of landing on this symbol.\""
] |
[
"Why can we use microwaves to heat our food, when microwaves have less energy than visible light?"
] |
[
false
] |
[deleted]
|
[
"The general idea is correct here, in that we use microwaves to excite water molecules, which then relax from that excited state by dumping energy into translational energy, which is heat. But one clarification to make is that microwaves don't excite water into vibrationally excited states, but rather rotationally excited states. See ",
"\"Electromagnetic absorption by water\""
] |
[
"It's not really so accurate to say microwaves have less energy than visible light. You can have any amount of energy in microwave radiation, just like you can have any amount in visible light. It just comes in smaller increments (i.e. photons) when you use microwaves.",
"In a nutshell, visible light doesn't work because it gets reflected, whereas microwaves tend to make it into the interior of the food. (Of course that's only the simplest version of the story.) See ",
"here",
" for more information."
] |
[
"Basically its like microwaves are pingpong balls (lower energy) and light would be basket balls (high energy). If I hit you with one ping pong ball, nothing. If I hit you with a basketball, you notice it. If I pelt you with 10million ping pong balls you will really notice it. Kinda the same thing. It is not the amount of energy EACH photon has to deposit, it is how MANY photons there are to deposit energy. A microwave creates a high density flux of a large number of lower energy photons that will each deposit all their energy into the substance causing the average kinetic energy of the substance to increase. That is what heats it up. "
] |
[
"Is burning gasoline carbon-neutral?"
] |
[
false
] |
Presumably, (this is where a potential correction fits) most of the carbon dioxide released from the burning of crude oil and its derivatives was at one point stolen from the atmosphere and used by living organisms that became fossil fuels. Wouldn't this mean that burning gasoline is, in fact, a carbon neutral activity? (I am not considering the toxins and other compounds it would release and obviously have an ill effect on the air. It's very obvious that gasoline is a proven pollutant, I'm asking whether it's carbon-neutral.)
|
[
"Gasoline comes from petroleum, which is taken from the ground. On a human timescale it's not carbon-neutral at all, but on a ",
"timescale of 100-200 million years",
", then yes, carbon does recycle. But that's not the intent of the term \"carbon neutral\", it's intended to refer to the human habitat."
] |
[
"Others have answered well regarding the timescale.",
"The big semantic issue is the term \"carbon neutral\" - the term is generally used in the very-short timescale. On the order of \"less than a year.\" Such that the amount of CO2 in the atmosphere is the same \"immediately before that quantity of fuel enters the economy\" as it is \"after that quantity of fuel is consumed.\"",
"So biodiesel created from waste vegetable oil is carbon neutral, because the CO2 from the vegetable oil came from a plant source that grew extremely recently, was harvested extremely recently, and turned in to fuel recently. That CO2 was in the air less than a year ago. Now it's in that vegetable oil. Once the biodiesel is burned, it is in the air again - the same state as a year ago. (Of course, that grossly simplifies - the actual CO2 molecules are destroyed and reformed a few times in the process, so it's not like you can trace a single CO2 molecule through the process. But it would be theoretically possible to trace a single Carbon ATOM from CO2 in the air to some compound in the plant (a sugar, most likely) to some compound in the vegetable oil, back to CO2 out the exhaust pipe. Theoretically, anyway - it's beyond our technology at the moment to trace that single atom...)"
] |
[
"I don't mean the atmosphere as a sentient being. Sorry it sounded that way. I mean it, and the ocean, as a thermoregulator. But you're right, that was poorly phrased on my part. ",
"And of course all life isn't going extinct. It will just displace billions of humans, especially poorer ones, and kill off many more species than we already do. Life as in what's here now will suffer greatly. What may come after, who knows? If there's massive human die offs from crop failures and desertification, maybe in a few hundred years there'll be a massive increase in biodiversity to fill the void dead people left. It's that kind of extreme disruption we're trying to prevent.",
"It's not the isolated effects of any one gas or any one action, but the net total of our activities resulting from fossil fuel extraction and use. "
] |
[
"Why are there no freshwater octopuses or squid?"
] |
[
false
] |
There are freshwater crustaceans, gastropods, and bivalves. Why are there no freshwater cephalopods?
|
[
"Living in freshwater requires that you have the ability to excrete fluid from your cells efficiently, because osmotic pressure causes water to constantly move into the cells. Why they haven't evolved to deal with this.....honestly I have no idea. It really doesn't make sense does it?",
"I will note that this isn't a rare phenomenon. The majority of groups of animals are limited to marine environments."
] |
[
"Interesting observation.",
"You might try soliciting opinions from ",
"r/biology"
] |
[
"The majority of groups of animals are limited to marine environments.",
"That really makes sense when you think about it. Freshwater is innately familiar to us, since we drink it and live near it, but it's easy to forget how niche of an environment it is.",
"Follow-up question: I wonder if the general rareness of freshwater adaptation can be attributed to the fact that, in evolutionary terms, sources of fresh water are relatively fleeting. What's a temperate river today is something completely different during an ice age, for example."
] |
[
"If an object is approaching you at a speed close to the speed of light will the color frequency of the reflected light change due to the Doppler effect?"
] |
[
false
] | null |
[
"Yes. It's called relativistic blueshift, or in general relativistic Doppler shifting. "
] |
[
"Sorry, my wording was confusing. I meant that in the example about measuring relative velocities of stars we were concerned with radiated light rather than reflected light.",
"Reflected light is shifted, too. This is how radar guns (like the police use to catch speeders) work. That would have been a better example, but I was reading something about stars right before I replied so that's the example that came to mind. In the speeding car/radar gun example the shift is small enough not to be noticeable visually, but you can still measure it."
] |
[
"EDIT: We learn the relative velocities of stars using this phenomenon, although in the case of measuring relative velocities of stars, we are concerned with radiated light rather than reflected light.",
"We can look at a spectrograph of a star and by the relative positions of the peaks, we know what each peak corresponds to and what its wavelength should be in the absence of relativistic shifting. By comparing what the wavelength of the peak should be and what the wavelength of the peak actually is, we can determine the relative velocity towards or away from Earth."
] |
[
"If you stack layers of graphene does it turn into graphite?"
] |
[
false
] |
I've heard things like, "graphene as thin as saran wrap could hold an elephant balancing on a pencil". But if graphene is just a single layer of graphite how could you stack it and still have graphene (if that's even possible).
|
[
"Correct. Graphene layers only weakly bond to each other via ",
"Van der Waals",
" forces and once you have several layers you really cannot call it graphene anymore. The properties of multilayer graphene are drastically different from single layer. Stacking enough layers to make saran wrap would result in a graphite material that would definitely just break when an elephant stood on a pencil. In fact I work with pieces of graphite this thick (its how we get graphene) and they easily break when you try to grab them with tweezers.",
"What these statements are actually trying to say is that if you scaled up a single layer of graphene, it would have extraordinary mechanical properties. You make a good example of how bad, confusing and unphysical this analogy is. Scientists tend to struggle to explain things to others without just confusing the matter."
] |
[
"One word answer: yep. "
] |
[
"Thanks for clearing that up. It would have been embarrassing telling people how tough graphene is if you stacked it up."
] |
[
"Can a vacuum cleaner interfere with the broadband connection?"
