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|---|---|---|---|---|---|
[
"Do any non-human animals use other animals to do their work?"
] |
[
false
] |
Are there examples of animals domesticating other animals or at least using the work of another animal to their advantage?
|
[
"there are ",
"ants that enslave other ants",
"there are also ",
"baboons that have pet dogs"
] |
[
"Some types of ants are known for farming ",
"aphids",
" for their honeydew."
] |
[
"Helminths",
" and other parasites might qualify."
] |
[
"What is the relation between becquerels and Sieverts?"
] |
[
false
] |
With the Fukushima incident, we hear of becquerels and sieverts as ways to measure, I guess, radioactivity and dosage, respectively. How should I think about the relationship between these quantities? If there are readings of 100 bq (?) in a room, and I am there for 5 minutes, is there a way to compute how many millisieverts I will absorb? Is there an equivalency between becquerels and sieverts, eg, 1bq = 1 sv/h. ?? Something like that? just trying to get my head around these units.
|
[
"There is no direct relationship.",
"1 becquerels is 1 decay per second, irrespective of the decay type.",
"A sievert, on the other hand, is an empirical measure with a number of weighting factors, based on the type of radiation, energy level, and what tissue was exposed.",
"For example, alpha radiation is weighted 20 times more heavily than beta radiation. So an alpha source and a beta source might have the same measure in becquerels, but the sievert exposure of the alpha source would be 20 times higher. "
] |
[
"Very helpful."
] |
[
"http://www.johndcook.com/blog/2011/03/18/understanding-radiation-units/"
] |
[
"How strong would a magnet have to be in order to repel another magnet on which a person is sat on?"
] |
[
false
] |
I have a pretty basic knowledge of magnets but as a designer i'm curious to their application in seating, so say if a person sat on top of a seat that had a magnet underneath, then beneath the seat was a magnet that was repelling it, could you create the illusion that the seat was hovering? and how could you achieve it? much like this: thanks!
|
[
"There's a lot of variables here. In short, the magnet would have to be strong enough to produce a force that is greater than the force of gravity*the mass to be moved. The force produced by a magnet depends on several factors and is not always easy to calculate. I did find an online calculator that can give you some estimates for different types of magnets ",
"calculator link",
"If the main question is \"can this be done?\", I would say yes, but it may difficult to keep the object floating if the object is a person, because a slight shift in the body's position can throw off the equilibrium of the system."
] |
[
"Your best bet will be to buy/build an electromagnet"
] |
[
"thanks for the reply, im off to look into that calculator. buts it's good to know that it is achievable."
] |
[
"Why does converting from one video format to another so often cause the sound to go out of sync?"
] |
[
false
] |
I presume this has something to do with the fact that sound is more a continuous stream and video is made of frames, but why is this problem so notoriously difficult to solve? I would think that the streams have to have a time index of some sort, so why isn't the conversion just a straight algorithmic operation? Why should it ever get out of sync?
|
[
"There is nothing fundamentally difficult about getting it right, just a lot of details for the programmer to keep track of. On top of that many audio/video codecs and file formats are proprietary, vaguely defined, and/or poorly documented. And to add even more confusion the program that originally created the file may have been buggy and written the file in a workable but somewhat-out-of-spec way."
] |
[
"Modern media is stored in what's called a digital container. Common containers you may have seen are .mkv, .avi, .ogg or .mp4. A container is usually a flexible file format that allows all sorts of media streams to be stored within it. A container might have 1 video stream and 2 audio streams, or 2 video streams and 8 audio streams, or 1 video stream and 19 subtitle streams. Those video and audio streams might be of different formats (different codecs used to store them) and the container format tries to be agnostic about which specific audio or video technologies are used within it.",
"Audio and video streams are broken up into chunks and stored (often not together) in the container. The container format will provide some way to group disparate chunks together to be synchronized together into what's called a physical stream. For instance, one chunk of video data and one chunk of audio data might be synchronized together to form one segment of physical video playback that contains sound. In order for these to be synchronized properly, the video and audio have to be broken down into chunks properly ",
" their chunks have to be tagged in the container with time synchronozation information properly. When video and audio get out of sync, one of those two things has not happened properly.",
"As ee58 said, it's all down to software bugs and poor documentation, but hopefully that gives a better idea of where those documentation problems can come about. Every container does synchronization differently (e.g., .mkv uses actual wall-clock timestamps whereas .ogg has no concept of \"time\" at all) which means you have to code for each container differently."
] |
[
"I was going to comment on this but that pretty much covers that I was going to say. ",
"However the hardware is also responsible. A lot of slower machines like netbooks will get out of sync when playing high resolution video because of the processing power required.",
"edit: You know ",
"r/programming",
" would understand this in more detail."
] |
[
"How is entropy quantified?"
] |
[
false
] |
What makes something classified as more orderly than something else, and is there a way to quantify the entropy of a system?
|
[
"Entropy is difficult to explain because it is a fundamentally useful mathematical number that corresponds only loosely to what people intuitively think of as entropy.",
"When I think entropy, what immediately springs to mind is the density of states. Basically entropy is a measure of how many possible configurations a system can be in. This is complicated by a bunch of technical details (microstates, DoS definition, entropy is defined to be a logarithmic function, etc).",
"Here's",
" one of the more basic ways of stating the definition of entropy. It corresponds to the handwavy explanation I gave in the last paragraph."
] |
[
"It's actually quite beautiful. It's the number of possible states a system of a fixed energy can have (actually, the logarithm of that number).",
"The more states that a system can be in, but still be indistinguishable as far as energy goes, the higher the entropy."
] |
[
"If we take the macroscopic state as the number of heads and tails, then a 2-2 H-T distribution could be the result of:",
"While a 4-0 H-T distribution would be the result of ",
"So a 2-2 H-T distribution can be the result of any one of 6 microstates, while a 4-0 H-T distribution can only be the result of 1 microstate.",
"We can quantify the disorder using the entropy via",
"With kb the Boltzman constant and w the number of microstates associated with the measured macrostate. So for 2-2 H-T, S = kb ln 6, approximately 2.5*10",
" while the entropy for 4-0 H-T would be S = kb ln 1 = 0.",
"So a macrostate is a given result, for example, a 2-2 H-T distribution or a 4-0 H-T distribution, while the microstates associated with this macrostate are the microstates that result in 2 heads and 2 tails or 4 heads and no tails respectively."
] |
[
"Are there any examples of natural hybridization, or any possibilities of this occurring in nature?"
] |
[
false
] |
Have there ever been any naturally found examples of hybridization in wild animals? I know hybrid animals cannot breed and so would be rare one offs effectively but have we ever found an example of hybrid animals existing naturally rather than being the result of captive breeding?
|
[
"Yes, it's actually a pretty standard avenue toward speciation, aka the recognition of a new species ",
"https://en.wikipedia.org/wiki/Hybrid_speciation",
"It's more common in plants but it happens in animals as well. Hybrid animals actually can sometimes reproduce (that's how they can become a new species, after all), depending on the genetic qualities of the parents. Ligers (lion-tigers) are fertile and can mate with other ligers, tigers, or lions. The well-known example of the sterile mule is due to the fact that horses and donkeys donate 32 and 31 chromosomes to their offspring, respectively, so the child of a donkey-horse pairing (a mule) will have an odd number of chromosomes, leaving them unable to reproduce."
] |
[
"Completely normal. Even hybrid bears have been known (polar bear x grizzly). The Dingo population in Australia is suffering because they mix with feral dogs. It made the news when a 100% pure Dingo puppy (of a certain sub species) was found, he is now fathering little Dingos to help the species. Other animals also sometimes mix. I guess we never notice most hybrids as they either stand out and are preyed upon or just are not seen because it's obviously impossible to monitor this everywhere at all times."
] |
[
"Only female ligers are fertile, males are sterile. This is part of why we still consider lions and tigers to be different species not subspecies"
] |
[
"Plastic in the microwave, is there really any danger to it?"
] |
[
false
] | null |
[
"The majority of plastics can withstand microwave radiation and won't break down nor melt. If you are buying commercial products, you might be able to find \"microwave safe\" symbols on them, which I think look like three squigly lines. Essentially, if you're buying plastics that look like they were designed for storing food, then they are microwave safe.",
"Generally, for a plastic to be safe in a microwave it can't react with the food, leach out components in the food, or decompose. A lot of storage containers such as plastic butter containers, sour cream containers, etc., can either melt or decompose. This also includes water bottles- don't reheat water bottles if you want to be safe. Also, plastic bags from the grocery store should not be placed in the microwave.",
"Sometimes plasticizers in the plastic may leach into your food when microwaving. Specifically, fatty foods can make diethylhexyl adipate plasticizer (what makes the plastic soft) leach out. However, the FDA is on top of things when it measures and approves the amounts of chemicals that leach into the food, and these are quite small amounts. The FDA doesn't have \"microwave safe\" labels on all of the plastics, but that doesn't mean they aren't safe for use. They just haven't been sufficiently tested to know for sure.",
"The testing generally takes some assumptions into account, like what surface areas of plastic:food are going to interact, and the temperature that the plastic will reach, which is around 100",
" C for the boiling of water, which most foods are composed of. Some things get much hotter, though, like dry foods with oil (popcorn bags).",
" BPA is used as a precursor to the plastic called polycarbonate (PC). PC is used for those ",
"Nalgene",
" style bottles you see in heavy use. In other drinking bottles with different plastics, the levels of BPA are negligible. BPA is being studied for ill effects on the human body, however I'm not sure what the current consensus is in the biology field. It is potentially bad for the environment and some test animals, but I believe the testing on humans isn't fully conclusive. But it is probably harmful to some extent. I'd rather a biologist talk more about that one, though, don't take my word for it.",
"As far as the contamination is concerned, they believe that the temperature of liquid the bottles have been exposed to has a large effect on the release rate of this chemical. They took PC bottles and placed them in boiling water, after which the bottles released BPA anywhere from 15 to 55 times faster than if the bottle were only used with colder liquids. ",
"Source",
". If the bottles were used with only cool liquids, there was still a very small leak of the BPA into the water. It doesn't seem like the amount of BPA will cause damage, however those tests are still being conducted.",
"BPA can be found in polycarbonate plastics, and anything labeled {3} or {7} for the recycling code. BPA simulates the action of estrogen and \"",
"A growing number of scientists are concluding, from animal tests, that exposure to BPA raises your risks of heart disease, obesity, diabetes and childhood behavioral problems such as hyperactivity.",
"\"",
"Given all of that, I wouldn't put a Nalgene bottle in the microwave because the hot, boiling water may help leach what tiny amount of chemical there is in there, however something tells me it's not ",
" dangerous."
] |
[
"Thanks a lot, that was quite interesting."
] |
[
"I can't add anything to Sad_Scientists excellent comment but I have, out of concern for BPA, started using all glass storage ware (got a stack of nested bowls at Walmart that are thick and microwave safe). and have been quite happy with them. I eat a lot of leftovers so I use these bowls almost everyday and I at least feel safer.",
"Here are the bowls I bought:",
"Glass Lidded Bowls",
"Basic Storage Bowls"
] |
[
"How does star size affect planetary characteristics?"
