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[
"If refraction is caused by the absorption and reemission of photons, why don't materials only refract certain wavelengths of light?"
] |
[
false
] |
It's my understanding that light seems to slow when entering a more optically dense medium because the photons are being reabsorbed and re-emitted by the atoms of the medium, which takes time and results in a lower average speed of light. However, don't atoms also only absorb photons of certain energies? Wouldn't this mean that only certain photons with the right frequency would be refracted and those with other frequencies would carry on straight through?
|
[
"Refraction is an emergent property of a material, that is, it only arises when there is a large number of atoms that the light is passing through. For instance, if you have 5 carbon atoms, that group of carbon atoms will not have the refractive properties of a diamond which is made of carbon. ",
"So, how do large collections of atoms have continuous absorption patterns? Because large collections of atoms have many more absorption methods than simple electron promotion. Collections of atoms can vibrate and oscillate, using two to n atoms (where n is some large number) which allows a nearly continuous absorption pattern as they are many, many combinations. "
] |
[
"The argument in the linked site is very flawed. He ignores all other excitation states other than electron excitation (such as vibration and oscillation) and his argument about how it should only depend upon the type of atom ignores the fact that solids have structure, thus there are different excitation states available between diamond and graphite. ",
"See my larger answer on the topic ",
"here",
". "
] |
[
"The argument in the linked site is very flawed. He ignores all other excitation states other than electron excitation (such as vibration and oscillation) and his argument about how it should only depend upon the type of atom ignores the fact that solids have structure, thus there are different excitation states available between diamond and graphite. ",
"See my larger answer on the topic ",
"here",
". "
] |
[
"If someone gets a stroke and one side of their body suffers, how does the inside of their body react to that?"
] |
[
false
] |
I'm having a bit of trouble phrasing this question correctly, but it was inspired by the Family Guy where Peter gets a stroke and walks around with half of his body limp and numb - and yes I know that's completely fictional but... Does a stroke affect those functions of your body like breathing or digestion that are span across both sides of your body? If it does or does not, why is that? Thanks! Please let me know if I can rephrase this question better.
|
[
"It can, but often not so much. Breathing and such is often not affected unilaterally because the brain stem (which controls things that keep you alive, like breathing, heart rate, etc) is not divided into left and right easily (because it isn't fed by left and right blood vessels in many parts), and also if you have a brain stem stroke, you are likely to just die instead of having noted left or right deficits. ",
"Ability of the body to adapt to a stroke depends on the magnitude. If you lose most of the left side of your brain, and survive, the odds of you ever being able to speak, understand language, move or feel the right side of your body, are almost zero. That function is probably lost forever. ",
"But say you had a very small stroke on one side, you may only lose certain functions and can go about life compensating. Many people become paralyzed and lose sensation to one side of the body, but can continue to speak, swallow, interact, etc. Some people only lose strength, and can learn to do things again with the opposite side becoming dominant. "
] |
[
"A stroke will affect areas downstream from where the incident occurs where collateral circulation is not sufficient to perfuse the tissues. So for the example of a stroke in the brain stem, probably the basilar artery. At the point of insult (either a clot for ischemic, or rupture if hemorrhagic), everything downstream will die. This means areas above the point, and other directions as well (such as the lower temporal lobes and occipital lobes, which are fed by the posterior cerebral arteries, which themselves branch off of the basilar artery). This is what makes a basilar infarct incredibly destructive and not compatible with life (for the most part). ",
"Another example would be a a middle cerebral arterial infarct. This will knock out blood flow to most of the middle of the brain (most of the parietal and frontal lobes, as well as the top of the temporal lobes). Back the clot up a little bit into the main internal carotid artery, and not only do you stop blood flow to the MCA, but also the ACA (anterior cerebral artery) which knocks out the blood flow to the top and front parts of the brain. ",
"Theoretically, you could only lose function to some parts of the brain stem and lose others depending on where the insult originates, but the problem is way more catastrophic because of the lack of flow downstream from there. There are also more factors at play than simply the blood flow the the areas, too. As brain cells start to die, brain swelling starts to occur, pressure builds up in the brain tissue which squeezes other blood vessels shut, decreasing blood flow to other parts of the brain, more cell death, more swelling, etc. Eventually enough pressure can build up in the skull to cause brain herniation (the worst example of which being that pressure in the top of the brain pushes the brainstem down into the spinal cord, destroying it). ",
"This is why major strokes are not really survivable.",
"Edit: And to clarify a bit on your question, strokes affect the brain specifically, and can damage or destroy parts of the brain that control parts of the body. When you hear stroke, think brain. Strokes can be called \"brain attacks\" to make the comparison to heart attacks."
] |
[
"Watch this 20 minute video, also. It's a great watch. It's a chick telling the story of herself having a stroke.",
"http://www.youtube.com/watch?v=UyyjU8fzEYU"
] |
[
"Why do some fields measure angles in degrees while other fields measure angles in radians?"
] |
[
false
] |
What are the advantages and disadvantages of each system, and why do they outweigh the benefits of widespread unit conformity?
|
[
"Radians are a natural angle based on a circle, and the sine and cosine functions make more sense when expressed in radians. For example, cosine of x in radians is 1-x",
" /2 + x",
" /24 ... while in degrees it has a bunch of unit conversion factors. As such, radians are used in mathematics, and also physics to describe vectors in either the complex plane or a vector space.",
"Degrees, however, are much easier to measure in the real world (imagine if your protractor only had six points instead of 360), so they tend to be used in engineering and architecture."
] |
[
"Well, okay. Yeah. That's what I meant."
] |
[
"This right here. Radians are derived from the circle itself, so they're a natural measurement. Degrees are more practical, but they're arbitrary. 360 is used because it can be divided into a lot of smaller increments. "
] |
[
"Can insects eventually evolve into higher, more intelligent lifeforms?"
] |
[
false
] |
Why haven't they done this already? The Earth was dominated by reptiles, now mammals... why not insects next?
|
[
"Your question presumes that the world is dominated by mammals, but by all measures of biomass, productivity, and evolutionary longevity, insects have been much more successful than mammals.",
"EDIT: I also think you have a flawed view of evolution. Modern insects are quite advanced evolutionarily, with many examples of highly derived features. Further, evolution doesn't have any \"goal\" in mind beyond encouraging survivability of species. Your question is akin to asking \"I think humans would be better if they could also see in infrared. Why haven't humans evolved infrared vision?\"",
"The wikipedia page on evolution gives a great rundown of the concept, as well as common misconceptions."
] |
[
"I believe a big problem is lack of size for a large brain. Insects don't have lungs; they absorb oxygen through diffusion pores in their bodies, which severely limits their size. So they would need to evolve another way to increase their size, and there doesn't seem to be selection pressure for that right now. "
] |
[
"Sure, why not.",
"Why they haven't? They've got it pretty good already, survival wise. Insects are already some of the most \"highly evolved\" things out there, with some extremely sophisticated strategies in play.",
"Evolution doesn't go \"forward\" to \"more intelligent\" species. Evolution goes towards what is immediately useful for survival."
] |
[
"What factors decide if the \"memory\" antibodies for a previous virus is permanent, or just temporary?"
] |
[
false
] |
As the topic states, I'm curious to as why the antibodies for some infections, e.g. chickenpox, persist for life and confer immunity, while other infections only grant immunity for a certain amount of time?
|
[
"It's not really the antibodies that persist, but the cells that created those anti-bodies. In an active infection, such as chicken pox, the infection persists for long enough to have your immune response \"class switch\" from immunoglobulin M to immunoglobulin G. (Immunoglobulin being another word for anti-body)",
"Immunoglobulin M (IGM) is less effective, but faster to produce that immunoglobulin G (IGG). The key step in getting anti-body producing cells (B-cells) to switch from IGM production to IGG production is T-Helper cell involvement. Once T-Helper cells meet up with a B cell that is cranking out IGM, it causes that B-cell to proliferate (divide). Some of those new cells that were just created produce IGG, and others become memory cells that just chill in your lymph nodes sometimes for decades. The IGM producing B cells chill for a while in the lymph nodes (around 6 months) before they decay. ",
"In some infections are just not around long enough, though this is more the case with small doses of dead vaccines. (Hence why you need twinrix vaccines quite often) Some infections can get you again and again and again - like this cold I am currently suffering from - by changing their surface markings, so you have no \"memory\" of this cold, even though you totally had this EXACT same thing like a month ago. Kinda like a Bond villain putting on a hat and using a different passport... 100% perfect disguise. ",
"So the mutability of the infection is really the determining factor. You are probably lifelong protected from every cold you've ever had, the cold just mutates often enough to keep you guessing. Something like Chicken pox doesn't change its \"hat\" (antigens) much/ever, so lifelong immunity.",
"When a doctor says \"6 months\" it means really \"in the study, the range of protective benefit went from 3 weeks to 4.5 years, the majority of people were protected for around 6 months.\"",
"TL DR: B-cell class switch from IGM to IGG via T-cell modulation. IGG class switch causes memory cell production, which can persist for life."
] |
[
"Another wrinkle is that,when you are reinfected with a virus or bacteria to which you have previously been exposed, it takes time for the memory B cells to produce large numbers of active plasma cells. As the length of time since your last exposure increases, the population of surviving memory cells wanes, and the speed with which they can remobilize an effective response slows. At the same time, when a pathogen first gets into your body, it has to go through a process of reproducing and multiplying from what may be an initial innoculum of just one or several virions or bacterial cells into what may eventually be trillions (or more). Whether or not you ever actually get sick depends on who wins this race: your immune system or the disease. For a disease that reproduces relatively slowly, your immune response may always be fast enough, even with a significantly depleted population of memory cells. For a very fast-growing pathogen, however, you may only have effective immunity when you have a large population of memory cells, and that immunity may rapidly become insufficient as that population wanes."
] |
[
"Yes that plays an important role. As ",
"/u/SheHasAGirlfriendNow",
" said, the mutability of an infection is probably the most significant player in the likelihood of reinfection. For example, rhinovirus is responsible for most of the common colds. There are hundreds of serotypes of rhinovirus, each stimulating production of different antibodies. So if you get the common cold, chances are you will be immune to that serotype for life, but can be 'reinfected' by any of the others.",
"Contrast that with influenza virus which is responsible for a more severe flu. Over time, the flu virus changes and presents different antigens on its surface, requiring yearly immunizations to the current strains. Currently our most common seasonal flu vaccine is trivalent, offering protection against 3 of the most likely serotypes. "
] |
[
"Has TV/Photography had an impact on the way our brain works, compared to before these forms were invented?"
] |
[
false
] | null |
[
"What exactly do you have in mind? ",
"We don't have brain recordings except the most primitive kind of EEG from that long ago. We don't really put adults who have had no exposure to photographs or TV into scanners (and such people are harder and harder to find). It's possible to do this with children sometimes..."
] |
[
"Fair point, didn't really think about that. Just watching Videodrome and it got me thinking how we perceive what is basically a perfect perception of reality but through a 2d screen. I just was curious on how that may have affected our brain, possibly creating new region in conjunction with this technology."
] |
[
"Well we've had 2D painting for thousands of years and have no problem with that..."
] |
[
"In countries where wild poliovirus had been recently eradicated, weakened poliovirus oral vaccines have caused new vaccine derived outbreaks. Why were attenuated, subunit, mRNA,recombinant or other safer vaccine technologies not used for curb polio spread?"
] |
[
false
] | null |
[
"The oral vaccine is easier to administer and doesn't require intense refrigeration. You're talking about travelling to the most rural regions of placed like Afghanistan.",
"Easier to store, easier to administer, and the need to get it to places with a lack of infrastructure means that the oral version is just about the only option (and far cheaper to deploy)."
] |
[
"Cold chain issues are a big problem. I read about people carrying insulated packs of dry ice on donkeys to maintain vaccine stocks. It's a heroic effort.",
"I had hoped that sugar-vitrification might have solved the problem of room temperature stability, but it's not turned out to be applicable.",
"Review on vitrification",
"https://www.sciencedirect.com/science/article/pii/S0939641116306956"
] |
[
"The live virus vaccine has a number of advantages that are thought to outweigh the small chance of vaccine-related infection. It's much easier to transport and store because it doesn't need to be refrigerated. It's given orally so nobody needs to be trained to give injections (and you don't have to worry about supplies of clean needles). It also generates stronger immunity.",
"For all of these reasons, the oral (live virus) vaccine is preferred in higher risk countries which tend to be developing countries without much infrastructure to support cold storage, transportation, etc.",
"In most developed countries, the other vaccine is used. Transportation and cold storage is less of a problem in California than in rural Pakistan. The weaker immunity is also less problematic because the risk of being exposed to polio in developed countries is much smaller."
] |
[
"How long does it take a crab to die when boiled?"
] |
[
false
] |
I have a friend who is growing increasingly worried about how much the food he eats suffered before it's, err, food. He's particularly fond of blue crab, and wanted to know how long they're alive while being boiled. I've found several sources which say that crustaceans probably feel pain, and indeed that they may feel it longer than we would because of how their nervous systems work, but a lot of this is mixed up with sources like PETA. So, how long does it generally take a crab or lobster to die in boiling water? And are they in pain the entire time?
|
[
"I'm not sure, ",
"this article",
" goes into great detail on the topic of killing lobsters, and is also cited on wikipedia with the claim that lobsters will continue to live when the first ganglion ist damaged. The article has as far as I could see no real references though.",
"After quick glace, [this article](",
"www.vliz.be/imisdocs/publications/231732.pdf",
") (pdf) seems to adress this problem very well. There is more info to be found googling, too."
] |
[
"Alton Brown, the host of the show \"Good Eats,\" had a great episode about this. The transcript can be found ",
"here",
". CTRL+F \"Geneva Convention\" and read those two paragraphs for what he has to say about killing crustaceans."
] |
[
"Yup, its basically putting a chefs knife right through the head of the lobster/crab before boiling it."
] |
[
"Are plastic containers (with or without BPA) relatively safe as long as they're not used to store/heat up food?"
] |
[
false
] |
I'm into organizing, and plastic containers, storage boxes, and trays are the most commonly available items for that. I've recently become aware of the dangers of BPA (Bisphenol A), as well as BPS (Bisphenol S) (which is not flagged equally but considered just as dangerous) as endocrine disruptors and carcinogens. I've learned that it is strongly recommended to replace plastic when it comes to food containers. However, I have a few questions about the use of plastics in every day life that I have found opposing answers for when searching and I'd appreciate your take on:
|
[
"As for part 1. I would use only glass to store food or find BPA free alternatives. As for part 2, if you do some research you will see that women have higher rates of exposure bc of the products they use. I’d assume that anything you rub on your skin could contain these compounds. For clothes and storage of items, I would assume that containers are fine. Usually if you set the containers in the sun a few days the chemical smells go away. As for part 3, you could look for articles on pubmed. My off hand reaction is that it’s hard to say, some people smoke cigarettes their whole life and love to 100 and others die of lung cancer at early age. My assumption is the real threat and damage is in babies and early developing children where phthalates can really start to affect hormone development with life long consequences."
] |
[
"Seems research is all behind paywalls these days, but as an example see: Biedermann, S.; Tschudin, P.; Grob, K. Transfer of Bisphenol A From Thermal Printer Paper to the Skin, Analytical and Bioanalytical Chemistry, 2010, 398(1), 571-576. (pubmed.ncbi.nlm.gov/20623271/ )"
] |
[
"Seems research is all behind paywalls these days, but as an example see: Biedermann, S.; Tschudin, P.; Grob, K. Transfer of Bisphenol A From Thermal Printer Paper to the Skin, Analytical and Bioanalytical Chemistry, 2010, 398(1), 571-576. (pubmed.ncbi.nlm.gov/20623271/ )"
] |
[
"1. What are some examples of solids through which sound waves travel transversely, and solids through which sound waves travel longitudinally? Why the difference if it’s the same medium?"
] |
[
false
] | null |
[
"Sound can travel both transversely and longitudinally in any solid. Transverse and longitudinal waves look ",
"like this",
". The reason that liquids do not allow transverse waves is that they have zero ",
"sheer modulus",
", while solids do not. ",
"A more intuitive explanation is this: imagine that you are in zero gravity, and you have a rod of metal and a rod-shaped mass of water. If you displace the metal or the water along the direction of the rod (longitudinal), a pressure wave is created which propagates through the material, moving the mass in the direction you push. However, if you displace the metal or water rod sideways (transverse), the metal rod will have a sheer wave which quickly reaches the other end of the rod, forcing it to move sideways as well, but no such effect occurs in the water (disregarding effects such as surface tension, which do actually permit small transverse waves in liquid).",
"As to your question on how to visualize sound, a speaker consists of a moving metal diaphragm which creates vibrations in the surrounding air. The vibrations of the diaphragm are mostly transverse, but the propagation of the sound in the air is longitudinal, and form a spherical wavefront when far from the speaker."
] |
[
"Think of hitting the middle of a snare drum or throwing a pebble into calm water. You'll get circular waves. Just expand the same idea to 3d. Or think of an exploding deathstar. "
] |
[
"Oh that first example actually cleared a lot up thank you. ",
"the propagation of the sound in the air is longitudinal, and form a spherical wavefront when far from the speaker",
"Spherical? I thought longitudinal waves were one dimensional and transverse waves were only two dimensional. How do you get a three dimensional wavefront from that? Thank you for answering my questions by the way. I love music and I’ve found the scientific aspects in what creates that experience to be extremely interesting."
] |
[
"What is occurring in the brain when we solve novel problems?"
] |
[
false
] |
What is happening in the brain when we solve problems, how does it come upon solutions for problems that are novel and have no idea how to solve, does it just try random ideas rejecting ones that don't work till it comes up with one that does by trial and error?
|
[
"Not my field, but can queue you into ways to optimize searching?",
"Insight learning is the \"aha\" moment we arrive at when the solution isn't nec. something we can deduce from our past experiences. It's a specific extreme of a novel problem, as that term can be used for things we can deduce solutions to.",
"Here're the three most-relevant studies that came up for a boolean search for fMRI + \"insight learning\"\n",
"https://d1wqtxts1xzle7.cloudfront.net/44317146/Neural_correlates_of_the_Aha_experiences20160401-21193-1spnnjf-with-cover-page-v2.pdf?Expires=1665970463&Signature=Th~81e18UNi4YyTdBPH74al0VYAohwD9pw8rVs-z1lSXekdLayAcud6adesF-RumDXysyadlO8N92GsPWFZT4J2psbypDhlT43jl2QVRo~NNgO~yHJ44l5Bo1IIP-r1AUIWzv62fnJ7qHjC1PpboPr4DGj-92VuHZPZZuFoyd1upd6K6gtdrYqKM1FDp3fjRZlMmVS2Xdcea3a7lnMbEAoka7ENn7RE69hP-s99MtSbVOrrboBrhNFpn~QfgnapWzfzAgYhGjK~3VeN547Is83P6~jnsyBPoIXiwS1GFdjoHmIyLCmSoTpoBwS8rYS05Hwzi2j5a9dYy8HICyipsjQ__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA",
"https://onlinelibrary.wiley.com/doi/pdf/10.1002/hbm.24073",
"https://www.frontiersin.org/articles/10.3389/fnsys.2016.00046/full"
] |
[
"While I don’t have time to discuss it in depth like I would, the neurological process for learning anything is called “long term potentiation (LTP)” which is the process of your brain creating new receptors and synapses as it learns to solve novel problems. ",
"With Additional practice in things that are not novel LTP still occurs which is how you get really good at something as you begin to learn additional intricacies of the problem. ",
"However with truly novel tasks, there is also a dopamine / reward loop which occurs in a part of the brain for all novel things (I believe the striatum? Can’t remember where right now)."
] |
[
"I don’t have any recommendations I’m sorry. ",
"I learned about it best through a “psychology of learning” course I took while doing my bachelors degree. But that was years ago so I don’t think I still have it. ",
"Best of luck!"
] |
[
"Could plasma (state of matter, not blood plasma) be a solvent?"
] |
[
false
] |
So, I'm a chemistry major in college, and last week I was randomly thinking about more abnormal solutions (normal being liquid-liquid or solid-liquid), when I came up with this question, which managed to stump my professor. Now, we all know gases can dissolve gases, but what if you made that gas into plasma? I imagine another plasma could dissolve into it, right? But could you possibly or even plausibly dissolve a gas into a plasma without the solute gas converting to plasma?
|
[
"Plasmas are unstable in general, and adding a neutral gas to them threatens to cool it down to a point of recombination, and the ions and electrons would recombine into regular atoms and molecules",
"If you maintain the plasma while adding a neutral gas, the plasma would ionize the gas and you would have new ion species in your plasma. ",
"The sun is a giant ball of plasma that is very stable (it isn't going to just turn into gas anytime soon). Anything you want will definitely dissolve in the sun, but molecules will be broken into their constituent atoms and those into ions. For example, if you wanted to dissolve ethelenol in a plasma you would most likely end up with carbon ions and hydrogen ions and oxygen ions. ",
"TL;DR plasma will dissolve stuff, but the stuff doesn't always remain very stuff-y. "
] |
[
"The ionization state of a gas is a function of temperature and density. Neutrals can exist in a plasma, but are dynamically ionized and recombine into neutral particles (gasses). ",
"A mixture of two species could have reasonably different ionization fractions at some temperatures and densities."
] |
[
"Saint_Oliver hit the nail on the head. This subject is sometimes called \"Plasma Chemistry\" and it is being studied heavily in the nanotech/nanoengineering field right now. You can get strange reactions to occur which wouldn't normally be possible. ",
"Neutral species occasionally take part in plasma reactions, such as in reactive ion etching or doping of substrates. The solid substrate (usually a wafer of something) is having plasma ions \"dissolved\" in it, even though it is remaining a solid for the most part."
] |
[
"What is the reason some medicine has to be taken at a certain time of day, such as right before bedtime or at breakfast/meal?"
] |
[
false
] | null |
[
"Most medicines I'm aware of that should be taken at night may cause drowsiness, or other side effects that would interfere with daily living. Sleeping through the peak dosage ameliorates the bad side effects. ",
"Most medications I'm aware of that are suggested to be taken in the morning are antidepressants or antipsychotics. This is done in the hopes that the optimal therapeutic dose will be experienced during daily life, so that the user would get the full benefit of the drug when they need it, rather than while they're asleep. The only other drugs I know that should be taken a good while before sleep are cough medicines, so that any phlegm coughed up can be eliminated from the body rather than coughing being suppressed or the phlegm simply swallowed again in the users sleep.",
"Afaik it's just due to the aims and side effects of the drug. I can't think of any reasons that a drug would not be metabolised or taken up by the body at a certain time, and haven't heard of anything to that effect. ",
"Edit: missed the part about taking with meals. As others users said, it's to do with absorption, needing more acidic condiotions/absorbing better alongside certain nutrients, or some medications need to be taken on an empty stomach for opposite reasons, also some drugs are enzymes for the purpose of helping digest e.g. Pancreatic enzymes are taken with food"
] |
[
"some medications are absorbed better with a fatty meal or have fewer side effects when taking with a meal. Other medications might be better tolerated on an empty stomach ",
"Some medications might be a stimulant and give energy in the day and at night it might cause problems and vice versa. Some like laxatives take a while to work so bedtime they can be taken and by the morning they will have done the job with minimal disruption.",
"Sometimes the instructions are more like guidelines and adjustments might be needed. "
] |
[
"some antipsychotics can be activating, as well, which isn't good for sleep hygiene. abilify is notorious for being less sedating than many others, to the point of akathisia being a calling card. "
] |
[
"Will the CMB ever go dark?"