] |
[
false
] |
Whenever I'm in my girlfriends house and she vacuums, her broadband connection goes down. It happens every time and I find it a bit strange. Can anyone explain this?
|
[
"Suppression. Capacitor across motor supply. "
] |
[
"Electromagnetic interference. The vacuum cleaner generates electromagnetic emissions (radio signals) that interfere with the broadband device(s) by inducing currents that aren't supposed to be there.",
"What kind of broadband? Fiber-optic to the house? 3G? ADSL?"
] |
[
"Fiber-optic broadband to the house. Its very annoying because it keeps going regularly afterwards."
] |
[
"Is it possible that previous human civilizations used technologies made from organic materials, leaving no trace, therefore were more advanced than we are currently aware of?"
] |
[
false
] | null |
[
"I've asked my self that question many times as well. A new question rised for me as well. We now use technology to store our knowledge such as servers, hard-drives, memory chips/cards. Say civilization sort of collapses in on itself, and thousands of years later new advanced civilizations sprawl. Will they have the ability to extract the data from said technology, or will they think it was just furniture of some sort?"
] |
[
"Yes. At least from a cultural perspective. ",
"Look at cultures like the ",
"Mound Builders",
". If they hadn't decided to move earth around, we would probably have no idea they existed. ",
"Just like we don't know much about who built ",
"Gobekli Tepe",
" becuase it was the only stone thing they seemed to have built. ",
"If a culture didn't leave behind permanent writing or structures, the can be lost to history."
] |
[
"OP is asking a different question than the one in the link. Although taken literally this question is unanswerable, the closely related question:",
"'Is it possible that previous human civilizations used technologies made from organic materials, leaving no trace ",
", therefore were more advanced than we are currently aware of?'",
"is very interesting and the subject of ongoing archaeological research."
] |
[
"GPS requires microsecond clock accuracy in the satellites but the terrestrial receiver accuracy doesn't matter at all. Why is this?"
] |
[
false
] | null |
[
"Think of it like a math problem where you are trying to solve for your coordinates (X,Y,Z) ",
"If you know the current time, you can solve this by comparing the signals from 3 satellites, which have a known position and broadcast their current time.",
"It takes longer for the signal to reach you from satellites that are further than you, so you can compare the current time to the time the signal was set out and determine the distance. ",
"Each satellite signal creates an equation involving X, Y and Z. With 3 equations, 3 unknowns, you can solve for all 3. ",
"This would work if you had a super accurate atomic clock on board. ",
"But you don’t.",
"So instead, treat the current time as an unknown variable as well. Now you have 4 unknowns. With signals from 4 satellites, you can solve for 4 unknowns (X,Y,Z,T) ..... where T is the current time. "
] |
[
"The GPS satellites transmit their clock signals. Anyone can pick them up and have a super-accurate stable clock.",
"I used to work in a radio-frequency lab. An RF lab that needed a stable frequency reference, typically 10MHz, which was carried on coax cables to all the instruments in the lab. Each instrument would use that 10MHz reference frequency when generating any other clocks or carrier frequencies that it needed.",
"Before GPS, the lab would have had a single highly-accurate and expensive frequency reference feeding the system. Possibly a crystal oscillator, like in your Timex, but housed within a temperature-controlled \"oven\" to reduce drift. Or, if the ultimate in accuracy was needed, they would have an atomic clock of some kind.",
"After GPS, you had dozens of super-stable atomic clocks floating around overhead. You just needed one small, inexpensive box to tune in the reference clocks from the GPS satellites."
] |
[
"To add to this, because each satellite transmits the time it is possible to compare the arrival times of signals from different satellites. If you can see more than 4 satellites, you can work out the actual time at the receiver without needing a super-accurate clock at the receiver.",
"By comparing the delays (due to the speed of radio signals which is roughly equal to the speed of light) of each signal you can work out the distance from each satellite. If you draw imaginary spheres at the distance from each satellite, they will intersect at 2 points. Only one of these points will make sense by being somewhere near the surface of the Earth, so that is the point where the receiver must be."
] |
[
"Are there any reactions more exothermic than nuclear fusion of hydrogen to helium?"
] |
[
false
] |
is hydrogen the only element that can be tempted to fuse?
|
[
"I suppose it depends how you're defining 'exothermic.' Do you want the most energy/fuel mass? Or most energy/reaction? ",
"To the extent of my knowledge, hydrogen fusion is the most exothermic nuclear reaction ",
". It's pretty easy to see from the ",
"binding energy per nucleon.",
" Since the sun fuses by the proton-proton chain, which is actually a series of hydrogen fusions, let's just focus on something we know goes from hydrogen to helium: the ",
" reaction. Judging by that plot, the average binding energy per nucleon goes from about 3 to 7 MeV. Sure enough, this reaction releases about 17 MeV. ",
"You can make other elements fuse exothermically as well, but not with as high an energy release per nucleon. Just reading that plot, fusing to any product smaller than iron will release energy. ",
"But other reactions containing many more nucleons can give you more energy, for example, Uranium-235 contains literally hundreds of nucleons. The fission of U235 releases about 200 MeV, nearly 10x more than hydrogen fusion!",
"And aside from fusion, matter-annihilation comes to mind as ",
" exothermic, but that's another story. "
] |
[
"In a sense, yes, there is exactly one of me."
] |
[
"To fuse, nuclei have to collide. I'm not saying it's impossible, but how often do you think 26 nuclei collide at once?"
] |
[
"Is it feasible to grow plantlife on mars in an attempt to oxygenate the planet?"
] |
[
false
] |
If not, is there any other idea as how to Increase martian oxygen levels to 20% or higher? Some sort of "Total Recall" scenario?
|
[
"We would have to increase the atmospheric pressure first. As is, there is not enough atmosphere to support any earth-based plant life."
] |
[
"Most organisms that can ",
" in space cannot actually ",
" in space. They go into some sort of hibernation or other method for surviving extremely harsh conditions. Even if they can function to some degree, it would be only for a limited amount of time.",
"And a cyanobacteria/tardigrade hybrid doesn't really make sense. One is a photosynthetic bacteria, one is a multicellular animal. "
] |
[
"There are a number of exceedingly challenging things about mars that are often glossed over in the popular science/scifi presentation. There's no magnetic field, so little protection from the charged cosmic rays. The planet's small and the combination of small planet and no magnetic field seems to mean that the atmosphere is likely to be stripped quickly. While there still seems to be ",
" water on the planet, it doesn't seem like ",
" water to do anything significant with. ",
"If we do stuff on Mars, I'd bet on caves and greenhouses. Not the whole planet."
] |
[
"Could there be any subterranean oil or gas reserves deep enough to come in contact with mantle? If so is there any evidence of an underground explosion occurring?"
] |
[
false
] |
Just a random thought I had. Couldn't find anything about it online.
|
[
"No. Oil and gas form within a rather restrictive temperature range, commonly referred to as the ",
"\"oil window\" of between ~60-160 degrees Celsius",
". A normal ",
"geothermal gradient",
" in the earth is ~25 degrees C per kilometer, so the oil window in most places occurs at depths of ~2.5 - 6.5 km. By the base of the lithosphere and the upper portion of the mantle, the temperature is ~1700 degrees C, so a bit out of the temperature stability range for oil and gas. At the higher temperature part of the oil window (sometimes instead called the gas window), ",
"natural gas forms through thermal cracking",
". Beyond that temperature, hydrocarbons are not produced."
] |
[
"One thing to keep in mind, heat + oil does not equal an explosion. You still need some form of oxidizer to create combustion and an explosion, like Oxygen."
