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[
"Why do our pupils dilate/constrict when under the influence of CNS depressors and stimulants?"
] |
[
false
] | null |
[
"Stimulants typically cause a release in norepinephrine, a crucial neurotransmitter that is normally involves in the sympathetic nervous system that mediates the fight or flight response. As part of that response, the pupils dilate to allow more light to hit the retina, hopefully providing a clearer image of whatever threat is being perceived so that the threat can be most accurately appraised"
] |
[
"It isn't all depressants and stimulants. ",
"Pupil dilation is regulated by the nervous system, not the eyes. So it makes sense that some psychoactive drugs will affect pupil size. ",
"The drugs that most powerfully impact pupil size are those that increase activation of the 5-HT2A receptor in the brain. 5-HT2A is a serotonin receptor. Those would be MDMA (a serotonergic stimulant) and all classical psychedelics (LSD, psilocybin, mescaline, ayahuasca). It seems like activation of this receptor produces pupil dilation. I am not exactly sure why. Pupil dilation may be an effect of serotonin in certain areas of the brain: serotonin syndrome is also known to dilate the pupils.Stimulants that increase dopamine and norepinephrine activity are associated with some, but less pupil dilation. For example, cocaine and methamphetamine can cause pupil dilation- but not nearly as much as MDMA. If I had to guess, increased arousal and/or wakefullness should increase pupil size to let more light in- evolutionarily speaking. But I am not sure why pupil size responds more to serotonin impacting drugs than dopamine or norepinephrine. ",
"As far as depressants- that is mainly opioids (which bind to the same receptor as the endorphins: the mu receptor). Alcohol does not really affect pupil size much and benzodiazepines do not cause the pinpoint pupils of opioids. Antihistamine/anticholinergic depressant drugs actually dilate the pupils."
] |
[
"Eye is mostly under the influence of parasympathetic nervous system,so if you take a ganglion stimulants this increases the parasympathtic influence(that causes conctriction),if you take a ganglion inhibitors that depresses the already in charge parasympathetic system(that causes dilatation)",
"Sorry for my english"
] |
[
"Why is poor ecosystem peat moss used in potting soil?"
] |
[
false
] |
I've always thought it odd that peat moss harvested from some of the slowest growing ecosystems (bogs, or ancient bogs), is used in potting soils, sometimes at greater then 50% of a mix. Why?
|
[
"Peat moss is used not because of its nutrient content, but rather for its ability to absorb and retain water. Also because it allows roots to expand into the soil stratum more easily due to the low large pore size/low density of peat."
] |
[
"Peat moss adds acidity to soil. Based on species, the plant will thrive or suffer, but most plants love acidity in soil."
] |
[
"Thanks. I was under the impression it was mostly due to water retention. "
] |
[
"Backwards galaxy NGC 4622"
] |
[
false
] | null |
[
"Correct, from the point of view of the image of that galaxy, it would normally rotate counter-clockwise, however it is rotating clockwise. As to how they measure the rotation, unfortunately I'm unsure of that, hopefully a physicist will be able to explain that :) "
] |
[
"Correct, from the point of view of the image of that galaxy, it would normally rotate counter-clockwise, however it is rotating clockwise. As to how they measure the rotation, unfortunately I'm unsure of that, hopefully a physicist will be able to explain that :) "
] |
[
"if the galaxy not not at a 90° angle with us, we can see with redshift that one half is moving away and the other half moving towards us."
] |
[
"How do we take pictures of our galaxy if we are in our galaxy?"
] |
[
false
] |
So we have pictures of the Milky way but we are in the Milky Way? Edit:Rip my inbox Thanks for the replies everyone!!!
|
[
"Assuming you mean plan view, we don't.",
"All images like that described as showing our galaxy are either digitally (or manually) produced images, or images of other galaxies similar to ours.",
"We can see the milky way in our sky, and being on the outer limb of one of the arms we see it as a broad band of stars dominating one hemisphere of sky. It's in profile. We have no photograph taken that shows the milky way in plan view."
] |
[
"It's more like taking a photo of a landscape, using trigonometry to work out how far away the trees are, then drawing a map to show where the trees are relative to each other. You never need to go and touch the trees; it's all just angles and distances."
] |
[
"Oh i get it. It's like taking a selfie of parts of your face and putting them together and photoshopping them and smoothing them out."
] |
[
"Is 95% confidence in a result the same as \"the result is 95% likely to be true\"? Or are confidence and probability different in this aspect?"
] |
[
false
] | null |
[
"In statistics, there is a very precise notion of a \"confidence interval\". This notion is as counterintuitive as it is unhelpful when it comes to communicating statistics to the lay public.",
"As a result, language involving confidence intervals and % confidence often gets repeated without understanding, and distorted in the process. So, depending on where \"95% confident in a result\" shows up, it could mean a lot of different things. In mass media, who knows. In scientific literature, you can usually assume that the scientists used it correctly (but even this isn't always true, because the notion is counterintuitive to the point of commonly confusing even scientists, unless they are more training in vagaries of statistics than many do).",
"In its precise statistical meaning, the notion of \"95% confidence\" is definitely ",
" the same as \"95% likely to be true\". As a result, odds are that whether this shows up in its precise statistical sense, or in some distorted-through-misunderstanding sense, \"95% confidence\" does not mean \"95% likely to be true\".",
"And to answer the inevitable question about what 95% confidence means statistically in a way that I think is helpful to a general audience, the answer is that being 95% confident in the result of an experiment (or a study) means that if the experiment / study is repeated (with a different selection of subjects, whatever that means in the context of the study), you'd expect 95% of the repetitions of the experiment to yield the same result.",
"For example, if I check resting heart rates of 1000 athletes against resting heart rates of 1000 non-athletes and find with 95% confidence that athletes' heart rates are lower, that means that if I repeat the experiment with 99 other groups of 1000 athletes and 1000 non-athletes, I would expect ~95 of those 100 experiments to yield the same finding (that is, that athletes' heart rates are lower). ",
"The difference between that and the result being 95% likely to be true is that 95% confidence is a statement about experiments about something, and likelihood-to-be-true is a statement about that something. And in ways that are beyond the scope of my leisurely evening, those two aren't the same.",
"But if you would like to read more about this (ranging from casual to highly technical), look up \"criticism of confidence intervals\"."
] |
[
"Just to add an example that illustrates the difference between a 95% confidence interval and being 95% certain the something is true, suppose that we want to estimate the average resting heart rate of an athlete. I can construct a 95% confidence interval as follows:",
"Suppose that we flip a coin and get tails -- the result is an (empty) 95% confidence interval, but there is a 0% chance that it contains the true average heart rate."
] |
[
"There are two viewpoints on this, but neither of them is that the result is 95% likely.",
"In frequentist statistics, the true value of something you are estimating is fixed (albeit unknown) so it doesn't make sense to assign a probability for it to be in a certain range. The 95% means that 95% of the time you make a 95% confidence interval (i.e. with a series of experiments), it'll contain the true value.",
"In Bayesian statistics, there isn't a fixed true value, but the estimated quantity is drawn from a probability distribution: in this case you need use Bayes' law to invert the probability statement about the interval to one about the value. Having 95% confidence in something with a very low prior probability will result in a still somewhat low postierior probability.",
"https://en.wikipedia.org/wiki/Confidence_interval#Meaning_and_interpretation"
] |
[
"Why does a dog cock its head to the side when confused?"
] |
[
false
] | null |
[
"I believe it is because it is easier for an animal with ears on each side of its head to localize sounds in a horizontal plane. If there is some mysterious sound they are trying to learn more about, it's hard for them to know how high/low it is without changing the orientation of their head."
] |
[
"If you've had a dog for a long time and have regular verbal commands, try speaking those in distorted pronunciations to the dog, it'll cock its head at you."
] |
[
"If you've had a dog for a long time and have regular verbal commands, try speaking those in distorted pronunciations to the dog, it'll cock its head at you."
] |
[
"Is there a limit to how fast a black hole can spin?"
] |
[
false
] |
[deleted]
|
[
"Yes, there is a limit. The limit of how large its spin can b depends on the mass of the black hole. Its angular momentum J needs to be ≤ M² in suitable units (where M is the mass). If that condition is broken, you get an overextreme Kerr solution (Kerr black holes are rotating ones) which has a naked singularity, ie a singularity not hidden behind a horizon.",
"This post on stackexchange explains it in more detail: ",
"https://astronomy.stackexchange.com/questions/20276/maximum-spin-rate-of-a-black-hole"
] |
[
"Thank you for leading me to discover the “Deep Astronomy” podcast. I have been looking for astronomy and physics based podcasts with content intended for an audience with greater than average knowledge of the topic. I’m no academic but it gets old to constantly hear the sentence “black holes are objects SO MASSIVE that nothing, not even LIGHT can escape them!”"
] |
[
"Thank you. After going down that rabbit hole I've only gained more questions haha. I should have expected this."
] |
[
"Why do fizzy drinks served on planes not fizz everywhere when opened? (Or explode when they get up there?)"
] |
[
false
] | null |
[
"Cabins are pressurized to prevent health problems that arise from being in high altitudes (many commercial planes fly at 40,000ft). They are pressurized by siphoning some of the air that is compressed in the engine (before ignition). The cabins are pressurized such that the maximum altitude you feel is at most 8,000 ft, but many stay below 6,000 ft. At this altitude, the pressure is 75-80% what it is at sea level. That isn't low enough for the gas in soda to depressurize is a violent way.",
"You could calculate how the equilibrium concentration of the dissolved gas in your soda changes with pressure. Or, you could just take my word for it. I live in Colorado, US, and have had carbonated beverages above 8,000ft without any problems."
] |
[
"While flying at such a high altitude, it is difficult to make the cabin pressure so high. Also, most people don't experience health problems until 8000 ft or more.",
"Here is the procedure: The plane starts out at the pressure of the airport it takes off from (say, sea level), then it decreases the cabin pressure as the plane rises to the minimum cabin pressure corresponding to 6000 ft. Then, they increase the pressure as the plane descends to the pressure at the airport they are arriving at (Say, Denver, which is ~5200 ft). Its probably easier for people to get acclimated this way."
] |
[
"The ",
"SyberJet SJ30",
" can provide sea-level pressure at cruising altitude. It also costs $7.5 million.",
"Planes must be stronger to resist larger pressure gradients. "
] |
[
"How does our circulatory system handle losing a limb?"
] |
[
false
] |
Edit: thanks everyone for their comments. And someone in the comments pointed out my view of a circulatory system is pretty bad and probably comes from horrible drawings of it in textbooks. Also I now know more about our body so thanks!
|
[
"Forgive me if I'm wrong but I suspect you are asking this because of a simplified understanding of our circulatory system, thinking that arteries carry blood all the way to the end of our limbs, then the blood is carried all the way back to our heart by veins, and severing this flow means arteries are no longer connected to veins.",
"In reality there are smaller vessels branching off of the main artery constantly which go through your muscles and organs then reconnect to veins at the end. The arteries get progressively smaller as they reach the end of your limbs because of all the branches so they are carrying less blood. Similarly, veins grow larger as they get closer to your heart because of branches entering them and the veins carry more blood.",
"When you lose a limb the artery is suddenly ended and the blood simply goes through the other branches. There is higher blood pressure in that area for a little while as the artery is now too large for the amount of body it needs to feed but over time the system equalizes. "
] |
[
"Follow up question(totally ignorant on the subject). We typically have like 5L of blood in an adult. if you lose a limb(and survive obviously) do you produce less total blood? Makes sense that we would but nice to "
] |
[
"The mechanism simplified is all about meeting aerobic metabolic needs. In other words, \"Is enough oxygen being delivered to the cells of your tissues to allow them to function appropriately?\" If not enough oxygen is being delivered to the kidneys they release a hormone called Erythropoietin that starts the process of, you guessed it, erythropoiesis (AKA red blood cell production mostly within bone marrow). If you lose large amounts weight (and I'm assuming you mean adipose tissue, or fat) your body will adjust the amount of circulating blood to meet minimum need. That being said different tissues in your body require vastly different amounts blood with fat requiring very little (as opposed to muscle requiring large amounts). Therefore, your body would not need to adjust your total blood volume much if you lost 30 pounds of fat as opposed to if you lost a thirty pound limb."
] |
[
"Why is the symmetry of fundamental forces temperature dependent?"
] |
[
false
] |
If i understand this right, above a certain temperature there is a symmetry between the electromagnetic and weak nuclear forces, meaning they operate identically and are for all intents and purposes a single force. Now, really this is just a specific example of a larger concept so i guess trying to understand why temperature is even a factor in any symmetry, or "why do symmetries in general break based on temperature?". I'm not sure if specificity is my friend here. I'm just trying to form some sort of intuition so hopefully this doesn't require getting too technical. If i could get some explanation for how these temperature sensitive symmetries relate to Noether's theorem that'd be the cherry on top. I know in a general sense that physical symmetries correlate to specific conservation laws (or the conservation is the expression of the symmetry? I'm not sure what's the correct way to parse this). so does that mean that above the electroweak symmetry temperature there was some property that was conserved that is no longer a conserved property in our colder reality? if so, I'd love to know what it is if it's articulable. on a more general note, any resource that has a concise listing of symmetries and their corresponding conserved properties would be very much appreciated. preferably something neater than some dense dissertation where I'll get lost in the weeds XD
|
[
"In the vaguest sense, it goes something like this. Consider a potential V(x) = x",
" - x",
". Go ahead and pull up your favourite graphing calculator and plot it. You'll see that it is symmetric with respect to flipping along the y axis, but if you put a marble at zero, it will decay to one of the two wells on the right and left of the origin.",
"At low temperatures, a system is only able to explore the vicinity of a local well of the full symmetry-breaking potential. At high temperatures, this changes, and the whole potential can be explored. (Or, of course, a larger well, that is still symmetry-breaking, but of a smaller subgroup.) ",
"Of course, the example I gave has a finite symmetry group (C_2), and in reality it's a Lie group that will get broken, but the basic idea is there."
] |
[
"Maybe this can help motivate it a little bit. The energy for a field has terms which are conventionally called kinetic and potential terms. These terms are called that because the equations look a lot like linear elastic waves in media, and in those continuum mechanics theories, the terms literally represent kinetic and potential energy densities. The kinetic term is usually a function of the derivatives of the fields, and the potential term of the field value itself, whether that's a scalar, a vector, a spinor, or whatever exotic type of object you want your field to take values on.",
"When we're talking about there being a symmetry-breaking potential, we quite literally mean that the potential term in the energy is symmetry-breaking.",
"E: In fact, have a look at the Higgs sector of the ",
"Standard Model",
" Lagrangian (if you don't know what a Lagrangian is don't worry too much about it, as far as we are concerned here T - V instead of T + V where T is kinetic and V is potential). You can see the symmetry-breaking term ",
"right here",
". (I know that notation is ugly, but the term is pretty much literally (x",
" - v",
")",
"."
] |
[
"my background is materials science, so i can understand the intuition of overcoming an energetic barrier.but let me take you a step back.why does this logic apply to symmetries in the first place?",
"i can understand a phase breaking down into two different phases when the temperature drops, but this seems a LITTLE more fundamental than a free energy calculation.",
"I mean we're talking fundamental laws of reality here.",
"to put it bluntly, it's like if we sat down to play a board game and you tell me if i crank up the thermostat the French and German text in the instructions will merge into Indo-European. or maybe more accurately some of the rules get rewritten so as to make other rules redundant? it's hard to come up with a decent analogy here, but you get my point.",
"like... HOW? i hope this is not a philosophical question. i'm assuming it's not."
] |
[
"How big would the Sahara desert have been 1000 years ago?"
] |
[
false
] |
I know Sahara desert had a wet period a few thousand years BCE but didn't manage to find any information about the rate of desertification in recent (so to say) years.
|
[
"I have been taking a class on Africa and its early peoples. I don't have much to tell you but you should look into the Inter Tropical Convergence Zone (ITCZ). We learned that this line, delineated by the meeting of SW and NE winds across the continent, determines how much and where rain falls. As you can see ",
"here",
", this convergence line has been migrating south (and at an alarming rate) increasing the desertification of Western, Eastern, and Central Africa. ",
"Here",
" is another website with more information regarding the topic and a clearer explanation of the migration of the ITCZ."
] |
[
"It was smaller. Herodotus records many of the areas as being fertile. Yes, I know Herodotus also described flying snakes and ants that could chase down camels, but it is one of the few sources we have to go off of.",
"It is generally believed that the Sahara was expanded by continuous goat farming in the area. Goats eat the roots of plants, instead of just the top growth. "
] |
[
"A thousand years isn't even halfway to Herodotus' time."
] |
[
"Is it possible for an Asteroid to be composed of Gold or other Precious Metals?"
] |
[
false
] |
To my understanding, Asteroids are usually iron/nickel, but is it possible for asteroids to be composed of less heavy metals?
|
[
"statistically, yes. there is so many asteroids that one of them is bound to be made of gold",
"That's not how statistics work. ",
"Do you have a source or anything to back up your claim?"
] |
[
"If you mean only gold or nickel, no. Iron asteroids, which probably represent the cores of small parent bodies will have enhanced amounts of elements that follow iron into the core as it forms (",
"siderophile elements",
"). These include Ni, Co, the ",
"PGEs",
", and Au. Gold in iron meteorites (samples from iron asteroids) rarely has concentrations higher than 2 ppm. ",
"[ref",
"]"
] |
[
"if you take a regular quarter, and flip it a million times, it is bound to to land on tails once",
"No. That's not how statistics work. There is a non-zero chance of flipping only heads. ",
"Also, even if statistics did work that way, it wouldn't be relevant: the analogy is completely inaccurate. The composition of an asteroid is in no way like a coin flip. It is not random and separate outcomes are highly linked. "
] |
[
"Question about dark matter, dark energy and unseen dimensions for reddit physicists and cosmologists"
] |
[
false
] |
Background: Roughly 5% of our universe is directly observable, another 23% is made of dark matter and a whopping 72% is composed of dark energy (percentages are approximate). We can indirectly measure the effects of dark matter - the gravity it exerts bends light and we measure the resulting distortion. We can't measure dark energy but infer its existence by the accelerating expansion of our universe. Also extra (unseen) dimensions are required for String Theory to work. The most promising version of String Theory (M Theory) requires 11 dimensions total, of which only 4 we can observe. Some would argue whether String Theory and M Theory are really theories since they haven't predicted anything and are not testable yet. However since M Theory appears to be the best candidate to explain quantum behavior and gravity in a unified theory, let's assume for the moment M Theory is correct. Question: If these theories are correct, we have 95% of the universe that we can't directly measure and 7 dimensions that we can't observe. Is there a connection here? Is some or all of this unobservable matter and energy possibly tied up in the unseen dimensions of M Theory? I could be way off base here but curious nonetheless.
|
[
"Observational cosmologist/astrophysicist here. ",
"There is no mathematical reason that dark energy and the extra dimensions of string theory would be connected. The dimensions of string theory are confined to the smallest scales, whereas dark energy only exerts its influence on the largest scales.",
"There isn't much connection, apart from stuff being unexplained. We ",
" measure dark energy, we have a very accurate measurement of how powerful it is - we just don't have a theoretical underpinning for it. There has been some attempt to develop a small (quantum) scale explanation for dark energy involving the energy of the vacuum, but it was nothing to do with extra dimensions (and it has failed dramatically, so we have to concede that we don't understand dark energy at all!). "
] |
[
"Thank you for that explanation. Another response from an expert - two for two today.",
"Follow up question to your statement that we can measure dark energy. How is that done? Is the estimate based on the acceleration of the universe's expansion or some other method? "
] |
[
"First of all we should separate the idea of extra dimensions and string theory. Certainly string theory popularized the idea, but if we find extra dimensions it in no way means that string theory has been validated. ",
"The most popular theory for dark energy in extra dimensions was put forth by ",
"Greene and Levin",
". The idea is that the dark energy could be a Casimir energy that arises in these small dimensions. The ",
"Casimir energy",
" is a quantum phenomenon where the vacuum energy appears between two plates placed very closely together. In the Greene and Levin model this the size of the dimension acts as the plates, and a vacuum energy (Casmir energy) appears. There are many forces that balance out to such that the size of the extra dimensions is stable. The number I've heard thrown around for the size of the extra dimensions is ",
"10 microns",
". These sizes are considered large considered large when compared to the ",
"Planck length",
". ",
"This extra Casimir energy in the extra dimensions can interact gravitationally with matter in our observable dimensions. The problem is that gravity is a very weak force and is very hard to measure. The signature of extra dimensions would be a deviation from the inverse square law (1/r",
" at small distances. The reason that there is a 2 in the power of r",
" instead of some other integer, or a fraction, is because we live in ",
"3 dimensions and surfaces scale as r",
". If there were extra dimensions we would see the power law change as these extra dimensions emerge. The ",
"Eot-Wash",
" group has done some amazing work looking for deviations from the inverse scale law. I think they've checked down to about 100 microns, but I'm a bit behind on this stuff. If they see a deviation around 10 microns, the size of these extra dimensions, it would be very strong evidence for Greene and Levin's theory. "
] |
[
"Light speed and time travel."