] |
[
false
] |
I'm most interested whether the current model(s) of star/planet formation show any trends between star size and the characteristics of planets in orbit. For example, are larger stars expected to have more or fewer planets? Is there a negative or positive correlation between star and planet size? Are "Hot Jupiters" more or less likely with giant or dwarf stars? Answers to any and all of these questions would be very helpful since I've had a devil of a time looking them up.
|
[
"Hi,",
"This is an interesting question, before I get started have a look at [exoplanet.eu](",
"www.exoplanet.eu",
") which has a list of planets in a nice table format, with web plotting tools :-).",
"Also it should be noted that (almost) all of the data we have on planets is biased towards massive planets which orbit close to their star and whose host star is small. This is because they are easier to detect with the main techniques we use; ",
"radial velocity",
", and ",
"transit photometry",
". There are other methods but these two are by far the most prolific. This is called a selection effect (or selection bias) and is very annoying when talking about populations because you have to try to adjust for it ",
"here's a paper that proposes a model for the underlying planet population",
".",
"It's observed that there are more planets around stars with ",
"higher metallicity",
" (which measures how much stuff in the star isn't hydrogen of helium). And many models assume that there are more planets around larger stars just because the protoplanetary disk of larger stars has more mass. Unfortunately due to selection effects we have little data on the planetary population around stars larger than a few times the mass of our sun.",
"One of the most popular 'zeroeth order' (guesstimate) models for the underlying planetary population is a power law with a negative index. I.e. N_{planets} = A*M",
" where A is some constant, and b is always negative. This model gives you lots of low mass planets and only a few high mass planets.",
"If you want to talk about habitable planets, then you have to start looking at the habitable zones and the lifetimes of the host stars. The habitable zones around high mass stars has more area than around low mass stars, but the lifetime of high mass stars is much shorter than the lifetime of low mass stars. This tradeoff actually works out in the low mass star's favour. But then you have to consider number of planets around stars w.r.t mass. If you assume that more massive stars have more planets (which many models do) then you end up with stars which are approximately solar having the best chance of habitable planets.",
"Here are a few links for further reading:",
"Loads of papers on planetary occurance rate",
"Population analysis of Kepler exoplanets",
"Example of building a population synthesis model",
"Trying to infer the actual population of planets from our noisy incomplete data",
"The Exoplanet Handbook - pretty good book",
"Nasa Exoplanet Archive",
"TL;DR - We have models, and reasons to think that higher mass stars will have more planets. However due to selection effects in our data it is very difficult to actually find out.",
"Hope that helps :-)"
] |
[
"Its a constant. In reality it probably does vary with stellar mass but we cant tell with our data"
] |
[
"Thanks very much for the answer and the data link.",
"I understand the problem with the bias in observable data towards planets that are easy to detect from earth. That's why I was interested in what the current guesstimate models would indicate too. Thanks for the handy rule of thumb equation. Just for clarification, is the negative exponent (b) in this equation dependent on factors specific to the given star system, or is it treated as more or less constant across most stars?"
] |
[
"Could desiccant packets (from food boxes etc) control humidity in a larger space, and how long would they be effective?"
] |
[
false
] |
So a desiccant packet can apparently keep my shoes dry in a shoebox. If I gather 100 of them and put them in a can or something, would that keep a filing cabinet full of pictures and papers dry (assuming the filing cabinet is in a residential place, not a jungle) and act as a 'humidity safeguard'? How long would it work? a year, 5 years?
|
[
"It depends on the ambient conditions, the size of the desiccant particles (and therefore surface area), and any other number of conditions. Desiccants can be reused, simply by placing them in an oven until the moisture is driven off."
] |
[
"We have a (no longer used) desiccant cabinet in the lab. It has roughly the interior volume of a dorm fridge and it has a lower shelf for desiccant that sits in a tray. It used to be used to keep really hygroscopic stuff moisture free, and we just had to bake the desiccant every few weeks.",
"I also use the same kind of stuff in our anaerobic chamber; the reactions that sop up excess oxygen produce water vapor, and if the vapor pressure is too high it can slow the reaction. Again, we just take the cartridges out ever few weeks and bake them (200C for a few hours).",
"As for you...it depends. How often do you open it? How well is it sealed? How humid is it normally? "
] |
[
"So, we do have desiccant wheels in some ventilation systems. These are energy recovery ventilators designed to take moisture out, or put moisture into a space. There is an outbound and inbound air stream and the wheel rotates to adsorb then evaporate the moisture, taking it in our out of the space, depending on what the needs are.",
"Check out ",
"this",
"and ",
"this diagram"
] |
[
"Does There Exist A Series Smaller than the Harmonic Series that STILL Diverges?"
] |
[
false
] |
Is there such a thing, or is the Harmonic series the smallest series we know of that still diverges? I'm not exactly sure if I'm phrasing the question properly, in the sense that it may not be exactly clear what "a series smaller than the harmonic series" even , right? I mean, there's an argument to be made that the harmonic series is greater than the series: 1/2 + 1/2 + 1/2 + 1/2 + 1/2... which clearly diverges, right? Because they can be re-written as: 1/2 + 1/4 + 1/4 + 1/8 + 1/8 + 1/8 + 1/8... 1/2 + 1/3 + 1/4 + 1/5 + 1/6 + 1/7 + 1/8... But some way to describe how "large" a series is? Or how quickly it diverges? And if there is, is there a series that is "smaller", or diverges slower, but still diverges?
|
[
"Yes, there is a way to talk about how fast a series converges or how slowly a series diverges. Let's consider only series with positive terms. So a",
", a",
", ..., a",
", ... is a positive sequence and the partial sums",
"s",
" = a",
" + ... + a",
"are positive and increasing.",
"We can compare the rate at which two sequences diverge or converge by examining the limit of their ratio. That is, we look at the limit",
"L = lim",
"(a",
"/b",
")",
"Since we are assuming all of the sequences are positive, this limit exists and is equal to some extended real number in the interval [0, ∞]. If L is some positive, finite number, we say the sequences a",
" and b",
" grow at the same rate. If L = 0, we say b",
" grows much faster than a",
". This is because, in that case, by definition of limit, it is possible, given any ϵ > 0, to choose ",
" sufficiently large so that a",
" < ϵ b",
". If L = ∞, we say a",
" grows much faster than b",
".",
"The limit comparison test that you learn in Calculus II actually tells you that you can compare the growth rate of two partial sums s",
" and s",
"' by computing the growth rate of the associated sequences a",
" and a",
"'. If two sequences grow at the same rate, then their series both converge or both diverge. If a",
"/d",
" --> ∞ and the series whose terms are d",
" diverges, then the series whose terms are a",
" also diverges. If a",
"/c",
" --> 0 and the series whose terms are c",
" converges, then the series whose terms are a",
" also converges.",
"Okay, so let's look at the harmonic series and then consider a very interesting general question. If ",
" < 1, then you know that the series",
"1",
"+2",
"+3",
"+...",
"diverges. This series, in fact, diverges ",
" than the harmonic series. Why? The limit",
"lim",
" (n",
"/n",
") = lim",
"( n",
" ) = ∞",
"tells us so. But that seems obvious, right? After all, the sequence n",
" goes to 0 slower than the sequence n",
" (as the above limit shows). So it makes sense the associated series also diverges faster. The terms are bigger, so the sum is bigger! (Of course, both sums are infinity, but we are talking about how the partial sums scale here.)",
"Can you come up with a series that diverges but does so ",
" than the harmonic series? One example as provided by ",
"u/functor7",
" is the sum",
"1/2 + 1/3 + 1/5 + 1/7 + 1/11 + 1/13 + ...",
"which is the sum of the reciprocals of the positive prime integers. Clearly, the associated sequence goes to 0 faster than 1/n, but it's a non-trivial fact that the series still nevertheless diverges.",
"But that seems almost too exotic. ",
"u/functor7",
" gave a good heuristic for finding a series that diverges or converges as slow or as fast as you want. Here's another way to do it, and this method will answer the following interesting question:",
"*",
"For the particular case of ",
"-series, that is, sums of the form 1",
"+2",
"+3",
"+..., there is an answer to this. The sequence that satisfies the desired property is a",
" = 1/n. This is commonly known in Calculus II as the ",
"-test. If p < 1 then the series diverges, and if p > 1, then the series converges. But I want to answer the question above in general, for ",
" series, not just ",
"-series.",
"It turns out that there is no such boundary. Let's see why.",
"Suppose a",
" is a positive sequence such that the series Σa",
" diverges. Let s",
" be the ",
"th partial sum as usual. Now consider the two following sequences:",
"b",
" = a",
"/s",
"c",
" = a",
"/s",
"(The sequence c",
" always has limit 0. The sequence b",
" has limit 0 if a",
" has at most polynomial growth, but generally b",
" is bounded.) If you've taken analysis, you should be able to prove yourself that Σb",
" diverges and Σc",
" converges. Note that is true for ",
" positive sequence a",
" whose series diverges. (In the case of a",
" = 1/n, the sequences defined above are roughly equal to b",
" ~ 1/(n log(n)) and c",
" ~ 1/(n log(n)",
").)",
"What's so interesting about these sequences? Well, since s",
" --> ∞ as n --> ∞, we have the following:",
"b",
"/a",
" --> 0",
"c",
"/a",
" --> 0",
"That is, both b",
" and c",
" are much ",
" than a",
". Yet Σb",
" diverges and Σc",
" converges.",
"Similarly, suppose a",
" is a positive sequence such that the series Σa",
" converges. Let r",
" be the ",
"th remainder sum. That is,",
"r",
" = a",
" + a",
" + a",
" + ...",
"Now consider the two sequences",
"b",
" = a",
"/r",
"c",
" = a",
"/√(r",
")",
"(Both of these sequences have limit 0.) Again, if you've taken some analysis, you should be able to prove that Σb",
" diverges and Σc",
" converges. (In the case of a",
" = 1/n",
", the sequences defined above are roughly equal to b",
" ~ 1/n and c",
" ~ 1/n",
".)",
"What's so interesting about these sequences? Well, since r",
" --> 0 as n --> ∞, we have the following:",
"b",
"/a",
" --> ∞",
"c",
"/a",
" --> ∞",
"That is, both b",
" and c",
" are much ",
" than a",
". Yet Σb",
" diverges and Σc",
" converges.",
"This method shows that given ",
" positive sequence a",
" whose series diverges, there is always a pair of positive sequences that are both smaller than a",
", but only one of which whose series diverges also. Similarly, given ",
" positive sequence a",
" whose series converges, there is a always a pair of positive sequences that are both larger than a",
", but only one of which whose series converges also.",
"Think about what this means. Start with your favorite divergent series. Mine is a",
" = n. That's ",
" divergent. Then compute the sequence b",
" = a",
"/s",
". In this case, we have exactly",
"b",
" = 2/(n+1)",
"but more generally",
"b",
" ~ 1/n",
"Then Σb",
" diverges more slowly than Σa",
" but it still diverges. Okay, not let b",
" become your new a",
". So a",
" = 1/n. Now compute b",
" = a",
"/s",
". In this case,",
"b",
" ~ 1/(n log(n))",
"Then Σb",
" diverges more slowly than Σa",
" but it still diverges. Now do it again. We get the sequence",
"b",
" = 1/(n log(n) log(log(n)))",
"The associated series diverges ",
" than all the previous ones. And we can do this for as long as we want. We will keep getting divergent series that diverge slower and slower and slower.",
"The same can be done in the opposite direction. Start with a convergent series and you can keep making larger and larger series that still nevertheless converge.",
"So, alas, there is so such \"boundary sequence\" for divergence of series."