] |
[
"What would happen if a Volcano formed through a deposit of natural gas or oil? "
] |
[
"What causes you to feel terrible during a hangover?"
] |
[
false
] |
I know a big part is dehydration, but what exactly makes you feel like sh*t?
|
[
"Besides dehydration, it's also the highly toxic and carcinogenic chemical ",
"acetaldehyde",
", which enzymes in your liver turn alcohol into. Normally this is turned by your liver into acetic acid, the primary ingredient (besides water) of vinegar, but not all of it is properly converted, which contributes to hangovers."
] |
[
"Alcohol is turned to acetaldehyde by alcohol dehydrogenase, and then acetaldehyde is turned into acetic acid by acetaldehyde dehydrogenase. The production of alcohol dehydrogenase is usually sufficient, but the production of alcohol dehydrogenase is limited, so acetaldehyde accumulates in human body and causes negative effects."
] |
[
"This cannot be a good answer because I (like many people) have been *much* more dehydrated for non-alcohol related reasons than I ever got drinking alcohol, but the hangover symptoms are very distinct."
] |
[
"Could you feel the gravity from a teaspoon of Neutron Star matter on Earth?"
] |
[
false
] | null |
[
"I would strongly recommend against this experiment as I am a little overly attached to this planet. A few things first.",
"The instant this mass was removed from the star it would expand violently, very violently, so violently that I cannot even begin to explain it in real life terms, if this helps here is the math 10",
" J...<- this is about 1,000,000,000,000 nuclear bombs being detonated in the same square centimeter at the same time. Put it this way the resulting explosion would definitely destroy all life on this planet, and possibly knock it out of orbit.",
"Assuming it could be contained, the gravity from said world ending teaspoon of doom would have a exponential gravitational effect around it. Its like a sliding scale, @ 50 feet you might not feel it, @ 30 it would tug slightly, @ 20 it would feel as if you were at 1.5 Gs, @ 10 feet 2 gs, @ 1 foot it would compress your hand as you reached for it, pull you into it and compress you to a single point. THESE NUMBERS ARE ESTIMATIONS, PLEASE REFER TO YOUR OWN NEUTRON STAR FOR ACTUAL RESULTS.",
"Edited for formatting."
] |
[
"A teaspoon of material from a neutron star will be terribly unstable outside a neutron star. However, if it were stable, than the force on a 50 kg man from 1E12 kg 1 meter away would be 7E-11 * 50 * 1E12 / 1",
" = 4000 N (all calculations done with 1 significant digit). The force of attraction will be equal to the weight of 400 kg on Earth. That's roughly the weight of a big motorcycle."
] |
[
"Having trouble understanding your result.",
"Are you saying someone would feel as if they were being pulled with the force of 400 kilograms? Or that the attraction is equivalent to standing near a large motorcycle? ",
"Or is it something else because both of those results make no sense.",
"A teaspoon full of neutron star matter is generally given as something around the mass of Mt. Everest (give or take). Clearly standing near Mt. Everest has nowhere near the effect of pulling those near it with 400 kilos of force. Hell...the whole earth pulls a 50 kg man with...you guessed it...50 kg. Where is the other 350 kg coming from?",
"If you are saying the pull is akin to a large motorcycle that cannot be true either. The matter has the mass of a mountain, not a motorcycle.",
"Maybe I am missing something..."
] |
[
"Do chickens or other birds have unique “voices”?"
] |
[
false
] |
[deleted]
|
[
"Yes! I can easly tell apart some of my chickens by the way they cluck. Its a change in pitch in some. In others they may draw out their cluck longer or just cluck in a certain definable pattern. Somechickens sound very similar however.",
"Edit: cant to can"
] |
[
"Yes, my parrots most definitely did. It was most noticeable when they would talk. I could tell who was speaking very easily, even if I couldn’t see them. Their calls were also quite different, even if they were the same breed. As for chickens, I can’t speak to their differences.",
"Edit: also, flicks in the wild have completely different “dialects” that they speak. One flock would have zero idea what another flock was saying due to sound, call differences, etc. "
] |
[
"Interesting, I wasn’t aware about the dialects either"
] |
[
"Is there a limit to how strong materials can theoretically be?"
] |
[
false
] |
People have managed to create extraordinarily strong materials such as carbon nanotubes and graphene, as well as other materials such as carbyne. But, in the future, it is possible that we could create future materials that are even stronger? Is there a limit to how strong a material can be, and of scientists were to create the strongest substance that is physically possible, how strong would it be?
|
[
"Yes, if you're talking about regular molecular matter that we can create and manipulate. You're limited by the strength of molecular bonds, which have well-defined and known values.",
"It turns out that carbon-carbon bonds tend to give you the best strength to weight ratio due to their high intrinsic strength and the low density of carbon. There are stronger bounds but due to the greater material density and/or geometric limitations in how those bonds form networks, the strength to weight ratio isn't as good.",
"Carbon nanotubes and graphene are scraping the limit of how strong a material can be. There are some largely theoretical materials with boron and/or nitrogen than might be slightly stronger in some measures but it's very unlikely that we're going to create anything that significantly exceeds the theoretical strength of nanostructured carbon.",
"Of course, it is possible that there are forms of matter possible that do not rely upon molecular bonds for their integrity such as nuclear bonding, weird quantum mechanical voodoo, etc. However, we know of no such materials and if we ever do get the ability to construct such materials, it is a long, long way in the future."
] |
[
"The highest possible strength-to-weight ratio is c",
" = 9*10",
" kYuri. The highest known is the colossal carbon tube at 59,483 kYuri. So the theoretical limit is about 1.5 trillion times stronger. ",
"Source"
] |
[
"How is that ratio calculated?"
] |
[
"[Mathematics] What is the problem with a dictator in the Arrow's Paradox in ranked voting? Wouldn't the dictator change each election, so that it isn't just one person forever in charge?"
] |
[
false
] | null |
[
"I am not sure what you are asking. \"Dictator\" in this sense means that the overall rank-order, which is ostensibly a function f(x",
", x",
", ...., x",
") where x",
"'s are the individual rank-orders of the voters, is really just a function of one particular voter's rank-order. That is, f = f(x",
"). That one voter is fixed and called the dictator.",
"\"Dictator\" does not mean that the overall rank-order happens to match an individual voter's rank-order. If x",
" is the dictator, then the overall rank-order always matches x",
" no matter what."
] |
[
"Ok, so in a system with the two conditions needed to ensure a dictator, even if everyone else voted E>D>C>B>A, in this case if x put A first, then A would still win?"
] |
[
"Yes that's what it means for X to be a dictator. "
] |
[
"If a blackhole has the same gravitational pull of a star with the same mass, how come light can excape the star but not the blackhole?"
] |
[
false
] | null |
[
"The gravitational pull depends on the mass that is pulling and the distance to the pulled object.",
"A black hole is much more concentrated than a star.",
"In the case of a star if you get close to its surface most of the star is still far from you.",
"In the case of a black hole, since its so concentrated, if you get close to its event horizon, you are very close to all of its mass.",
"Hope this helps."
] |
[
"While the comments here are fine, it is important to note one thing that makes black holes very different from stars. In general relativity, gravity is the geometry of space time. A star will form a gravity well, but not a singularity. The singularity is what makes a black hole so different. Since all its mass is concentrated into one point (at least in the simplest version of a black hole), the event horizon is where space time actually bends back on itself, and thus nothing can escape (barring Hawking radiation)."
