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[
"Why don't the chemicals in pills get totally dissolved by our stomach acid? How do they make it to the blood stream?"
] |
[
false
] |
When I take a vitamin, or a melatonin gummy, or aspirin, why doesn't our stomach acid just totally destroy those chemicals, rendering them useless. Are some molecular bonds immune to acid? I probably don't understand how acid actually works, so maybe you can start there.
|
[
"So a lot of drugs are actually designed to be activated in your stomache. I.e. they are in a \"pro\" form, or prior to use form, where upon entering your stomache and interacting with the fluids there, they get turned into the final form of the drug as they enter into your blood stream. ",
"Also, yes. There are many bonds that are immune to acid. Acid work as proton donors and bases work as proton acceptors (there are different interpretations, but we can start using this one), meaning that acids release a positive hydrogen atom, while bases accept this atom. However, not all chemical bonds are sensitive to this, and can in fact remain unaffected by acids (something has to store them after all). However, a lot of biological material can be broken down into simple nutrients (thus why we have stomache acid in the first place), but not enough to completely destroy the nutrients. "
] |
[
"Well I'm glad it helped you understand. There is another definition of acids and bases, which is electron donors and acceptors, which is a bit more comprehensive, but the proton thing works in most cases"
] |
[
"This is part of he reason that certain drugs (such as insulin for diabetes) cannot be administered orally. Insulin (as an example) is a peptide. While peptides are relatively able in stomach acid, they are rapidly digested by enzymes, so they must be injected. By the way, stomach acid is dilute enough that it does not destroy most molecules."
] |
[
"How do we define the solar system boundaries?"
] |
[
false
] |
I keep reading voyager 1 is about to leave the solar system. How exactly does science define the boundaries of the solar system? First thought would probably be "any object under the gravitational influence of the sun is in the solar system" but that does not seem right as the gravity formula applies whatever the distance and is never zero for two objects of any mass > 0. It's my understanding that even stars are under each other's gravitational influences anyway. So what is exactly that "limit of the solar system" everyone seems to talk about? How can an objet such as voyager 1 be said to be "inside" or "outside" the solar system? Thank you !
|
[
"Yes -- when people are talking about voyagers reaching the edge of the solar system, they are talking about the heliopause. The sun whips out energetic particles in a highly magnetized solar wind. The sun is also moving through through a low-density interstellar medium (ISM), a partially ionized gas. At some point, they come into a kind of equilibrium, which is the heliopause, the contact surface between the solar wind and the ISM. As someone who studies the ISM (see flair), and has worked on the significance of the heliopause, I find all this rather interesting! "
] |
[
"In the case of Voyager 1 (which is about 120 AU from the Sun, i.e., 120 times as far from the Sun as the Earth is), what you are hearing about is that Voyager 1 is approaching the ",
".",
"From the Sun comes a stream of charged particles; this is the solar wind. There is a bubble in space, essentially, in which you find these charged particles. This region is the heliosphere.",
"Outside the heliosphere is the so-called ",
", the material that fills the space between the stars.",
"The heliopause is the boundary between the helisophere and the interstellar medium, where the pressure of each balances the other.",
"This ",
"piece from NASA",
" indicates some of the evidence that Voyager 1 is nearing the heliopause.",
"So this is the sense in which Voyager 1 is nearing the edge of the solar system. Since within the heliopause, the particles you find in space come predominantly from the Sun, and outside the heliopause, the particles you find in space generally do not, the heliopause is one way to define the limits of the solar system."
] |
[
"Thank you all for your answers ! It's definitely clearer for me now and it bugs me traditional media don't explain this clearly and just use general throwaway sentences \"woah it leaves the solar system\" style. AskScience is an amazing community."
] |
[
"Would it be possible to propel a man in an open aired vehicle fast enough to travel along side a bullet?"
] |
[
false
] | null |
[
"Yes, this is possible, however very hard to do. Vacuum would make things even harder. You just need a very fast care like ",
"this",
". The hardest part would be to fire a gun close enough to the driver to reach it.",
"From the driver's point of view, he would see a bullet simply falling on the ground. The amount of time a bullet spends in the air does not depend on the bullet speed. A bullet fired, let say horizontally, spends the same amount of time flying as the bullet dropped on the ground from the gun's height. "
] |
[
"I would think a vacuum would make things easier(assuming it's easy to get a large vacuum to work with). The vehicle and the bullet is going to slow down at different rate in the atmosphere, you won't have that problem in a vacuum."
] |
[
"You would need a different kind of propulsion, as ordinary methods need air to work, driver would have to wear a space suit, because people can not be exposed to vacuum for more than a couple tens of seconds and building a vacuum chamber big enough is far beyond current technology. "
] |
[
"If the sun emits heat into our solar system like a giant space heater then why is the universe so cold if it has trillions of stars like the sun?"
] |
[
false
] |
My dad brought this up to me when we were talking about space and I didn't really have the answer for it. I'm not sure how dumb of a question it is but I couldn't think of a reason.
|
[
"The nearest star to the Sun is a few light years away. We're about 100 million miles away from the Sun, about a millionth of the distance to the nearest star, and even here the Sun barely warms space. The Earth is warm because it has an atmosphere to trap the Sun's heat, but leave the atmosphere and you'll freeze your buns off. And that's a millionth of the distance to the next star over!"
] |
[
"The universe is bigger than stars are hot."
] |
[
"Doesn't it dissipate proportional the surface area of that sphere, r",
", instead of its volume r",
"?"
] |
[
"How does this \"Mitochondrial Eve\" thing work? Did humanity really population-bottleneck to a single female at one point in time?"
] |
[
false
] |
So, every single living human being today, can have their lineage traced back to a Mitochondrial Eve. How does that even work? Did we really come that close to extinction that at some point, there was only one female human on the entire planet whose descendants didn't die out before making contact with others? That's some cosmic horror level stuff right there. Every other pocket of human population dying, only the children of one woman living on... Holy crap... Shouldn't this show some lower than normal genetic diversity tho? I heard cheetahs have debilitatingly low genetic dieversity due to a bottleneck in their population thousands of years ago... yet I never heard of humans having such.
|
[
"It's a little more complicated than that - it's not that nobody else alive had any descendants, it's that there are no other unbroken maternal lines. The other women alive at the same time as MtEve may have descendants, it just so happens that they hit a generation that was all sons. ",
"You only inherit your mitochondrial DNA from your mother anyway, and there's not a ton of variation, so it doesn't really impact genetic diversity.",
"As much as anything, it's a statistical phenomenon - you will eventually find a common maternal ancestor - rather than indicative of some horrible event."
] |
[
"Here is a nice essay on the subject. It says basically the same thing as ",
"u/mikelywhiplash",
"http://pages.ucsd.edu/~dkjordan/resources/clarifications/MitochondrialEve.html"
] |
[
"Oh, that's really a good way to put it. Thanks!"
] |
[
"Ask Anything Wednesday - Biology, Chemistry, Neuroscience, Medicine, Psychology"
] |
[
false
] |
Welcome to our weekly feature, Ask Anything Wednesday - this week we are focusing on Do you have a question within these topics you weren't sure was worth submitting? Is something a bit too speculative for a typical post? No question is too big or small for AAW. In this thread you can ask any science-related question! Things like: "What would happen if...", "How will the future...", "If all the rules for 'X' were different...", "Why does my...". Please post your question as a top-level response to this, and our team of panellists will be here to answer and discuss your questions. The other topic areas will appear in future Ask Anything Wednesdays, so if you have other questions not covered by this weeks theme please either hold on to it until those topics come around, or go and post over in our sister subreddit , where every day is Ask Anything Wednesday! Off-theme questions in this post will be removed to try and keep the thread a manageable size for both our readers and panellists. Please only answer a posted question if you are an expert in the field. . In short, this is a moderated subreddit, and responses which do not meet our quality guidelines will be removed. Remember, peer reviewed sources are always appreciated, and anecdotes are absolutely not appropriate. In general if your answer begins with 'I think', or 'I've heard', then it's not suitable for . If you would like to become a member of the AskScience panel, . Past AskAnythingWednesday posts . Ask away!
|
[
"are there any breakthroughs in diagnostic testing and treatment for fibromyalgia (or it’s more recent title) in 2022-23?"
] |
[
"Why does fusion stop releasing energy after iron?"
] |
[
"At the point a star makes Iron it's dying. Fusion costs energy when you hit iron, or so I have been told."
] |
[
"Does evolution move toward reproduction at earlier stages in life?"
] |
[
false
] |
So, evolution as we know it, in simplest terms is survival of the fittest. But really all that matters is that the animal is fit only until it reproduces. After reproduction, the success of various traits in the animal don't get passed on and therefore, evolutionarily don't matter. Wouldn't it make sense if more animals evolved to reproduce sooner in life? Do animals that reproduce at earlier stages in life evolve more quickly? Is this happening already and I just don't know about it?
|
[
"Older animals would be better suited to caring for offspring, increasing the chance of their children's survival."
] |
[
"There are two main types of reproductive strategies, r and k.",
"r-strategists live short lives, are smaller, have many children, and replicate early.",
"k-strategists live long lives, are bigger, have few children, and devote lots of time to caring for them.",
"Having lots of offspring early is good if your environment changes a lot or it is easy to find enough resources, since there would be little competition.",
"When you're larger and live longer, competition is a serious problem. You can't have hundreds or thousands of children- with competition, too many of them will die. So instead, they care for their young to make sure they do well, and live long enough to reproduce."
] |
[
"It depends on the life-strategies of the organism. You are refereing to ",
"r/k selection theory",
"R species - grow fast, reproduce fast and reproduce many young. They are well suited for fast changing unpredictable environments. And yes, they do accumulate mutations faster - and the changing environment selects for/against them faster. Thus evolution happens at a quicker rate - sometimes within one or two generations. However, there is a crux, reproducing many many young means unfortunately that there is a greater chance that any given offspring will die. So while the parents may produce 1000'2 or even 100,000's of young, perhaps only a few dozen if that become adults. Also, adults tend to die faster, some after just one reproduction event (salmon, octipii...), thus they put all of their available energy into one reproduction event - and if this is the strategy of your species then its best to produce as many offspring as possible in hopes that at least 2 make it to the next generation. (2 is needed to replace the parents, to avoid population decline) Great example are: bacteria, insects, fish, rodents, octopuses...",
"K selected species: slow growing, slow to reproduce and when they do, they only have a few young. They are well suited to stable environments with predictable food, water...sources. These accumulate mutations slower, because fewer offspring are produced. The crux here is that any given offspring needs a lot of additional investment - nursing, caring, protecting, feeding, teaching - until it is capable of surviving on its own. However, the chance of that one offspring surviving is much higher. In this case, animals may only have 20 offspring in their lives, but 5 - 6 - 7 make it. Unlike R selected species, K selected species tend to be long-lived, thus have more chances for reproduction, producing a few offspring each year is stable, knowing that the next year the same will happen. Examples of K selected species include: us, elephants, whales, large mammals, cats, slow growing reptiles...",
"So in summary, the stability of the environment, the predation regime (including diseases), the chances of finding a mate... effects the optimal growing and reproducing strategy of each species. This in turn effects how fast organisms grow, the first age of reproduction, how many offspring are produced at each conception, the lifespan of each individual, the likelihood of offspring surviving to first reproduction..."
] |
[
"Do blind people require less sleep because their eyes don't stress the brain as much?"
] |
[
false
] | null |
[
"No. We really don't know why we sleep. We think it may have to do with clearing out unneeded information, consolidating memories and recovering energy, but it's really not known for sure. We can rest without sleeping to recover energy as well. You can also get sleepy from eating a heavy meal; that has nothing to do with your eyes. "
] |
[
"No. We're not sure why we need to sleep, but it's not because we have eyes."
] |
[
"Well, sleep is the body's way of recovering from a day of spent energy, primarily because of stress on the brain from visual, physical, and mental activity, right? I don't think you understand my question. A large portion of my cue to fall asleep is the heavy sensation under my eyelids. I feel like if that sensation wasn't there, I would wait until I am physically and mentally tired to fall asleep. If the brain didn't have visual stress to worry about, it would make sense that a person would feel less fatigued in comparison to a person who has been using their eyes all day. I can't find any articles online dealing with this topic. "
] |
[
"Why do lights flicker as you turn them on?"
] |
[
false
] | null |
[
"Are you talking about normal light bulbs? Like when you flip the switch slowly and it flickers a bit? That's due to a poor connection within the switch. The circuit is closing/opening over and over.",
"Or, are you are you talking about fluorescent bulbs? The kind that take a few seconds to warm up sometimes? It's exactly that, warming up. The gas inside isn't warm enough to allow for a consistent connection between the ends.",
"In both cases the flow of electricity is unstable. In one case it's unstable in the switch, in the other it's unstable within the gas."
] |
[
"Hi, thanks for the response!",
"The thing that got me wondering was a regular bulb flickering, so the first case. It's not the case that I flick the switch slowly, I would guess that it's a normal flick. When I do, however, there is a slight delay, and then the bulb flickers into shining. ",
"Do you know why the flow of electricity in the switch stabilizes after a while, and why it's unstable to begin with?"
] |
[
"Switches have springs in them, and that makes the contacts bounce off few times when it changes position. Normally this isn't visible to the naked eye, though electronics have to deal with it anyway. If you have a device where a key press registers as several presses sometimes, bad denouncing is probably at fault.",
"Your switch is probably worn out or badly made."
] |
[
"Do we burn more calories when we work our brain harder?"
] |
[
false
] |
[deleted]
|
[
"From what i have read the answer is yes, but it is not neccesarily the brain itself that burns the extra calories, it could be the rest of the body. It could be result of increased heart rate and general awareness/stress levels in the body as a result of the difficult task putting us into some kind of \"performance mode\". ",
"",
"A similar question has been asked some time ago, with some good answers: ",
"https://www.reddit.com/r/askscience/comments/38v6i5/is_the_metabolic_rate_of_a_person_who_is_doing/",
" ",
"This link is also great and has good references within it: ",
"https://www.scientificamerican.com/article/thinking-hard-calories/",
" ",
"PLaying video games vs baseline metabolic rate:",
"https://www.ncbi.nlm.nih.gov/pubmed/16585487",
" ",
"Hard stroops test vs easy stroops test compared:",
"https://www.ncbi.nlm.nih.gov/pubmed/15041139"
] |
[
"Is there a source for this fact about grandmasters?"
] |
[
"Is there a source for this fact about grandmasters?"
] |
[
"What is the evidence for the existence of other universes or that ours is but one universe in a greater \"multiverse\"?"
] |
[
false
] |
is there any real evidence for this idea? i know many very highly reputable physicists have proposed it as a possibility I just have never seen any evidence for it.
|
[
"None yet. But the theory has made testable predictions for what it should look like if another eternally-inflating bubble universe should look like if it collides with ours. Most of these predictions have to do with looking at the structure of the cosmic microwave background, so these people are anxiously awaiting data being taken by the Planck telescope.",
"Edit: ",
"Here",
" is a recent paper on the subject, where astrophysicists combed through the WMAP cosmic microwave background data looking for bubbles and didn't see any strong evidence for one. But they talk briefly about their hopes for more sensitive data from Planck."
] |
[
"It is the product of a certain model of quantum mechanics.",
"No direct evidence has yet been observed for the concept."
] |
[
"I'd clarify that there is no evidence that distinguishes Many Worlds from other modern interpretations, since all tests of QM so far have been evidence for all of them, since they all make similar predictions for a single observer. Though some (myself included) argue that Many Worlds contains fewer assumptions and is therefore more likely to be correct."
] |
[
"How do rockets keep themselves stable during liftoff when the main source of thrust if coming from the bottom?"
] |
[
false
] |
[deleted]
|
[
"First, whether the source of thrust comes from the bottom or from the top doesn't make any difference for stiff objects like a rocket. That's a bit counter-intuitive perhaps: generally we're used to the notion that pushing something tends to make it rotate, while pulling something tends to straighten it. But that's because in those cases the direction of pushing/pulling is disconnected from the direction of the object itself. But if, like in the case of the rocker, the direction of thrust necessarily follows the direction of the object, the location of the thrust (along the direction of the thrust) is not important. (This is known as the ",
"Pendulum rocket fallacy",
").",
"Also, reaction wheels (als OutRunOutdoors describes) work in Kerbal Space Program, but are in real life not effective enough for balancing a complete rocket during lift-off.",
"And I'm not so sure about differential thrust as Harcsas describes: I don't think most rocket engines are capable of fine-tuning there thrust precisely and quickly enough.",
"Other things do help make a rocket stable:"
] |
[
"In addition to adjusting thrust, many (maybe most?) rockets use gimbaled engines that can swivel side to side to correct drifting."
] |
[
"Don't forget that most rockets have their direction of thrust aimed at center of mass, in case an engine fails, it doesn't cause the whole assembly to go off-axis."
] |
[
"When will quantum processor's be in every PC and how will they differ from todays technology."
] |
[
false
] |
Just curious as to how long until they are made commercially available? Best guess is all I'm looking for... Are you talking about 10-20 or 100 years? How will they differ from the tech we have today? How much faster will they be? How will quantum processors change the computing world? What is the latest news/progress on QP's? Thanks guys!
|
[
"More than likely, if made practical, quantum processing will be for solving niche problems and may not be the most efficient way to work with general computation. I imagine that the transister CPU will live alongside the quantum processor, the latter being used at universities and governments while your CPU + GPU combo plays all the latest games.",
"For example, I knew someone who tried to run a MUD game (think text based WoW) on an underutilized older Cray supercomputer at a university in the mid to late 90s. This computer's architecture was designed to run big computation jobs consisting of floating point units, but the game was written so that it does integer math (damage is random(50) then minus your hitpoints (another integer)). The game ran incredibly slow even though it was running on a supercomputer. Slower than a 486 PC running linux, but if you were to feed it floating point based problems like weather simulation it would blow the doors off anything under 1 million dollars."
] |
[
"Thank you."
] |
[
"A quantum computer allows a different range of algorithms (Called quantum algorithms) to be used. Currently these algorithms can only be simulated with classical computation in a very limited way.",
"The most significant real word applications are probably in breaking large prime number encryption and searching large unsorted databases."
] |
[
"The strength of the mentally handicapped."
] |
[
false
] |
[deleted]
|
[
"My sister is special needs, so I have always had a theory. She is shorter than everyone in my family, but is very strong (We have to restrain her at the dentist, which takes about 3 grown people).",
"I think, with her, and probably others, is that their motor skills are so messed up. Leading them to learn to function in a different way. What I'm trying to say is they might use only one motion for everything they do, kind of like lifting weights. ",
"It also has to do with perception. They don't know the correct force to put on something in every day life. "
] |
[
"I'm not an expert, but I'd agree with your thoughts on them just not being able to control their own strength. ",
"I think it's just relative, and in the end, we seem to remember the mentally handicapped who grow up to be rather large grown men with lots of muscle on them (possibly because a person with so much strength and no ability to discernibly control that strength can be alarming and memorable).",
"I've met mentally handicapped people who are quite small and not very strong, and I wasn't thinking the whole time that they could pick me up or squeeze me too hard."
] |
[
"We all have the capacity to use enough strength to injure ourselves. An average person starts to hurt herself with over exertion and eases off. Someone with a mental handicap might not receive or interpret those signals telling her that the force she's using is hurting herself too. ",
"That's one possible source. I think the others mentioned here are also involved. "
] |
[
"How does invisibility of a material to a wavelength of light work?"
] |
[
false
] | null |
[
"Following your answer, why would light just pass through if it doesn't have the energy to promote the electron? As I understand it colours are given off by an atom because the when the electron is promoted it may be in an unstable state and then drop back, releasing a photon of energy in the process. If a photon hits and electron but doesn't have the energy to promote wouldn't it just raise the energy of the photon to a small degree but not quite enough to put it into the next state and hence be visibly \"black\" or when dropping back to its normal state release another photon of lower energy?"
] |
[
"Lets talk about solids. In solids, you have nearly an infinite number of atoms, which all have atomic orbitals. To form the solid we take in phase and out of phase combinations or each individual orbital. If we start will 1,000,000 atoms each with 1 atomic orbital, we will end up with 1,000,000 'molecular orbitals'. Since the number of atoms in a solid is usually very very large (~10",
" for a gram of solid) there orbitals blend together in practically a continuous band of bonding and antibonding orbitals. ",
"We call the lower energy bonding band the valence band. The higher energy band is the conduction band. For most materials, the valence band is full and the conduction band is empty. The lowest energy transition is to take an electron from the highest energy state of the valence band and put it into the lowest energy state of the conduction band. ",
"If the incident light does not have enough energy to promote a electron by this amount, the light will not be absorbed. Most insulators have a band structure such that visible light is not energetic enough to promote electrons, so they are transparent to visible light. Glass is a perfect example of an insulator that is transparent.",
"Semiconductors have a smaller gap between bands, and are able to absorb some light giving rise to colors.",
"Metals can absorb practically any energy of light and are opaque to visible light."
] |
[
"Energy levels in atoms are quantized. If you don't have the right energy, the atom won't absorb. In solids, there are no electronics states between the bands, so if the light is of insufficient energy to promote an electron, then it is transmitted. Now, very low energy light interacts with lattice vibrations in the solid and is absorbed, however, that isn't visible light in the first place."
] |
[
"What would weigh the most: all whales, all humans, or all ants?"
] |
[
false
] |
What would weigh the most if you added up all the whales, all the humans, and all the ants alive on the planet?
|
[
"It depends on how you measure biomass. Do you mean dry biomass, wet biomass, or number of carbon atoms? Wikipedia has the answer, and you can sort the table in different ways. ",
"http://en.wikipedia.org/wiki/Biomass_(ecology)",
"Ants have the most wet biomass (of animals on your list), but humans have the most dried out biomass."
] |
[
"Use backslashes before parenthesis.",
"http://en.wikipedia.org/wiki/Biomass_\\(ecology\\)"
] |
[
"It's not the underscore but the parenthesis."
] |
[
"Why are there both on-center and off-center receptive fields in the retina & LGN?"
] |
[
false
] |
Center-surround receptive fields are useful for detecting changes in luminance across the visual field, but I would think just one organization (i.e. excitatory-center inhibitory-surround inhibitory-center excitatory-surround) would do the trick. What is the point of having both organizations? Is it redundant coding? I have read some places that the two organizations are because you want to be able to detect a dark object on a lighter background or a light object on a darker background, but again, I can see this done by the same organization. In an ON-centre cell, a dark spot would decrease firing, and a light spot would increase firing. Done, and done. I need another example to help me figure this out. Thanks in advance... :)
|
[
"The reason is basically that baseline firing rate for post-retinal neurons is close to zero. In cortex, baseline ",
" zero. So, a zero-contrast stimulus gives you zero firing rate. That means there's no room in the response range to represent both positive and negative polarity stimuli.",
"Even if it were possible, it would make for some strange encoding, which might not square with visual experience. We see dark and bright contrasts as qualitatively opposite things, not as different points on a continuum. The fact that they are encoded by separate populations of neurons helps to make sense of that.."
] |
[
"What I'm going on is having seen many plots of response as a function of contrast; even for RGCs, the response begins near zero for zero contrast, then increases monotonically. e.g. Kroner, Purpura and Kaplan PNAS 1993, or Kaplan & Shapley, PNAS 1986 (that one is good, one of the first figures is an average over a few dozen cells; it goes from near zero to ~70 spikes/s as contrast increases).",
"If you look at the distribution of baseline firing rates, they'll be massed near zero, with a long tail (i don't have an obvious reference for that but i can look). In the early days (Kuffler, Barlow, etc.), the cells with higher baseline firing rates would be the easiest to find, for obvious reasons. Once we got better at finding what neurons were tuned for, we find that most of them are quiet unless stimulated."