] |
[
false
] |
I'm a high school astronomy teacher and I'm trying to teach about the Big Bang. Reddit was extremely helpful with a previous question! So now I'm wondering if we will ever stop receiving the CMB from the early universe. If I understand this correctly light was released when the first atoms of H and He formed at 380,000 years after the big bang. I assume that we're getting the red-shifted light from objects that we're distant from us in the early universe. So the CMB light that arrives every day is from an object that was one light-day farther away from us. Will the last bit of light from the farthest object ever pass us? Thanks!
|
[
"In principle, no. There are really two separate questions here, so I'll address them one by one.",
"1) Will the CMB ever go dark because we've stopped seeing light from the farthest point? The answer is almost certainly no (or not any time soon), because we don't know that such a farthest point exists. The Universe could very well be infinite. And even if it is finite, it's quite likely to be so absurdly large that any edges or any changes in its character are farther away than we'll ever be able to see. (I can expand on that if you'd like.)",
"There's another way of answering that question (just to give you an idea of some different ways to explain this to your students). This is the more accurate answer for our Universe, actually. As you probably know, the expansion of the Universe is accelerating. Now consider a part of the Universe so far away that it's receding from us faster than light. Because the expansion is accelerating, that point is accelerating away from us. So it will ",
" appear to be moving away faster than light. This means it will always outrun any light it sends to us. Because of this, there's a ",
" past which we'll never be able to see. If the Universe does have an edge, it's (far, far) past that horizon, so we'd never see the CMB from it. As time goes on and on, we'll see CMB photons emitted from closer and closer to (but never quite ",
") that horizon.",
"By the way: you phrased this as \"the farthest object\", but that's not really correct. The CMB wasn't emitted by ",
" as we normally think of them. Heck, there weren't any objects back then at all. The Universe at 380,000 years was a plasma of radiation, dark matter, neutrinos, and some scattered atoms (mostly hydrogen and helium). No objects had formed. The CMB was emitted ",
".",
"2) Will the CMB ever go dark because it's just gotten so dim we can't see it? Well, like any light source, the CMB dims as it travels further out and gets diluted (or, more precisely, as the Universe expands, so there are fewer photons per volume). It will also redshift to more and more unobservably small frequencies (long wavelengths). But there's not any particular point where it becomes dark all of a sudden - it just depends on how good your telescope is!"
] |
[
"The Universe is expanding. This means that if you look at a faraway galaxy, you'll see it getting farther and farther away from us. It turns out that the speed of that galaxy is related to its distance - the farther away it is, the faster it seems to be moving away, as you might expect. So there's some distance beyond which any galaxies you find appear to be moving away at a speed greater than the speed of light.",
"(This doesn't cause any problems with relativity: these galaxies aren't ",
" moving through space, but rather space itself is expanding.)",
"Naively you'd think we can't see such faraway galaxies because they're receding from us faster than their light can travel to us. And you'd be right, but if the expansion were slowing down, such galaxies might one day be receding at speeds slower than light, and they would come into our view. The fact that the expansion is ",
" means that this won't happen, and there are some galaxies (at some times) which we'll just never be able to see."
] |
[
"I get mostly all of what you wrote in here, except the part where you say there is a horizon past wich we'll never be able to see. Now I'm far from being an expert, but what exactly do you mean by receding faster than light? "
] |
[
"We've seen comets/asteroids hit Jupiter, and while they resulted in spectacular impacts, they didn't really do any damage to the planet that we are aware of. What would it take to do serious damage to a gas giant like Jupiter? What if one of its larger moons impacted with it?"
] |
[
false
] |
Someone in another thread was talking about a moon's decaying orbit resulting in an impact with its planet. I've often wondered, what would happen if one of Jupiter's moons were to have its orbit decay and it impacted with Jupiter? Is it at all possible for the impact to severely damage the planet or is it practically impervious to all damage because of its size, gravity and physical make up?
|
[
"To add to this. What is 'damaging a gas giant' in your opinion? The planet is a ball of gas held together by its own gravity. There is no real definite 'edge' to the planet. No crater can form. Kinetic energy can heat the atmosphere and little else... ",
"It's a little bit like watching a rock falling into the ocean and noting that even though the rock was big, it didn't really seem to affect the ocean that much or asking what size rock it would take to 'damage' the sea. ",
"The answer is an object of comparable size to Jupiter with enough energy to send a large portion of Jupiters atmosphere on escape trajectory. Even then, a violent merger is far more likely than any catastrophic explosion. ",
"But the remains would probably collapse into a smaller gas giant and once it had settled down (millions of years) you would be hard pressed to tell that a large impact had happened except perhaps from the measured orbital trajectories of debris.",
"A black hole could concievably destroy Jupiter if it were to come close enough but I feel that's cheating a bit."
] |
[
"This is a question that is hard to answer because we don't really think of impacts damaging other planets like we do Earth. When we talk about asteroid collisions with Earth we like to consider mainly the effects on the biosphere. That being said, it's worth mentioning that all the planets and moons in our solar system are created from these impacts over time, and while these may affect celestial bodies in the short term, they generally just add mass to the planet. ",
"Now to answer your question (incompletely) to \"destroy\" a planet one would have to move all of its mass to outside its gravity well. So you're looking at a magnitude of energy many orders of magnitude higher than the magnitude of the of the mass of the planet. ",
"Edit: A celestial body of equal mass traveling at a significant proportion of the speed of light would be required to destroy Jupiter. "
] |
[
"If said speeding object were traveling in the direction of the sun whilst hitting Jupiter, It could bump them both into the sun, no? I realize this wasn't the actual question I just really hate Jupiter."
] |
[
"How do we imagine stuff we've never seen before?"
] |
[
false
] |
[deleted]
|
[
"Imagination and sight are two domains working in paralel. \nYou can imagine only that what you have seen (all kinds of shapes/colors), for it was alrerady associated (preserving the structure (pathways) of a specific picture corresponding to reality). But as new research shows the way the brain organizes its visual sense remains intact even in people blind from birth, and the pattern of functional connectivity between the visual area and the topographical representation of space (up/down etc.) can develop on its own without any visual experience. Here: ",
"http://www.sciencedaily.com/releases/2015/04/150413130823.htm",
"Blindness from birth does not eclude imagination.\nI think they experience shapes and objects in imagined / projected 3D worlds same as we, what differs is the actual organization of their associations (method) with the real world inputs (other senses)"
] |
[
"This is quite a tricky question, but here is one potential answer that has been proposed.\nThe 'Tabula Rasa' or 'Blank Slate' idea proposes that you can't imagine things you've never seen (or sensed) before. You can however put together elements of things you have experienced to imagine something new. \nIt is summed up by the quote \"There is nothing in the mind that was not first in the senses.\" which I think is attributed to the philosopher John Locke, although I may be mistaken.\nThis school of thought has it's supporters and opponents both in science and philosophy, and there are many other conflicting ideas out there, so it is still quite a controversial area."
] |
[
"When I hear questions like this, it reminds me of reading books that eventually become movies. Before the movie is released, I have a preconceived idea of the setting of the book. I can picture it in my mind as I read, and it always seems to be made up of places ive already seen before, whether in real life or other media. But once I see the movie, its hard to go back to the false idea I had of it before. Its a similar concept I believe..."
] |
[
"What wavelength of electromagnetic radiation is given off by iron just due to normal impacts of room-temperature air molecules?"
] |
[
false
] |
Essentially, the iron in this example (and literally everything else) is constantly giving off electromagnetic radiation, right? But since it's absorbing so little energy from the room-temperature air it should be really low wavelength, and thus certainly not in the visible spectrum. But I'm wondering what wavelength it emitting? Is it radio waves? Microwaves? ULF? SLF? ELF? Thanks in advance!
|
[
"The peak wavelength of something at a given temperature is given by Wien's law (basically just calculus applied to the Planck blackbody spectrum to find the maximum). For something at room temperature, it emits 9 micron / 30 THz radiation, which is in the infrared. "
] |
[
"To tag along with this, you are assuming the iron is in thermal equilibrium with its environment. In that case, it's not just collisions with air molecules that give it the energy to radiate; it will be absorbing thermal radiation (at the same wavelength, 9 microns) from its surrounding as well (even though the iron also emits this radiation). "
] |
[
"It's worth mentioning that while the peak of emitted radiation is not visible, any black body radiates light from all over the spectrum (but less intense higher frequencies)."
] |
[
"Does rinsing with cold water after burning yourself actually prevent blistering?"
] |
[
false
] |
My mom always told me to quickly run cold water over my hand after burning myself on a stove/oven/whatever. It definitely helps curb the pain, but I'm not convinced it helps to prevent/slow down blistering. Hmm, maybe I should have ...
|
[
"I thought it was more to prevent swelling and excess inflammation along with the pain because cold water constricts blood vessels and the such. I'm not sure though; seems like we need more real doctors on this subreddit."
] |
[
"I always thought the reason for running your hand under water was the same reason you put blanched vegetables into an ice bath - to prevent it from continuing to cook. I guess that may depend on how bad the burn is."
] |
[
"I've always heard Luke warm is best. "
] |
[
"What happens to alpha particle (~5.3 MeV from 210-Po) if they hit stainless steel surface? Do they bounce off? Do they go away?"
] |
[
false
] |
What happens to alpha particle (~5.3 MeV from 210-Po) if they hit stainless steel surface? Do they bounce off? Do they go away?
|
[
"Most likely: It will go a bit into the steel and stop quickly. After that it can slowly move through the steel from thermal motion - it might leave the steel over time or get deeper into the steel. That happens on a much longer timescale (days to months).",
"While bouncing off is not impossible it is very unlikely. See the Rutherford experiment."
] |
[
"The penetration depth of 5.3 MeV He in Fe is 10.4+-0.3 micrometers, according to a simple TRIM simulation. The probability of backscattering for these conditions should be in the order of 10",
" (very hand-wavy upper estimation). So the bouncing off is very unlikely, and implantation is the most probable."
] |
[
"Thank you. I roughly guessed that alpha particle would loss on the metallic surfaces."
] |
[
"Can a fetus still undergo development if the mother is in a coma?"
] |
[
false
] |
Or what if the mother is on life support in a vegetative state assuming the growing baby/fetus hasn't been damaged.
|
[
"Yes; happened in ",
"2010,",
" in fact."
] |
[
"Yes. It has also happened.\n",
"http://articles.nydailynews.com/1996-01-26/news/17993581_1_raped-coma-pregnant"
] |
[
"Can one impregnate the comatose?"
] |
[
"Is there color at the atomic/sub-atomic scale?"
] |
[
false
] |
[deleted]
|
[
"Color is what we perceive. Each different frequency of (visible) light is assigned a color in our brain. There are frequencies of light both above and below the visible spectrum (radio, x-ray, gamma, etc) that we cannot see with our eyes.",
"So color is a psychological thing right? It's a conscious experience. A certain frequency photon hits your retina and you end up experiencing what we call color. But it's really the frequency of light we're talking about here. An atom is not light, and therefore it doesn't really have a \"color.\" Atoms are made up of protons, neutrons and electrons. ",
"However, atoms have certain colors (frequencies) of light that they can emit. Because photons do have color, and atoms can emit photons. This is called an atom's ",
"emission spectra",
".",
"An atom has a one or more electrons hanging out around the nucleus. These electrons can have different amounts of energy. There is a lowest amount of energy they can have, but it's some amount greater than zero. This lowest amount is called the ",
". Whenever an electron is at an energy level above its ground state, it can spontaneously transition to a lower energy state. Its energy has to go somewhere, right? (energy is conserved and all). This energy leaves in the form of a photon. Maybe this photon hits your eye and you experience a color. That would be the closest thing to an \"atom having color\"",
"Hope this makes sense."
] |
[
"Color is really a portion of the electromagnetic spectrum we call visible light (you may recall ROY G BIV for the colors of the rainbow, that range of colors is visible light). Each section of the EM spectrum has a certain energy, and depending on what energy is present you have a different color. Things that are really small like a single atom require a wave with so much energy (alternatively, such a small wavelength) that their \"color\" lies outside the bounds of visible light.",
"All forms of electromagnetic radiation (better known as \"light\") move at the same speed--to wit, the speed of light. Special relativity tells us light moves the same speed no matter what reference frame the person measuring it is present in. Energy of a wave isn't the usual 0.5mv",
" you're used to seeing, but instead given by some constant times the frequency of the light, so speed never enters the equation."
] |
[
"Color does exist on the scale of atoms, if we ask if a single atom can emit or absorb a photon. Just consider the deep red hydrogen alpha line, which is good for observing nebulae. Perceived color is an interaction with a pigment in the human eye. There is not just one frequency, but a peaked distribution of frequencies that can interact with a pigment. We have four pigments, and they cover about 400-700 nm wavelenghts.",
"In atomic scale, colors are produced by emission or scattering. Emission is either glowing hot, fluorescence or phosphoresce. Stellar nebulas or incandescent bulbs, fluorescent lights and phosphor, respectively. The last two happen at very sharp frequencies. Scattering requires light, but it can be frequency-selective. On the sky, we get blue scattered sideways (daytime) and red left traveling forward (sunset). This is at broad distribution of frequencies.",
"Organic dye-type absorption doesn't happen at atomic scale. Dye molecules are all multiatom. Metals and metal ions can have colors, though, since they are large enough. And interference-caused structural color requires even larger structures. So, an atom can glow, scatter, fluoresce and be an inorganic dye but not be an organic dye or a butterfly wing."
] |
[
"Do apoptosis-inducing viruses or retroviruses have a potential role in our ability to slow or stop cancer?"
] |
[
false
] |
It is apparently possible to do with bone marrow cancer: But I'm wondering if any of the mechanisms listed below would be high enough on the (Ras?) cascade to make this a little more general: In addition, shows a potentially useful apoptosis-inducing compound that targets cancer cells. Is this possible to infect cancer cells using a virus or any other vector to achieve utilization of this protein?
|
[
"Yes. Most definitely yes.",
"Many, many researchers are looking into adapting viruses for cancer treatment, whether by using the virus directly or by using it as a delivery method/gene therapy (like for your third link). Different viruses cause cancer cell death in different ways, so there is no need to focus exclusively on one pathway. Different viruses can also be more or less useful for different cancer types.",
"Take a look at the wiki for ",
"oncolytic viruses",
" and this snipet on ",
"viruses for gene therapy",
" for a little more general information.",
"Animal studies have been very promising and clinical trials are always underway to find safe and effective options.",
"I can answer any specific questions you might have."
] |
[
"Essentially you are asking if we can use viruses to deliver genetic material to cancer cells. The answer is yes. ",
"There is a whole journal dedicated to this research!",
". Retroviruses have been used for this purpose and so have pseudotyped viruses -- which are engineered viruses that have the shell of one virus and the interior of another. ",
"In your example, what we need to achieve is targeting of cancer cells by the viral vector used (specific targeting of ONLY cancer cells would be nice!), delivery of the apoptin gene and expression. The TAT in that paper seems to only be used as way of getting the apoptin gene into cells (without a virus), so we wouldn't really need that. ",
"This is a cool example",
" I've always liked, which essentially uses a retrovirus to stably deliver a gene that makes cancer cells into factories that convert a drug (that is delivered separately) into an even more potent drug. So the tumor cells end up expressing the object of their demise. "
] |
[
"Thanks for the response! Much appreciated. ",
"A follow up question: Do you think we are near the technological (bioengineering) means to \"vaccinate\" (not really the correct term, I think, more like infect?) against cancer by usage of retroviruses? Or at least using the integrase enzyme along with the cDNA as shown in the above nature paper (3rd one)? ",
"The main reason why I ask about retroviruses in particular is the integration of this TAT-apoptin compound (or something similar to that). If we are able to integrate it into cancer cells, the retrovirus will ensure that the production of more of these cells will mean more of the compound, so supposedly the compound will be gone (mostly) after all the cancer cells die. The growth of cancer is proportional to the overall production of TAT-apoptin? Perhaps a self-regulated negative feedback type of deal? (Sorry if I'm using any of this jargon incorrectly)",
"I don't see any retroviruses in the oncolytic viruses link above..."
] |
[
"How much of our depth perception is from focal point vs intelligent reasoning?"
] |
[
false
] |
I was reading about how much of what we think we see is constructed by the brain based upon learned understanding of what we expect from the world and it got me wondering about how the brain judges distance. With something like a truck my brain knows what size a truck is and so it can probably make a fair guess at distance based upon the size that it appears to be. Is there a distance when depth stops using the brain analyzing the focal depth of the eye muscles and relies upon contextual clues?
|
[
"Known/familiar size is definitely a depth cue that we can use. You can find a list of many cues ",
"here",
". ",
"Accommodation (focusing of the lens) is helpful only up to around 2m. There are physical limitations on the extent to which your lens can change shape. Binocular disparity is useful to around 6-10m practically, although there isn't really a limit (the distance between two object just needs to be larger and larger the farther apart they are, but by then there are more effective cues like motion parallax). The rotation of your eye (in or out) is also only useful up to around 10m; beyond that you're just looking straight ahead."
] |
[
"I just attended a very interesting continuing education course on depth-perception and the instructor made a very interesting point:",
"Our brains develop depth-perception before the age of 5, and it’s not lost due to loss of vision in one eye. Example: you can cover one eye, walk around, and probably not bump into anything, even though you supposedly need binocular vision for depth perception.",
"Once a brain has developed a sense of self/other; distance; texture; and proportion, our brains can figure all that out with just some very basic analysis of visual information.",
"This help us react much faster in the real world because our brains are taking shortcuts, but unconsciously, not through reasoning, per se.",
"It also means small children who have limited access to their environment before the age of five often have delays or impairments in visual processing.",
"So cool!"
] |
[
"Thank you."
] |
[
"How many generations of random mating are needed to make two populations indistinguishable?"
] |
[
false
] |
[deleted]
|
[
"1) Why are you assuming that the genes are linked? But anyway, 2) HWE doesn't say anything about genotype at one locus predicting/being correlated with genotype at another--HWE examines one locus at a time. 3) So, after one generation of random mating, in an infinite population, no selection, no mutation, no migration, the loci will be in HWE. If you're interested in more than one ",
" at a locus, ",
"Wikipedia",
" has generalizations for the >2 alleles."
] |
[
"This is a ",
" question!",
"The non-random association you are referring to is known in population genetics as \"linkage disequilibrium\".",
"Let's take your extreme case where all individuals in one population are AA/BB, and all individuals in the other population are aa/bb. I'm writing diploid genotypes here, but the objects we really need to be thinking about are the individual chromosomes. One population is full of AB chromosomes, and the other is full of only ab chromosomes.",
"When the populations come together and interbreed, you're correct that random mating will restore HWE almost immediately. If the populations mixed in equal proportions, then the ratio of AA:Aa:aa will be 1:2:1, and similarly for BB:Bb:bb.",
"Random mating does nothing to break down the association between A and B, however. This requires recombination. When heterozygotes are created by random mating, they have a two locus genotype of AB/ab. In other words, they have one chromosome that is AB, and another chromosome that is ab. The A and B loci are located some distance away from one another on the chromosome. If, during the course of meiosis in this individual, a recombination event happens somewhere between these two loci, then the offspring will inherit either an Ab chromosome, or and aB one.",
"The probability that a recombination event will occur between locus A and B during any given generation is a function of their distance apart on the chromosome. If the two loci are far apart, then recombination between them will happen frequently, then all of the linkage disequilibrium introduced by the admixture between the two ancestral populations will decay quickly. If they are very close together, then recombination events will occur rarely, and thus it will take a long time for the association to decay.",
"In reality, most genetic loci will not show these sort of extremely strong differences between populations (i.e. it won't be the case that one population has all AA/BB individuals, and the other all aa/bb individuals), but the association is still there and can be measured, so long as the two parental populations are at least somewhat diverged.",
"It actually turns out that this is an active area of research in populations genetics.",
" Imagine you have some population that you believe to be a mixture of two or more ancestral populations, but you don't know which populations, nor how long ago they mixed. By comparing the population in question to other present day populations that are believed to be closely related to the ancestral populations that mixed, we can estimate how much of the linkage disequilibrium introduced by the mixture event has decayed, as well as how much there originally was, which essentially gives a way to get a guess of what were the proportions of the ancestral populations that mixed (i.e. 80/20 vs. 50/50), as well as how long ago.",
"To directly answer your question, I can't cite offhand any theoretical analysis that indicates exactly how many generations we expect these signals to persist for, but I can tell you that generally, for human populations, it is on the order of thousands of years, but not tens of thousands. For example, we can use these sort of techniques to help reconstruct relationships between a variety of present day populations (i.e. events within last few thousand years), but these sort of approaches are not particularly useful for learning about ",
"relationships between modern humans and Neandertals",
", for example. Those mixture events occurred tens of thousands of years ago, and as such much of the signal of linkage disequilibrium has been erased by recombination, and thus we must use information from allele frequencies alone to get estimates of modern human/Neandertal interbreeding."
] |
[
"How long would it take if all pairs of loci were unlinked?",
"Not very long.",
"The recombination rate between alleles on different chromosomes is 1/2. If you have an individual who is AB/ab, but where A and B are on separate chromosomes, then independent Mendelian segregation guarantees that an offspring will be Ab 25% of the time, aB 25% of the time, AB another 25%, and ab the other 25%.",
"As such, 50% of the association breaks down every generation, and thus all of the linkage disequilibrium vanishes within just a handful of generations.",
"If your question truly is \"How long until the populations are indistinguishable?\" however, then the answer is still the one I gave above, as opposed to just a few generations. While the association between alleles on different chromosomes breaks down rather quickly, these associations can persist for quite some time between two alleles that are perhaps millions of base pairs apart on the same chromosome, and these associations allow us to determine that certain present day populations are the result of mixtures between ancestral populations."
] |
[
"What information can be gathered from a properly constructed phylogeny (and what can't easily)?"