] |
[
"When you move inside a spherical shell of uniform mass, the gravitational pull cancels out. If you were able to push an infinitely insulating and strong tube through the center of the sun, you could hang out in the center with zero gravitational pull from it. In a black hole, all the mass from the star is concentrated in a tiny space, a few km to a few hundred km in diameter, so you can't get inside any shell of matter, since you would need to be past the event horizon."
] |
[
"How long it takes for crude oil to form?"
] |
[
false
] |
I'm just fighting with this god believer guy As far as I know it takes millions of years for it to form but why does it take that long? How do scientists tell that it takes that long?
|
[
"The amount of time it takes to form crude oil is variable because it depends on how quickly you can bury organic material, how deeply it becomes buried and heated, and the pressure that the organic material experiences. These processes are controlled by tectonic activity, which occurs on millions-of-year timescales. "
] |
[
"It takes that long because it is such a complicated and complex process. You need the perfect tectonic environment which has formed over millions of years to create a basin structure capable of trapping carbon-rich material. You then need the perfect climate to cause this material to decay without being eaten by biogenic organisms or be diluted and washed away. Then this material must be buried, so you need another change in climate/sea level/tectonic regime to bury this carbon-rich, organic material under kilometres of sedimentary rock. This sedimentary rock then must be further buried (caused by another change in tectonic regime), to bury the organic material to such a depth (many many kilometres) that it starts to break down from its kerogen form and generate hydrocarbons. These hydrocarbons then need to be able to migrate out of this \"source rock\" and find their way into a porous bed of rock where they are hopefully trapped and accumulate. ",
"As you could imagine, the only way this feasibly is possible is if it occurs over millions of years.",
"Proof: Attempts to recreate oil/gas generation in the lab by simulating similar pressures and temperatures such a source rock would experience results in the carbon usually being lost or \"over-cooked\". i.e. there is no substitute for time in this instance. "
] |
[
"The correct answer is ",
" - people who have strongly held (wrong) beliefs don't change their mind when presented with evidence. It's like arguing that night is day. There is a link to another reddit discussion on the subject of belief vs reality:\n",
"http://www.reddit.com/comments/262n45"
] |
[
"Is storing energy mechanically (like winding a watch) less efficient or expensive than batteries?"
] |
[
false
] |
I was wondering if it would be practical to store solar energy using a mechanism that is analogous to a wind-up watch. Would a mechanism like this have to be enormous? Would it have to be made of incredibly expensive components? Would it break down easily?
|
[
"Springs can be very reliable and efficient over a small travel range when unimpeded. However, they're not very dense – particularly if you're trying to use them over their more-efficient range or in an open, unimpeded arrangement.",
"Watch springs are fine if you're only trying to store a couple joules. You only need one spring and lightness is more important than raw density.",
"Chemical (battery) and thermal (molten salt) storage are likely to be far simpler when you're dealing with a scale of multiple mega or giga joules of energy storage which would be typical of medium to large solar energy storage systems.",
"There are specific kinds of spring arrays that have been proposed for dense energy storage:",
"http://en.wikipedia.org/wiki/Carbon_nanotube_springs",
"but as with everything involving carbon nanotubes, it's mostly speculation/pipedream at this point in time."
] |
[
"When you say turbine I've got no clue what you mean. Do you mean an object with mass that is set to spin on a spindle? ",
"Like if I had a big cylinder of granite on a spindle? ",
"If that is what you are talking about, having low friction bearings and a vacuum on the inside ends up being a very efficient form of storage. It's good to help balance out when power supply is unable to adapt well to demand. Solar and wind energy both benefit from having energy buffers. "
] |
[
"From the description that would be one type of turbine. Stores energy as mechanical energy traveling in a circle.",
"It is good for short term, long term that bearing is a bad vampire load, and no vacuum is ever perfect and creates new loads.",
"Very efficient for short periods of time but your energy leaks away surprisingly fast. With solar we know it will be accessed within a few hours, so not that long. Living in California I would have trouble trusting the reliability of turbine for home energy storage, an earthquake would wipe out a lot of energy or require such sizeable isolation that the size would be a problem."
] |
[
"How close could two planets get before they would start to pull each other apart?"
] |
[
false
] |
I know hypothetically they would just crash into one another but if they were somehow held at a distance would their gravity wells pull things off the surface of the other planet? Would there be some sort of gravitational limbo in between where they mixed?
|
[
"it's called the Roche Limit and it depends on the masses",
"http://en.wikipedia.org/wiki/Roche_limit"
] |
[
"http://en.wikipedia.org/wiki/Roche_limit"
] |
[
"very cool thank you sir!"
] |
[
"How does the human body separate waste into solids and liquids and why?"
] |
[
false
] | null |
[
"Urinary 'waste' and gastrointestinal 'waste' are two fairly distinct entities.",
"Stool is mostly just the undigested portions of whatever you ate, with some added biliary secretions and gut bacteria added in. Think of it as 'waste' in the sense that some of what you're eating is 'wasted', rather than getting digested.",
"Urinary 'waste', on the other hand, is the result of a very complex filtration/reabsorption/secretion system from the blood to the urine. Urinary solutes include many metabolic byproducts, as well as electrolytes to maintain body homeostasis. Urinary regulation is also a key component of body water content control, as its variable range in concentration can help the body conserve water when needed and excrete water when there is excess. Think of urinary 'waste' as more like 'toxic waste' - something the body is actively trying to remove. (This is a grossly oversimplified view of the kidney - look ",
"here",
" for more info.)",
"To sum up, everything you eat goes through the GI tract and some of it passes through unabsorbed and becomes stool. Some of what you eat and absorb into your bloodstream can eventually be excreted through the kidneys as urine."
] |
[
"So basically if the GI tract was more efficient we'd need to poop less often?"
] |
[
"It does not have much to do with efficiency- the majority of the nutrients that you eat are absorbed (unless you have some kind of inflammatory bowel disease or other problem). They've calculated the efficiency, and it was mentioned in a lecture back in med school. I can't remember the number but it was over 90% for fat/carbs/protein.",
"If you eat food with almost no fiber the number of bowel movements you will have will go down significantly. It won't be zero though, because feces is also formed from the continual sloughing of the mucosal surface of the lining of the intestines. I would not recommend doing this though, as there are health benefits from dumping regularly.",
"source- doctor"
] |
[
"What is gained from having different blood types? [Biology]"
] |
[
false
] | null |
[
"There may not be any advantage at all. (Not everything that is around today has evolved because it is advantageous.) ",
" (Edit: we have a good idea of why they exist, which I wasn't going to get into, but short version is that different people in different parts of the world were exposed to different kinds of disease, and over many generations blood types evolved specific to that part of the world.) It is just one of many MANY traits that animals have, that appear to have arisen randomly from disparate populations of the same species. ",
", I should point out that the process of evolution is dumb. It does not have an end goal. It does not \"know\" anything. It is just an observable phenomenon, like gravity, that we can describe and measure. It doesn't have a \"purpose\".",
"Evolution produces a lot of stupid shit. ",
"A few great examples here",
" but there are so many more, and very possibly blood types is one of them.",
"I can explain more about evolution and provide useful links if you have questions."