] |
[
false
] |
I understand the concept that the faster you travel the slower your personal time travels relative to someone who is stationary. However take the following scenario. I get into a rocket ship and travel to Alpha Centauri and back (8.6 light years round trip). If i complete the trip traveling at 99.99% the speed of light does it still feel like it took 8.6 years to me and 600 or so years for earth or does it feel like a couple of minutes to me and 8.6 years for earth?
|
[
"from the Earth's reference frame, your trip takes 8 years and 7 months. From the reference frame of the ship, it takes just over 44 days to complete your trip."
] |
[
"How do you calculate this?"
] |
[
"To figure out the time as measured on Earth, take the distance as measured by Earth and divide it by the speed as measured on Earth. Since the ship is going at 99.99% the speed of light for an 8.6 light-year trip, it should take just about 8.6 years.",
"To figure out the time on the ship, multiply that number by the square root of 1 - (v/c)",
" where v is the speed of the ship. In this case, v = 0.9999c, so v/c = 0.9999. Plugging that in, you get about 0.01415, which gives about 44.3 days when multiplied by 8 years and 7 months."
] |
[
"What would happen to the Earth if the sun suddenly went out?"
] |
[
false
] | null |
[
"That really depends what you mean by \"went out\". Do you mean completely disappear or fusion stopped?",
"Either way, we wouldn't notice anything until a little over 8 minutes after it happened. ",
"If fusion stopped and the solar mass still exists, all the radiation expended by the fusion would cease. Sunlight and heat would be the most obvious. The solar winds would also die, preventing comets from having tails, and it may alter the weather here on earth. Otherwise the solar system would be pretty much the same.",
"If the solar mass completely went away, then after the 8 minutes all the above changes would happen as well as every object held into orbit by the sun would lose orbit and fly off into space at whatever direction it was moving at that particular time. I believe the likelihood of planets colliding into each other is pretty slim though planets like Jupiter and Saturn might alter the velocity some, but not enough to put them into an orbit around them. Asteroids from the belt would strike some planets.",
"All objects would continue on their trajectory until some other body changed it or collided with it."
] |
[
"As far as the temperature, I could only speculate. ",
"Here",
" is an article that seems to be backed by research that might help answer that. ",
"As for what could cause fusion to stop? Once a thermonuclear fusion reaction starts, the only way to really stop it is to cool it enough to prevent further fusion or for it to run out of fuel, which could technically mean the same thing.",
"It is the sun's mass that caused enough heat to start fusion in the first place. So if enough mass was removed by say a black hole absorbing or another body large enough to disrupt it collided with it (essentially ripping it apart), fusion would stop. This would of course be a different scenario then what your original question asked.",
"The second way is for it to run out of fuel. This will happen naturally to our sun in time. The process of fusion creates heavier elements from lighter elements. The heavier elements fuse into heavier elements and this process continues until there isn't enough energy to continue fusion (it runs out of fuel, or is no longer hot enough to fuse the elements that are left). At this point, the star becomes a brown dwarf.",
"It is worth noting that our sun will expand most likely past the earth before this happens meaning the earth will be destroyed before fusion halts."
] |
[
"Thank you! What would we expect the temperatures to drop to on Earth, without a Sun? Also, what kind of things could cause the fusion to stop?"
] |
[
"If the universe is expanding, why do the stars look like fixed objects in the sky?"
] |
[
false
] |
Are we just not capable of seeing their movement relative to each other? It just seems to follow that if something is expanding, its visible dimensions have to be changing. Yet the night sky looks completely and utterly static apart from planets and meteoroids. Similarly, if the solar system was created earlier/closer to the origin of the Big Bang, would the expansion be apparent to the naked eye?
|
[
"Their movement relative to one another (due to intragalactic dynamics, not cosmological expansion) occurs over thousands of years, so people can't notice it in their lifetimes. If you look in a telescope at multiple-star systems, then you can see them moving."
] |
[
"As iorgfeflkd said, that motion is \"due to intragalactic dynamics, not cosmological expansion\". Cosmological expansion is an effect that occurs only between very, very distant objects. It simply isn't the case that expansion is causing the stars in our galaxy to move away from us."
] |
[
"Lots of things in this.",
"Human eyes can't see movement of the stars on human timescales because they're too far away. Telescopes can see movement. For example, ",
"this",
" is a list of stars that have high proper motions (or motion in the plane of the sky, as in, not coming towards us or away from us). ",
"The universe on the local level is dominated by gravity. On local levels (a few megaparsecs, or Mpc, or less), things are gravitationally bound, so they won't expand away from one another. For example, our Local Group (Milky Way, Andromeda, Triangulum galaxies, and assorted satellite galaxies) will all eventually merge, and then we'll all merge with the Virgo supercluster of galaxies. However, (I believe) all other superclusters will then expand away from us because they're not gravitationally bound. (I don't think the Virgo supercluster will merge with another supercluster.)",
"The Big Bang occurred ",
". ",
"Dark energy, the thing that causes the expansion of the universe, only became dominant 5 Gyr ago (5 billion years ago). Before then, matter was dominant, which meant that the universe's expansion was slowing. Once dark energy became dominant, the universe's expansion started accelerating. Unless something unexpected happens in the future, the universe's expansion will continue to accelerate for the rest of time. ",
"You will never see the expansion of the universe with the naked eye in any realistic time (which means excluding the totally unlivable early universe)."
] |
[
"How do two interfering EM waves conserve energy in all cases?"
] |
[
false
] |
Intro below, question in bold at the bottom. Hey guys! I'm a molecular electronics PhD student at Leiden university, and before that did a nanoscience MSc at Groningen university. I've asked this question to my fellow students and also some postdocs and professors. We have not yet found a satisfactory answer. The problem is a gedankenexperiment where two laser beams or more specifically: two electromagnetic waves meet each other and interfere perfectly destructively or constructively. So we have two (for example) laser beams that cross paths. Now, each laser sends out an oscillating electromagnetic field with a certain power (say 100 watts). We set up the system in such a way that they interfere constructively completely. When the two beams cross and in, we can add the beams vectorially and end up with an EM-field with twice the amplitude. Since the energy density of an EM-field goes with the amplitude squared, we suddenly have four times the output power. Now, I know that this cannot be. We could simply use quantum mechanics and say: well, we just have twice the amount of photons now, and photons do not interfere with each-other, only with themselves. Therefore, you just have twice the power, not four times. However, this answer sounds unsatisfactory to me. I think that the answer should be simpler and should not require quantum mechanics, or anything other than the laws of Maxwell. Furthermore, I'm not interested in answers along the lines of: "Well, you can never make a laser like that, since the laser linewidth is never infinitely sharp". I think that the solution should not be practical one, but a principle one. I have looked at thread. However, the answers here are among the lines of: well, the intensity goes somewhere else. So that means that whatever interferes destructively somewhere, should interfere constructively somewhere else. This sounds like the most valid answer, but I don't see how this is a fundamental and not a practical issue. For example: we could take a laser beam with a width of half the wavelength (I now this is not practical, but I don't see why we cannot do this fundamentally), and at 90 degrees cross another identical laser beam, so that they interfere constructively. Twice the field amplitude, four times the power. We could also interfere destructively and end up with no power.
|
[
"You have the answer on your hand, you're just dismissing it on no basis. The answer is actually really on point because the issues you claim are purely \"practical\" are actually consequences of Maxwell's equations, the equations of motion for the EM field, which are absolutely fundamental. In fact, conservation of energy, which is a Nöther charge, only holds if the equations of motion are satisfied.",
"For example: laser beam with the width half the wavelength: the profile of the field is not how you would naively think it is, precisely because you're not anymore in the \"geometric\" regime where the wavelength is much smaller than the typical lengths. That's exactly why the thing ends up not working. To see this, imagine you have a perfectly cilindrical laser beam like you want it, and think about Maxwell's equations on the surface of the beam. Are they satisfied?",
"Now, I know that this cannot be. We could simply use quantum mechanics and say: well, we just have twice the amount of photons now, and photons do not interfere with each-other, only with themselves. Therefore, you just have twice the power, not four times. However, this answer sounds unsatisfactory to me. I think that the answer should be simpler and should not require quantum mechanics, or anything other than the laws of Maxwell.",
"There is no reason to bring QM in and moreover what you say about photons is false."
] |
[
"Below is an answer I've previously posted on the subject to provide a bit of background for others who might come across this thread. The answer to your question, OP, is closer to the bottom of this post, but you may find some insight in what's below as well",
"When doing anything related to interferometry, you have to add the electric fields as vectors ",
". As long as the amplitudes of the electric fields of the two interfering waves are the same, they will completely destructively interfere. Put another way, their electric fields will sum to zero only when their magnitudes are equal and opposite.",
"when two beams of light destructively interfere, they only do so in one location. There will always be other locations in the optical system where there is constructive interference which contain the seemingly missing energy. The entire system still contains the same amount of energy when you take all interferometer outputs into account.",
"Let me take the example of a ",
"michelson interferometer",
". As per usual, an interference pattern is seen at the detector in the image linked (sometimes called the ",
" of the interferometer). HOWEVER, there is ",
" an interference pattern seen that is directed back toward the laser, often called the ",
" output of the interferometer. If you were to look at both these interference patterns, they would be opposites, that is, where there is a bright spot on the symmetric output, there is a corresponding dark spot in the antisymmetric output. The combined energy in these two interference patterns is the same as the input intensity of the beams (minus any losses due to scattering, absorption, etc)",
"The reason for this is that if you carefully follow the path of each ray in the diagram, the beams that interfere at the symmetric output have acquired a pi difference in phase relative to the antisymmetric output.",
"When the two beams cross and in, we can add the beams vectorially and end up with an EM-field with twice the amplitude",
" in areas of constructive interference. In areas of destructive interference, the Electric fields will add to zero, so everything averages quite nicely as expected over all space.",
"why should this not depend on the geometry if the setup",
"The geometry of the set-up only governs the ",
" of the interferogram, ",
" the energy contained in it - and the shape of the interferogram can tell you A LOT about the optical system under test. For example, in the Michelson interferometer, when the two beams travel exactly the same geometric path, you will get a uniformly illumination interference fringe at the symmetric detector (and the anti-symmetric detector will see the opposite). If, now, you were to extend one arm of the interferometer, you change the geometric path of that beam relative to the other, and the pattern you see will resemble a bullseye (again, the symmetric and antisymmetric outputs will be opposites. If you were to take your perfectly aligned interferometer and now tilt one mirror relative to the other, you would see bars corresponding to lines of constant height on the tilted mirror. In this way, interferometry is a very powerful tool that is used to align optical systems with extremely high precision. You simply look at the interference pattern generated by an optical system and tilt and translate your optics to generate a null interferogram."
] |
[
"You certainly can. But you can't cause just ",
" interference pattern with laser beams-- i.e., ones that don't conserve energy. EM waves are governed by Maxwell's equations, and those equations are ",
" intimately linked to fundamental principles."
] |
[
"What makes fish smell like fish?"
] |
[
false
] |
Also why do fish that smell like fish smell different than shellfish who also smell like fish?
|
[
"The smell of fish is caused by ",
"amines",
" produced by decay. Fresh fish has no odor, the natural revulsion to fish smell is due to its indication of decay. The majority of the odor comes from cadaverine, which produces a general odor of death and rot, while trimethylamine provides the extra \"edge\" of the specific odor of fish. Shellfish produce different compounds, and usually decay slower as well, so the odors are different. ",
"EDIT: Should have said \"fresh fish has no odor of trimethylamine\"; it certainly smells, however this is due to the bacteria in the slime coating, not decay. "
] |
[
"Exactly correct. This is why it is common to squeeze lemon juice onto fish. The citric acid in lemon juice protonates the amines to produce ammonium salts. Ammonium salts aren't volatile and thus the fish will have a less \"fishy smell\" after the lemon juice is added. And as a bonus, lemon juice is tasty. "
] |
[
"Damn...I've been studying chemistry for 3 years and never realized that, I always assumed it was just for flavour"
] |
[
"Can something be in a superposition between existing and not existing?"
] |
[
false
] |
[deleted]
|
[
"That can surely happen, but the correct way to phrase it is that there is a bigger system which has states where your object does not exist and states where it does, and this bigger system is in a superposition of two states from these classes.",
"For example, the (quantum) electromagnetic field can be in a superposition of having 0 photons and having 1 photon.",
"For a physically relevant example, consider an isolated excited atom. As time passes the \"probability of a photon having been emitted\" grows, but this just means the atom + EM field system is in a superposition of the states",
"1) atom excited, 0 photons",
"2) atom ground state, 1 photon",
"and that the \"importance\" of (2) in the superposition is growing. This superposition is actually an entangled state between atom and EM field.",
"Only when you place a detector or any other macroscopic system to try and see if there is a photon there does the state of the system collapse to either 1 or 2, but until you do then it reasonable to say the photon is in a superposition of existing and not existing."
] |
[
"It wouldn't. If you measured it you'd observe EITHER state 1 or state 2, NOT a still-excited atom and an emitted photon. The two states have the same total energy; there is zero probability of finding the system in the atom-still-excited-and-photon-emitted-state."
] |
[
"Observing the system.",
"Some might tell you that it entails some kind of 'wave function collapse', but it would be more accurate to say that the two quantum systems - the observed system and the observer or measurement apparatus him-/her-/itself - become coupled. This coupling of a quantum system with its environment (in what is fundamentally all one larger quantum system) is an example of quantum decoherence.",
"Although this does not explain exactly what happens causing you to observe a particular realization of the system (the apparent wave function collapse), it formalizes it within the framework of quantum mechanics. What 'actually' happens largely depends on your interpretation of quantum mechanics."
] |
[
"Can a planet sustain life without a hot core? What would happen if the earth's core suddenly produced no heat?"
] |
[
false
] | null |
[
"Howdy! I've removed your post for its open-ended and speculative nature. I can redirect you towards ",
"/r/AskScienceDiscussion",
" which better suits such posts."
] |
[
"How would you recommend I rephrase it, to be acceptable? Or is such a question not suited to this sub?"
] |
[
"The first question is alright on its own, but it'd be better to rephrase it more like, \"What ecological roles does the Earth's core play?\"",
"The second question can't be helped. The only way would be to rephrase it more along the lines of asking what happened to Mars when it's core cooled--to which I suggest you also check out our search bar, there may be valuable previous discussions on the topic."
] |
[
"Since the honeybee is an foreign species in North America, would it really be that bad for NA if all honeybees died out?"
] |
[
false
] |
Since honeybees are technically an invasive species, would it have much impact? How did pollination work before bees in the Americas?
|
[
"Bees aren't the only pollinators; before european bees were introduced in 1600, other pollinators played a central role.",
"But you can't feed the US of today with the technology of 1600s. Crops that depend on pollination by bees today include apples, avocados, onions, oranges, pears, and pumpkins.",
"Eliminate everything from your life that includes any one of those, see how much is left, and then decide whether that is \"much impact\". "
] |
[
"The problem is that WE are an invasive species, too. And, we have a bunch of crops that we use to survive that need pollinating. And, the native bees are just not as good at it as the invasive honey bees we use.",
"Essentially, native bees would not pick up the slack if all our honeybees died off."
] |
[
"I might add....",
"The crops that Native Americans used for most of their food were corn (wind pollinated) and squash (self pollinated) and various berries that are good for native bees to pollinate. "
] |
[
"Andromeda is the closest galaxy to the Milky Way but is the Milky Way the closest galaxy to Andromeda?"
] |
[
false
] | null |
[
"The Local Group basically consists of two large galaxies - the Milky Way and Andromeda - each with their own systems of dwarf galaxies orbiting around them. There are also a few other small galaxies floating around. Then there's quite a lot of open space around the Local Group with very few \"field\" galaxies, until you reach other groups and clusters.",
"So the Milky Way and Andromeda are the closest ",
" galaxies to each other. But for each galaxy, the closest galaxy ",
" would be one of the small satellite galaxies. Some of these satellite dwarf galaxies are even merging with the more massive host galaxies."
] |
[
"M33/Triangulum is \"another small galaxy floating around\" :P",
"It's closer in mass to the Large Magellanic Cloud (M33 is ~5x bigger) than to the Milky Way or Andromeda (M33 is ~20x smaller than either)."
] |
[
"Why no love for Triangulum?",
"I know it's smaller than the Milky Way and Andromeda, but it's definitely a full-on spiral galaxy player when it comes to the Local Group."
] |
[
"Why is my current cold \"a cold in the nose\", whilst others are \"in the head/chest/throat\"? Is the infection localised there, or is it that different cold viruses exhibit different symptoms?"
] |
[
false
] | null |
[
"There are certainly countless different viral strains, whose infectivity may range from asymptomatic to severe. Especially with the common cold. It also mutates all the time so new \"mixes\" of different phenotypes are always transpiring around you.",
"The infection might very well be more localized, but most likely due to differences in the virus' own capacity to infect different tissues/wreak havoc, and not due to some sort of bad luck that \"got it stuck to only infecting a certain location\". AFAIK, rhinoviruses (common cold) have the respiratory tract as their pathogenic route of entry. This may be due to the specific molecular mechanism of pathogenesis in those tissues (tissue specificity), or due to much simpler things like how the virus \"likes\" colder temperatures than the human body so it is able to be successful only in the airways that are constantly being cooled by breathing.",
"Viruses are so tiny that, as a general rule, their infectivity and area of infection are much more tied to the actual capabilities of the virus and natural selection allowing it to proliferate and infect whereas another cannot. A localized infection is certainly a thing, but it's not usually because a virus failed to infect those parts, but that it just can't."
] |
[
"Rhinoviruses aren't the only virus that causes colds. Coronaviruses, adenoviruses, and among children sometimes respiratory syncytial virus and parainfluenza look like colds. Heck, Epstein-Barr (which causes mononucleosis) looks like a cold among younger children. Most of the cold symptoms aren't actually viral symptoms, but rather caused by the immune response to the virus. So a lot of times you'll have a pretty broad overlap of most symptoms that people would call a \"cold\", but one might be caused by a rhinovirus with more general malaise and upper respiratory problems, while the other might be caused by a coronavirus that can more easily infect the bronchi and lungs.",
"tl;dr: It might be more useful to think of the \"common cold\" not as a disease but as a syndrome, a collection of symptoms caused by several diseases we just call a cold. Variation in symptoms can be because they're not even the same virus!"
] |
[
"Excellent response. Thank you."
] |
[
"How can a virus cause a person to Vomit?"
] |
[
false
] | null |
[
"Well, for example ",
"norovirus",
" attacks the lining of the stomach and small intestine, causing an immune response (inflammation). Vomiting is a natural defense mechanism when the stomach lining is irritated, as it indicates a problem and getting the substance that's causing the problem out of the body is usually a good idea. When the virus attacks the lining of the small intestine it causes fluid to leak out of the cells and into the intestine, which leads to diarrhea."
] |
[
"This is the correct answer. In most cases vomiting and diarrhea are caused by our own body trying to get rid of a pathogen, although some viruses like Polio will hijack this response in order to spread."
] |
[
"Also it's not so much the virus that is causing you to throw up but rather those are symptoms your body has in response to trying to get rid of it. In the same manner of having a cough or stuffed nose your body is causing these to occur in order to rid of the virus."