] |
[
false
] | null |
[
"There are actually a lot of ways to construct a phylogeny and some give more or less information. ",
"The most basic tree will give you information about divergence in lineages.",
"\n",
"Here's an example.",
"\nIn that tree you can see a lot of information about the evolutionary relationships among the great apes.",
"\nYou can see that the orangs are an outgroup to all of the African great apes. Then within the African apes you can see gorillas are an outgroup to the human/chimp clade. Etc. ",
"So that tells us that the Asian great apes are outgroup to the rest of us, but it doesn't paint any picture about timing or evolutionary history. ",
"Here's another example.",
"\nIn this example we're looking at the ",
" (old world monkeys and apes) with the ",
" (new world monkeys) as an outgroup. ",
"And in this example we're also getting a lot more information. The most important thing is that branch lengths matter in this tree. Meaning that they attempt to describe divergence ",
" as well as relationships. And there is a scale at the bottom marked off in millions of years.",
"\nClades are color coded by the circles and the branches are also color coded.",
"\nThe colors on the branches tell us what 'adaptive regime' is present in the clade. So if you look at the top of the image you'll see all of the lime-green lines. And you can see that that is the ancestral trait. Those colors are mapping hand morphology. The lime green branches are a human like hand used for manipulation. And you can see that gorillas share that trait with us. But you can also see that ",
" and ",
" have converged on a different morphology which they use more for locomotion than for manipulating.",
"\nYou can also see that the gibbons (",
") have a different morphology. ",
"And these are just a couple of ",
" trees.",
"\nIt can get a lot more complicated; but you can see that the amount of information you can get from a tree will vary a lot by the tree; and it's really up to the people who constructed the tree. You can put in as much or as little information as you'd like. ",
"Edit: I just realized I left out the interesting reason that I chose those trees. ",
"Because branch length matters there we can get a little information about the history as well.",
"\nYou'll notice that the gibbons and ",
" are older lineages than the African great apes. But they aren't more closely related to each other than one might expect given that they are both Asian. ",
"As it turns out, the common ancestor to all of the apes was African and it migrated out and settled in Asia sometime between 20 and 30 million years ago. Apes diversified there, which is why the only extant lesser apes are endemic to Asia.",
"\nSometime later, around 15 million years ago, another creature migrated back ",
" Africa and that creature eventually gave rise to the African great apes -- including us. ",
"So the common ancestor to all of the apes was African, but the common ancestor to the great apes was actually Asian. ",
"That tree actually has a lot of other more subtle information as well.",
"\nPhylogenetics is awesome. "
] |
[
"I'd love to!",
"\nI find this subject a little fascinating. ",
"This scenario is the best explanation for a lot of the facts that we have. The next parsimonious explanation would require at least 6 migrations into Asia. ",
"That's because, like I said above, the apes diversified in Asia.",
"\nAnd Asian fossil apes come in a very wide variety that Africa can't come close to matching.",
"\nJust a brief list of extinct Asian ape genera: ",
": 3 spp.",
"\n",
": 1 sp.",
"\n",
": 5 spp.",
"\n",
": 3 spp.",
"\n",
": 3 spp.",
"\n",
": 1 sp.",
"\n",
": 2 extinct, 2 extant ",
"That's an amazing diversity and it includes ",
"this impressive beast.",
" ",
"And those are just the Asian ",
".",
"\nThere's a whole other group of apes there that live nowhere else. ",
"So an African great ape origin would require many more assumptions about migrations and would also need to come up with a very good reason for the sheer diversity of the Asian apes, and would also need to go to some length to explain the relative paucity of African apes. ",
"The Asian great ape evolution is just the best explanation given the data at hand. ",
"Edit: I have no idea how I missed the opportunity to throw another tree into this comment. ",
"Here's a tree that shows the extant and extinct apes.",
"\nAnd here's the caption that would go along with that figure: ",
"Simplified phylogenetic scheme of the Hominoidea indicating the main changes in RET (from intermediate thin or thin to intermediate thick or thick, and vice versa). Two different topologies are depicted for the three most problematic taxa (Ouranopithecus, Dryopithecus and Hispanopithecus); see text for further explanations. RET values reported in table S2, electronic supplementary material. Black arrow, (int.) thick; grey arrow, (int.) thin, question mark, uncertain; asterisk, variable; 1, our preferred hypothesis; 2, Begun (2009).",
"I just wanted to point out the difference in the Asian and African lineages.",
"\nEverything under the line labeled \"",
"\" is African. ",
"Everything else is Asian."
] |
[
"Quite a few things, if you know what to look for.",
"First, it's important to note that a phylogeny is ",
" the history of the species or sequences you're considering. To reconstruct the history, you'd need a time machine. It's just a model for understanding their relationship. As the maxim goes, all models are wrong, but some are useful.",
"Anyway, some examples of things you can use a phylogeny for:",
"Testing the molecular clock hypothesis. If the sequences you're investigating are all more or less equidistant from the root of the tree, that tells you it might be plausible that they've been evolving at a roughly constant rate. Violations of the clock could be for any number of reasons, such as a sequence being under positive or negative selection along some branch.",
"Testing the hypothesis of biased speciation rates. If your phylogeny is massively skewed, so that one side of the tree branches more frequently than the other side, this can indicate that the speciation rate for that lineage is higher, for whatever reason. One common, simple model for phylogenies in which speciation occurs at a constant rate is the Yule model. Topology-dependent statistics such as the Sackin and Colless indices can be useful for testing this model (or others) as a null hypothesis.",
"Gene and species trees can be compared to test for convergent evolution: for a recent example, see ",
"Parker et al. (2013)",
", which found evidence for convergent evolution in echolocating mammals (at certain genes, bats and dolphins cluster rather closely).",
"Gene and species trees can also be compared to test hypotheses about how the history of ",
" differs from the history of ",
". One example of this is incomplete lineage sorting, where coalescent effects become important: due to recombination and short internal branch lengths, gene and species trees can have different topologies. Another example is simple outcrossing. ",
"Churakov et al. (2009)",
" failed to find a unifying phylogeny for retroposon insertions in mammals, which suggests that early mammals lived in a state of more or less constant reproductive connectedness as the continents separated. Both of the preceding processes were likely important. A third and more obvious example is horizontal gene transfer.",
"Testing the hypothesis that a particular trait is somehow related to an important evolutionary event. ",
"Alexandrou et al. (2013)",
" tried to ascertain whether a whole-genome duplication event in salmon had something to do with the evolution of complex migratory behavior (it probably didn't, but it's not like this is a totally half-baked hypothesis).",
"Related to the above, violations of clock models can be tested. ",
"Lee et al. (2013)",
" found that arthropod radiation during the Cambrian can be explained by a slight (five-fold) elevation of the morphological and molecular evolutionary rates.",
"If the phylogeny is of a non-recombining component in a single population (i.e., it's a genealogy), the phylogeny can be an indicator of selection. Genealogies in a neutrally evolving population follow the statistics of the Kingman coalescent, so deviations from these statistics indicate deviations from neutrality, including adherence to an alternate coalescent model like the Bolthausen-Sznitman coalescent––this can arise either from very strong selection at many loci or from skewed offspring number distributions. (This is the subject of a paper I'm working on.) They can also indicate more trivial processes like population expansion. There's a relationship here between genealogies and standard population statistics like Tajima's ",
".",
"Hopefully some of this is useful.",
"One thing to be wary about: A well-constructed phylogeny can still be useless for estimating the time of divergence events, provided you don't have a good estimate of the substitution rate along the entire tree (or else some kind of very good fossil calibration). ",
"Graur and Martin's \"Reading the entrails of chickens: molecular timescales of evolution and the illusion of precision\" (2004)",
" is a hilarious overview of this problem. Apparently people thought they could use molecular evolution rates inferred from mice and humans to date (e.g.) the divergence of nematodes and arthropods, with very high precision. This is nonsense. Graur compares it to \"trying to decipher Demotic Egyptian with the help of an odometer and the ",
"\" and opines that, with the illusory precision such techniques sometimes grant us, \"we might\nultimately be able to tell whether the human–chimpanzee divergence occurred on a Monday or not.\""
] |
[
"Is there \"high-speed sound recording\" the way there's high-speed video recording?"
] |
[
false
] |
If you slow down a normal video, the frame rate drops, and if you slow it enough it looks choppy because there aren't enough frames per second to trick the human eye. But you can shoot a video at high speed and get enough frames that you can slow it way down and still have smooth video. If you slow down a sound recording, the pitch drops. I vaguely understand that that's because sound recordings somehow reproduce vibrations. And you can electronically raise the pitch back to where it was. But if you keep slowing down the sound and re-raising the pitch, things get choppy again. Is that because re-raising the pitch is just chopping up the continuous low sound into "frames", shortening those frames and leaving gaps in between? Is there a way to record sound at "high speed" or is that nonsensical? I'm pretty sure the answer is "no" but I couldn't at all tell you why.
|
[
"You might be interested to read about ",
"Sampling rates",
"."
] |
[
"As foretopsail hints, if you are dealing with an electronic recording, you will get \"gaps\" and \"choppy\" when you try to keep the same pitch & slow down the recording. ",
"You can indeed continue to slow down the sound recording as far as you want, given the correct equipment. "
] |
[
"The pitch of the sound is based on its frequency, which is time dependent. When you play a recording at a slow speed, you end up decrease the frequency, which lowers the pitch. I don't know the algorithm one uses to increase the pitch artificially (or what technique you're referring to), but since what we hear is so dependent on frequency, increasing the sampling rate won't improve anything in ",
" regards. It only allows the Fourier transform to distinguish between frequencies better - which might be useful in audio modification techniques such as ",
"phase vocoding",
".",
"Video, on the other hand, does not have this dependency. The quality of individual frames are not affected by the speed of playback. The audio analogue of this would be that the ",
" of the sound wave at a particular sampling point is not affected by speed of playback."
] |
[
"Force/Acceleration question."
] |
[
false
] |
Short version. You have a book sitting on a table, gravity is exerting a force on it in newtons (kilogram meters per second squared), the equal and opposite force is from the table. So your book sits there nice and happy doing nothing, not moving. I was discussing this with my physics teacher and she was very insistent that there are no acceleration. This makes no sense to me. Usually this means I'm wrong, which happens regularly. How can a force be applied without an acceleration? Force even implies that there is acceleration both in that Newtons are kilogram (meters per seconds squared), and F=ma, if a is zero, then there can be no force. So which is the correct interpretation, a force being applied without acceleration, or, in the case of my stationary book, the equal forces imply equal accelerations making the net acceleration zero?
|
[
"You need a ",
" force to get acceleration, not just any force.",
"When your book is sitting on the table, the force due to gravity and the normal force are of identical magnitude and opposite direction: end result is no net force.",
"For simple situations it's the same result if you consider each individual force to produce an acceleration and the accelerations 'cancel', but it's not something I've ever seen done."
] |
[
"Is this a compulsory-school physics course we're talking about? The reason I ask is because the topic in question is actually ",
" deeper than you might suspect, in a way that is far beyond the appropriate curriculum for an introductory class for teenagers.",
"The short version is that you've hit upon one of the fundamental insights that helped take us from purely classical physics into modern physics. It's this: Why is it that, when we stand perfectly still on the surface of the Earth, we feel an acceleration?",
"Consider the Apollo moon astronauts. When they boarded their spaceship, they climbed into the capsule and strapped themselves in, lying on their backs as it were because their rocket was pointed straight upward.",
"As they sat there waiting for the countdown to reach zero, they felt the sensation of being pressed down into their seats. This is obvious; it's ",
" We all feel it every day.",
"But then when the rocket motors were started and the rocket lifted off, the sensation of being pressed ",
" As the capsule climbed skyward, the astronauts felt themselves being ",
" pressed into their seats than they had been when they were sitting on the launchpad. Same sensation — the sensation of acceleration — but now more so. As if they were accelerating somewhat before, and that acceleration merely increased.",
"Thinking along these lines is what led Einstein to the principle of equivalence: gravity and acceleration are the same thing, basically. If you're standing in a room with no windows, it's impossible to tell whether you're on the surface of the Earth or in a rocketship in deep space accelerating at a constant rate. And it's not merely a perceptual problem; there's ",
" that can tell you which is the case. There is absolutely no difference between accelerating at a constant rate through what's called ",
" — space far from sources of gravitation — and standing still on the surface of the Earth.",
"And out of that insight fell the entirety of the modern theory of gravitation, which while quite simple in many respects is also in other respects hellishly complicated and is ",
" beyond what's appropriate for an introductory course.",
"So really, you're both right. If you're in a room, and that room is on the Earth, and in that room there is a table, and on the table is a book, then you will not be able to measure ",
" any motion on the part of the book, which means you cannot say that the book is accelerating.",
"However, if you employ an accelerometer — and it can be a fancy electronic thing, or something as simple as a weight on a spring — then you can determine with absolute certainty that the book, the table, you, and the room you're in are ",
" accelerating at a constant rate.",
"To understand this, you basically must divorce, in your mind, the concepts of acceleration and motion. We define acceleration, trivially, as the rate of change of motion over time. But that definition only works in some contexts. It's better, overall, to think of acceleration in general as ",
" If your accelerometer measures an acceleration, then an acceleration there is, and that's the end of that."
] |
[
"Newton's second law is that the acceleration is proportional to the ",
" force. Since the ",
" force is zero, the acceleration is zero."
] |
[
"Do beauty creams really help 'reduce the signs of aging' or are they all a load of rubbish?"
] |
[
false
] |
Do the more expensive creams work? What 'active ingredients' are proven to work? What do dermatologists use on themselves?
|
[
"I cannot speak for the majority of the creams, but I do know that one of my better-liked health compounds, DMAE (Demethylaminoethanol) has been shown to treat the skin ",
"very nicely",
". (And ",
"two",
" other ",
"sources",
")",
"That being said, if skin cream marketing is anything like supplement marketing, then the active ingredients could very well work, but they would be overhyped. I would say look for ingredients that are common to all face creams (glycerol, vitamin E, etc.) and those are most likely the active ingredients. The fancier stuff that comes out in the 'newest' products would be very hit or miss."
] |
[
"As a casual observation, the beauty cream and cosmetic companies seem to have a \"major breakthrough\" every two months or so and release a new product claiming to be better than everything before. I wonder how much of this is just re-marketing of their successful (perhaps even effective) products and now much of it is actual attempts to produce something with passes the consumer test."
] |
[
"Many of them have an irritant in them that makes your skin get puffy... this will temporarily obliterate fine wrinkles and slightly reduce the deeper ones."
] |
[
"Why is there no measurement for sharpness?"
] |
[
false
] |
[deleted]
|
[
"Because there's no quantifiable definition of sharpness."
] |
[
"While not explicitly there is the scientific measurement of ",
"hardness",
". This dictates which materials can damage or scratch/cut other materials.",
"Then there is ",
"toughness",
" which dictates how much force can your material take before breaking or fracturing. Though the measurement of toughness is itself a composite of other measurements, you get the picture.",
"Combining and taking into account these two (or several if you break toughness down to basics) you can scientifically determine which knifes are better at their job than other knives.*",
"*Ignoring the other uses for knives besides cutting."
] |
[
"Here's a machine that claims to do it. ",
"http://www.catra.org/pages/products/kniveslevel1/st.htm"
] |
[
"If all marine viruses were joined end-to-end, they would stretch further than the nearest 60 galaxie. Is this a true statement?"
] |
[
false
] |
This was the opening paragraph to an article I was reading regarding marine viruses for a Biology class. "At abundances routinely greater than 10 million particles per milliliter, viruses are the most numerous biological entities in the oceans. To put the sheer abundance of marine viruses in context, we note that they contain more carbon than 75 million blue whales and, if such viruses were joined end-to-end, they would stretch further than the nearest 60 galaxie" I could figure it out, but I thought it was an interesting fact (possibly true) and I would share it with Reddit. Also, I don't have the time at the moment. Also, obviously viruses are different sizes so that would make a huge difference. But I was thinking to use an average size. This is my first time to by the way. Just front paged!
|
[
"http://en.wikipedia.org/wiki/Virus#Structure",
" gives a size range for a virus between 20nm and 300nm. Let's call a virus a sphere with a 100 nm diameter. ",
"This gives us 100 nm * ",
"10",
" viral particles",
" = 10",
" meters.",
"This is 5,000 times farther than the nearest galaxy",
" - PS Wolfram Alpha is awesome"
] |
[
"Yeah, the biggest assumption is the size of the viral particles. I couldn't find any concrete data on a viral size distribution.",
"Also, re-reading the OP, the viral concentration is given as \"routinely greater than 10 million particles per milliliter\" - this sounds like more of an upper bound and is going to cause us to overestimate the distance."
] |
[
"Just to clarify, your distance is about 100 million light-years which is much much farther than any of the nearest galaxies as OP's statement suggests."
] |
[
"Question about tooth decay and evolution."
] |
[
false
] |
Did humans get cavities before inventions like refined sugars? Were our teeth designed to last a lifetime without brushing and cleaning?
|
[
"Yes, we got cavities. Go google image some pics of old skulls."
] |
[
"I don't have the study in front of me, but I remember learning in college that the native Ohio river valley people had a much lower incidence of cavities during their lives as hunter-gatherers, however once they began agriculture the researchers found an enormous jump in the number of cavities due to the increase in starch and sugars in their diet.",
"You also have to remember the life expectancy of someone back in those days was early 40's - so a life time without brushing isn't that long compared to today."
] |
[
"I agree with Jedu. I have always wondered about this myself, but now i realize that our modern diet is most definitely the cause of cavities. Like Sid says, there probably where instances of cavities, but the advent of agriculture greatly increased their occurrence. Oh, a comment on the way you phrased your question. Be careful when you use the word designed. Evolutionary, nothing is designed. Evolution is not a linear progressive process like many people might believe. Some times we trick ourselves into thinking how great and awesome the human body is, but the reality of it is that we are not perfect."
] |
[
"What made cells become 'alive'?"
] |
[
false
] |
[deleted]
|
[
"Read 'The Selfish Gene' by Richard Dawkins. He describes (at least one) theory about how life may have gotten it's start. I'll paraphrase to the best of my ability...",
"As Mediumtim mentioned, amino acids formed in the primordial oceans. These and other particles were jostled about by the water, and some had a tendency to stick together. Some of these had more of a tendency to stick together for longer, and thus would necessarily become more prevalent (think like, molecular natural selection.) The number of 'ingredients' in the oceans were finite, obviously, and the molecular chains that were the most stable would eat up most of these ingredients. Through random chance, one of these 'species' of molecular chains became capable of reproducing itself. This new self-replicating molecule would immediately become 'dominant', gobbling up all the free-roaming ingredients and propagating through the oceans.",
"Perhaps some other 'competing' molecule was introduced - whichever had a higher likelihood of staying together would stick together. Flash forward a bunch, and these molecules developed cell walls, etc, etc... Long story short, organisms are simply survival machines for their DNA. We all are. Every life form exists solely to propagate it's genetic material, and we are the result of a molecular arms race to build the most efficient, effective survival machines. I'm sure I've done Dawkins a disservice here, as he describes this hypothetical chain of events in a much more elegant way than I ever could. Check it out."
] |
[
"Didn't life start with amino acids forming basic protein blocks and working up from there? "
] |
[
"Speaking to an event which happened in the past is always hypothetical, please keep this in mind, there is no way to prove any of this happened, though it is generally considered proven that it is all possible.",
"Urey and Miller were able to generate amino-acids by simulating conditions such as found before there was any life. Given a very long period of time, it is possible for proteïns and enzymes to form spontaneously. It is also possible that \"soapy\" molecules were formed, which form mycella (\"membrame bubbles containing other molecules\")",
"In such \"bubbles\", self replicating proteïn sequences (such as modern prions, but with considerably more matabolic potetial) may have accumulated, allowing the growth of biomass, initially generated by chance, to increase exponentially as biomass becomes self replicating.",
"Sustained growth allowed for more complexity to appear, both due to mutations and the spontaneous appearance of new macro molecules within the safe boundaries of membrames. This went on until enzymes and fragments of membrame began to form the first self replicating micro-organelles. ",
"Micro-organisms are little more than a group of such organelles in a \"smart\" protective shell. Evolution, as defined by the first line of the previous paragraph, allowed for better and better MO's to form, spawning forth new lifeforms. This path leads from single celled organisms to multi celled organisms and organisms with differentiated cells, the last group posing the greatest boundary since the beginning. No longer direct products of spontaneous generation, but designed by already existing life.",
"Now we have arrived at the most difficult choice to be made before this question is answered: What do we consider alive? There are multiple definitions, I personally only demand: \"A contained body, capable of metabolism and autonomous procreation based on contained information\", but there are many other opinions on this matter. You'll have to define for yourself what you consider to be \"alive\", and find the point in history where these criteria are met.",
"Sources: College courses in biochemistry, genetics, cellular biology and microbiology"
] |
[
"Is there 2 longest and 2 shortest days at the equator or do they differ?"
] |
[
false
] |
[deleted]
|
[
"At the equator, every day is exactly 12 hours long (if you ignore some ",
"minor variations",
"). Earth's axial tilt doesn't matter in this case. It's a very common misconception that the days are shorter at the equator when the sun is not directly overhead, but that is simply not the case. At the equator, every star is above the horizon exactly half of the time and the sun is no exception, no matter how far north or south it is."
] |
[
"For example, look at ",
"Quito",
", which is about 20 kilometers south of the equator, and you can see that the day length does not change."
] |
[
"Remember that Earth spins about a tilted axis. Look at the geographic \"tropics\". These describe, in a sense, the variation of 90 degree sun angle position as a function of latitude.",
"So yes. They vary sinusoidally."
] |
[
"How is it that powders are compressible when they are made of solid particles?"
] |
[
false
] |
I find powders really interesting, how they are made of solid particles but almost act like fluids etc. They seem really complicated though, and scientific publications are a bit over my level of understanding. Can anyone please give me a crash course in compressibility?
|
[
"compressing powders may just rearrrange the way the particles are organized, occupying less space. i used to conduct wood fiber alignments experiments in my lab and i got up to 60% increase in density just by organizing the particles in a specific way rather than just pouring them in a vessel. Also, the compression was much more efficient with organized particles.",
"just imagine you fill a box with randomly poured straws and another box with straws all pointing to the same direction. you would get a greater density on the second way."
] |
[
"Well, first of all, powders are not (always) like marbles - super round and individual particles.",
"The smaller the size, the attractive force between particles get really strong compared with others (like gravity) and so you get some clusters.",
"you can break them down so you can get the particles closer together.",
"Also you can reshape some of the particles, so they get closer together.",
"I did a presentation about roll granulation soem time ago. Hered is a part of it where you can see the two big ways you can get the particles closer together: ",
"Pic",
".",
"If you want to know more about details, I can try to help you out, but this is generally how you can get the powderparticles closer together by applying force."