] |
[
"That list of \"The ten worse evolutionary designs\" is simplistic to the point of almost being wrong. To be sure, each of those things is likely not an ideal design, but it is a trade-off of a larger advantageous adaptation. To deconstruct a few of their examples:",
"[1]. It basically says that \"sea mammals should have gills, because that makes sense!\" but totally ignores the ",
" advantage of breathing air: namely, oxygen is at much higher concentrations in air than water, which is the (hypothesized) reason that the biggest animals in water tend to breathe air. ",
"[2]. The pseudo-penis of female hyenas is an ",
"adaptation that prevents forced copulation, which allows the females to choose partners that are fitter and not related to them, preventing incest.",
"[7]. Humans stomachs can't digest cellulose not because the bacteria needed for that are too \"busy helping termites,\" but because our digestive systems have evolved to digest a greater variety of foods. Evolving to eat cellulose ",
"would require such specialization(think cow stomachs or horse colons)",
" and change in our digestive systems that we would likely lose the ability to digest the variety of foods we now eat."
] |
[
"That said, one idea that comes to mind for how it might be advantageous is that having diverse blood types might make life a bit harder for blood-borne pathogens. If everybody's blood cells are flagged with different antigens it becomes more complicated trying to disguise itself from the immune system when hopping from host to host.",
"Just an idea that pops to mind. Probably not easy to test a theory like that, though perhaps one might study blood-borne pathogens in animal species with more or less diverse blood type systems to see if there's a correlation.",
"Here's an article listing off blood type groups in some nonhuman species",
", might be useful."
] |
[
"What would be different if Homo Sapiens had gone extinct instead of Neanderthals?"
] |
[
false
] |
[deleted]
|
[
"This is really an impossible question to answer without wholesale speculation. We do not actually know enough about Neanderthal intelligence or culture to make many conclusions."
] |
[
"Since this is purely speculation, I strongly recommend ",
"The Neanderthal Parallax",
" trilogy by Robert J. Sawyer. ",
"This deals with your question in detail. An alternate Earth where Neanderthal man survived into a technological age, and a rift opens between that world and ours. It's a bit utopian, but good hard science fiction, more solid science speculation than much in the genre."
] |
[
"why?"
] |
[
"Do animals seek a variety in their diet?"
] |
[
false
] |
For animals, why are feeding chains organised into food webs, rather than chains, is it because having more than one source of nutrition provides a security in the case of an extinction occurring to a species you're feeding on, or do they simply enjoy having a variety in their diet, much like the way we humans do?
|
[
"This largely depends on the animal! Some animals, like the leatherback turtle, whose diet consists solely of jellyfish, are specialists that lack variety in diet. Animals that DO eat several food sources generally do so because of two main reasons. ",
"First off, many food sources are only seasonally available. Many animals exhibit diet switching as different fruits and animals become more abundant! Birds often feed their chicks a regimen of food based on the age of the chick (i.e. insects -> berries); i'd lump this into the same category. ",
"The second reason is food availability. If you eat more types of food, there is more food. Just look at animals like racoons and seagulls! ",
"Hope this answers the question"
] |
[
"Suppose an animal has access to two types of food: A and B. Will it always choose its favorite one, or will it ever think: \"I've had a lot of A recently, I'll have B today for a change\"?"
] |
[
"Animals catagorically do eat for pleasure. If you offer a pet a treat as opposed to their normal fare they'll scarf the treat eagerly. Pleasure from good tasting stuff is not human exclusive."
] |
[
"Why does copper form a nice coating on iron, but not silver during electrolysis?"
] |
[
false
] |
Okay that was a retarded title, but I did a chemistry experiment at school which involves the electrolysis of substances in solutions. One we did was the electrolysis of an iron nail in a CuSO4 solution, and we observed copper forming on the surface of the iron nail, which can be filed off. We also tried doing the same experiment without the battery, and we got the same result. This is purely my speculation, but would iron ions not go into solution in the experiment involving the battery, and would go into solution in the one without a battery? Because of the battery constantly supplying electrons to the iron nail preventing Fe losing electrons and becoming Fe2+? Secondly, we did another experiment, but this time with AgNO3 instead of CuSO4. I observed silver forming on the surface of the nail, however unlike copper, this can be wiped off very easily, and flecks of it would fall off the nail even while the circuit is connected. Why does copper adhere better than silver? Furthermore, we also did an electrolysis of copper in CuSO4. In this case, copper was formed much more significantly at the edges of the cathode, not the flat surfaces, and unlike the formation of copper on the iron nail, the excess copper falls off quite easily too. Why? I've read that this reaction is referred to as the purification of copper. Why? We also tried using graphite electrodes, instead of metallic electrons, and throughout the experiment carbon kept falling out. Shouldn't the graphite be non-reactive? Also, copper and silver forming on the graphite appears to be in a flatter, smoother structure, as opposed to the crystal-like structure observed elsewhere. However, it wasn't too hard to remove... How is graphite different from other substances? Thanks for the science.
|
[
"It's a crapshoot whether an electroplated metal will stick or flake off the electrode surface. Metal atoms pack in a regular pattern called a crystal lattice. Not all atoms adopt crystal structures that mesh well together. It just happens that silver and copper don't fit well together, but iron and copper do. ",
"How well your graphite electrode holds up to the electrolytic conditions is highly dependent on the quality of the electrode. A cheap piece of polycrystalline graphite is going to fall apart more easily than glassy carbon.",
"The other stuff sounds like homework questions that you should be able to answer if you read through the electrochemistry chapter of your textbook."
] |
[
"The rougher parts of the surface tend to be the places where the number of surface defects is higher. These spots also tend to have higher reactivity."
] |
[
"I read that in graphite the carbon is bonded with three other carbons and has an electron that is delocalised. Could it be that during the electrolysis process that the electron is removed?"
] |
[
"Why don't elements consistently increase in stability as more neutrons are added?"
] |
[
false
] |
From what I understand, neutrons and the strong nuclear force are what allow elements other than hydrogen to exist. However, as an example: U238 has a half-life of about 4.4 billion years, but U240 has a half-life of about 14 hours. I am aware of the existence of neutron and proton shells, but that's about the limit of my knowledge.
|
[
"Neutrons are fermions. If you keep adding them to an atom, they will go to higher and higher energy states as the lower states are already filled with neutrons and the Pauli principle exclude those states from being occupied by multiple neutrons. Their binding energy is therefore much lower and they can escape easily. Hence, unstable."
] |
[
"I believe you are; the strong force actually gets stronger or is constant with distance.",
"The nuclear force (which is to a certain extent, as van der Waals is to the electromagnetic force) does however decay with distance, Yukawa style."
] |
[
"Naw, the similarities don't go that far."
] |
[
"The Indian subcontinent was an isolated landmass for millions of years. Did unique dinosaurs or wildlife evolve during that time?"
] |
[
false
] | null |
[
"Barapasaurus,Indosaurus and a Ichthyosaur species have been found in India. Also not to mention one site that had over a thousand fossilized dinosaur eggs. There’s new evidence suggesting that hoofed mammals of the horse and rhino linage evolved near or on the Indian subcontinent. Also the primates we see in India today evolved on the subcontinent way before it smashed into Asia. That’s bare bones basics of what I know. Also here’s some sources ",
"https://www.google.com/amp/s/www.csmonitor.com/layout/set/amphtml/Science/2020/1202/India-s-fossil-heritage-is-vast.-It-s-also-under-threat",
"https://www.sciencedaily.com/releases/2020/11/201107133927.htm",
"https://www2.palomar.edu/anthro/earlyprimates/early_2.htm"
] |
[
"Also here’s a good article on early India biological interchange. ",
"https://www.nature.com/articles/ncomms12132"
] |
[
"That first article mentions how India has the highest number of fossilised Dino eggs and nesting sites, from 68 Ma ago “deposited during a time of volcanic activity”. Do they mean the Deccan Traps? I didn’t realise it was already active at that point. Whether they mean that or not, I wonder if nesting continued when the Deccan Traps did first become active and if so, what sort of proximity any nesting sites had to the outgassings?"