] |
[
"Sure, let's see what I come up with:",
"Say we have a simple pattern of light, consisting of three vertical bars, like this: |||",
"The outer bars are black (very little light), the inner bar is white (lots of light), and they're surrounded by a mean gray field (an intermediate amount of light).",
"There are neurons in primary visual cortex that REALLY like this kind of stimulus, and if you put it in their field of view (their 'receptive field), they'll respond vigorously, firing lots of action potentials.",
"To decrease the contrast of the pattern, you add some light to the black stripes and subtract some from the white stripe. When you do this, the response of the neuron will decrease. At some point, when the bars are nearly the same luminance as the gray background, the neuron won't respond at all.",
"So, we see this neuron as encoding this kind of simple light pattern: a bright streak flanked by dark streaks. If the neuron responds, then a similar pattern is present; if it doesn't respond, then no similar pattern is present.",
"If we ",
" the contrast of the pattern, we're making the center bar (which was white) black, and vice versa, so now there's a black bar flanked by two white bars. Call this the 'negative' pattern, and the previous one the 'positive'.",
"Depending on what kind of neuron our neuron is, two things can happen when we show it the 'negative' pattern. If it doesn't respond, then it's a \"",
"simple cell",
"\" - if it does respond, then it's a \"",
"complex cell",
"\".",
"These two types of neurons are encoding different types of information. The simple cell cares about the polarity of the stimulus, so we say it's encoding polarity - one neuron likes bright bars (flanked by dark), and another likes dark bars (flanked by bright). The simple cell doesn't care about polarity (or position), just about the structure - so long as the bars are shaped the right way, it don't matter if they're black or white.",
"But whatever kind of neuron it is, it won't respond if you give it a zero contrast pattern (i.e. a blank). It will sit there quietly, waiting for its preferred pattern to appear, and then it will fire in proportion to the contrast of the pattern: higher the contrast (in absolute terms: i.e. \"very negative\" is also \"high\"), bigger the response.",
"Because of this kind of behavior, we know that e.g. brightness and darkness must actually be encoded in separate neural channels - there aren't any neurons that encode light level ",
", e.g. \"black is no response, white is big response\". It's all encoded around the local mean luminance.",
"How the local mean is included is.. complicated. That's another story."
] |
[
"Is light a wave, or is it packages of energy?"
] |
[
false
] |
I'm a little confused.
|
[
"Hi, unfortunately the answer is that it is in fact both! Light is made up of little packets called photons, each with one quantum of energy.",
"The reason for this is that light demonstrates a lot of properties- in some cases the photon (packet) explanation is the only one that works. In some cases the wave explanation is the only one that works.",
"They are both equally valid, scientifically speaking.",
"An example of a packet-only explanation would be the ",
"photoelectric effect",
". Simply put, sometimes when light shines on a metal it can emit electrons. ",
"Now there is a minimum amount of energy that the electrons need to absorb before they can be emitted- that's called the ",
"work function",
".",
"It turns out that there will be a certain frequency of light that corresponds to the energy of the work function. That frequency and higher will remove electrons.",
"Any ",
" than that and you won't see any electrons being emitted.",
"At the characteristic frequency, increasing the intensity of the light increases the number of electrons emitted, but not their energy.\nThe same applies for decreasing the intensity at that frequency. Energy of electrons stays the same, but number decreases.",
"At lower frequencies than the energy of the work function, there are ",
" excitations.\nIf light were just a continuous wave you would imagine a continuum of energy being absorbed by the electron, allowing it to be emitted in a continuum of small steps (if the intensity were high enough).",
"That doesn't happen.",
"It only makes sense if light were made up of little packets, and the packets can only be absorbed one at a time. So electrons will only be emitted if the packets absorbed are of the right energy (depends of the colour/frequency/wavelength).",
"But there are times when a pure particle explanation doesn't work either. For instance in simple ",
"intereference",
").\nIf you take the ",
"double-slit experiment",
" and send the light in one photon at a time, and measure then position of where the photons hit the detector screen over time, you'll see an interference pattern.",
"The only explanation for this is that the photon has an associated ",
"wave-function",
" that becomes sufficiently spread out that it allows the photon to potentially pass through ",
" slits, and interfere with ",
".",
"Trying to measure which slit the photon went through actually destroys this interference pattern- an experimental observation of the ",
"uncertainty principle",
".",
"TL;DR",
"It's really both. It just depends on the circumstances you put the light into."
] |
[
"wow thanks for the answer :D Now I am a little more cleared up :)"
] |
[
"It's neither, really. Light is what it is and we choose to ",
" it as either behaving as a particle or behaving as a wave. It's important to emphasize that light doesn't exist as a particle in some cases and as a wave in other cases. It's us humans who choose one model or the other to describe an interaction because it makes more sense to ",
".",
"I can't think of any behavior of light that can not be explained using both models, though one will always be simpler and more intuitive than the other."
] |
[
"If you eat a fish with cancer, is it unharmful?"
] |
[
false
] | null |
[
"The tumors were right behind the gills so I guess it would be lung cancer.",
"Haha, epic. Eating a cancer-ridden animal is fine, cancer isn't contagious across our own species, let alone across species lines. The reason cancer is so strong is that it evades our immune system by looking almost exactly like our own cells. A cancer cell from a fish would be destroyed very quickly by your white cells, assuming they weren't digested first"
] |
[
"You have to remember that all diseases aren't the same. Rabies is a virus, a very powerful one that can gain virulence (infectious power) though crossing species lines. This is what happened with H1N1 flu.",
"Cancer is simply rogue cells that have lost contact inhibition, apoptosis ability, and divide very rapidly. They aren't designed by evolution to be infectious."
] |
[
"You have to remember that all diseases aren't the same. Rabies is a virus, a very powerful one that can gain virulence (infectious power) though crossing species lines. This is what happened with H1N1 flu.",
"Cancer is simply rogue cells that have lost contact inhibition, apoptosis ability, and divide very rapidly. They aren't designed by evolution to be infectious."
] |
[
"What makes human altruism different than, say, other hominids or even avian?"
] |
[
false
] |
If the same underlying principles of evolutionary biology hold, shouldn't we expect the same "feel-good" nature of altruism in other hominids? Furthermore, why do Homo sapiens perform self-sacrificial acts of altruism at a higher rate than other species (again, assuming similar selective pressures)? If it's due to our complex brain, then why have such a brain in the first place? It consumes an inordinate amount of our body's resources, leaves our infant state unbelievably vulnerable, and thanks to our huge noggins, makes child-birth deadly to this day.
|
[
"There is some extent of scientific agreement that altruism arises in organisms only if there is some amount of relatedness between the organism helping and the one being helped. Why does it happen more in humans than their immediate relatives? (Well, we're definitely not the MOST altruistic organisms though: look at bees and ants) That probably became a thing when humans started living in (somewhat) large groups, perhaps village-sizes. This is speculation though, but it would make sense that when you are in a village with 250 people or more, you kind of cannot remember everyone, you would rather help anyone you have a good sense of belonging to \"your group\". Of course how do you define a \"group\" then? I think (underline think) that our brain is not very specific here. We have more cognitive capabilities and I feel that our mind can redefine who belongs to our group very differently. Thats why we still try to find a sense of patriotism, thats why people still fight between states and clans (in some places) and are more closer to people of their own race. Of course, in today's world our sense of belonging extends almost to entire humanity and that could explain why some of us are just altruistic in an almost \"global\" sense, helping out anyone just because they're human (or even just because its a living thing!)."
] |
[
"Taken my vastly different approaches to education (bs in biology and minor in leadership studies which is heavily philosophy based), I have somewhat conflicting notions of the idea's of altruism. While Biology does in fact characterize humans as an altruistic creature (we will lower our fitness to temporally improve others), the other side of of the coin states that there is no altruistic act...which leads to reciprocal altruism. If you consider all altruism as reciprocal altruism, then it could be an evolutionary step, or merely cognitive choice, to invest in our future fitness when our current fitness is seeing boons.",
"Vampire bats share food because they realize that should their own food run low, the sharing will be returned. Birds commit to warning calls because, should they be in the inner portion of a flock, they will benefit from it later down the road. However, as to the reason why humans tend to exhibit this at a higher rate, I would imagine that it is heavily dictated by cultural norms and expectations, and not so much evolutionary gain."
] |
[
"But what then sparked our social revolution? Is it some luck of genetics that ancient ancestors developed sufficient cognitive ability to \"create\" culture? Did other hominids not also have the same opportunity?",
"I guess my question also ties into the classic question of why we tip, even if we don't plan on visiting the same restaurant again. Reciprocal altruism doesn't play a part (no further contact), and Hamilton's rule certainly doesn't explain this. ",
"I guess then I'm looking for the sinuous connection between our freak genetics and complex social customs and hierarchies."
] |
[
"What exactly are the differences between Active ingredients and Inactive ingredients in medicine?"
] |
[
false
] |
Obviously (or so I think), the active ingredient is the one that is the most important for chemical reaction to help out the taker's problem. But what exactly is the purpose of the inactive ingredients? Is the name "inactive" a misnomer because the name seems to imply it does nothing? For example I have some flavored Tums (an antacid) in front of me: Active Ingredient: Calcium carbonate USB Inactive ingredients: Sucrose (for flavor?), Calcium carbonate, corn starch, talc, mineral oil, natural and artificial flavor, adipic acid, sodium polyphosphate, red 40 lake, yellow 6 lake, yellow 5 (tartrazine) lake, blue 1 lake. I'm guessing the lakes are food coloring.
|
[
"Yup, those are things for coating/dissolving/stabilising/making taste good the active ingredient so it can do it's work."
] |
[
"Ah I see. That makes sense. I guess the wording \"inactive\" just was confusing because that implied (to me) that they were useless. I guess it was more meant to be \"inactive in helping the main cause of the medication\"."
] |
[
"They don't eat foods that are artificially colored."
] |
[
"Why does birth control fail?"
] |
[
false
] |
If a woman takes it exactly as prescribed, or has an IUD, then how can they get pregnant? Why is it only 99% effective?
|
[
"A full answer to this would be extremely long and involve a lot of \"we don't know.\"",
"There are a lot of mechanisms geared towards reproduction, and biological mechanisms aren't precisely engineered. They have some tolerances built-in, and these vary from person to person (and from cycle to cycle!).",
"The combined oral contraceptive contains progestin and estrogen. These work to inhibit ovulation. When the pill was first released, the dose was four times higher than it is now, which wasn't safe, long-term. The dose was decreased. Estrogen is still a risk factor for clots -- but so is pregnancy. So we have it at a tolerably safe dose with a significant reduction in risk of pregnancy.",
"Let's get more specific: the most effective form of birth control we have is Nexplanon. When Merck was replacing their previous contraceptive implant, Implanon, with Nexplanon, they did a study on all the causes of failure in Implanon.",
"Again, this is the most effective contraceptive we have. It is 99.9%+ effective. It is more effective than tubal ligation.",
"Of the 127 causes that they found:",
"84 were a failure to insert implant -- one of the biggest changes between Implanon and Nexplanon was a package redesign to make it much harder to neglect to insert the device, plus changes to protocol to require the provider to check that the device is present in the needle prior to insertion and absent after insertion.",
"19: incorrect timing -- that means that the patient was either already or imminently pregnant at the time of insertion, or became pregnant in the first week after insertion.",
"8: interaction with hepatic-enzyme-inducing meds -- progestin is digested by a set of liver enzymes that some other medications up-regulate.",
"3: expulsion -- the device came out because it was poorly-inserted.",
"13: product/method failure: as in, unexplained.",
"There were some theories that those 13 unexplained cases may have been related to obesity, because fat tissue is hormonally active and increases the volume of distribution of the medication.",
"For IUDs, it's also the case that most failures are due to the IUD not actually being there, or placement being poorly-timed."
] |
[
"This has been studied for the copper IUD and it is the most effective form of emergency contraception, yes.",
"However, it is still less effective as emergency contraception than as non-emergency contraception."
] |
[
"OP said that the effectiveness of the contraceptive was over 99.9% effective. Surgery always carries risks of being performed improperly, and our bodies are always trying to heal ourselves, so for tubal ligation to be less effective than this birth control you're looking at a surgical failure rate of 1 in 2000 (two separate tube surgeries per woman). ",
"Your body is full of tubes that look like other tubes, and not all doctors are infallible. "
] |
[
"When we break the sound barrier, we get a sonic boom. Theoretically, what would happen if we broke the light barrier?"
] |
[
false
] | null |
[
"Since nothing can go faster than ",
" your question can not be answered. (No physics exists for it.)",
"Edit: In the situation of light in a medium where it is less than ",
" and can be overcome - the result is ",
"Cherenkov Radiation",
"."
] |
[
"It's kind of like asking where the lungs are on a coffee table. "
] |
[
"Theoretically, you can't. It isn't a \"barrier\", it's simply impossible to reach no matter how more force you give something.",
"There is a whole lot of interesting stuff that happens when you are going ",
" to the speed of light though."
] |
[
"Can there be gas moons?"
] |
[
false
] |
There are both terrestrial and gas planets, so can a planet's moon be gas as well? How would this form?
|
[
"This couldn't happen, because a gas body is quite large. You could end up with a gas binary planet system, with bodies of similar size, but not anything of the size disparity to be considered a moon/planet system. Either the moon would dissipate or it would be consumed by the planet.",
"If you had something the size of Neptune orbiting a huge gas giant, I think that would still be considered a binary planet system, it if were even possible due to the gravity of the gas giant.",
"A similar question is answered here: ",
"http://www.spaceanswers.com/deep-space/2323/why-does-a-gas-planet-not-have-a-gas-moon/"
] |
[
"That is true, the definitions are ambiguous. I do not know the math behind the stability of certain mass gas giants orbiting much larger mass gas giants. I suppose at this point we would call them both \"gas giants\", the defining factor perhaps would be which one orbits which. If they each orbit each other, they would be a binary system. If one orbits the other, it would be a planet/moon.",
"The real problem would be how would such a system form? I suppose a smaller gas giant could be captured by a much larger one, but I don't see how they could both form in proximity. A gas giant is a super vacuum cleaner in its orbit and nearby vicinity. Gasses would be hoovered up by its rocky moons and the gas giant itself eventually.",
"I'm sure though that there are examples of this out there, I just think it unlikely. Not that that means much in our extremely limited knowledge of extra solar planetary mechanics."
] |
[
"That is true, the definitions are ambiguous. I do not know the math behind the stability of certain mass gas giants orbiting much larger mass gas giants. I suppose at this point we would call them both \"gas giants\", the defining factor perhaps would be which one orbits which. If they each orbit each other, they would be a binary system. If one orbits the other, it would be a planet/moon.",
"The real problem would be how would such a system form? I suppose a smaller gas giant could be captured by a much larger one, but I don't see how they could both form in proximity. A gas giant is a super vacuum cleaner in its orbit and nearby vicinity. Gasses would be hoovered up by its rocky moons and the gas giant itself eventually.",
"I'm sure though that there are examples of this out there, I just think it unlikely. Not that that means much in our extremely limited knowledge of extra solar planetary mechanics."
] |
[
"How subjective is the human sense of smell?"
] |
[
false
] |
Describing odors is probably one of the most difficult things to do (outside of organic chemistry where everything smells "camphor-like"). But is it really more subjective than the other senses?
|
[
"I think it's mostly an issue of vocabulary. Anyone with significant experience with organic solvents can easily identify the smell of a new solvent in relation to other solvents, in my experience.",
"I can tell you when something smells \"like Xylene\", which generally means it's an aromatic hydrocarbon, or \"like Trichloroethylene\", which implies a chlorinated hydrocarbon, or \"alcoholic\" (self explanatory, I hope).",
"These observations seem pretty consistent across different smellers. I've experienced a similar thing in Wine tasting, actually. People will use a lot of the same descriptions for the same wines, though it's less precise there."
] |
[
"sound can be high pitched or low",
"Though, if you combine multiple sounds - creating a harmony for example, you will often get something",
"complex or hard to describe",
"Also harmonies are very subjective as they trigger different emotions (or memories, or whatever) in people. So is there really such a big difference to smells?"
] |
[
"Probably because a sound can be high pitched or low, light can only be so many distinguishable colors, but smell can be anything of 10,000 recognizable unrelated things!",
"Or to put it in language people like me understand, the state space of scent has thousands of basis vectors."
] |
[
"Is there an \"average\" temperature in space?"
] |
[
false
] |
The other thread about black holes got me thinking. There was another thread 3 years ago that mentioned that space is about 2.7 Kelvin, very close to absolute zero. But that temperature is measured in a vacuum, meaning that the radiation received would be similar to that of matter at 2.7 K. So, we know about stars and how big and warm they are (there is warm matter) and black holes (the bigger the colder) which also has matter. Then there are rocks floating around, the side towards the sun is warm and the side facing away is cold and so on. I don't even know about super novas, I mean there is matter there we can see it, but at that point it's all separated particles which surely can't keep warm for long, until they fall back into each other and form new stars or such. So, if we only take matter into account, what is the estimate here? Is there more warm than cold, on the account that space itself is full of energy, or is there more "dead" matter than alive, lowering our average?
|
[
"Yes, you could compute something and call it the 'average' temperature in space. But it depends on how you want to frame the problem.",
"My default interpretation would be this: ",
" So, for example, if you were dropped somewhere near the Earth's orbit, you would settle at a temperature around 250K because you happen to be relatively close to a star (the Sun). If you were dropped into an active star forming region like the Orion Nebula, you'd settle at a temperature of around 10,000K. You could do that type of calculation for every point in the universe, and average over the volume. But you'd probably end up with an ",
", and almost certainly below 10K. Why? Because the universe is incredibly empty. Most of the volume of space is taken up by the empty regions between galaxies where not much is happening and your main source of equilibrium radiation would be the Cosmic Microwave Background (CMB) of 2.7K. So even though that's the minimum temperature and the equilibrium temperature in galaxies and near stars is much higher, those regions take up an incredibly small volume of the universe."
] |
[
"the vast majority of matter in the Universe is in stars.",
"That's definitely not true.",
"The vast majority of matter in the Universe is in dark matter. Here's a breakdown of just our Milky Way Galaxy:",
"Mass of Interstellar Medium (Gas & Dust): 7 x 10",
" Solar-masses",
"Mass of Stars: 5 x 10",
" Solar-masses",
"Mass of Dark Matter: 1 x 10",
" Solar-masses",
"It's more than a bit difficult to pin an exact temperature down for dark matter, but based on the clumpiness of the matter we can see, cold dark matter models fit the data better, and ",
"cold dark matter must be very cold",
"."
] |
[
"the vast majority of matter in the Universe is in stars.",
"That's definitely not true.",
"The vast majority of matter in the Universe is in dark matter. Here's a breakdown of just our Milky Way Galaxy:",
"Mass of Interstellar Medium (Gas & Dust): 7 x 10",
" Solar-masses",
"Mass of Stars: 5 x 10",
" Solar-masses",
"Mass of Dark Matter: 1 x 10",
" Solar-masses",
"It's more than a bit difficult to pin an exact temperature down for dark matter, but based on the clumpiness of the matter we can see, cold dark matter models fit the data better, and ",
"cold dark matter must be very cold",
"."
] |
[
"Are there any patents on the covid vaccine or are all companies sharing \"open source\" tech at this point?"
] |
[
false
] | null |
[
"Currently no, however there is a ",
"vaccine",
" in phase 2 trials that is open source. Once approved it will still need to be manufactured by proper facilities so not as open source as something like software. At this point any facility capable of producing a covid vaccine is already making them under contract or ownership of a company with an approved vaccine so an open source vaccine isn’t likely drastically to increase the supply."
] |
[
"There are many patents covering the vaccines and their manufacture. The notion that they should make their core IP public domain was ridiculously naive. The mechanism for providing access, or more manufacturing capacity is to license the relevant patents. ",
"My analogy is that you can let someone stay at your house for free without giving them (or the world) title to your house."
] |
[
"Here",
" is one that is published but not granted in Russia.",
"There is probably more but I don't really want to look through it all: ",
"https://patents.google.com/?q=Covid-2019&oq=Covid-2019+"
] |
[
"How often do cats lose their whiskers?"
] |
[
false
] |
My cat lost a whisker last night. From what I've found online, this is normal (as long as it doesn't happen too often), but I can't find anything from a reliable source on how often this happens. Any veterinarians out there who can give me an answer?
|
[
"The worst that might happen if they lost several whiskers would be that they might have some balance/coordination issues.",
"This is a myth. Feline whiskers' purpose has nothing to do with balance. Like all mammals, a cat's sense of balance are from the inner ear. Whiskers are used as \"feelers\", mainly to judge distances and how tight of a space they can fit into. If you notice, in some cases the fatter the cat, the longer it's whiskers. "
] |
[
"I am not a veterinarian, but two things: First, giving/requesting medical advice in this subreddit (if not all of reddit) is frowned upon. With that being said, your question ",
" scientific.",
"Second, yes, this is normal. Your cat will be fine, at least as far as losing one (or a couple) whiskers. Their whiskers are basically specialized hairs so they have a replacement cycle just like normal hair. The worst that might happen if they lost several whiskers would be that they might have some balance/coordination issues.",
"Losing a whisker every now and then is normal just like any other hair. If you don't notice any other signs of illness or injury then there is no reason to think there is anything wrong with your cat.",
"EDIT: Also, I defer to any actual vets if they know differently. Hopefully a veterinarian or biologist will give you a more \"official\" answer."
] |
[
"This is what I know the whiskers are for as well. It appeared in a primary school textbook, and it mentioned that if you cut a cat's whiskers off it could potentially trap itself in a tight space because they lose the ability to judge the 'width' of their bodies properly."
] |
[
"Rain Question"
] |
[
false
] |
I have a really random question. Has there been any studies done to see how far a single drop of rain moves on its descent from the cloud it originated in? So basically if I drop something depending on weight and whether it flies or glides it will land directly below the point where I released it, how close to directly below the cloud it came from does a raindrop land?
|
[
"This is almost impossible to answer.",
"It's impossible to give an exact answer, but we can give some rough limits. Raindrops form at an altitude of about 100 to 5,000 meters. On average, they fall at a constant terminal velocity of 10 meters per second, meaning that they take about 10 to 500 seconds to reach the ground. Average wind speed is typically somewhere between 1 and 10 m/s. Therefore, assuming the wind is blowing steadily in one direction at the average rate, a raindrop will typically hit the ground at a point that is ",
" from where it started its final descent. 10 meters displacement would be for a low rain cloud and slow wind, and 5000 meters displacement would be for a high rain cloud and a moderately fast wind. Of course, extreme wind speeds will place the value outside this range.",
"Note that this assumes a steady wind in the same direction the whole time. More realistically, the wind will change direction, so that the raindrop will lose some of the lateral displacement it first gained. This means that the range would be reduced."
] |
[
"I think rain is most heavily affected by the wind - starting from the initial motion of the cloud it forms in (also caused by wind), then any individual raindrop is going to be moved around by whatever the wind is doing on its way down, potentially even moving in different directions if the wind or gusts are different at different altitudes.",
"So you'd need to know a lot about the wind situation for any given raindrop to give an accurate estimate of how far its going to travel horizontally from its origin point.",
"If the rain was occurring in a no-wind situation then the drop should pretty much just fall straight down."
] |
[
"This is almost impossible to answer. Keep in mind there are an infinite amount of variables in this equation.",
"\nFirstly, a particle of dust is the beginning of the \"drop\" of rain, cold air rising into a warm front (or vice versa) creates condensation, so other small molecules of water begin to collect on the dust particle. As more water molecules join the drop it becomes heavier and begins its decent toward the Earth due to gravity. Along the way there are many other factors: wind, pressure, sunlight, differing temperatures in the troposphere, etc. you would have to isolate all of those to obtain a definitive answer. Now, if we were in space, the answer would be easy - it would boil then freeze as all its energy is released as heat.",
"I would say the drop of rain travels a great distance both in the vertical and horizontal planes before landing."
] |
[
"Why is it that if two cars run into each other while both going 60mph the collision isn't equivalent to a 120mph crash?"
] |
[
false
] | null |
[
"That 60 mph crash is into a solid, unmovable wall, and all the energy is absorbed by 1 car. In the two cars colliding head on at 60mph each, each car delivers as much energy as the other, but also absorbs as much, so while the total energy in the collision might be double, it's also absorbed by double the number of cars, and each car absorbs the same as just 1 hitting a solid object.",
"When the cars have different masses, it changes, and something like a big rig at 60mph smoking a smart car at 60mph, head on, becomes much more like a 120mph accident, for the smart car. On the other hand, the big rig won't see it as nearly a violent accident as it would had it struck a solid wall instead.",
"This has great implications in crash-test ratings. The Smart has great forward crash test rankings, thanks to it's high-tech and ultra-rigid body. But against something like an F-350...mass wins."