] |
[
"The Nth term in the Harmonic Series is approximately equal to log(N) (where this is the natural log). In fact, the difference between 1+1/2+1/3+...+1/N and log(N) is pretty small and measured by the ",
"Euler-Mascheroni Constant",
".",
"But there are definitely series that diverge slower than the Harmonic Series. In particular, if you do the Harmonic series, except using only prime numbers, then the series still diverges. That is, 1+1/2+1/3+1/5+1/7+1/11+... diverges. The Nth term in this series is approximately log(log(N)), which grows much slower than log(N). The difference between log(log(N)) and the sum up to the Nth term is measured by the ",
"Meissel-Mertens Constant",
"In general, you can construct series to grow as slow as you want. Just take any differentiable function f(x), and take the values f'(1),f'(2),f'(3),f'(4),... and then make the sum f'(1)+f'(2)+f'(3)+...+f'(N). The value of this will be approximately f(N), and its difference is measured by the ",
"Euler-Maclaurin Series",
". ",
"If f(x) is a function that \"levels off\" as you go to infinity, then f'(x) will approach zero, so to get it more in a \"Harmonic Series\" form, you could take the 1/n-type value that is closest to f'(N) without messing with the divergence rate too much."
] |
[
"Yes."
] |
[
"Why does recently gained muscle mass deteriorate when there's a lack of physical activity?"
] |
[
false
] | null |
[
"Muscles consume large amounts of energy. Your body attempts to be as energy efficient as possible to survive for long periods without food. Therefore it rids you of unused muscle mass, which is just wasting energy."
] |
[
"Your body knows, that keeping around enough ",
" is copasetic.\n",
" getting rid of \"unused\" muscles and converting the daily intake of excess energy into fat is the better long term plan for survival.",
"In return the muscles which you really ",
" will get used to carrying around more mass and adapt."
] |
[
"If you have muscle already and consume tons of energy but are not really using your muscles, it is ",
" more efficient to reduce muscle mass and store excess food energy in fat."
] |
[
"Do houseplants \"die from old age\"?"
] |
[
false
] |
[deleted]
|
[
"Have you repoted your yellowing plants into fresh high quality soil at least once every year or two for the duration of you owning it?",
"I got a plant from my sister once that had been good for a long time but had been slowly becoming less and less healthy. (yellowing and thinning leaves)",
"First thing I did was pull the plant out of the pot and find that ALL of the soil had been replaced by roots, the plant was living in vermiculite and sand.. I cut off about 50% of the roots, thinned them out, and replanted it in high grade soil, a few months later it was greener and more lush than it had ever been",
"I make it a yearly event to take all my plants, trim some roots and repot them with additional fresh soil. I haven't killed a plant in 15 years. "
] |
[
"This is all great advice but does not answer the scientific question at all. :("
] |
[
"From what I remember, its life cycle has two stages, polyp (immobile on the seafloor) and medusa (jellyfish form), and it is able to indefinitely switch between the two, renewing its own life in the process."
] |
[
"How do we know the rate at which atomic clocks lose time?"
] |
[
false
] |
If they are the most accurate means of measuring time, what are they being compared to when we say they drift x seconds in y million years etc.
|
[
"It's very simple, you compare them to each other. Because the error is random, calibrating a group of atomic clocks to the same time and seeing them drift apart gives you an indication of the individual error rate.",
"Of course, you don't know which one of them is right, most likely all of them will have lost time at some point, you'll only know how much on average they drift away from the right time. Which reminds me of an old joke: \"A man with one watch always knows what time it is, a man with two watches is never quite sure.\""
] |
[
"It's not a drift, it's an uncertainty. If it was a definite and constant change in one direction it would be correctable while a random error isn't. So if they say it's x seconds in y million years that's just a more relatable way of stating the estimated uncertainty in the measurement than saying that one tick varies from another by x parts in thirty million million (the number of seconds in a million years)."
] |
[
"More specifically, if you have many atomic clocks you can use the average to get an estimate of the true time, and then from that calculate the average drift by estimating the standard deviation."
] |
[
"How is all the antimatter in the universe not destroyed by the matter?"
] |
[
false
] |
[deleted]
|
[
"There really isn't much antimatter in the known universe. Why, is a mystery. Positrons are created fairly frequently and they often annihilate with electrons. If you look for positron-electron annihilation in the sky you see it ",
"coming from the galactic center",
"."
] |
[
"This has more or less been ruled out for two reasons:",
"If there were galaxies and other bodies made from antimatter, then there should be annihilation zones between these regions and matter-dominated regions. These would emit huge bursts of gamma radiation which we would be able to see from Earth. No such radiation has been detected.",
"A lot of the cosmic radiation we get from earth comes from distant galaxies. If there were antimatter regions out there then we should see more high energy antimatter particles being annihilated in the atmosphere than are observed in reality.",
"Source: senior physics student who had to answer this question in an exam this morning :P"
] |
[
"Nothing much different from an electron scattering off something."
] |
[
"Do Superpositions in QM solve the respective Schrödinger equation?"
] |
[
false
] |
Can someone explain for an engineer the mathematics behind superposition I get it that <psi|psi> = 1 and that you can get the superposition with the coefficients Infront of the Eigenstates but those work because they solve the SE. How does that work with superpositions? Thanks in advance :)
|
[
"The Schrodinger equation is a linear differential equation, so a superposition of solutions to the Schrodinger equation is still a solution to the Schrodinger equation."
] |
[
"Even when the SE is time dependent?"
] |
[
"Yes, it’s still linear."
] |
[
"What is the earliest song in human history that we are not only aware of, but have some idea of how it sounded?"
] |
[
false
] |
I don't mean the earliest that modern society is simply aware of once existing through references or mentioning in early history, but the earliest song that we could at-least make an honest educated attempt of recreating the sound of. This could mean ancient sheet music of some form, or other means of accurately guessing how it sounded in person. A separate question on a similar note; Do we have any idea when humans (or our non-human ancestors) first started making music recreationally?
|
[
"Not the earliest song, exactly, but the oldest musical piece we have a complete \"score\" for is the Seikilos Epitaph from Ancient Greece, from around the first century AD: ",
"https://en.m.wikipedia.org/wiki/Seikilos_epitaph",
"It's very short, but is actually a beautiful piece. "
] |
[
"Hurrian Hymn, about 3400 years ago.\n",
"http://www.greenwych.ca/evidence.htm",
"\n",
"https://www.youtube.com/watch?v=gKN7Em9Q6qY",
"I would disagree with other theories here by saying that music existed... before homo sapiens existed. There is evidence that animals make music too, from birds to whales."
] |
[
"While you live, shine",
"have no grief at all",
"life exists only for a short while",
"and time demands an end.",
"Wow. Two thousands years later, their little love song is still played and still remembered. ",
"Time, perhaps, is not the end that the lovers thought it would be."
] |
[
"Does every substance have a melting point, even crystals?"
] |
[
false
] | null |
[
"Crystals (at least what I imagine you mean by crystals) do eventually melt if you get them hot enough. For example, quartz melts at 1610 ",
"C and table salt (NaCl) melts at 800 ",
"C.",
"However, a lot of things will decompose before they melt, like big, bio-organic polymers. For example, paper will autoignite at 250 ",
"C in the presence of oxygen, and even without oxygen, it will undergo ",
"pyrolysis",
" to form a charcoal like substance.",
"If you want an arbitrarily hot solid, you have to apply ever increasing amounts of pressure. See, for example, the ",
"phase diagram of carbon",
". If you have millions of atmospheres of pressure, you can have 10000 Kelvin diamonds.",
"At any given pressure, though, everything will eventually melt or decompose with enough heat."
] |
[
"A simple example you can try at home is simple white sugar. Put some into a dry frying pan and put the heat on medium. It will eventually look \"wet\". This won't be water, it will be the sugar itself that is melting. Be careful though, since it will eventually burn."
] |
[
"Sugar is kind of a vague term in chemistry, but assuming you mean ",
"sucrose",
", that's actually a good example of the latter category.",
"Sucrose is a disaccharide (if you look at the chemical structure, there are two distinct rings connected by an oxygen atom), and it will dissociate into its constituent glucose and fructose monosaccharides at high temperatures. These parts are both liquid at the sucrose decomposition temperature, so this is the clear liquid you've seen.",
"If you heat that liquid further, those simple sugars will also pyrolyze (break down further under heat) into a caramel."
] |
[
"Botanics : How many leaves does a regular tree have on average? [e.g : oak tree]."
] |
[
false
] | null |
[
"This would be based on age of the tree, species, health of the tree, and the season."
] |
[
"But surely theres an optimum age when the tree stops growing? If so then at this age, approximately how many leaves would be on a healthy oak tree in the summer?"
] |
[
"Healthy trees are not like healthy people. They rarely \"stop growing\" just because they reach a certain stature or age. A tree's whole life is a process of systemically beating out competing plants for essential nutrients, like water and sunlight. Therefore, it is in a tree's best interest to continue expanding the size of its canopy and leaf cover, in the interest of getting the most light for itself and depriving those around it. There's a reason why some of the largest trees you see are also some of the oldest."
] |
[
"why isn't *all* of our vision as sharp as the center of our field of vision. wouldn't universal sharpness be a great adaptation?"
] |
[
false
] |
you could just cluster more nerves around the outside of the center point.
|
[
"But then you'd need a much bigger hole for all of the extra nerves to exit the eye and a lot more brain to process all of that information. Most importantly, however, there is a great advantage to pooling signals from many receptors - yes it does result in lower resolution, but it's also more sensitive to low levels of light, which is why it's easier to see stars at night out of the corner of your eye."
] |
[
"Great adaptations are only adopted if ",
" having them is a hindrance. If people without that sharp vision died, then yes, we would evolve to have it. As long as people without sharp vision everywhere stay alive and reproduce, we'll stay with that for most of the population."
] |
[
"In our eyes we have two types of sensors. We have cones which are for color detection and in bright light we see better with cones. Rods, which are just light detectors, are more sensitive to detecting light. The fovea, which is in the centre of the retina, has a lot more cones than it does rods compared to the surrounding area.",
" ",
"Color helps you better distinguish one object from another especially if they have the same frequency.",
"Are you saying we could just add more cones all over the retina instead of clustered in the fovea? Since I'm assuming you would replace the rods with cones, you do have to remember that rods are important because they are much better at detecting light than cones. So we need rods so we can see as well as we do in dim light so you could but if you were you would no longer be able to see as well as you do in dim light.",
"More info"
] |
[
"Does prolonged 'slight dehydration' have any long term affects on the body?"
] |
[
false
] |
I know all the symptoms of dehydration, but it seems to be an acute condition easily fixed by hydration. If one we do be dehydrated for years, what would be the long term consequences? (Let's assume the person has dark yellow pee and slight dry mouth for years, maybe an occasional headache, but otherwise leads a normal life).
|
[
"Specifics please? You have no evidence, no specific examples, no sources, and no explanations. "
] |
[
"One specific consequence that comes to mind is kidney stones. Chronic dehydration increases the risk that dietary minerals will concentrate into stones."