] |
[
"Are powders actually very compressible? I'm thinking of some common ones - such as flour or sand - and if you put either of those in a syringe, blocked the end, and pushed down, would you get any noticeable compression at all? I use that example as I remember a demonstration in school, years ago, where we did precisely that experiment, and it didn't compress! Even liquids don't compress terribly well.",
"Perhaps someone can quote some compressible powders, but I certainly can't think of any."
] |
[
"If you were on a circular space station that was spinning so that it produced artificial gravity, and you ran opposite to the spin at the same speed would you still feel the affect of the artificial gravity?"
] |
[
false
] | null |
[
"No. (And conversely if you ran spinward you would get \"heavier.\") The force of \"gravity\" you feel is the floor of the station accelerating you inward and thus preventing you from flying of at a tangent. If you run opposite the floor at its tangential velocity, your tangential velocity in an inertial frame becomes zero, so you are no longer accelerating and will be weightless, just like if you were sitting in space next to the station but not spinning with it.",
"On the other hand, you couldn't hover for long, since the air in the station would be trying to push you along with it, decreasing your velocity in the station frame/increasing it in the inertial frame, giving you weight again."
] |
[
"This is correct. You could jump and fly. You would then actually be at rest, relative to the center of rotation. After a while, air drag will put you back in sync with the rest of the space station and you \"fall down\"."
] |
[
"but you wouldn't be able to hover by running against the spin. in order to run you need traction and as your gravity decreases you lose traction necessary to accelerate yourself forward. ",
"carefully observe what you do when you start walking or running, you don't start by pushing back, you start by falling forward! you can't run without gravity."
] |
[
"What exactly is the \"amplitude of a photon\" of light?"
] |
[
false
] |
I recently watched a lecture on the quantum mechanical nature of reflecting light, and I was a bit confused by something. In the lecture, the professor speaks of the "amplitude of a photon" and how it effects the nature of reflected light. The amplitude of a wave of light, if higher, should create gamma or x-rays, and if lower, should create infrared and other similarly low amplitude waves (in the visual sector of light, I guess this would be more violet/blue colors vs red colors). I understand there is something of a paradox with the wave/particle nature of light but, I was wondering if anyone could shed some light (no pun intended) on what exactly the amplitude of a photon of light (a particle) is and how it works on a lower level. When light strikes an object, does the amplitude describe the energy with which the photon was first released from the source or something? If not, what initially determines the amplitude of a photon? Can this "amplitude" be described in some form as the energy, or vibration of a photon or something? Then, when it strikes an object, does it transfer that energy, causing the object to release a photon of similar energy which causes us to see a similar representation of things through a reflected surface? I understand this question may not be detailed enough to get a good answer but, anything you think can point me in the right direction of understanding this phenomenon would be useful. Thanks!
|
[
"\"Amplitude\" in the sense I think you're talking about is different from the \"amplitude\" of wave theory. They're somewhat mathematically related, but only abstractly.",
"The amplitude I'm pretty sure he's talking about is the \"probability amplitude\". It is represented by a complex number whose square of its magnitude is the probability. It helps to think of this complex number is the form z = |z|exp(it) instead of the usual x+iy, because in the former, the magnitude |z| and the phase t are both clear.",
"As a particle, such as a photon, marches freely in space, its phase changes. The higher energy photons ",
" their propbability amplitude faster than lower energy (lower frequency) ones. Feynman likens the probability amplitude to the hand of a clock. Some photons' hands move faster, some move slower, depending on their energy. The ",
" (the \"vector\" sum on the 2D complex plane) of probability amplitudes will give a different amplitude with a different magnitude (length) and phase (angle). But its magnitude never changes when it travels as a free particle.",
"In some of Feynman's lectures that I've seen, to avoid the tricky math, he treats the amplitude as simply an arrow, which behaves in certain ways under addition and multiplication. The arrow rotates in time like hands on a clock, and when you add more than one of them, you can get interference. Reflection is remarkable in that the photons are being absorbed, then emitted in a random direction from every atom, yet there is a nice overall behavior that results from adding up all these \"arrows\", which represent the photons emitted from each atom. The result is \"angle of reflection equals the angle of incidence\", which most people are familiar with, but becomes a mindfuck when you realize the particle nature of light."
] |
[
"It sounds like he was talking about the energy of light, not amplitude. E = h f. Extremely high frequency photons are x-rays or gamma rays, lower frequency is visual light. The amplitude of the electric field will vary depending on how the photon is confined, although the energy density is readily calculable ",
"here",
"."
] |
[
"It's the strength of the electric (in volts per meter) or magnetic (in tesla) fields that are perpendicular to the direction of propagation.",
"The energy is just the frequency."
] |
[
"How are warts formed"
] |
[
false
] |
As the title. How are they formed? They just turn out on your skin and there you go a wart
|
[
"Warts are caused by viruses (non-living infectious particles that infect cells and use them to reproduce), typically the human pappilomaviruses of the same family that cause cervical cancer. However the wart-causing HPVs are less harmful to humans than the cancer-causing types.",
"When the virus infects the cells of the skin, it causes the cell to lose regulation over its own growth and division leading to abnormalities in these processes. The cells divide and push up and out through the skin leading to the bumpy wart formations. "
] |
[
"Just to tack on a bit of STD education, wearing a condom does not entirely prevent the spread of genital HPV strains. Any skin contact, including around the groin or thighs, puts you at risk. You will not be screened for HPV during a routine STD check, and most people have no idea they are even carriers. Please consider talking to your doctor about* HPV vaccines for you and/or your children. "
] |
[
"When you answer, please distinguish between the two parts inherent in this question– the transmission of warts and their growth and development."
] |
[
"Why do p-values in statistical tests have a cut-off point of 0.05?"
] |
[
false
] | null |
[
"It's completely arbitrary and it depends on the field. Each field has its own conventions. There is nothing special about 5%, it's just generally seen as a reasonably small number.",
"In high energy physics, the convention for the discovery of a new particle is 5σ. I'm not in a place where I can convert that into a p-value at the moment, but suffice it to say, it's much smaller than 5%."
] |
[
"5σ corresponds to a p-value around 3.5*10",
", which is ",
" smaller than .05."
] |
[
"To expand on why it depends on the field, consider how silly it would be to have a 5σ criterion (one chance in 3.5 million that the result is a fluke) in, for example, psychology. If a test is administered to 100 people, then the error on the mean test results scales as sqrt(100), or about 5% for a mean test result of 50/100. So you won't get much better than a p-value of 5% in a lot of fields where you have limited statistics. Also, even if you did get a p-value of 5σ, it would likely not be very meaningful because other non-statistical errors due to selection effects and confounding variables tend to rarely be below the 5% level. So it makes sense to have a cutoff that is large in a lot of fields where both low statistics and difficult-to-control variables are at play. Unfortunately, in those fields, the phenomenon called \"p-hacking\" is made very insidious by such a large cutoff (if you don't get a p-value below 5%, just don't publish and repeat until you get a p-value below 5%). This is one reason why a lot of fields like medicine and psychology have such a poor track record with regard to reproducibility.",
"In particle physics, we have a very controlled environment in which we can observe very large numbers of data points, and so it is possible to have 5σ p-values be somewhat meaningful, though even then, no one takes too seriously that idea that the p-values are very realistic so far into the tail of the bell curve. "
] |
[
"How do we create vacuum in our mouths?"
] |
[
false
] |
I assumed that we suck in the air with our lungs and that creates vacuum, but I recently discovered that I can breathe through my nose while using a straw (of course not while swallowing). :)
|
[
"We don't create a vacuum at all. However, our lungs do create a region of lower pressure than the outside environment, or in other words negative pressure. This negative pressure \"sucks\" in air from outside through both our mouths and noses (because they are both are connected to the lungs), although it's probably better stated as outside air, due to its higher pressure compared to the lungs, forcing air into the lungs.",
"The reason why our lungs generates this negative pressure that sucks in air so we can breathe is because contraction of the diaphragm below your lungs (as well as help from other muscles like intercostal muscles in your ribs) causes the lungs to expand. Higher volume from this expansion causes reduced pressure (boyle's law), and as such the lungs now have negative pressure compared to the outside environment."
] |
[
"Well your lips can conform very well to the contours of a surface, and a layer of fluid can finish that seal off. When you think about it though, you're applying the strength of many muscles of your lung to a relatively small area.",
"Gosh, there's no way I can make that sound not dirty."
] |
[
"Thank you! "
] |
[
"Does storing batteries in the freezer actually make them last longer?"
] |
[
false
] |
A couple of my friends like to store their batteries (AA, AAA, D, 9-volt) in the freezer claiming it makes them last longer than they would if they were not in the freezer.
|
[
"Self-discharge rate is dependent on temperature:",
"http://corrosion-doctors.org/Batteries/self-compare.htm",
"So, yes."
] |
[
"Usually you take the batteries out of the freezer before using them."
] |
[
"Batteries rely on chemical reactions to produce a charge and once the reaction is finished the battery is dead. Cooler temperatures slow the rate of the reaction and lower the amount of charge produced. Also, the battery does not need to be powering anything to drive the reaction. Some discharge will occur spontaneously (not exactly sure why but I think the battery releases charge into the air) and to minimize this you should store them at cool temperatures. However, you should be careful storing them in the freezer because some battery types will actually freeze and crack their casing.",
"P.S. Never underestimate ",
"wikipedia",
" as a source for satisfying your curiosity (not my source, just thought it might help). "
] |
[
"How do airfoils allow planes to fly upside down?"
] |
[
false
] |
I understand the theory of the airfoil creating a low pressure zone above the wing. How does this let planes turn upside down and still work. Bonus question. How do model planes without an airfoil work?
|
[
"Bernoulli's principle only partially explains lift. While Bernoulli inspired airfoils tend to increase efficiency, they aren't necessary (as you pointed out with model planes). You can create lift by deflecting enough air down to compensate for gravity (following Newton's second and third law).",
"In practice, this means they just provide enough negative pitch to compensate for the negative lift from the inverted airfoil. Planes without an airfoil (or symmetrical airfoil, as in the full size Extra 300: ",
"http://en.wikipedia.org/wiki/Extra_EA-300",
") do the same thing, but use the same amount of pitch (+/-) whether flying right side up or upside down."
] |
[
"In all honesty, physicists aren't entirely sure of the fluid dynamics creating the lift force on a wing. ",
"Aerospace engineer and pilot here. This is utterly untrue; the aerodynamic theory explaining lift has been fully developed for the better part of a century.",
"This is a not-too-terrible explanation: \n",
"part 1",
" ",
"2",
" ",
"3"
] |
[
"In all honesty, physicists aren't entirely sure of the fluid dynamics creating the lift force on a wing. ",
"Aerospace engineer and pilot here. This is utterly untrue; the aerodynamic theory explaining lift has been fully developed for the better part of a century.",
"This is a not-too-terrible explanation: \n",
"part 1",
" ",
"2",
" ",
"3"
] |
[
"Why does Schrodinger's time dependent equation have infinitely many independent solutions while an nth order linear DE only has n independent solutions?"
] |
[
false
] |
The solution for Schrodinger's equation is y(x,t)=Ae but we can create a linear combination (i.e a wave packet) with infinitely many of these wave solutions for particles with slightly different k's and w's and still have it be a solution. My question is what is the difference between schrodinger's equation which has infinite independent solutions and say a linear second order DE who's general solution is the linear combination of two independent solutions?
|
[
"Schrödinger's equation is a partial differential equation. Only nth order ordinary (single-variable) differential equations have n independent solutions.",
"Alternatively, if you use the ",
"general Schrödinger equation",
", then it's back to being a first-order ordinary differential equation ... but it's a matrix equation, and so it has exactly as many independent solutions as the space it's acting on has dimensions. For the good ol' single-particle nonrelativistic Schrödinger equation, that space is an infinite-dimensional function space."
] |
[
"Things are a little hairy if you're working over a noncompact space. It's not correct, but is \"correct,\" to say that the e",
" are basis functions over R",
" . Things are much nicer when you're working over a compact space or domain: separating variables, you get countably many eigenfunctions, each in L",
" , and your solution is the appropriate linear combination of these.",
"On a noncompact space, again take R",
" , there is a continuum of such basis functions. The decomposition of a function into a \"sum\" of these \"basis\" functions is nothing more than the inverse Fourier transform of its Fourier transform. If you're interested in physically meaningful results, then you have to restrict your attention to functions with unit mass; no single such basis function will be a solution (unlike in the compact case) but you can have functions which are solutions. These are wave packets."
] |
[
"The Hamiltonian is only a differential operator in a very limited range of quantum theory. People are usually introduced to a form of Schrödinger equation with that kind of Hamiltonian first, though, for pretty much historical reasons: those were the kinds of quantum systems that early quantum physicists thought of first, before a full quantum theory that permitted other kinds of quantum systems was developed.",
"In reality, a purely differential Hamiltonian, as opposed to a more general matrix Hamiltonian, is only applicable in a pretty limited range of circumstances. As far as I can tell: quantum chemistry usually uses one, at least for some portion of their analysis. Solid-state physics often uses one. Condensed matter physics rarely uses one. Particle and high-energy physics, inasmuch as they ever use a Hamiltonian, rarely use a differential Hamiltonian. And quantum information/quantum computing pretty much never use a differential Hamiltonian.",
"The simplest and most common example of a quantum system where a matrix Hamiltonian would be absolutely necessary is a spin-1/2 system, like a (spatially confined) electron. The basis has two elements: spin-up and spin-down. The entire state space is just complex linear combinations of those two.",
"I apologize if that was a little jumbled or unclear; I'm pretty groggy right now."
] |
[
"Ask Anything Wednesday - Economics, Political Science, Linguistics, Anthropology"
] |
[
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!
|
[
"Somewhat linguistic related:\nDid the '.' (full stop) first appear in writing (to end a sentence) or in mathematics (as in 0.5 for instance)?"
] |
[
"It looks like using a dot to end a sentence is seen in ancient Greek, although a bit different from now - it didn't have to be level with the bottom of the writing, and different heights meant different things.",
"Using the point to indicate decimals, on the other hand, looks to be ",
"16th century",
", with earlier work only using fractions."
] |
[
"How does inflation not end up being logarithmic growth?"
] |
[
"Does the presence of two nostrils serve a specific purpose for olfaction (the way two eyes and two ears do for sight and hearing)?"
] |
[
false
] |
Two eyes are essential for depth perception. Two ears help determine sound location and intensity. Is there a similar significance for our two nostrils? Such as locating smells or determining smell intensity?
|
[
"I'm not sure about better \"smell perception,\" but your two-nostrils are essential for maintaining adequate hydration of the nasal tissues. Every 30 minutes (approximately), the tissue in one nostril will enlarge (cutting off air flow) and the tissue in the opposite nostril will become smaller (allowing for air flow). This process ensures that your nasal tissues do not dry out (and subsequently bleed or become thickened) due to uninterrupted air flow across them. ",
"http://www.todayifoundout.com/index.php/2010/03/about-85-of-people-only-breathe-out-of-one-nostril-at-a-time/",
" (not the best source but I'm on my phone)"
] |
[
"Wow. I always wondered why it was rare for me to be able to breathe the same amount through each nostril."
] |
[
"I've also heard that having one nostril with reduced air flow helps increase your overall sensitivity. IIRC the nostril with the reduced air flow gives the receptors on that side more time with the chemicals making up the odors. While on the open side there is less time and so some smells might be missed."
] |
[
"Why are there different types of neurons?"
] |
[
false
] |
Could someone explain why a unipolar neuron is specific somatosensory, a bipolar to sensory, and a multipolar specific to the CNS? i.e., what advantages (based on design) does each neuron have in each system and how each one relays information.
|
[
"Do you have a non-general answer?"
] |
[
"Well, for some bipolar cells in the retina, for example, some synapse with a very small number of cones (sometimes with even just one cone near the fovea). This means you have much more signal fidelity. This is good for resolution/detail. ",
"Ganglion cells and pyramidal neurons are multipolar and pool signal from many other cells; this is important for detecting weak signals as well for allowing integration of information that falls outside of the classic receptive field. As an example, some ganglion cells have center-surround organization meaning that they are \"looking for\" (respond maximally to) a region of light surrounded by a region of darkness (or vice versa), like a donut. There are no single cells that detect photons from such a large region. Instead, the ganglion cell is collecting many signals together to build up this more complicated feature. ",
"Far ranging connections across cells (i.e. with horizontal cells or mango cells in the retina or pyramidal cells in the cerebrum) allow for the influence of contextual / scene information outside of a cell's classic receptive field. So if you've got a cell that normally only responds when there is some feature (like a bar of light) in a particular part of the world (small receptive field), it might be good to somehow modulate this signal of there's another, adjacent bar of light parallel to this one and collinear with it (i.e. they form a line). This is exactly what happens. ",
"So it depends on what the signal is for: are you detecting one specific signal, or a few, or many?"
] |
[
"A general answer to your question is: because different types of neurons allow you to perform different kinds of computations. Even among a single type, like bipolar cells, there can be many different varieties."
] |
[
"How does ‘breaking’ something work? If I snap a pencil in two, do I take the atoms apart? Why do they don’t join together back when I push them back together?"
] |
[
false
] | null |
[
"To break something you are basically applying energy to overcome the molecular bonds in it.",
"Some materials will in fact join back up if you push them back together. But most everyday materials do not, mostly due to the molecules having been changed and requiring added energy to go back to the original state. Like many pure metals will “cold weld” back together, but in reality the surfaces will for example instantly react with the air, so they are no longer pure."
] |
[
"They have to have a 'pure surface' free of any kind of separating elements. Even a thin layer of oxidization will keep it from happening.",
"But, yes. If you put two pieces of clean iron together in space, they'll fuse and become one piece of iron."
] |
[
"Cold welding works in space and is something astronauts have to be careful of."
] |
[
"Are there any multicellular organisms that make unusual use of trace elements?"
] |
[
false
] |
By "unusual use" I mean atypical usage in cellular structures, metabolism, enzymes, or metalloproteins. I know there are all sorts bacteria that do strange things like Uranium reduction, but I'm more interested in instances of unusual usage in multicellular organisms.
|
[
"A lot. I'll just limit myself to humans.",
"\nWe use molybdenum to introduce metallic sites in active sites of enzymes. It's also used as a cofactor. Molybdenum is absolutely necessary for human survival.",
"\nSelenium is another essential element. It is used in the thyroid because it functions as a cofactor for thyroid hormones. ",
"Humans make use of cofactors a lot and these can be remarkable elements. Other trace elements used are copper, cobalt (part of vitamin B12), of course iron and manganese. ",
"Zinc can actually regulate brain functioning. It is one of the most important trace elements in the body (arguably after iron). Zinc is well known for being part of so-called ''zinc-fingers'' which are structures in proteins held together by zinc ions. They are especially important in binding DNA and regulating transcription. Zinc deficiency is actually a major cause of illness. ",
"Very interesting source if you're into this kind of thing:",
"\n",
"https://books.google.nl/books?hl=nl&lr=&id=jYn5baEnKBsC&oi=fnd&pg=PP1&dq=trace+elements+in+human&ots=fJHsSPw4F4&sig=BDnbQEfkw-wNNsjSICpdffDkhVs#v=onepage&q=trace%20elements%20in%20human&f=false"
] |
[
"Some wasps that parasitize wood dwelling insects have an ovipositor with a tip of metallic zinc that let's them drill through wood.",
"Edit: Almost forgot! Some chitons, a type of marine mollusk, make their teeth out of magnetite, a very hard type of iron oxide. They have enough iron ",
"that their teeth that are black in color",
" and will even stick to a magnet."
] |
[
"Thanks. I was actually asking a different question, which is if there were organisms which were unique in some way."
] |
[
"If free neutrons decay in 15 minutes how are neutron stars stable?"
] |
[
false
] |
I assume the neutrons in the star are unbound so what mechanism is keeping the neutrons from decaying?
|
[
"For the neutron to decay",
"N -> P + e",
" + antineutrino",
"and for this reaction to be energetically favourable, the electron has to have an unoccupied low energy quantum state to fall into. But all these states are already occupied by the free electrons which are highly degenerate. So the reaction cannot proceed in an energetically favourable way. So that's why neutron stars are stable against decay."
] |
[
"Hmm, yeah that's not right. This is why you shouldn't pull numbers off the top of your head. I remembered that one of the processes was faster that the actually normal decay, and 900 s seconds came to mind. ",
"I went back and looked at one of my old rate calculations for the Urca processes. At 10",
" K the rate is lifetime is closer to 10",
" seconds. But, these transitions rates have a T",
" temperature dependance, so at 10",
" K the lifetime of the neutron is closer to 10 seconds, and at 10",
" K, the temperature right at the birth of the neutron star, the lifetime is only 100 microseconds!",
"Sorry for the confusion. "
] |
[
"Just to clarify, the decay actually does occur in neutron stars, but at an exponentially suppressed rate. For those who want to read more, the name of the suppression that nicksauce described is called Pauli blocking. ",
"In addition, there's also the inverse process occurring,",
"p + e^- -> n + neutrino.\n",
"Together, these equilibrium processes occur quickly enough that the energy lost through neutrino emission is the mechanism responsible for cooling the star for much of its life. ",
"Together these processes are called the direct Urca processes. In reality some modified Urca process is suppressed less than they are and is probably closer to the actually cooling processes. "
] |
[
"Which specific advancements have led to the size decrease of household AC-DC adapters?"
] |
[
false
] |
In the last 15-20 years AC-DC chargers and power adapters have shrunk significantly with many offering substantially higher power output. I have an older iPod charger (pre-iPhone) that is as big or even bigger than the current crop of MacBook chargers and it outputs a measly 2.5W and the current phone chargers are much smaller and can do 10W at least with fast chargers going much higher.
|
[
"The biggest improvement is when we went to switching converters - instead of having a transformer that needs to handle line voltage (120/240VAC) - which by their nature require some physical size to the transformer to be able to withstand the relatively slow AC wave.",
"Transformers are a type of inductor, and in order to keep currents reasonable and not generate too much heat in the wires, you need a lot of inductance. Large inductances that can handle more than a tiny bit of current need more mass.",
"Modern switching converters don't operate at 50 or 60Hz, they operate at much higher frequencies, and don't need as much physical size to hold the current back for the shorter duration of the switching pulses.",
"Another way of looking at it, the bigger a transformer is, the more energy it can transfer per cycle - cheaper electronics have made it practical to use smaller transformers with a faster cycle speed generated inside the power adapter, rather than linking it to the line frequency.",
"Beyond that, capacitors have gotten smaller as well, semiconductors have gotten more well integrated, and various other things have been incrementally shrunk. But the biggest single improvement is the smaller transformer.",
"However, to directly answer your question, we've been using switching converters for most products in the last 15 years, and certainly for all of the devices you mention. In that timeframe, it's been a lot of those incremential improvements, including efficiency improvements that allow the adapters to be smaller. Efficiency is a huge deal because you can only dissipate as much heat as you have surface area or airflow, and without cranking up efficiency, it's hard to get small, especially when surrounded by an insulating plastic case. But it's difficult to point at any one improvement for that.",
"Here's a list of the general areas that improved:",
"/u/Doctor_Mudshark",
"These are just some general areas that have gotten better - I'm probably forgetting a bunch!"