] |
[
"Because kinetic energy is given as 1/2 mv",
"In the collision between 2 60 mph cars, assuming both cars are the same mass you end up with kinetic energy proportional to 2(1/2 60",
" ), or 3600",
"In the case of 1 car going 120 mph, you end up with kinetic energy proportional to 1/2 120",
" or 7200.",
"In each case, that number will then be multiplied by the mass of one car.",
"Additionally, as others have said, it has to do with whether or not it's an elastic or inelastic collision - whether or not the cars rebound from each other or from a wall.",
"(Edited to fix formatting)"
] |
[
"At least in the US, road maintenance is paid for primarily by gas taxes. ",
"The fuel efficiency of a smart car is 36 mpg combined. The fuel efficiency of an F-150 is 17 mpg. Assuming an equal number of miles driven, the F-150 driver is paying about double the road taxes of the smart car driver. "
] |
[
"Does Gravity move slower in different mediums? (e.g. water, air)."
] |
[
false
] |
If the speed of light = the speed of gravity, but the speed of light moves slower in different mediums, then is the same true for the speed of gravity?
|
[
"light moves slower in water, etc. because light interacts with the water, not because \"the speed of light\" changes in water. \"the speed of light\" is the speed that an object would have to travel in order for the vector describing the velocity to be null (g_{ab} v",
" v",
" = 0). the water doesn't actually change \"the speed of light\" so the gravitational waves would move through water as if it weren't there.",
"if you found a medium that did change \"the speed of light\" in it, then yes, gravitational waves would move at a different speed through that medium."
] |
[
"wait, that's kind of an absurd question, of course gravity interacts with water. gravity interacts with anything that has mass.",
"The reason it SEEMS like light slows down is because the atmospheric particles absorb, reflect and discharge light. So the light you see is not a straight beam from its source. It's the same as if you asked a person to run through a hedge maze vs. on a straight track. Point a and point b may be equidistant in both cases, but it will take longer for the person to navigate the hedge maze even if both people are equally fast"
] |
[
"You are basically asking whether gravity is a quantized force (or whether there is a graviton) or not. That is still not certain. Moreover, quantum gravity does not necessarily mean that matter slows its propagation.",
"What empathetica1's explanation means is that there is some ultimate limit of all information travel, ",
", and light only reaches that limit in a vacuum. If there is some material that causes gravity to propagate at less than ",
", or if that material has a lower ",
" overall, then gravity would be slower.",
"I don't think we have seen any evidence of such a material, and I haven't heard of any theory that would predict its existence."
] |
[
"Is it possible to land safely in a wingsuit without a parachute?"
] |
[
false
] |
From what I've heard (and this could be off), a person generally travels forward twice as much as they drop in a Wingsuit. The speeds look pretty fast. Would it be possible however to arc yourself upwards to a point where your velocity and momentum would be offset by gravity to come to a soft (and safe) landing?
|
[
"The performance varies, but a measured reference was 300km/hr horizontal speed with a 30km/hr descent.",
"A person with enough speed built up can temporarily slow their descent, but it's temporary, soon the loss of velocity makes them drop, more than they were dropping initially. ",
"Gary Connery's record-breaking \"landing without a parachute\" into a mass of cardboard boxes slowed him down to 80 km/h horizontal and 24 km/h vertically by flaring back near the end. This is not a sustainable drop, and should be assumed as the slowest rate possible with the current soft \"wingsuits\". Sure, we could make them larger, and/or give 'em a rigid framework, but then you've got a parachute or hangglider and we KNOW those land just fine. ",
"Well bellyflop onto a runway at 24km/hr downward speed is very problematic. That's the kind of downward speed you'd get from dropping from a 9m roof. Now jumping off a 3m roof is easy enough if you land on your legs, but you'll be seriously injured if you bellyflop onto the ground.",
"But the problem of 80 km/h horizontal velocity is there. That's the same as being tossed out of the back of a pickup on your belly at 80 km/hr, an almost certainly fatal tumble.",
"If you have wheels on your belly (and brakes) and were able to use them successfully, the horizontal speed wouldn't matter too much. You'd roll with no additional stress and slow gradually, provided you had enough runway to brake.",
"But practical wheels would be really tricky. Small wheels running at 80 km/hr are really hard to control even on a rigid frame. ",
"Even if you had working wheels on your chest to take the horizontal velocity, it still doesn't change the fact that you are belly-flopping onto the runway like you just jumped off a 3rd story roof. You'd need a huge shock absorbing suspension with substantial distance from wheels-to-body to make this survivable. ",
"It'd be hard to fly with that much drag, and implausible that you wouldn't flip headfirst when the wheels made contact. A large landing gear framework would probably not fit the intent of \"landing in a wingsuit\".",
"Some aerodynamic improvements may be possible (and esp when working with a very small, light person) to better these numbers. But IMHO it would have to be like HALF the current kinetic energies to make this remotely possible.",
"The jet-powered rigid wingsuit ",
", in theory, land. But as far as I can tell it needs ~200 km/h to create enough lift for level flight. Skating along on wheels at this speed would be almost impossible even with a smooth runway, you'd likely flip and tumble on the runway which would be fatal. ",
"For reference, jet-powered street luge (small-wheel sled) HAS achieved a record of 183 km/h. However, landing is substantially more stressful- having some side motion or rotation is likely. The wheels could be spun up to speed before contact so they're not having to go from like 0 to 10,000 rpm instantly.",
"But that street luge record speed would likely be fatal coming down from a tiny 3\" hop in the air at that speed. More speed, trying to land... oh it's possible to be successful, but more than likely it looks like you'd tumble and die."
] |
[
"Well the easiest solution would be to just build a setup similar to ski-jump hills. The landing zone is sharply graded. Obviously though some people might take exception to that as a \"landing\"."
] |
[
"Well the easiest solution would be to just build a setup similar to ski-jump hills. The landing zone is sharply graded. Obviously though some people might take exception to that as a \"landing\"."
] |
[
"Will the Sun's gravitational pull on Earth change as it expands?"
] |
[
false
] |
I was thinking about the Sun's expansion at the end of it's life- as it expands a portion of it's matter will become ever closer to us, but on the opposite side of the sun from us matter will be getting farther away. Will the change in distance of matter on both sides cancel itself out? Will the Earth experience a greater pull, sucking it in before the expansion would have consumed it? Is this a completely misguided question? Thanks in advance. Edit: I did Google the question before asking, but none of the answers addressed the matter / mass getting closer to Earth, which to me would change the gravitational force. The answers simply stated that gravity wouldn't change. The volume is increasing, but not mass.
|
[
"It will, but for a different reason - the Sun loses mass in the same process. If the mass would stay the same it would stay the same. This is the result of the ",
"Shell theorem",
". Any mass distribution with a spherical symmetry (a good approximation for the Sun) leads to the same gravitational force as a point mass with the same mass in the center - if you are outside. If you are inside then everything with a larger distance to the center than you doesn't contribute to the force you get."
] |
[
"because the center of mass grew",
"The center of mass is, by definition, a point. It does not grow."
] |
[
"because the center of mass grew",
"The center of mass is, by definition, a point. It does not grow."
] |
[
"Has science (engineering) come up with a better microphone than a human ear?"
] |
[
false
] |
If so, can it compete with natures best?
|
[
"If by better, you mean more sensitive, then yes. "
] |
[
"\"Better\" is qualitative. There are sound capturing devices that are able to capture sounds (both directional and not) that an ear couldn't. Whether it is \"better\" or not is a function of the use of the ear and the purpose it serves which is impossible to answer. For examples \"is it better to have 1 child or 2?\" Science can't answer that without some sort of further goal or weighted analytic matrix being defined.",
"If so, can it compete with natures best?",
"My mouse trap car from grade 7 can compete with Usain Bolt in the 100m sprint. What information can you glean from that that would make you ask that question?"
] |
[
"Based on ",
"Fletcher–Munson curves",
", the human hearing range is from 20 Hz to 20 kHz (or < 20 kHz less if you're older). The dynamic range of our hearing is, at best, around 130 dB. For starters, let's ask if there's a microphone that beats that..."
] |
[
"How does water get to the top of huge trees through adhesion, cohesion, and capillary action, when atmospheric pressure should limit the uptake to 10 metres?"
] |
[
false
] | null |
[
"Imagine a long vessel starting at the roots and ending in te stomata of leaves. The roots take up water by osmosis after which it moves into the xylem vessel. It would not go far indeed but for evaporation at the leaves (trough stomata) which creates a lower pressure at the top of the water “column” thereby drawing it further upwards."
] |
[
"It would make it less effective but there is still an enormous surface area across all the leaves for evaporation to occur. And even at 100% humidity water will still evaporate since it is a state of equilibrium and is not static, but it will be very slow."
] |
[
"I have heard this explanation too, but I have to wonder what would happen in conditions where the air is saturated with water vapor. That would mean the evaporation wouldn't work. Should that not make this mechanism muss less effective?"
] |
[
"If a chemical is turned into plasma, does it keep it's uses/properties?"
] |
[
false
] |
[deleted]
|
[
"From ",
"https://en.wikipedia.org/wiki/Plasma_(physics)",
"\"A plasma can be created by heating a gas or subjecting it to a strong electromagnetic field applied with a laser or microwave generator. This decreases or increases the number of electrons, creating positive or negative charged particles called ions,[2] ",
".[3]\""
] |
[
"In the case of NaCl, if you pefectly exclude all water and oxygen for the entire process you may recover NaCl after cooling to room temperature. You wouldn't be able to cook the eggs with it. The Na and Cl nuclei would react with whatever they touch upon quenching and you'd lose at least a portion of the saltiness to other end products like sodium oxide/hydroxide and chlorate/chlorite/organo-chlorides. "
] |
[
"So do the Na and Cl seperate as well?"
] |
[
"Could non-carbon based lifeforms be living among us without us being able to recognize them as life-forms?"
] |
[
false
] | null |
[
"Not having proof of something's existence does not mean that it doesn't exist or that has to be false. I'm not saying this about ghosts but it's the way you put it...Back in the days there was no proof that the earth was round. Proofs come when you look for them when trying to validate/invalidate a theory"
] |
[
"It's important to get a good definition of \"life\" for this question. Assuming the scientifically used definition of \"being manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally\", all of these phenomenon would be observable in the matter forming the life, and thus, we'd recognize it as life.",
"If you're talking about forms of \"life\" that don't exist in standard matter, or this dimension, (not sure what specifically you mean by \"ghosts\") then I can't really say anything. \"Unable to fully interact with what we have created as humans\" is an odd definition, but I take it you mean that it would not interact with the matter we're familiar with, in which case - no, we have no way of observing it, as it would be outside our reality, as well as having no bearing on it."
] |
[
"You're right, except this post asks for \"the scientific angle\" regarding \"if 'ghosts' could be life-forms\". Science says we have no proof that they exist."
] |
[
"Why are people more likely to be right handed than left handed?"
] |
[
false
] |
I hear people say it's a "right handed world" where a majority of items are designed for right handed people as default. How did it even get to be like that in the first place? Why are we not inclined to be more left handed, or even ambidextrous?
|
[
"So a lot of people are touching on the idea of where language is localized. Most of the time we have language localized in our left-hemispheres. The left side of our brain controls the right side of our body, and vice versa. It is commonly believed that this is why most people are right-handed. ",
"This article",
" says that: ",
"Functional hemispheric language lateralization has shown to be correlated with handedness: 95% of right-handers show left-sided functional hemispheric language lateralization, while 15% of left-handers show right-sided functional lateralization (Lurito and Dzemidzic, 2001 and Pujol et al., 1999).",
"Also a weird fact that left-handers tend to have shorter life expectancies for some reasons that are currently not well understood. ",
"Here",
" is one study on that topic."
] |
[
"I thought the reason for left-handers living shorter lives than right-handers was because in the past left-handed people were forced to use their right hand, so left-handers that have died recently have tended to be on average much younger than right-handers that have died recently.",
"This was covered in my intro stats class, though I'm not sure if it is related to the study you linked."
] |
[
"A couple of theories floated in the literature:",
"It might be a secondary/derivative trait, a result of the lateralization of language in the brain: ",
"http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=176485",
"It might be a primary trait, with an allele all of its own: ",
"http://psycnet.apa.org/psycinfo/1997-30055-003",
"Interesting facts about ambidexterity: humans are the only strongly handed species, but as you get to our nearest ape relatives, handedness starts to appear (eg chimpanzees ",
"http://psycnet.apa.org/psycinfo/2004-21410-003",
" ). Evidence from the chimp studies seems to indicate that handedness is ",
" related to language lateralization, but rather has to do with other lateralization of motor function. ",
"So your question may reduce down to: why are certain capacities lateralized in the brain? A full answer to that, I'll have to leave to a better specialist, but roughly, it's to do with the fact that we have limited brain real estate, and specializing on one side leaves the parallel structure on the other side free to do other things (as opposed to other species, where parallel structures on both sides serve roughly the same purpose).",
"Note: a truly fantastic book on the subject is ",
" by John Napier",
". Reading ",
" is where I found out about the handedness in chimpanzees."
] |
[
"Is there one \"color\" on the light spectrum that is more prevalent in our universe than the others, or is each represented equally?"
] |
[
false
] | null |
[
"My first thought is that the cosmic microwave background would dominate on a cosmic scale. These photons were generated everywhere in the early universe during the recombination epoch: the time when the universe cooled enough to allow electrons and protons to form stable hydrogen atoms.",
"One interesting thing to note about the CMB is that when the photons were first generated, the wavelengths were much shorter! Only due to the expansion of spacetime over ~14 billion years have the photons redshifted as microwaves.",
"The answer below goes into good detail without being overwhelmingly technical:",
"https://physics.stackexchange.com/questions/199680/what-is-the-most-common-photon-wavelength-in-our-observable-universe-not-includ"
] |
[
"This isn't a direct answer to your question, but is along the same lines of thought, and also kinda neat.",
"A survey of the colour of all light in the universe added up to a slightly beigeish white, called \"Cosmic Latte.\""
] |
[
"Damn that's awesome thanks man! "
] |
[
"If I drop a boat into a hollow air-filled shaft in the ocean, then move it into the water and let it rise back up, can it generate net energy this way? If so, how?"
] |
[
false
] | null |
[
"No you can't make energy this way. Pushing the object in/out of the airlock and pumping it would take as much (actually more if you account for losses) energy as what you would gain from dropping the object and letting it float up."
] |
[
"That seems correct only if the volume of the float is completely replaced with water inside the lock, then you would have to pump all that water out, but this seems avoidable.",
"Consider that the lock is a cylinder and the float a very close shape. Say there's a quarter inch space difference all the way around and the float is some large diameter like 10'.",
"We have the float at the bottom of the air shaft. We open the air side of the lock and push the float into the lock and close the door behind it. 99% of the lock volume is filled with the float.",
"Now we open the water side door and allow the water to rush in filling that remaining 1% around the float.",
"With that done, there's no water pressure we're pushing against to move the float horizontally into the sea, it's already pressure equalized. So we're not losing or spending any significant energy to push the float into the ocean.",
"Furthermore we can push it out of the lock using a piston the same shape as the lock that prevents water from rushing into the lock as the float exits. ",
"Then we shut the water side door, the piston retracts but draws in air from the air side, and only that 1% of water leaks into the air side shaft, ready for another float to enter.",
"It seems like it works, what am I missing?"
] |
[
"The pressure on your push piston is proportional to the water column heigh so all you energy would be spent pushing the object out. It doesn't matter the voIume of water you let into your lock. Your idea is a pretty classic \"free energy\" machine concept that people have. You should be able to find plenty of explanations online if you look for buoyancy energy machines."
] |
[
"What was the radiation exposure per year for a \"Radium Girl\", who painted clock dials with luminous radium-based paint in the 1920's?"
] |
[
false
] |
Most of the dial-painters died either of anemia or cancer. My daughter is designing a program for a play, "Radium Girls", about the struggle of these women for recognition and recompense, and wants to use either as the cover or as the center page. She'd like to note the estimated yearly dosage in sieverts a dial-painter would have collected on the chart. I didn't realize it would be so hard to figure out - our heads are spinning with all the units - rads, rems, curies? sieverts? becquerel? gray(?)??? After asking a couple of physics students, I realized I needed to ask YOU, reddit. Here are some resources I've collected. Using an estimated exposure per year of 500 microcuries of radium, how to figure the "dose" in sieverts? Thank you for your help. I used to be intelligent, but I'm feeling pretty confused now.
|
[
"This is actually an extremely complicated calculation. External exposure calculations are easy - you look at how much radiation hits which parts of the body, and calculate the effective dose. For ingestion, you need to know the following things:",
"How much was ingested?",
"What fraction of the ingested activity travels to which parts of the body? (e.g. bones, liver, skin, lungs, etc)",
"How long does it stay in the body? What is the biological half-life?",
"Then you take the activity in each part of the body and multiply by the weighting factor for that part. Then you calculate it over time, factor in the particle type, yada yada...",
"Anyway, what you actually do is to look up the \"committed dose coefficient.\" People have already done the complicated calculations for each isotope and figured out a conversion factor to change \"grams ingested\" into \"effective dose.\"",
"I have a bunch of handy textbooks that list these values, but unfortunately they are at my office. The only decent source I can find at the moment is ",
"this ANL report",
" (big pdf warning). If you skip to the tables at the end, they calculate roughly 10 cGy of bone dose per 1 microcurie. They show lots of the higher-exposed women at around ",
" of committed dose. For your value of 500 microcuries, that would be 5 Gy or roughly 5 Sv.",
"As far as your daughter's program goes... well this is complicated as well. The radium dial painters were exposed over many years working at the factory. The XKCD radiation chart shows the effects of instantaneous exposure. 10 Sv, delivered all at once, would almost certainly result in acute radiation poisoning. But spread out over a year, there isn't any acute effect. You could say they received ~500 times the yearly limit of radiation, perhaps.",
"edit: got my textbook out. 9x10",
" Bq converts to 20 mSv, so 500 microcurie per year would be about 4 Sv per year."
] |
[
"Here is a very rough calculation which excludes the decay products of Radium: ",
"Assuming 500 microCuries( .005 Ci) of nothing but alpha decays from radium through out the year and an alpha particle energy of around 5 MeV according to Wikipedia then we get a total absorbed energy of 9.2*10",
" MeV/sec or 1.4 * 10",
" J/sec or 441 J/year , and it all will be absorbed because alpha radiation has a penetration depth measured in millimeters. ",
"For the whole body dose(this is wrong, all the dose would have been in the mouth and so much higher) over a year we will assume the woman masses 75 kg. This leads to a dose of 5.8 Sv for the year in whole body exposure or 98 Sv for the year all focused in the head. ",
"I can make a more detailed post on my calculations if anyone wants a better accounting, I skimped typing out some steps.",
"edit: thetripp says for references that I got in the ballpark, go me. ",
"Wow, I can't believe how high that is. I'm almost crying thinking about what would happen to those poor girls."
] |
[
"The problem with radiation units is that they are not that complicated but they are confusing. I started a very lengthy response that just got out of hand. One issue as you point out is that there are different unit systems. In the US we used to deal with curies (activity), roentegens (energy delivered to nearby materials), rads (actual energy absorbed by target) and rems (a measure of the actual damage and thus danger to humans). ",
"But the activity has another unit that is the preferred SI unit, the becquerel. Kerma is the chosen SI unit to replace roentegens and sieverts now the SI unit for rems. ",
"Some conversions are simple. A sievert is 100 rem. A curie = 3.7 × 10",
" becquerels. The kerma and the roentegen do not have an exact conversion because they represent slightly different things. A rad = 0.01 grays. ",
"Long story short there is also the issue of how the units are used by experts. Your question asks about \"exposure per year of 500 microcuries of radium\" and might be an ill-formed question. \"Exposure\" is slightly ambiguous because we measure the exposure in terms of the rads or the grays. The curie measures the \"activity\" of the sample. But it might be a common lingo in some circles to say \"curies\" and mean something different because those experts may have some standard conversions or conclusions based on a figure for the activity (for example if we are talking about the activity of a sample that someone ate or inhaled). ",
"The \"activity\" is a property of a sample of your radioactive sample. But it has that activity whether or not you are in the room with it. What you absorb is what matters. But even this has an ambiguity. ",
"A sievert tells you about the potential ability of a sample to cause damage while the gray describes the actual damaging energy absorbed by a target. Yet these are sometimes used interchangeably and perhaps more perplexing 1 sievert = 1 gray, yet they don't represent the same thing. Don't let this bother you. ",
"My advice, I would do some more research and look for these terms. Try to find a source that describes the rads, rems, sieverts or grays that these people are suspected to have absorbed. Texts from different points in history or from different disciplines may have differing conventions in the use of these terms. If you can get any statements about these women in those terms it might be easier to answer the question. If you repost or edit this post with some more info it might be easier to answer this question. "
] |
[
"Why aren't solar panels mounted on wind turbine towers?"
] |
[
false
] |
My thoughts so far: A 3 MW wind turbine tower (ca. 5.000.000 kWh / year) is 100 m tall and about 5 m wide. It should be possible to mount about 500 m facing east to west with a 45° mounting angle to produce roughly 50.000 kWh / year in northern Germany (where I am) up to 100.000 kWh / year in Texas (I used an online calculator and tried to maximize output by changing locations). So only 1% - 2% solar in comparison to wind. very little shadows on the panels electrical installations are mostly (?) already in place low installation cost, as the towers are produced en masse and in factorys slightly higher grid stability (when the wind blows and it's overcast / when the sky is clear and it's calm) (also solar should be able to produce at least more than the 1 - 2% of wind stated above, since thats per annum and in reality solar produces power only half of the day -> 2 - 4% ?) vertically mounting solar panels reduces efficancy by roughly 35% (but panels could be mounted at an angle) maintenance difficulty due to hight wind puts high stresses on panels
|
[
"Because adding fixings for solar panels adds a stupendous amount of complexity to tower manufacturing, transport and erection.",
"First of all, these towers are not mass produced in factories, at least not like cars or electric appliances. There are many different kinds of towers (steel, concrete and hybrid) to start with. Steel towers are pre-built in segments that have to be shipped to site. Shipping giant steel tower segments with attached brackets for panels is asking for transport damage (which already happens quite often). Concrete towers are often built on site, so that is certainly not mass production.",
"Assembling the brackets on site takes away valuable manpower that is desperately needed to erect the turbine.",
"Maintenance of the panels would be near impossible on the outside of the tower.",
"The panels could interfere with the turbine blades that pass the tower - they would reduce their clearance to the tower which means they must not flex too much which means the wind turbine would need to cut off in lower windspeed than without the panels (although it is most efficient at high wind speeds). So you would gain a bit of power from the panels but would lose a lot because you can't run the turbine at full power.",
"Also, wind turbines tend to be in areas where land is fairly cheap, so there is no reason for a super expensive installation on the tower when there is loads of empty field right next to the tower."
] |
[
"So imagine that wind turbines did not exist in the world and you are sitting in your office and you think...",
"\"Would it be a better idea to build solar panels on the ground or 300 feet in the air? ",
"Also, solar panels (those that do not rotate with the sun, are still in a fixed direction to take advantage of the most direct sunlight. Wind turbines rotate with the wind direction which would have nothing to do with efficient solar panel use. The solar panels would only get optimized sunlight occasionally and by coincidence. "
] |
[
"The problem with solar power has never been \"where do we put it?\" The problem is that solar panels are really expensive and the electricity they produce often doesn't offset that cost."
] |
[
"If the atmosphere of Mars is less than 1% the density of Earth, why are there large dust devils and sand dunes?"
] |
[
false
] | null |
[
"Two factors are involved there. First, windblown sand is not like beach sand, it's much finer, almost like talcum powder. Desert sand dunes are not much like beach sand dunes, which is a very common mistake people make, as most people are more familiar with the latter rather than the former.",
"Second, since the grains of sand are very small, they don't take very much wind to get aloft. The faint whisper of the winds of Mars is sufficient to shift its very fine sands."