] |
[
"Yes, Chronic dehydration is actually a risk factor for a whole bunch of illnesses!",
"Water is used in the body in LOADS of different ways and in tons of biochemical pathways.",
"Just think about all the different uses in the human body there are for water: Cell life, metabolic reactions, excretion, temperature regulation.\nIf any of these are even slightly slowed down or stopped then it can have serious long term effects!"
] |
[
"I hope this isn't a dumb question. I am taking ekg classes to further my education, and one lead on the machine I am using is the \"ground\". My question is this: what is the natural \"ground\" for the electrical activity in the heart?"
] |
[
false
] |
Everything I have learned about closed electrical systems has come from working in an automotive shop for 15 years, and replacing or installing electrical outlets in my home. I do understand how the heart works, chemically and anatomically. I just can't seem to understand the grounding part. I have a BSN, but this was never touched on that I can recall. Google gives me nothing, no matter how I ask. Edit: google, rrrrrr.
|
[
"One thing I'd like to add is that voltage is used to measure a ",
" in potential. When you have circuits measuring change in potential caused by something like the heart, they need to compare this change to a baseline value. This value is often called ground, allowing different parts of the circuit to know how the input (or other components) potential is changing."
] |
[
"Thank you for your sincere answer. You did a great job explaining electricity, and putting it in terms that are easy to understand. Thank you very much for your effort. I value it."
] |
[
"The ground in an EKG is used as a baseline. It is derived from a location on the body far from the heart so that it does not pick up the signals that are generated there. The EKG can then \"subtract\" the ground measurement which removes any noise in the environment giving a much cleaner reading of the hearts electrical activity."
] |
[
"How would Quantum Mechanics apply in complex situations?"
] |
[
false
] | null |
[
"Physics"
] |
[
"Physics"
] |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):"
] |
[
"If dark matter is found and proven, how much would that change physics and math?"
] |
[
false
] |
I don't have a math background so the in depth answers will be above me, but I just wonder if a discovery like proving dark matter will have a fundamental shift in math and physics theories?
|
[
"Most people already work under the assumption that dark matter exists. All of cosmology just assumes it is there. The real question comes down to what is it made of, what sort of things make up dark matter. We believe it is there but currently only have guesses as to what it might be, what other properties it has and therefor how to even begin looking for it.",
"So the better question would be how would our understanding change if we discovered it and learned what sort of properties it has, but we jsut don't know enouhg to answer that question yet. Until we can learn more about what dark matter is, we just don't know what consequences it could have on our understanding. Maybe it turns out to be something we have already guessed, in which case maybe not much changes. Maybe it will turn out to be something completely different than anyone ever guessed which could change our understanding quite a bit.",
"Sorry if it isn't a great answer, but we sadly just don't know much about dark matter yet, and thats why people are trying to find it."
] |
[
"Definitely possible. But the simplest alternative is that it's basically one thing. The properties that we can observe or deduce about it seem to be pretty uniform. Now obviously, simplest doesn't always mean much, but it's the best place to start. ",
"For instance, let's say a book falls off of your desk for no reason that you can see. You might blame a ghost, or a poltergeist, or a gremlin, but you're not going to bother to blame all three of them at once. There's no reason why it ",
" be a collusion of all three, but that requires three suppositions where one would do. "
] |
[
"Am I wrong, or is it possible that dark matter isn't just one thing, but actually a collection of various different things that we can't currently see or measure?"
] |
[
"Since retroviruses inject their DNA into the host's DNA, if the host's genome is sequenced, will the retrovirus DNA show up in the results?"
] |
[
false
] | null |
[
"Yes, in fact the human genome consists of a ",
" of retroviral related DNA sequences, as in, almost half of the genome. Since ",
"this paper is behind a paywall",
" I've put the relevant figure on ",
"imgur",
", you can see that transposable elements (which come from retroviruses either directly or are inserted into the genome then copied over and over) make up almost half of the genome. Of course having your DNA jumping around all the time isn't a good idea so your genome/cells have evolved methods to suppress this activity (while the movable elements have evolved methods to escape repression...it's an ",
"evolutionary arms race",
". Most of these elements in the human genome are inactive but about 100 are still active and some diseases are caused but one of these elements copying/inserting itself into a gene and interfering with that gene.",
"Another example, someone with HIV will have HIV DNA sequences inserted into their genomes and the sequences will show up if you sequence their cells (if it's cells that have been infected). Genome sequencing is a way to study HIV DNA insertion. ",
"reference"
] |
[
"If the virus infects a germ cell & that germ cell leads to progeny then yes. It is one of the most convincing pieces of evidence in favor of evolution because we share the viral fingerprints of many such events with our closest relatives: ",
"https://en.wikipedia.org/wiki/Endogenous_retrovirus",
" also... if any individual cell that had been infected is sequenced then it is quite likely to have the viral dna show up."
] |
[
"In the case of most genome sequencing, you provide a saliva sample. Many believe it is the epithelial cells in the saliva that is used, but most companies use the white blood cells commonly found in saliva. If infected with HIV, it seems it would show up in the sequencing. Hmm. Interesting. Thanks for the reply!"
] |
[
"What are ants doing when they run into each other?"
] |
[
false
] |
I was just looking out my back window onto the porch and saw a line of ants running across the concrete. Every time an ant runs into another ant while traveling opposite directions they stop very briefly and then continue on. I understand the line they are following is made through pheromones and it's probably going to food. However, I wonder if they are communicating something when they run into each other or just checking the other ant out or what? What really struck me is how mechanistic the whole process seemed. Each ant would pause what seemed liked the same exact amount of time (below 1 second), then continue on.
|
[
"I have always wondered this too; I think it has to do with a chemical exchange between them that lets the other one know that they are a part of the others colony. If I'm on the right track, Reddit, let me know."
] |
[
"That's kinda what I was thinking as well. "
] |
[
"\"Excuse me\"",
"\"Pardon me\""
] |
[
"What if everyone in the world got in the ocean at the same time? Is there enough coastline?"
] |
[
false
] |
If you could have everyone in the world hold hands, and enter the ocean, how much would the water level raise? Also us there enough coastline to pull this off?
|
[
"The world has a total of about 300,000 km of coastline",
"Are you ",
"sure",
"?"
] |
[
"The world has a total of about 300,000 km of coastline, which is not nearly enough space for 7000,000,000 people. ",
"However, all the people of the world have a total volume of about 700 million cubic meters, which is nearly insignificant considering the 1.3 billion cubic kilometers. With a total area of 360 million square kilometers, all the people in the world jumping into the ocean would result in the ocean rising about 2 microns, or 2 millionths of a meter."
] |
[
"Good point."
] |
[
"If muscles can only contract, how do we expand our ribcages when taking a deep breath?"
] |
[
false
] |
I understand how the diaphragm can move to pull in air, but from a mechanical point of view this suction should pull one's chest in, not balloon it? I know I'm missing something...
|
[
"You are right, muscles only contract, and muscles do expand our ribcages with deep inspiration.",
"It's called 'bucket handle movement'. By contracting the intercostals, the ribs are pulled together and up. Due to their curvature, that means the chest gets wider laterally, increasing the overall intrathoracic size."
] |
[
"To add to this: when the diaphragm is relaxed, it gets pulled upward into the rib cage toward the lungs. ",
"When it contracts, it pulls tight and flattens. The muscle is pulling away from the rib cage cavity, which creates negative pressure and allows air to flow in."
] |
[
"Inspiration means breathing in, expiration is breathing out, and respiration is the whole process."
] |
[
"Which is the appropriate statistical test?"
] |
[
false
] | null |
[
"Use the chi square goodness of fit test. You'll perform it multiple times for the different potential genotypes. To look extra classy when you're done perform a Bonferroni correction to adjust for the fact that you have done multiple tests. ",
"For example you could try flipping a coin and getting 8/10 heads. If you only did one set of 10 flips it is unlikely, if you did thirty sets of 10 flips it becomes quite likely. "
] |
[
"Why is your data in ratios? If it is weight or something you can just use a t-test. ",
"If you want to compare expected vs observed results use a chi-squared goodness of fit test.",
"Is this just the weight of black sees compared to the weight of yellow? If so perform two tailed t-test. "
] |
[
"It's the number of mature seeds. I think it's called kernels.\nMy aim is to find the possible genotypes of the parents, so I am comparing the ratio observed (black to yellow) with the theoretical ratio of possible genotypes of parents.\nI hope I clarified any misunderstandings, if the information I'm giving is still to vague or irrelevant please do ask."
] |
[
"Why do some people handle spicy foods better than others?"
] |
[
false
] | null |
[
"I'm a scientist that studies the proteins that your taste buds and nerves use to detect various substances that give most spices their distinctive taste. Most of these proteins are TRP-channels, and are activated by many diverse substances (",
"chili",
" for TRPV1, ",
"cloves",
", ",
"thyme",
" and ",
"oregano",
" for TRPV3, ",
"garlic",
" for TRPA1, ",
"mint",
" for TRPM8 and so on). ",
"Your tolerance to spicy foods can stem from two distinct factors. One of them is the aforementioned genetic background, for example TRPV1 can be mutated so that it is less suspectible to capsaicin. Since most spicy foods contain more than one spice, the mutation of either of the proteins mentioned above can influence the final taste sensation. Another important factor can be ",
"splice variants",
", which means that even if there are no detectable mutations the TRP proteins can be assembled differently during synthesis, leading to different sensitivity to spices.",
"The other important factor is how often you eat spicy foods. Most spice receptors show ",
"desensitization",
", in other words if they are exposed to compounds that activate them regularly, they will stop responding to any stimuli that would activate them."
] |
[
"I have been told (I cannot cite sources) that eating spicy food will lead to endorphins being released as the body attempts to counter the 'pain'. Over time, people who enjoy spicy food have learned to associate their favourite food with the rather pleasant feelings associated with endorphin release.",
"See also - the pleasant feeling of wellbeing triggered by having mild sunburn."
] |
[
"There are proteins expressed in certain tissues that are taste receptors. These receptors, like any other protein in your body, may be slightly different than other peoples' receptors. This, because of Genetics. An example of a \"spicy\" tasting molecule would be capsaicin:",
"http://en.wikipedia.org/wiki/Capsaicin",
"Capsaicin binds to temperature receptors AND \"spicy\" receptors. For some people, their \"Hot\" receptors may not bind to it as well, rather the capsaicin might not bind to the receptor as well. Thus, they may not get the same level(or rate) of neuronal spikes(gustation). This is a fact, I just finished studying this in a Neuroscience course. For the non-science person: Genetics is the answer."
] |
[
"Do I have more fat cells in my abdomen, or do the ones there just get bigger faster?"