] |
[
"I agree that transformers are one of the main reasons, but the old linear power supplies also needed large heat sinks for the series pass transistors. The switch mode supplies have much smaller heat sinks. The low voltage capacitors also have smaller values than in old linear supplies."
] |
[
"Plug ten reasonable quality wallwart switch modes into a distribution strip and plug the strip into a power meter that goes down to 0.1w. It will read zero, so the psu s are drawing at most 10mW each while idle. ",
"I have been lectured by one of the “I am so green science does not matter” on how psus draw full current all the time. Asked how they stay stone cold while idle, answer came there none."
] |
[
"How do we know for sure that a species is extinct?"
] |
[
false
] |
[deleted]
|
[
"We don't exactly know, but biologists will be monitoring species that are near extinction and when they stop seeing any specimens/signs of specimens for a long period of time they will declare it extinct. Many examples of them being wrong exist too."
] |
[
"Monitoring of species close to extinction also allows for designations such as functionally extinct which the northern white rhino is currently classed as because only 2 females are left alive so there will be no more born to continue the species. For lots of aquatic species tho science has been wrong as there's a whole lot of ocean that we can't really see."
] |
[
"That has to be such an amazing feeling, finding an animal everyone thought was extinct. There’s a show called Extinct Or Alive that is pretty incredible when they find an “extinct” species."
] |
[
"If you had a perfectly spherical earth and an indestructible ring that has an internal diameter larger than that of the earth's diameter, would the ring float around the earth?"
] |
[
false
] |
By indestructible ring I mean one that could support its own weight without any flex. I am implying perfect geometry as well.
|
[
"It's actually an unstable system, and this was something that was pointed out about the Ringworld novels.",
"Basically, it's impossible to have a perfectly circular orbit. If you have a moon or satellite in orbit, then sometimes it will get a little closer to Earth. But that makes it speed up a little, which makes it go a little further from Earth, which makes it slow down a little, which makes it go closer... and so on. You basically just end up with an elliptical orbit. Small errors don't build up.",
"But with a ring, it's a single connected object. Speeding up or slowing down its rotation doesn't change its position. So if one side gets a little bit closer to Earth, all that happens is that it starts to feel Earth's gravity a little bit stronger, which brings to closer to Earth, which makes the gravity stronger, and so on and so on. Speeding up or slowing down the ring's spin doesn't change how it moves, so you don't get things \"cancelling out\" like you do for an object in orbit.",
"So: unfortunately, no, not for any long amount of time: the ring would crash into the Earth."
] |
[
"That is why you mount Bussard ramjets on the ring. And no pilfering to make spaceships. You don't have a sun to flare in this case."
] |
[
"If the ring was spinning, it might resist motion a bit more, however the same effect still applies, if one side becomes minutely closer to the earth, then that side would crash into the ground. "
] |
[
"Physics class rocket project"
] |
[
false
] |
Hey everyone, so for my physics class we were given an A8-3 model rocket engine and let loose. The only rule is that there can be no parachute (a parachute is define as a piece of flexible material attached at 2 or more points i.e. streams are ok). Winner is judged by who's rocket stays in the air the longest. Any advice or ideas would be greatly appreciated.
|
[
"Helium balloon attached by a single string."
] |
[
"Lol just dangle the rocket engine from a string?"
] |
[
"Yes."
] |
[
"Do Humans Sweat While They're Underwater?"
] |
[
false
] | null |
[
"As a former competitive swimmer I can definitely answer this as a \"yes\". Sometimes I'd sweat for nearly an hour after a workout as well as while in the pool. It really helps to have a cool pool. Most pools are kept at around 82F which is nice for just floating around. Swimmers' pools are nice around 72-73F. ",
"Water is MUCH more effective at pulling heat from a body than is air, but if you're working hard enough your muscles will still generate enough heat to start sweating."
] |
[
"You never really stop sweating per se. You are constantly expelling water (and other stuff) through your skin. Sweat is actually seen as one of the enemies of being comfortable in the cold, because nothing saps body heat like water.",
"Here's a similar",
" ",
"/r/askscience",
" post for a little further reading."
] |
[
"In addition to this, there are regulations for competitive events that require a temperature range. See here: ",
"http://en.m.wikipedia.org/wiki/Olympic-size_swimming_pool",
"Water temperature while swimming is a big deal during competitive events because it's much harder to tell when you're overheating. The water is such a good thermal conductor and it feels like sweat and your body can't do any better. The upcoming Olympics has a small controversy over water being too warm to compete in because it can fairly readily lead to heat stroke in the competitor. A U.S. Swim team member, Fran Crippen, died in an open water event like the upcoming one and the consensus seems to be that the water was too warm. ",
"http://en.m.wikipedia.org/wiki/Fran_Crippen"
] |
[
"Why does sweat stain?"
] |
[
false
] |
What is responsible for that icky yellow stain on the armpits of my white t-shirts? Or is it deodorant? Or is this unusual and someone just been using them as diapers? (eww)
|
[
"I'd read it was as a result of an aluminium salt in antiperspirants.",
"\"When mixed with sweat, aluminium zirconium tetrachlorohydrex gly is known to stain clothing with a yellowish tint. It can also cause a stiffening of the affected areas of clothing. If excessive amounts of aluminium zirconium tetrachlorohydrex gly mixed with sweat come in contact with a material, bleach marks may develop.\"",
"http://en.wikipedia.org/wiki/Aluminum_zirconium_tetrachlorohydrex_gly"
] |
[
"I heard that 100% of urine is pee."
] |
[
"Sweat is responsible for that. Sweat isn't pure water, it is more like piss.",
"From Wikipedia:",
"Sweat contains the chemicals or odorants 2-methylphenol (o-cresol) and 4-methylphenol (p-cresol), as well as a small amount of urea."
] |
[
"How would I go about testing if a Vitamin B12 supplement actually contains Vitamin B12?"
] |
[
false
] |
Vitamin supplements are not as strictly regulated as drugs. Take a this Vitamin B12 [1] supplement for example (chosen at random from a Google search). How would I go about finding out whether it really contains what it says it contains, which is 1000 mcg of Methylcobalamin (a form of B12)? Out of curiosity, I called a local drug testing lab and they said that because it's not a drug, they do not have any norms to test for and that the supplement could have any number of ingredients effecting false results. Are there any methods I could either personally perform or use a lab for? If not, why not? Any pointers on terms I could read up on as well as reading material? Thank you for your time. [1] Edit: corrected typo. Edit 2: Thank you all for your thorough and interesting answers.
|
[
"I used to work for a contract research organization that tested food and pharmaceutical products. I specifically worked in a group that tested B vitamins in food and supplements. The most common method we used to test for the B vitamins was bacteria growth. Basically, the food or supplement is taken through an extraction method that dilutes the vitamin to a desirable concentration (usually you are provided with \"claims\" on the vitamin content from the manufacturer and can figure out the proper dilution to perform. If there are no \"claims\", you take a guess at where you think the content will be and may end up performing the test several times before getting a proper result). After the extraction and dilution precess, your samples are sterilized to prevent any unwanted bacterial growth. The samples (along with a set of standards of known concentration) are then inoculated with bacteria that will grow in proportion with the amount of B vitamin present in your sample. The standards and samples are put through an instrument that measures turbidity. A calibration curve is created from the standards and the samples are compared to the curve to give the amount of B vitamin in the sample. These are tried-and-true methods that work for all for all of the B vitamins (with modifications in the extraction process and bacteria used for each vitamin). There is an HPLC method developed for the B vitamins too, but it does not work for all of them and the concentrations need to be pretty high. Hope this helps answer your question!"
] |
[
"The bacteria is Lactobacillus leichmannii. Here is the USP entry on the vitamin B12 assay: \n",
"http://www.pharmacopeia.cn/v29240/usp29nf24s0_c171.html",
"It is kind of ridiculous to read through since they make it so generalized, but most testing methods are based off of USP assays with whatever little changes necessary to optimize the method. "
] |
[
"The way around doing such an experiment would be to use a chelating agent that specifically binds to the methylcobalamin, such as a specific enzyme, which you could then attempt to quantify through weighing/etc. or the enzyme may break the methylcobalamin down into a fragment that may be easier to measure. It may be worth looking into the ",
"pdb database",
". Ive linked you to a possibly relevant structure, but may need further research as to finding an appropriate agent.",
"The main problem with this process is that the chelating agent may bind to other molecules.",
"Another method may be to run a ",
"GLC",
" or some similar method to break the mix of compounds up in to its different ingredients, and then measure the amount that comes out at the correct corresponding time. ",
"Cant really think of possible problems with this, may be the best bet."
] |
[
"Is there a theoretical limit to the energy density of lithium ion batteries?"
] |
[
false
] |
Title basically says it. Is there a known physical limit to how energy dense lithium ion batteries could possibly become? If so, how do modern batteries compare to that limit?
|
[
"Yes, there is. Lithium ion batteries work by the lithiation and delithiation of an anodic material through electrochemical processes. So far, the energy density is dictated by how well the anodic materials will alloy with Lithium. For example, when you charge a lithium ion battery with a graphitic anode, the graphite alloys with Lithium to form LiC6. This tells us that the anode has a theoretical capacity of 372mAh/g. Typical Li-ion cathode material has theoretical capacities in the ~270 mAh/g due to the challenge of ionically transmitting Li ions from the cathode to the anode. ",
"Now, as for there being a known physical limit, this does not seem possible to calculate in my opinion. Since the Li-ion battery discovery by Sony in '91, better and new materials have been discovered. These materials have improved all facets of battery life: ionic conductivity, electric resistivity, storage capacity, cell stability, columbic efficiency, and energy density. On top of this, there are a wide array of lithium ion technologies for different solutions. A Li-S battery is hardly comparable to a normal Li-C battery. Not to mention that LFP batteries have different properties from an NMC battery as well. ",
"For what its worth, we do know that Li-S batteries have a maximum theoretical capacity of 1675 mAh/g",
"EDIT: Thank you all for the awards and the questions! I'm happy to be answering as much as I am capable of, but I just want to remind people to do their due diligence. I am a PhD student in material science right now, and there are far, far more knowledgeable experts on Li-ion batteries out there. To anyone looking to read some papers on the technology, I highly recommend looking into Jeff Dahn out of Dalhousie University and Yi Cui out of Stanford University."
] |
[
"He’s talking about the number of ions that can enter or leave the cell, per gram. An amp is a unit of ion flow rate, and an amp-hour is a number of ions (literally 3600 coulombs or 2.24 x 10",
" ions).\nThe voltage depends on the cell chemistry, how charged the cell is, the discharge rate, and the internal resistance of the cell."
] |
[
"I know that battery capacities are often told with Ah, but wouldn't Wh be better unit in this case since you didn't tell us what the voltage was ?"
] |
[
"Is alcohol as bad (nutritionally) for you as its caloric count would have us believe?"
] |
[
false
] |
I have often heard alcohol described as "empty calories", and I wonder what exactly that term means. I know that originally the calorie (or Kcal) content of a food item was determined using incineration in a calorimeter, however now it seems the industry uses the Atwater system, which uses averaged values of previous incinerations to assign caloric values to macronutrients and ingredients (4 Kcal/g for protein, 4 Kcal/g for carbohydrate, 9 Kcal/g for fat, Kcal/g for alcohol, ). That being said, I also know from both chemistry and first-hand experience that alcohol and most hydrocarbons are extremely flammable (though I'm not sure if this means higher than avg amounts of energy are released upon combustion). So it follows naturally that alcohol would have a high calorie content. However, it doesn't seem intuitive to me that flammability and malnutritiousness should be so closely linked. I have read in articles such as that alcohol is more a culprit of indirectly causing weight gain (decreased insulin sensitivity and reduced lipolysis due to the conversion of alcohol to acetate) than actually converting directly to fat/sugars. (But can still lead to weight gain indirectly)
|
[
"That makes sense. But what about my comments in the thread description? Is the caloric content in alcohol somewhat arbitrarily higher than its lack of nutrients would imply, simply due to its natural tendency to combust and release higher amounts of energy than say, a glucose molecule?"
] |
[
"Okay, I see what you mean. Haha I'll be honest, it was kind of a highdea that turned into an actual scientific thought. But yeah I think I am reading into it a bit too much. Thanks for the help!"
] |
[
"\"Empty calories\" refers to the fact that it doesn't have any micro nutrients."
] |
[
"I found those spots on the cover of a book that has been sitting on my shelf for a while. What are they?"
] |
[
true
] |
[deleted]
|
[
"Agree. Not with the spider part necessarily, but with the \"bug poop\" part. "
] |
[
"It's possible that it's an early stage of foxing, somewhat resembling ",
"this stock picture",
". If so what they are is apparently ",
"not fully understood",
"."
] |
[
"As a bibliophile that has combated fly infestations before, I was going to say the same thing. Looks like fly poop."
] |
[
"Why does the JWST need to be in a \"halo orbit\" at the L2 Lagrange point?"
] |
[
false
] |
I thought the point of putting satellites at the Lagrange points was that the unique gravitational properties of these spots in 3-body systems allowed them to basically sit still with little to no fuel needed to keep them there in a stable manner. Reading about the JWST I saw that the telescope will be in a constantly moving Halo Orbit, which requires fuel to maintain stability. Why would that be necessary?
|
[
"The L2 point is an equilibrium point, but it's not a stable one like L4 and L5 are (the other Lagrange points we use a lot). You can think of stable vs unstable equilibrium points like hills and valleys. Put a ball at the top of a hill or the bottom of a valley, and in both cases, the ball should stay there. However, in a valley, if the ball gets kicked a little bit, it rolls back down to the bottom, but if you have a ball at the top of a hill and it gets kicked a little bit, it will just keep going. The gravity at L2 is like the top of the hill. If it gets bumped away from it, it will just keep going. ",
"However, L2 does has a semi-stable orbit near it. It will still take some station keeping, since it isn't fully stable, but it's better than trying to just park right at L2. And then you still get the benefits of still orbiting the Sun every 365.25 days just like Earth does, so you can stay in constant radio communication with it.",
" Realized I said L3 and 4 were stable, instead of 4 & 5, so changed it above."
] |
[
"This is a common misconception. The halo orbit JWST is going into is many times the size of the Earth. It is not in Earth's shadow."
] |
[
"I read it has enough fuel for 10 years"
] |
[
"How do we know so much about the Earth's layers and it's core?"
] |
[
false
] |
The deepest hole we have ever bored is less than 8 miles deep. The crust alone can be as deep as 50 miles in some places. How do we know about the mantle, the outer core, or the inner core?
|
[
"Our luck in that matter is that our Earth is a dynamic planet with active plate tectonics. This means that oceanic crust expands from ocean ridges, and gets subducted at convergent margins. ",
"One result of this is that Mountain belts form at convergent margins. The tectonic processes active in mountain building (orogenesis) deform the rocks through folding and thrusting. This results in a stacking of layers which can bring to the surface rocks from a much deeper crustal level. In general, thrust belts add older material on top of younger rocks, and older generally implies deeper (There are exceptions of course). In some cases, such as when an ocean is closing, complete sections of the oceanic lithosphere reaching into the upper mantle are obducted on the continental crust in what are called \"Ophiolitic complexes\". These ophiolites provide us with large continuous exposure of rocks from the upper mantle. Important ophiolitic complexes are found in Cyprus, Oman, and the Appalachians amongst other places.",
"Another source of samples from even deeper crustal levels is deep rooted intrusions and crustal xenoliths. Some types of magmas such as kimberlites and some lamprophyres come from very deep levels within the middle mantle. The chemistry of these intrusions provides information about environments at those levels. Those magmas will also rip parts of the rocks they travel through on their way to the surface and carry them up to the surface. These chuncks are called xenoliths and some of them come from very deep indeed.",
"This provides us with a fairly extensive sample collection all the way down to the middle of the mantle. The way we get information from lower down is through measuring the physical properties of the Earth and interpreting this data in integrated models. The types of geophysical data mostly used are density (from gravity), the propagation of waves through the Earths layers (seismic surveys), magnetic data, and geothermal flux.",
"The model we have for the formation of the Earth is through accretion of primitive material in the early solar system. We have a pretty fair understanding of the average composition of this primitive material through the study of the chemistry of météorites. When we combine our understanding of the materials in the outer Earth, with the physical properties of Earth and a whole and of the average chemistry of météorites, we our able to infer the composition and structure of the inner Earth."
] |
[
"When there is a massive seismic event this induces two different types of waves in the material of the earth: P and S waves. P waves are compression waves, like sound, where S waves are tranverse waves (like a water wave). These waves propagate through the core of the earth and can be detected at different seismographic sensors around the world. When a wave travels through a medium and then moves into a new movement with a different density it refracts, it bends. Thus by having knowledge of the time of arrival and relative intensity of the seismic waves around the planet that result from a single localized seismic event one can piece together the number, location and change in density of the various interior layers but piecing together how they were refracted.",
"Along this line, nuclear weapons testing has actually yielded a fair amount of data for this because the location, yield and time of a \"seismic event\" can be known very precisely and thus the refraction picture of the P and S waves can be assembled with more accuracy.",
"Such things will tell you how the DENSITY changes, one then has to look to things like planetary formation theory to try and piece together what materials at what temperature produces said density."
] |
[
"(at least much better than an earthquake)",
"We had a seismic station at my university; the managing researcher told me that the seimic research community had an understanding with the military about those nuclear tests. The military would in some instances warn the researchers ahead of test explosions, and give them at first a rough estimate of when the test would happen (say within a couple of days). After the test, they would circulate a very precise time of detonation (down to fractions of a second) which would allow quality research to be performed."
] |
[
"Does the expansion of the universe cause time dilation?"
] |
[
false
] |
If space and time are part of the same thing, does the expansion of the universe make time expand to? If so, does it make time slow down, similar to the time dilation around a black hole?
|
[
"See the sidebar of ",
"/r/math",
" to render equations.",
"Does the expansion of the universe cause time dilation?",
"Time dilation is really just difference in elapsed times according to different observers. It should be understood as ultimately arising from the different coordinate systems of the two observers. Time, just like space, is really just a coordinate. ",
"In this earlier post",
" I give much more detail about how to find time dilation factors for general metrics. In the case of cosmological expansion (say, for a flat universe), the metric has the form",
"[; ds^2 = -dt^2+a(t)^2(dx^2+dy^2+dz^2) ;]",
"To understand how time is measured by different observers, we should first understand what the coordinates in this metric even mean. The coordinates ",
"[; (t,x,y,z) ;]",
" are so-called co-moving coordinates, or cosmological coordinates. ",
"This graph",
" shows what's going on. There is a fixed spatial grid which simply expands along with the universe as time increases. If an observer remains at a fixed grid point for all time, then that observer is a so-called co-moving observer or isotropic observer. According to this type of observer, the universe looks spatially homogeneous and isotropic. That is, no matter what direction he looks and no matter which grid point he is at, the universe looks the same (on large scales). Cosmological time is, in fact, defined by those two properties. All co-moving observers, no matter their location in the universe, have synchronized clocks.",
"Okay, what if you are not a co-moving observer? Then you have coordinates that are not the same as the co-moving coordinates. In particular, you have a different time coordinate. ",
", some elapsed time acoording to a co-moving observer is not the same elapsed time for you. In that sense, cosmic expansion of space does cause time dilation (but that's nothing special really).",
"Let's look at the time dilation factor now. Suppose we are a co-moving observer and, in our coordinates, another observer moves along the path in spacetime given by",
"[; x^{\\mu} = x^{\\mu}(t) ;]",
"So the time dilation factor is",
"[; \\gamma = \\left(1-a(t)^2v(t)^2\\right)^{-1/2} ;]",
"where ",
"[; v(t) ;]",
" is the speed of the second observer. Note very carefully that we are describing the second observer in terms of ",
" co-moving coordinates. For instance, a speed of ",
"[; v(t) = 0 ;]",
" does not mean that the second observer does not ",
" to move. What that means is that the second observer's spatial coordinates (in our cosmological coordinates) do not change. In other words, ",
"[; v(t) = 0 ;]",
" means that the second observer is ",
" a co-moving observer. Also note that the local speed of light is not ",
"[; v_{light} = 1 ;]",
", but rather ",
"[; v_{light} = 1/a(t) ;]",
", which can be greater than or less than 1. ",
"To describe the apparent change of distance between co-moving observers, we use a different spatial coordinate system based on ",
" distance. The co-moving distance ",
"[; \\chi ;]",
" and the proper distance ",
"[; d ;]",
" are related by",
"[; d(t) = a(t)\\chi(t) ;]",
"The time derivative of the proper distance is",
"[; V(t) = d'(t) = a'(t)\\chi(t) + a(t)\\chi'(t) ;]",
"The first summand on the right-side is sometimes called the ",
" (as described by Hubble's law) and the second summand is sometimes called the ",
". The peculiar velocity is the apparent velocity of an object as measured by a local co-moving observer. That is, if we want to measure the peculiar velocity of some object, the co-moving observer who happens to be right next to that object has to measure its speed. (We cannot measure the velocity of distant objects.)",
"Note that ",
"[; \\chi'(t) ;]",
", being the time derivative of the co-moving coordinate, is bounded above by ",
"[; v_{light}(t) = 1/a(t) ;]",
", the local speed of light in co-moving coordinates. Hence the peculiar velocity ",
"[; V_{pec}(t) = a(t)\\chi'(t) ;]",
" is bounded above by 1. That is, a local observer always measures objects right next to him to move at less than 1 (or ",
" in dimensional units). (This is what is meant when we say that in GR we have to modify the rule that light always has the same speed. It is only true that objects right next to us cannot travel faster than light and that light rays right next to us have speed 1.)",
"Anyway... back to time dilation. The time dilation factor is written above in terms of ",
"[; v(t) = \\chi'(t) ;]",
", which is the time derivative of the co-moving coordinate. It is more natural to write the time dilation in terms of the peculiar velocity since that is what we can measure directly. We then have that",
"[; \\gamma = \\left(1-V_{pec}(t)^2\\right)^{-1/2} ;]",
"which looks an awfully lot like the usual time dilation formula from SR. As an example, suppose we are a co-moving observer that sees some other observer with a constant peculiar velocity ",
"[; V ;]",
". Then if we measure a time interval of ",
" between two events, that other observer will measure the time interval",
"[; \\tau = t\\sqrt{1-V^2} ;]",
"The second observer measures a smaller time interval. So whereas all co-moving observers agree that the universe is about 13.8 billion years old, a non-co-moving observer measures the age of the universe to be younger, how young depending on how fast he moves with respect to the co-moving observers.",
"Let's consider a fun example. We operationally define the co-moving observers to consist of the frame in which the ",
"CMB",
" is homogeneous and isotropic. We see the CMB with a prominent dipole anisotropy, which occurs because we are not co-moving observers. Our peculiar velocity through the CMB causes some of the CMB to appear redshifted and some of the CMB to appear blueshifted. The peculiar velocity of Earth through the CMB is about 600 km/sec. A time period of ",
" for the co-moving observers is, for us, a time period of",
"[; \\tau = t\\sqrt{1-V^2}\\approx t\\left(1-\\tfrac{1}{2}V^2\\right) \\approx t\\left(1-2\\times 10^{-6}\\right) ;]",
"So if the age of the universe is about 13.8 billion years for co-moving observers, then the age of the universe for us is about the same, minus 28,000 years.",
"Now I can quickly answer your remaining questions.",
"If space and time are part of the same thing, does the expansion of the universe make time expand to?",
"The FLRW metric in cosmological coordinates (and for a flat universe) has the form",
"[; ds^2 = -dt^2+a(t)^2(dx^2+dy^2+dz^2) ;]",
"In these coordinates, the timelike coordinate is left undistorted. Only space is affected by the expansion factor. However, we can easily change to a different coordinate system in which the timelike components of the metric are not trivial. If we use so-called ",
", then the metric has the form",
"[; ds^2 = a(\\eta)^2(-d\\eta^2+dx^2+dy^2+dz^2) ;]",
"So it appears in these coordinates that time does get distorted, but that the entire metric is really just a scaled version of the Minkowski metric from SR. But be careful! The coordinate ",
"[; \\eta ;]",
" is not the same as cosmological ",
" or Minkowski ",
". So to say that ",
"[; \\eta ;]",
" is distorted by the metric doesn't really say much. Indeed, ",
"[; \\eta ;]",
" has the interpretation of the time as measured by a clock whose ticking rate decelerates along with the expansion of the universe. So ",
"[; \\eta ;]",
" is, in a sense, ",
" measuring a time coordinate that gets \"distorted\". The lesson here is that in GR, the coordinates are rather arbitrary, and in the absence of preferred coordinates, there really is no definitive way to answer the question of whether cosmological expansion also affects time. It depends on your coordinates.",
"If so, does it make time slow down, similar to the time dilation around a black hole?",
"Again, it's important to be precise about what we mean by \"time slowing down\". As I explained above, all co-moving observers have synchronized clocks that tick at the same rate. An observer that has a non-zero peculiar velocity with respect to the co-moving observers does, in fact, measure a smaller time interval, given a fixed time interval as measured by the co-moving observers. In this sense, the time dilation should remind you more of time dilation in SR, where it arises only as a result of relative motion. The co-moving observers are somewhat analogous to a set of inertial observers of SR. A set of observers that are moving at some constant peculiar velocity with respect to the co-moving observers are somewhat analogous to a second set of inertial observers of SR. (The analogy breaks down, however, for several reasons. For one, time dilation, length contraction, and all that is not symmetric between different sets of observers. The co-moving observers, for instance, really have experienced the most proper time since the big bang.)",
"Contrast this with time dilation in the spacetime of a black hole, e.g., the Schwarzschild metric. Suppose we are a Schwarzschild observer (more or less, an observer infinitely far away from the singularity) and there is a second observer, who has fixed spatial coordinates in our coordinate system. Now if we were talking about the FLRW metric in cosmology, we would know that the second observer would necessarily also have to be a co-moving observer, and so everyone's clocks are synchronized. But in the Schwarzschild metric, our clock is not synchronized with that of the second observer. The second observer's clock runs more slowly than ours simply for being closer to the singularity. Mathematically, this happens for various reasons. For example, the ",
"[; dt^2 ;]",
" part of the Schwarzschild metric gets multiplied by some non-trivial factor that depends on the spatial coordinates. Physically, the spacetime of a black hole is neither homoegeneous nor isotropic about any point. That is, the universe does not look the same in all directions. So we should not expect observers at different coordinates (even if they are fixed) to have synchronized clocks."