] |
[
"Why aren't there \"snow thunderstorms?\""
] |
[
false
] |
We all know about rain thunderstorms, but I don't ever remember experiencing a snow thunderstorm. Do they not exist? If so, why?
|
[
"They exist, they're just rare. Thunderstorms are typically driven by a temperature differential between two bodies of air. The cold air in winter typically doesn't produce the right conditions for a thunderstorm, but it can happen occasionally. ",
"https://en.m.wikipedia.org/wiki/Thundersnow#Formation"
] |
[
"It's a once in a lifetime thing, we had a thunder snow storm in Vermont a few years ago and it was kind of a big deal"
] |
[
"Drove through one last week near Cleveland. It was really cool except for the blinding lake effect snow"
] |
[
"Is is possible to tan through a (glass) window?"
] |
[
false
] |
I've been debating with my father whether it is possible to get a sunburn, or tan in general through a window or if direct sunlight is required? Similar with a car window?
|
[
"\"Ultraviolet light, which causes both erythema (sunburn) and tanning, ranges in wavelength from 4,000 angstrom units (A) down to about 100 A. (Light with wavelength greater than 4,000 A lies in the visible spectrum.) The most potent rays for burning and tanning lie in the 2,900-3,050-A range, with radiation of 2,967 A supposedly being most effective of all.",
"Ordinary window glass, however, is pretty much opaque to wavelengths below 3,000 A. From this we deduce that the intervention of a window will significantly reduce but not halt the burning/tanning process. \"",
"http://www.straightdope.com/columns/read/152/can-you-tan-through-glass"
] |
[
"Ordinary window glass, however, is pretty much opaque to wavelengths below 3,000 A.",
"Then why do printed materials in a video rental store fade especially near the window? Are there just enough UV rays to fade the inks, but not enough to tan? ",
"I work for a printer and we add an anti-UV coating to materials to prevent fading. Some colors fade more than others (reds I think). Not only do printed materials fade, but so do paintings, fabrics on furniture, and even paint on walls and on cars. This is why it's impossible to get a perfect paint match for old paint on a car or wall: UV rays have changed the color to something lighter. (On my 10 year old car, my touch up paint is a very very close match, but is not perfect.)"
] |
[
"No, you can just end up looking like ",
"this guy",
"."
] |
[
"How is it possible that the strong coupling constant is greater than 1?"
] |
[
false
] |
I can't wrap my mind around this at all. Basically for strong interactions, the more complex the interaction, the more likely it is to happen. But you can always make an interaction more complex by adding more propagators which make it more likely to happen. Doesn't this make an average strong interaction an infinite order interaction? Does is max out eventually and the probability starts to decrease?
|
[
"Doesn't this make an average strong interaction an infinite order interaction?",
"When the strong coupling constant is large, the assumption that you can treat interactions perturbatively breaks down. That's why when you expand in a power series in the coupling constant, you reach this conclusion.",
"Using the technique of expanding the S-matrix in a perturbation series is no longer useful, because you have infinitely many gluon self-couplings, and each contributes a vertex factor on the order of 1 or greater.",
"So instead of using perturbation theory, you would use Feynman path integrals. You can put QCD on a ",
"lattice",
" to simplify things, and use Monte Carlo methods to do very high-dimensional integrals.",
"These techniques allow you to study QCD at low energies, before the onset of asymptotic freedom, where standard perturbation theory doesn't really work."
] |
[
"Generally when you are doing perturbation theory you assume higher order terms are smaller than lower order terms. If coupling is strong this is not the case and perturbation theory is typically no longer useful. "
] |
[
"While the coupling constant of a quantum field theory is smaller than one, it means you're in the semiclassical (or perturbative) regime. Sure, the theory ",
" quantum, and the quantum effects ",
" matter, but you can still describe the system as \"basic classical system plus quantum corrections.\" How many corrections you need depend on how precise you want your result to be, but there's this picture of a classical system that is the ",
" description with some extra quantum effects added. Particles in fact only exist under this notion. They are used to describe these corrections.",
"When the coupling constant of a quantum field theory becomes greater than one, it indicates you are now in the realm of genuine quantum physics. You can't treat the system as \"classical plus quantum corrections\" because the corrections are bigger than the base classical system. Everything is completely and utterly quantum; there's no notion of a classical \"scaffold.\" There aren't even particles anymore. That's why it's meaningless to describe such system in terms of Feynman diagrams and propagators and whatnot. There are ",
" virtual particles in this regime. "
] |
[
"What causes sleep paralysis? And why do some people see demons when they experience sleep paralysis?"
] |
[
false
] |
I spoke to someone this happens to frequently, not the demons, but sleep paralysis. I just curious to how it works.
|
[
"I have it frequently. The brain \"shuts of\" certain muscles when you sleep so you wont move around when youre dreaming and the brain sometimes shut them of too late. "
] |
[
"Freaky!"
] |
[
"I have it as well, and I really don't care for it at all. Not because it scares me, because I can instantly tell what it is, but because I could imagine it happening during an emergency (fire, robbery) and not being able to respond. I wonder if there has ever been a study linking sleep paralysis with sleep apnea/oxygen deprivation? Me and my friends used to make each other pass out by suffocating each other (I know stupid) and it causes the same feeling as sleep paralysis.",
" We would squeeze the carotid to cut off oxygen to the brain. "
] |
[
"Hey askscience, r/trees needs your help."
] |
[
false
] | null |
[
"Heating aluminum and inhaling isn't harmful to your lungs in and of itself. A lighter doesn't get aluminum hot enough to vaporize or oxidize which would be necessary in order for it to be inhaled. Your oven, for instance, gets hotter than your smoking device would, and there's no harm in using an aluminum baking pan or using foil to wrap food. ",
"There is some concern that there is a link between aluminum exposure and Alzheimer's, but the research is incomplete and inconclusive. Also, people have to be aware that they may be allergic to it. ",
"Bottom line, though, is that you're exposed to a much higher level of aluminum if you, say, wear deodorant every day than if you smoke out of a device that uses aluminum."
] |
[
"Your oven, for instance, gets hotter than your smoking device would",
"I'm not sure I've understood you correctly here. A candle flame is around ~1200C and the melting point of Al is ~650C. No domestic oven I know can reach 650C let alone 1200C"
] |
[
"It's found in many ",
", but nowadays most deodorants contain antiperspirants so the distinction doesn't really matter.",
"http://en.wikipedia.org/wiki/Aluminum_zirconium_tetrachlorohydrex_gly"
] |
[
"Can someone explain Maxwell's Equations?"
] |
[
false
] |
Reference Image: I am interested in the Integral form specifically. Can someone explain what each equation means, and how it can be used/applied? Bonus Question: I can't for the life of me figure out what ds represents
|
[
"The first equation is an expression of Gauss' Law and states that the integral of the electric field over a closed surface (specifically the component of the field normal to the surface) is proportional to the charge enclosed by that surface. This law can be used to, among other things, derive the inverse-square-law for electric fields (field strength goes down with the square of the distance).",
"The second equation expresses the same for the magnetic field. But since magnetic charge doesn't exist in isolation (as far as we know), the righthand side is always zero. From this equation, we learn that the surface integral of a magnetic field over any closed surface has to be zero.",
"The third equation is Faraday's Law. The \"ds\" term indicates that we're dealing with a line integral. Specifically, this equation expresses that the line integral of the electric field over a closed loop is equal to the negative of the change in magnetic flux through the area enclosed by that loop. The line integral of the electric field represents a voltage and from Faraday's Law you can see that a changing magnetic field through the area enclosed by a loop generates a non-zero voltage in that loop. This is the core principle behind electric generators.",
"Finally, the fourth equation mirrors the third, somewhat. It expresses that the line integral of the magnetic field over a closed loop is proportional to the rate of change of the electric flux over the area enclosed by the loop. Plus an additional term that represents the electric current that flows through the surface enclosed by the loop. This is Ampere's Law.",
"It is interesting to note that there some degree of symmetry between the equations 1 and 2 and between 3 and 4. The difference between them is the presence of electric charge, but the absence of isolated magnetic charge (or magnetic monopoles). If magnetic monopoles would exist, the necessary modifications to the Maxwell equations would turn them into 2 pairs of symmetric equations where magnetic and electric components may be interchanged."
] |
[
"That's the final two, yes. The first two state the essential nature of electricity and magnetism, respectively. The first states that closed systems with an electrical charge have a field that is related to that charge. Conversely, magnetic fields in a closed system always balance (think north and south poles), with a net force of zero."
] |
[
"And..in super simple terms..",
"A changing magnetic field makes an electric field",
"A changing electric field makes a magnetic field",
"This can be self-sustaining, resulting in electromagnetic waves like light"
] |
[
"When there is a momentum transfer between two charged particles (via a virtual particle) is that transfer instantaneous?"
] |
[
false
] |
So if particle A emits a virtual photon that interacts with particle B, is the transfer of momentum from A to B instant, or does it take time for the information about the interaction to reach particle A?
|
[
"It doesn't make sense to talk about \"emitting\" and \"receiving\" virtual particles. ",
"The process is not instantaneous, the particles interact for some finite time just like in classical mechanics."
] |
[
"The other thread is quite correct, but I get the feeling OP might still be a little confused.",
"Here is a slightly broader picture.",
"When you begin talking about virtual particles being able to carry momentum by themselves and transfer it from one particle to another, there tends to be quite a lot of confusion among the lay public regarding what exactly is going on. In fact, virtual particles are entirely a mathematical construct -- they are part of the theory, and not at all like the particles we can \"observe\". Real particles (but not virtual particles) leave tracks inside our detectors that look like ",
"these",
". (If you get the chance to visit CERN, in one of the exhibits on site you actually have a really cool live bubble chamber with an alpha-particles source and you can observe such tracks yourself!) More mathematically, you may know the equation E",
" = m",
" * c",
" + p",
" * c",
" (which reduces to the much more famous E = m * c",
" at p = 0); this relation is is not valid for virtual particles.",
"In quantum field theory, in any fundamental interaction, you generally have two particles \"colliding\" with each other, in the sense that they can exchange momentum with each other (or else generate entirely new particles). Keep this in mind, because this is important -- the experimental observation is \"a set of particles shot at each other, and a possibly different set of particles being detected.\" Anything more than this is part of the mathematical model, not an experimental observation. To model the \"collision\", there is a mathematical prescription to build up a sequence of ever-better approximations. This mathematical prescription uses \"Feynman diagrams\" that look like ",
"these",
"; to calculate the probability of one particular final set of particles, at each successive step of the approximation, we draw all possible diagrams with the given final set, follow the prescription to calculate a number associated with each such diagram, and finally add up those numbers.",
"In the early days of quantum theory, physicists thought it might be a good idea to develop a better intuition for such diagrams. In the diagram that I linked, for example, it is possible to think of the two particles on the left (a real electron and a real positron) as merging together into the wavy line (a virtual photon), which then turns into the combination of particles on the right (a real meson -- which one depends on what quark is represented by q). This may take some brief time, but the time is not measurable; there is also the opposite process in which the vacuum spontaneously emits mesons and a virtual photon, and this virtual photon \"cancels out\" the electron-positron pair some time later -- so, in a sense, this alternate version of the process to get to the same result takes negative time. This latter process of the vacuum spontaneously creating new particles can be thought of as a temporary violation of energy conservation -- by Heisenberg's uncertainty principle, this is allowed but only for a small time period, so it can only happen if there are virtual particles present to cancel out the particles and preserve the energy balance. (Energy and time, just like momentum and position, are complementary variables).",
"This intuition (particles \"merging together\" into virtual ones) was especially useful in the early days of quantum field theory, because physicists took this particular method of approximation quite seriously and, at the back of their minds, treated Feynman diagrams as representing real series of interactions. But we no longer have such a strong emphasis on interpreting the intermediate lines in the diagram as virtual particles, because there are many results that are most clearly proved by more exact techniques -- for examples of such results, look up ",
"Ward identities",
".",
"This was all about virtual particles. Now when we come to actually modelling two interacting particles, we typically model them as so-called \"wavepackets\". These wavepackets are constructed so that they overlap most strongly with each other at the moment of the collision and then stop interacting later. It doesn't make sense to ask how long the collision took place -- it's just a parameter that makes no difference to the final result by construction, and we pretend it doesn't exist. Depending on your maturity in physics, you may want to look at Chapter 3 of Weinberg's Quantum Field Theory, vol 1."
] |
[
"That doesn't make any statement about the time.",
"And you could equally say the other particle emitted it, gaining momentum p, while the first one absorbed it, losing momentum p. It is literally the same process. It is meaningless which direction you use in English, in the equations there is no such direction."
] |
[
"If people sweat to cool down during physical activity, then is it best to leave the sweat alone when you're working out instead of drying off with a towel periodically?"
] |
[
false
] |
When I'm running I sometimes use a towel to dry off my arms, neck, etc. Does this raise my body temperature? Should I just leave the sweat alone?
|
[
"Well first and foremost, it matters where you are running. If you're on a treadmill with no wind, your sweat -once it has been perspired - won't be doing a great deal help in terms of thermoregulation. Get rid of it.",
"You should know, however, that sweat cools in two ways. The most well known and noticeable is evaporation. Sweat does however, help to cool in the simple act of leaving your body. Water has a special property called \"high specific heat\" that means that its temperature has high inertia, ie. it takes a good deal of energy to change. You know this from experience: it takes a good deal longer to heat up a pot of water than a piece of metal (metals tend to have low specific heat). So when sweat leaves your body, it will takes with it a portion of your body heat. Thus, if you're not running with a source of wind, by all means wipe off that nasty salty sweat before your skin boils in it.",
"The same property gives water its cooling effect in evaporation. In a system where water evaporates, such as on your skin, the majority of the heat energy present will tend to remain with the departing water vapor than with your on fleshy armor.",
" I figured that if you have a towel with you, you're not on a full blown run around the block."
] |
[
"The towel does absorb most of the sweat, but also distributes a fine layer or moisture across your skin and hairs. Combined, this causes more evaporation close to the skin, and the loss of heat due to evaporation is not lost on cooling down sweat drops."
] |
[
"In theory, yes, leaving the sweat will help maintain lower body temps. However, I have no idea how significant (i.e., what temperature difference) the effect would be."
] |
[
"When I cut something, what is happening to it at a small scale? Atomic/molocule level?"
] |
[
false
] |
When i was cutting up chicken today I got thinking about what was actually happening between my knife and the chicken at the smallest scale?
|
[
"Molecular physics/engineering PhD here. Most of the comments here have unfortunately been, I believe, a little misleading. ",
"When you cut something, in general, you are ",
" melting it. A scientist will describe you as \"applying stress\" to the material. Imagine breaking a twig with your fingers. You're not \"cutting\" the twig at all: simplying applying enough force/energy to overcome whatever forces are holding the fibres together. ",
"Kinves are, in general, not sharp enough to have an edge that is thinner than the molecules you are cutting. What you're doing is applying a really big force on a really little area - this gives you a high enough stress (i.e. concentration of energy) that the nearby interatomic \"bonds\" (whatever they may be) are indeed ripped apart.",
"For the scientists: perhaps the addition of heat from friction may contribute ",
" to increasing the rate of bond breakage through increasing thermal fluctuations. I'd like to see an order of magnitude analysis comparing it to the energy imparted by the stress field before I believe it, though.",
": Although I hope I'm slightly excused by the fact I put it in speech marks, thanks to everyone for pointing out that in the penultimate paragraph I probably chose the wrong word with \"bonds\". You are correct, yes, I should have replaced 'interatomic \"bonds\"' with something like 'whatever interatomic forces are holding the material together'. What is happening when these forces are overcome is a ",
" complicated question that entirely depends on the material you're considering. If it's butter, then you're essentially untangling polymer chains like pulling apart one big lump of spaghetti with two hands. If it's some sort of metal then it is indeed probably bonds you are pulling apart. In the case of a steak: well, I'm not sure what steak consists of so I can't comment on it, sadly!"
] |
[
"You're right, it's basically all about introducing enough strain (by applying stress) to force the bonds to break. ",
"Certain cutting techniques may actually result in enough local heat transfer to melt a very small volume of material around the cut. In general though, you're not melting paper when you cut it with scissors. "
] |
[
"Technically, scissors shear; they don't cut.",
"Edit: from below",
"There is a difference. Cutting is applying pressure along a thin line, shearing is applying pressure from two sides along adjacent thin lines.",
"The resultant separations look different."
] |
[
"Is dark matter just an invention by scientists to support existing \"knowledge\" of physics? Are there more mysteries that support the idea that scientists have done some false adoption earlier and now we accept it as a law of physics resulting in such terms as \"dark materia\" and \"dark energy?"
] |
[
false
] |
The whole concept of dark matter sounds really strange to me. I mean, it sounds like we make up a name for things we cannot find or even for sure know exists. Could it that there is some major error hidden behind all of our laws we follow in physics? Also interested in hearing about unsolved mysteries in physics like the 'Pioneer anomaly'. Ironically, this whole dark matter issue, seem similar to explaining the unexplainable by believing in God, making up stuff. Also want to thank all of the people answering the questions to the less informed here on , easily my fav subreddit!
|
[
"Whenever your observations of the Universe disagree with your predictions, there are two possibilities: either your equations are wrong, or the stuff you've put into the equations are wrong. Dark matter postulates the latter, that we got incorrect answers for things like galactic rotation rates because we assumed matter in the Universe consisted mostly of things like gas and stars.",
"You're asking why we haven't considered the other possibility, that the equations of physics we use are wrong, and of course people have spent lots and lots of time looking into it. The problem is that it turns out that all of the modified gravity theories we've proposed to solve the dark matter problem either have conflicts with data, require there to be some amount of dark matter anyway, are extremely inelegant, or some combinations of those.",
"One important recent observation is of the ",
"Bullet Cluster",
", a pair of colliding galaxy clusters which we recently observed in both x-rays and gravitational lensing. The x-rays trace the visible matter in the galaxy clusters, and gravitational lensing traces where most of the matter is. Incredibly, they're not in the same place! Normally dark matter (which takes up most of the mass of a cluster) is in more or less the same places as the visible matter, but when the clusters collided, the dark matter got flung out further because it can't be slowed down by electromagnetic interactions like friction. So the Bullet Cluster confirms the predictions of dark matter very nicely, while modified theories of gravity require some dark matter to explain these results anyway, since they otherwise would have a lot of trouble having most of the gravity coming from somewhere where there isn't much matter! It also bears mentioning that while there's a non-relativistic theory of gravity, ",
"MOND",
", which purports to explain away dark energy, the only relativistic theory which has MOND as its limit is called ",
"TeVeS",
" and requires you to add three new matter fields, so you could call it a theory of dark matter anyway.",
"The concept of dark matter itself isn't too strange, by the way, there are plenty of types of fundamental particles which don't interact electromagnetically (and hence don't emit any light) and which could conceivably exist in large amounts in the Universe.",
"Dark energy is something entirely different, and could very well be explained by a modified theory of gravity like ",
"quintessence",
"). However, in this case the difference is somewhat semantic because practically any modified gravity theory can, by a bit of mathematical wrangling, be recast as standard Einstein gravity with extra matter, albeit matter with some very unusual behavior."
] |
[
"The wikipedia article",
" on dark matter should give you some idea of the evidence supporting dark matter. All of these issues can be explained by there being mass which we simply cannot see. 'Dark Matter' fits all of the available evidence, even if we haven't detected it directly yet. It's not really 'making up stuff'. There is work going on to investigate other possible solutions, such as alterations to our current theories of gravity but, so far, nothing fits the data as well as some kind of 'invisible' matter. Science is not an orthodoxy is this sense, the reason that dark matter theory is so accepted is because it is the best explanation we currently have. If another valid theory appears it would replace dark matter as quickly as it could be verified (by most people anyway).",
"Also, not too sure of how reliable they are but several sources, including ",
"this one",
" report the Pioneer anomaly as 'solved'"
] |
[
"To put it simply (and I'm probably repeating what adamsolomon has already said), when the rotation of galaxies doesn't match the predictions of orbital mechanics, either the orbital mechanics theories are wrong, or our knowledge of what galaxies are made of is wrong. Orbital mechanics can be tested very accurately in our own solar system (for example, before Einstein improved the theory of gravity, there was unexplained motion of Mercury that amounted to 0.01 degrees per ",
"), whereas our knowledge of what's in between stars is less firm. The dark matter conclusion was that there is a component to the interstellar medium that we weren't aware of. ",
"This has happened before, for example, when people noticed that galaxies appeared larger the farther away they were, and concluded that there was interstellar dust."