] |
[
"This was a big thing when they made the islands in Dubai (",
"http://en.wikipedia.org/wiki/Palm_Islands",
"). The prince of Dubai, who came up with the idea, at first thought it would be easy. Just take some sand from our huge desert and dump it in the ocean in this pattern. It then had to be explained to him that desert sand is not the same as beach sand and it wouldn't be that simple."
] |
[
"Since others have already covered the dunes, I'll talk about why Mars has so many dust devils.",
"Dust devils can only occur where there's strong convection. Warm plumes of air rise from the surface, pulling in lots of the surrounding air in the process, which then proceeds to \"spin-up\" due to conservation of angular momentum, similar to an ice skater pulling in her arms and ",
"spinning faster",
".",
"In order to get surface convection, though, you need to have an atmosphere that's sufficiently warm at the bottom and cold higher up. Here on Earth that doesn't happen too often near the surface - our atmosphere is fairly opaque to infrared radiation (especially in humid regions), so radiation emitted from a lower warm layer of the atmosphere is quickly absorbed by a cooler layer above, warming it up in the process. This tends to equalize the vertical temperature near the surface, discouraging convection.",
"On Mars, however, the atmosphere is very thin, so it's much more transparent to infrared radiation. Most of the radiation emitted by the surface can escape directly out to space without ever getting absorbed by colder layers of atmosphere just above it. In fact, our landers have frequently measured temperature differences up to 20 C just between the surface and 1 meter above the surface. Imagine going outside when your feet experience 25 C (77 F) while your arms experience 5 C (41 F), and you get some idea of the difference here.",
"Those big temperature differences over small vertical scales is the perfect setup for convection to get going. All this surface convection on Mars produces lots and lots of dust devils.",
": The thin atmosphere of Mars actually helps create dust devils."
] |
[
"What causes the glow in nuclear reactor cores?"
] |
[
false
] | null |
[
"It's Cherenkov radiation, coming from charges particles moving faster than c/n in some material of index of refraction n."
] |
[
"Next question. Would we see anything if it was outside the liquid? Besides all the usual health hazards. "
] |
[
"There would be a lot less Cherenkov radiation. Whether it would still appear to glow blue, I'm not sure."
] |
[
"Do our eyes stop sending signals to our brain when we sleep?"
] |
[
false
] |
It seems like it would be a waste of energy to be continually sending a signal to the brain while we sleep. Do the eyes cease this signaling while we sleep?
|
[
"As you can imagine, this question requires a pretty complicated answer since the physiology of the brain is not yet completely known. For example, of the four types of wave forms drawn from an EEG, the alpha wave grows in amplitude when the eyes are shut.",
"There are several theories in how we perceive light, and one of the leading theories actually suggests that two colors are perceived by one cone. The way the brain distinguishes between these colors is the frequency of the neuron firing(the signal, you are talking about). When one color is seen, it fires faster. But when the other color is seen it fires slower. ",
"The cones affect the signal through processes called excitation and inhibition. Red and green are thought the be one of the opponent process pairs. So, when red light comes in, the photon hits the photoreceptor, and it gets 'bleached'. This bleaching causes the receptor to fire an excitatory signal into the brain through the ganglia, increasing its firing frequency. Green has the opposite effect. When its receptors become bleached, the signal to the ganglia are inhibitory, and slow the firing frequency down.",
"Now, the most interesting thing about this theory is that it directly explains the presence of an after image. If the red becomes too bleached, it stops sending signals to the ganglia, so the inhibitory signals from the green cone become prevalent, and you thus perceive a green image.",
" The eyes are constantly sending signals to the brain. Some of these signals are strongest when your eyes are closed, while perceiving certain colors is, physiologically speaking, a decrease in the firing of your ocular nerve."
] |
[
"We can easily find out how the light interacts with our cells, but how these intracellular signals create the perception of a color is unknown. We know quite a bit about the mechanisms for turning the external stimulus into an internal signal. There are ",
"three cones",
" responsible for seeing color. The provided graph shows how sensitive the three cones are against the visible light spectrum. ",
"Now, somehow, through what seems to be magic, your brain converts a electrical impulse from a cell containing a very specific chemical into the sensation of red, or blue. How your brain does this is as of now a mystery."
] |
[
"Most of the body becomes paralyzed during sleep. Other wise we would flail wildly during our dreams.",
"But our sensory pathways are still active. Otherwise you wouldn't be able to wake someone up."
] |
[
"Carbon Dating?"
] |
[
false
] |
Sophomore in high school now and science has really grabbed my attention. I'm currently reading Life Ascending by Nick Lane. At the start of the book he says, "... a few grains of rock from that bygone age have survived the restless aeons to this very day. Inside them are trapped the tiniest specks of carbon, which bear in their atomic composition the nearly unmistakable imprint of life itself." What about carbon makes it so special? What properties does it have that makes it preserve stuff well? I heard that carbon dating after a few thousand years becomes inaccurate so how does he know how such a long time has passed?
|
[
"The passage you are quoting is not referring to carbon as a geochronometer using the decay of the radioactive isotope C",
" but to the properties of two of its stable isotopes, C",
" & C",
" . And you are correct, C",
" geochronometers are useless outside a rather narrow range of values, down to about 12 000 years. Much if not most geochronology is carried out with U-Pb methods on zircons in igneous rocks. Those get accuracies in the order of about ±1 My all the way down to the earlier terrestrial materials dated at 4.4 Gy; they would presumably still be reliable on older samples if we had any. But there are other methods as well depending on the nature of the materials at hand.",
"Normally, most chemical processes will not discriminate between isotopes. The reaction products of these (mostly) inorganic processes will present C",
" /C",
" ratios which are pretty close to those of a standard (Say the PDB) and within a very narrow range of values within about 5-10 ‰. But some biological processes (photosynthesis in particular, see ",
"Park & Epstein, 1960",
", ",
"Usdowski, 1982",
") ",
" discriminate between isotopes and integrate C",
" and C",
" at different rates, leading to much greater variation of C",
" /C",
" ratios, which might be as far from the standard as, say, 50 ‰, and within a quite larger range of variation. This ratio has been used for a lot of things, such as a proxy of paleo-oceanic températures (",
"Grossman & Ku, 1986",
") and for quantifying the composition of the diets of extinct creatures (",
"de Niro & Epstein, 1978",
").",
"The end result is that when considering carbon-containing rocks and minerals, it is quite straightforward to discriminate between biogenic and abiogenic materials, particularly for carbonates (shells, limestone, and like that) from the signature of their carbon stable isotopes. And that this is completely independent of what happens with the ",
"stable isotope C",
" ...",
"For some reason, the applications of stable isotope geochemistry receive considerably less press than the applications of unstable isotopes. I can't understand why."
] |
[
"Thanks for the awesome reply! Had to look up a few words, but I'm glad I full understand what you wrote."
] |
[
"A pleasure! ",
"Enjoy your reading!"
] |
[
"Is there any material that is accessible to humans (either natural or artificial) that can withstand the temperature of our sun?"
] |
[
false
] |
And if so, how have/can we used/use it to our benefit?
|
[
"Short answer: No.",
"Long answer: We currently don't know any materials able to withstand the temperatures of our sun, but the prospects of finding such a material depend on where on or in the sun. ",
"The surface of the sun has a temperature of roughly 5800K. The material with the currently known highest melting point, ",
"tantalum hafnium carbide",
", has a melting point of 4488K. So while there is no material currently known to withstand 5800K as a solid it's at least close enough that it is conceivable that such a matrial could exist and could be found with some research effort.",
"If you mean deeper inside the sun, then definitely not. Temperatures there are in the millions of kelvins. Everything will turn to a plasma at these temperatures."
] |
[
"If you think about heating a material from 0 up to the temperature of the sun, something must remain, though it could be a gas! Rhenium is a liquid at 5778 K, and I wouldn't be surprised if there were some metal oxides still solid at that temperature. Most of the oxides have useless mechanical properties though."
] |
[
"As far as I understand:\nThe sun fuels itself through nuclear fusion. This is where matter changes state to plasma, so from cold to hot you have three phases of a material, Solid, Fluid, and Plasma. Within fluid, it changes state from liquid to gas, but both are fluids, just with different properties.\n(for nit-pickers there are phases between these for materials such as glass and blu-tac which are solid/liquid but still withstand shear forces - a liquid is defined as a material which can't withstand shear)\nwhen something is plasma, it is no longer nice neat atoms, but all of the constituents are like a very hot soup of quantum stuff which glows with the energy within it. Any material at this temperature would completely disintegrate to something not on the chemical periodic table. It would become would be a quantam physics situation.\nthere are materials with a very high melting point, such as Rhenium as mentioned by Jeeebs but the sun is millions of degrees in temperature internally, both in C and K (there is only 273 degrees difference).\nHowever, there are fusion reactors on our planet, ITES in france and JET in the UK (I think) which heats deutirium and tritium up to a few million degrees at pressure to create fusion conditions. this is done by holding the plasma in a donut shaped electromagnetic coccoon created by supercooled (superconducting) coils.\nwhen this goes wrong and the plasma breaks out and hits the wall, it melts it. From my visit there i remember the walls are made of the same/similar stuff the tiles at the bottom of the space shuttle are made of and these partially melt away when hit by plasma, and they are one of the most heat absorbing materials that exists!\n",
"http://www.youtube.com/watch?feature=player_embedded&v=Pp9Yax8UNoM",
"\n2200 F = 1478 K\nsun surface temperature = 5,778K - hotter as you move inwards\nso overall, i don't think such a material exists, and if it does then it will be gaseous/liquid at that temperature, which would be as useful as a chocolate teapot if you wanted anything structural from it."
] |
[
"How exactly do we know that nuclear fusion occurs on the Sun?"
] |
[
false
] |
I know that nuclear fusion occurs on the sun thanks to the vast amounts of hydrogen atoms fusing together to form helium and other elements, releasing a great amount of energy in the process. What I want to know is, in whatever detail necessary, how did scientists come to figure this out (through what observations and experiments)?
|
[
"When in doubt, go to the source, especially when that source is Hans Bethe. Bethe won the Nobel Prize for his determination in 1939 of the mechanism of energy production in stars. His 1967 Nobel lecture gives a clear account of how these ideas developed. The first couple of pages are quite non-technical, and you can probably follow much of the logic of the rest, even without absorbing the technical details.",
"Anyway, you can read the lecture in this PDF: ",
"http://www.nobelprize.org/nobel_prizes/physics/laureates/1967/bethe-lecture.pdf"
] |
[
"I think it was first that they calculated the energy the sun could produce via gravitational potential energy and found it wasn't high enough for the sun to have lasted much more than a few hundred million years(I can't remember actual numbers). They already knew the earth had been around for billions of years from radioactive decay and geology, so there had to be some other power source. ",
"Later it was found four separate hydrogen nuclei(four protons) were heavier than a helium nucleus(two protons, two neutrons) so there had to be some energy loss in the binding. From spectra we knew the sun was mostly hydrogen so we have a suspected source. Then we question whether the sun is hot enough for atoms to overcome coulomb repulsion(yes), and calculate the added lifetime to the sun we could add from this power source.",
"Edit for clarity."
] |
[
"Beyond the already-mentioned work by Bethe, the most direct evidence is the observation of ",
"neutrinos coming from the sun.",
" They are emitted in the various beta decays as protons turn into neutrons after certain fusions.",
"Ray Davis",
" got a Nobel Prize for ",
"the first observation of them,",
" and since then we have been able to ",
"image the sun in its neutrino \"light\"",
", which comes straight from the core."
] |
[
"Why does human perception seem to follow a logarithmic scale?"
] |
[
false
] | null |
[
"Perception of what? Not all things do. Perception of length is on a linear scale. Perception of electric shock is exponential."
] |
[
"I was wondering about loudness and musical notes, which seem to be logarithmic along with brightness. I kinda assumed all things were like that.",
"I didn't know electric shock was exponential, that's really interesting"
] |
[
"See ",
"Weber-Fechner law",
" and ",
"Steven's power function",
". ",
"As to why it is logarithmic for brightness -- we can come up with stories: detecting some amount of light vs. no light is very useful and seeing in low light is also very helpful, so we want big jumps in perceived brightness as a function of change in luminance when there is little or no light. Rod photoreceptors are very sensitive in scotopic (low-light) conditions, especially after dark adaptation. ",
"At higher intensities it doesn't matter so much (from an information/evolutionary standpoint) what the brightness is and there are corresponding limitations in response of rods where they simply become saturated. "
] |
[
"How did Einstein work out that the speed of light is the fastest speed there is? How do we know this?"
] |
[
false
] | null |
[
"The origin is in Maxwell's equations. These are the equations that describe how electric and magnetic fields evolve and interact. They were compiled together by James Clark Maxwell, who basically took everything that was known about electric and magnetic fields and put them together into a single set of equations.",
"Putting them together like this, he noticed there was an interesting symmetry going on. A changing magnetic field will generate an electric field, but a changing electric field will generate a magnetic field. He wondered if it was possible to set up a changing magnetic field that generates a changing electric field that generates a changing magnetic field and so on, to produce an electromagnetic wave that propagates itself. He figured out that not only was this possible, but that that this hypothetical electromagnetic wave just happened to move at the speed of light. It didn't take long for this idea to catch on, and it was soon universally understood that light is an electromagnetic wave.",
"However, there is something a bit odd about the speed of light in Maxwell's equations. It turns out that light ",
" to go at the same speed. This is very different to other realms of physics, like projectile motion for instance. Here, a ball can go at any speed in any direction, and no matter what your relative speed is, the equations of projectile motion stay the same. If you see a ball being thrown in the air by someone on a train, then you can follow the same equations for projectile motion, provided you add the velocity from the train to the velocity of the ball relative to the train.",
"This doesn't work for Maxwell's equations. If you have a car moving forward and shining its headlights, you can't just add the car's velocity to the speed of light. This isn't a valid solution of Maxwell's equations, which simply state that light ",
" go at ",
", the \"speed of light\".",
"So what's going on here? The simplest solution was that light moves at ",
" in some universal reference frame, and that this is the only reference frame where Maxwell's equations correctly apply. However, this was disproven by experiment, in particular by the Michelson-Morley experiment, which showed that Maxwell's equations seem to be correct for everybody everywhere.",
"And this is where Einstein comes in. If you assume that Maxwell's equations are the same everywhere, and that ",
" sees light at moving at ",
", then that simply does not add up if you use the classical ideas about space and time and velocity. So he had to invent an entirely new system, where time is relative, and where velocities are added in a new way. This seems like a drastic measure, but uprooting the basic principles of space and time was the only possible way to fit the observation that Maxwell's equations apply everywhere to everyone at every velocity. This new paradigm was called Special Relativity.",
"Special Relativity has been thoroughly tested experimentally over the last century, and it has passed every test. It is rock solid. And one of the many parts of this is there are new equations for motion, and it turns out that these produce a speed limit to the universe - ",
".",
"Now it's difficult to answer ",
" there is a speed limit to the universe - it's almost a philosophical question - but this is ",
" it was determined: through Maxwell's equations of electromagnetism."
] |
[
"For those who are interested, Einstein's original paper is available ",
"here",
" (translated from German to English), and he quite explicitly states that relativity comes from Maxwell's equations:",
"It is known that Maxwell’s electrodynamics—as usually understood at the\npresent time—when applied to moving bodies, leads to asymmetries which do\nnot appear to be inherent in the phenomena... Examples of this sort, together with the unsuccessful attempts to discover\nany motion of the earth relatively to the “light medium,” suggest that the\nphenomena of electrodynamics as well as of mechanics possess no properties\ncorresponding to the idea of absolute rest. They suggest rather that, as has\nalready been shown to the first order of small quantities, the same laws of\nelectrodynamics and optics will be valid for all frames of reference for which the\nequations of mechanics hold good. We will raise this conjecture (the purport\nof which will hereafter be called the “Principle of Relativity”) to the status\nof a postulate, and also introduce another postulate, which is only apparently\nirreconcilable with the former, namely, that light is always propagated in empty\nspace with a definite velocity c which is independent of the state of motion of the\nemitting body. These two postulates suffice for the attainment of a simple and\nconsistent theory of the electrodynamics of moving bodies based on Maxwell’s\ntheory for stationary bodies."
] |
[
"Just cleared up ten years of confusion, thanks!"
] |
[
"Are planets with more moons less likely to be struck by asteroids?"
] |
[
false
] |
This leads me to think it may deter impacts.
|
[
"Depends on the spatial orientation of the moon, planet and the asteroid at that time.A moon might slingshot an asteroid not colliding with the planet into the planet or it might actually push it out of the way of a planet like in the case of the animation. Both are possible."
] |
[
"The gravitational pull of the moons may or may not help. It may move an asteroid out of the path of a planet, or move it into the path. All we know for sure is that it will (slightly, depending on distance and size) move it somewhere.",
"However, the moons themselves can be hit. Any asteroid that lands on a moon is one less that can land on the planet. So in that sense, yes, planets with more moons are slightly less likely to be struck by asteroids, assuming all other variables are equal.",
"It's likely that other factors would much more strongly relate to frequency of impacts, such as the size of a planet."
] |
[
"Well of course it depends on the exact trajectory of each particular asteroid, but I think OP is asking if planets with more moons, on average, are hit more or less often than planets without satellites."
] |
[
"What prevents bighorn sheep from constantly getting (fatal) concussions? And could we adapt this anatomy for improvement of football helmets?"
] |
[
false
] |
I was watching NatGeo last night and they mentioned bighorn sheep slam head-to-head at about 20mph, which would instantly kill a human. And then I immediately thought of the the concussion controversy going on in the NFL (and other sports) and I wondered if why bighorn sheep were less susceptible to concussions (or are they?) and if we could use this knowledge to improve football helmets.
|
[
"Bighorn sheep slam their horns together during mating season and they do not drop to the ground. They have unique adaptations that allow them to slam or bang their horns together and not drop dead. Concussions in humans are caused by events that make the brain hit the inside of the skull. This is why a human get concussions from being jolted around in car accidents or sports injuries in which collisions are involved. Bighorn sheep have three ways of protecting themselves. The first way they are protected is the fact that their horns bend. Since their horns bend this draws out the time of the collision and reduces the force. The second way bighorn sheep are protected against concussions is the fact that their skull bones shift and rotate around the sutures. The skull is double-layered and the sutures are in a honeycomb pattern. This makes the sutures spring-like, this helps reduce the force when the sheep collide. The third adaptation bighorn sheep have to prevent concussions is that most of the energy from the collision goes to their neck muscles. Bighorn sheep have developed very strong neck muscles. These are the adaptations that animals have developed. These unique adaptations are the reason that a concussion is very unlikely."
] |
[
"just asking because you sound like you may be able to help. ",
"looking at all of those adaptations, would it be reasonable to think that this species might be fertile ground to research against prion disease?"
] |
[
"I think you may have misread something. Prion diseases are caused by misfolding of proteins in the brain, which leads to cell death and the formation of tiny cavities in the brain. Hence the other name for them - Transmissible Spongiform Encephalopathies. Blunt trauma doesn't really play into it, so the physical adaptations of bighorn sheep won't have any effect."
] |
[
"How can light be always at a constant speed but still be influenced by gravity?"
] |
[
false
] |
Lets take a black hole for example. The gravitational pull is so large that not even the speed of light is a sufficient escape velocity. But wouldn’t that mean that light escaping from gravitational pulls is slowed down? That in theory a gravitational body with the same escape velocity as the speed of light would make light "stand still"?
|
[
"This is a pretty subtle question. Light travels at a constant speed in the sense that if you perform measurements ",
", in a box small enough that you can ignore the effects of gravity, then you'll measure the same speed of light no matter where you are or how you're moving.",
"When you look ",
" at a larger patch of spacetime, where gravity is significant - i.e., where the patch is large enough that you notice the curvature of spacetime, which is equivalent to gravity - you can't even really define a single notion of what speed is. Think of spacetime like the surface of the Earth - if you restrict yourself to small regions where the curvature of the Earth isn't noticeable, everything looks flat, and you can use standard Euclidean geometry to measure distances. But if you consider a really big region where the Earth's curvature comes into play, then some of those tools are no longer applicable.",
"So if you consider a small patch of spacetime around a black hole, locally you'll find a constant speed of light, but as you string all those local patches together, you'll find light behaving in non-trivial ways. For example, the fact that light can't escape the event horizon is due to the fact that, inside the horizon, time and distance exchange roles. If you look at some local patch inside the horizon, you'll see light moving in a straight line at a constant speed, but when you connect your local picture (or coordinates) to the global picture, you'll find that what looks to you like light moving forward in time becomes, in the global picture, light moving inwards in radius. And since every physical object moves forward in time (by definition), within the horizon everything moves inexorably towards the center."
] |
[
"This makes sense, thank you!"
] |
[
"The time it takes to the center is indeed finite, as long as you use a sensible time coordinate. For massive objects, which travel below the speed of light, there's a very natural time coordinate to use, namely the time measured by a clock carried by that object.",
"With light (and other massless particles), there's no such time coordinate, because there's no notion of time as seen by a photon. (This is because light travels at the same speed in every reference frame, meaning there's no frame where light is at rest, but this is precisely the frame you're working in when you talk about the clock time measured by an object.) But if you use some other suitable time coordinate, then sure enough the light will reach the center in finite time."
] |
[
"Is sound/soundwaves affected by gravity?"
] |
[
false
] |
As far as I know light is affected by gravity, according to what I've been taught anyways, and so as I was watching the music video of Chris Hadfield in space playing a song I was wondering if gravity or lack of gravity affected sound.
|
[
"A sound wave is different from light. Photons have momentum and are affected by gravity. A sound wave is a true wave in that it's a traveling disturbance in a medium. There are no \"sound particles.\"",
"So sound is affected by gravity only in the sense that whatever medium sound is traveling through is affected by gravity."
] |
[
"Ah ok I see thanks."
] |
[
"Technically it should.",
"Gravity effects mass, and sound is disturbance waves propagating through mass.",
"As far as Gas and liquid go, higher gravity means higher density -- the force which pulls particles towards the ground becomes larger, while the force which pushes them apart remains the same (almost, it is stronger, but only because they are closer. The point is that they need to be closer for the gravity and the rejective forces between particles to balance).",
"If you somehow doubled the weight of the earth, but left everything else the same, the density of the air would increase which would cause a decrease in the speed of sound. The same way sound travels slower through water due to their high density compared with air."
] |
[
"Does the quantity of water on the Earth fluctuate?"
] |
[
false
] |
[deleted]
|
[
"When people are talking about wasting water, what they mean is wasting clean/drinkable water and or polluting water systems.",
"There is a lot of water on earth, but most of it is unusable, usually because it is either vapor, frozen, or salty. Overusing fresh water means there will be less water that we can use. The water that is wasted does not disappear, but it does become useless (until it is recycled either by treatment facilities or through the natural water cycle).",
"Wasting water can also pollute natural water systems like rivers, lakes, and seas/oceans. This damages the environment and hurts wildlife."
] |
[
"Does this mean there is finite amount of drinking water? Or could we potentially turn undrinkable water back into drinkable water thus eliminate worries about wasting water?",
"Would this be like what Dean Kamen's 'Slingshot' is doing, or would that have its limitations too?"
] |
[
"You can filter/distill undrinkable water to make it drinkable. Water can be reused indefinitely. The only question is if there is enough for everyone to use at once. "
] |
[
"Why does the universe care if it is observed?"
] |
[
false
] |
In quantum mechanics, apparently it is the case that everything is described as a probability function, or in other words: the universe itself does not know the state of itself... it only knows the chances of itself being in certain states (like Schrodinger's cat). The universe only lands on a particular conclusion when it is observed... does anyone have insight into why the universe should only land on a definite outcome when a clump of chemicals, such as the human brain, tries to make sense of it, and not when in the presence of other - lifeless - chemicals... does this mean that logical beings are truly distinct from everything else since they somehow make the universe jump to conclusions... Seriously blows my mind, is there any explanations or even acceptable theories?
|
[
"This is a common misunderstanding. Observation = interaction.",
"It has nothing to do with life / non-life (which has nothing to do with physics). ",
"In the case of the photon, it just means whether the photon has interacted with anything. Before it interacts = superposition, after it interacts, that superposition collapses."