] |
[
false
] |
I was also wondering if the body can make more fat cells. I heard somewhere that we always have the same amount, they just get bigger and smaller depending on how much you're storing. However, if someone has liposuction and then gains weight again would theyexpand in different areas of their body, or create new fat cells?
|
[
"In short, both size and number.\nUsually the size of your body is not given by the size of the cells, it is given by their number. So a 100kg guy will have his cells almost the same size as a 50kg guy but in a larger number.\nWhite adipocytes (the ones that make you fat) can vary in size. The cell has a lipidic droplet in its center that pushes the nucleus and the citoplasm to the side. The size of that droplet will give the cell it's size. The more fat your body has, usually from overeating or some diseases, the more fat will be stored in your cells. These cells will increase in size, as the amount of fat stored in them increases, and then start to divide. There is only a limited amount of space in them.\ntl;dr Your cells will be bigger both in number and size."
] |
[
"So if someone weighs 300 lbs and then loses 150, the number of fat cells will stay the same but their size will get smaller?",
"Also, does this mean if someone were to weigh 150 consistently they would have less fat cells and therefore look \"leaner\" than a similarly weighed/muscled person?",
"If yes, will liposuction be the only way to get rid of the excess fat cells?"
] |
[
"any sources for this?"
] |
[
"If you expose a baby to stressful experiences (eg music), would the child develop better coping skills, or be more prone to stress later in life?"
] |
[
false
] |
[deleted]
|
[
"Attachment theory suggests that it would make the person worse at dealing with stress. The idea is that children who feel that the world is safe, learn to deal with distress is functional/productive ways - fixing the underlying issue and seeking emotional support. However, kids who receive inconsistent or insufficient care develop coping strategies that are ineffective later in life. Children who receive inconsistent care tend to hyperactivate later in life (when they get upset, they become to loud/emotional that they ",
" care). Conversely those who receive insufficient care tend to deactivate later in life (emotionally numb themselves to protect from pain)."
] |
[
"cool question though"
] |
[
"I have got exposed to heavy music.According my mother, my father used to turn the volume up of our home radio, whenever I started to cry.Then there I am stopping the crying part and within few minutes I'm dreaming according to them :) Now I am 27 and I am the most calm and collected person in such situations, among the people I know :)"
] |
[
"What is the economic theory behind \"one price for all sizes\" regarding coffee sales."
] |
[
false
] |
Do customers trend towards larger sizes? Is the one price for a predicted average volume sale? It's an interesting sales tactic and I was wondering if any sales or economic research existed on the topic.
|
[
"When I worked in a small cafe, I asked my boss why it was worth it to brew an entire cup of coffee for only one cup. ",
"The amount of coffee grinds used for a pot of coffee, including water and energy, is significantly less than the cost that single cup is being sold for ($1.35 for a small). In fact, the cup and lid costs more than the coffee contained at roughly 20 cents per cup. "
] |
[
"To the business selling the cup of coffee, the cost of the actual cup (paper or styrofoam) is normally much higher than the contents of the cup themselves. The additional cost of brewing a pot of coffee or drawing a cup of soda is nominal.",
"Cup: $0.15\nLid: $0.03\nSoda: $0.12 \n --------------\nTotal $0.30\nPrice $1.50 (conservative)\nProfit $1.20\n",
"This is also why \"free refills\" on soda are so common - once you have bought the cup, you'd have to get around 130 refills before the restaurant takes a loss.",
"Now, obviously there's lots of overhead to a business; Rent, power, cleaning kids' boogers from under the railing, initial purchase of coffee/soda equipment, etc. However, drinks are generally one of the items with the highest gross profit margin.",
"See ",
"PDC's \"Costing Out Soda\"",
" (PDC is a foodservice product distributor). The same generally applies to coffee, although straws are not needed. Note that 'specialty coffee' generally has more labor cost, higher setup cost, and some additional materials cost (milk, syrups, etc) but the gross profit margins are also fairly high. Simple brewed coffee is similar in cost to soda."
] |
[
"How can it be the \"price of the smallest size for any size\"? They're all one price, so you could just as well say it's the price of the largest size for any size.",
"Edit: changed size to price."
] |
[
"If mass alters the curvature of spacetime, does charge distort spacetime or some electromagnetic analogue?"
] |
[
false
] | null |
[
"The source of gravity is the stress-energy tensor, which includes things like mass, momentum, pressure, etc. Charge itself does not contribute to the stress-energy per se, but the electromagnetic field does.",
"Roughly, a gravitational monopole (a point mass) has a corresponding Newtonian gravitational potential that is like -1/r, and an electric (or magnetic) monopole (a point charge) has a corresponding Newtonian gravitational potential that is like +1/r",
". So generally speaking, for a compact distribution of charge, the relativistic effects on gravity due to a body's electric charge are negligible for sufficiently far distances."
] |
[
"Charge affects the curvature of the fibre bundle associated with the electromagnetic field, which is, roughly speaking, a point on a circle for every point in spacetime (the circle is called U(1) and is the gauge group of electromagnetism). This is very similar to the case of mass producing curvature in spacetime, more specifically curvature between the tangent spaces at each point (which define the directions you can trace from that point to neighboring points). ",
"I'm just starting to learn about this so hopefully somebody else can add more. ",
"There's a good physics stack exchange page about it here"
] |
[
"thanks for that. but to be more specific. I was curious about the following. gravity distorts spacetime, this is how it is able to act over large distances. while EM acts over scales much smaller, it still acts over distances.",
"Both the gravitational and the electromagnetic forces have infinite range. Even though the electromagnetic force is arguably the most prevalent force in our daily lives, it is negligible on cosmic scales because stars, planets, etc. are generally electrically neutral.",
"so how do charges know about each other?",
"The motion of charges is determined by the local electromagnetic field, which itself is, in turn, determined by the distribution of charge and current. Changes in the EM field generally propagate at the speed of light.",
"are electromagnetic fields an identical concept to spacetime curvature, just concerning the EM force as opposed to gravity?",
"Since different objects interact with the EM field differently based on their electric charge, we cannot describe the EM force in quite the same way as we can with gravity. The equivalence principle, which roughly says that all masses are affected the same way by gravity, is what lets us formulate gravity in the language of geometry."
] |
[
"Do groups of galaxies orbit some larger attractor?"
] |
[
false
] |
We can tell there are billions of galaxies. From planets to dwarf galaxies, there is always the gravitational nature of things to always want to orbit something more massive. Do large clusters of galaxies also rotate forming the classic accretion disk or at least about an axis?
|
[
"No. ",
"Galaxy clusters",
" form the largest gravitationally bound objects. Superclusters are not gravitationally bound any more - their individual galaxy clusters fly away from each other over time."
] |
[
"By the same logic a planet is just a system of atoms, which are themselves systems of smaller particles."
] |
[
"Galaxy clusters form the largest gravitationally bound objects",
"Semantics maybe, but why do we call galaxies \"objects\"? Wouldnt it be more correct to call them \"systems\"?"
] |
[
"Is the universe finite or infinite."
] |
[
false
] |
I know the whole deal about the "observable universe" and all, but is there any evidence that beyond the observable limits there is an actual "hard" limit where nothing exists. And if not why doesn't the universe has a center, anything that is finite must have a geometrically calculable center unless it wraps around itself or something weird. Am I the only one totally confused by this?
|
[
"First, we observe that as far as we can tell, physics is the same everywhere. The universe is homogeneous and isotropic, meaning roughly \"the same everywhere you look.\" Sure there are pockets of galaxies and pockets without, but on the large scale, it's pretty uniform. But this also gives us a second concept. Because physics is the same everywhere, then to have an edge to the universe would mean that physics would have to be ",
" at the edge. ",
"Given this homogeneous and isotropic nature of the universe we can construct a set of rules over its shape and how it will evolve. These rules have the name of the FLRW metric. Friedman and Lemaitre came up with the math, and Robertson and Walker proved that this math is the only type that describes the homogeneous and isotropic nature of the universe. The FLRW metric allows for one of 3 shapes of the universe, depending on the intrinsic curvature of the universe. (intrinsic just means we're not curving into another dimension). ",
"These three shapes are usually described in 2 dimensional examples, because they're easier to picture. Positive curvature is like the 2-D surface of a sphere. Negative curvature is harder to visualize, but a part of the space will look like a saddle or pringles-chip. 0 curvature is flat and.. well.. flat. Normal Euclidean Geometry. Regular mass tends to pull things together as tightly as it can (gravity) and tends toward positive curvature. Dark energy tends to push things apart, so negative curvature. If the forces pulling things together balance the forces pushing out, it's 0. ",
"Now let's look at some details. A sphere has no boundaries. So that takes care of all the stuff above. And it can have finite space. A plane and the saddle-shape ",
" have boundaries or they are infinite. We've already said that boundaries don't make much sense physically. So if curvature is less than or equal to 0, space is almost definitely infinite.",
"Well the best data available to us from the WMAP satellite has measured to a remarkable accuracy that the universe is ",
" to be flat. This means our universe is plane-shaped and infinite. Space and the stuff in it just keeps on going on and on for forever. And it has likely always been infinite. Even the big bang was an infinite region of extremely high energy density.",
": Sorry, I was a bit short last night. While the evidence is not conclusive in favor of flat infinite Euclidean universe, it's highly suggestive of it, and it is, at present the scientific consensus on the matter as far as I can tell. In the future of course, we may know more and have reason to select a different shape to describe our universe. And while consensus is no sure sign of truth, I'm willing to accept it here because it is out of my expertise."
] |
[
"Ths short answer is we don't know, but infinite is the baseline assumption, because if we assume it is flat (which is supported by observations to high accuracy) then (almost) all calculations work ",
" it was infinite euclidean space. Long answer as follows:",
"Eiichiro Komatsu",
" is the lead co-author of the NASA reports from the ",
"Wilkinson Microwave Anisotropic Probe (WMAP)",
", one of the main empirical efforts to increase our understanding of the shape of the universe (among many other things).",
"He (co-)wrote a general overview of what results can be expected from WMAP and what is already known about that subject ",
"Here",
" (I link to the preprints because they are publicly accessible, but the scientific articles are published in peer-reviewed journals). I strongly recommend interested people read the first two pages and the conclusion of this paper. On page 2:",
"Naturally, the near flatness\nof the Universe ",
". Remarkably,\nthe largest matching circles in most of the topologies we\nmight hope to detect will be back-to-back on the sky\nor nearly so. This immediately reduces the search space\nfrom six to four dimensions. This result is exact and easy\nto prove for nine of the ",
".",
"From the last page of that article:",
"Has this search ruled out the possibility that we live\nin a finite universe? No, it has only ruled out a broad\nclass of finite universe models smaller than a character-\nistic size. By extending the search to all possible orien-\ntations, we will be able to exclude the possibility that we\nlive in a universe smaller than 24 Gpc in diameter. More\ndirected searches could extend the result for specific man-\nifolds somewhat beyond 24 Gpc. With lower noise and\nhigher resolution CMB maps (from WMAP ’s extended\nmission and from Planck), we will be able to search for\nsmaller circles and extend the limit to ∼ 28 Gpc. If the\nuniverse is larger than this, the circle statistic will not be\nable to constrain it shape.",
"This (PDF!)",
" In particular after reading this, you should be able to understand what finite flat topologies look like, and how it's possible that finite universes have no center (under the standard assumptions: isotropy, homogeneity - which implies \"no edges\").",
"Despite what the \"cosmology experts\" in this subreddit claim, curvature (or manifold) does not mean two different things for mathematicians and physicists, it means ",
" the same thing.",
"Also, Robertson and Walker have ",
" proved that in the flat case the FLRW metric doesn't allow other Euclidean topologies - metrics are local constructions.",
"How could we detect a \"non-trivial\" (e.g. finite but flat) universe? If a \"wraparound\" is small enough (less than a few orders of magnitude bigger than the observable universe) we can do it by searching for certain inference patterns in the cosmic microwave background radiation. As of today there has nothing been detected, which means we can exclude a lot of possible topologies, but ",
"we're not done yet",
", especially if the universe is flat:",
"These upper bounds on the deviation from antipodicity demonstrate that in a flat universe for some classes of topology there remains a substantial fraction of observers for whom the deviation from antipodicity of the matching circles is considerably larger than zero, which implies that the searches for circles-in-the-sky undertaken so far are not enough to exclude the possibility of a detectable non-trivial flat topology.",
"Again I recommend at least reading that summary, or the corresponding ",
"article",
" itself.",
"tl;dr: Besides some strange irregularities in the CMB power spectrum and the fact that curvature measurements tend to be slightly positive, but still in the error range that makes flatness most likely, there is little reason against an infinite universe, but also little reason against some kinds of finite universes. We don't have to pretend ",
" more than we actually do know today."