] |
[
"Is there a way of diagnozing Schizophrenia without ever talking to the patient?"
] |
[
false
] |
Neuro connectivity is affected but is there a specific part of the brain activity that can be measured to show symptoms of schizophrenia or is it strictly diagnozed after talking to a patient.
|
[
"It's still a clinical diagnosis. That is to say, other tests are useful for ruling out other causes, but talking to the patient (plus other sources of practical information, especially family and close contacts) is at the core. Remember that schizophrenia is a chronic condition by definition, and evaluating someone at a single point in time is always going to be limited next to getting information about months to years. ",
"There have been efforts to find other methods, but even testing for large numbers of genes that each add slight risk hasn't been good enough. Imaging doesn't generally give you the answers you'd want, not in a way that's meaningfully reliable for individuals."
] |
[
"To expand on the use of medical imaging:",
"MRI scans of the brain are generally unable to diagnose mental illnesses, be they psychological like schizophrenia or depression, or more clearly degenerative like Parkinson's or dementia. Physicians will order a head MRI mostly to rule out other things that might present similarly to those illnesses: things like tumours or malformations that could restrict or affect brain function. ",
"Diseases of the brain and mind can be very difficult to diagnose positively, and even people that are confirmed to have a disease can have normal-seeming brains on imaging. Sometimes you can diagnose certain forms of seizure syndromes from imaging, multiple sclerosis, some kinds of dementia. But a lot of times, whatever diseases a person has, the changes it causes are too subtle to show up on imaging."
] |
[
"a clear neural signature of schizophrenia would be kind of like a holy grail of schizophrenia research, because it would mean that we have the beginnings of a clear neurophysiological explanation for the disorder. and to put it simply, we do not have that. not at all."
] |
[
"Does the lagging Strand of DNA code for something else?"
] |
[
false
] |
So in DNA A=T C=G. So a strand running 5'-3' that is AATGACTG would be attached to a 3'-5' strand going TTACTGAC. ATTGACTG would could for a different protein pattern than its opposite strand (flipped to 5'-3') of CAGTCATT. Some my question is does the opposite strand even code for the same information as the other side? Is the entire DNA strand a palindrome so that as long as you read 5'-3' the results will always be the same? Is there some other mechanism involved?
|
[
"Genes are encoded on both strands of DNA - there are both sense and antisense transcripts. We wouldn't usually call it the lagging strand - that is a term referring to the mechanism of DNA replication.",
"Some areas of the genome have sense and antisense transcripts right on top of each other; so the same sequence of DNA is encoding different things in different directions. This is sometimes two classic protein-encoding genes, sometimes it's a non-coding RNA in one direction (which may have roles in controlling the expression of the gene that is encoded in the other direction). "
] |
[
"So if i am understanding correctly the opposite side varies. Sometimes it codes for nothing, sometimes a whole other protein, and sometimes its structural shape just influences gene expression of the first side.",
"Would it be wrong to think that introns connect to this in any way?"
] |
[
"As ",
"/u/rastolo",
" mentioned, the leading and lagging strand simply refer to the directionality of replication. However, during translation, either strand may contain the template sequence for synthesizing the various RNAs (this is the template strand). The opposite side of the strand (the coding strand) will contain a complementary sequence of DNA.",
"While I am unfamiliar with regions of the genome where the same sequence of DNA encodes different things in different directions, the idea of introns would be similar.",
"Introns are basically portions of a gene sequence that are removed shortly after translation. Exons are the regions of the gene sequence that form the mature RNA. However, different combination of introns and exons can yield different proteins. Essentially, you can have one gene encoding multiple products."
] |
[
"Is it possible to have one eye color blind and the other one not?"
] |
[
false
] | null |
[
"For most common (i.e. genetic) forms of colorblindness, no. However, if you have some degenerative disorder that specifically affects one eye or optic nerve, then you can selectively impair color processing in that eye. ",
"Also, you could be colorblind in one hemifield (left or right visual field) if colorblindness is cortical in nature (i.e., caused by brain damage). This is called ",
"achromatopsia",
" and can either apply to the whole visual field or to just one hemifield."
] |
[
"I just wanted to add that genetic conditions such as mosaicism and chimerism should also be able to result in colorblindness in one eye only.",
"I also want to add, as ",
"/u/albasri",
" alluded to, that it's not as straightforward as 'left eye sees the left and right eye sees the right'. Each eye has a left field and a right field, and for both eyes, the left field ends up being processed in the right hemisphere and the right field in the left hemisphere. There is a decussation at the optic chiasm. "
] |
[
"The body isn't symmetrical, and while certain portions of the body are created at different times, some parts are actually created in unison. Both eyes are formed at the same time, now in the situation we have damaged genetic material governing the growth of a fetus's eyes (or one eye) in the womb, said Fetus could have abnormalities. In the same way that Heterochromia iridum can exist, DNA can contain mutations or errors that can cause one eye to be colourblind or lack clarity due to a misshapen lens.\nWe can also take into account various other methods such as cancer, parasites, virii, and bacterial infections which can infect the eyes, the plausibility of unilateral colourblindness is very possible.",
"I direct you to the following previous reddit post: ",
"https://m.reddit.com/r/askscience/comments/32a2o3/can_you_be_colorblind_in_just_one_eye/"
] |
[
"Could we tell a message is maximily encrypted from random?"
] |
[
false
] | null |
[
"No, and a simple thought experiment can show why. But first, a bit of explanation on how we're going to treat the concept of \"maximally encrypted\".",
"The most secure encryption method available is known as a One Time Pad (OTP). In an OTP, you have two parts -- a message you want to convey, and a one-time, pre-shared key that is at least as large as the message (typically, we consider them to be the exact same size). You generate the encrypted message form by doing a bitwise XOR",
" between the key and the message.",
"Here's a quick example. Let's say we secretly want to secretly send the message \"Hello, world!\"",
"(1) 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21\n",
"This message has 13 bytes, so we generate a one-time pad of random digits (the ",
") also 13 bytes long. Say these bytes are as follows",
"(2) 3f 52 3f 34 a9 20 05 32 d2 bb c6 08 b5\n",
"If we XOR (1) and (2) together, we get the following encrypted message:",
"(3) 77 37 53 58 c6 0c 25 45 bd c9 aa 6c 94\n",
"The nice thing about OTPs is that they are symmetric, and we can decrypt this message simply by bitwise XORing the encrypted message with the key again, to get:",
"Hello, world!\n",
"...back out.",
"With that bit of understanding, on to the thought experiment.",
"Generate a long string of purely true random digits",
" Take a message of exactly the same size. We can XOR our message with the random string of digits, and get a result. If you use ",
" as the key, and encrypt the message with it, you'll get the random digits we generated at the start. We now have an \"encrypted message\" that is ",
" from (as it's identical to) the ones we generated at the beginning of the thought experiment.",
"Again, a simple example. Let's try the message \"Encryption is fun!\". This is an 18 byte message, with the following hex representation:",
"(1) 45 6e 63 72 79 70 74 69 6f 6e 20 69 66 20 66 75 6e 21\n",
"And we'll generate a bunch of random digits:",
"(2) d7 30 d6 0a b0 a3 0c ac ae d2 e8 d3 f6 19 5a 82 f1 23\n",
"Now XOR them together to get:",
"(3) 92 5e b5 78 c9 d3 78 c5 c1 bc c8 ba 90 39 3c f7 9f 02\n",
"Now XOR the original message with the result from (3) above, and we get:",
"(4) d7 30 d6 0a b0 a3 0c ac ae d2 e8 d3 f6 19 5a 82 f1 23\n",
"...which is the randomly generated bytes from (2) again.",
"Hence, the answer is no -- we ",
" distinguish between a maximally encrypted message and truly random data, because you can always generate a OTP key that will result in what is indistinguishable from completely randomly generated data",
"Hope this answers your question!",
" -- for the sake of the thought experiment, assume they are ",
" random, and not pseudo-random.",
"\n",
" -- XOR == ",
".",
"\n",
" -- bitwise XOR follows simple rules: if both bits are different, return 1. If both bits are identical, return 0.",
"\n",
" -- This was obtained by running ",
"dd if=/dev/urandom bs=13 count=1 | hexdump",
", so for this example the bytes are pseudo-randomly generated.",
"\n",
" -- note that in practice, we don't use OTPs, powerful as they are. The big problem with OTPs is in sharing the keys in a secure manner, and never re-using them. More commonly used encryption algorithms ",
" be differentiable from random data, and algorithms to detect encrypted data from random data is an active area of research. But you asked about ",
" encrypted data, and for that you need a OTP.",
"\n",
" -- Should you want to play with OTPs and XORing data, the examples above were computed using ",
"this online XOR calculator",
"."
] |
[
"To be clear, otp is the \"best\" algorithm that exists for our weakest notion of symmetric encryption security, IND-EAV. In the real world we want better security such as AEAD, which OTP cannot achieve on its own."
] |
[
"Thanks for this detailed answer. I had the intuition only."
] |
[
"What is the density of the Carina Nebula?"
] |
[
false
] |
In a James Webb photo thread, someone posted that the Carina Nebula has a density of a few atoms per cubic meter. This seems off to me, as this is close to the average density of the intergalactic medium of one atom per cubic meter, which is much less than the interstellar medium average density of one atom per cubic centimeter, which is much less than the average density of a planetary nebular (100-10,000 atoms per cubic centimeter). But I can't find any information on the web to help me understand this better.
|
[
"Variable, but I think in line with or higher than your planetary nebula figure:",
"I did a search for NGC 3324 column densities, and this was an early hit: ",
"https://academic.oup.com/mnras/article/456/3/2406/1089538?login=false",
"The gas column density has values in the range from 6.3 × 10",
" to 1.4 × 10",
" cm",
"The depth is rather non-obvious, though assuming a ~15 lightyears from the associated star clusters, those would imply a density of molecular hydrogen of ~44 - 9865 per cubic cm."
] |
[
"Glad it helped. You can \"just\" divide them by a reasonable depth to get from number per square cm to number per cubic cm. But barring something like this situation where 1 or 2 sig figs is fine, figuring out the depth is rather hard. And, like, I'm ignoring that it'll vary depending on where in the cloud."
] |
[
"Ah, thanks for that. I saw those gas column density numbers but could not begin to make sense of how those numbers convert to a cubic density volume."
] |
[
"Do cross-eyed people see the world in double and learn to operate that way? Or does their brain learn to process the image correctly?"
] |
[
false
] |
I assume it's the latter because I doubt anyone's eyes are 100% straight/aligned, but in that case - when a baby is born cross-eyed, do they already know how to correctly interpret images, or do their neurons have to learn?
|
[
"The first several years of a childs life is when the neurons are learning how to work properly. If it is severe enough, the child will essentially block out the image from one eye. They will see single, but it will be monocular. If it isn't fixed early enough in development, the eye that is ignored will never be able to see as clearly as the other eye."
] |
[
"The above is correct (see link at bottom of post). To expand a bit, crossed eyes (or more accurately, any mis-alignment of the eyes) is called strabismus. Strabismus not discovered early enough in a child's life can lead to amblyopia, where one of the eye's signals to the brain will not be processed properly, essentially to prevent double-vision. Amblyopia can also be caused by one eye being more near or far-sighted than the other. In my case when I was young, my left eye did not align correctly with my right eye, and my left eye was also less far-sighted than my right eye. I had both conditions that were not discovered until too late to prevent my brain's visual pathways from developing \"improperly\", making my right eye \"dominant\" and my left eye \"lazy\". Even a surgical realignment of my left eye when I was 13-years old did not correct the amblyopia, including the use of corrective glasses to \"fix\" the difference in my far-sightedness between the two eyes. It's hard to describe to people who don't have amblyopia what you do \"see\", but the below article explains it in more detail (several pages in the article). For me, I do have peripheral vision that I use from my left (\"lazy\") eye, but of course that is a blurred peripheral vision...not focused in any way. My left eye has no \"focal point\" in the center of it's vision...my brain basically ignores any left eye focal point inputs. If I close my right eye and try to just use my left eye, it's like having my eye's blind spot exactly where my focal point would be at the center of my eye's vision when trying to read something or focus on an object. The brain \"ignores\" any left eye focal point for me...just doesn't display it for me...to prevent me from experiencing double-vision, but I still have full peripheral vision with it and can walk around without tripping over things or running into walls. Just can't use it to read or see details because there is no focal point to discern the acute shape of letters that make up words in books, etc. It would be like trying to use your peripheral vision to read something...you couldn't do it. For about a week after my eye was \"re-aligned\" when I was 13-years old, I did have double vision, but after about a week, that went away with no left eye focal point improvement and no binocular vision. Still have dominant right eye and \"lazy\" left eye at 62-years old today. But I still have depth perception. My brain just uses different cues, like the \"vanishing point\" perspectives artists use when drawing or painting pictures, to perceive depth. I passed a medical eye test to get my Private Pilot's license when I was 18-years old by demonstrating during a flight test I could determine depth...somethng significant if trying to land an airplane. Still visually medically qualified to fly today. ",
"https://nei.nih.gov/health/amblyopia",
"EDIT: Added below link for \"vanishing points\" if someone is unfamiliar with the concept.",
"https://en.wikipedia.org/wiki/Vanishing_point"
] |
[
"Yes the brain is amazing that way. If a child has crossed eyes, they will see double in the beginning but the brain will try to correct this in several ways. Usually, one eye's signals to the brain will get suppressed, so that they only see from a single eye. They also move their head and body in certain ways to get a better view of their surroundings and this becomes part of their mannerism. "
] |
[
"If the symptoms of flu(fever, coughing) are from the immune response, rather than the virus. Why don't we get flu like symptoms after a flu vaccine?"
] |
[
false
] | null |
[
"Those that are applied by injection are inactivated flu viruses, so they don't have the ability to actually infect cells. They serve as a \"sample\" for the immune system to develop defenses against the pathogen. So, symptoms-wise, you may develop fever, headaches, fatigue, etc ",
" but nothing specifically in the airway dept because the virus is not reproducing. (The immune response is not localized in the usual tissue)",
"If it's the nasal spray vaccine, in that case it uses attenuated viruses (meaning live ones, but a strain that doesn't cause disease). Some people get a runny nose, but that's the closest to the flu-like symptoms you mention."
] |
[
"That's why I HATE it when someone says they won't get the vaccine because last time they got the flu shot they got the flu. No you didn't. Well, you might have gotten a different strain than what you got vaccinated for, but the actual vaccine didn't give you the flu!"
] |
[
"They may have also just actually been unlucky and contracted influenza during the latency period (usually a couple of weeks) between when they got the vaccine and when it actually became effective. ",
"In any case, I agree - still not a reason to not get the flu shot!"
] |
[
"What IS a flame?"
] |
[
false
] |
I understand that fire/flame is a relese of chemical energy through an exothermic reaction, but what IS the flame itself? WHAT is it made from? In researching it, all I can find is explanations of how they occur and what causes a flame, but not its actual make up. Can anyone help?
|
[
"what is fire?"
] |
[
"Have a look at previous discussions ",
"here",
"."
] |
[
"thank you very much!"
] |
[
"Does surgery leave any air bubbles inside the body?"
] |
[
false
] |
I was wondering, when they open the body and sew it back together, how do they get rid of the air that would be introduced inside the body? If they can't remove it, does that pose any risk to the health of the person?
|
[
"Yes surgery regularly leave air bubbles, and that is perfectly safe. ",
"Regular room air usually takes about a week to dissolve back into the blood, while gas from laparoscopic procedures which is CO2 dissolves a lot faster usually disappearing after 48 hours or so.",
"That is why people who have abdominal surgery commonly experience bloating sensation, or pain the shoulders after surgery. The shoulder pain is caused by diaphragmatic irritation, which the brain interpret as shoulder pain because your brain is not used to feeling anything like this from the diaphragm."
] |
[
"Yup. It is a referred pain from the phrenic nerve crossing over at the cervical plexus to supraclavicular nerve to the shoulder. It's more commonly termed as the Kehr's sign."
] |
[
"Is that referred pain?"
] |
[
"How does an egg (reptile, chicken, etc.) embryo develop?"
] |
[
false
] |
I understand that during pregnancy in humans, the fetus receives nutrients through the umbilical cord and this helps sustain them. In an egg, how does this work? Is the embryo strong enough to 'eat' parts of the yolk and gain nutrients from that? Or does it absorb them some other way?
|
[
"The yolk of the egg is essentially the embryo, the albumen (white bit) contains the sustenance the embryo needs during development up until hatching. As far as the method of \"eating\" the albumen i would assume it gains sustenance from it via absorbing the proteins present until it has developed a mouth and digestive tract. The second part i guess someone with a better grasp of embryonic development might have to answer."
] |
[
"You might enjoy this ",
"video",
". Basically, it all comes down to ",
"blood vessels",
". A network of blood vessels branches out through the yolk, circling around the ",
"germinal disk",
". The nutrients within the yolk and albumen travel to the rest of the body via the blood vessels. "
] |
[
"This answered my question perfectly, thank you so much!"
] |
[
"Can a password be complex enough to be future-proof?"
] |
[
false
] |
If I wanted to choose a password that nobody could ever break, how big must my haystack (search space) be? Is this at all possible? I would assume that there must be some kind of physical limit related to the amount of computation the universe itself can perform in its life, but is there a smaller limit? For example, the search space size of one of my passwords is approximately 2×10 and, according to it would take "6.29 billion trillion centuries" for a massive cracking array to crack it. But that assumes current technology. If I can't make my password future-proof, when would this password become easily crackable (e.g., in one day), given current trends?
|
[
"while one may disagree with Schneier on a principle, I guess, he made a point in one of his books:",
"https://www.schneier.com/blog/archives/2009/09/the_doghouse_cr.html",
"One of the consequences of the second law of thermodynamics is that a certain amount of energy is necessary to represent information. To record a single bit by changing the state of a system requires an amount of energy no less than kT, where T is the absolute temperature of the system and k is the Boltzman constant. (Stick with me; the physics lesson is almost over.)\n Given that k = 1.38×10-16 erg/°Kelvin, and that the ambient temperature of the universe is 3.2°Kelvin, an ideal computer running at 3.2°K would consume 4.4×10-16 ergs every time it set or cleared a bit. To run a computer any colder than the cosmic background radiation would require extra energy to run a heat pump.\n Now, the annual energy output of our sun is about 1.21×1041 ergs. This is enough to power about 2.7×1056 single bit changes on our ideal computer; enough state changes to put a 187-bit counter through all its values. If we built a Dyson sphere around the sun and captured all its energy for 32 years, without any loss, we could power a computer to count up to 2192. Of course, it wouldn't have the energy left over to perform any useful calculations with this counter.\n But that's just one star, and a measly one at that. A typical supernova releases something like 1051 ergs. (About a hundred times as much energy would be released in the form of neutrinos, but let them go for now.) If all of this energy could be channeled into a single orgy of computation, a 219-bit counter could be cycled through all of its states.",
"** These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space.** "
] |
[
"Couple points:",
"First, your password is likely stored hashed. That is instead of recording ",
"P@$$W0Rd!",
" in the database they store in the db ",
"ef890b$f919b1dabb6d3c196b1e631dad4b209ad15c85a7d0824fcfedc91ce7762d3136",
" (where ",
"ef890b$",
" is a random salt). That is they store ",
"salt ++ Hash(salt ++ pw)",
", where ",
"++",
" is concatenation, salt is a short random string per user, and Hash is a hash function (I used sha256 for simplicity). Ideally, the hash function would be key-strengthened through multiple rounds of sha256 like in the shacrypt or bcrypt protocols. Since your password is stored as a finite length hash (256-bit in my example), there's no point in using a password with more than 256-bits of entropy -- once they try roughly 2",
" different passwords they may not find your particular password, but they'll find one that collides with your particular hash and be able to access your data.",
"Second, use of 2",
" bit (~ 10",
" ) security is probably secure for your lifetime with classical computers. If you every person on earth each had a million computers, each attacking your password at a rate of a trillion (1000 times faster than current rate) checks per second, it would take about 1800 years to crack a 2",
" bit password. 2",
" bit (~ 10",
" security is definitely secure within your lifetime with classical computers.",
"However, from fear of general-purpose quantum computers it makes sense to double hash/key size (note D-wave systems only can do quantum annealing and cannot do Shor/Grover's alogirthm). ",
"Grover's algorithm",
" basically says you can find a match to a hash in O(sqrt(N)) time instead of O(N) time; that is breaking a 128-bit hash should take ~ 2",
" (10",
" ) time with a powerful quantum computer instead of 2",
" ~ (10",
" ). That's why you see proposals to use 2",
" bit hashes/keys.",
"So to answer your question: yes its fairly easy to make a password that's uncrackable -- make it very high entropy. (E.g., have a 19-word diceware passphrase with an entropy of about 256-bits). Granted if you knew the password ",
"someone likely could use force/bribery",
" to get you to give up the password."