] |
[
"In the case of the photon, it just means whether the photon has interacted with anything. Before it interacts = superposition, after it interacts, that superposition collapses.",
"A subtle, but important, clarification here. When a photon interacts with a particle, the superposition doesn't collapse, the photon and the particle become part of a 'combined' superposition known as entanglement. It's a common misunderstanding that interaction 'causes' a collapse. In reality it's a matter of the system becoming entangled with the environment. "
] |
[
"It is just a confusing language of quantum mechanics. QM is primary designed to describe outcomes of experiments. It uses terms of the system and the observer. Observer isn't a person. It's a piece science equipment which interacts with the system in order to measure something. It can also be natural phenomenon which the system interacts with and yields some information in the proces. It is not that universe cares if someone is watching it's that we cannot see without interfering with the system."
] |
[
"If you forced different gases through a whistle, would it change the sound?"
] |
[
false
] | null |
[
"It can do.",
"An organ pipe produces a sound with a certain wavelength, depending on the length of the pipe. But your ears perceive the pitch of a sound based on its frequency. If the speed of sound in the gas in the pipe changes, then the pitch will change.",
"A real-world example of this is the calliope or steam organ, traditionally used in circuses. The temperature of the steam varies somewhat, which varies the speed of sound, and that's why calliopes always sound out-of-tune no matter how hard the operator tries to tune them.",
"Changing the composition of the gas can also change the speed of sound in it. (Changing the ",
" actually has no effect on the speed of sound in a gas.)",
"EDIT PS: But this may not apply to all instruments or whistles. In \"free reed\" instruments such as harmonicas and accordions, the pitch depends on the size and shape of a piece of vibrating metal. I would predict, but don't know, that a free reed instrument would produce the same sound whatever gas you put through this. The experiment would be easy to do with a gas source, regulator, and a melodica."
] |
[
"Yes. Helium changes the sound of your voice because the speed of sound is different in helium and air at the same pressure. I don't see any reason why that effect wouldn't apply to whistles or any other wind instrument."
] |
[
"Here's a technical paper describing an instrument that uses the frequency of a small whistle to precisely measure the composition of an unknown mixture of gases.",
"https://www.ncbi.nlm.nih.gov/pubmed/24420261"
] |
[
"Are Blue Whales actually the biggest ever?"
] |
[
false
] |
We are taught that blue whales are the largest mammals/vertebrates to ever exist. Isn’t this unlikely considering our lack of seabed fossil evidence?
|
[
"Any time scientists say anything you can assume there is a \"*to my knowledge\" attached to it. They have never found a mamal bigger than the blue whale so that is currently the one they say is the largest to ever exist. No serious scientist waould say there's no chance of finding another one. As far as how in depth the sea bed has been explored, I have no idea."
] |
[
"We actually have a lot of seabed fossils. We find them all the time on land, including whales in the Sahara and seashells on Mt Everest. The oldest current oceanic plates are only about 200 million years old. Older crust gets uplifted to become dry land, or subducted and destroyed in the mantle. Considering the patterns of deep-ocean fossils found on land, we can be reasonably confident in our assertion about the relative size of the blue whale. As you point out, nothing is ever 100% certain, and I think we would all be very excited to find something bigger."
] |
[
"This. Biology always deals in 'as far as we know this is what happened', specially with such a difficult field as paleontology that, like the OP said, depends on the fossils found and the stimations done."
] |
[
"A coin of unknown fairness is tossed N times. It results in H heads. What can you tell me about the coin?"
] |
[
false
] |
Imagine it's tossed 10 times. Even if it's all heads, you can't be sure if it's unfair, or to what extent. All you know for certain is that it is not an "all-tails" coin. I'm not entirely sure how the answer would be framed, but I assume there is some probability distribution for the "heads-ness" of the coin that changes with each result. Eg: After N flips, given some results, there is a Q% chance the coin is P likely to give you heads... I'd love to see the answer derived. And, also, how would one go about this if it were a S-sided coin -- that is, we're not even sure how many possible outcomes there can be? For a coin, even with all heads results, I assume we'll think there is chance of tails coming up -- does that means there is chance of some other side turning up? If so, aren't there infinitely many possible sides, how is this resolved?
|
[
"You are asking about statistical estimation: given some observed data and an underlying statistical model, what is the best estimate for parameters that define that model? (There are other ways to phrase the problem if you don't know exactly what the underlying statistical model should be.) One particular method of estimation is ",
". Note that there are other estimation procedures, such as ",
". I am just going to talk about maximum likelihood. There are also related problems, like \"how many flips of the coin do you need to determine whether the coin is fair?\" For that type of question, we need to introduce concepts like ",
". For a general introduction to these topics, I suggest any intro text on statistical theory. The Wikipedia pages on terms like ",
" are a good start too, as is ",
"this page",
" that discusses the general problem of determining whether a coin is fair.",
"In your example, we consider a coin that has some unknown probability ",
" of showing heads. In fact, it doesn't even have to be a coin. Just some machine or computer program which, on each trial, outputs either \"H\" or something else. The number of \"sides\" (which are really the alternatives the program can spit out) has no bearing on the problem right now. (See the next section for the more interesting problem of an unknown number of sides.) There can even be an infinite number of possible outputs. All we care about is that the computer program has some probability of outputting \"H\". Our goal is to estimate ",
" given some observed data.",
"The idea of maximum likelihood is to first assume a fixed underlying statistical model for the data. In this case, we assume that each trial is an independent Bernoulli trial, and so the distribution of N trials is the binomial distribution Bin(N,p). The true value we seek is ",
", but all we have is some observed data (e.g., after 10 trials, we observe 10 heads). Next, we consider a so-called likelihood function ",
" = f(",
",p), which is a function of the observed data ",
" (a vector of the outputs) and the true parameter ",
". We then simply maximize ",
" with respect to ",
", and the value of that maximizer is what we use to estimate the true value of ",
". In the case of an underlying model which is discrete (like the binomial distribution), we end up actually maximizing the probability of observing the data ",
". That maximizer is called the ",
" (MLE).",
"For a binomial distribution, it turns out that the MLE is just what you would expect. If we observe ",
" heads in ",
" trials, then the MLE is θ = ",
"/",
". So if we observe 10 heads in 10 trials, our best estimate is just p = 1, in the sense described above. That is, our best guess is that the coin or program or whatever just always outputs heads.",
"Yes, of course, the program could actually have p = 1/2 and we just happened to get 10 heads in a row. Statistical theory can make more precise how well we know whether our MLE is actually good. Suppose we keep flipping the coin, thereby letting ",
" increase. After each trial, we update our MLE to the current value of h/N. So if the next flip gives something other than heads, we will update our MLE to θ = 10/11. If we get heads again after that, we update to θ = 11/12. If we get non-heads after that, we update to θ = 11/13, and so on. So we actually get a sequence of estimators for ",
". Under some suitable conditions, you can show that the sequence of MLE's will actually converge to the true value of ",
". (The sense of convergence is rather technical. Generally, we have convergence in probability, but under stronger conditions we get convergence almost surely.) Furthermore, we also know that the MLE will converge in distribution to a normal distribution, with mean equal to the true value of ",
" and variance that decays like 1/N. ",
"What does all of that mean? Well, for your coin-flipping example, if we simply estimate the probability of getting heads as θ = h/N, then as N gets larger,",
"So as we flip the coin more, we get more confident about our estimate of ",
".",
"Now let's finally go back to the number of sides of the coin, alternatively, the number of possible outputs of the computer program. Assume that at the end of each trial, the program outputs one of ",
" possible outputs, and all outputs have equal probability of being chosen. (The equal probability simplifies things.) So our goal is to estimate ",
" after ",
" trials. This problem is actually rather complex, and is formally identical to the ",
"capture-recapture problem",
".",
"In the capture-recapture problem, we want to estimate some population (of wild animals, say). We capture ",
" animals, mark and release them, then come back later and capture ",
" animals. If ",
" of those animals were already marked, an estimate of the population is M = AB/C. (This is called the Petersen estimate.) The problem gets more complex if you consider multiple visits. That is, we mark the animals in the second batch (the already marked animals now have two marks), release them, and capture another sample. The estimate of the population is a function of several variables: the total number of captures, the number of animals in each capture, the number of animals in each capture already marked, the number of animals in each capture marked for the first time, etc. There is considerable literature on this problem.",
"How does this relate to your problem? Think of it like this. Behind some screen there is some collection of ",
" animals... or a set of possible computer outputs, like {H, T, 4, @, Q, ...}. One trial (or coin-flip or capture) consists of revealing one random animal (or output or symbol). We are allowed to write down the sequence of outputs. (This is like marking and releasing the animals.) We may also want to take note of the first time we see a symbol, how many times we have seen it, etc. This is exactly the capture-recapture problem, in which we perform multiple captures, all of which are of a single animal. The ",
" offers one method to estimate the population ",
", and details can be found on ",
"this page",
". Since we only ever \"capture\" 1 animal at a time, there is an easier way to write the formula on that page.",
"We conduct ",
" trials. Let s(j) = number of unique outputs seen so far just before the ",
"th trial. Let R = total number of trials which showed a previous output. Then the Schnabel estimate is",
"M = [s(1)+...+s(N)] / R",
"Let's look at an example. Suppose we conduct 7 trials, and the sequence of outputs is {H, @, T, H, 3, T, $}. The values of s(j) are",
"s(1) = 0 (remember this is number of unique outputs seen just ",
" this time, so this will always be 0)",
"s(2) = 1",
"s(3) = 2",
"s(4) = 3",
"s(5) = 3",
"s(6) = 4",
"s(7) = 4",
"Twice we encountered an output we had already seen (on trial #4 and trial #6), which means that R = 2. Hence the Schnabel estimate of the population is",
"M = [0+1+2+3+3+4+4] / 2 = 8.5",
"We actually have to round down, so we estimate that there are a total of 8 unique possible outputs. (Note that we only saw 5 of them.) You can actually show that as the number of trials goes to infinity, this estimate converges to the true population (assuming that M is finite). One can also calculate the variance and perform all the usual statistical analysis.",
"There are some nuances though. Suppose there are 3 possible outputs {A, B, C} and we conduct N trials. What if the trials were {A, A, A, ...., A, B, C}? That is, all of them were \"A\", except the last two which finally revealed the other outputs. Then we have",
"s(1) = 0, s(2) = 1, s(3) = 1, ...., s(N-1) = 1, s(N) = 2",
"We have R = N-3 since all trials are repeats except the first and last two. So the estimate is",
"M = (0+1+1+...+1+2)/(N-3) = N/(N-3)",
"Note that M --> 1 as N --> infinity. So the estimator somehow predicts that there is only ",
" possible output as the number of trials gets large. That doesn't make much sense. In fact, the estimator is less than 3 for all N > 4 and less than 2 for all N > 6. What happened? Shouldn't the estimator estimate at least as many unique outputs that you saw? Yes, of course. But...",
"This example was cooked up to highlight the importance of the assumption that the outputs are equally probable. Think about what it means to have a sequence of trials that is all \"A\" except the last two. Suppose you had 10 million A's, then 1 B, and 1 C. Okay, sure you would say there are at least 3 possible outputs. But would you also say that the three outputs are equally likely? No way. Clearly, \"A\" is much, much more likely to be drawn than either \"B\" or \"C\". The Schnabel estimator assumes that the outputs are ",
". If the outputs are not equally probable, then the formula changes. It's as simple as that.",
"Also, remember that the statement that the Schnabel estimator converges to the real population does not mean that the sequence of estimators converges for ",
" possible sequence of trials, but only ",
" of them. That is, there is a probability of 0 that you get a sequence of trials for which the associated estimators do not converge to the real population."
] |
[
"/u/midtek",
" talks about MLE's. An alternative formulation of your problem is the Bayesian point of view.",
"TLDR version: If you say \"I got a fair coin and got 10 heads,\" I would say \"you got lucky, it happens sometimes.\" If you say \"1 in every trillion coins is magic and always comes up heads! I got heads 10 times, is this coin magic?\" I would say \"Possibly, but you probably got a regular coin and are getting lucky.\" If you say \"Half of all coins are magic and always come up heads! I got heads 10 times, is this coin magic?\" I would say \"Probably is.\" My answer changes based on my knowledge of how rare / common magic coins are. You didn't specify this in your problem, so there's not enough information to answer your question.",
"The way Bayesian analysis works is you start out with a belief (the ",
") which is your model of the coin before the experiment is performed; this is essentially how rare magic coins are. For example your prior might be \"This coin has some probability p of coming up heads, and all values of p between 0 and 1 are equally likely.\"",
"Once you have the observed sequence, you can compute P(observation | p=x) for every value x which parameter p could take, which can then be converted to P(p=x | observation) with Bayes' rule:",
"P(p=x | observation) = P(observation | p=x) * P(p=x) / P(observation)\n",
"The value P(p=x) is simply the value of the prior at x, and the value P(observation) can be computed as the sum of P(observation | p=x) P(p=x) over all values x.",
"Basically the idea is to extend the problem: Pretend you have a bag of 100 coins, with probabilities 0.5%, 1.5%, 2.5%, ..., 98.5%, 99.5%. You pick one at random from the bag and flip it 10 times. You can view the situation as 100 * 2",
" = 102,400 possible universes (you could view the last two decimal digits of the universe number as encoding which coin you got, and the leading digits as encoding which of the 2",
" = 1024 possible 10-flip sequences you got). Each universe has a probability weight associated with it, for example the one where the p = 0.5% coin comes up heads every time will be very \"light\" (unlikely) but the one where the p = 0.5% coin comes up tails every time will be very \"heavy.\"",
"Now when you have your observation -- when you know you have 10 heads -- you eliminate from consideration all the universes inconsistent with this possibility. Then classify the remaining universes according to the variable you're trying to measure (which coin you have), and add up the probability weights of the universes in each classification.",
"There's not enough information in your statement of the problem to run this calculation; you have to make a guess about the contents of the bag. If you said the bag contained only 50% coins, then the Bayesian analysis would say you have a 50% coin with 100% probability. If you say the bag contains a trillion 50% coins and a single 99% coin, the analysis would say you probably have a 50% coin and got unlucky (although there is a small probability you drew the 99% coin, and the probability you have the 99% coin, although still small, would be very much larger than 1/1000000000000.) If you say the bag contains half 50% coins and half 99% coins, the analysis would say you probably have a 99% coin (although there would still be a small probability you have a 50% coin).",
"Bayesian analysis tells you ",
", but in order to actually run the calculation, you have to specify your mind's initial belief (by picking a prior, i.e. guessing the contents of the bag)."
] |
[
"This type of approach is used in the real world in engineering in quality control fields, in which you have to check a certain percentage of products and make a determination as to whether or not a process is flawed or not. (In practice, you dont go through the theory everytime. You just determine a z value and use the charts to determine confidence interval)"
] |
[
"If wave nodes are static, how do they transfer energy?"
] |
[
false
] |
I had this question way back in high school and I was never able to get a good explanation. So, a couple of basic assumptions: The "node" of wave is where motion/amplitude is zero. Energy travels through motion or heat, right? So how can the energy on one side of a wave travel through each node if each node doesn't move, and there's an assumption that there's no heat buildup?
|
[
"Well, it's a standing wave, so the energy for each non-stationary region (between the nodes) just stays where it is. If you want energy transfer, you have to talk about propagating waves instead, where there aren't any nodes."
] |
[
"Actually, no net energy is transferred through a node in a standing wave. The equation for the power transfer is proportional to the rate of change of the displacement, which for a node is zero."
] |
[
"So I guess my question is if no net energy is tranferred, how does the actual transfer happen. Energy MUST transfer, because the movement continues through a node."
] |
[
"A Few Questions About Schizophrenia"
] |
[
false
] |
I've recently been studying mental disorders, and Schizophrenia really interests me. A few questions in no particular order: 1) Normal people (as far as I know) DO (sub)vocalize their thoughts, but they realize it's their own "voice in their head". From discussing with people, they also don't always think in the same voice or sound the same, depending on mood, situation, subject, etc. What's the difference between this and "voices in your head"? Just that you don't see them as originating from your own thoughts? Or do they actually sound like someone was standing right next to you speaking aloud? 2) How do the schizophrenic mind and a normal mind under the influence of a hallucinogen such as LSD/psilocybin compare? I've heard there are similarities. 3) I guess with as much specific detail as possible, how does the schizophrenic mind process things differently than a "normal" mind? I guess I've always thought if it as a lack of structure to your thoughts, almost. Like things just randomly fly at you out of context? Thanks! Sorry, not the most awesomely-phrased questions, but I'd love to know more about any of the above.
|
[
"I've worked with numerous schizophrenics in the last few years in an inpatient setting:",
"1) It depends on the person. Some may have a whole bunch of voices in their head both male and female that aren't necessarily theirs. It may seem to them that they are actually hearing them with their ears or that the voices are of the subvocalized persuasion, but aren't their thoughts. These voices may tell them to do things, put them down, complement them, make no sense at all, or just be annoying sounds. I read recently that when they hear voices, it shows up as activity in their auditory centers in their brain as if they're actually hearing something with their ears. Sometimes they think they're thoughts/voices are so loud that everyone can hear them.",
"2) Again, every case is different. Schizophrenics may perceive illusions when watching TV, thinking that the news anchor is saying things to them that he didn't say at all. One guy I met saw an alien run around his room. Another couldn't get the smell of burning flesh out of his nose. With some patients there are going to be incredible similarities, others not so much. Every trip is different, every patient is different.",
"3) It depends on the type of schizophrenic. It's often an issue of your brain having a problem prioritizing and filtering things. If you try to talk to a schizophrenic man, he may not be able to focus on your conversation and may get hung up on a word here and there or just get totally caught up in his thoughts if they're racing or if he's hearing voices. There's a lot going up there that isn't structured. He might reply to you with a sentence that doesn't make any sense on its own but will include a reference to one of the words that you said. This is called loose association. One woman kept yelling \"blueberries\" at me because my eyes are blue. You might just get a word salad or something. Or echolalia, when they just repeat something over and over again. "
] |
[
"Med student here. I got off an inpatient psych rotation a few weeks ago. I'm not expert obviously but...",
"Patients who had been previously diagnosed with schizophrenia tended to show up in the ERs after quitting their meds when the voices became too much to ignore. They recognized they were in trouble and were usually admitted to get back on their meds.",
"Sorry I couldn't help more. Kinda in a braindead state right now."
] |
[
"Pretty much. One guy I worked with had a psychosis where the voices made him laugh hysterically all day long, while this other woman was stuck hearing her children being continuously tortured. ",
"This",
" is kind of an interesting video by Janssen, the makers of antipsychotics Risperdal and Invega. Mental illness sucks."
] |
[
"How does Truvada (PREP) prevent people from getting infected with HIV?"
] |
[
false
] |
For example, say two guys have unprotected sex and the "top" is HIV+ and does not have an undetectable viral load. How exactly does PREP work to prevent the "bottom" from contracting HIV? Wouldn't it be impossible for Truvada to be inside/protecting every single one of his T Cells? If just one immune cell is infected, would a viral reservoir be established? Why can this HIV drug stop a person from becoming infected even if that person has been directly to exposed to HIV, yet these drugs can't cure people who have already been infected? And finally, lets assume the same scenario except the "bottom" isn't taking Truvada. Why is the infection rate (I think) around 1%? Why wouldn't it be 100%? Thanks.
|
[
"PrEP is a combination of two Reverse Transcriptase inhibitors (if you don't know what reverse transcriptase is, please ask and I'll be happy to explain what the enzyme is and the actual mechanism by which the inhibitors work).",
"The reason the drug works is because the drugs were designed to specifically target the viral enzyme and are essentially not used by human enzymes of similar function, and the reason this seemingly small amount of drug functions is partially explained by the answer below, but also because the viral exposure is very small as well, and the immune system is actually very very good at recognizing and killing virally infected cells. ",
"HIV has such a low infectivity rate because compared to other pathogens, it is very very new. The general trend that is found with new pathogens is low infectivity and high lethality, and given the time to co-evolve, you see infectivity rise and lethality decline (this is good for both us and the pathogen, so given enough time most diseases become follow this trend)",
"https://aidsinfo.nih.gov/drugs/406/truvada/0/professional#S12.4"
] |
[
"So reverse transcriptase is a viral enzyme which converts RNA to DNA. Both of the drugs are analogs of nucleotides (one is an adenine analog, another a cytosine analog, but that isn't really important) and these nucleotide analogs get taken into the reverse transcriptase, they are added to the DNA strand, but because they aren't real nucleotides, reverse transcription stops. The viral genome cannot be incorporated into the nucleus, and the viral life cycle stops. The clever part is that the nucleotide analogs do not get used by our DNA replication enzymes. "
] |
[
"I see, that's in fact simultaneously brillant and very simple (at least theoretically, i'm not talking about the process, which I know nothing about). Thanks a lot for your time."
] |
[
"How does vocabulary influence cognition?"
] |
[
false
] |
......................
|
[
"So an early version of this theory is the [Sapir-Whorf hypothesis] -- the strong version of which is that the language that you speak determines the kinds of concepts / thoughts that you have. The strongest form of this is linguistic determinism which is that language ",
" determines cognitive processes; no one really adheres to this; the weaker version is linguistic influence or relativity. There is some nuance to the history and what claims were actually made and I would recommend reading that section of the Wiki article. I think we can safely say that initial evidence for this was rather weak and including such things as Whorf's infamous claim of the many different Inuit words for snow ... which turns out to be a pernicious misconception and isn't true. Another classic example was the lack of grammatical distinctions for past tense in Hopi -- this was interpreted as \"no sense of time!\" but actually there are other ways that they can convey time and future/past events.",
"That being said, there do seem to be some differences across languages that can ",
" how we perceive and categorize the world. For example, having certain color category distinctions can make it easier / faster to distinguish between colors that cross the category boundary for speakers of the language than for non-speakers. For example, in Russian there is a special word for light blue \"goluboy\". In an experiment, observers had to match one of two colored squares to a target. Observers were faster at matching one of the squares to the target if the other square was goluboy (between-category distinction between a blue and goluboy square) than if the two choice squares were different shades of blue (within-category distinction). This advantage did not exist for non-Russian speakers (",
"Winawer et al. 2007",
"; see ",
"Davidoff 2001",
" for a review). ",
"Another perceptual example comes from Liz Spelke's lab. In Korean, there is a distinction between a tight fight and a loose fit (either inside or around something) which we don't have in English -- we just say \"in\" or \"on/around\". English-speaking adults do not make this distinction, but Korean-speaking adults do. In their experiment, they had 5-month-old infants view a series of actions: a hand putting an object inside another one or putting a ring around another object. After habituating to seeing an object enter or fit around another object loosely, infants spent more time looking at a tightly fitting event than another loosely fitting event (and vice versa if they adapted to the tightly fitting event first). That is, infants' looking patterns resembled those of adult Korean speakers, not English speakers (even though these were children of English-speaking families). The implication is that the language that we ultimately learn guides the kinds of perceptual / event categories that we eventually have / keep (",
"Hespos and Spelke, 2004",
"; see also ",
"Choi and Bowerman 1991",
"). ",
"Another interesting example is the associations that we form with words based on their gender (in gendered languages). For example, Spanish and German speakers were asked to come up with three adjectives that they would use to describe a key. In Spanish, the word \"key\" is feminine (\"la llave\") and the words that often came up were: golden, intricate, little, lovely, and shiny. In German, the word for key is masculine (\"Schlussel\"), and the words they used were: hard, heavy, jagged, metal, and serrated (",
"Boroditsky et al. 2003",
"; although note this recent failure to replicate, ",
"Mickan, Shiefke, and Stefanowitch 2014",
"). ",
"I would be curious to hear a linguist's take on this recent wave of linguistic relevatism. This is how we talk about it in cognitive psychology."
] |
[
"I'm not a linguist, but if you don't get a response consider posting your questions to ",
"/r/linguistics",
".",
"They might be more responsive than ",
"/r/askscience",
" (since it's a subreddit devoted to a specific field of science), and you might get more in-depth answers."