] |
[
"The links I provide are from leading cosmologists (e.g. at NASA), not \"navelgazing topologists\".",
"All that RRc provides to prove me wrong (he would thereby also prove the WMAP team at NASA and the head of Astrophysics at Oxford wrong, btw) is condescension, hubris and insults. I have never seen RRc provide any support for his claims in scientific journals or books.",
"I really don't get why so many people here fall for that, even though I do realize he usually gives very good explanations of other topics in physics, maybe people just think if h's usually right, he'll be right here as well."
] |
[
"What would happen if one human body appeared inside another human body?"
] |
[
false
] | null |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):",
"guidelines.",
"If you disagree with this decision, please send a ",
"message to the moderators."
] |
[
"/r/AskScience",
" is a heavily moderated subreddit. All submissions are initially placed into a moderation queue where they are let out individually, provided they follow the submission guidelines. This process can take several hours, depending on the influx of submissions, time of day, and which mods are available during that time. It's holiday so not many people are around."
] |
[
"Well it certainly took you long enough"
] |
[
"Sociology/Psychology: Motivational effects of shame?"
] |
[
false
] |
In arguments like the recent Dan Savage / fat people shitshow people often say "Shame isn't motivational." This is presented as dogma, but I haven't seen any of these arguments supported with more than ideology and the debater's personal experience. I feel like shame is a very powerful motivational force, albeit with some decompensating because of it. What has research in this area shown?
|
[
"Shame is a function of society..."
] |
[
"I'm a Psychology Ph.D. student and my research focuses on the self-concept and personality. From reading William James awhile back (arguably considered the father of modern psychology), he argued that shame/guilt is a emotion brought about when one behaves in antisocial ways. Basically, James said that it is the ",
" of shame or guilt which creates a negative emotion that keeps people from behaving in socially undesirable ways (avoiding gluttony, not indulging in excessive pleasures, or what have you). This seems to be the motivational component you speak of.",
"Such emotions are influenced by the idea of who we ought, could, or should be (James' notion of the \"must\", or \"ought\" self). Shame also varies by culture, as many Eastern cultures believe shame can be beneficial in bringing honor to their family or group. ",
"James said that the self-feelings we have ultimately depend on what we back ourselves to be and do. In this sense, they are motivational insofar as we hold such ideals close to heart. ",
"This is an interesting topic and I'll try to reply later with some more recent advances in this area, after I read and research a bit more on the topic for you. Hopefully this is a helpful start to thinking critically about the issue, though!"
] |
[
"In response to your conclusion, I would clarify this by saying that shame is motivational in that one does not want to feel shame (we want to avoid experiencing a negative emotional state), so we engage in behavior that minimizes the likelihood of shame occurring (i.e. we behave in socially desirable ways). ",
"This is just basic operant conditioning (reinforcement) theory. Here, frequency of a given behavior (i.e. one that produces shame) is decreased because we want to avoid a punishment (the negative emotional state that shame produces). In this light, shame would indeed be considered motivational, since motivation here would mean \"I am motivated to not get fat because I want to ",
" the shame resulting from the social stigma people give to fat people\"."
] |
[
"Could I escape Earth's gravity by constantly traveling upwards at 1 m/s?"
] |
[
false
] |
Okay, this is a question that has been bothering me for a very long time. It doesn't seem to make intuitive sense or to fit in with what I know about physical law so I was hoping someone much smarter than myself could help me understand it. Here's the scenario: Let's say you're sitting in your own personal magic rocket ship that has infinite fuel that doesn't add to the weight of your craft. This magic rocket ship also happens to have the quirk that when you turn it on, it always generates the exact right power and thrust to reliably maintain a constant upwards velocity of 1 m/s. So you hop in your new magic rocket ship and hit the gas and begin your extremely rapid ascent of 1 m/s. So here's my question. If you can sit in this spacecraft for an infinite amount of time, if you maintain this 1 m/s directly upward ascent, will you eventually escape Earth's gravitational pull (point where g < 1 m/s)? If the answer is yes, does that imply that you don't actually have to reach 11 km/s (escape velocity) to escape a gravitational body? If yes, what does that imply for escaping a black hole's event horizon? From my understanding, the schwarzschild radius is set as the point where escape velocity for a given body is equal to c.
|
[
"Escape speed is the speed required for a particle to escape a gravitating mass (like a planet or star) under the following assumptions:",
"The last point is most relevant to your question. When we say the escape speed of Earth is ~11 km/s, we mean that for a particle to escape Earth without any additional thrust, it must be launched at a speed of 11 km/s from the surface of Earth. (Also note that escape speed is position-dependent. The escape speed of Earth from a distance of, say, 1 AU, is not 11 km/s, but much less.)",
"It is perfectly possible for a particle to escape Earth by moving at literally a snail's pace. As long as the particle or magic rocket ship has a thrust to maintain a constant speed of 1 m/s, your scenario is physically allowed. Indeed, the rocket can travel in such a way that it ",
" surpasses the escape speed at any distance. In other words, the rocket can just throttle its thrust to lower its speed gradually forever, never at any time exceeding the escape speed at its current location.",
"Now for your last question.",
"If yes, what does that imply for escaping a black hole's event horizon? From my understanding, the schwarzschild radius is set as the point where escape velocity for a given body is equal to c.",
"Your reasoning actually shows that a black hole is emphatically ",
" an object whose surface escape speed is the speed of light. A black hole is also not an object for which the interior escape speed is greater than the speed of light. If either ",
" the case, then such a definition would not actually preclude escape from within a black hole! If escape speed is just the speed required for a particle in free-fall to escape, then a statement like \"black holes are objects whose escape speed are greater than ",
"\" would just mean that no ",
" could escape.",
"Indeed, ",
" particle can escape from within a black hole once it has passed the event horizon. It doesn't matter whether this particle is in free fall or on some properly accelerated path (i.e., is a rocket applying some thrust). This is quite the distinction between an actual black hole and an \"object whose escape speed is greater than ",
"\".",
"Part of the confusion arises because many people simply don't understand what escape speed really is. But a large part of the confusion arises also because of the ",
" false proofs and misleading statements in pop-sci and seemingly reputable sources that claim the escape speed of a black hole is greater than ",
". In fact, you can even mess around with the equations for gravitational and kinetic energy and eventually derive some formula for the Schwarzschild radius by starting with the assumption \"escape speed is ",
"\". (The Wikipedia article on Schwarzschild radius used to contain such a false derivation. You can even read my ",
"ignored corrections on the talk page",
". It's not longer there, yes, but has now been relegated to ",
"this article",
". No, Wikipedia, Karl Schwarzschild did not derive the Schwarzschild metric using junk science.)",
"If you are interested in details about why the false derivation is really, ",
" wrong, you can read ",
"my explanation here",
". The coordinate \"r\" doesn't even ",
" the same thing in the relativistic problem as it does in the Newtonian problem. It's ",
" bad of a false proof."
] |
[
"The concept of escape speed does not apply at all to the interior of the black hole; that is the point that I was making. If a particle is beyond the horizon, it cannot come back out, regardless of anything else. Period."
] |
[
"I don't really know what you're asking. \"Minimum gravitational influence is ",
"\" doesn't make any sense. So I will just summarize what I've already explained.",
"The statement \"the escape speed of this object at its surface is ",
"\" means that a particle in free-fall can escape from this object only if its speed at its surface is ",
". There is no preclusion of a particle not in free-fall being able to escape. (Indeed, in Newtonian gravity, the only theory where escape speed even makes sense, it is perfectly possible for a particle to escape without ever reaching escape speed.)",
"That is not what a black hole is. No particle, no matter what, can escape from within the black hole. The concept of escape speed is just not applicable."
] |
[
"Is it true that day care kids are healthier later on in life because they are exposed to more illness ?"
] |
[
false
] |
[deleted]
|
[
"This is somewhat of an outdated point of view. Recent research has shown that you actually need immune stimulation at an early age in order to get your immune system going. In other words, it isn't about developing specific immunity against an individual antigen, but rather about activating the whole immune system. Said another way, individuals who are kept in germ-free environments during early development actually have stunted immune systems. There is a huge literature on this out there on this subject. If you're actually interested, you can start with the work by ",
"Jeff Gordon",
" on gnotobiotic mice grown in germ-free environments."
] |
[
"Actually, kids who crawl around in the dirt and get exposed to multiple pathogens have fewer allergies. Allergies and asthma are almost exclusively a first world problem. ",
"I'm on my phone so I don't have a citation, but google \"allergies third world\" and decide for yourself. "
] |
[
"I encouraged Redditors to google and decide for themselves, therefore indicating that the jury is still out."
] |
[
"Do psychopaths know that they're psychopaths?"
] |
[
false
] | null |
[
"Some do, some don't. It depends on whether they notice how differently they react to emotional situations than others, and how they interpret it. Many atribute their behaviour to supernatural causes and think that their lack of instinctive emotional reaction is because they are the descendants of the devil, or them being cursed."
] |
[
"There is apparently a case of a neurologist, who studied the brains of psychopaths and then found out he has the same \"brain structure\" as a psychopath, ",
"as described in this article.",
" "
] |
[
"Didn't realise someone answered to this, but thanks a lot! This is some awesome insight"
] |
[
"HAARP gets a lot of attention by conspiracy theorists, who I don't believe whatsoever, but I still don't know what HAARP is. Can a qualified person explain what's actually going on?"
] |
[
false
] |
HAARP gets a lot of attention by conspiracy theorists, who I don't believe, because these people seem like idiots most of the time. I don't believe for one second that most of them have ANY understanding of the technology behind . It is often the blame for some ridiculous claims such as earthquakes and what not, which I think is absolutely hysterical, HAARP has a website full of but honestly I don't understand what any of it means. I'm pretty sure I'm not alone in this.
|
[
"Very briefly, it sends a very high-power radio signal to the ionosphere (an upper layer of the atmosphere which reflects radio waves) and then measures the reflected signal in order to study the ionosphere. You can think of it kind of like a powerful radar station. Understanding how the ionosphere varies over time (night/day, and due to things such as the solar wind, etc) is important for optimizing various communications technologies. HAARP's radio waves are powerful enough to cause brief physical changes in a very tiny part of the ionosphere, but any claim that it can significantly alter weather or cause disasters is bunk. ",
"It's a bit odd that it would be the target of so many conspiracy theories, considering how open and transparent it is."