] |
[
"You should note all the exponentiation was lost above quote.",
"One of the consequences of the second law of thermodynamics is that a certain amount of energy is necessary to represent information. To record a single bit by changing the state of a system requires an amount of energy no less than kT, where T is the absolute temperature of the system and k is the Boltzman constant. (Stick with me; the physics lesson is almost over.) Given that k = 1.38×10",
" erg/°Kelvin, and that the ambient temperature of the universe is 3.2°Kelvin, an ideal computer running at 3.2°K would consume 4.4×10",
" ergs every time it set or cleared a bit. To run a computer any colder than the cosmic background radiation would require extra energy to run a heat pump. Now, the annual energy output of our sun is about 1.21×10",
" ergs. This is enough to power about 2.7×10",
" single bit changes on our ideal computer; enough state changes to put a 187-bit counter through all its values. If we built a Dyson sphere around the sun and captured all its energy for 32 years, without any loss, we could power a computer to count up to 2",
" Of course, it wouldn't have the energy left over to perform any useful calculations with this counter. But that's just one star, and a measly one at that. A typical supernova releases something like 10",
" ergs. (About a hundred times as much energy would be released in the form of neutrinos, but let them go for now.) If all of this energy could be channeled into a single orgy of computation, a 219-bit counter could be cycled through all of its states."
] |
[
"Is tooth erosion an irreversible process?"
] |
[
false
] | null |
[
"I actually got so happy when I saw this post haha. So I am currently doing research that involves answering your question. In short, no, tooth erosion is not necessarily irreversible. What I am currently working on is exploring how our own DNA can template hydroxyapatite mineralization. A few years ago, my team came to a conclusion that, yes, DNA can act as a template for HAP to mineralized on. In case you were wondering we are now exploring what about the DNA causes this (which we have found that G quadruplexes promote mineralization).",
"But as far as accessible treatment, we aren't quite there yet, but hopefully sometime down the road!"
] |
[
"Nothing active and available as far as I know, but according to the article, it may be coming soon. Basically filling the decayed area with an alzheimer's drug seems to regrow teeth. ",
"http://www.dailymail.co.uk/health/article-4101996/Could-end-fillings-Scientists-create-method-regrow-teeth-using-Alzheimer-s-drug.html",
"\"Tests in mice showed that when applied directly inside tooth cavities, the drug stimulated regrowth of dentine. The filling method involves inserting a biodegradable sponge soaked with the drug in the tooth cavity.\""
] |
[
"First of all, erosion is very different than caries or abrasion.",
"By erosion we're referencing the damage tissue suffers after being repeatedly exposed to chemical substances. In terms of enamel; The remineralization process somewhat handles erosion within specific parameters, that is to say, the combination of water, flour and certain minerals creates hydroxiapatite which bonds to the surface of the enamel matrix.",
"Basically it's a patch job on the surface of the tooth. So you could say that dental erosion is irreversible because the tooth itself will never be restored to it's original state .... BUT, it can be patched and repaired over time assuming that the damage is not too extensive. ",
"Now if we're talking about dentin or especially cement, then yes, tooth erosion is completely irreversible and requires dental treatment. ",
"Dentin itself has the ability to create new dentin (Tertiary dentin) but this is more of a protective method for the pulp rather than a restorative method for the organ. ",
"So yeah, enamel has a slight 'reversible' nature to erosion but dentin and cement do not.... obviously if the erosion has reached the pulp, the tooth is dead so the pulp is irreversibly affected."
] |
[
"In Japan, it is common for people with cold infections to wear surgical masks in public. Does this affect the rate of infection in Japan? If so, why does no government elsewhere promote them?"
] |
[
false
] | null |
[
"A 2008 study published in the International Journal of Infectious Disease concluded that when used correctly, masks are highly effective in preventing the spread of infections. Family members of children with flu-like illnesses who used the masks properly were 80 percent less likely to be diagnosed with the illness. The difference between types of masks used was insignificant (MacIntyre, 2008).",
"Another study published in the Annals of Internal Medicine reported similar results. Researchers looked at 400 people who had the flu. They found that family members cut their risk of getting the flu by 70 percent when they washed their hands often and wore surgical masks (Benjamin J. Cowling, 2009). ",
"Edit: And another ",
"http://cid.oxfordjournals.org/content/49/2/275.full",
" "
] |
[
"Actually, many of the masks in Europe and Asia now have antimicrobial coating on them that release toxic O3 when exposed to light. The O3 only travels a few micro meters from the surface to kill germs and not you. It's very effective, even against tough germs such as MRSA. It's currently being evaluated by the FDA for use in the US. They have also put the compound in floor wax for use in hospitals. Let me see if I can provide a good source link from my phone. ",
"Edit:I think this link might work for the scientific paper. Information about it's use is more anecdotal because one of the inventors is my professor and he's spoken about it at length with me. ",
"http://onlinelibrary.wiley.com/doi/10.1002/pola.10556/full"
] |
[
"It's hard to say because it doesn't just stop with the masks. Japanese are taught to gargle and wash their hands EVERY time they come in from outside, in addition to wearing masks when they get sick or want to prevent illness when a flu or going around.\nIt's said that this behavior accounts for the low rate of SARS infection during the Asian epidemic, however."
] |
[
"If an airplane acts as a Faraday Cage, why can we still use 4G and GPS reasonably well inside (at low altitudes)?"
] |
[
false
] | null |
[
"An airplane has windows which are large enough the 4G radio wave energy can pass through without being attenuated 'much'.",
"",
"Two things.",
"",
"Radio's can work 'fine' with extremely attenuated signals. In the big scheme of things being inside an airplane with windows doesn't effect the signal very much. ",
"One the other hand when you talk about a plane being hit by lightning. That's different. The aircraft skin being aluminum just shunts all the current around the passengers. Lightning isn't radio waves. It's direct current."
] |
[
"In order to act as a Faraday cage, any openings in the conductive material must be no larger than a fraction of the wavelength of interest (I think it's 1/2). GPS and cell phones operate at frequencies that have a wavelength on the order of 12cm or less, so they easily penetrate the windows."
] |
[
"Cell phone repeaters were introduced in 2004. When you make a call from your phone on board an aircraft you're connecting to the on-board repeater. Prior to 2004 calls from aircraft above 10k feet was impossible."
] |
[
"AskScience AMA Series: We're Rob Dagle, Bob Wegeng, and Richard Zheng - experts in extracting low carbon hydrogen from natural gas from the Pacific Northwest National Laboratory and STARS, LLC. We're here to answer your questions. AUA!"
] |
[
false
] |
Hi Reddit, tomorrow is National Hydrogen and Fuel Cell Day, and we hope you'll have a gas! Hydrogen and fuel cells are the perfect partners for clean, fuel-efficient transportation and a secure energy future. Here at Pacific Northwest National Laboratory (PNNL), we have expertise in creating low carbon hydrogen from natural gas, and we have two projects we want to discuss with you. The first is Solar Thermochemical Advanced Reactor System-or STARS, a technology that converts a record-setting 70 percent of solar energy into chemical energy, such as hydrogen. STARS uses thermal energy from the sun to break down natural gas and water into hydrogen and carbon dioxide in a steam methane reforming process. PNNL licensed STARS to a spin-off company, STARS, LLC, who is working with Southern California Gas Company to implement the technology. Read about STARS here: . The second project uses a low-emission process to convert natural gas to hydrogen, carbon fiber, and carbon nanotubes. The process creates hydrogen that can be used in fuel cell vehicles and industrial processes, as well as carbon fiber that can be used in applications from medical devices and aerospace structures to building products. The goal of the project is to make hydrogen-fueled cars and trucks cost-competitive with conventional gasoline and diesel vehicles. In addition, this technology would virtually eliminate CO2 emissions from the methane-to-hydrogen process. The technology development and commercialization team includes PNNL; West Virginia University; Southern California Gas Company; and C4-MCP, a Santa Monica-based start-up company. Read more about project here: . We invite you to look over the information in these links, and we will be back at 11-1 PDT (2-4 ET, UT) to answer your questions.
|
[
"On STARS, ",
"How is the 70% efficiency noted in the article defined, and what's the overall energy balance? If we think of the solar input as \"free\" and the methane as the input energy, what is the efficiency in producing hydrogen, and how does that change if we also consider the chemical energy stored in the chemical products produced?",
"Also, an alternative would be to run the methane through a combined cycle gas turbine to produce electricity, run a carbon capture and sequestration process on the exhaust, and use PV over the same collection area. Then that electricity could be used in EVs. How does the efficiency compare between those two, from methane input to electric output from the batteries or fuel cell. I'd say well to wheels, but the well-to-methane-delivery part should be the same and the electric propulsion system should be the same so we can compare pretty well without going all the way to well to wheels.",
"Given that STARS can produce useful chemical feedstocks, and given that fuel cells are good for some niche applications even without a highly efficient way to produce hydrogen, STARS sounds like a great process even if the answers to my questions are not favorable, but I would like to understand how broadly it could make sense to use it."
] |
[
"Why is STARS creating hydrogen and carbon dioxide from water and natural gas rather than just hydrogen and oxygen from water ? I would assume that when you want to create a clean energy source you would want to avoid carbon dioxide completely ?"
] |
[
"On the carbon fiber process,",
"CO2 emissions are said to be virtually eliminated. I'm excited to have them significantly reduced at all, so virtually eliminated sounds great, but it is also vague. I appreciate that there's likely some uncertainty, and so I don't expect you can give me an exact number, but can you say what order of magnitude the emissions reduction is? Is it safe to assume that the climate emissions would be dominated by methane leaks in the drilling, pipeline, and distribution systems?",
"What's the scale of the hydrogen production from this is all the carbon fiber production is the US or in the world was switched over to this process? That's only a baseline scoping question, as carbon fiber production is presumably increasing fairly rapidly, but will give and idea of the scale of the contribution this could make."
] |
[
"How do scientists know what elements distant planets are comprised of, without actually going/sending probes there?"
] |
[
false
] |
I always see reports describing the chemical composition of distant planets and their atmospheres, but I've never discovered how we know. Help? Thanks!
|
[
"Spectroscopic analysis (mentioned by Zito773) can currently only be used on planets that are a) transiting, b) not too distant from Earth, and c) close to their host star, otherwise we just don't get enough light to make any meaningful measurement.",
"The first-cut way to guess at a planet's composition is to measure it's mass (typically from radial velocity measurements) and it's radius (from the transit depth, so a planet is must transit to do this) and combine this information to get the planet's density. One can then compare to models of how materials behave and thus guess at it's composition. ",
"Here's an example",
" from ",
"Winn et al",
"."
] |
[
"Woooosh."
] |
[
"Thank you! "
] |
[
"How does an eraser on a pencil work? And why can you erase pencil but not pen?"
] |
[
false
] | null |
[
"The eraser works in a way that when you rub it, it removes the graphite from the papers surface. The rubber is more \"sticky\" then the paper and thus the graphite preferes the eraser over the paper. Another way some erasers works is the eraser damges the top layer of the paper effectively removing the graphite that way. \nYou can't use an eraser on ink (what a pen leaves behind) because the paper more or less absorbs the ink deeper making the erasers funcion useless. For these you can use ink remover that either changes the chemical compound of the ink removing it from the paper, or dying it white"
] |
[
"The ink in most erasable pens is also not absorbed into the paper as well as regular liquid inks."
] |
[
"What about pens with erasers? How do they work? "
] |
[
"If birds have hollow bones, and bone marrow produces red blood cells, how do birds make red blood cells?"
] |
[
false
] |
I'm sure there's a simple answer to this but I've been wondering it for years.
|
[
"This",
" is what I found from searching for \"cross-section of a bird bone\""
] |
[
"Birds have different types of bones, Pneumatic are those which are \"hollow\", ",
" like a straw but composed of a matrix of criss-crossing bone-fibres, these are the bones connected to the air sacs. \nCancellous bone is at the epiphysis and makes some red blood cells. \nBut that doesn't make all the marrow birds have a lymphoid/epithelial organ \"Bursa of Fabricius\" this makes alot of the blood cells. \nAlso kinda relevant, Birds have an extremely efficient lung setup with 1 large pair of lungs (respective to bird size) aided by 5 pairs of air sacs spread throughout the body: back to front: Abdominal, Dorsal, Anterior, Interclavicular and cervical. This basically allows them to use the air they breathe more efficiently and extract more oxygen from it. ",
"So basically; bone marrow not so important in Aves but moreso the Bursa of Fabricius for Blood cell production. hope that helps some bit."
] |
[
"Do you have any good images or diagrams of this? I'm having a hard time visualizing this, thank you! "
] |
[
"Who would die first of starvation? Fat people or thin people?"
] |
[
false
] | null |
[
"I'd have to go with the skinny person dying. The body stores glucose (fuel needed to get electrons to power mitochondria) in a few forms. There's glycogen which acts as a quick reserve, and then there's fat. The body actually converts glucose to a fatty acid through several steps and stores it in adipose tissue. When glucose demands aren't met the body can convert these fatty acids back to glucose.",
"So in this scenario you put forth, the skinny man dies first because he doesn't have the reserves of fatty acids and he can't make anymore energy. His body will actually start to convert amino acids from protein into glucose (note the body does this when we eat too much protein, but this protein comes from tissue, the body starts eating itself!)"
] |
[
"But he got vitamin supplements. How long could an obese person survive with a vitamin deficiency? "
] |
[
"But he got vitamin supplements. How long could an obese person survive with a vitamin deficiency? "
] |
[
"What element is the best at generating plasma?"
] |
[
false
] | null |
[
"\"Best\" in what way? The easiest element to achieve full ionization is hydrogen. But that's not the only way one might qualify an element as being \"best\" for it."
] |
[
"In what other ways could an element qualify as \"Best\"?"
] |
[
"Well what did you have in mind for the original question?"
] |
[
"What factors affect mosquito bite size and duration?"
] |
[
false
] | null |
[
"Mosquitoes release a protein that helps keep your blood from clotting. This protein causes swelling and itching. The more protein the more significant the reaction, so the longer the mosquito is attached, or the larger it is, may indirectly factor into the bite's total size. As far as how long it lasts, the more protein/bigger it is, the longer it will last. Factors such as temperature and pH could also influence how long the protein remains active in the area as well though. For example the protein denatures at warm temperatures, which is why a hot spoon placed on the site can relieve itching. "
] |
[
"If I ignore a bite it seems to go away almost instantly. If I itch one it seems to stay for days. ",
"Is this just a placebo effect?"
] |
[
"I don't think it's placebo, when I was tested for allergies with a skin prick test they told me not to touch the welts that formed because if I did they would get bigger and much worse."
] |
[
"If the moon is tidally locked to Earth, how are there craters on the side facing Earth?"
] |
[
false
] | null |
[
"For reference"
] |
[
"I assume your question is essentially how do things hit the near side of moon if the Earth is in the way. The Earth is far enough away from the moon that there is definitely room for an object to slip past. In fact the gravity of the Earth can bend the trajectory of objects into the moon."
] |
[
"You can ",
"fit every other planet in the solar system in between the Earth and Moon",
" with a little wiggle room to spare!"
] |
[
"Why does a lacrosse ball skip forward on the second bounce?"
] |
[
false
] |
When a lacrosse ball is thrown (out of a stick or out of someone's hand), it's first bounce is normal. Following that its second bounce will skip forward and with a much lower trajectory than expected? Lacrosse balls are solid rubber so I'm not sure why it would have the abnormal second bounce unlike any other ball that I am aware of.
|
[
"Are you imparting any backspin on the ball as you release it? If so, one would naturally expect the first and second bounces to behave differently. In a way, it is the first bounce that ",
" normal, as you have a ball with backspin that fights the forward motion that it has."
] |
[
"Probably because of spin. When you throw it, there is little spin, but on the first bounce, it picks up lots of spin. The second bounce shows the effects of the spin.",
"The effect is explained well here ",
"http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/the-spin-of-a-bouncing-ball/",
"To test this, draw a visible line on your ball so that you can observe the spin. "
] |
[
"The ball gains topspin on the first bounce causing it to jump forward on the second bounce. The same thing will happen, but slightly less noticeable on the 4th bounce. It is also much more noticeable on blacktop."
] |
[
"Why is it that generally people lose their high frequency hearing as apposed to their low frequency hearing when they're going deaf?"
] |
[
false
] |
I don't really know how true this statement even is. I would just like a little enlightenment on it.
|
[
"I didn't answer OP's question. I just said that the source that you cited did not seem credible."
] |
[
"4th year med student here. The most common reason for this is due to aging called Presbycusis. ",
"It has to do with how the cochlea turns sound into something our brains can understand. Put simply the cells that are sensitive to high frequency sounds get damaged more than those that are sensitive to low frequency sound as we age. ",
"Here's a good link I found that explains it ",
"http://www.baylordoctor.com/2012/02/why-grandpa-cant-understand-his.html"
] |
[
"Thanks a lot. That's very helpful."
] |
[
"Is there dielectric breakdown in a liquid? What are the effects of the breakdown on it?"
] |
[
false
] |
If a solid undergoes dielectric breakdown, I’ve read it permanently damages the solid usually, and for a gas, it ionizes the gas but doesn’t “damage it”. What about for a liquid?
|
[
"Yes! (Depending on the liquid)\nSpecifically, the insulating mineral oil used in transformers is subject to very high voltages. This oil has a resistance in the range of 10kV per centimeter seperation. When the dielectric breakdown limit is surpassed occasional arks happen. The fluid serves to cool and quench the ark but some of it is converted into gasses such as hydrogen, acetelyne, and various volitile organic compounds.",
"Over time these byproducts can contaminate the oil reducing its dielectric strength or produce an explosion hazard."
] |
[
"If the voltage exceeds the dielectric breakdown strength of the liquid, it will \"cut\" through and form a momentary ark. \nFor a sustained ark, there would need to be alot of current present to fight the cooling and collapsing bubble that is formed. Enough that whatever conductor/contacts the ark is originating from probably wouldn't last long."
] |
[
"Interesting, thanks! Also, with enough voltage, can an arc be sustained through the liquid? And/or could the arc “cut” through the liquid and sustain itself?"
] |
[
"Are certain insects evolving (or evolved) to have the idea of not banging mindlessly onto windows wired into their heads?"
] |
[
false
] |
Glass is a brand new invention and getting stuck behind a window and possibly sprayed with raid is not nearly as much of a factor as soot covered tree trunks but it certainly is a factor nevertheless. I guess I just feel bad for them, I imagine them going "WORLD! Y U NO MOVE? I'm flying ffs!" I know they still can't tell lightbulbs from the sun so I assume changes in bevior take longer than changes in appearance but, well, who knows, that's why I'm wasting your time asking: are there any glass-smart bugs?
|
[
"Interesting side note on this. I'm pretty sure the deer in my neighborhood have learned to be cautious around roads and vehicles.",
"When I was young, I remember deer would often panic and jump into a vehicle's path. Deer carcasses would line the street at different times of the year. Some time ago I witnessed a few deer standing beside the road waiting for cars to pass until they crossed.",
"Then, I realized that I haven't seen many dead deer on the side of the road in a long time. This isn't evolution, but still -- clever deer."