] |
[
"Does the Sun experience \"tides\" due to influence of it's satellites (the planets)?"
] |
[
false
] |
Water on earth is notably affected by the Moon and the Sun. Does our favorite big ball of plasma experience an equivalent distortion caused by the planets?
|
[
"There are tidal forces from the planets, yes, because the gravity of the planets affects the sun, but they are very small. The largest tidal acceleration would be from Jupiter, and it's on the order of GM_J / a_J",
" * R_sun / a_J ~ 2x10",
" m/s",
" . I may have left out a factor of 2 in there somewhere, but the point is that it's very small because Jupiter is far away relative to the size of the Sun, while closer planets aren't very massive. Just for fun, we can do Mercury's: GM_m / a_m",
" * R_sun / a_m ~ 8x10",
" m/s",
" which is actually much closer to Jupiter's than I would have guessed! That 1/a",
" term really swings it."
] |
[
"This is the formula for the tidal acceleration on the sun due to jupiter on the axis connecting the two centres of mass. Where M is mass of jupiter and a is the distance between the two centres of mass and R is the radius of the sun and G is the gravitational constant. It could also be written as (2",
"M*R)/a",
" Yes he did forget a factor of two but it isn't really relevant. My other comment may also be helpful"
] |
[
"This is the formula for the tidal acceleration on the sun due to jupiter on the axis connecting the two centres of mass. Where M is mass of jupiter and a is the distance between the two centres of mass and R is the radius of the sun and G is the gravitational constant. It could also be written as (2",
"M*R)/a",
" Yes he did forget a factor of two but it isn't really relevant. My other comment may also be helpful"
] |
[
"How do self-focusing projectors work?"
] |
[
false
] |
[deleted]
|
[
"They have a built-in rangefinder that determines the distance between the lenses and the screen. Using the data that the rangefinder supplies, the little computer inside the projector calculates how it should position its lenses so that the light focuses on the screen. Once the calculation is done, small motors attached to the lenses adjust their position."
] |
[
"How does the microcontroller calculate the lens position? Is it just an equation based on the distance from the screen? What if I moved the piece of paper I wanted to project closer to the lens, how does it correct for that, since the distance from the lens to the screen wouldn't change?"
] |
[
"Yes the lens configuration is probably calculated using a function with the distance as a parameter.",
"Is it an overhead projector we are talking about here? Because when you say simply \"projector\" I imagine you're talking about a device that takes digital signals as input and projects an image onto a screen, like you know, the thing that you plug into your computer. If we are talking about an overhead projector, I don't know the answer. I'm not even sure overhead projectors have AF modules..."
] |
[
"Is the maximum velocity of every atom the same (i.e. is the maximum velocity of an iron atom the same as the maximum velocity of a plutonium atom?)?"
] |
[
false
] | null |
[
"Yes; the maximum velocity of anything, no matter what it is, is the speed of light (nothing massive can ",
" the speed of light, but anything can get arbitrarily close to the speed of light). "
] |
[
"No, all atoms behave the same at any velocity, owing to Einstein's (special) relativity; there is no such thing as absolute motion, and no way to tell whether the atom is moving or whether it is you who are moving. It sounds like you might be confusing velocity with acceleration (which is change in velocity, due to a force). Atoms do come apart when they are made to collide at high velocities, and yes, different elements behave differently when they are made to collide. But collisions are more complicated that just a simple acceleration of the whole atom, involving electrons and protons and neutrons smashing into one-another. If you were to accelerate an atom and all of its constituents equally, then it would again behave the same no matter what, and again because of Einstein's (general) relativity, specifically the equivalence principle. "
] |
[
"An interesting example of this is the ",
"\"Oh-My-God-Particle\"",
", which was a particle of cosmic radiation, proton for example, that was found that traveled incredibly close to the speed of light, so close that if this particle traveled in parallel to a photon, it would take 200,000 years for the photon to gain a 1 centimeter lead.",
"Its observation was a shock to astrophysicists (hence the name), who estimated its energy to be approximately 3×1020 eV (3×108 TeV, about 20 million times more energetic than the highest energy measured in radiation emitted by an extragalactic object and approximately 1020 (100 quintillion) times the energy of visible light);[4][5] in other words, an atomic nucleus with a kinetic energy of 48 joules, equivalent to a 142 g (5 oz) baseball travelling at about 26 m/s (94 km/h; 58 mph).[6]",
"This particle had so much kinetic energy it was travelling at 99.99999999999999999999951% the speed of light. This is so near the speed of light that if a photon were travelling with the particle, it would take 220,000 years for the photon to gain a 1-centimeter lead.[1]"
] |
[
"Question About Global Warming"
] |
[
false
] |
[deleted]
|
[
"I hesitate to get into conversations about anthropogenic global warming because of my stance on the subject, namely that global warming is a definite reality but us being the sole cause or the biggest contributor is a little dubious.",
"However this one's an easy answer. The ice melting that they're afraid of is all the ",
" ice. Like glaciers and what-have-you. Essentially the ice that isn't simply floating in water the way icebergs do - you've got displacement volume to think about there, but landlocked ice doesn't have the same problem.",
"So if the glaciers melted, flowed down the mountain ranges and into the oceans then yeah, there'd be a net rise in sea level. Probably a big one too."
] |
[
"As others have mentioned, land locked ice will melt and put water into the oceans, increasing the level.",
"Even if we were only considering floating ice, the water level would still not go down (or up, for that matter). An ice cube floating in water displaces its weight in water (with the rest floating above the surface). When the ice cube melts, the volume of ice displacement lost would be equal to the volume of liquid water formed from that lost ice volume, so the water level would remain the same.",
"That, of course, assumes equal densities between the water and the ice. I believe there's a (negligible but non-zero) difference in the densities of (Antarctic) glaciers and sea water."
] |
[
"my geochemistry book disagrees with this notion, but, sure, i'll look at what you have. ",
"According to what I have: basically, the concentration of O-18 decrease toward the inland of continents, and with an increase in latitude due to Rayleigh fractionation. So, as you get to the poles, you have less O-18 than you would at the equator. Another notable factor is that O-16 accounts for 99.76% of all oxygen isotopes, while O-18 accounts for .20%. Essentially we're talking about minor variations in the overall water chemistry that will not affect the overall density. ",
"\nThe only way you would be able to made the density higher than water, would be to create what is known as heavy heavy water, or deuterium (an isotope of hydrogen) oxide -- something that is required for nuclear reactors, and does not occur naturally. hydrogen also accounts for 99.985% of all hydrogen isotopes, while deuterium accounts for .015%. So, it's unlikely enough would be able to be created.",
"this link might be helpful as well:",
"http://www.sahra.arizona.edu/programs/isotopes/oxygen.html"
] |
[
"Why do people appear to not be bleeding in images/videos of surgeries in progress?"