] |
[
"I'm not a linguist, but if you don't get a response consider posting your questions to ",
"/r/linguistics",
".",
"They might be more responsive than ",
"/r/askscience",
" (since it's a subreddit devoted to a specific field of science), and you might get more in-depth answers."
] |
[
"Why does glucose require insulin to be metabolized but not fructose since they are so similar in structure?"
] |
[
false
] | null |
[
"glucose doesn't require insulin to be metabolised; rather insulin causes cells to absorb glucose. It's a significant distinction since fructose and glucose metabolism are similar in that the same kind of chemistry occurs, albeit with different enzymes.",
"The short answer is that the small differences between fructose and glucose are enough for the body to differentiate them. IIRC, glucose has an aldehyde functional group whereas fructose has a ketone. ",
"It's more complicated than that however because insulin ",
" affect fructose uptake in skeletal muscle cells, just not in the liver. This is because fructose is not normally metabolised in all cells and is processed by the liver first, so presumably it's beneficial for all fructose to enter the liver so that it can be phosphorylated into a form which can be used elsewhere."
] |
[
"Just because chemicals are similar in structure doesn't mean they are necessarily processed the same way. Indeed, even differences in enantiomers (ie same exact chemical structure but just mirror-images of each other) can lead to very different effects. The classic case is thalidomide, where one enantiomer was teratogenic while the other wasn't. ",
"As for glucose, insulin's active site contains residues that can directly bind/interact with the chemical structure of glucose but probably not fructose. Why it evolved this way, one can only speculate. There are many other examples where similar chemical structures can have very different effects (and also very different chemical structures can elicit very similar effects)."
] |
[
"Just wanted to confirm that glucose is an aldohexose while fructose is a ketohexose. "
] |
[
"Why is sapphire an electrical insulator as well as a thermal conductor?"
] |
[
false
] |
[deleted]
|
[
"thermal and electrical conductivity typically go hand in hand in ",
" (see ",
"Wiedemann-Franz law",
" ), for insulators there is no such correlation. If you look at this chart of measurements from wikipedia: ",
"thermal conductivity.svg",
" you'll see that diamond (while also being an insulator) has a very high thermal conductivity as well.",
"The answer here is that in highly ordered bodies like sapphire or diamond ",
"phonons",
" can act as heat carriers as well.\nYou can basically picture these as sound waves, propagating through the lattice of atoms/molecules.",
"It's worth noting, that phonons also exist in metals (which are themselves highly ordered) but on top of that the free electrons also carry heat and that's where the correlation between electrical and thermal conductivity stems from."
] |
[
"Yes. In disordered materials, phonons can't be described in the simple way we can model them for crystals, and long-range phonons don't really exist there. Basically, phonons are a collective motion of many particles. If the material is disordered, this will disrupt the collectivity, so large-wavelength phonons cannot either not form at all or not propagate. There is also a large probability of phonons being scattered due to defects or lattice irregularities, which reduces heat transfer even more.",
"Metals actually have different \"regimes\" of thermal conductivity, as electrons can interact with phonons. The thermal capacity of electrons is very nearly proportional to the temperature T.",
"Phonon excitation is strongly temperature-dependent at low T, it is proportional to T",
" Since electrons can scatter at phonons, this reduces their mean free path length and, therefore, their thermal conductivity. So at low to intermediate temperatures (\"intermediate\" here meaning a few K to several ten K), thermal conductivity goes down as T",
" In reality, at very low temperatures heat conductivity usually decreases again due to impurities.",
"At higher temperatures, phonon excitation is better described as being proportional to T, so thermal conductivity becomes constant.",
"Here is a site with some nice graphs: ",
"http://physicsmadeeasy.wordpress.com/physics-made-easy/condensed-matter-iii/",
"You can find a graph for copper ",
"here",
". As you can see, the low-temperature behaviour strongly depends on the purity of the sample (RRR means residual resistance ratio and is a measure for purity, large RRR is more pure)"
] |
[
"Yes. In disordered materials, phonons can't be described in the simple way we can model them for crystals, and long-range phonons don't really exist there. Basically, phonons are a collective motion of many particles. If the material is disordered, this will disrupt the collectivity, so large-wavelength phonons cannot either not form at all or not propagate. There is also a large probability of phonons being scattered due to defects or lattice irregularities, which reduces heat transfer even more.",
"Metals actually have different \"regimes\" of thermal conductivity, as electrons can interact with phonons. The thermal capacity of electrons is very nearly proportional to the temperature T.",
"Phonon excitation is strongly temperature-dependent at low T, it is proportional to T",
" Since electrons can scatter at phonons, this reduces their mean free path length and, therefore, their thermal conductivity. So at low to intermediate temperatures (\"intermediate\" here meaning a few K to several ten K), thermal conductivity goes down as T",
" In reality, at very low temperatures heat conductivity usually decreases again due to impurities.",
"At higher temperatures, phonon excitation is better described as being proportional to T, so thermal conductivity becomes constant.",
"Here is a site with some nice graphs: ",
"http://physicsmadeeasy.wordpress.com/physics-made-easy/condensed-matter-iii/",
"You can find a graph for copper ",
"here",
". As you can see, the low-temperature behaviour strongly depends on the purity of the sample (RRR means residual resistance ratio and is a measure for purity, large RRR is more pure)"
] |
[
"Are thoughts physical objects?"
] |
[
false
] |
I know it's a strange question, but I have a hard time thinking about it. How do we see and hear thoughts, are thoughts three-dimensional? Thank you very much for the enlightening responses, they help tremendously. For anyone else interested, this might set you on the correct path- . If somebody has a more legitimate source, please post a link. I'm very skeptical of that website.
|
[
"Thoughts are to your brain like Walking is to your legs.",
"Thoughts are an emergent property of brain activity, just like walking is what happens when you legs work.",
"Your brain is a physical thing, your legs are a physical thing. Thoughts aren't, walking isn't. Can you pick up \"walking\" and put it in a jar? Where does \"walking\" go when you stop moving your legs? Do you see?"
] |
[
"Thoughts are webs of electricity traveling through different sections of your brain. Most are reactionary responses to outside stimuli, and then chained together based on memories that surface during the process... ",
"I try not to think about it really; that our entire lives are just a chain of reactionary electric charges pulsing on a lump of meat...",
"I'm sad now. "
] |
[
"They're physical, but not objects exactly, more like patterns of energy and matter moving around inside the brain.",
"Think about a flashlight. It's a physical object. Turn it on and electricity flows through it and light comes out one end. The electricity and light are physical, but they're not objects (unless you count electrons and photons as objects).",
"Because our brains (and the rest of our bodies) are three-dimensional, all brain (and other bodily) activity is three-dimensional.",
"It ",
" really hard to think about this stuff!",
"I'm going to try to answer what I think is the heart of your question: we perceive physical objects \"out there\" in the universe. Since we perceive thoughts in a similar way, is that because they're like the \"real objects\" we perceive with our senses?",
"The answer is no, you've got it backward. We don't really perceive physical objects directly, we just perceive the effect they have on our brain. Thoughts, already being in the brain, aren't perceived by the senses, but more directly by the brain. So it's not that thoughts are things, more like things are thoughts."
] |
[
"Can patients diagnosed with serious psychopathologies (i.e. schyzophrenia) lose their pathological traits after a brain trauma, coma or amnesia?"
] |
[
false
] | null |
[
"This is a really complex question and has multiple layers, and no single answer. ",
"First of all: we'd have to consider the pathology. A lot of the ones that are in the forefront of science right now (like Schizophrenia, Alzheimer's, Parkinson's, etc.) are either problems with particular proteins which are found throughout the brain and CNS (such as in the case of schizophrenia, which is thought to be a mainly dopamine and nicotinic acetylcholine receptor pathology) or neurodegenerative (such as Parkinson's disease or Alzheimer's, both of which involve neuronal death.) In these cases, I think brain trauma or other injury wouldn't help/change the pathology, because they're more ",
" rather than brain area centered (for example, though we see a lot of the first effects of Parkinson's in the dopaminergic neurons coming from the striatum, it's more those neuronal pathways, and less the brain area itself, that the disease affects.) ",
"Second of all: I think that brain trauma, coma, and amnesia are all vastly different. I will say that I don't know a whole lot about amnesia or what causes it, so I can't really comment on that, but trauma is the most simple to understand- actual physical injury to the brain that causes -usually prolonged- cell death due to excitotoxicity from the glutamate released by damaged neurons. I can't really think of a case in which this would be good, especially because of how wanton this sort of destruction tends to be, except perhaps in the case of epilepsy/chronic seizures. Epilepsy is one of those diseases where there is synchronous hyperactivity of neurons, and a last-ditch effort among neurologists and neurosurgeons for some years now has been to cut the corpus callosum (the tract that connects the two hemispheres of the brain to each other.) It's conceivable to think that, in some rare cases, brain trauma occurred in juuuuuuuuuust the right way/magnitude, some pre-existing epilepsy may be alleviated. In the same sort of vein, the potential neuronal changes following a coma also might have some effects on mood disorders such as depression and anxiety, but again, the chances of this are quite low and I, personally, have not heard of any cases where this has happened. "
] |
[
"except perhaps in the case of epilepsy/chronic seizures. Epilepsy is one of those diseases where there is synchronous hyperactivity of neurons, and a last-ditch effort among neurologists and neurosurgeons for some years now has been to cut the corpus callosum ",
"But then the cutting of the corpus callosum can cause right & left brain personalities which is a whole other issue. Just to clarify for the OP."
] |
[
"Thank you for your answer.\nI'm sorry as my knowledge in the matter is that of an absolute outsider, and I discover with your answer nuances I ignored in my question: I'm talking exclusively about psychological disorders for which we're not sure they are \"protein-related\" or at all caused by autonomous physical changes of the brain (therefore not neurodegenerative diseases). That could include bipolar disorder, post traumatic disorders, eating disorders... ",
"I originally wanted to add that I figured amnesia was the most likely to erase pathological trait in the way that it could reset their learning and thinking patterns, and possibly pathology-starting (or fueling) traumatic experiences from their memory. Any accidental amnesia in a patient with previous pathological traits in medical record would seem to me as a way to learn about the origin of said pathologies.",
"I added trauma because I thought it to be an accepted \"personnality\" changer in some cases, so I figured it could have changed a patient's personnality enough that they would'nt exhibit said pathological traits. Coma doesn't seem a logical addition to my question anymore :/"
] |
[
"Why are the Earth's magnetic and geographic poles so close together?"
] |
[
false
] |
I searched for previous answers on the sub but none had what I was looking for.
|
[
"In very simple terms (and simple is all I can do as the details of geodynamo theory are beyond me), the geographic pole is the rotational axis of the planet. The magnetic field is generated via a ",
"dynamo",
", which basically means that the geomagnetic field is induced by currents within the liquid Fe/Ni outer core. If you look at the ",
"'formal definition bit'",
", you can see the discussion that the rotation of the earth (and via the Coriolis effect, the rotation of the outer core) controls the alignment of the fluid motions within the outer core, which tend to form into columns aligned with the rotational axis, meaning that the induced magnetic field resultant from these columns will also be aligned with the rotational axis."
] |
[
"The rotation is only one part of the equation, so the Earth does not have a magnetic field solely because of the angular momentum of the accretionary disc, but the rotation of the earth is a part of why we have a magnetic field. Dynamos are incredibly complicated and can get weird. We see similar patterns in several other planets (e.g. Earth, Jupiter, Saturn), but then you have planets that don't have much of a coherent magnetic field (e.g. Mars) and planets with oddly oriented magnetic fields (e.g. Uranus & Neptune). Here's a ",
"set of class notes",
" on magnetic fields on other planets, and here's some more detailed discussion of one of the weird ones, ",
"(Uranus)",
". Ultimately though, this question would be better answered by someone more familiar with this topic (I work on the Earth and not on anything having to do with magnetic fields or dynamo theory)."
] |
[
"I mostly stick to atmospheres, so while I don't work directly on planetary dynamos/magnetospheres, quite a few of my friends do.",
"While Uranus and Neptune's magnetic fields are very tilted, we think we understand the basic idea why. Their magnetic fields are generated in a shallow superionic water sea (rather than deep in a metallic core or mantle), meaning that there's a very strong quadrupole moment that modifies the direction of the typical dipole field.",
"One thing that's not well-understood, though, is that an unusual number of planets all have just about a 10 degree tilt between their intrinsic magnetic poles and rotational poles: Mercury, Earth, Jupiter, and even Ganymede (moon of Jupiter). Perhaps even more strange is that Saturn alone is the exception, as it appears to have no magnetic tilt whatsoever."
] |
[
"Are there non continuous macroscopic physical quantities?"
] |
[
false
] |
Temperature, speed, pressure, energy... are all continuous in any situation, to the best of my knowledge. Even square waves are in practice continuous. Do non continuous macroscopic physical quantities exist? Why or why not?
|
[
"Yes. Look at ",
"this",
" diagram of typical superconductor during phase transition.",
"Superconductivity is quantum effect but it can be seen on macroscopic scale because piece of superconductor can be of arbitrary size given that you can cool it down below critical temperature. "
] |
[
"Yes. Ie. all material properties are discontinuous at the edge between two materials. (Ie. diffraction and reflection at the border between air and water is due to a discontinuity of the speed of light in those materials). Also, every phase transition has discontinuous properties related to it, ie. if you melt ice there's a jump in density between the molten and liquid state. ",
"Now, ofc. these discontinuities might seem fuzzy if you look at them on a microscopic level, but one needs to remember that the respective properties are only well-defined on a macroscopic scale, so within a macroscopic description these discontinuities are real. "
] |
[
"This is probably the best example of a completely discontinuous phenomena in physics. Many discontinuous phenomena end up looking reasonably smooth anyway but this one is just so striking"
] |
[
"Does CERN use proton-proton collisions in the accelerator? If so, why not proton-antiproton collisions?"
] |
[
false
] |
The only things I have been able to find seem to suggest that they use proton-proton and proton-heavy ion collisions. Wouldn't a matter-antimatter collision make more sense as their entire mass would also be converted into energy which would go into making particles? Also, are there any other experiments today that do use matter-antimatter collisions?
|
[
"For some particles the production cross sections differ significantly between proton-proton and proton-antiproton collisions. See for example this standard plot: ",
"http://www.hep.ph.ic.ac.uk/~wstirlin/plots/crosssections2013.jpg",
" .\nThe thick red line labeled sigma_top has this weird jump at 4TeV, which is caused by changing the calculation from proton-antiproton to proton-proton.",
"Generally production crosssections increase when using proton-antiproton collisions. The reason why LHC is (mainly) running on proton-proton, is simply that it is very hard (and expensive etc.) to create enough antiprotons for a full LHC fill and generate a stable, well behaving beam out of them. Protons however are very easily available by stripping Hydrogen of its electron. \nActually this ( ",
"http://void.printf.net/~conor/sa/LHCb/LHCfuel.jpg",
" ) is the hydrogen bottle that contains all of the protons LHC will ever need. I believe during standard operation around 13nanograms of hydrogen are used up per day..."
] |
[
"It does proton-proton, lead-lead and proton-lead. The Tevatron at Fermilab near Chicago used proton-antiproton. The predecessor at CERN, the LEP, did electron-positron. The protons have about 7000 times as much kinetic energy as mass energy, so that extra part doesn't really make a difference."
] |
[
"Thank you, makes sense.\nThat is so crazy that they'll only need that much, it looks so insignificant compared to what they do there. Really a wow moment!"
] |
[
"Is nuclear waste management a big problem with nuclear energy?"
] |
[
false
] |
A few days ago, me and my colleagues were discussing nuclear energy v/s conventional energy and I was debating for the nuclear side. Of all their arguments (including nuclear disasters, escaping radiation etc.), I was able to provide counter arguments and references, but when one of my co-workers mentioned nuclear waste management and said that as of now we don't know how to handle nuclear waste effectively, I did not have an answer. Simply, I had never thought of it or read extensively about it. So, what I wanted to know is, are our current methods of nuclear waste management essentially ineffective or largely inefficient?
|
[
"It depends on the type of nuclear reactor used. Breeder reactors do not produce much waste at all, but for whatever reason these are not the preferred reactor in most countries. In the uk waste is a big problem, Sellafield reprocesses waste into new fuel which reduces some of the problem. ",
"Storage of waste in deep underground facilities seems to be the most sensible option at the moment, but this is extremely expensive and generally these facilities have to be made tamper proof from nature and animals for 100,000 years. ",
"There is a lot of information available on this if you're interested. If you go onto the Scandinavian Netflix if you can and watch Into Eternity which looks into Finland deep geological storage site. On (at least) the UK Netflix there is a documentary called Pandora's Promise which interviews a collection of environmentalists who have changed their viewpoints and are now pro nuclear power. ",
"The UK government have a lot of resources on nuclear waste and decommissioning, maybe try with this website as they have a lot of external links: ",
"https://www.gov.uk/government/policies/managing-the-use-and-disposal-of-radioactive-and-nuclear-substances-and-waste",
"Also the IAEA: ",
"http://www.iaea.org/OurWork/ST/NE/NEFW/home.html"
] |
[
"I will try and expand this comment later, but...",
"There are technologies to handle waste management (e.g., reprocessing), but the US in particular does not use them, due to an executive order by Jimmy Carter. Geological storage (namely Yucca Mountain) was also a viable solution (though not very good compared to a \"closed\" cycle), but it was canceled.",
"Edit:\n",
"/u/GAMEOVER",
" described reprocessing and the history well, so I will just add to it. Obviously weapons proliferation isn't really a concern in the US, but the motivation was as a symbolic gesture. The US would have a \"once-through\" cycle, which was to serve as a model. A once-through cycle is valid, but it generates much larger volumes of waste (there's no recycling), for which you must build a geological repository. This never happened, of course. The government told most of the utilities there would be, then never followed through, so utilities have been left holding large amounts of spent fuel on site. It's not a danger at present, but it's not safely sustainable indefinitely.",
"So, the options are either to reprocess or to go through with building a repository. Regarding cost, it's not so much that reprocessing itself is expensive (it ",
" expensive, but the costs are only a bit more than normal fabrication), but more that uranium is very cheap in terms of GW/ton. Economically, there isn't a need to reprocess at present. The general consensus is that eventually American utilities will be forced to switch to reprocessing, both for economic and sustainability reasons. At that time, the spent fuel laying around will be an asset, as the vast majority of it can be scrubbed and fabricated into new fuel. Reprocessing also dramatically cuts down on the long-term containment issues. What's left as waste after reprocessing is much shorter lived than non-reprocessed waste, and dramatically reduced in volume (can be 95-99% reduction) so disposing of it is less complex. (Note based on ",
"/u/fizzle_dizzle",
"'s observation: Reprocessing isn't magical, there is still ultimately ",
" waste left over. Generally it is easier to handle, but not trivial.)",
"Amusingly, reprocessing technology was largely developed in the United States. When the decision to not reprocess was made, we sold it to countries like France. Recently, the French offered to sell us the same tech back (I will try and dig up a source for this, I heard this in a presentation so I don't have it handy).",
"Tl;dr, we can deal with it, but due to inconsistent political policy (in the US), we currently don't do a very good job. The technology exists, though, France, Japan and the UK all use it, though not at full scale. Eventually, the US will be forced to, but it won't happen until it's an economic and safety necessity."
] |
[
"Yes, that's what I meant by volume reduction, though I guess I should have spelled that out (I had to head out the door, so my original answer was a placeholder). Reprocessing doesn't burn everything, though you can get very high burnups with a full implementation of a closed cycle using fast reactors, breeders etc. What's left over is mostly the very highly active fission products, but the volume of waste is cut down by quite a lot. In addition, because the byproducts are mostly high-activity, they decay away much faster and aren't as persistent as some of the more annoying isotopes in once-burned fuel. Ultimately, however, you ",
" still have some waste to dispose of, but the considerations for designing such a disposal facility are dramatically different, and generally much easier.",
"Plutonium is usually the main concern in reprocessing, the worry is that separation of plutonium can yield lots of weapons-grade plutonium to fuel a lensing-based weapon. This is less of a concern in a reprocessing cycle derived from a power-oriented cycle, though. To get weapons-grade material, you have to pull out the fuel after only a very short irradiation period, which is the exact opposite of what you want to do in a power cycle. When you leave it in for longer, the Pu-239 (weapons-grade stuff) is contaminated by Pu-240 due to neutron activation. Pu-240 is bad because it has a really high spontaneous fission rate, so if there is too much of it in your bomb core there's a good chance that it might detonate early. Because they have similar chemical properties, it's very difficult to separate Pu-240 from Pu-239, so once it's there then it becomes very hard to scrub it back to weapons-viable material.",
"In other words, it's hard to make weapons with a sensible fuel cycle for power generation. You have to actively modify the cycle to get weapons-viable plutonium.",
"It's also worth noting that the longer we let spent fuel sit around, the harder it is to reprocess it into fuel, especially if there is a significant amount of plutonium present. The decay products of plutonium can be a big radiological hazard, so after it's more than about 5 years old it starts to get really annoying to refabricate it into fuel. ",
"PS: I'm an academic nuclear engineer. My PhD is in nuclear engineering, but my real specialty is mathematics. I design algorithms for efficient reactor simulation and engineering design under uncertainty. I also have a decent background in weapons and non-proliferation, as I participated in a DOE training program focused on detection of weapons and prevention of proliferation, we spent a lot of time discussing reprocessing technology from that point of view."
] |
[
"Why do ice cubes crack when liquid is poured over them?"
] |
[
false
] | null |
[
"Rapid temperature change, which also leads to a change in density.",
" The density change pushes and pulls the molecules around it, applying mechanical force within and around the ice cube that breaks its crystal structure. Water is actually most dense at 4 deg C. It's very similar to you snapping a pretzel.",
"LTP: Glass refrigerator shelves can do the same, and produce a lot of glass pebbles as a result."
] |
[
"LTP",
"Life Tip Pro?"
] |
[
"Fun tidbit, the forces at play can, on rare occasion, cause the ice to crack with enough force to make a piece jump out of your drink. I’ve witnessed a grand total of one time in my life. It was spectacular.",
"The forces at play are similar in some ways to the “prince rupert’s Drop”. Variable stress within the ice (or glass, in he case of the drop) pulling in multiple directions can sometimes cause explosive results."
] |
[
"Would a body encased in cement decompose?"
] |
[
false
] |
I was watching The X-Files on Netflix and they discovered that someone had buried a body in concrete. When they went to dig the body up, it had completely decomposed and all that was left was the bones. Would this happen in real life? Or would the concrete sort of preserve the body from anything that would decompose it?
|
[
"concrete is quite porous and will allow moisture to escape slowly over the years (which means given enough time the flesh would become desiccated), in the mean time anaerobic bacteria would go to work on any energy source it can digest"
] |
[
"Thanks so much for your response! Do the bacteria need oxygen to decompose the flesh? If so, would the porous nature of the concrete allow for oxygen to reach the bacteria?"
] |
[
"not enough to sustain very many bacteria and these would be out-competed for resources by the anaerobes "
] |
[
"Why is it reptiles and certain animals are able to survive such long periods on a single meal, compared to humans?"
] |
[
false
] |
I was watching a documentary and saw that a Nile Crocodile could last an en entire year after a meal. How is this possible? Even if the animal it ate was large, surely the eaten animal couldn't sustain a big crocodile for such a long time?
|
[
"Cold blooded. Humans use up massive amounts of energy keeping ourselves at optimum temperature, humans also have higher than average energy requirements than most other mammals due to our larger brains. Humans can survive months without eating btw."
] |
[
"Yeah, I didn't deny that? They can't last as long from a single meal though and instead rely on large fat stores."
] |
[
"There are plenty of warm-blooded animals that can last months without food "
] |
[
"When I flick a lighter/flint next to an already-burning fire, the sparks cause extra sparks to go off in and around the flame. How/why does this happen?"
] |
[
false
] |
I could not find any videos, but it is really easy to see the phenomenon with a candle and a lighter. Light the candle, and then flick the lighter a couple of times near the flame. "Extra" sparks will go off around the flame up to a second after you stop flicking. This completely baffles me--why does it do this?
|
[
"The flint dust that was leftover gets kicked up and ignited.",
"It's made of ",
"Ferrocerium",
".",
"Basically you're burning metal shavings."