] |
[
"I think your observation about the deer might be closer to evolution than flies and glass widows: Cars actually can put selective pressure on a population of wild animals, whereas a glass window really doesn't put much pressure on a population of insects - they can't leave the room for a while, but thats only rarely lethal. Usually there is enough food sources inside the room anyway.",
"I'm not a biologist, but the extinction of individuals prone to panic (jumping in front of cars or freezing in fear when caught in headlights) doesn't seem to far off to me.",
"Then again, I don't know much about how fast deers can learn (by observation, survival rates are too low to learn from expedience) and whether they can teach their young..."
] |
[
"... they can't leave the room for a while, but thats only rarely lethal.",
"Tell that to the wasps who come down my bathroom vent, then die as they cannot escape.",
"Another matter with my case is that selection in hive insects are different. The death of one individual basically has zero impact on the hive - and it's only pressure on the hive that can lead to selection on specific genes."
] |
[
"Why are there so many children allergic to peanuts these days?"
] |
[
false
] |
When I was a kid growing up in the 80s, peanut allergies were non-existant! I new nobody who was allergic to peanuts. I know a primary school teacher and she said that this year there are 6 in her grade, and more than 30 kids school wide who are allergic to peanuts. Why is this so?
|
[
"The prevailing theory (and I stress theory.. nobody really knows) is that today parents and people in general keep their bodies/living spaces as clean as possible (anti-bacterial such and such), avoiding contact with kids until they wash their hands, etc. Babies, and everyone really, need to be exposed to as many germs as possible in order for the body to develop an immune response.",
"A study I can't find right now found that children in households with pets, along with children in daycare, were more than 50% less likely to develop asthma.",
"TL;DR: Exposing yourself and your children to many types of environments is a good thing."
] |
[
"Media sensationalism has given rise to an over-reaction on the part of concerned parents. ",
"The public seems to have an exaggerated perception of the risks of food allergy, probably spurred on by the media. Recent headlines in national newspapers in the United Kingdom include: “One bite and he dies,” ",
"British Medical Journal, 31 August 2006"
] |
[
"One reason I see besides the ",
", is that the last 200-400 years, people have started to get exposed to a lot more foreign foodstuff. Northern Scandinavians are not used to peanuts and citrus fruits. Australian Aborigines did not eat bread or any grain at all, this is why they get health problems keeping a European diet. ",
"My point is that we today eat many things we've had in our diet for less than fifty years, which is far from enough time for our guts to adapt."
] |
[
"Ok scientists, I am working on bringing science to students with special needs. I would love to know 1) what is the \"essential questions\" of your field and 2) who are the scientists that people should know about from your field and why?"
] |
[
false
] | null |
[
"Thanks for spreading scientific education! ",
"What level of science knowledge are the students at? Many of the \"essential questions\" of specific fields are incredibly difficult to explain without some prior knowledge. ",
"In general, I think the biggest thing you could do for a science (especially physics and mathematics) syllabus is training someone to reduce complex problems by learning to spot symmetries. Some people learn it far too late in their education which makes many essential basic concepts seem more challenging than needed. This skill carries across the sciences and even out to actual life problems. "
] |
[
"You might get more responses in ",
"/r/AskScienceDiscussion",
".",
"May I ask what sorts of students we're catering towards here? The needs of your students will affect the concepts we ought to communicate to them and how we should frame them."
] |
[
"Historian of science here.",
"Maybe start with a discussion of what science is, then follow with some of the ideas here, by discipline. I don't know the age of the folks you're working with, nor their cognitive abilities, but taking an organized approach with a simple definition of scientific thinking followed by concrete examples is probably the way to go. As for definitions of science, see books by Steven Shapin or Peter Dear, or PM me for more help. ",
"U/I_Cant_Logoff has the right idea -- this stuff is important as academic information but also good.for life skills. Keep us.updated and keep fighting the good fight!"
] |
[
"How does the excess salt from salting roads affect the environment? Things such as bodies of water or soil quality?"
] |
[
false
] | null |
[
"Seattle and Portland have actually found that this is a huge problem. Salmon return to the same area every year to make babies, and they guide themselves by the gradient of salt to fresh water as they move upstream. When there is too much salt from the roads, they get lost and spawn in the wrong place so their babies die too, leading to population decline, which is obviously terrible for the salmon industry. ",
"AFAIK they use some non salt alternatives that don’t work as well to protect the salmon."
] |
[
"I attended a seminar on this. After winter storms, salinity can spike in stream reaches, and temporarily become fatal to the life there. Additionally, after a while the salinity spike dissipates, but each year, the baseline salinity rises.",
"They still didn't know or understand much about the long term effects."
] |
[
"I don't know what Seattle uses, but on the Olympic peninsula we just use plows and sand. We don't get a lot of snow, or very often so they just try and keep the levels down on the main roads and throw sand for traction. "
] |
[
"How does ISON's speed, currently estimated at > 0.1% the speed of light, compare to the fastest things in our Solar System?"
] |
[
false
] |
Is ISON currently the fastest object (let's say bigger than a baseball) in our solar system? If not, what is?
|
[
"Most things in the solar system go at 10s of km/s. Earth goes at about 0.01% of the speed of light, or about 30 km/s. Even Mercury is going at less than 60 km/s. So if ISON was going at 0.1% of the speed of light, it's going at 300 km/s, which is more than 5 times faster than the next fastest thing.",
"This speed basically comes from the Sun's gravity: the comet has \"dropped\" towards the Sun and picked up a lot of speed. The greatest speed you can get from falling downwards (assuming you start pretty stationary) is the same as the escape velocity from the Sun's surface - a bit over 600 km/s. So you'll not likely see anything natural in the solar system going at 600 km/s or faster."
] |
[
"What's an order of magnitude or two between friends?"
] |
[
"Wouldn't that be ~10% of the speed of light?"
] |
[
"How do they polarize glass?"
] |
[
false
] |
Evert time i google how polarized glass is made i get some non-technical stuff about how they are good for fishing, reduce glare, remove horizontal light waves. One article mentioned something about iodine crystals but didn't elaborate. I just want to know how they treat the glass to get the polarization effect.
|
[
"There are many different ways, one is simply placing a PVA (poly vinyl acetate) foil on the glass. This material approximately has the consistency and thickness of plastic wrap. The manufacturing process begins by heating and stretching PVA to five times its natural length, making it even thinner. This lengthens PVA's long chain molecules, causing them to align. The stretched PVA is then dipped in iodine solution. The iodine is absorbed into the molecular chains forming long grids of parallel, darkened lines that are not visible to the human eye. The film is then dyed to the color of the desired finished film. The darker the film, the more polarization it provides. In the last step, the film will be applied on the glass."
] |
[
"You stretch it in a single direction?",
"Yes. The \"Pull Direction\" is the direction which the molecules align with."
] |
[
"But how does one choose the direction of polarization like this? You stretch it in a single direction?"
] |
[
"Bear domestication."
] |
[
false
] |
[deleted]
|
[
"See ",
"my thread from last year",
" for a discussion."
] |
[
"Bears are quite a bit bigger than wolves (60-100lbs compared to 90-370 lbs for black bears-grizzlies are larger). If a tame wolf gets aggressive with you, you stand a chance of fighting it off especially if you have a stout stick. A bear could maim you almost accidentally with a single swipe in a moment of irritation. No large carnivore makes a good pet for this very reason.",
"Bears also don't live in groups, which means there's no pre-adapted social structure to plug humans into."
] |
[
"You have to understand that there's no absolute division between possible and impossible in things like this. With decades of work, huge expense, and probably a number of human deaths you could breed a bear down to something approaching a domestic animal. It would probably in some ways resemble a cat more in temperment than a dog (or horse or elephant for that matter) since unlike most other domestic animals bears are not group living creatures."
] |
[
"How can my brain know that it's being fooled, yet still continue being fooled?"
] |
[
false
] |
I apologise for the badly worded question, so I'll give an example. The reason I may throw up when dizzy is to empty my stomach because my brain thinks I am being poisoned. How can my brain still make my body throw up when I - the brain - know I have not taken poison and am just dizzy? If I still haven't made myself clear, I'll try to explain some other way :) Thank you in advance.
|
[
"Well, in this instance your stomach is largely autonomous. The brain is responsible for decision making and processing new information, but it has little control over processes like the beating of your heart, gastrointestinal digestion or the movement of the sphincters along your gastrointestinal system. Thus, disconnects can occur where the physical response is a result of autonomy, not conscious efforts on part of your brain to contradict what you see.",
"The brain is also compartmentalised - sometimes information processed by one part confuses another - the two regions can only share certain information and thus, the problem cannot be resolved and you have to compensate somehow. Take the hollow face illusion, it is an example of competing means of processing the types of visual stimuli from the occipital lobe and is known as the ",
"two streams hypothesis",
" and the two ways in which we process information results in us being tricked into thinking what we see is a concave face is viewed as a convex face due to one stream taking priority over another with processing. Thus, you can have one region aware that something is not right, but the other convinced it is."
] |
[
"Your brain isn't just one thing. It is a whole constellation of (usually) coordinated processes. What one part of your brain 'knows' is not automatically used by all parts of the brain. This is especially true when we are talking about physiological reactions such as nausea, heart beats, breathing, pupil dilation, etc as these reactions are controlled largely by neural structures that predate the more 'advanced' parts of the brain.",
"When this disconnect or conflict happens at a higher level with two different cognitive systems or behavior profiles, it is called ",
"cognitive dissonance",
"."
] |
[
"Thank you very much, this was a great help!"
] |
[
"How is the amount of matter in \"the vacuum of space\" represented? Moles per km^3 ?"
] |
[
false
] |
I asked this in a thread and got a couple upvotes but no answer so apparently this question is on a couple peoples minds. When you read about a gas cloud I assume that the center can either be the same "pressure" as the majority or it can be 10's of thousands of PSI if it decides to become a star. For example, there is a cloud containing trillions of gallons of ethanol. How many m would you have to condense to get a gallon of ethanol?
|
[
"Typically in astrophysics we use grams per cm",
" or number of particles per cm",
" If you look at the interstellar medium in our galaxy. A very typical density is one particle per cm"
] |
[
"And 1 particle/cm",
" is 1.6E-9 moles/km"
] |
[
"For example, there is a cloud containing trillions of gallons of ethanol. How many m3 would you have to condense to get a gallon of ethanol?",
"Are you asking about a specific cloud? Otherwise, the question doesn't make sense. Some clouds are more dense than others.",
"If you were referring to the massive cloud Sagittarius B2, wikipedia states:",
"The mean hydrogen density within the cloud is 3000 atoms per cm3, which is about 20–40 times denser than a typical molecular cloud.",
"In this case, a gallon of ethanol would occupy about 80 million km³, assuming all 3,000 atoms/cm³ of H were in the form of ethanol."
] |
[
"How would the life support system on a Mars habitat be different from thr life support systems seen on the moon landings and in the iss?"
] |
[
false
] | null |
[
"Hi fellxcatking thank you for submitting to ",
"/r/Askscience",
".",
" Please add flair to your post. ",
"Your post will be removed permanently if flair is not added within one hour. You can flair this post by replying to this message with your flair choice. It must be an exact match to one of the following flair categories and contain no other text:",
"'Computing', 'Economics', 'Human Body', 'Engineering', 'Planetary Sci.', 'Archaeology', 'Neuroscience', 'Biology', 'Chemistry', 'Medicine', 'Linguistics', 'Mathematics', 'Astronomy', 'Psychology', 'Paleontology', 'Political Science', 'Social Science', 'Earth Sciences', 'Anthropology', 'Physics'",
"Your post is not yet visible on the forum and is awaiting review from the moderator team. Your question may be denied for the following reasons, ",
"/r/AskScienceDiscussion",
"There are more restrictions on what kind of questions are suitable for ",
"/r/AskScience",
", the above are just some of the most common. While you wait, check out the forum \n",
" on asking questions as well as our ",
". Please wait several hours before messaging us if there is an issue, moderator mail concerning recent submissions will be ignored.",
" ",
" "
] |
[
"Mars\nSpace\nISS\nInternational space station\nMoon\nLife support."
] |
[
"'Engineering' 'Physics'"
] |
[
"AskScience Panel of Scientists XVII"
] |
[
false
] |
This post is for new panelist recruitment! The previous one is . The panel is an informal group of redditors who are . All panelists have at least a graduate-level familiarity within their declared field of expertise and answer questions from related areas of study. A panelist's expertise is summarized in a color-coded AskScience flair. Membership in the panel comes with access to a panelist subreddit. It is a place for panelists to interact with each other, voice concerns to the moderators, and where the moderators make announcements to the whole panel. It's a good place to network with people who share your interests! Are studying for at least an MSc. or equivalent degree in the sciences, AND, Are able to communicate your knowledge of your field at a level accessible to various audiences. Choose exactly one general field from the side-bar (Physics, Engineering, Social Sciences, etc.). State your specific field in one word or phrase (Neuropathology, Quantum Chemistry, etc.) Succinctly describe your particular area of research in a few words (carbon nanotube dielectric properties, myelin sheath degradation in Parkinsons patients, etc.) Give us a brief synopsis of your education: are you a research scientist for three decades, or a first-year Ph.D. student? Provide links to comments you've made in AskScience which you feel are indicative of your scholarship. Applications will not be approved without several comments made in itself. Ideally, these comments should clearly indicate your fluency in the fundamentals of your discipline as well as your expertise. We favor comments that contain citations so we can assess its correctness without specific domain knowledge. Here's an example application: Please do not give us personally identifiable information and please follow the template. We're not going to do real-life background checks - we're just asking for reddit's best behavior. However, several moderators are tasked with monitoring panelist activity, and your credentials will be checked against the academic content of your posts on a continuing basis. You can submit your application by replying to this post.
|
[
"Username: ",
"/u/bencbartlett",
"General field: AMO Physics",
"Specific field: Quantum Physics | Quantum Information ",
"Particular areas of research currently include simulating realistic (error-prone) quantum networks and designing error correction schemes for them. Will likely transition to more experimental work soon.",
"Education: BS, physics and computer science, Caltech. 1st year PhD student, applied physics, Stanford. I also have ",
"/r/science",
" flair.",
"Comments: ",
"1",
", ",
"2",
", ",
"3",
", ",
"4",
", ",
"5",
", ",
"6",
", ",
"7",
", ",
"8",
", ",
"9",
", ",
"10",
", ",
"11",
", ",
"12",
", ",
"13",
", ",
"14",
", ",
"15",
", ",
"16",
", ",
"17",
", and about 100 more comments in ",
"/r/askscience",
" "
] |
[
"Username: ",
"/u/contact_fusion",
"General Field: Astrophysics",
"Specific Field: Magnetohydrodynamics",
"Particular Areas of Research: Star Formation. Magnetized turbulence, scientific supercomputing, submillimeter polarimetry, fluid mechanics. Also work on impact diversion of asteroids and x-ray interactions with high-Z materials. General areas of interest include astronomy, physics, planetary science, and applied mathematics, especially extreme (high energy density) physics.",
"Education: B.S. Physics/Mathematics (2013); M.S. Astronomy (2015); current astrophysics Ph.D. student nearing graduation. ",
"Comments: ",
"1",
", ",
"2",
", ",
"3",
", ",
"4",
", ",
"5"
] |
[
"I just want to say I'm really psyched about you becoming a fellow panelist. I've seen your comments in various climate-related threads, and they're always on point."
] |
[
"Is ALS more common today than in the past?"
] |
[
false
] |
I’m willing to consider the fact that we’re just more aware of it as a society after the ice bucket challenge, but it seems much more common today.
|
[
"Yes, scientists believe ALS is slightly more common today than in decades past, but they can't explain why. In my opinion, although it may actually be more common today based on statistics, it also appears to be because of things like the ice bucket challenge, and the fact that it's starting to appear in younger people more commonly than ever before. People in their 30s and 40s, and sometimes even in their late 20s start to show symptoms of ALS (another thing scientists cannot explain why).",
"ALS is the most common motor neuron disease. Apart from Lou Gehrig himself, the most well known person with ALS was Stephen Hawking, and he lived with the disease for decades, which a lot of people fail to understand is EXTREMELY rare. Over 50% of people diagnosed die within 30 months. There is no cure, and there is no treatment that will prolong life by more than a few months at best.",
"It is a terrible disease that has not always gotten much attention, partly because at any given time, there are not a lot of people living with the disease. Most of them die within a couple years of the diagnosis, and are sadly forgotten about.",
"My dad was diagnosed with it in 2012, and did not make it past 2013. I got to see first hand what it can do to people and I really hope a cure, or at the very least, some substantial treatment options, can be found sooner rather than later."
] |
[
"I'd be happy to.",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4334292/",
"https://www.alzforum.org/news/research-news/lou-gehrigs-disease-rise"
] |
[
"I want to add to the discussion the strange hypothesis that ",
"cyanobacteria can cause ALS",
".",
"Cyanobacteria, or blue-green algae, produce the amino acid BMAA. The idea is that some people have a genetic predisposition for having this amino acid accumulate in their brains. This leads to excitotoxicity and the death of neurons associated with ALS (and Alzheimer's).",
"There does seem to be a connection. There are higher levels of BMAA in the brains of people with ALS and Alzheimer's Disease. ",
"People have an increased likelihood of developing ALS if they live near lakes with poor water quality",
". For instance, the prevalence of ALS is more than 40 times higher along the shores of Mascoma Lake in New Hampshire.",
"The most notorious example comes from Guam. There were villages where 25% of the inhabitants died from neurological diseases such as Parkinson's, Alzheimer's, and ALS. Cyanobacteria is produced by the cycad three. Inhabitants grind its seeds into flour. So there's a bunch of BMAA. There are also high levels in flying foxes, which they eat. So they have a diet increadibly rich in BMAA, increadily high levels of BMAA in their brains, and incredibly high rates of neurological diseases."
] |
[
"What separates animals that recognize themselves in a mirror from ones that don't?"
] |
[
false
] |
[deleted]
|
[
"It's thought that animals that can pass the ",
"mirror test",
" have ",
"theory of mind",
" (ToM). This idea however has many critics who, among their qualms, accuse the test of being overly anthropocentric. "
] |
[
"Generally this is associated with higher intelligence. As far as I've heard it has to do with them being able to process the concept of them existing outside their environment as an individual. This is similar to object permanence in children. At a certain point children develop to where they realize things exist outside their experience. (This is why peek a boo is so fun when they are younger. You literally cease to exist and then magically reappear a moment later as far as they are concerned.) \nMaybe think of it as the reverse of that. Animals that would fail the mirror test may just not have sufficient processing power or sufficiently developed brains to recognize that they exist outside their environment as an individual. That would explain why the animals often associated with high intelligence can pass this test.",
"Feel free to correct anything I got wrong!",
"Here's a good post I found that has a lot of information and some cool videos on the topic. \n",
"http://www.slate.com/blogs/wild_things/2014/10/24/what_do_animals_see_in_the_mirror_self_recognition_and_social_behavior_video.html",
"(Edit: added confirming source.)"
] |
[
"My dog can recognize herself in a mirror, I have proven this by placing a treat on her head. She sees the treat in the mirror and in order to get said treat she shook her head till it fell. @xarhtna"
] |
[
"Before antibiotics were bacterial diseases just not curable?"
] |
[
false
] |
If you got pink eye, chlamydia, strep throat, or any other type of bacterial infection, was there no hope? Is the human immune system not capable of healing itself?
|
[
"Some things the body will eventually take care of. Maybe. Like strep throat. But without antibiotics, a patient ",
" have a higher likelihood of developing further problems, secondary issues, that come with having that infection.",
"Before antibiotics, there were other things that were used sometimes. Salt rinses (saline) for surface wounds/infections are actually fairly useful, even today, at helping to limit infections."
] |
[
"Strep throat will get better without treating it. Treating only reduces the duration of the illness by less than 1 day, but it prevents some of the bad complications of it such as rheumatic fever, rheumatic heart disease, and post-strep glomerulonephritis."
] |
[
"Before antibiotics, you would either live or die. Depending. Often because of associated complications. Now we can avoid many of those complications.",
"I did have a friend though who had strep throat which was misdiagnosed as mono. He ended up getting scarlet fever and had to get part of his throat surgically removed."
] |
[
"How do I, as somebody outside higher education, get access to published journals?"