] |
[
"This is an awesomely detailed answer. Thank you."
] |
[
"Wow, thank-you for your very clear and thorough explanation of that aspect of spacetime. I really like the analogies back top SR, and how you so elegantly point out the issue of coordinate choices in GR."
] |
[
"Asking here because /r/AskStatistics won't answer my question. Women are more likely to suffer from depression than men: true or false?"
] |
[
false
] |
My question : The Mayo Clinic states that " " Other less reputable sources ( , ) argue that men and women experience depression at similar rates but that male depression often goes unrecognized. I would have thought that this is something taken into consideration by statisticians?
|
[
"True. See ",
"this study",
".",
"BAM!"
] |
[
"Also, interesting enough, ",
"Almost four times as many males as females die by suicide."
] |
[
"the BAM! made me lol :D",
"Thank you for answering my question!"
] |
[
"What happens when our stomachs grumble?"
] |
[
false
] |
[deleted]
|
[
"It’s usually associated with the beginning of the gastric phases. Before we even eat, the thoughts and smells of food cause our vagus nerve to begin sending signals to our digestive tract. It’s essentially Priming the tract to get ready fo food. Muscle contractions begin and so does the secretion of lots of mucous and other stuff to help us digest. "
] |
[
"Gas and and remnant liquids being moved by peristalsis in the stomach and upper intestines, which vibrates intestinal tissue.",
"This wonderfully alien video is gastric peristalsis in the stomach - the digestive tract is a moving, contracting squelchy tube.",
"https://youtu.be/o18UycWRsaA"
] |
[
"But how exactly is the ",
" produced?"
] |
[
"Why is copper used for hot water piping, even though it's an excellent thermal conductor? Wouldn't an insulator be more energy efficient?"
] |
[
false
] |
[deleted]
|
[
"Not easy to bend. Not easy to cut to length. Using a blowtorch to solder a stainless steel pipe will likely affect its properties, it would be untempered."
] |
[
"Copper is used for plumbing because it is relativly easy to work with and fairly chemically resistant. If you want to conserve heat you can always wrap the copper with insulation. The most important thing is that you want to be able to contain the water without it leaking. ",
"New homes have replaced copper with ",
"plastic piping",
", it is cheaper than copper, but not quite as durable."
] |
[
"So why not use stainless steel as they do in the pharmaceutical environment where being clean is crucial."
] |
[
"Do we use the same muscles for resting inhalation as we do for forced inhalation?"
] |
[
false
] |
[deleted]
|
[
"That only works in the direction you've typed it. We use the same muscles in forced inhalation as relaxed, but we add several accessory muscles, namely the ones I'd mentioned. Also, as I'd said, the scalenes appear to be used in relaxed inspiration."
] |
[
"The scalenes are used in normal breathing as well, the serratus however are certainly an accessory muscle, sternocleidomastid, pectoralis major and traps also come to mind off the top of my head.",
"There's definitely some debate over some of these though, I don't think we have any good consesus on some of this."
] |
[
"Regardless of the ",
" muscles used, we use the same muscles during relaxed and forced inhalation, correct?"
] |
[
"Military Grid Reference System and Curvature of the Earth."
] |
[
false
] |
The Pythagorean Theory states that A + B = C can be used to find a distance between two points on a grid system. Using the Military Grid Reference System, is it necessary to additionally account for the curvature of the earth? If so, how? Thank you for any help.
|
[
"To see a demonstration of this, consider the triangle formed by taking the north pole ",
", and two distinct points ",
" and ",
" on the equator. Clearly the distances ",
" and ",
" are the same, and ",
" is some positive distance. ",
"Now, since the line of longitude connecting the north pole to point ",
" meets the equator at a right angle, we have a right triangle (and actually, this is also true for the longitude from ",
" to ",
" as well, so this triangle ",
" and another angle, demonstrating that a triangle in a curved space needn't have angle sum of 180 degrees).",
"Anyway, the pythagorean theorem then wants to say ",
" + ",
" = ",
" but we know ",
" and ",
" have ",
", and we know that ",
" > 0 as well, so the theorem fails."
] |
[
"To see a demonstration of this, consider the triangle formed by taking the north pole ",
", and two distinct points ",
" and ",
" on the equator. Clearly the distances ",
" and ",
" are the same, and ",
" is some positive distance. ",
"Now, since the line of longitude connecting the north pole to point ",
" meets the equator at a right angle, we have a right triangle (and actually, this is also true for the longitude from ",
" to ",
" as well, so this triangle ",
" and another angle, demonstrating that a triangle in a curved space needn't have angle sum of 180 degrees).",
"Anyway, the pythagorean theorem then wants to say ",
" + ",
" = ",
" but we know ",
" and ",
" have ",
", and we know that ",
" > 0 as well, so the theorem fails."
] |
[
"He wasn't asking about projectiles, he was asking about mapping Cartesian coordinates onto the surface of a sphere."
] |
[
"When I sleep after a night of drinking, I wake up every couple of hours. Is this the alcohol impacting my body's handling of the transition between sleep cycles, or something else?"
] |
[
false
] | null |
[
"Alcohol inhibits glutamate's action, which is arousal. In response your body releases much more glutamate to make up for what alcohol has inhibited. The alcohol is cleared out faster than the glutamate and all of a sudden you can't sleep. That's why you can be tired after a night out drinking but can't seem to fall asleep."
] |
[
"{Forgot to put this in the body} I am aware that, technically, a sleeping person will wake up every time a sleep cycle ends, but usually won't be awake enough to notice/remember it. When I go to sleep after having had a few, the percentage of transitions that I remember/notice goes way, way up--why is this?"
] |
[
"Keep the conversation on topic. Provide sources when you can, and focus on giving scientific answers to the question at hand.",
"If you're not sure of your answer, please note that you aren't in your comment. Uncertainty can be great for discussion, but we need to know when you're not sure."
] |
[
"How big of an asteroid would it take to completely destroy all evidence of life on Earth?"
] |
[
false
] |
Also, how fast would it be going approximately 10 hours before impact? I'm writing a black comedy on the destruction of Earth via asteroid and I could really use all the hard science I can get. Any information would on the subject would be incredibly helpful and appreciated.
|
[
"Try playing with ",
"this little doohickey",
".",
"It's been around for a while now, and I can't guarantee its complete accuracy but it convincingly describes an unfolding chain of events that could give a pretty good idea of what it might take to bring about an apocalypse. ",
"Anyway it takes a variety of inputs including object size, density, velocity, angle of impact, what sort of surface it impacts, and your distance from the impact site. Then it calculates your fate."
] |
[
"Be hard to get rid of all the bacterial life in the crust without melting the entire thing."
] |
[
"A body the size of Mars slammed into the Earth and all it did was create the moon."
] |
[
"How would a springboard work on Moon?"
] |
[
false
] |
I've been wondering about this for quite a long time. None of my physics teachers could tell me how would be the effect. The gravity on the Moon lets us to bounce 6 times higher, but reduced falling speed would reduce the pressure put on the trampoline surface by our body. So, how would it be?
|
[
"I'm going to guess it's because the average Joe can't go to the moon to attempt acrobatics."
] |
[
"First: ignore the fact that it's notoriously difficult to do acrobatics on the moon. And friction.",
"To figure this out, we need to look at energy. When you jump, you add kinetic energy to yourself. This is converted into gravitational energy when you move up, slowing you down. As you fall back, the reverse happens and you speed up again, ending with you falling down with the same energy and speed as you began. Note that the strength of gravity doesn't change your final energy/speed: lower gravity just takes a bit longer to get you back.",
"The trampoline does something like gravity: it converts kinetic into elastic energy and back again. Gravity plays no role in this. Without friction you'd bounce forever. If you jump, you add some more energy to the mix. The first jump about doubles it, the second triples etc.",
"So the trampoline would work perfectly fine on the moon, and you'd jump and bounce at least six times as high as you would on earth. As there's no air friction on the moon, it might even be higher."
] |
[
"it's notoriously difficult to do acrobatics on the moon.",
"Interesting. Why is that? Is it because of being unaccustomed to the reduced gravity? Is it the lack of atmosphere? Or is it due more to the fact that space suits are very bulky and stiff?"
] |
[
"Are there any signs punctuation is simplifying due to social media?"
] |
[
false
] |
Is there any sign that English punctuation is simplifying in general from omission/elision in social media due to practical constraints? For example apostrophes and capitalization are often omitted because they take too long to select/type on phones: is this beginning to carry over in an evolutionary (and not just lazy/uneducated) way to English in general?
|
[
"What you're seeing is communication over electronic devices forming its own ",
"register",
") of speech. We already have several, you speak differently at home vs at work vs doing a presentation vs in the bedroom etc. Other languages like Japanese have a register for when someone outranks you. Lack of punctuation and capitalization and abbreviations are hallmarks of this new register.",
"Much like other registers, it's generally inappropriate/taboo to use a register in the wrong setting. So just like you wouldn't use colloquial speech in an office presentation, you shouldn't use social media speech/writing to write a book (unless that's a specific, stylistic choice you want to make).",
"In short, English is getting more complex, if anything, not simplifying. Of course, language is in constant flux and you may see aspects carry over, such as \"sus\" being used in colloquial speech. This is normal.",
"Side note: the subfield which studies this is called computer mediated discourse analysis."
] |
[
"Do you mean code switching? That usually refers to switching languages entirely or at least dialects, so it's a little more extreme. There's so much overlap and fuzziness between registers it's hard to concretely classify them, let alone say when someone has \"switched\". They're more useful as an abstract concept to explain how people adjust their speech in various ways based on the situation."
] |
[
"So is this like \"Context Switching\"?"
] |
[
"Does Jerry Woodalls' new aluminium alloy work and why is this not being used?"
] |
[
false
] |
Could this be made by a haz-mat geek like myself and safe for individual households. I was blown away by the fact that this could be the one big find that could literally change the world and our dependence on fossil fuels. Lloyd Layman style please and thank you for responding in advance.
|
[
"An alloy of gallium and aluminum will produce hydrogen when brought in contact with water.",
"Could this be made by a haz-mat geek like myself and safe for individual households.",
"Sure, though gallium is a little expensive.",
"the one big find that could literally change the world and our dependence on fossil fuels.",
"Not really, aluminum requires massive amount of electricity to produce (so much that it's called congealed energy) and that electricity is mainly produced by fossil fuel power plants. So you're not getting rid of the problem, you're just moving it somewhere else."
] |
[
"Thank you langfan for your time."
] |
[
"not an answer but can you link me to the article?"
] |
[
"Could a box of kleenex take someones head clear off given terminal velocity?"
] |
[
false
] |
This question originates from a story from a driver's ed instructor who claimed that someones head was taken off from a kleenex box during a collision. While I fully do not believe this story, I am having trouble convincing someone that a kleenex box could in fact take someones head off and would appreciate some expertise on the subject. edit: When I said terminal velocity, I was referring to terminal velocity under normal atmospheric conditions.
|
[
"This isn't actually terribly difficult to calculate. We have V = sqrt(mg*2/pAC). (The only reason I remember the equation like that is because it looks like a milligram of Tupac. Hehehehehe.)",
"In this equation, m is the mass of the falling object, g is acceleration due to gravity, p is the density of the fluid, A is the cross-sectional area, C is the drag coefficient, and 2 is two.",
"The drag coefficient for a cone is ~0.5, so we'll be generous and assign a drag coefficient of 0.25 to build in a safety factor. (This will result in an overestimate of the terminal velocity.) Air is about 1.25 kg/m",
" The cross-sectional area is less than half of the surface area of a Kleenex box, which, for the one I am currently using, is 0.035 m",
" (Again, this will result in an overestimate.) I also took out a scale and measured the weight of a mostly full box of Kleenex, which came out to around 0.3 kg. And we know that g is 9.8 m/s",
"So we plug all of this into our favorite calculator and get that the terminal velocity of a box of Kleenex is around 11.6 m/s, which is around 26 mph. And remember, this was designed to be an overestimate. If a box of Kleenex at terminal velocity kills you, ",
"."
] |
[
"They did a ",
"Mythbusters",
" episode in this one, so if anyone wants to see proof of this I'd have a watch."
] |
[
"While I agree with your conclusion, I don't think you can use that equation to describe the highly non-equilibrium conditions during and shortly after a collision.",
"The moment you collide, the \"frame\" of your car gets decelerated, but the kleenex box, the air inside the car and ",
" will continue forward due to inertia. So, simply disregarding the turbulent air after a few milliseconds and assuming that you are being stopped by the seat belt and the air bag, ",
"Still way not enough to kill you, but I'd say terminal velocity has little to do with that."
] |
[
"Is it possible for intelligent life to be of a different sex system?"
] |
[
false
] |
I always watch movie or games, and while I realise that they are poor examples or representations of actual alien life, I always witness that aliens have the same genders as us, male and female. What are the chances that they follow us in this?
|
[
"Completely random. They could be single sex, two sexes or something completely different. "
] |
[
"Tons. Check out the Ringworld series by Larry Niven, or the Ender's Game series by Orson Scott Card. "
] |
[
"Are there any articles of fiction that support this idea? Not that I don't believe you, but I'd want to know if the thought was been put into a story yet. "
] |
[
"What if a blind person took some kind of hallucinogenic drug?"
] |
[
false
] | null |
[
"Anyone on hallucinogens can hear things that aren't actually there, you don't have to be blind.",
"That said, a large portion of a hallucination has to do with perception of experience, as much as changes to visual/auditory cues. A good portion of the 'trip' would be in how they think about things/perceive the world, rather than what they see."
] |
[
"I'm an optometrist - I agree! Auditory hallucinations will be present regardless of the person's visual status. However, when we think of blindness, most is not total darkness (this is what's no known as 'no light perception' and account for a small percentage of those classified as 'legally blind') and so most individuals would have some sort of perception of size of objects, colours, textures. And if a severely blind person, who doesn't have any light perception, hallucinates it would likely be similar to having a dream. It doesn't take much to stimulate the visual cortex, all you have to do is press on your eyeball and voila - phospenes (coloured lights!!). "
] |
[
"Do you have a source for these claims? If someone is 100% blind then surely they can't see anything in the physical world around them."
] |
[
"If the Measles virus erases the immune system for a few years, could this have implications for autoimmune diseases?"
] |
[
false
] |
I read this article on NPR: It prompted me to think about the implications this could have for autoimmune diseases. Essentially, my understanding of autoimmune diseases is that the body mistakingly identifies a part of the body as a foreign invader(Ex. Rheumatoid arthritis and small joints) and attacks it. Could measles be used to 'reboot' the immune systems of people with autoimmune diseases so that their bodies no longer attack themselves? BTW, I dont much of a science background, so this question could be ridiculous.
|
[
"What if someone already had an autoimmune disease?"
] |
[
"Couldn't find any studies on it, but it seems theoretically possible. \"Those who contracted the measles virus are less likely than a controlled match to develop AA disease.\"? Great study question!",
"That being said, measles encephalitis is bad and vaccination is key. I would say that the measles disease wouldn't alone be worth the theoretical benefit of decreasing ones risk of AA diseases."
] |
[
"lelo1248, actually, most antibodies are secreted and do not have any membrane associated with them - a non-secreted antibody is called a B-cell receptor. Antibodies are structured to have a \"variable region\" which is designed to recognize and bind a target, which is often something that has been deemed foreign in the body. Healthy individuals are born with the ability to select out self-reactive antibodies (and T-cell receptors) so that autoimmune diseases do not occur. However, sometimes this goes wrong, and it is also thought that autoimmune diseases can occur as a side effect of an immune reaction against something that is foreign in the body, and that healthy tissue is sometimes caught up in the immune system's dealing the foreign tissue and is guilty by association. This is only one of many theories on the subject.",
"Secondly, membranes are not built from genetic code - the proteins in the membrane are, but since antibodies are not in membranes (with the exception of IgD, which is weakly binding, anyway), this point is moot. You may be thinking of T-cell receptors binding to their corresponding MHC on another cell. In this case, there are mechanisms in place for a T-cell to recognize that the MHC is wrong, and that the target cell is thus foreign. This is the basis of organ rejection, for example.",
"Antibodies are actually used as a therapeutic to treat many diseases these days - they are not required to be made from the same patient they are treating. It is still possible to develop an immune reaction against these therapeutic antibodies if they were not present in the patient before, but steps are taken to prevent against this."
] |
[
"What are the odds of mankind domesticating certain wild animals over many years?"
] |
[
false
] |
[deleted]
|
[
"You are talking about two different things. ",
"Habituation is easily done in a variety of animals. It is a process that occurs within the lifetime of an individual and just requires that one animal to act socially acceptable towards a particular human.",
"Domestication is highly unlikely for any animal. It requires generations of selective breeding and a propensity towards socially acceptable behavior, loss of fear and the like. Think about how few animals have gone this route out of the number possible. ",
"I am not sure we can calculate numerical odds on this sort of thing but must talk about the likelihood in a qualitative manner. It is more likely for a fox than a bear because x,y and z."
] |
[
"The problem is that we are not selectively breeding these things for the propensity to habituate. One bear may habituate in the zoo and even breed if we are lucky. But what we need to do to domesticate bears is to have a bunch of animals breeding and select the ones that are the friendliest.",
"One problem is the longer the generation time is the longer it takes to go through this process. Another is the fecundity of an organism. The more offspring they have the harder you can select for the traits you want. Bears have relatively long generation times and low fecundity. This, together with their generally ornery and skittish ways makes it almost impossible to do this. Also, coolness is not going to be much of a driving force for this since the dangers and lack of functional utility to a domesticated bear make the likelihood approach zero."
] |
[
"It would take many generations and you would end up with something akin to what you have selected for over many generations. take the well known example of the russian scientists domesticating foxes; over many generations of artificial selection the scientists have breed foxes that are similar to domestic dogs. Note that an unexpected product of this selection have been the lightening of the color of the coats of the animals and a more floppy 'dog like' ear"
] |
[
"Can you see the real size of stars in this picture?"