] |
[
false
] |
Pictures/videos of surgeries always have no blood anywhere, and a gaping wound...they might wipe every now and then, but for the most part, there is no blood. Is it all anti-anticoagulants? Do they clamp off all the capillaries? Am I answering my own question? Why do folks need multiple transfusions then? (during/after surgery)
|
[
"While it depends on the type, surgeries do tend to bleed a lot. Superficially there may not seem to be much bleeding, however a large amount of blood may have been suctioned away or hidden, collecting at the bottom of a cavity (e.g. abdominal surgery). However, unless there is truly significant blood loss, there is a tendency towards replacing fluid rather than using blood transfusions.",
"There are several methods used to reduce blood loss. Most of the surgeries I've seen used a diathermy (rather than a scalpel) - It's a device that uses an electrical current to cut and cauterise tissue, closing the smaller blood vessels/capillaries and preventing excessive bleeding. Laser surgery presumably achieves a similar thing. ",
"For larger blood vessels, the surgeons can throw a few stitches (a ligature) to tie off the blood vessel, or close the vessel with a clip. Prior to tying/clipping the vessel, haemostats might be used to clamp close the vessel. For orthopaedic/limb surgery, a tourniquet can be used to completely occlude blood supply to that limb (which presents risks of ischaemia/damage).",
"Positioning the patient in a certain way can also reduce bleeding (e.g. a downwards slope with the head and chest lower than the abdomen and legs).",
"In terms of pharmaceuticals, the anaesthetist can use drugs to maintain a lower blood pressure to reduce bleeding. In scenarios with a high risk of blood loss, drugs can be used reduce bleeding (i.e. antifibrinolytics and coagulants). However, for the most part, patients are often given drugs to ",
" coagulation. While excessive bleeding is undesirable, surgery also carries the risk of deep vein thrombosis and pulmonary embolisms - so anticoagulation/DVT prophylaxis is usually preferable."
] |
[
"Hope that answers your question\nSource: I am an orthopaedic surgery resident in the US"
] |
[
"Lets also add a major factor, surgeons try very hard to avoid slashing through major blood vessels. Surgeries tend to be planned to avoid them."
] |
[
"[Weekly Discussion Thread] Asking for Suggestions"
] |
[
false
] |
We have had eight installments of the weekly discussion thread and I think this is a good time to ask for suggestions from the community. So instead of a regular thread this week this is the place to offer your ideas for future topics and I will do my best to use them all in the future. Here is last weeks thread:
|
[
"This could be fun or it could be a disaster (disaster seems more likely but I'll just throw it out there): \"Scientists, what's a 'fringe hypothesis' that you find really interesting even though it's not well-regarded in the field?\""
] |
[
"What is one thing you would change about the way science is done (wherever it is that you are)?",
"What about your science keeps you motivated on a daily basis?",
"What is something really cool about the science in your field that gets you excited when talking about it."
] |
[
"Not the best questions, but I figured I'd suggest something:",
"Scientists, what are some of your favorite books & authors from your field?",
"How do you deal with blocks? As in, when you are trying to figure out a problem and you just keep hitting walls/blocks. What are some of your tricks in dealing with this that you could recommend to younger students?",
"What's your take on citizen science? Over rated, under rated? ",
"What websites (besides that of TED or Khan, or Udacity, or OCW MIT, or Coursera) relating to your specific fields do you wish more people would visit?"
] |
[
"What are Neil Degrasse Tyson's contributions to the field of physics?"
] |
[
false
] | null |
[
"NdGT was not a heavy-hitter as a scientist like those other names you mentioned. He has spent almost all his career in outreach. His ",
"PhD thesis",
" was about measuring the number of a certain types of stars in our galaxy. He was also like the nineteenth author on some big astronomy papers in the 90s like ",
"this one",
". He's only published like two or three papers. ",
"This",
" is probably the most important where he is the main investigator."
] |
[
"So basically he is a personality that people have come to accept as a face for astrophysics. I just dont get the hype. I saw his picture on a shirt with the aforementioned bigger names, and it got me wondering what he actually did to become so famous. Thank you for the info! "
] |
[
"Such questions are better suited for our newish sistersub ",
"/r/asksciencediscussion",
". Please post there instead."
] |
[
"What do you think science/humanity's goal should be?"
] |
[
false
] |
What do you want to see in the world? A planet of super intelligent, super organized people? A planet with no hunger? I have personal goals. I want to graduate from college, make some money, be secure in my life, have fun, etc. But does our collective humanity have a goal? What are we gravitating towards? Asking this in because I envision a generally happy, science fictionized future.
|
[
"For humanity's goal, I want to see a peaceful society unmarred by endless war, where instead of destroying, people strive to use our greatest gift of intelligence and promote knowledge, education, and enlightenment. There should eventually be a time where everyone is granted certain rights and freedom and people think of others and treat them with kindness and respect rather than encouraged selfishness and bigotry/ignorance. I want to see a society where policies are made with informed decisions, listening to our collective knowledge and understanding of what would be good for society as a whole rather than catering to the capitalist desires the top elite."
] |
[
"Ubiquitous pantsless monkey butlers."
] |
[
"http://singinst.org/upload/CEV.html"
] |
[
"What happens to fields as they approach and reach the speed if light?"
] |
[
false
] |
I am curious what physically happens to a field as it approaches and then hits the speed of light. Like say a magnetic field, does it change? Or electromagnetic? Or are there any fields that behave strangely as they speed up and hit C? I asked this question once in a FB group and got a smartass remark about “Maxwell’s equation tells you, duh.” Sadly, I am not well versed enough in math or physics to be able to understand his equations and what they mean “in the real world.” I’m not exactly a layperson, but I am by no means well-versed.
|
[
"So \"fields\" no matter what are everywhere in the universe regardless if it is being interacted with or not. If there is a disturbance of that field, that shows up as a wave or a particle depending on how you look at it. Everything that is a wave is a particle and everything that is a particle is a wave and it only shows up differently depending on how we observe it (look up the double slit experiment for an explanation on that). So if the field is a field for a massless particle, that particle will always move at the speed of light, no faster, no slower. If it is a massive particle it will never be able to go at the speed of light because to do so would require more energy than is possible. The universe does a weird thing with massive particles the faster they go. The closer the particle gets to the speed of light the heavier and slower it gets requiring more energy to push it faster, eventually you get to infinite energy required and that can't happen. But at the end of the day it is still just a particle which is observed as a disturbance in that field, not the field itself because fields don't move, they just exist"
] |
[
"Yeah, another post made me aware of the problem with my thinking. I was thinking, basically, take a magnet and accelerate it to C, what happens to the magnetic field, as if the magnetic field is only around the magnet. Thank you for taking the time to throughly explain it."
] |
[
"I feel like you are thinking the same thoughts as \"What happens if a car travelling at C, turns on its headlights?\". If so, i would direct you to this article, it explains it extremely well and simple.",
"As even though we can't have a car, or a magnet in this case travelling at C, you can still wonder:",
"https://www.physlink.com/education/askexperts/ae169.cfm"
] |
[
"How can images from particle detectors inform scientists about the nature and properties of particles?"
] |
[
false
] | null |
[
"For example, the size of the ",
"swirls in a bubble chamber",
" depends on the charge-to-mass ratio of the particles, and the direction they swirl depends on whether the charge is positive or negative."