] |
[
false
] | null |
[
"This might be better suited for ",
"/r/AskAcademia"
] |
[
"Thanks for the tip. I wasn't even aware of that sub"
] |
[
"Hi the_dummy thank you for submitting to ",
"/r/Askscience",
".",
" Please add flair to your post. ",
"Your post will be removed permanently if flair is not added within one hour. You can flair this post by replying to this message with your flair choice. It must be an exact match to one of the following flair categories and contain no other text:",
"'Computing', 'Economics', 'Human Body', 'Engineering', 'Planetary Sci.', 'Archaeology', 'Neuroscience', 'Biology', 'Chemistry', 'Medicine', 'Linguistics', 'Mathematics', 'Astronomy', 'Psychology', 'Paleontology', 'Political Science', 'Social Science', 'Earth Sciences', 'Anthropology', 'Physics'",
"Your post is not yet visible on the forum and is awaiting review from the moderator team. Your question may be denied for the following reasons, ",
"/r/AskScienceDiscussion",
"There are more restrictions on what kind of questions are suitable for ",
"/r/AskScience",
", the above are just some of the most common. While you wait, check out the forum \n",
" on asking questions as well as our ",
". Please wait several hours before messaging us if there is an issue, moderator mail concerning recent submissions will be ignored.",
" ",
" "
] |
[
"What is the lowest frequency that can be transmitted in a vacuum?"
] |
[
false
] |
For example, given a variable oscillator, and an antenna placed in a vacuum. What is the lowest frequency (electromagnetic wave) that can be radiated from the antenna? The antenna can be of any size or shape. I realize most usable frequency transmitted are in 300kHz and above, but what about the lower frequencies < 1kHz, does it have do with amount of energy in the wave?
|
[
"None. There is no lowest frequency.",
"Just by coincidence, I was working on a problem related to this just a few hours ago. I wanted to have numbers handy, so I was working out the Hawking radiation of Sagittarius A*. That's a black hole that radiates (assuming my maths are correct here) at a peak frequency of 0.00148111 Hertz, corresponding to a wavelength of about a hundred and twenty five million miles, or about one and a half times the average distance from the Earth to the sun. And it emits one of those roughly twice a week.",
"There's no lowest-possible-energy of a single photon. There's just longest wavelength you can "
] |
[
"There is a technicality that you run into when you start dealing with wavelengths that are close to or longer than the Hubble length. The math stops those modes from being oscillatory; modes with a wavelength longer than the Hubble length are \"frozen\"."
] |
[
"Photon. With a fffff."
] |
[
"How does plate tectonics explain continental drift?"
] |
[
false
] |
This has always confused me. So there are these huge stone plates that can slide on top of liquid mantle. They are very tightly packed and there are forces that cause them to move in relation to each other. Where they collide they either deform (forming mountain chains) or slide on top of each other (usually resulting in trenches). I suppose they can also deform at other places causing the land to either rise or sink, although plate tectonics don't seem to allow for that. But isn't that how Sahara used to be at the bottom of an ocean? What I don't understand is how this explains actual continents moving closer or away from each other. They are not somethings floating on water, they are protrusions on respective plates. Why would South America and Africa fit each other, and why would that make sense in the light of plate tectonics? How could they have been one continent and later drift apart if both plates have underwater sections as wide as half an ocean? And why would super continents form every few hundred million years? Thats one huge bulge on one side of the planet. Kind of strange with all that gravity acting on the plates and the mantle trying to keep the shape of a spheroid.
|
[
"What I don't understand is how this explains actual continents moving closer or away from each other.",
"The continents are parts of the plates, as the plates grind together or separate, the continents move closer and further apart. The plates move relative to each other.",
"Why would South America and Africa fit each other, and why would that make sense in the light of plate tectonics? ",
"At one point they were next to each other, but the plates they were on moved apart like two convener belts.",
"How could they have been one continent and later drift apart if both plates have underwater sections as wide as half an ocean? ",
"Plates are created on one side and melted on the other. In this case, solid rock was created between SA and Africa and so they drifted apart.",
"http://en.wikipedia.org/wiki/File:Plates_tect2_en.svg",
"Do you see those arrows? Those represent how plates are moving. Where the arrows point apart, land is created. The opposite is true for when the arrows point at each other.",
"And why would super continents form every few hundred million years?",
"Eventually the continents will collide at the places where the arrows point at each other.",
"Thats one huge bulge on one side of the planet. Kind of strange with all that gravity acting on the plates and the mantle trying to keep the shape of a spheroid.",
"Not really, continents are geologically flat and light. They might seem large to you, but are not to the earth."
] |
[
"Continental crust is lower density than ocean crust so it doesn't get sucked under usually. "
] |
[
"I am too tired to do a full response to this but one correction is needed.",
"The mantle is not liquid. The mantle is solid. Near the surface of the earth, just under the crust (the lithosphere) is fairly runny but still a solid (like plasticine)"
] |
[
"What is the difference between an organism made up of smaller individuals(e.g. sea salp, pyrosome) vs. a multicellular organism?"
] |
[
false
] | null |
[
"A salp ",
" a multicellular organism. ",
"I think you’re confusing two concepts uni-& multi-cellular organisms vs colonies.",
"Typically a multicellular organism will have specialized cells. This is what differentiates, say, a group of choanoflagellates from a sponge. A choanoflagellate is one cell type - basically a choanocyte , though they may group up. A sponge is made of choanocytes, amoebocytes, pore cells, etc. furthermore tissues work together.",
"Organisms that are unicellular may come together and form colonies, for protection. Protists do this, bacteria do this, but so do multicellular organisms. We see this mostly In sessile organisms- for example coral. Once organisms become more motile (eg fish) it’s harder for them to “stick” together but they may still group (eg forming schools)."
] |
[
"Isn't the defining characteristic whether the cells have the same DNA or not?",
"Many cells with different functions but same DNA -> multicellular organism",
"Collection of very similar cells with different DNA -> colony"
] |
[
"Most (nearly all) colonial organisms are made of clones and also have identical DNA in all cells."
] |
[
"Since there is evidence of our universe expanding, does that mean that the universe is finite?"
] |
[
false
] |
If anyone could link articles are other informtion about it I would appreciate it.
|
[
"Imagine the real number line. Now consider some interval along the line, say from -1 to +1.",
"Now multiply the whole thing by two.",
"The interval you were considering is now twice as large as it was; it goes from -2 to +2. But the line started out infinite, and it was infinite after you doubled it.",
"In other words, no. The expansion of the universe does not require that the universe be finite."
] |
[
"It's easier to think of the universe just getting less dense with time."
] |
[
"That doesn't follow. Just because the universe is infinite it doesn't mean it will have such and such density. Also, there probably is a star in all and every angle, it's just that we're limited to the observable universe."
] |
[
"Why are angles measured in radians?"
] |
[
false
] |
Is there a particular reason why in trigonometry, angles are measured in radians? Is it just a standard, or is there some "sense" or "logic" behind it? Wouldn't it be simpler to measure angles in "turns" so as 1 turn =360 degrees = 2 Pi Radians? I mean, we still end up expressing everything in fractions of Pi.
|
[
"Are you familiar with calculus? Radians make a lot of sense when you start thinking about the derivatives of trigonometric functions.",
"For example, if theta is measured in radians, the derivative of sin(theta) is cos(theta). If theta is measured in \"turns\" (as you propose), the derivative of sin(theta) is 2pi*cos(theta). Most expressions involving trigonometric functions end up looking much simpler if angles are measured in radians."
] |
[
"A radian is the angle formed by taking an arc whose length is exactly the same as the radius of the circle. This means it's really simple to go back and forth from the angle to the circumference to the radius to the area. ",
"There are alternative ways to measure angles, but none of them are as easy as radians when you're doing any sort of trigonometry."
] |
[
"sin(x)=~x",
"Near zero, that is."
] |
[
"What purpose does skin-to-skin contact with the mother and baby after childbirth serve?"
] |
[
false
] |
I'm learning a lot about childbirth today because my close friend is having her first baby any moment now. Is there a physiological response between the baby and the mother? What happens when the baby or mother does not have skin-to-skin contact?
|
[
"There are only hypotheses at this point, but there are a lot of observed benefits. ",
"From survival to weight gain and attachment.",
"And it costs very little to accommodate,unless you're in the US when its an extra fee."
] |
[
"I was told skin to skin, particularly soon after birth, helps to calm the baby and mother. The baby can hear the same heart beat it was exposed to in utero, and the resonance of mothers voice through her ribs is closer to the in utero experience also. This will calm the baby, and that calming has a positive feedback to mother.",
"This continues for at least the first 6 months, and works best if it's done regularly. It can also work with other adults, but is best from the primary care giver.",
"Not an expert, but in the UK Children's Centres give free parenting classes to everyone who wants to attend, and that's the explanation they gave us."
] |
[
"No, babies are exposed to the microbiome naturally as they come out. And it’s been tested pretty extensively in the US, where healthy babies are being born via C section about a third of the time"
] |
[
"why do anti-depressants take time to work?"
] |
[
false
] |
ie, they usually are advertised as taking 1-2 months to show any real effect. Why is this?
|
[
"Yeah, of course it isn't. As i fully acknowledged, \"more research is needed before we'll understand it completely.\""
] |
[
"Yeah, of course it isn't. As i fully acknowledged, \"more research is needed before we'll understand it completely.\""
] |
[
"Did you see ",
"this?",
" It happens to be asking your exact question. ",
"This sort of depends on what you mean by \"work\". If you mean a noticable mood change then it's important to note that moods are controled by many more chemicals and receptors than are affected by antidepressants. Individual chemicals and receptors may be 'on' or 'off' in status, but that doesn't mean that as a whole you're mood is 'on' or 'off'. ",
"The brain also likes consistency and patterns it's used to, so if you've been stuck in an emotional rut, you will likely still be in that rut, even as your brain is pushing back to less well worn 'emotional pathways'. (",
"Though you may be able to change that",
" )",
"Your emotions also can in fact go two ways at once (hence why we even have a word like 'ambivelant'), and what you experience is a reflection of how your 'mental chemical soup' is balanced. That balance will shift over time, and you'll still have to contend with other emotional triggers. If you have thoughts that inflict strong emotions on you, and you don't get rid of those thoughts, you might as well be poping another pill by having those thoughts, one that functions to keep you depressed. ",
"Cognitive behavioural therapy is based on the idea that mood is very affected by your thought patterns",
". (For further reading on thought patterns, ",
"here's",
" an article on psychcentral that deals with the topic.) ",
"Also, and this is very important, chemicals don't affect everyone the same way, and some people may feel near immediate relief, and others may be resistant and not feel the effects for quite a long time after, or not at all, or it may work against their mood rather than helping it. This is why psychiatrists have to try and very carefully balance and regulate medications in the first place.",
"I hope that answers your question at least to a degree.",
"Edit: Another article that covers the topic: ",
"By Jonathan Crowe"
] |
[
"If ball bearings are continuously dropped at 1-second intervals from some height above the ground, will they also hit the ground at 1-second intervals?"
] |
[
false
] |
If not, would would be the formula to calculate what the time intervals between consecutive ground hits would be given a drop height and regular interval between consecutive drops? Edit: I feel kind of silly asking this since I'm almost sure that the answer is "yes". The reason I ask is because in trying to have an intuitive understanding of gravitational time dilation, I came across a diagram that showed an accelerating rocket ship with a photon emitter near the front pointed at the back, and a photon receptor in the rear. The diagram explained that since the rocket was accelerating, if photons were emitted at (locally to the front) 1-second intervals, then the photon receptor in the back would receive photons at intervals longer than 1 second. I then thought if this were true, it would also have to happen with anything, wouldn't it? But I don't think it would happen with ball bearings.
|
[
"Ah, well given your question I assumed a classical system in an inertial reference frame where nothing was moving besides the ball bearings. That logic can't be simply applied to other situations like the rocket example you give. To take a classical example:",
"Suppose I am in an elevator with an open top and you are standing on top of the building dropping ball bearings down the elevator shaft ever 1 second. If the elevator is not moving, they will indeed arrive every 1 second. However, if the elevator is moving upwards, each successive ball bearing has to travel less distance before it lands in the elevator, meaning each successive ball bearing will arrive less than 1 second apart.",
"The rocket example you are talking about is often used to demonstrate changes in the rate of time in a gravitational field, or equivalently in a non-inertial (accelerating) reference frame. The photons will indeed not arrive at 1 second intervals, despite being sent at 1 second intervals, because the rocket is accelerating."
] |
[
"Interesting that everyone is saying yes.",
"Really, it depends on who is counting the seconds:",
"If the person at the top of some tower is dropping them at one second intervals, the person at the bottom of the tower will definitely measure them as hitting the ground at less than one second apart. This is the simplest way to envision the manifestation of curvature in spacetime.",
"Edit: If the balls themselves are counting the seconds, then one second at the top will be one second at the bottom."
] |
[
"The rocket example you are talking about is often used to demonstrate changes in the rate of time in a gravitational field, or equivalently in a non-inertial (accelerating) reference frame. The photons will indeed not arrive at 1 second intervals, despite being sent at 1 second intervals, because the rocket is accelerating.",
"I'm still lost as to why this wouldn't happen with ball bearings though. If you set up the exact same thing (a ball bearing emitter in the front of the rocket and a detector in the back), why would they arrive at the detector at 1-second intervals, but light pulses wouldn't? I think it has something to do with the speed of light being constant, but I can't quite make the connection as to why that would cause this effect. I'm also not sure how the constancy of the speed of light really works in an accelerating reference frame. I've heard it said both that it's always constant even in accelerating frames, and that it's only constant locally in accelerating frames, but in some senses can be seen as not really constant in the overall picture. Since gravity can bend light, it seems that at least something non-constant is going on, whether this goes beyond just a change in direction I don't know."
] |
[
"If you have the flu, does every exhale you take contain the flu virus?"
] |
[
false
] |
I was talking to a friend and they were arguing that if you don't get the virus within 10 minutes being around someone, then you can't get the virus. I didn't think that was true though because 1.). I'm not inhaling every particle that the sick person is exhaling, so it could take longer than 10 minutes. 2.). Not every exhale that the sick person exhales contains the virus particles/amount of virus particles. But I'm not sure if these are actual facts or not, hopefully someone can give me more insight, thank you.
|
[
"Yes. ",
"... viruses are very efficiently spread through aerosols by the patient’s breathing only. It is not necessary for the patient to cough or sneeze. ... The fact that the spread of different viruses occurs through normal breathing of infected persons has now also been proven by various other working groups. ",
"—",
"Breathing Is Enough: For the Spread of Influenza Virus and SARS-CoV-2 by Breathing Only"
] |
[
"There's also the matter of how much you get exposed to at once.",
"The body takes time to respond to infection. ",
"If you start out with 1000 infected cells, you'll end up exponentially more sick before your body has the response ready to go.",
"If you start with one infected cell, the body has a lot more time to ramp up before the virus can spread all over your body.",
"A small serving of virus will make you less sick than a big serving, unless it was going to grow until you were dead anyway."
] |
[
"I don't know about #2, but #1 is certainly true. ",
"Also, there's an incubation period for every disease. Even if you acquire the disease within minutes, you won't know about it for a little while. For influenza/the flu, the incubation period is normally 1-4 days. During this time the virus replicates until there are enough copies of it to start causing symptoms. Also during this time, your immune system tries to kill it before it starts making you sick. You might feel extra tired or have minor symptoms indicating that your immune system is responding to something, like a runny nose or mild cough, or you might not. If the immune response is successful, you don't get sick. If it isn't, you do."
] |
[
"Quantum erasers: is one single photon capable of interfering with itself?"
] |
[
false
] |
[deleted]
|
[
"You may be looking for ",
"this paper",
".",
"The single-photon experiments are just creepy and feel very intuitively wrong; there's no way to interpret it without a particle being in more than one place at once. The worst of them all is probably the ",
"delayed-choice version of this experiment",
" wherein the which-path information is erased ",
" the photon has hit the screen."
] |
[
"Yes, you see that interference patterns you see in Young's double-slit experiment or a Michelson interferometer are still there even when you work with individual photons. "
] |
[
"I feel the poster made that assumption when writing \"if we don't try to measure which path\". But I guess it bears underlining that a \"measurement\" in QM doesn't neccesarily mean an deliberate attempt to measure in the everyday sense."
] |
[
"What do the x and y stand for when I hear about the new HxNy flu threat?"
] |
[
false
] |
H1N1, or "Swine flu", was the first time I had seen this systematic naming, but now, apparently, there's an outbreak of H7N9, and I seem to recall a concern over H5N1. What information is conveyed in these names?
|
[
"Hemagglutinin (HA) and neuraminidase (NA) are the two large glycoproteins on the outside of the viral particles. HA is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell, while NA is involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles.[53] Thus, these proteins are targets for antiviral drugs.[54] Furthermore, they are antigens to which antibodies can be raised. Influenza A viruses are classified into subtypes based on antibody responses to HA and NA. These different types of HA and NA form the basis of the H and N distinctions in, for example, H5N1.[55] There are 16 H and 9 N subtypes known, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.[56]",
"For more: ",
"http://en.wikipedia.org/wiki/Influenza#Virology"
] |
[
"To add to this, Birds have a large pool of H and N varients lots more than humans have (as stated). These H antigens are responsible for attaching mainly to your respiratory cells thus inducing the infection. Therefore if a human does not have say H6 then the disease will not be readily transferable. However in close contact with birds there is a chance that a human will contract bird flu but and a big but is that it will not able to transfer from human to human. ",
"What is possibly worrying is the virus infecting another animal or human and mutating so that it does become infective directly to humans eg develops H1,2 OR 3. Pigs for example have some H variants found in both humans and birds and there is a possibility that a bird virus say for example H5N2 and and human disease say H1N3 can mutate together in a pig and combine DNA and produce say H1N2 which could then have potential to spread to humans and will be transferable from human to human."
] |
[
"And this is what makes these viruses so annoying to combat. The strains can be mixed and matched in any which way (in theory). The flu vaccine is actually just a big probability game where they decide which strains are most likely to give you the flu this season. Other viruses besides Influenza (which is an orthomyxovirus) can do this too. For example, Arena virus, Bunya virus and Reoviruses have similar segmented genomes."
] |
[
"How does a breathalyzer test represent your blood alcohol content? How does a test of the breath give accurate information on the alcohol content of the blood?"
] |
[
false
] | null |
[
"Deep in the lungs are alveoli which have very small capillaries all over them. This is where gas exchange occurs. The oxygen we breathe in crosses over the capillary membrane and into our blood (mostly bound to hemoglobin) and carbon dioxide crosses from the blood and into the lungs to be exhaled. But carbon dioxide isn't the only thing that crosses this membrane. The ethanol (from the alcohol you drank) also crosses this membrane to be exhaled, where it can be easily detected and quantified using a breathalyzer."
] |
[
"This answer is correct, but I just wanted to provide some numbers in case OP was interested. About 95% of the ethanol we take in is metabolized enzymatically, whereas the remaining 5% is excreted unchanged in the lungs. Breathalyzers take advantage of this to calculate BAC from the breath sample.\n-An interesting side note about the alcohol that is metabolized: while most of the metabolism of ethanol is done in the liver, about 15% of the metabolism is actually performed by enzymes in the stomach lining. Women tend to have less of the enzyme responsible (alcohol dehydrogenase), which explains why women have higher blood ethanol levels than men after drinking the same amount per body weight."
] |
[
"A breathalyser is not not accurate enough to charge someone with DUI (in Australia at least)\nIf you test over the limit on a breathalyser you get taken to the station for a blood test, it's just a less invasive initial test."
] |
[
"How many genetically diverse people would it take to successfully colonize a new world without the effects of inbreeding?"
] |
[
false
] |
[deleted]
|
[
"I recall learning about this in my ecology classes but didn't recall offhand what the numbers were so I did some searching. ",
"It appears",
" that between 50 and 500 individuals would be needed for a successful population. 50 individuals would prevent inbreeding while 500 would ensure genetic variability. The site I linked just gives a brief overview of minimum viable population sizes but has citations if you want to read further. As for the male to female ratio I'm assuming an equal ratio but am unaware if this is the case."
] |
[
"I'm not sure what the number would be, but I'd like to point out it would be a far smaller number if you brought along a bunch of sperm and eggs. Each new offspring from the colonists could be genetically distinct. "
] |
[
"Here's a ",
"similar post",
" and the response I gave there. The take-away is the number's higher than you might think, dependent not only dependent on the initial diversity of the population, but also the size in general (see the bit about genetic drift)."