] |
[
"well sure, it's little splinters of metal that fly up and get ignited and combust. You can call them mini-fires if you like.",
"Thought what shavera said may be true, I don't believe this is what happens most often. It would look different i'd think - much more of a mini-fireball-woosh effect."
] |
[
"You can achieve the same effect by holding the lighter upside down and grinding the flint slowly onto your fingernail, then flicking the accumulated shavings into the flame. I loved doing this when I was a kid."
] |
[
"How does the 'security pattern' in the Canadian polymer bills work?"
] |
[
false
] |
Short description of what I'm talking about: In 2011, the Bank of Canada started issuing the Frontier Series of Canadian banknotes, which were the first to be produced on polymer. If you look through the circle in the middle of the frosted maple leaf, there is kind of a 'display' of the denomination of the bill. How does this work? Specifically, why can't I see it on the actual bill (i.e. in the reflection of the circle), and why does it stay centred on the point of light? For that matter, why does it only work with a small light source anyways? Examples: ,
|
[
"It uses a ",
"diffraction grating",
" to produce the images. The diffraction pattern in this case produces the denomination amount. The second video produces a much clearer image of the denomination amount because they used a laser, which is a source of coherent light."
] |
[
"I guess a bank could have the note pass over a laser on one side with a detector on the other side to measure the shape of the diffraction pattern. A focused laser would allow the pattern to be more easily deciphered at smaller distances."
] |
[
"My guess is this works similarly to how the glasses which make bright small light sources appear differently work. For example, making the lights on a Christmas tree look like snowflakes. ",
"What such glasses show while driving, for example.",
" Or for a more natural explanation, how when you look at a traffic light at night the light may appear \"smeared\" in some pattern.",
"I would LOVE an explanation of exactly how these things work, though, and I hope someone chimes in."
] |
[
"How can the universe be both infinite and expanding?"
] |
[
false
] |
If the universe is expanding then that implies that there is something to expand into which would imply an end to the universe. I just cant wrap my head around this.
|
[
"Think of the number line. Multiply every number by two. The distance between any two points has just increased (by a factor of 2) yet the number line is still the same length."
] |
[
"Metric expansion",
" is entirely analogous to this. I was trying to give the simplest example where",
"If the universe is expanding then that implies that there is something to expand into",
"(i.e. the first premise) is incorrect"
] |
[
"Yeah? And that's how you try to make sense of abstract concepts."
] |
[
"How large can stars get?"
] |
[
false
] | null |
[
"There is not a single answer to this question. Previously stars were expected to top out at around 150 solar masses, in part due to the Eddington luminosity which describes the point where the radiation pressure would exceed the pressure due to the gravitational force.",
"https://en.wikipedia.org/wiki/Stellar_mass",
"https://en.wikipedia.org/wiki/Eddington_luminosity",
"If a star exceeds this limit it will shed excess mass due to the net outward pressure from the radiation it generates. But there are stars much larger than this, that reach around 300 solar masses. ",
"http://www.eso.org/public/news/eso1030/",
"They were either \"born\" giant or formed from the merger of several stars, and they do shed lots of mass in the form of stellar wind, though the amount does not seem to agree with what should be lost in theory.",
"So there is not a clear answer for stars in general since there are many variables like composition, luminosity, formation, that all can complicate how large that star can theoretically be. However for specific types like white dwarf stars there can be hard limits, namely the Chandrasekhar limit in that case",
"https://en.wikipedia.org/wiki/Chandrasekhar_limit",
"Black holes can get ",
" bigger, reaching sizes of 50 billion solar masses, and thats not even a hard limit really, since the black hole could still theoretically grow at that point. They also have the interesting property that they lose less and less mass by radiation as they grow larger, in contrast to stars.",
"https://arxiv.org/abs/1511.08502"
] |
[
"Stars can get very large indeed. The largest star yet discovered is UY Scuti in the constellation Scutum (Latin for shield) near the constellation Sagittarius. ",
"Our Sun is enormous. It accounts of 99.8% of all the mass of our solar system. If you wanted to fill the volume of the Sun with planet Earths you would need 1,300,000 Earths to do so. While our Sun is truly massive it is dwarfed by UY Scuti. ",
"UY Scuti is so massive that if you were to replace our Sun with it, the photosphere of UY Scuti would engulf Jupiter. This means that the planets of Mercury, Venus, Earth, Mars, the asteroid belt and Jupiter would all be inside that star. Because of its massive size it has a volume nearly 5 billion times the volume of our Sun. ",
"But UY Scuti is not the largest single object we have discovered. The super massive black hole at the center of NGC 1277 is mind bending in scope. The best way to put this into perspective is this: if I wanted to travel around the circumference of our Sun at the speed of light (186,000 miles per second or 299,792 kilometers per second) it would take me about 14 seconds to do so. If I wanted to do the same with UY Scuti it would take about 7 hours to do so. But if I wanted to do the same to the super massive black hole at the center of NGC 1277 it would take an astonishing 11 days.",
"Our universe is amazing and full of wonders. Vistas reaching out across not only space but of time await you. Everything we are and see was formed in the hearts of dying stars. You are the universe looking out at yourself. I would highly recommend looking up your local astronomy \"star party\" or outreach event. Some of these events have professional grade telescopes open to the public and often free of charge. Looking thru a top quality telescope can be a breath taking experience. "
] |
[
"The limit outlined in that paper is the practical limit because the black hole won't grow appreciably if it cannot form an accretion disk. But there are known black holes close to this limit (the same order of magnitude) at 40b solar masses",
"https://en.m.wikipedia.org/wiki/List_of_most_massive_black_holes",
"There are also some apparent issues with unexpectedly early formation of supermassive bh in the early universe.",
"https://arxiv.org/abs/1511.05494",
"So it seems like this is still an area where there are unanswered questions"
] |
[
"When someone has an operation to remove an organ, what fill the space left behind?"
] |
[
false
] |
Say I get a lung or kidney removed, they obviously took up space and had blood vessels and muscles attached to them, what fills that empty space, and what happens to all of the now unnecessary blood vessels?
|
[
"Here's a book that describes what happens with lung resections. It's a special case because the lung cavity is subjected to a big variance in pressure when you inhale and exhale.\n",
"http://www.intechopen.com/books/topics-in-thoracic-surgery/pathophysiology-of-extravascular-water-in-the-pleural-cavity-and-in-the-lung-interstitium-after-lung",
"The “postoperative residual pleural space” refers to the fate of the volume left free by lung resection (Misthos et al, 2007). As much as in physiological conditions, the main variable setting the volume of the postoperative residual pleural space is the absorption pressure of the pleural lymphatics. If their capacity to drain flow and generate a subatmospheric pressure have remained unchanged, they will still tend to reduce pleural liquid volume to a minimum. However, the new “minimum” will reflect a state equilibrium resulting from the modified lung-chest wall coupling and the actual filtration/absorption balance of pleural fluid. ",
"Fig. 7",
"If you are curious about a specific organ, I'd google \"(organ of interest) resection cavity.\" Also, failing kidneys are left in place unless there's a specific need to remove it (e.g. cancer)."
] |
[
"Usually when a kidney fails it because it's not filtering blood properly anymore. One of the complications is that the kidneys are attached to major arteries/veins so it's usually better to leave them in place. If the tissue itself is undamaged (just not functioning) then it's better for the patient to leave the kidney in place rather than perform a resection. People who receive kidney transplants usually get a third kidney placed in their pelvis instead of one in place of one of the previous kidneys. "
] |
[
"A lung is a pretty dramatic example nowadays - if you have an organ removed, you normally replace it. But let's say it's your spleen which is relatively dispensible. ",
"You have a total splenectomy. The blood vessels are ligated and cauterised to prevent them leaking blood into the peritoneal cavity. The muscles.. well in the case of the spleen you have limited musculature. The muscle that is there will slowly degrade without use, but I can't imagine anything major happens. All the empty space? Your intestines fill it almost immediately with no ill effect."
] |
[
"What are the epidemiological reasons for immunizing healthcare workers before at-risk populations?"
] |
[
false
] |
"Frontline healthcare workers and elderly residents of long-term care facilities will receive the very first COVID-19 vaccinations, a Centers for Disease Control and Prevention (CDC) advisory board recommended Tuesday. These groups will make up Phase 1A of U.S. vaccine recipients who will receive the first 40 million or so doses that could be available by the end of the year. The Food and Drug Administration (FDA) is currently reviewing two vaccines, from Pfizer-BioNTech and Moderna, for emergency use authorization." - Time I am wondering why we're not immunizing older folks or people with pre-existing conditions first, from an epidemiological standpoint (not asking if it's moral to do so). Does the overall amount of deaths actually decrease if you immunize healthcare workers first? What are the epidemiological implications and justifications of this decision and how will it alter the trajectory of the pandemic, as compared to if we immunize the at-risk population first?
|
[
"Health care workers are the most likely people to spread the disease because they have to work with people who actually have it. Other demographics, especially at risk people, aren't likely to visit lots of people and aren't likely to deliberately visit people who do have it. Vaccinating the people most likely to spread the disease first means it will spread much slower, giving you more time to vaccinate more people before they catch it."
] |
[
"Also to protect capacity within the healthcare system. You don't want outbreaks removing nurses and doctors when they're needed most."
] |
[
"If the health care system crumbles, who will care for the sick? (Including the non-covid sick). People still get into car accidents, have heart attacks, need emergency surgery, need prescriptions.",
"Also, because we see so many people in such a short period of time, we are very likely to be asymptotic carriers. We would expose hundreds, if not thousands, before we even knew we were infectious.",
"I am a health care worker. Because I cannot easily be replaced, my workplace has a policy: no symptoms, no positive test: your fine, get your ass to work. Regardless of exposures.",
"I’m terrified that I will be the carrier."
] |
[
"What you see traveling near the speed of light"
] |
[
false
] | null |
[
"The joke is you would suffocate because you were traveling through the vacuum of space (with no mention of a spacesuit/oxygen -- the comic character specifically not having one drawn) and has nothing to do with relativity.",
"Sort of similar to ",
"xkcd 669",
".",
"Note for the other panels, you don't have to go to the economics of government debt, or the neurophysiology of humans, but its a trivial reason why the standard absurd \"thought-analogies\" are implausible."
] |
[
"Image",
" Experiment",
" The other two are still lost on the infinite plane of uniform density.",
"Comic Explanation"
] |
[
"I am officially very stupid, hahaha. I don't know why I thought that one was real fact, knowing the other ones were spoofs on fun facts. ",
"Thanks for pointing out my sillyness"
] |
[
"Some of the most massive craters on the surface of the moon and elsewhere throughout the solar system seem relatively \"shallow\" considering how wide the craters are. What gives craters this wide and flat shape?"
] |
[
false
] | null |
[
"This is not entirely consistent with discussions in the literature on impact craters, e.g., ",
"Robbins et al, 2017",
" discuss relationships between crater depth and impact velocity (and angle) with references to more detailed studies."
] |
[
"The ",
"impact depth",
" is approximately (Length of Object)*(Density of Object)/(Density of Target). One thing you will note is that the velocity or energy of the impactor does not effect how deep it will penetrate. As more energy is added the crater gets wider but doesn't penetrate any better, this is the reason armor piercing rounds are long and skinny and made out of high density materials."
] |
[
"The Modification stage of crater formation. See figure ",
"here",
" of simple crater formation.",
"So long as there aren't huge density differences, the initial excavation from the impact is pretty close to a hemisphere centered on the impact point. However, the resulting crater walls are usually a fair bit steeper than the ",
"angle of repose",
", especially for all that newly fractured rock. That material slumps back into the bottom of the crater, leaving behind a wide, relatively shallow depression."
] |
[
"How does the body maintain the same amount of blood that it contains?"
] |
[
false
] |
I'm not sure if this is a silly question or not. I have just given blood and, as a male, I have about 11 pints of blood left in my body which will (hopefully!) increase back to 12 pints. It got me thinking about how the body maintains this consistent level of blood and doesn't produce too much/too little?
|
[
"There is a lot that goes into the formation of blood. In your blood you have: water, rbc's/wbc's/platelets, plasma proteins, and electrolytes (there are some other things, but they are not as fundamental). ",
"When you lost that pint of blood, the first thing that happens is constriction of the blood vessels and activation of your sympathetic system. This activates the renin-angiotensin system in your kidneys which will now absorb many Na+ ions that would normally be excreted. Water will follow the Na+ ions and will also be recirculated in the blood. Also, the water you drink after you donate blood will quickly enter your blood stream. Your kidneys will also be able to restore K+ levels in the blood by absorption of more K+. Now you have a good blood volume and a good Na+/K+ concentration. ",
"In a state of hypocalcemia and hypophosphotemia, your kidneys can reabsorb any Ca2+ or PO4 that may be secreted. Also, your parathyroid will secrete more parathyroid hormone which will signal the break down of bones and release of Ca2+ and PO4 into your blood stream. Now your calcium and phosphate balances should be fine. ",
"Because you had less blood, your body was in a state of hypoxia. When the kidneys enter hypoxia, they will release erythropoietin which stimulates the synthesis of blood cells. This eventually fixes your blood cell counts.",
"Finally, your liver will begin producing plasma proteins because of the deficit in proteins in your blood. ",
"Now your blood is back to normal. All of this happens at the same time."
] |
[
"In case of excess fluid in your blood vessels, the same system works in reverse: the renin-angiotensin-aldosterone system becomes less active, the kidneys will excrete more Na+ ions, and because of that will also excrete more water. At the same time, lack of hypoxic stress (because of the sufficient availability of red blood cells) results in low EPO production, eventually causing the number of red blood cells to normalize as they die off naturally. However, the body does not speed up the destruction of red cells.",
"\nExcess production of red blood cells on the other hand results in a disease called polcythemia vera, which is potentially lethal because patients become prone to the development of blood clots. PV can be caused by overproduction of EPO or because the bone marrow is overactive despite low EPO levels. The main treatment for PV is bloodletting. "
] |
[
"Blood is a mixture of water, cells and some molecules and ions, each of one has its amount rigorously controlled. When you donate blood, you lose about 400 mL. The first thing your body does to compensate this loss is to drive water from the tissues to inside the vessels. This will keep your blood pressure. After that, you will need more erythrocytes. Your kidneys will produce EPO (erythropoietin) that will lead the bone marrow to accelerate erythropoiesis. And this process goes on and on for all the components until the previous levels are reached."
] |
[
"How does climate effect human behaviour? eg how would living in tropics change society will it change sexual behaviour?"
] |
[
false
] | null |
[
"Hello,",
"This question would be more appropriate for ",
"/r/AskScienceDiscussion",
".",
"Best."
] |
[
"Have I been shadow banned?"
] |
[
"No, this thread is just removed."
] |
[
"What has caused the characteristic shape of eye among alot of native asian people?"
] |
[
false
] |
We all know that native africans have darker skin than other people because exposure to strong sunlight during millions of years, but what has caused the characteristic shape of eye that is common among native asian people?
|
[
"It has been described as a defense against the cold and winds of northern Asia, or the harsh sun's rays in tropical parts of Asia.",
"But, it is also seen in a number of medical disorders (such as Down Syndrome, Turner Syndrome, Fetal Alcohol Syndrome, and more) and in fact all people have these folds in the womb, but they typically disappear by birth. "
] |
[
"Ha yes I will admit that it's partly Google, but I'm also studying developmental biology, so I come across things about this from time to time. "
] |
[
"I think it is safe to say that it is always okay to google a lot. :)"
] |
[
"Could humans circumvent breathing by directly oxygenating their blood?"
] |
[
false
] |
I've always wondered if it would be possible to do this. I can see it being incredibly useful for a navy seal team (for example) if, instead of holding their breath or using an 02 tank, they could directly oxygenate their blood. In my head it would involve connecting a device to a major artery or vein (idk which would be more efficient/useful) and blood would make a short trip out of the body and into this machine which would draw the oxygen out of the water and infuse it to the blood, eliminating the need to breath. I'm not sure what the process would have to be for pulling the 02 out of the water or what it takes to oxygenate the blood (though, since your blood is always red when you cut yourself, i would hazard a guess that simple exposure to oxygen would do it) What do you guys have to say?
|
[
"Promising future research aside, the only clinical way to accomplish this currently is via extra-corporeal membrane oxygenation (ECMO); also known as a heart-lung bypass machine. It is similar to what you're imagining in that it inserts into major blood vessels - in the surgeries I've taken part in, usually the IVC and the ascending aorta, but the femoral veins and internal jugular veins are also common. ",
"The problem with your execution is twofold - it requires a large amount of hardware as well as an experienced technician to monitor and operate the equipment. ",
"The other problem is that oxygen doesn't diffuse very well into water, which is why our blood has carrier proteins (hemoglobin) to transport oxygen through the bloodstream effectively. I can't imagine pulling oxygen out of the water directly could satisfy the body's metabolic demands, least of all while swimming with a huge load of equipment."
] |
[
"Graphene seems to be very popular right now. A machine that makes oxygen + graphene out of airborne co2 would probably net you a couple of bucks.",
"Also, I think the machine you describe is called \"a tree\"."
] |
[
"You have to get rid of the CO2 produced though cell respiration. Even if you oxygenate the blood, CO2 will build up and throw you into ",
"acidosis",
", eventually resulting in death. A 140 lb person exhales about 42 grams of CO2 per hour through basal metabolism, so you can see it would build up quickly if not disposed of."
] |
[
"What'd happen to the charge of the plates if you insert a dielectric between the capacitor, then remove the battery 1st then the dielectric after?"
] |
[
false
] |
I know that the charge would increase in the capacitor in the presence of dielectric and battery. But what if I remove the battery with dielectric still in place, then remove the dielectric after the battery is removed. So after I remove the dielectric, the capacitance would drop so it wouldn't be able to hold that much charge, but the extra charges can't flow back to the battery because it's removed so what would happen to those extra charges it's holding? Thanks.
|
[
"Just remember the capacitor equation. Q = C*V",
"Q is charge",
"C is capacitance",
"V is voltage",
"If the plates aren't connected to anything, and I assume there is no arcing, that means that Q, the charge, is locked and cannot change. If Q is constant, and we know C goes down, the V (the voltage) must increase to satisfy the equation. So the voltage is increased.",
"Using the same logic, you can see that if the plates were still connected to the battery, then V is constant, C decreases (like before) so Q must decrease as well. Or basically, the removal of the dielectric would still increase the voltage of the plates, but since there is a battery maintaining the voltage, charge would quickly drain TO the battery till the voltage is the same."
] |
[
"Thanks a lot :)"
] |
[
"This is correct, except that a dielectric ",
" the capacitance. So:",
"1: Constant Q --> V decreases",
"2: Constant V --> Q increases."
] |
[
"Why is the mole the SI unit of measurement for amounts of chemicals?"
] |
[
false
] |
We are currently covering stoichiometry in my chemistry class, and this question doesn't seem to be answered in the course content, so I was wondering if anyone would be able to answer this question for me.
|
[
"Well consider the following: You know the elements, and you know their weights relative each other. Hydrogen is 1 U or 'Dalton' and oxygen is 16 U, so H2O weighs 18 U and the increase in mass on burning oygen to water is 18/16 = 112.5%. So you know 1 pound of oxygen will burnt with hydrogen to produce 1.125 pounds of water. ",
"So you can work on the molecular and the macroscopic scale. Which is how chemistry could get along for nearly a century before anyone had determined Avogadro's number to any good accuracy (circa 1910 by Perrin). ",
"A mole is a counting number, a ",
", like a 'dozen' (12) or a gross (144). It just happens to be a very big group (6.022*10",
" - Avogadro's number)- a very big number. ",
"This the number that gets you from the small scale to the big scale, 1 mol is the number of atoms in 1 g of hydrogen, or in 12 g of carbon, etc. Since we know the relative weight of the atoms and their compounds, counting in moles gives you the means to calculate how many atoms you have, or how much they weigh. ",
"Say you counted your atoms in something else, you'd still have to use Avogadro's number. If I have 10",
" dozen carbon atoms, how many kg is that? 10",
" dozen is ~5 times Avogadro's constant (5 moles of C atoms). 1 mole of C has the mass 12 grams, so you have ~60 grams of carbon. ",
"By counting the number of atoms/molecules in moles instead of dozens, or any other quantity, you avoid to have to use Avogadro's number. 5 moles of carbon atoms weigh 12*5 grams. ",
"So when you talk about the number of atoms, it's most useful to use moles rather than talking about sextillions or septillioms of them."
] |
[
"Isn't your example a bit off? 12x10",
" is two moles of carbon. so 24 g."
] |
[
"To put it simply, the inverse of Avogadro's number is the atomic mass unit in grams. Avogadros number is the conversion factor between atomic mass units and SI mass units."
] |
[
"Why can't we do massive blood replacement to help fight diseases in the blood such as HIV/AIDS?"
] |
[
false
] |
This might be a super stupid question, but I was just thinking, with diseases like HIV/AIDS which slowly 'infect' your blood. Could you not do a massive blood transfusion? Essentially removing most of your current blood and replacing it with a donor's blood which doesn't have your white blood cells massively infected? I realize this couldn't cure you because the infected and fresh blood would have to touch since you can't be drained of all your blood and survive (or can you?), but it seems like if you did do this you could at least remove a wide majority of the infected WBCs and supply it with a fresh supply, slowing the total assimilation of your blood and reducing the risk you would die from an opportunistic infection? Could this also work with other diseases of the blood? I am assuming it can't, but why not? Edit: Thanks all for the great answers, it seems like you can do this technique, but only for diseases existing only in the blood and I had mistakenly though HIV was one such disease. I LOVE ASK SCIENCE /moar
|
[
"The blood replacement would not work because blood transfusions only occur in the peripheral bloodstream. The WBCs that HIV infects (CD4 lymphocytes, macrophage, dendritic cells) are located in the bloodstream and other areas. ",
"If you transfused all of someones blood you would be left with HIV infected cells in lymphoid tissue, bone marrow, ect. These infected cells would proliferate normally after the transfusion. ",
"The person who was cured of HIV went through a different process. HIV resistant bone marrow was given to someone with HIV. During the bone marrow transplant, many of the infected WBCs are wiped out in the process with chemotherapy/radiation. After the transplant, the patient made new WBCs that were resistant to HIV so he was cured. "
] |
[
"HIV uses the bodies immune cells to replicate itself, so removing all blood would not remove all of the HIV virus."
] |
[
"There is a technique like this called ",
"exchange transfusion",
" which is sometimes used for severe malaria (which infects red blood cells). ",
"It wouldn't work to cure HIV because it hides away as others have mentioned. Anti-retroviral drugs can get HIV in the blood down to almost undetectable levels. "
] |
[
"What would the lifespan of someone born in the past two decades be?"
] |
[
false
] |
[deleted]
|
[
"Here are the ",
"actuarial tables",
" the US Social Secuirty administration uses.",
"A 21 year old male can expect to live to be 76, a female 81."
] |
[
"our increasing lifespans are largely due to medical advances, new drugs and treatments..",
"So unless somebody wants to go out on such a limb as to predict the next 50 years of medical advances, the question can't be answered reliably.."
] |
[
"I don't know, but I heard that the majority of children born today here (Sweden) can expect to reach 100.\nEDIT: Actually, this is not a Swedish source. It comes from the Lancet. See for example, ",
"http://abcnews.go.com/Health/WellnessNews/half-todays-babies-expected-live-past-100/story?id=8724273"
] |
[
"If antihistamines can reduce swelling and vasodilation without negative side effects, why do our bodies do this in the first place?"