] |
[
"It's not merely images which a particle detector produces -- the detector produces a lot of data about those particles, such as their mass, momentum, and charge. The images produced are not actual photo images, they are computer-generated representations of limited parts of the data; there is much more that cannot be shown via images.",
"Aside from that, what we're interested in are the particles that are produced in these high-energy collisions. The presence of so much energy allows excited states of matter to exist for brief periods of time, but these excited states quickly decay into many normal types of matter. The decays are so fast that these very excited particles cannot be detected directly -- we have to infer their existance from the decay products.",
"When a particle is detected after a collision, a lot of information about the detection event is recorded. This information combined with the equations governing conservation of certain properties allows us to reconstruct the decay chain and determine properties about the very-excited states of matter that we couldn't detect directly."
] |
[
"Conservation laws are key. For example, if you measure the energy and momentum of some resulting particles, then you know the energy and momentum of the particles that created them. Similarly with charge. So you can work through a process of what you detected, and piece together how they could have been produced. (Sometimes things don't add up, and that tells you that there is a particle you've not seen, which is carrying some of the momentum and energy -- but because of the conservation laws, you know how much momentum and energy, so you can figure out its properties.)"
] |
[
"Why is digital camouflage preferable to more traditional styles?"
] |
[
false
] |
I always thought that camouflage is supposed to conceal its wearer by breaking up his/her figure with naturally occurring shapes and forms. Is there any scientific reason why randomly placed squares conceal the wearer better than say, woodland patterns? Also, if digital camouflage is more effective than a traditional one why don't more hunters use it?
|
[
"Some references ",
"here"
] |
[
"I'd just like to point out that camouflage has a variety of use cases with distinct considerations. Camouflaging is different for objects of different size, either still or active, in different environments and against different observers in different ranges.",
"Military camouflage on individual fighters is often enhanced with stuff like tree branches (or anything comparable, depending on the environment), which serve to break the form of the fighter. In this case the pattern has to provide a good base. Military camouflage is also not only used against humans observing with naked eye, but also optical enhancements and various imaging tech.",
"There are a variety of patterns, digital and otherwise, that are widely used with no consensus on what's \"most effective\". The Latvian army uses an interesting pattern with ",
"really big squares",
", the Germans use random dotted shapes, and some armies still don't bother with camo patterns at all. There are also fractal patterns, striped patterns, and traditional looking new patterns. MultiCam is not a digital pattern, but the US Army has started issuing MC equipment to some troops over UCP. The British introduced a new pattern with familiar distinct British camouflage pattern shapes but MultiCam-style colors.",
"Many people are willing to bet that digital is not better. New patterns are being created and tested in new ways all the time.",
"I can't say what the considerations are when it comes to hunting. One thing that comes to mind is that different animals see and pay attention to different things.",
"I don't have better sources than what I've read in the past and short military service, but in my own experience it's pretty hard for the human eye to focus on the kind of random scrabble of digi patterns. The most important job of the camouflage is not to resemble some natural object, but to blend in as \"noise\" and not be picked out.",
"Also, military camouflage needs to be feasible to produce in mass quantities at a reasonable cost, and provide a good compromise in a variety of environments (which are different for each organization). In addition, military-issue equipment needs to be very durable, which might or might not create some limitations on what kind of patterns can be issued."
] |
[
"I see! Thanks for the response!"
] |
[
"Were the Supermassive Black Holes at the centre of our Galaxy (and others) Stars at some point?"
] |
[
false
] |
And if so, how big were they? Do galaxies form around the supermassive star or do they have to collapse into the Black hole in order to form the Galaxy?
|
[
"This is one explanation, and probably the most likely, but the reality is that we don't know. ",
"Other explanations include primordial black holes (formed immediately after the Big Bang) and black holes formed through large-scale collapse of gas clouds in the early universe."
] |
[
"When the universe originally formed, the abundance of hydrogen was much greater than that of today, resulting in massive stars that exhausted their fuel supplies over millions of years; collapsing violently in hyper novae explosions and the first black holes. These black holes eventually accumulated and merged, forming the first supper massive blacks of which galaxies would later be built around. "
] |
[
"It's possible, but unlikely (at least in my view). ",
"There is a limit to how quickly a black hole (or any object) can acquire mass. Disregarding problems with dissipating angular momentum (and this is no easy problem to solve) infalling matter will heat up quickly by friction as it falls down the gravitational well and will begin to glow xray hot. The radiation pressure will act to expel the matter behind it, in the least efficient case, by Thomson scattering.",
"From this, the Eddington limit, we can work backwards from estimates of the current mass of SMBHs (and their age) to say how big the progenitors must have been. We arrive at masses in excess of 400 solar masses. [see e.g., ",
"http://arxiv.org/pdf/1003.4404.pdf",
"] This is bloody enormous, but in the metal-poor environment of the early universe, perhaps possible (though not seen in far redshift observations?). Although physical models of these systems are extremely poorly understood, the temperatures at the centre of these \"stars\" will be relativistic; i.e., sufficient to produce electron-positron pairs. Pair production reduces outward pressure: the star will not last long and we will have an intermediate size black hole soon. This method of collapse probably captures about half the progenitor mass in the black hole, making it very efficient compared to smaller supernovae. But the black hole still has to suck in a lot more mass to get to its present size. In the shallow potential wells of halo galaxies this is not an easy proposition. They are also feeing a lot of energy back into the local medium, slowing their rate of growth. (Quasars, for instance, are exceptionally bright. They also expel a lot of matter beyond their host galaxies.)",
"Could black hole mergers help? Perhaps, although estimating the collision frequency at early times is not easy. Today, galaxy collisions are quite rare in our local universe (but much more common than star collisions, for instance, which probably only occur ~ once in a spiral galaxy.) Even if you get a couple of black holes within a few light years of one another, there are big problems with shedding angular momentum again (maybe gravitational waves, which might be detectable!).",
"The real problem I have with this model is the ultimate fate of these SMBHs. They occur at the centre of every galaxy we observe, and only the centre. Does exactly one happen to form in every galaxy? Do loads form and they all somehow merge, thousands of light years away from their origins, without any being expelled to higher orbits by the galactic tugs of war? Why and how are they so stable in the centre? In spiral galaxies, why are SMBH masses so tightly correlated with the total galaxy mass?",
"These questions to me suggest that SMBHs form as part of early galaxy formation objects, as very massive (>10",
" solar masses) objects initially, and, though they may shine very brightly during their collapse phase, they would never go through a stable \"star-like\" phase at all."
] |
[
"What happens when we crack knuckles?"
] |
[
false
] |
Why do some people's knuckles crack while others' don't? What happens when a crack happens? Are there any proven methods for avoiding the build-up of whatever cracks? Is cracking knuckles safe?
|
[
"The cracking noise is produced by the collapse of bubbles of synovial fluid vapour caused by the movement of the joint.",
"There is no evidence it causes any problems.",
"\"Habitual knuckle cracking in children has been considered a cause of arthritis. A survey of a geriatric patient population with a history of knuckle cracking failed to show a correlation between knuckle cracking and degenerative changes of the metacarpal phalangeal joints.\""
] |
[
"This study",
" suggests that habitual knuckle cracking was associated with manual labour, alcohol and smoking. However, that may not be causative, and I think it means associated with the habit of cracking, rather than the actual bubble formation. ",
"This is a more technical article",
" that might have the details you are interested in"
] |
[
"From the second link:",
"Repeated performance of articular release may decrease the occurrence of arthritis.",
"Am I reading it correctly that there is some evidence that knuckle cracking may actually be helpful in decreasing the chances of getting arthritis?"
] |
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