] |
[
"would there be muzzle flash if a firearm was fired in a vacuum?"
] |
[
false
] | null |
[
"The muzzle flash of a firearm is the explosion that is propelling the bullet out of the barrel following the round and exiting behind it. That explosion is produced by the propellant inside the bullet's casing (or the casing itself for caseless ammunition). The explosion would still be present, though it may appear differently in a vacuum.",
"Edit: Also, the propellant contains its own oxygen and would ignite within a vacuum."
] |
[
"Most modern gunpowders contain their own oxydiser. So if you were to fire a gun in space, it would shoot. The flash may look different, but it would still be there."
] |
[
"thank you. "
] |
[
"With current electricity production methods that are in use, how much greener is an all-electric car compared to a petroleum car?"
] |
[
false
] |
Obviously a traditional petroleum car produces CO2 and other nasty gases, but if I plugged in a car, wouldn't some power plant be burning something else producing similar gases somewhere else in order to power the car? In this case, the only difference is where the fuel burns. Because I am primarily concerned with greenhouse gases, I am asking from a chemistry perspective. From what I understand nuclear power only requires cores and cooling usually with water, though waste is a problem, so though not totally true could we assume that power from nuclear plants is pure and without side effects [for this question at least]? In the US nuclear power roughly accounts for 19% of all power production. Also in the US renewable energy accounted for 12.5% of power production in the US in 2012 apparently. All these factors included, who wins the green fight? Whoa, thanks for the massive response. Just got home so I'll be reading through the night
|
[
"You are certainly right that switching to electric cars doesn't directly eliminate the problem of greenhouse gas emissions if the electricity used to power them was generated through methods that themselves release CO2. The main advantages of electrical power, however, are two-fold. Perhaps the most immediate advantage is that they are much more efficient than combustion engines. If gasoline powered engines have an efficiency typically around 15-20%, the value can be four times higher for electrical cars. ",
"Secondly, electrical cars have the advantage that they are at least compatible with clean energy sources, while gasoline engines by design are not. What this means is that as the share of \"clean\" energies, e.g. stemming from nuclear and renewable energy sources will increase, only electrical cars would benefit from this change in terms of environmental impact. "
] |
[
"Some very rough math to conservatively estimate the carbon intensity of an electric car like the Nissan Leaf, if you were to charge the battery from exclusively coal-fired power stations:",
"Carbon emissions per kWh produced by average U.S. coal power station:",
"~ 1 kg CO2 / kWh",
"Natural gas is ",
"roughly half of that",
"Losses from transmission: ",
"say ~7%",
"So let's just call it 1.1 kg CO2 emitted for each coal-based kWh that actually gets into the battery of the car, what with some losses in charging and all that. Say 0.6 for e- from only natural gas. ",
"A Leaf in rigorous driving conditions gets maybe ",
"3.5 miles per kWh.",
"So call it a worst-case carbon intensity of: 1.1 / 3.5 = ",
" for the Leaf.\nBest case (still fossil-fuelled) would be ",
"A gasoline powered sedan that gets 25 mpg has a carbon intensity of ",
". ",
"8.92 kg CO2 per gallon / 25 mpg",
"\nA car that gets 30 mpg is on-par with the coal-only electric. To get to the natural gas sourced electric you need 50 mpg on conventional petrol. ",
"In all this, I am ignoring a bunch of stuff, including lifecycle emissions, but this is a ballpark answer.",
"However, if you look at the (current) economics, what we really should be doing is making all ICE vehicles into hybrid electrics - you get the most emission reductions per mile traveled for the least increase in vehicle base price.",
"TL;DR: Even powered solely by coal-fired power stations, the higher power-train efficiency of an electric car results in CO2 emissions comparable to a 30 mpg conventional ICE vehicle. ",
" Since many commenters below have noted that I disregard emissions generated by battery production and other upstream processes, I would like to refer everyone to this excellent study done at UCLA ",
"Link to Study - PDF",
"Not only do they account for many of these upstream or lifecycle emissions, but it is overall a very comprehensive and rigorous analysis of this whole question."
] |
[
"Missing from this analysis is the fact that gasoline is responsible for significant additional emissions before the fuel even makes it to the car.",
"The refining process in particular requires a boatload of electricity and other fossil fuel energy. The EIA estimates about 6 KWH per gallon refined. ",
"That's enough energy by itself to take an EV 20-30 miles, yet gasoline needs that much energy just to prepare the fuel. ",
"At least electric vehicles don't burn a side of oil with their electricity."
] |
[
"Why do human males have nipples? What purpose do they serve?"
] |
[
false
] | null |
[
"Because in theearly stages of pregnancy the fetus is sexually ambiguous, and the 'stronger chromosome' essentially decides the sex of the baby during later stages of the gestation period "
] |
[
"To add to this, it's called a ",
"spandrel",
", borrowing a term from architecture."
] |
[
"From conception until 6 weeks, the fetus is asexual. This means physical traits of the body develop the exact same way. Around this time, a genetically male fetus (XY chromosomes) starts producing testosterone & continues as a male (no mammary glands develop), while a genetically female fetus (XX) produces estrogen & continues as a female (mammary glands develop but remain dormant until puberty). ",
"TLDR: For 15% of human development, we're all growing the same cells...nipples too."
] |
[
"Could a massless particle gain mass?"
] |
[
false
] |
[deleted]
|
[
"Well, what you're basically describing is the Higgs mechanism (ever heard of the Higgs boson?).",
"The basic idea is that there is a quantum field, which we call the Higgs field, permeating all space. It's similar in concept to other fields you're probably more familiar with, like the electric or magnetic or gravitational field. A 'field' has some strength at all points in space, which in turn affects matter. For example, a stronger magnetic field will more easily deflect the needle in your compass, for example. Some particles are magnetic, and so they'll 'feel' the field, while others are not magnetic, so they won't. ",
"Similarly, the 'strength' of the Higgs field sets how strongly it interacts with other particles, which in turn gives them mass. As an analogy, imagine the Higgs field is a giant ocean of honey or something sticky that fills space. By default, you could imagine all particles are massless, but many particles will 'interact' with that honey will get stuck in it, and be slowed down. They'll travel slower than the speed of light, and have mass. Meanwhile, particles that don't interact with this field at all will 'remain' massless, such as photons. ",
"This is part of the reason that the observation of the Higgs boson at the LHC is such a big deal- it verified this model of mass generation. Everything I told you is very proximate to the actual physics at work, if a particle theorist thinks they can give a more technically accurate but still accessible explanation I encourage them to respond. ",
"So, if you're some sci-fi writer who wants to have magic under the guise of physics, just have a machine that turns the strength of the Higgs field up or down so that the masses of particles change- or maybe even they become massless and whiz away some distance at the speed of light."
] |
[
"I recall reading something about how the Higgs field only accounts for a small portion of the mass in atoms, and that the rest of the mass is binding energy.",
"Yes.",
"If the elementary particles in an atom were to stop interacting with the Higgs field, what would happen to this binding energy? Would it stay, maintaining mass? Or would the atom fall apart? Or something else?",
"Physics would be different because you would now have systems of massless particles and it's unclear why they should form the same bound structures."
] |
[
"But can a particle that doesn't interact with the Higgs field somehow change and begin to do so?"
] |
[
"Can we talk yet to any other species in our world?"
] |
[
false
] | null |
[
"No non-human species has language as we do, so in that sense no. "
] |
[
"We will be able to make it happen? Or no one is interested? I mean that could lead into helping other species to evolve it makes me wonder and dream big."
] |
[
"As far as we know, no known species currently have the capacity for language. We can communicate with them, but their methods of communication do not have the hallmarks of language. ",
"Here",
" is a discussion of some of these features. This theory is a bit outdated at this point, but the general ideas are still there in terms of how we distinguish between language and communication."
] |
[
"Questions on the relativity of motion."
] |
[
false
] |
[deleted]
|
[
"According to the special theory of relativity, if you are traveling at absolute constant velocity, you can claim that you are stationary and the rest of the universe, and observers can claim they are stationary and you are not, and both claims are right.",
"Yep.",
"But how can they both be right? If the rest of the universe is travelling around you, then the total energy of particles would increase, increasing mass, and gravity, and we would be able to detect these things. ",
"In fact, you do measure an increase in the energy of objects that are moving relative to you. Of course, this was true in Galilean relativity as well, where an object at rest relative to your coördinate system had zero kinetic energy and zero momentum, while one moving relative to your coördinate system had both nonzero.",
"As for increased gravity, that's more complicated and is basically the entire point of the general theory of relativity. Basically, gravity is like an ",
", and the special theory of relativity does ",
" say that an accelerating observer gets to consider themselves stationary and everything else to be accelerating."
] |
[
"Gravitomagnetic",
" effects come to the rescue and are equal and opposite to the increased gravitational field. It's basically the same thing as in electromagnetism, where changing reference frames turns magnetic fields into electric fields and vice-versa. Think of two charged particles in the lab frame, where they are repelled due to their electric field. Now shift to a moving reference frame where they also have magnetic fields. The magnetic fields exactly cancel with the lorentz-contracted electric fields so that measurements are consistent in each reference frame. "
] |
[
"Gravity is the curvature of spacetime, but Einstein's equivalence principle asserts the equivalence of an accelerating reference frame to one in a uniform gravitational field. The effect of gravity is ",
" like an acceleration.",
"To quote Einstein, \"we shall therefore assume the complete physical equivalence of a gravitational field and the corresponding acceleration of the reference frame.\""
] |
[
"Building Height math.."
] |
[
false
] |
How tall would a building in Washington DC have to be, to see the skyline of Philadelphia?
|
[
"Man, I really wish reddit had a decent search function, because a few weeks ago I answered almost this exact comment, except it was with New York and San Francisco.",
"Anyways, the idea is this. The distance between New York and Philadelphia is 129 km as the crow flies. The circumference of Earth is 40,000 km, so the angle between New York and Philadelphia, as seen from the center of the Earth, is 129/40000=x/(2*pi), or x=.02 radians. If you look at ",
"this picture",
" and think about it for a moment, it should be clear the height we want will be r(sec(x)-1), where r is the radius of the earth and x=.02 radians. We end up with 1.27 km. The tallest building on Earth is the Burj Khalifa, at .83 km. ",
"So you need a tower that's as tall as the tallest building that currently exists and then half as tall again to see New York from Philadelphia or vice versa. I don't know much about engineering; maybe the Burj Khalifa is at the absolute limits of our current materials technology, maybe it isn't. But that doesn't seem too unreasonable.",
" Man, did I mess up, you said DC and Philadelphia, not New York and Philadelphia. Oops. That distance is 219 km, so 6378(sec(219*2pi/40000)-1)=3.78 km"
] |
[
"They are 220 km apart. According to an approximation, the height required to see a distance d is (d/3.57 km)",
" , so about 3.8 km high."
] |
[
"Nobody has factored in realistic cases of refraction yet. Is the effect significant?"
] |
[
"Would it even be possible to revive the people who have been cryonically preserved?"
] |
[
false
] | null |
[
"Modern cryo attempts circulate anti-freeze compounds before bringing to liquid nitrogen temperatures to minimize crystal formation. So crystalization isn't that much of an issue. See ",
"here",
" \nThe short answer to OP's question really is that we don't know either way if cryonically preserved individuals will be able to be revived but successful similar work has occurred with individual organs such as kidneys. See ",
"here",
". "
] |
[
"For preserving whole humans there are still a few things to worry about. One is that you have to use a ",
" of the cryoprotectant chemical (since humans are quite large compared to a single organ, or an even tinier cluster of cells, and you have to be certain that it penetrates to every part of the body), and the chemicals are themselves toxic in high concentrations (which shouldn't be a problem as long as you stay frozen ...). The other is that, at least in the U.S., you aren't allowed to cryopreserve people until they have been declared legally dead, which means they are obviously going to be in a worse state than someone who was frozen while still healthy. You wouldn't want to thaw them until you knew you could fix things. ",
"On the other hand, for freezing clusters of cells smaller than an entire human, cryopreservation is in use right now, and it works very well. Embryo cryopreservation is now commonly used as part of IVF, and ",
"implanting thawed embryos appears to be either just as good or even better than implanting freshly fertilized embryos",
". There have been over ",
"300,000 children worldwide born from frozen embryos",
". "
] |
[
"Currently, no. One of the biggest problems with freezing someone is that we are primarily water, and when frozen, water crystallizes. Imagine billions of water crystals forming in your blood vessels, brain, and other organs. These would (and do) tear these tissues to shreds. Thawing someone out just turns them into a pulpy mess. When we can effectively freeze someone without this crystallization, thawing them will be the next step. "
] |
[
"Why are rockets/ spacecraft corrosion resistant, if there is no oxygen in space?"
] |
[
false
] |
I was reading about the different types of alloys used in rockets, and many of them are labeled as 'corrosion resistant'; does this actually matter or is it just a useless byproduct of the alloys that rockets use? (btw, sorry if I used the wrong flair.)
|
[
"There are 3 main aspects that are relevant there on why you would want corrosion resistant materials.",
"First while the goal is to send the thing to space it will spend a significant amount of time on the ground first. Even in a clean room environment you can get corrosion. Moreover contrary to a lot of other vehicles and systems you can't easily protect the materials with paint, oils or other rust inhibitor because they are typically not vacuum compatible. Rockets also often launched close to the coast where they can have to spend days in hot humid environment where there can be salt water spray.",
"The second thing for some parts is that a lot of propellants are very chemically agressive and corrosion resistant materials are usually also pretty resistant to chemical reactions.",
"The last one is that there is actually oxygen in space! It's a very tiny amount but at the very top of the atmosphere you get what is called \"atomic oxygen\", basically instead of O2 it's single atoms of oxygen. Those are way way more agressive than normal oxygen and it can be a significant problem after years in low earth orbit. Plastics might get attacked for example. Using stainless steel or passivated aluminium really helps cutting this effect."
] |
[
"Thank you! I can't believe I forgot that rockets aren't always in space lol"
] |
[
"Trust me, everyday I whish we could build spacecraft directly in space. It would make a lot of things so much easier."
] |
[
"How does curved spacetime cause acceleration?"
] |
[
false
] |
If I'm free-falling, I feel no forces. So how would I explain the fact that the earth is accelerating towards me? I understand that I'm on a geodesic that minimizes my action, but how does the fact that spacetime is warped cause me to move in the first place? Edit: Please only reply if you have a working knowledge of general relativity
|
[
"For simplicity, let's treat a universe with just you and one other person, so we don't have to deal with orbital stuff.",
"You and this person are initially at rest relative to one another. If spacetime were flat and didn't respond to your masses, then your worldlines (essentially, the collection of all spacetime points you occupy) would remain parallel: you and this person, both at rest, would appear to remain at rest relative to one another.",
"Now, in the presence of curvature, those worldlines, following the curvature, bend toward one another. In either reference frame, the distance between you gets smaller over time. Moreover, the rate at which the distance decreases gets bigger over time, so there's an apparent acceleration.",
"An analogy:",
"You and your friend start out on the equator and both walk due North. If Earth were an infinite flat plane, you would remain the same distance apart. But as it's not, you find that the further North you get, the closer you get, and that the rate at which this happens increases as you get further North. In this analogy, your distance North of the equator corresponds to ",
"."
] |
[
"coördinate",
"I had never heard of this before.",
"https://en.wiktionary.org/wiki/coördinate",
"Alternative spelling of coordinate"
] |
[
"Moreover, the rate at which the distance decreases gets bigger over time, so there's an apparent acceleration.",
"Also, you don't ",
" the acceleration the way you would with, say, a rocket engine. It feels like your moving along a straight line, because in the metric, you technically are. ",
"(Of course since you're made out of lots of objects, you do feel tidal forces) "
] |
[
"How quickly does boiling water sterilize a surface?"
] |
[
false
] |
[deleted]
|
[
"Disclaimer: Don't take any medical advice from reddit, and yes this kind of counts because you made it relevant to yourself. However I'll explain because it's telling you not to.",
"A common sterilisation technique used in science is autoclaving which involves steam at around 120C for around 20 minutes to sterilise glassware. So no, stirring boiling pasta water will not truly sterilise your fork. It will ",
" kill ",
" bacteria. Although there are also endospores which are still perfectly happy at boiling temperatures, and there are also bacteria that are happy up to 80C (Thermus aquaticus). USE SEPARATE UTENSILS IT ISN'T THAT HARD."
] |
[
"So how is it that chicken need only be cooked to an internal temperature of 165 F to be considered safe? That is significantly less than the 212 of boiling water."
] |
[
"This might not be 100% accurate, but I would assume it has to do with the main thing you are getting rid of is salmonella. It is recommended to heat at 167F for 10 minutes to kill salmonella. "
] |
[
"Do we know the general lifespan for dinosaurs?"
] |
[
false
] |
Of course, it would differ from species to species, but have we been able to date bones? Or are we only able to compare them to modern reptiles/birds...
|
[
"Most dinosaurs have lines of arrested growth in their bones which most scientists think are laid down annually (think tree rings essentially). This allows us to estimate ages based on the assumption that one LAG = 1 year (this is not necessarily true, but it is not a bad assumption and even if it is wrong, i.e. because the lines are laid down biannually or something, it would still give us correct relative ages between dinosaurs).",
"Obviously lifespan differed between species. \"Sue\" the famous ",
" is believed to have been 28 when she died. Some sauropods may have lived as much as a century, while hadrosaurs (duck bills) and ceratopsians (horned dinos) were likely in a similar range to ",
", around 3 or 4 decades. This is a rough estimate, but I believe it's fairly accurate.",
"If you guys have any other questions about dinosaurs, please feel free to ask me ",
"here",
"!"
] |
[
"Amazingly the answer is no. One year has stayed roughly the same length in seconds, but the day has been growing steadily longer due to ",
"tidal friction",
". The Moon is slowing us down. ",
"But this effect wouldn't be huge. ",
"This paper",
" estimates the change at only 2 hours a day over 650 million years. So (roughly) a day would only be 20 mins shorter 100 million years ago."
] |
[
"did one year 100-200 millions year ago last around the same amount of time as now? e.g. did the earth have about the same number of days and did each day last about the same duration as the days now?"
] |
[
"Why do people with dementia experience hallucinations? In particular, why do the vast majority experience seeing children?"
] |
[
false
] |
My grandmother has dementia and whenever I visit her she talks about seeing children in the room with us. I’m particularly interested in this phenomena because she is blind.
|
[
"There are probably two things going on here; dementia in combination with Charles Bonnet syndrome.",
"\"Charles Bonnet syndrome\" (",
"http://www.rnib.org.uk/eye-health-your-guide-charles-bonnet-syndrome-cbs/understanding-charles-bonnet-syndrome",
") is a relatively common syndrome in people with deep blindness in their central visual field, usually caused by bilateral macular degeneration, but possibly due to other causes. Patients with this kind of blindness will often experience hallucinations, frequently in the form of people (often small people or children). They will report that the people are just standing still or milling around nearby, not moving very much or very far. It sounds very disconcerting.",
"Patients who are simply blind will typically recognize that they are hallucinating, but they are often embarrassed to admit that they are having hallucinations, for fear that it is a sign of mental illness - but Charles Bonnet syndrome is not really a psychiatric problem, it is a consequence of blindness. The human visual system is ",
" primed to see certain kinds of objects, especially people and faces, that in the total absence of input it may begin to interpret neural noise from the now-dormant central visual field as actual stimuli (making CB syndrome a kind of pathological pareidolia; cf ",
"https://hal.archives-ouvertes.fr/hal-00587980/document",
").",
"So normally, someone with CB would know they are hallucinating and would not actually believe that there are e.g. non-existent children there in the room. But combined with dementia, you would expect that the patient's ability to distinguish hallucination from reality is going to be pretty degraded (but I don't have any expertise in dementia so can't say more about that side of things)."
] |
[
"Where did you hear it? Explanations for phenomena are only useful if the phenomena is backed up with strong evidence."
] |
[
"Thanks for the thorough reply! It makes a lot of sense.",
"You wouldn’t happen to know why dementia patients see children in particular though do you? I’ve heard it is the most common hallucinations attributed to the disease."
] |
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