] |
[
false
] |
[deleted]
|
[
"Just because we have allergic reactions to things like pollen, it doesn't mean it evolved because it was advantages. It could be a negative offshoot of an overall beneficial biological process. The \"swelling and vasodilation\" are very important in the bodies response to harmful stimuli. This is how the body removes whatever is causing the harm, or speeds up the process of healing or fighting off an infection, and would not be able to do so without it.",
"The benefits to having an immune system are obvious, and it's possible that allergies are a price we have to pay for having this great way to fight off nasty invading bacteria, etc. ",
"Also to note, antihistamines only affect one type of Histamine receptor. And some people do get a variety of negative side effects. "
] |
[
"Here is the reason life has not figured out a workaround yet. It hasn't had the need to, nor has it had the time to if there was enough selective pressure to eliminate them. Allergies are a relatively new phenomenon. Children in third world countries do not develop allergies nearly as frequently as children in first world countries because they are exposed to a more natural set of allergens and in more appropriate amounts from a young age. The US center for Disease Control and Prevention noted an increase in the prevalence of asthma in children in the US from 3.6% in 1980 to 5.8% in 2003, that's crazy! Allergies are on the rise and it is brand spanking new in terms of an evolutionary time-scale. Plus, with modern medication, there isn't much of a selective pressure to remove allergies from the gene pool. We actually don't know that much about allergies compared to other things because they are so new. Another interesting thing to note is that the allergic response is mediated by IgE, an almost vestigial antibody type in first world countries. IgE is responsible for defense against parasites and helminths that are uncommon to western countries. If you had a deficiency in IgE, you would likely not have any clinical symptoms beyond a total lack of allergies. Now if a child in the Amazon or Uganda did not have IgE, it would be a very different story, but those children are far less likely to have allergies."
] |
[
"Here is the reason life has not figured out a workaround yet. It hasn't had the need to, nor has it had the time to if there was enough selective pressure to eliminate them. Allergies are a relatively new phenomenon. Children in third world countries do not develop allergies nearly as frequently as children in first world countries because they are exposed to a more natural set of allergens and in more appropriate amounts from a young age. The US center for Disease Control and Prevention noted an increase in the prevalence of asthma in children in the US from 3.6% in 1980 to 5.8% in 2003, that's crazy! Allergies are on the rise and it is brand spanking new in terms of an evolutionary time-scale. Plus, with modern medication, there isn't much of a selective pressure to remove allergies from the gene pool. We actually don't know that much about allergies compared to other things because they are so new. Another interesting thing to note is that the allergic response is mediated by IgE, an almost vestigial antibody type in first world countries. IgE is responsible for defense against parasites and helminths that are uncommon to western countries. If you had a deficiency in IgE, you would likely not have any clinical symptoms beyond a total lack of allergies. Now if a child in the Amazon or Uganda did not have IgE, it would be a very different story, but those children are far less likely to have allergies."
] |
[
"What mammals are more plentiful than humans?"
] |
[
false
] |
I'm wondering about the number of individuals of various mammalian species. There are currently just below 7 billion humans on Earth. Are there any mammalian species that come close to or exceed this number of individuals of the species? Mice, rats and possibly certain species of livestock come to mind as possible candidates, but I can't find any estimates of the number of individuals of these species.
|
[
"Birds are not mammals. "
] |
[
"Would you consider, for example, all species of mice just \"mice\" or all species of bat just to be \"bat?\" It's an important distinction, and I think that if you were to consider all species of mice or bat, both groups would out-populate humans. "
] |
[
"It was an interesting piece of information anyway. I had no idea there was such a vast number of chickens."
] |
[
"When you dissolve NaCl in H2O, and then boil the solution so that the H2O molecules begin to evaporate, what keeps the evaporating molecules from carrying Na and Cl atoms away with them?"
] |
[
false
] |
When the water boils away, there is always NaCl left over. Why are the ions not carried away with the water? Don’t the ions bind to the polar ends of the water molecules?
|
[
"The energy you put into the water when you boil it breaks the bonds between the ions and the water molecules. Gaseous molecules, to a decent approximation, are not bound to anything but themselves. ",
"Bonus fact: because the bonds between the ions and water are stronger than the bonds between water molecules, it takes more energy to break those bonds and the boiling point of salt water is higher than that of pure water. "
] |
[
"Na+ and Cl- have a higher boiling point than water. If the salt water was heated to 1621 degree Fahrenheit then the salt would evaporate too.\n The water boiling away is a type of distillation. For more information on distillation using differing boiling points, look up fractional distillation or go to ",
"http://en.wikipedia.org/wiki/Fractional_distillation"
] |
[
"Got a source for that?"
] |
[
"What underlying force caused the expansion of the universe after Cosmic Inflation ended but before Dark Energy became the dominant factor in the universe?"
] |
[
false
] | null |
[
"Was it some kind of ‘inertia’ ",
"Basically, yes. "
] |
[
"That seems about right."
] |
[
"Thanks, I appreciate the answer! This definitely pointed me in a good direction.",
"I’ve kept reading with inflation and inertia as my search terms, and my real rough understanding is that space expands because vacuum energy has negative pressure which provides an effect that acts like the opposite of gravity. (I also understand that we don’t know the origin of the really fundamental things yet (like time, space, energy, phyics etc,) so I’m not worried about that here.)",
"So I think then this all means that, in a universe where inflation occurred and dark energy is real:",
"At or before 13.8 billion YBP, a very large state of vacuum energy somehow comes into being (the inflaton field) and this causes the universe to very rapidly expand.",
"After a fraction of a second, this unstable inflaton field decays into the quark gluon plasma of the Hot Big Bang.",
"With the inflaton no longer powering expansion, the universe continued to expand due to the inertia of inflation. (This part had been my “????” point.))",
"But immediately the gravity of all that newly minted mass-energy began to slow down this inertia, very rapidly drawing it down from the still extremely fast rate it had just been (which would explain why the universe was expanding so much faster back right after the Hot Big Bang.)",
"For the next ten billion-ish years, with matter the dominant factor, gravity continued to slow down cosmic expansion, even as it clumped matter together into stars and galaxies, that themselves were spread out through the cosmos. ",
"But because, unlike the density of matter, the density of dark energy is constant (more space means more dark energy) as the universe expanded, the influence of dark energy grew as well in something like a positive feedback loop, until about 4 or 5 billion years ago when there was enough space/dark energy to overcome gravity, and the expansion of the universe began to accelerate again.",
"Is this generally correct or is there something that I am missing?",
"Cheers!"
] |
[
"How does oral plaque originally form inside humans?"
] |
[
false
] | null |
[
"You're right, plaque is definitely (predominantly) bacteria. In fact, it's composed of not one but hundreds of different bacterial species as well as protein, polysaccharides, and even some of your own cells. ",
"This is a really complete review on the subject of plaque",
". It's a little long but even just reading the beginning is worth it.",
"So how do we get the germs that cause plaque? Babies develop in a germ-free environment so that means we have to pick up all the bacteria that live on and in us from somewhere else. This first happens during the process of being born. The type of bacteria you get initially ",
"depends on delivery method",
". Babies born vaginally acquire vaginal bacteria and babies born by cesarean section acquire mostly skin bacteria from their mothers. Interestingly (and perhaps unsurprisingly for parents), the same types of bacteria that are on a baby's skin are also going to be present in its mouth. ",
"As a person grows, the bacteria present may change. After a while, your method of delivery will no longer influence the bacteria you have as you pick up more from contact with other humans, dirt, air, etc. but there will always be bacteria in your mouth and they will always be forming plaque!",
"TL;DR: Every person has always had a mouth full of bacteria since birth, many (if not most) of which are capable of causing dental plaque. "
] |
[
"Thank you for the interesting reply! ",
"One of the reasons I asked this question is because I was wondering if there was a particular bacteria that we could force into extinction. I see now that's not the case. ",
"Thanks again."
] |
[
" and ",
" are reportedly the most common bacterial species responsible for early dental plaque.",
" It is unclear exactly how children are colonized. One study found a significant number of those born via c-section acquired ",
" earlier than those born vaginally.",
" There is evidence for transmission from mother to child although rates are highly variable.",
"http://www.textbookofbacteriology.net/normalflora_5.html",
"http://www.ncbi.nlm.nih.gov/pubmed/16109988",
"http://www.podj.com.pk/Dec_2012/p-28.pdf"
] |
[
"I don't know if this is specifically a science question, but why does NASA seem to assign such short missions to probes that can apparently operate for much (even decades) longer than that mission?"
] |
[
false
] | null |
[
"The answer will depend on what probe you're talking about. For space telescopes which operate in the infrared, they need coolant, often liquid helium, which will eventually leak out and boil away, making it impossible to keep the telescope cooled to temperatures where it can continue functioning.",
"There are other instances where various probes (such as the heroic ",
"Spirit Rover",
", profiled by XKCD) have dramatically exceeded their expected operational lifetime. Part of this is that the expected lifetime is often the ",
" expected lifetime, and well-engineered probes can last a long time."
] |
[
"Underpromise, overdeliver."
] |
[
"Two reasons, one financial, one cynical. First, the financial, the initial mission funding is for a specific time, NASA may want to run longer, but it would have to be approved in next years budget, so, they design it to last as long as possible, in the hope of continued funding. The second, is that they deliberately give an initial projected lifespan publicly that is much less than what they expect, so if it fails no one can be criticized, but if it lasts as long as they expect it to internally, they can claim undeserved extra success, and use it to get a bigger budget."
] |
[
"How come different allotropes of an element have such drastically different characteristics? For example, the major differences between Diamond and Graphite."
] |
[
false
] |
I have never studied chemistry, but I know that allotropes differ from one another based on the way the atoms are structured, but how come some of them are so different? For example, diamond vs graphite - both are made of Carbon, but Diamond is very hard and clear while graphite is soft and black.
|
[
"The carbon atoms in diamond and graphite bond to each other differently, which leads to different crystal structures. Graphite is a layered 2-dimensional structure, and graphite is a more 3-dimensional structure (you can easily find pictures on the internet of how the atoms are arranged).",
"Graphite is soft in the out-of-plane direction -- the layered planes are not very strongly coupled to one another so they can slide and cleave pretty easily. But within a single plane, it is in fact very strong, even stronger than diamond. Diamond, in contrast, is not a layered structure so all of this sliding doesn't take place.",
"Their crystal structures also lead to different electronic properties. In diamond there is a large energy gap between the electrons' highest occupied state and the lowest unoccupied state. In graphite, the energy difference is equal to zero. So diamond is transparent to visible light because the energy of light is smaller than the size of the large energy gap -- an electron in diamond needs more energy than a photon of visible light to be excited across the gap. Graphite is black because its gap is zero and it absorbs all frequencies of light.",
"This is also why graphite has higher conductivity. Electrons can be easily excited and moved around in graphite, but require a large kick before you can move them at all in diamond -- they act as insulators because it's hard to excite the electrons across that energy gap.",
"The different size energy gaps come about because of the crystal symmetries of the lattice -- atomic orbitals bond or hybridize in different ways to become crystal orbitals -- and the way this happens differs according to the geometrical arrangement of the atoms."
] |
[
"The molecular arrangement of the atoms is very important as well. Graphite is composed of sheets of carbon atoms ",
"layered",
" on top of each other. Diamond, on the other hand, is arranged in a 3D structure called a ",
"diamond lattice",
", which makes it both hard and transparent."
] |
[
"Elemental silicon has the same crystal structure as diamond, but is definitely not transparent. It sounds like the effect you're likening it to (the cloudy lens) is a scattering process due to disorder on a much larger scale (10's of nm's to um's). ",
"The \"transparency\" is due to the lack of energy level differences in diamond falling within the visible range of the spectrum."
] |
[
"Are we born with the cells of the adaptive immune system?"
] |
[
false
] |
[deleted]
|
[
"Yes, more or less. They're actively produced (and dying) throughout our lifespan. Adaptive immune cells are primarily B and T lymphocytes. These cells, in their development (again, constantly happening throughout your life) edit their individual genomes to generate the receptors which detect specific pathogens, introducing regions of random structure at a specific part of these proteins. So the necessary structure to recognize a pathogen you've never been exposed to probably already exists among the B/T cells in your body, but it may be one or a handful of cells among all the different B and T cells in your body (sorry, no number on hand, probably millions to hundreds of millions throughout your body). It just needs to be activated and proliferate.",
"EDIT: Actually I just thought to google \"number of lymphocytes in the human body\" and apparently the number is like 2 trillion... so a lot more than I thought. Either way, a lot of potential diversity in epitope recognition there. Probably fewer earlier and later on in the lifespan though."
] |
[
"Cool thanks, I don’t remember the source I read on it, but it must have been 10+ years ago, and I only got the gist of it"
] |
[
"If I understood your question right the answer would be no, you dont have an uncountable number of naive cells waiting to be activated by their antigen. The adaptive (and inate) immune cells are constantly produced from hematopoietic stem cells in the bone marrow, with consequent maturation in spleen (B cells) and thymus (T cells). While maturing the cells pass thru a process called somatic hypermutation, this process induces randomized the mutations that lead to receptors for specific antigens. This means you dont have a pool of naive cells from birth waiting for their fated antigen to be activated, the immune cells are constantly produced and after maturation are usually found in lymph nodes for the antigen. They also have a limited lifetime so the old naive cells are recycled."
] |
[
"How does computer memory work?"
] |
[
false
] |
[deleted]
|
[
"I see some wavy hand explanations....but as someone who has an ECE degree, and engineers enterprise storage servers, I may be able to provide some insight. Here is the list of memory types we will go through...i'll skip over optical and tape media...as they're largely obsolete:",
"SRAM (static random access memory)",
"\nDRAM (dynamic ram)",
"\nFLASH (ssd...solid state drive)",
"\nSpinning disk (HDD...hard disk drive) ",
"SRAM:",
"SRAM is the fastest type of memory in a computer. It is used in the CPU cache exclusively (it's ON your cpu, as opposed to on the memory sticks). It requires constant power, and will hold its data indefinitely given that power. As has been mentioned, every piece of computer information is stored as a series of 0s and 1s, and every group of 0's or 1's (these days, groups of 64 of them) has an ADDRESS. the \"random access\" thing just means that we can access any address we want directly. So in your SRAM, you have groups of 64 CELLS, each of which can individually store a 0 or a 1, and when you ask for a given address, you are returned the data on each one of the 64 cells stored at that specific address.",
"So how do those sram cells work? They are effectively composed of two inverters. What is an inverter? when you pass a 0 in on one side, a 1 comes out on the other (so long as they have power). So if you hook two of them up in a loop, you'll have 1 on the output of one inverter, and 0 on the other, continuously feeding back into eachother. Here's an image:",
"https://upload.wikimedia.org/wikipedia/commons/3/31/SRAM_Cell_(6_Transistors).svg",
"M1/2 form one inverter, M3/4 the other. To read the cell, the WL line is charged, opening M5/6, which allows the current stored output of the two inverters (which is opposite) to come out on BL and BL-bar (bar = opposite). To WRITE the transistor, the BL and BL-bar lines are set with the 0 and 1 that we want to store on the cell, and the WL is charged...allowing us to override the state of the two inverters when M5/6 are opened.",
"This is all very fast, and quite small....but the main issues of SRAM are:\n1) always need power. those inverters need power...or the 0->1 inverter in each cell would have no way of working! (or the other one...for that matter)",
"\n2) it takes 6 transistors....which doesn't seem like a lot, until we get to DRAM",
"\n3) you need a lot of logic to be able to select the address you want...since you want it to be very fast. This all has to fit ON the CPU (remember we're using this for CPU cache....This is one of the reasons why so much area of a CPU is cache:",
"\n",
"http://images.anandtech.com/reviews/cpu/intel/IvyBridge/review/Ivy-Bridge_Die_Label.jpg",
"It takes up almost as much room as the cores themselves!",
"DRAM:\nDram is the smallest type of RAM, but a bit slower than SRAM. This is why it is used for your main system memory (the sticks that you have to add in). It has a lot of the same properties of SRAM (accessible via whatever address you want, 64 bits at a time, etc). So what is different? Instead of 6 transistors to store a bit, we can do it with just a single transistor and a capacitor! about 6x smaller: ",
"http://www.cse.scu.edu/~tschwarz/coen180/LN/Images/dramcell.bmp",
"when the WL is charged, T1 is opened, and whatever value is on DL is stored on C1. T1 is then closed and the charge on C1 stays there for a decent amount of time (about .3 seconds), before the charge LEAKS and the capacitor loses the data. So how do we prevent this? The way you read a DRAM cell is very unique and solves this.",
"To read a DRAM cell, you open T1 again, but you don't put any value on DL, so instead, whatever value was on C1 ends up on DL as well. The problem is C1 has been leaking charge this whole time, so the value on DL now is far \"weaker\" than the value which we put on DL when we first wrote the cell. We think of digital as 0 or 1, but here, we are VERY analog in that we can have weak or strong 1's and 0's (voltages closer to halfway between a 0 and 1). So because we might have a weak 1 or 0, we need to first SENSE the value with a very sensitive circuit to determine whether it is a 0 or 1, and then we need to AMPLIFY it to make the signal \"strong\" again. That combined circuit is called a sense-amplifier. There are many circuits that do this. here is one:",
"http://teamx.emulationzone.org/vlsi/pages/senseamp/senseamp.gif",
"A funny thing, though, is that the input of the SA, DL from the dram cell, is also the OUTPUT of the SA. So if a weak 1 is on DL, the SA will make it a strong 1, and same with a 0. At this point, the cool thing is T1 of our DRAM cell is still open, so the strong 1 or 0 is written BACK to the capacitor, and we have another .3 seconds of repreive! Hooray!",
"1) dram is super small, and relatively fast. hence its use in main memory",
"\n2) every cell of dram must be read several times a second to recharge all the capacitors so they don't lose their data",
"\n3) because you constantly need to refresh, you always need power. ",
"SSD:",
"SSDs use flash memory, and for the first time, we are venturing away from storage we need power to maintain...hence \"non-volatile\". SRAM cells involve a floating gate transistor. ",
"http://www.eeherald.com/images/flash_cell_structure.jpg",
" On a normal transistor, you apply a voltage to the gate, and this allows electrons to flow from the drain to the source (depending on type). With a floating gate transistor, we maintain charge on the floating gate, so it simulates having a voltage....so charge can always or never flow. To write a 1 to a floating gate, we put a massive charge on the control gate. This forces the electrons to tunnel (yes, quantum tunnelling...super cool) from the substrate to the floating gate, where they stay indefinitely. Now the transistor is permanently \"on\" and current will flow between drain and source. To write a 0, the opposite happens, and the electrons tunnel back OUT of the gate.",
"The transistor is now permanently off. The insulator surrounding the floating gate is \"strong\" enough that the amount of tunnelling on and off the floating gate when no voltage is applied to the control gate is insignificant. This means that we don't need power to maintain the state of the data. ",
"The big problem with SSD is that when you force electrons to tunnel across the insulator, you actually cause damage to the insulator. Therefore, after you write to a particular SSD cell enough times, that particular cell will break down to the degree that it will no longer hold your bit of data. To combat this, SSDs use wear-leveling, which try to balance writes across the drive so cause it to last as long as possible.",
"1) slower than DRAM",
"\n2) smaller than DRAM \n3) doesn't need power! ",
"HDD:",
"Your classic hard drive. It has a spinning disk coated in a magnetic surface. It spins around, and there is an arm. In modern drives, whether a 1 or 0 is stored is whether a specific area of the disk is magnetised with North or South up (assuming modern perpendicular recording)",
"https://upload.wikimedia.org/wikipedia/commons/8/8a/Perpendicular_Recording_Diagram.svg",
"A specific address is translated by the disk to a certain platter surface (there are 3-5 stacked up in there), radius, and angular displacement. The arm ",
"http://thecomputerstoreofthedesert.com/wp-content/uploads/2013/03/HDD-Inside.jpg",
" swings out to the right radius, and then waits for the disk to spin so the right angular displacement is underneath the electromagnet. As that location whirs by, the electromagnet is charged in one of two directions, causing the little magnetic particles on the disk to flip in the right direction.",
"To read the bits, as the disk spins, we don't charge the electromagnet, but read the voltages that are induced by the magnetized disk. These are then reinterpreted back to 0's and 1's",
"1) super slow. the arm has to physically move, and you have to wait for the disk to spin to the right position....plus you're physically limited by how fast the disk can spin",
"\n2) really good at sequential reads and writes, as the arm only has to move once (seek latency) to the right radius, and the disk rotate once to the right position (rotational latency) and then you can read a lot of data at the maximum hdd data rate (determined by the rotation speed and the number of bits per rotation at that particular radius. Note that because the outside of a disk has a greater circumference, we can store more bits on it. Since the disk spins at the same speed regardless of where it is reading, this means we can read and write data faster on the outside of the disk (since there are more bits in the same rotational time, they go by the head faster).",
"\n3) super super small. We don't need individual transistors for each cell (all the logic is built into the head). All we need is a space small enough that we can discern one magnetic bit from the next. and this is a really really small space.",
"\n4) persistent. once written, it will last effectively forever.",
"Hope this helped."
] |
[
"There are many many layers to this. I can't really cover the details, but I can give you some information. But an important distinction is that it depends on the type of memory. There are four basic types that I know of:",
"RAM",
"magnetic disk",
"solid state drive",
"optical drive",
"I assume you're less curious about optical drives and as far as I'm concerned, solid state drives are black magic. So I'll focus on RAM and magnetic drives.",
"RAM stands for random access memory. It's very fast and it can hold files, but it forgets everything when you turn your computer off. So it's basically just used for things you need to access quickly while your computer is running. ",
"If we sweep lots of details under the rug, RAM is made up of a bunch of self-sustaining circuits called flip-flops. (See: ",
"https://en.m.wikipedia.org/wiki/Flip-flop_(electronics)",
" ) Basically, a flip flop outputs either +5 or 0 volts. And you can get it to switch between the two by sending 5 volts in. In other words, the flip flop remembers what state it was in and switched between the two. ",
"A computer file is encoded as 1s and 0s. And reach flip flop is a bit that can be set to either 1 or zero 0 depending on the voltage. ",
"Source",
" apparently this type of RAM is static RAM, which is used in places. But there's another type, dynamic RAM, where the 1s and 0s are encoded as village across a capacitor. (In a very simplistic picture, a capacitor is a tank that holds electrons.) Thanks for the correction, ",
"/u/Kinnell999",
"A non solid state computer hard drive is basically a disk of magnetized metal. Using a strong magnetic field (created using an electromagnet), were can make the magnetic field on different parts of the dish aligned in different directions, either up or down. Then an up is a binary 1 and a down is a binary 0.",
"Again, I am glossing over a ",
" the physical structure is a lot more complicated than I'm letting on.",
"Source",
"And a video",
"information is encoded as 1s and 0s. File size, then, is counts in the number of 1s and 0s, called bits.",
"physically, the 1s and 0s can be recorded in different ways, but the size of the memory corresponds to how many 1s and 0s we can fit.",
"Hope this helps.",
" Please read the answer by ",
"/u/uh_no_",
" it's really good."
] |
[
"The RAM you are describing is called static RAM and is generally only used in caches these days. Dynamic RAM is almost always used for main memory. This works by storing each bit on a capacitor and periodically refreshing the value by reading it back and rewriting it. Dynamic RAM is significantly smaller and more power efficient efficient than static RAM."
] |
[
"Would the way Dexter (from the TV series) covers up his murder scenes (from the actual site of murder to the disposal of bodies in the ocean) be constantly elusive to actual police forensics or are there some flaws in how he does it? (In terms of DNA, blood spatter etc.)"
] |
[
false
] | null |
[
"In case anyone wants to answer, but isn't sure of the specifics. ",
"off the top of my head Dexter does this:",
"Surprises victim in a secluded area (sometimes more secluded than other times) and injects them with something that immediately knocks them out. He loads them into the trunk of his SUV",
"Brings his victim into an already prepped 'kill room'. He's already covered everything in the room in plastic sheets (except for some photographs of the victim's victims and such that he uses to taunt his victim basically). He's been doing this for years and years AND he's a blood spatter analyst by day, so we assume that he's very good at covering the room, right?",
"He cuts their face to obtain a drop of blood that he keeps as a trophy. Then, he stabs them in the chest once to kill them.",
"He chops the body up into pieces and puts those pieces into several different trashbags. Then, he disposes of the body in the ocean. In later seasons, he modifies this to disposing of the bodies along some current in the ocean (as I understood it)"
] |
[
"Good point. I never really thought about it that way. What do you think about from when he actually has the victim in the murder scene though?\nEDIT: From a scientific perspective."
] |
[
"Good point. I never really thought about it that way. What do you think about from when he actually has the victim in the murder scene though?\nEDIT: From a scientific perspective."
] |
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