] |
[
"That's interesting, because public universities are invited to do research using HAARP's facilities all the time, and you can actually schedule a tour of the facility as a member of the public by contacting their office. They only tour at certain times during the year as I understand it, but they always seem to be willing to open their doors to researchers, the press, and interested civilians.",
"I called and asked all about it. I also was directed on how I could obtain a copy of their Environmental Impact Statement, which is on government record and can be requested legally by any civilian via written letter.",
"And I dug a little deeper and found out some statistics on what the satellite arrays were actually capable of. A lot of people claim that the satellite arrays can reflect off of the ionosphere to travel across the whole world to create earthquakes and nasty things like that. But after digging, it turns out they can beam radio waves at a 15 degree angle perpendicular to the Earth's surface, at most. Turns out, if they got the maximum angle, and reflected a radio wave off of the ionosphere, it would bounce off the ionosphere exactly once, before hitting the ground again directly. And it wouldn't even be very far -- it could only reach about as far as the next major city, Anchorage.",
"Not to mention the importance of its research is publicly posted right on HAARP's website.",
"Turns out they actually are ",
" open. That they are \"not open\" is a complete and utter myth, which seems to be perpetrated only by people who haven't actually mustered up the courage to phone and speak to someone directly.",
"Reminds me of that quote from the movie ",
": \"What's the easiest way to get a gun away from a Grammaton cleric?\" \"You ask him for it.\""
] |
[
"I think the fact that it can actually modify the ionosphere is a lot of what feeds the conspiracy theories.",
"That, and the fact that they can modulate the auroral electrojet to generate powerful VLF/ELF signals. Lots of folks with an idea that very low frequency EM radiation can be used for mind control.",
"I have to say, a talk on this topic (given by Dennis Papadopoulos ",
"http://umdphysics.umd.edu/people/faculty/188-papadopoulos.html",
") is one of the most jaw-dropping physics talks I've ever seen. ",
"If someone sets off a nuke in the Van Allen belts there would be a lot of really energetic and very very dangerous charged particles that would be trapped there. Destroying satellites, exposing astronauts to very high levels of radiation. They spiral around lines of Earth's magnetic field and bounce from pole to pole. Some of them have enough velocity parallel to the field lines to get to the atmosphere and dissipate. But those that have more of their kinetic energy in the perpendicular \"spiralling\" direction than the parallel \"shooting toward the atmosphere\" direction could be trapped for a very, very long time. Decades.",
"The ratio of perpendicular to parallel velocity can be changed by scattering processes between very low frequency electromagnetic waves and those charged particles. But it's hard to generate enough power at such low frequencies; antennas close to the earth and of reasonable size are extraordinarily inefficient.",
"So you use HAARP's ionospheric heater to modulate the auroral electrojet, which is a huge high current that kind of encircles the pole, and that modulated current is both your amplifier and antenna to generate the VLF you need to scatter dangerous charged particles and clean out the Van Allen belts.",
"It's pretty good conspiracy-theory fodder."
] |
[
"Why don't we launch for space from a high flying aircraft?"
] |
[
false
] |
[deleted]
|
[
"That's how SpaceShipTwo is launched. It's fine if you don't care about orbiting, just getting high up. But the energy you need to get fast enough to orbit is about 25 times the energy you need just to up and fall back down."
] |
[
"Check out the answers to ",
"a very similar question",
". In short, getting into orbit requires going at very, very high speeds. Launching from a jet won't make that part any easier."
] |
[
"It would be. However, you also have to account for the amount of energy required to lift the aircraft and spaceship to launch height, so you'd want to make the aircraft as small as possible. And while you're minimizing the size of the aircraft, do you ",
" need it to have independent controls? Why don't you just strap two aircraft onto the side of the spaceship, remove everything except fuel (preferably energy dense rocket fuel), and have them just drop to the ground once they get the spaceship to launch height?",
"Of course, that's precisely what the space shuttle did."
] |
[
"How accurate is the usual picture of the atomic nucleus of a ball/mass of protons and neutrons? What's really happening in the centre of atoms?"
] |
[
false
] | null |
[
"Well, a nucleus ",
" just a collection of protons and neutrons.",
"However as is usually the case with quantum systems, cartoonish depictions of them don’t accurately show their structure.",
"A nucleus is a strongly-interacting quantum many-body system, with very rich and interesting structure. But it’s not easy to draw a simple picture of what a nucleus “looks like”, so people just draw protons and neutrons as billiard balls sitting near each other. In some cases it’s fine to treat nucleons in that way, and in others it’s not.",
"Just like electrons in atoms have quantum-mechanical orbitals, nucleons in a nucleus do as well. However in a nucleus, the forces are much stronger and take a much more complicated form than in the case of atoms."
] |
[
"Nuclear charge densities, at least for spherical nuclei, look ",
"like this",
".",
"So they’re essentially uniform out to some radius, where they quickly drop to zero. This is why nuclei can be approximated in simple models to be like liquid drops.",
"The technical term used for the “fuzziness” of the surface is “diffuseness”, so thats actually not so far off of a guess.",
"But the nuclei shown in that figure are the ones which arguably have to most boring structure. They are doubly-magic, spherical nuclei (except molybdenum-96).",
"Nuclei off of magic numbers are the ones where all the cool stuff happens. For example, deformed shapes, halos, skins, intruder orbitals, etc.",
"For nuclei which are deformed, their densities are not uniform in all angles.",
"For halo nuclei, you have a tightly-bound core, and then one or two nucleons with vastly extended wavefunctions in space. A common example of a halo nucleus is lithium-11, which has a two-neutron halo. For a sense of scale, the RMS matter distribution of lithium-11 is about the same size as that of lead-208.",
"The neutron skin of a nucleus is the RMS neutron radius minus the RMS proton radius, and for heavy nuclei it can get very large. This means that the neutrons in average stick out farther than the protons.",
"All of these density distributions are probability densities, just like how you have probability densities for electrons in atoms.",
"Nucleon orbitals are similar to electron orbitals, but a little more complicated. And in addition to single-particle orbitals, you can have collective motions of nucleons, like rotations or vibrations (“jiggling” of the entire nucleus).",
"Nucleons can do all kinds of crazy things in nuclear reactions, especially in high-energy collisions."
] |
[
"Would it be more correct to imagine protons and neutrons as a force distribution that increases the closer you get to the center, like a ”fuzzy ball”? ",
"Or do they have an actual ”surface” as in some point where their effect on other particles goes from 0 to 1 instantly?",
"Or do they have a ”fluffy” surface where it goes from 0 to 1 over a gradient (but then stays at 1 behind that gradient).",
"Basically, what does the curve of their effect on other particles over distance look like? I’m assuming it’s uniform in every direction, is it?",
"Would such a curve show absolute values or is it a probability distribution like with electrons?",
"Also, how do orbitals work in the nucleus? I always thought protons and neutrons were kinda stuck in place (if jiggling). Do they ever move around, other than jiggly motion or during nuclear reactions?"
] |
[
"Ask Anything Wednesday - Biology, Chemistry, Neuroscience, Medicine, Psychology"
] |
[
false
] |
Welcome to our weekly feature, Ask Anything Wednesday - this week we are focusing on Do you have a question within these topics you weren't sure was worth submitting? Is something a bit too speculative for a typical post? No question is too big or small for AAW. In this thread you can ask any science-related question! Things like: "What would happen if...", "How will the future...", "If all the rules for 'X' were different...", "Why does my...". Please post your question as a top-level response to this, and our team of panellists will be here to answer and discuss your questions. The other topic areas will appear in future Ask Anything Wednesdays, so if you have other questions not covered by this weeks theme please either hold on to it until those topics come around, or go and post over in our sister subreddit , where every day is Ask Anything Wednesday! Off-theme questions in this post will be removed to try and keep the thread a manageable size for both our readers and panellists. Please only answer a posted question if you are an expert in the field. . In short, this is a moderated subreddit, and responses which do not meet our quality guidelines will be removed. Remember, peer reviewed sources are always appreciated, and anecdotes are absolutely not appropriate. In general if your answer begins with 'I think', or 'I've heard', then it's not suitable for . If you would like to become a member of the AskScience panel, . Past AskAnythingWednesday posts . Ask away!
|
[
"I'd say the reason it seems like it might be a good analogy is because it drastically oversimplifies DNA and the other processes going on around it. Using argument by analogy is not very persuasive since the analogy is only meant to explain complex concepts in simple terms.",
"While reducing DNA to its constituent bases AGTC makes it seem like code, it is fundamentally different and more complex. These bases are modified via processes like methylation, which adds to the number of 'bits' you have to consider. So instead of a 4-bit code it's more like >100. How about uracil and RNA? When you consider DNA wrapping around histones and that histones themselves can be acetylated and that there isn't just one type of histone it complicates things further.",
"What this means is the biggest problem is that DNA doesn't do anything on its own in isolation; there are alleles that induce mutation in other alleles via paramutation, some areas of DNA are more prone to mutation than others. There are just so many interactions going on that you're way beyond the 4-bit model ID proponents put forth.",
"DNA is further removed from the coding example in the sense that none of the steps are discrete; you can have many, many alleles being read at once, or being repaired or modified, which is different from the seemingly non-discrete computer that can compute so fast it ",
" non-discrete.",
"Computer code is much more fragile than DNA. Inserting random code into a program will likely crash it. DNA, conversely, can have viral DNA added to it and function completely normally, and chromosomes can combine or be split and still function properly, sometimes even better. You can move whole genes on chromosomes with no adverse effects too.",
"So at its core the code analogy is fundamentally flawed as an argument, and should simply stay as a descriptive tool for high school science classes.",
"Edit: fixed a word"
] |
[
"If DNA is a code, why isn't that an argument in favor of intelligent design? How could a code have arisen naturally? Why aren't there any other codes in nature?",
"I'm an atheist, but I don't understand why this argument for intelligent design is so easily rejected by other atheists. "
] |
[
"Cool: Mind is ",
" it can be easily manipulated.",
"Sad: Mind is so plastic it can be ",
"."
] |
[
"Why do bubbles in a glass or cup tend to move to the edges and stay there?"
] |
[
false
] |
I don't mean carbonation. When I pour a coffee or something, the bubbles are all spread out and then "migrate" to the sides of the cup, even if you slosh it around a little.
|
[
"Because of the surface tension of water.",
"The bubbles will tend to have a low surface area because of the surface tension. So if a bubble sticks to the sides of a cup, a part of the bubble doesn't make contact with the liquid. This is also the reason multiple bubbles stick together; to acquire less surface area."
] |
[
"Thank you. What causes the liquid to bulge up in the middle?"
] |
[
"This will only happen if the bubbles can form a group, or when your spoon is still in the liquid. For the same reason mentioned above.",
"Another reason for your bubbles to bulge up in the middle is because of stirring or right after stirring. This situation has nothing to do with tension or area. Because of the moving water, the liquid will form a centrifuge. This results in a higher pressure near the wall of the cup than in the center (centripetal force). So your foam will fload towards the middle."