] |
[
false
] |
I've been taught that stars are so far away that we see them as points. Even through a telescope (at least one an amateur could own) you see stars as points. If you see something that looks like a filled circle, you can be sure that you're seeing a celestial body from within the solar system. However, I've found this picture in the Wikipedia page for Alpha Centauri, and the stars appear clearly as filled circles. Some stars in the background would also appear to have area: I was wondering if the picture was just taken through a telescope with enough magnification to show stars as having size, or whether it's just some artifact from how the photograph was taken.
|
[
"No, you are looking at the ",
"point spread function",
" of the telescope. Real optical systems cannot be focused perfectly, and turn points of light into small circles that you see in the picture. ",
"The smallest angle that a telescope could resolve is called the ",
"diffraction limit",
". Larger telescopes have smaller diffraction limits. Hubble has a diffraction limit of about 0.05 arcsec (1 arcsec is 1/3600deg), so it can only see stars that appear larger than that. ",
"Wikipedia",
" only lists two stars (besides the Sun) that are larger than 0.05\", and they are both extremely large stars, and relatively close. You would need a telescope about 10 times as large as Hubble to resolve Alpha Centauri A, or 50 times larger for Proxima Centauri.",
"There are other techniques to resolve stars, mainly by using interferometry. If the telescopes are physically separated but the light they collect is combined (either physically or on a computer), they can attain the diffraction limit of the distance between the telescopes, rather than the size of the telescopes themselves. ",
"This",
" picture of Betelgeuse was obtained by a large array of radio telescopes, called ALMA. Just a few other stars have been resolved in this way.",
"Edit: ",
"/u/Stargrazer82301",
" is right that the brightest stars are larger than the PSF due to pixel saturation. When the pixels overflow with electrons, they spill into adjacent pixels and look bigger."
] |
[
"With very, very few exceptions, stars in photographs, including this one, are not resolved, and just appear as what we call \"point sources\" - what we call sources of light which, to the limits of our ability to measure, are indistinguishable from infinitesimally small points of light.",
"Because our telescopes don't have infinite resolution, even point sources will appear a bit \"smeared\" - their light appearing spread out around the star itself.",
"The reason why the stars appear to have discs here, is that the area surrounding the stars is \"saturated\" in the image. Imagine each pixel records the brightness of a given patch of sky, on a scale from 1-100. In any patch of sky bright enough to max out the scale, the pixel will read 100 - which will get displayed as a fully white pixel. And because of the smearing I described above, you will find a whole ",
" of pixels maxed out around particularly bright stars. Which is which is what happened here."
] |
[
"Let's do some napkin math.",
"WolframAlpha has a nice feature in which you can simply specify various quantities in English. What's ",
"Alpha Centauri's radius/Alpha Centauri's distance",
"? It's about 1.7E-8. Or, if you use the values posted on Wikipedia, ",
"2.06E-8",
". That's our angle, roughly, in radians.",
"Can a telescope possibly zoom in far enough? The usual angular resolution limit is the ",
"Rayleigh criterion",
", based on diffraction. D = 1.22 wavelength/angle. It turns out, that calculation demands a diameter of ",
"35 meters",
". That's almost reasonable for the largest telescopes on Earth, which are about 10 meters wide. This question isn't quite settled then.",
"Ok, let's try again. The orbital separation is roughly 17AU. Dividing that by the radius of the larger star, we find a factor of about ",
"3000",
". Does it look like the separation is roughly 3000 times wider than the dot? No, maybe a few dozen. The true radius of the stars should be much smaller, but it's bloomed much wider by optical fuzziness limits."
] |
[
"Is there a name for this phenomenon?"
] |
[
false
] |
Say you're driving and someone makes a mistake. We tend to think the absolute worst of them right away, that they're the stupidest, biggest jerk ever. It's like we antagonize people so much more when we aren't physically face to face to them so we don't give people the benefit of the doubt. Is this a studied phenomenon? Similar to cognitive biases?
|
[
"At its most basic, this is the fundamental attribution error. We downplay situational factors in evaluating other people's behaviors, and assume that their bad behavior comes from a bad personality."
] |
[
"Other people do silly things when driving because they're awful. I do silly things because of *insert reason here*"
] |
[
"Yes! This is a cognitive distortion. Many people do this. For example let's say a blue sedan is driving perfectly reasonably in front of you for 30 minutes, when all of the sudden they swerve and almost hit the car next to them. You now have 30 minutes of data in order to judge their driving. 29 minutes of good driving and 1 minute of bad driving. So why do we label them a bad driver? It's called mental filter, we filter out what doesn't fit our narrative."
] |
[
"How come there is no cure for lupus? Is anyone working towards one?"
] |
[
false
] | null |
[
"The diagnosis of Systemic lupus erythematosus (SLE) is a diagnosis of exclusion. You need to rule out many other diseases before you can just diagnose SLE in most cases.",
"Our current knowledge states SLE involves your own body recognizing itself as a foreign substance. It makes antibodies against itself which promote the body's self-destruction.",
"This is exactly what makes treatment so tricky. We have to find a way to try and shut off the body's immune response against itself ",
" reducing immune effectiveness against actual pathogens.",
"So far we have not found an effective way to do this and current treatment is based off of balancing drug dosage (such as steroids) and patient immune competence.",
"To answer your second question, there is tons of research in fields of medicine which have \"incurable\" diseases of western populations. This includes SLE."
] |
[
"I don't know why we don't have a cure, nor how close we are to one. But with ",
"15,800",
" papers showing up in google scholar for this year alone, someone certainly is doing something with lupus research!"
] |
[
"Lupus patient here\nVery little is known about the disease It wasn't until this year that there was a drug that existed solely to treat lupus. Most people treat it with malaria drugs (what I take) and steroids, or chemo in more severe cases. There are people working on it, people send me articles about it every once in a while. I know there's research going on with stem cells and lupus like this one: ",
"http://www.lupus.org/webmodules/webarticlesnet/templates/new_researchlfa.aspx?articleid=1144&zoneid=31",
"http://www.wchstv.com/newsroom/healthyforlife/2517.shtml",
"I know I read one involving rats a while ago I'll have to look for it"
] |
[
"Are communicable diseases rarer among small isolated tribes?"
] |
[
false
] |
It would stand to reason that small isolated tribes would rarely be exposed to new viruses and would almost never experience common colds, the flu, etc. Is there any evidence establishing this to be the case? How common were colds among more isolated Native American tribes, for example?
|
[
"Are communicable diseases rarer among small isolated tribes? ",
"Yes. The most important thing for the persistence of most infectious diseases is population size. When you only have a few people in a community, there's more chance for a disease to go locally extinct when there no non-immune people left or, by chance, the one or two infected people never manage to pass it on. This is called \"fade-out\". In a large community, however, those chance effects get smoothed over so there's always somebody passing it on. There's lots of evidence for this dynamic when you ",
"compare urban versus rural communities",
", for example. ",
"The main exception would be ",
"exposure to pathogens from other species",
". Isolated communities in sub-saharan Africa that depend on \"bush meat\", for example, have greater exposure to zoonotic HIV viruses because they butcher primates for food. There's a lot of blood involved. ",
"How common were colds among more isolated Native American tribes, for example? ",
"Before European contact, North America was a densely populated place",
". That's part of the reason smallpox spread so quickly. Aside from that, though, we're still only in the beginning stages of research on infectious diseases before good historic records. "
] |
[
"As a follow-up question, do you know if the incidence of infection of communicable diseases is increasing or decreasing? Population density is increasing and global contact is increasing, but this is offset by better medical practices (i.e., hygiene, flu shots, etc.)? (Perhaps the question is too general, given the wide variance in population density and practices in different areas and throughout history.)"
] |
[
"Infectious disease are definitely on the down-swing since the 1800s. The vast majority of this is due to better hygiene (sewage and air quality, mainly). Vaccines are the next-biggest contributor. As quality of life improves in developing nations, ",
"this trend will continue",
". ",
"More recently, ",
"emerging and re-emerging infectious diseases",
" have become a problem. For example, ",
"HIV is a very big deal in sub-saharan Africa",
". Political conflicts have also been creating opportunities for \"old\" diseases like cholera to gain new footholds in ",
"refugee camps",
". ",
"TL;DR: Things are getting better in the big scheme of things, but the past few decades have been pretty rocky. "
] |
[
"What aquatic mammals that reside in saltwater habitats drink?"
] |
[
false
] | null |
[
"Do you mean to say \"what do\" or \"which\"?"
] |
[
"My apologies I meant what do."
] |
[
"They drink some seawater, and get some water from their food."
] |
[
"If my biological evolution has befit me to have my sternum and ribs protect my heart and lungs, why has no protection formed for my brain stem?"
] |
[
false
] |
[deleted]
|
[
"Since when is the brainstem not protected? ",
"Isn't it inside the base of the skull?\n",
"As I understand it, ribs aren't purely for protection. They are also needed for lungs to properly inflate and deflate."
] |
[
"Additionally, without external pressure, several internal organs are just sloppy messes that don't hold their own form, where as the brain stem is far more structured and does not need the external force the maintain its shape. "
] |
[
"I stand corrected and shall thus delete my thread."
] |
[
"Could a planet be size of a galaxy? Whats the biggest one could be?"
] |
[
false
] | null |
[
"At a certain size, the planet would have so much mass that it would collapse and form a star, and if you added even more mass it would easily become a black hole. The cutoff point between a planet and a star (specifically a brown dwarf star) is around 11-16 jupiter masses."
] |
[
"Once a planet gets barely larger than jupiter it will have enough mass to collapse into a star. ",
"https://en.wikipedia.org/wiki/Super-Earth",
"As you see even so called 'super earths' are not insanely big.",
"BTW: a planet the size of jupiter might be possible but earthlike life would NEVER be possible on it. Gravitational forces alone would turn what we know as organic life into meat sauce."
] |
[
"It is actually 13.6 Jupiter masses before a gas giant would begin burning deuterium and enters the \"failed star\" classification of brown dwarf. That is for gas planets. For rocky planets it is as per the wiki."
] |
[
"Why is the ambient heat in the atmosphere not used as an energy source?"
] |
[
false
] |
I noticed that heat pumps are capable of using, say, 1kW of electricity to move 3kW of heat into a room. See: No laws of physics are broken because they are simply moving the heat from one place to another, not creating it. Why then, is there not a way of using a heat pump to boil water and generate electricity from the heat in the atmosphere? It would get energy from the sun the same way solar does, but not be limited to working at day time. Is there any reason this hasn't been done? I am sure I am missing something.
|
[
"From Wikipedia:",
"A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work.",
"We actually do use this process to harvest energy from the atmosphere; nature simply does the \"converting some of that energy to mechanical work\" part for us. We use ",
".",
"In your example of a heat engine boiling water to make energy, the boiling water would be creating steam, which rises because it is less dense than the surrounding air and turns a turbine. Wind is basically the same concept: warmer air rises and creates a difference in pressure in the air, leading to wind. The warm air cools (and cold air warms slightly), and the heat engine cycle is complete.",
"Now to address a misconception:",
"Why then, is there not a way of using a heat pump to boil water and generate electricity from the heat in the atmosphere",
"This could never have a net energy gain.\nThe \"heat pump\" you talked about is actually a heat engine in reverse, and the efficiency of an ideal heat engine is ",
"1 - T_hot / T_cold",
" and the coefficient of performance of an ideal heat pump is ",
"T_hot / (T_hot - T_cold)",
". The coefficient of performance is actually 1 / efficiency of the corresponding (opposite direction) heat engine cycle, so assuming we input x energy at T_hot and T_cold ( assuming T_hot and T_cold don't change temperature significantly), we get at most ",
"x * (T_hot/(T_hot - T_cold) * (1 - T_cold/T_hot = x",
" energy back. In reality, we don't have perfect heat engines so we lose some as \"new\" (not moved) heat and end up with less than x. It's impossible to make more, however, since the COP * the efficiency = 1.",
"The air can still make wind and \"lose\" energy because it's constantly getting more from heat from the sun (which mostly warms the earth's surface and therefore warms lower air more than higher air, leading to the temperature difference).",
"Source: Wikipedia Carnot heat engine page, high school physics"
] |
[
"Not quite what you're asking about but you may be interested in taking a look at the thermoelectric effect, or more specifically the ",
"Seebeck effect",
". This is a phenomenon whereby a thermal gradient (difference in temperature) is converted to an electrical potential that you can use to create current. There are several things making it difficult and impractical to get much usable energy out of realistic temperature gradients, and I could expand on that if you're interested."
] |
[
"Thank you, this explained it nicely. Also, good point about the windmills, somehow I managed to completely miss that connection."
] |
[
"is subatomic particle physics guilty of overfitting our observations?"
] |
[
false
] |
My apologies .. all i know about physics is what i learned some 40+ years ago in school. I have a degree in mathematics and have worked with computers and databases all my life. How boring, right? Recently I stumbled across the Standard Model of particle physics and was taken by aback by how many little subatomic particles that are referenced. When I was a boy the textbook only listed the proton, neutron and electron. Now I see many flavours of quarks, leptons, fermions, bosons. Not to mention umpteen you-name-it-inos being hypothesised. As a mathematician, i do have experience in fitting models to observed data. One of the problems faced in this area is that if you make your model too complex, you run the risk of "overfitting" the data. In essence, if you define as many parameters as you have data points ... you get a perfect fit ... but your model is pretty much guaranteed to be dung. My first impression when I came across so many particles in the subatomic model was that we may be going down a route of overfitting our observations. I'd like know what our particle physics hipsters think about this aspect. Again, apologies if this is a silly request.
|
[
"No. Much in the same way that combinations of just three particles (proton, neutron, and electron) explain the hundreds of atoms/isotopes in the periodic table, similarly combinations of just a handful of quarks explain the hundreds of hadrons that have been discovered in particle colliders. The theory is also highly predictive (not just post-dictive) so there is little room for over-fitting. Further more, there is fairly direct evidence for some of the particles in the Standard Model; top quarks, neutrinos, gluons, Z/W/Higgs bosons can be seen directly (from their decay products), and the properties of many hadrons that can be seen directly (such as bottom and charm and strange) are predicted from the quark model. "
] |
[
"The main things are:"
] |
[
"Yes, the way the quarks interact with each other gives another opportunity to describe how the Standard Model is not over-fit. Before the strong force (and ignoring gravity) the (pre) Standard Model contained two forces: electromagnetism and the weak force (which the Standard Model unifies into the electroweak force involving the Higgs mechanism). The way these forces are explained/derived is through what is called gauge theory. Basically (ignoring for simplification the Higgs mechanism) electromagnetism is the predicted result of U(1) symmetry and the weak force the predicted result of SU(2) symmetry, where U(1) and SU(2) are (very) basically the two simplest mathematical descriptions of internal symmetry. Amazingly, the Strong Force (the force between quarks) is ",
" by simply adding SU(3) symmetry. We therefore say the force content of the Standard Model can be compactly written U(1)xSU(2)xSU(3). I find it incredibly impressive and deep and very non-over-fitted, that basically all of particle physics can be motivated from such a simple and beautiful construction. "
] |
[
"did the ancient romans realize their society had collapsed?"
] |
[
false
] |
I was just wondering. It seems like they'd be in serious denial for decades, and then people would have just stopped caring. Does anybody think they know?
|
[
"Ancient Rome did not collapse it merely shifted to the east, Constantinople and the Holy Roman Empire of the Catholic Church are both offshoots of the Roman Empire, the \"collapse\" as it is understood by most people, is a myth, Rome was sacked and burned by barbarians but the bulk of Rome's power and influence had already shifted eastward. ",
"http://en.wikipedia.org/wiki/Fall_of_rome",
"And yes the people in western Rome were well aware of the decline in power and influence in the region, but keep in mind this was taking place over many centuries. So to the average person - avg lifespan 40 yrs, it was not something that could be easily observed. "
] |
[
"You might get a better answer at ",
"/r/AskHistorians",
" "
] |
[
"As Uzukiseed pointed out, you first have to consider the average lifespan of it's populace, and then consider how quickly the change happened. Also consider that there was no real \"news\" organization, or at least, it was very slow by modern standards. ",
"Consider the United States. It's changed immensly in the last 100 years. If you had no news broadcasters, all you'd really know about America's was that it went to war 4 or 5 times, and which leaders got elected. (other than technological changes). "
] |
[
"Resistance from a FET source-drain voltage vs. current curve??"
] |
[
false
] |
If given source-drain voltage vs. source-drain current curves for different gate biases, how do I find the resistance of the semiconducting materials? Edit: I'm trying to do this for a nanowire FET sensor, but I can't find any literature on the theory of how to calculate it for nanowires. So conventiona FETs will do.
|
[
"The effective resistance of your nanowire at any point on the curve is just V/I for that point. This resistance will change due to channel pinch-off and other effects, but is relatively stable for low absolute Vds (i.e. prior to saturation) differences given a fixed gate voltage."
] |
[
"If you know the carrier mobility and doping concentration you can ",
"calculate the conductivity",
". Knowing the dimensions then lets you ",
"calculate the resistance from this",
". Note that ρ (rho) in this equation is 1/σ (1/sigma) where sigma is your conductivity.",
"Electric fields complicate things by altering the carrier density in different parts of your nanowire. Without knowing anything about the FET it is hard to provide any advice, and my exposure to these problems is limited to silicon IC implementations. My guess (from looking at some of the ChemFET articles) is that it is a depletion-mode FET you're working with, whose resistance increases when the compound to be sensed is present.",
"If you just want to ensure your FET is operational, then perhaps you should just plot the V-I graphs when the compound is absent or present and see if it acts like a FET. The standard FET equations include abstractions like \"threshold voltage\", rather than physically modelling the formation of the inversion layer, etc.",
"There seem to be quite a few ",
"papers",
" that examine the properties of nanowire FET. A google scholar search or following the citations and references from articles you already have on the subject is probably your best bet at finding the information you are after."
] |
[
"I'm not sure exactly what you are asking. Are you looking for the effective R(DS) of a FET? Or something more intrinsic like carrier mobility? Resistivity in a FET channel is dependent on a lot of variables, so any single value you assign will only be accurate under specific conditions.",
"EDIT: ",
"/r/electronics",
" has more experts in this field. If it is research-related I can give you contact details for some semiconductor experts who will be able to assist you."
] |
[
"Why is heterochromia so common with cats, but so rare with humans?"
] |
[
false
] |
What's making the difference? I think heterochromia is a beautiful mutation, both animal and human-wise, but I can't help but wonder why cats have it so often compared to humans.
|
[
"is it more common in cats? Do you have a citation to back up this assertion?"
] |
[
"The reason is simple, people like the look in cats a lot, so cats with heterochromia are bred with other cats with the same gene, making about 5-10 kittens who will most likely have the gene. In humans, for long periods of time it was seen as a symbol of a witch and people could be killed, so it became rarer, and it is rather uncommon for 2 people with it to have kids, and even if they do, they wont have nearly as many kids. It just has to do with how often those with the gene breed, and how many offsprings those with heterochromia breed."
] |
[
"I don't have any citations, it's just an obversation I've made. "
] |
[
"Why don't birds fall of trees when they sleep?"
] |
[
false
] | null |
[
"Birds in the order Passeriformes have a specialized method for this. They have flexor tendons in their legs which, when perched, automatically closes their toes which allows them to grasps branches, wires, etc. with no extra effort. Passerines account for about half all birds so there are many that are not able to perch effectively. "
] |
[
"Woodpeckers will typically sleep in holes or creases in trees. One could deduce this is done because they're able to create a space if proper cover isn't around and provides better protection than simply perching on a branch. ",
"Owls are a bit different. They're nocturnal and are most active at night. During they day when they sleep, their sleep patterns are rather strange. The majority of the time spent sleeping is in shallow sleep which keeps them alert to their surrounds, but they do have short periods of deeper sleep sprinkled in, lasting less than a minute. "
] |
[
"Owls, at least, do have feet that are in a closed position when relaxed, so they are able to grip branches without much actual effort and sleep while perched on branches (they don't sleep soundly in long stretches like we do, though). Their feet are ridiculously strong, more so than other similarly-sized birds' feet. Great horned owl foot strength is comparable to bald eagle foot strength, and bald eagles are about 2-3x bigger than great horned owls. ",
"If a great horned owl is grabbing you, it's not going to let go until it wants to, because it can exert so much force that you won't even physically be able to pry its feet open. I train a pretty big female great horned owl who has a terrifying grip (I have scars to prove it). The first time she went for an annual vet visit, the vet tried to trim her talons (routine maintenance for captive raptors) but ended up having to briefly anesthetize her because she kept her feet clamped shut and none of us were physically able to open them! ",
"Part of the reason owl feet are so impressively strong is the zygodactyl toe arrangement, with one outer toe that can rotate to face either to the back or front...so they often grip and attack with two toes facing front and two facing back, which distributes force more evenly and gives them a sturdier grip. "
] |
[
"What exactly is electron and neutron degeneracy pressure?"
] |
[
false
] |
I'm still confused about electron and neutron degeneracy pressure. I know when a star is below 1.44 solar masses it undergoes electron degeneracy pressure anything bigger and it undergoes neutron degeneracy pressure and eventually at a certain point a black hole would be the result. I'm not sure exactly what's going on in these degeneracy pressures and how they are able to keep a star from collapsing for so long. What I think from what I've read is that in electron degeneracy pressure the star compresses only enough until the electrons refuse to get any closer and hold the star as a white dwarf. Does this mean the electrons are all squished together and are actually touching? If so does that mean in neutron degeneracy that the electrons are compressed inside one another? How does this support system provide energy and light? How does it collapse compared to the regular collapse of a star that's used up it's fuel and why can the star remain in this state so much longer than the regular star can before collapsing? I really appreciate any responses. I hope my theory isn't too stupid but I'm really grasping at straws because I'm just not sure how it works. Thanks in advance. :) Oh and I'd also appreciate it if you could dumb down the language for me, I'm pretty dense :S
|
[
"There are some theoretical ones but they haven't been shown to exist.",
"http://en.wikipedia.org/wiki/Quark_star",
"http://en.wikipedia.org/wiki/Electroweak_star"
] |
[
"I'm sorry but how exactly can that force be overcome when It's essentially impossible to have two electrons in the same quantum state? Am I missing something?"
] |
[
"I'm sorry but how exactly can that force be overcome when It's essentially impossible to have two electrons in the same quantum state? Am I missing something?"
] |
[
"Why is there no \"Human\" equivalent of dogfood or catfood?"
] |
[
false
] | null |
[
"There is, ",
"Ensure",
" for enteral feeding and ",
"TPN",
" which is parenteral, meaning it goes straight into circulation."
] |
[
"Well the dog food is manufactured from lots of other foods. If you watch any commercial they show you that they are made up of meats, vegies, etc. Conceivably, a really good meal replacement bar could do the same thing as long as it had all the vitamins and minerals we needed. The challenge in doing this is that humans is due to our more sophisticated/complicated appetite. Hunger can drive humans to eat less than appealing things, but appetite is what makes us crave variety in smell, taste, texture, etc. Even if science perfected \"man-food\" to answer the hunger/biological aspects of consuming food, it would also have to tackle the psychological/sensory aspects, and at that point the diversity would take us away from a one-food solution.",
"SOURCE: This specific topic was covered in a college level nutrition course."
] |
[
"This is AskScience, so if you're going to make a claim like that, be prepared to back it up with sources."
] |
[
"Could there have been other intelligent life on Earth, and we missed the evidence? (No spoilers for any movie I'm aware of)"
] |
[
false
] |
I'm going to step through my reasoning - please let me know if I've gotten something horribly wrong. As I understand it, it took just 65 million years to go from hamsters to humans. The Earth is four billion years old, and multicellular life started three billion years ago. While we have a decent fossil track from there to here, that track is pretty much made up of a relative handful of fossils here and there, and some strong inductive reasoning. Also, the fossil record is dependent on conditions, so there are areas of the earth where we have no fossil record. So - is it possible that, say, a billion years ago there was an offshoot of life that developed into intelligent life forms and died off or wiped themselves out over a few hundred million years, and either all evidence is gone, or we haven't found it? (of course, I'm interested in both "is it possible" and "is it likely")
|
[
" intelligent? I suppose so (though probably not a billion years ago). Intelligent and civilization-building? Almost certainly not. Though any structures or other direct signs of intelligence and civilization would be long gone, any industrial civilization would leave permanent traces in the geological record, and no such anomalous traces exist, to my knowledge. See ",
"Anthropocene Epoch",
"."
] |
[
"All evidence is that a billion years ago, life was barely multicellular..."
] |
[
"Can you elaborate on what sorts of traces there might be?",
"Something like this, perhaps: ",
"http://en.wikipedia.org/wiki/Yucca_Mountain_nuclear_waste_repository"
] |
[
"Question about relativity and the beginning of time."
] |
[
false
] |
I was having a conversation with someone about cosmology, and suddenly it occurred to me: if an infinite amount of material was compressed into a singularity at the beginning of time, wouldn't time have traveled infinitely slowly back then as well? We now say that the universe is 13-odd billion years old based on our arbitrary measure of times that the Earth has traveled around the sun, but from a relativistic standpoint, if time traveled infinitely slowly in the beginning, doesn't it also make sense to say that the beginning was infinitely long ago? I ask because the person I was talking with was speculating about what there was before the Big Bang (or rather, before the singularity). It made me question whether it made sense to argue that there was anything before the Big Bang. After all, if you were alive during those first few moments, with time traveling so slowly, wouldn't it still have seemed like the initial expansion was a long, long, long time ago?
|
[
"But isn't energy just another form of mass, where E=mc",
" Does energy in high enough concentration warp time?"
] |
[
"Well, that's not entirely true, the expansion was something like 370 million years to hydrogen. That was a long time. But most of it was very, very sudden. The laws of physics such came into play almost instantly as there was energy, now what does it do. So they were very instant. The one I personally am confused about is gravity, and gravity needs mass. The world was pure energy initially. Energy =/= Mass."
] |
[
"Couple of points to clarify!",
"if an infinite amount of material was compressed into a singularity at the beginning of time",
"The big bang was not a singularity; matter was infinitely dense at all points in space. It was the expansion of space and time everywhere.",
"We now say that the universe is 13-odd billion years old based on our arbitrary measure of times that the Earth has traveled around the sun",
"This is certainly not how cosmologists calculate the age of the universe =) One simple estimation is to look at the units of Hubble's constant (72km/s/Mpc found via Hubble's law (v=HD)). The units are km/s/Mpc, which are simply Length/time/Length, or 1/time. If you invert that you end up with time in seconds, convert to billions of years and you end up with a number close to 14 billion. ",
"Of course you can get a little more technical and calculate the lookback time via t_L=t_H integrate(dz'/((1+z')*E(z')),0,z); if you choose a redshift large enough you'll end up with the proper age of the universe given the accepted cosmological parameters that make up E.",
"There are also other independently verified observations that allude to the age of the universe, mainly the cosmic microwave background radiation (CMB). This was energy emitted as UV radiation when the universe became transparent at ~380,000 years after the big bang. The universe has expanded to the point where the photons have been redshifted into the microwave band.",
"And we have no idea what was before the big bang, if anything. From what we know time started at the big bang, so I guess talking about what was before the big bang has no actual meaning."
] |
[
"How is the Lorentz Factor in special relativity derived?"
] |
[
false
] |
Specifically, can it be derived mathematically from the postulates of special relativity or does it require physical observation of space-time dilation?
|
[
"It can be derived directly from the postulates of SR, and simple mechanics. ",
"Here",
" is Einstein's original paper."
] |
[
"Thank you!"
] |
[
"IIRC, that factor can be very simply derived from the \"light bouncing between two mirrors on a rocket\" thought experiment. It's simple Pythagoras in that scenario that yields the Lorentz factor."
] |
[
"I've heard that mantis shrimps have 12 types of photoreceptors in their eyes, while humans only have two. Can we understand or even fathom the additional colors that the shrimps can see? How there be additional colors not on the color wheel that humans cannot see?"
] |
[
false
] |
It's my understanding that the shrimps can see colors that humans cannot. How is this possible? Where would these colors lie on the color spectrum and is there any way to discern them, at least theoretically? Thanks!
|
[
"The color spectrum only contains the spectral (monochromatic) colors, i.e. colors made by light of a single wavelength. These are the same for you and the shrimp. But one can also create lots of colors by mixing light of various wavelengths and intensities. Many of these mixtures look the same to us, e.g. the color \"yellow\" could be made by mixing red and green monochromatic light, or by simply using yellow monochromatic light. These two \"yellows\" are physically different; they would look the same to you but not to the shrimp.",
"In general, to uniquely specify a light source physically, you would have to specify the mixture of spectral colors and their intensities. Lots of different such light sources look identical to us; we map the infinite-dimensional space of such light sources to a 3-dimensional space of human colors. The shrimp maps it to a 12-dimensional space of shrimp colors. So the shrimp has much richer color experiences, but both human and shrimp are missing infinitely many distinctions that the physicist knows about."
] |
[
"The shrimp maps it to a 12-dimensional space of shrimp colors. So the shrimp has much richer color experiences, but both human and shrimp are missing infinitely many distinctions that the physicist knows about.",
"That is wrong. You should never assume that the number of color receptor types in the eye of a creature corresponds to the dimensionality of its perceived color space. Mantis shrimp may have 12-16 receptor types, but not the brain power to build a 12-dimensional space. They do perceive a wider part of the EM spectrum, including ultraviolet, but they do it more coarsely - they have a worse color discrimination ability, i.e. two lights that emit slightly different spectra and look differently colored to us can look identically colored to them!",
"Source",
", ",
"article discussing it",
", and ",
"another one",
".",
"\nKey quote by one of the researchers: \"[mantis shrimp are] definitely not seeing the world of color in as much detail as other animals\".",
"\nAnd here's ",
"research about butterflies",
" with 6-8 receptor types being \"only\" tetrachromats (4-dimensional color space).",
"Furthermore, mantis shrimp have ",
"compound eyes",
", consisting of about 10000 ommatidia (eye units) per eye. A human eye has six million or so cone cells plus tens of millions of rod cells (for low-light vision). If you look at a ",
"mantis shrimp eye",
", you'll notice the \"midband\", a strip across the eye that is merely six ommatidia wide. The special photoreceptors of the mantis shrimp are located only there. Rows 1 to 4 have color receptors, rows 5 and 6 polarization receptors. The left and right halves of their eyes are basically colorblind. All of that means that mantis shrimp have a really low resolution / acuity compared to us. They do have a wider field of view, though.",
"Finally, color is a perception created by the brain, not a physical property. It's very hard to say what a creature does actually \"see\" in its mind. You and me are probably perceiving color slightly differently already, even though we are of the same species."
] |
[
"Great answer! I hope you won't mind some nitpicking.",
"Mantis shrimp see with not just 3 cones though, but with 16, 12 of which are color sensing cones with the other 4 being for color filtering.",
"Mantis shrimp do not have \"cones\". The photoreceptor cells in our eyes and those of some other vertebrates are called \"cone cells\", because they look that way. Mantis shrimp have compound eyes, which have ",
"crystalline cones",
", but that's not the same thing. Instead of \"cones\", you should call them pigment cells, or simply photoreceptors.",
"\nThose mantis shrimp that do have 16 types use 12 for color and 4 for polarization vision.",
"red-green colorblindness. This is caused by the green cone being defunct",
"That would be deuteranopia or deuteranomaly. But you can also have red–green color blindness with the L cone (\"red\") being defunct (protanopia, protanomaly).",
"There are some humans with a 4th cone as well. It is very rare and unfortunately many people who do have it don't use it because all of our technology, our screens and displays and such, use the 3 colors of our cones in varying degrees to \"trick\" our brains into seeing what ever color we want to. Without the need to use that 4th cone they never learn to use it. Usually those that do are people like artists, florists, or interior designers, people who are used to distinguishing between different colors and are exposed to large varieties of them. ",
"That sounds quite wrong. We aren't staring at screens 24/7 (I hope), so there should be plenty of opportunity to \"learn to use\" the additional cone. As you said, tetrachromacy is very rare. It took the researcher Dr. Jordan 20 years to find a woman with functional tetrachromacy, and I wouldn't blame screens for that.",
"\nIf somebody claims to be a a tetrachromat, I'd take it with a big grain of salt. Artists, florists, designers, etc. might be able to differentiate between colors better than the average person simply because of training, not because of tetrachromacy. ",
"See this,",
" for example.",
"Also, even for functional tetrachromats, claims like \"this woman sees 100 times more colors than the average person\" / \"99 million more colors\" are ridiculous. These figures come from a very rough estimation: a single cone can differentiate between about 100 different excitation levels, so if you have 3 cone types, that makes 100",
" = 1,000,000 colors. And with 4 types, 100",
" = 100,000,000 colors. However, that would only be true if all cone types were perfectly independent of each other. Which they aren't. Their outputs get combined in the ",
"opponent process",
", and it's not clear how a 4th cone type plays into that. Also, the sensitivity ranges of the cone types overlap, particularly for the M and L cones (\"green\" and \"red\"). Given the fact that the sensitivity curve of the 4th cone in human tetrachromats sits right between M and L, it is doubtful that it magically adds 99 million more colors. Here are the ",
"sensitivity curves",
" published in Dr. Jordan's research.",
"A good way of illustrating that would be through this test. A tetrachromat would have no difficulty with that test and may be able to even perform a similar test with even more gradients between the two poles.",
"Didn't you just say that our screens are unsuitable for tetrachromats? This test is good, but your performance depends a lot on the quality of the display you are using. Perhaps tetrachromats might perform better/faster at this test, or maybe not at all, because of the trichromatic nature of display technology. To determine whether someone is an actual, functual tetrachromat, you need a special lab test.",
"This means that they can not only see many different colors and lights but also the origin of light (polarization) as well as being able to see multiple aspects of the spectrum and distinguish between them all at once (multispectral).",
"Polarization isn't the origin of light, it's the plane/mode of its \"vibration\". But I see what you mean, with light bouncing off surfaces changing its polarization. And mantis shrimp vision is not multispectral, according to ",
"newer research",
". Also, mantis shrimp have compound eyes which are rather low-resolution compared to ours, and their special color and polarization receptors are confined to a narrow region, the ",
"midband",
". They're definitely not seeing the world of color in as much detail as other animals, as one researcher put it."
] |
[
"Are all (or most) Planck units equal to 1?"
] |
[
false
] |
I'm not talking about the normalized constants (e.g. c, G, h/2pi) but rather the length, time, mass and other units. For example, if we have sqrt(h/2piGc ) = Planck's length, and given that h/2pi = G = c = 1, wouldn't Planck's length equal 1 as well? The same would happen to mass, time, etc. This would result in h/2pi = G = c = ... = mp = lp = tp = Tp = Ep = ... = 1. It seems mathematically obvious, yet I haven't seen this stated explicitly anywhere, I've always seen all these units as lp, tp, mp, Tp... Any help?
|
[
"Yes, there's one Planck length in a Planck length... How many kilograms are there in a kilogram? How many metres in a metre?",
"The word \"unit\" comes from the Latin ",
" meaning the number one."
] |
[
"I don't mean 1 as in unit, I mean 1 as in mathematically equivalent to 1, i.e., that can be replaced with \"1\" anywhere in the equation.",
"In the metric system, the kilogram is not equal to 1, it is only equal to itself and the other units derived from the kg and others (m, s, A, etc.)",
"On the other hand, the Planck unit system is created by \"defining\" c, G, h/2pi, k and 1/e4pi as 1, so, stating that they are equal to one. The length, mass and other units are not ",
" but ",
" from those previous axioms. The way the constants are normalized and the equations for the magintudes are made, it turns out that those units are also equal to 1, but whereas it it explicitly stated that those constants are 1 under that system, the same is not said about the units themselves."
] |
[
"Oh, then it must be that. Now I wonder why wouldn't anyone writing about normalizing constants clearly state that. Why just not say \"we'll use c as our unit for speed\" instead of saying that we \"normalize it to 1\" and then in all equations containing c leave it as it is? Not only it'd allow that formula to be read by people who are not aware of the unit system in use but it'd also let you know the correct dimensions of a variable just by reading instead of guessing. There are some conventions that I can't seem to make any sense of.",
"Anyways thanks for your answer, as you can see I don't know much about physics and I assumed that's how it'd work."
] |
[
"If surface area doesn't affect friction, why do spinning tops have a tip?"
] |
[
false
] | null |
[
"Tops are slowed by torque, i.e. a force applied at a distance. The closer that distance is to the axis of rotation, the less torque is applied to the top. So by giving the top a tip, it experiences the same frictional force at a much shorter radius, and thus a much lower torque compared to say, an inverted cup where all the force is on the rim"
] |
[
"To add: A point also has lower velocity at the contact surfaces. Power is force * velocity. A point reduces the force and the velocity."
] |
[
"\"A point reduces the force and the velocity.\" The point doesn't reduce the force. As you said, it reduces the velocity. Or as zeraul98 said, it reduces the torque.",
"For example, lets say you had a \"top\" that weighed 3 N (I put \"top\" in quotes because this is going to make a very poor top). Now instead of a single point, say it has 3 points (or feet if you prefer) arranged symmetrically about the center of the top. With this arrangement, each point will have 1 N of force on it. If the sliding coefficient of friction is 0.5, then each point will have a 1/2 N frictional force.* It doesn't matter how far from the center of the top the points are -- 1 mm, 10 mm or 100 mm -- the frictional force will be the same. ",
"The torque will be different, and the velocity will be different (for the same RPM), and the distance traveled per revolution will be different. But these are basically 3 different ways of describing the same physical situation.",
"* Note: For simplicity, I'm ignoring the fact that at any significant rotation speed, a top with 3 contact points will likely wobble a lot with only one point touching a lot of the time. That's a different topic."
] |
[
"Is infinity in one direction (a ray) shorter than infinity in two directions (a line)?"
] |
[
false
] |
"Infinity in One Direction? Shoot me now."
|
[
"Almost none of the words you've described are unambiguously defined. ",
"I'll assume you're talking about sets of real numbers. ",
"By \"infinity in one direction\" I'll assume you mean something like the positive reals, or the interval (0, infinity). By \"infinity in two directions\" I'll assume you mean the whole real line. ",
"Now what do you mean by longer? ",
"These two sets have the same cardinality, in the sense that there is a bijection between them (log, in this case, is a bijection from (0, infinity) to ",
"). ",
"Neither of them have a well-defined length, so in one sense the question of \"do they have the same length\" is not meaningful. One can put a measure on the real line in which the intuitive idea that the whole line has twice the measure of the ray is true. One can also put a measure on it in which the ray has any nonzero and not full fraction of the full measure. "
] |
[
"They are the same 'size'.",
"When we talk about different infinities, we usually use mapping to describe their sizes. For example, consider the positive real numbers versus all real numbers. Because I can create what we call a 'bijective' map between the two, they are the same.",
"What is bijective? Bijective means the mapping is one-to-one (injective) and 'onto' (surjective). Injective means each x is only mapped to one y. Surjective means that all of the X space maps to all of the Y space.",
"Specifically, in this case, let's take the map from the positive real numbers to all real numbers:",
"f(x)= ln(x)",
"This function maps positive real numbers to all real numbers, which you can see in ",
"this graph",
".",
"Note, a bijective mapping means there is an inverse map, which in this case is x=Exp[y].",
"For more reading, look at the wikipedia page on ",
"bijective mapping",
" and ",
"countability",
". If you have more questions, or want more reading, let me know!"
] |
[
"OP may enjoy the concept of a ",
"Hilbert hotel",
", in which a hotel with infinitely many rooms may always accept one more guest ",
".",
"Infinity is not a number. It is an idea, and throwing it into a system of numbers causes some strange results like this one."
] |
[
"AskScience: Why do cameras struggle to deal with high-contrast images that the human eye resolves easily?"
] |
[
false
] |
This question struck me tonight when I tried to take a picture of the beautiful crescent moon and them again when I wanted to take pic of my daughter by a lamp (the light from the lamp blew out the picture). Why does the human retina easily resolve images that cameras can't deal with? Is it some mechanical difference? Is is something structural in the eye?
|
[
"First of all you have to keep in mind that your vision is continuous -- more analogous to video than to still, and while your iris adjusts fairly slowly, other adaptations to the level of illumination are much faster and will change as you shift your gaze across a scene. This means that when you perceive a high dynamic range scene, you typically have looked separately at different parts of it. ",
"Second, ",
"your vision is highly non-linear, which helps preserve shadow detail in comparison to bright high-lights",
"Third, consider the area of the human retina -- it is about 1100mm",
" That's a bit larger than a standard full-frame digital camera -- ",
"cameras that do not really \"struggle\" with high-contrast scenes",
"*Edit: Forgot to mention -- neuroscience is not my area of expertise, but digital cameras are. *"
] |
[
"There are a couple of things going on with your visual system that help explain this.",
"First, I know it's an old trope, but you see with your brain not with your eyes. Your eyes are actually really bad at seeing detail and color anywhere but in the dead center (the fovea). Not to mention the fact that your raw vision is obstructed by the famous \"blind spot\" where all the nerves come in and numerous blood vessels on top of the retina. The upshot of all this is that your perception of the world is entirely constructed by the brain via memory and continuous series of small field \"snapshots\" you get from your eye darting around a scene (these movements are called saccades). When you look at a high contrast shot, you (unlike a camera) can auto adjust your \"exposure\"'to every spot on the field of view.",
"On the flip side, your eye is actually intrinsically better at seeing high contrast than a camera. We all know we have pupils that set the aperture of your eye, but the retina itself compensates for brightness as well. The light sensitive proteins that trigger neuron firing have a finite refractory period after getting energized by a photon. This means that in high light environments, there are fewer receptors around to be stimulated in a given instant (and is also why it takes time to get night vision back after a flash of light). Ultimately, the \"gain\" between light intensity in to sensory stimulation out is a logarithmic curve and doesn't easily saturate at some max value. Cameras emulate this curve but ultimately, the electronics do saturate at a much lower level. The term for this is \"dynamic range\" and rigorously speaking, it is the ratio of the brightest possible input without saturating to the least bright without going black. All this to say your eye even in a static shot has a higher dynamic range than is possible on a camera sensor. \n",
"http://www.cambridgeincolour.com/tutorials/dynamic-range.htm"
] |
[
"All of the answers here have touched on the answer, but I think it can be summed up better.",
"You camera probably has a 12 bit digitizer to read off the CCD, so that at each pixel the \"brightness\" (term used loosely) can be coded between 0 and 4095. That is to say, black being 0, and bright being 4095. If you were limited to this, your camera would have a very poor range, however, because you can alter the exposure time or the aperture size, 0 might code for pitch black and 4095 coding for say, the brightness of the room... or 0 might be for room brightness and 4095 coding for the brightness of a halogen lamp. The point is, you camera (in various auto modes) tries hard so it doesn't spend a lot of time reading 0 pixels, or a lot of time reading saturated 4095 pixels. It tries to find a combination of settings where most of the pixels are seeing something, without too many being saturated. But ultimately, the algorithms will always have a hard time if you have a very bright source in a dark scene, because it will try to avoid saturating the bright pixels, and mean that the dark pixels will get almost nothing (not even including CCD blooming, where charge spills off one pixel, onto the neighbours on its row). Put another way, in a 12 bit camera, the smallest increment in brightness the camera can see is about 1/4000th of the brightest think in the scene. Expensive cameras can have 16 bit digitizers, meaning that each pixel brightness is coded between 0 and 65535, giving much more range... but still, the underlying problem is still there.",
"The eye still suffers from this, but not as badly, because each rod/cone, which are basically pixels, can alter it's signalling due to biochemical systems in the rod/cone that mean they signal more weakly once exposed to bright light. Moreover, there is no digitizer in the system. Finally, a camera reads off all the pixels synchronously, meaning that each pixel gets the same time to integrate the light signal. The retina does not do this, and each rod/cone can signal whenever it wants. However, the issue is still there. You can't make things out when there is a very bright light behind them, because your pupil closes to protect your retina, meaning the rods/cones seeing the darker parts of the image don't get much light.",
"TL;DR. Cameras digitize the data from the CCD, meaning that brightness can only take a small number set values. The eye does not do this, and can change the sensitive of the individual rods/cones."
] |
[
"Why don't celestial objects rotating around other celestial bodies (like the earth, rotating around the sun) end up colliding with the thing they're orbiting (like a marble, ending up in the center of a sloped surface)?"
] |
[
false
] |
I ask because i just saw some images of how gravity actually works, with gravity wells surrounding massive objects. So i got to wonder how something like the earth, doesn't eventually fall into the gravity-well of the sun?
|
[
"The other comments are being needlessly confusing. The marble ends up in the center of the sloped surface (rather than swirling around the edge forever) because it experiences friction. A frictionless marble would in fact be able to circle on the side of a depression forever.",
"Earth in its orbit is for all practical purposes not experiencing any friction whatsoever, so its orbit is stable and it doesn't crash into the sun."
] |
[
"Oh no, that's not really how gravity works. The gravitational attraction is what keeps Earth in a stable orbit, instead of Earth flying away in a straight line. The great thing about orbits is that they are perfectly stable. After one year (one orbit), Earth is back ",
" where it was the previous orbit. If Earth would go a bit faster or a bit slower, it would change the path of the orbit a bit, but it wouldn't change the fact that orbit would be closed.",
"If you want an analogy with that, imagine holding a ball at the end of a rope and spinning it around. Because you're pulling on the rope from the center, the ball at the end keeps spinning around you instead of flying away, but at no point would the ball actually collide with you, no matter how long you keep pulling the rope.",
"DISCLAIMER: that analogy is quite limited."
] |
[
"It depends, really. The analogy of the marble on a sloped surface stops here. This is the point where you have to take into account some really complicated effects in order to know what Earth's orbit is doing. Think about the emission of gravitational waves by Earth, the decreasing mass of the Sun due to nuclear fusion and the solar wind, Earth's drag through the interplanetary medium, etc...",
"Not accounting for those effects, but just considering Newton's theory of gravity, there's no reason why Earth couldn't keep orbiting forever. What exactly makes you think Earth has to get closer?"
] |
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