] |
[
"Is Allicin a possible cure for MRSA? Does anyone know anything about this topic?"
] |
[
false
] |
I have read about Allicin and it's effects on MRSA here and there but there doesn't seem to be much research out there. I'm a layman when it comes to research papers, so that's why I have come here to check with you guys. Below are some links that apply to Allicin & MRSA. Here's a Here is a . Please note this study was paid for my the same company that sells Allicin. Once again I'm a layman when it comes to these studies so me reading through them would be a huge waist of time. Just wondering if anyone knows anything about or could shed some light on the subject. Thanks
|
[
"Lots of things can get MRSA. I did my master's thesis on Daptomycin, which can knock out most MRSA, as can Vancomycin and some Gentamycin (I think we had a couple of strains that were Gent resistant). CA-MRSA is much easier to knock out than HA-MRSA. I also heard something about a Linezolid which might also work."
] |
[
"The simplified answer: It's resistant to the most nontoxic antibiotics. We can kill the bug, but we have to use more powerful antibiotics that have more serious side effects. For instance, gentamycin has a chance of causing serious hearing damage. Other drugs are toxic in other ways, and are hard for very sick people to tolerate.",
"Slightly less simplified answer: We're also worried about what comes next. For a long time we used methicillin to kill staph, and now staph resists methicillin. So now we use, say, vancomycin. But if we keep using vanc, then over time staph may come to resist that too. Since we have discovered a finite number of antibiotic drugs, it's possible that eventually, we would run out of things that can kill it."
] |
[
"I don't know anything about Allicin other than it is obtained from garlic. Garlic is historically known for its antibacterial properties and work is currently going on to use the active agents as novel antimicrobials. As for a cure for MRSA? That is a strong word, is it possible for this to kill MRSA, maybe! However there are better MRSA killers out there. ",
"The interest in garlic goes beyond just MRSA, it has been found to interfere with bacterial communication as well, stopping the ability of bacterial to \"talk\" and \"co-ordinate\" with each other. This can also be used to \"break-up\" and kill certain bacteria individually. ",
"MRSA is not my area, but another big killer is. The problem with MRSA, Methicillin Resistant Staphylococcus aureus, is that it is present in up to 20-50% of people's noses and that a high degree are MRSA these days. There are antibiotics such as Tigacycline, gentamycin, vancomycin, and as Kale said, some linezolids, can treat MRSA. The problem with using these antibiotics is that if we do get MRSA becoming resistant to them (and it has happened on VERY few occasions), we will have BIG MASSIVE problems. Many, many people will die. There are other bacteria in your gut called enterococci which adapt very quickly to antibiotics, and they share there adaptations with other bacteria. If vancomycin resistance breaks out in these bacteria, the fear is they will give it to MRSA to become VRSA. Only if the infection is very serious, do we use these as they also have a habit of nearly killing patients in the process.",
"Antimicrobials act in certain ways to kill bacteria, depending on the type of antibiotic. One problem is that they sometimes cant tell the difference between you and the bacteria, this causes toxicity side-effects and can cause serious liver damage. Linezolids in particular are known for really messing people up. These are known as last-choice drugs because of these side-effects. ",
"There has not been a new class of antibiotic developed in the past 30 years, where as the ones we have are starting to become obsolete due to resistance. If a pathogenic bacteria comes out which is resistant to everything we have now (",
"and there is already, but thank fuck it isn't that dangerous",
"), it will have free run to kill as many people as it wants until we can develop a new drug to kill it. Last time I checked, it takes over 10 years to develop a new antimicrobial and get it on the market. "
] |
[
"Why does Moderna require two 100 microgram doses while Pfizer only requires two 30 microgram dose to produce a slightly better effectiveness?"
] |
[
false
] | null |
[
"Each company made an educated guess as to what they thought the right size of the dose should be before widespread testing was done for this specific vaccine. They each tried to pick an effective dose size. With more testing, it might turn out the Moderna vaccine would work as well with a smaller dose. ",
"The same educated guess scheme went into the scheduling of the 2nd dose. They each picked a different time (3 weeks vs 4 weeks). Neither is sure that is the optimal timing, but it’s what they picked and it worked.",
"The compressed testing schedule did not permit much dose/timing variance in their testing, so they delivered what was tested."
] |
[
"The mRNA is delivered inside a lipid protective package which adds significant \"bulk\" to the actual mRNA deliverable product.",
"The two companies use different proprietary packages and Moderna's package seems to be bulkier (by 70 micrograms).",
"Ref: ",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5439223/"
] |
[
"Your article is from before the pandemic. Do you have a source that says the vaccine dose sizes include the LNP?",
"The EMA product characteristics say:",
"One dose (0.5 mL) contains 100 micrograms of messenger RNA (mRNA) (embedded in SM-102 lipid \nnanoparticles).",
"Which can be read both ways, but sounds more like it's 100ug without the LNPs."
] |
[
"Is chicken/lamb/beef broth a potential risk for prion disease?"
] |
[
false
] |
There's no chicken prion disease that I'm aware of, but seeing as we weren't aware of the cow one until 30 years ago, it seems completely plausible. Even discounting the chicken, isn't making broth a risk, since some neural matter is bound to get into the broth?
|
[
"Supposing some infected brain matter gets into your lamb or beef broth then it is possible that there is a risk of infection as prions are highly resistant to normal sterilization procedures like boiling (such as in your soup pot). (",
"source",
") Prion disease is caused by the induced misfolding of a normally present protein (called prion protein PrPc). The mechanism of how this mis-folding is propagated by prion particles in an infected host is not known. Boiling/heating usually denatures proteins, making them lose their specifically folded shape. Somehow prions do not lose the ability to induce folding changes in normal prion protein after boiling or decontamination with formaldehyde. A lot of these mechanisms are poorly understood. ",
"Here is an open access review of prions for further reading."
] |
[
"it is incredibly unlikely but possible; at least in cows US regulations prevent most of the potential BSE tissues from human food supplies and animal feed. That being said it's not impossible that a cow with BSE could be broken down improperly and cross contaminated. Spinal cord contamination happens from time to time. It was only a few months ago that a large amount of beef was recalled in the northeast due to potential spinal contamination. My understanding is that it was more based around improperly filled out paperwork but it had gone on for almost a year before it was caught. ",
"http://www.cnn.com/2014/06/12/health/beef-recall-mad-cow-disease/",
" ",
"In terms of chicken, there is no documented prion disease in birds and chicken prions are different enough from human prions that the transmission risk is assumed to be low. ",
"http://www.ontariogenomics.ca/sites/default/files/Episode%204%20-%20Prions.pdf"
] |
[
"US factory butchering techniques have, over the years, adapted to pretty much keep any brain or spinal material out of the process. Even pet food processing is very careful to segregate brain/spinal material.",
"Our local rendering plant (which used to pick up dead farm animals) has closed because it became too expensive to keep parts segregated."
] |
[
"Lead contamination in Uranium dating?"
] |
[
false
] |
With Uranium-Lead dating zircon crystals to determine age, finding the decay rate from U235 to Pb207. I read that when the zircon crystals form trace amounts of lead can get into the crystal and that chemically the isotopes of lead 204, 206, 207, and 208 are the same so the same proportions are present as when it was formed. I know that lead 204, which is not produced by radioactive decay provides a measure of what was "original" lead so its presence is irrelevant to my question; my question is: #1 can trace amounts of Pb207 be absorbed into the forming zircon crystal, and #2 if it could would it be enough to give an inaccurate dating? Either way I guess a follow up question would then be could there be ways that would cause reliability issues with U-Pb dating? (Please correct any mistakes or misunderstandings I might have projected in here)
|
[
"1 can trace amounts of Pb207 be absorbed into the forming zircon crystal",
"Yes.",
"2 if it could would it be enough to give an inaccurate dating",
"If you didn't correct for it, yes, in some cases.",
"There are a couple of ways you can correct for initial radioactive lead in a zircon. The first way is sort of in your question already, you said, \" I read that when the zircon crystals form trace amounts of lead can get into the crystal and that chemically the isotopes of lead 204, 206, 207, and 208 are the same so the same proportions are present as when it was formed. I know that lead 204, which is not produced by radioactive decay provides a measure of what was \"original\" lead so its presence is irrelevant to my question\", but the common lead (204) is not irrelevant, cause you can use the amount of Pb-204 in your sample (which shouldn't change) and an assumption about what the initial 204 to radioactive lead ratio was to correct for the initial lead. An alternative way that doesn't require any assumption about initial ratios is exploiting the fact that there are two separate decay chains of U to Pb, 235-U to 207-Pb and 238-U to 206-Pb via concordia plots. Or you can use an isochron method for a single decay chain (assuming you have a population of samples) or both decay chains simultaneously. ",
"This",
" chapter on U-Pb dating covers both the lead correction (PDF page 10) and using isochrons and concordia plots (PDF page 5) to deal with initial lead (and recognizing other issues, like lead loss, etc). "
] |
[
"I've had plenty of conversations online with faceless/nameless people who doubt the veracity of geochronologic methods because the results of such invalidate a strong personal viewpoint (e.g. billion year old zircons and young earth creationists don't mix well). Within the community of people who use these methods (e.g. geologists, archeologists, etc), nobody doubts the techniques because (1) they are based on relatively well studied and understood physics (e.g. radioactivity) and (2) they are internally consistent (e.g. comparing methods, etc). The bigger issue is what does a particular date mean. ",
"So, if you date a zircon, nobody generally questions whether the date is correct (assuming you've thoroughly documented how you obtained that date and followed standard procedures), but other scientists may question your interpretation of that date or if that date is meaningful. As a simple example, zircons are quite hardy little minerals and so can survive partial melting and pretty extreme metamorphism, but during this process the outer parts of the crystal may be removed and a new portion of the crystal may grow as a 'rind'. This can happen multiple times for extremely old zircons that have been 'recycled', so you can end up with a grain that is 'zoned', which you can see with certain types of imaging, ",
"e.g. these pictures",
". These different zones will generally record the time that they crystallized, which could mean that the difference in age between the 'core' and the 'rim' of a zircon could be hundreds of millions of years in extreme cases with various zones in between. Now, if you can use a laser ablation system coupled to a mass-spec, you could measure the U-Pb content of each zone (if it's big enough and your laser spot size is small enough) and get meaningful ages of different zircon growth events. If however, you dissolved that zircon whole and measured the U-Pb ratios of the entire crystal as a single, amalgamated sample, you would get some average age depending on the spread of the rim and core ages + the volume of each of these components. This age however would have no real geologic meaning. This is when people start talking about trust of ages, basically do they trust the interpretation of ages (or that the age was acquired in a way that produces a meaningful age)."
] |
[
"Thank-you. It was a bit of a mind workout reading that, but one day I hope to be more fluent (side note, impressive, list of sources at the end). Random question, have you ever had someone say that they don't trust uranium/carbon dating? Did they have any good reason not to?"
] |
[
"How do they determine the calorie count and nutritional information for foods?"
] |
[
false
] | null |
[
"Is this a bomb calorimeter? I've only ever worked with a DSC, so I'm just curious which type this is."
] |
[
"Is this a bomb calorimeter? I've only ever worked with a DSC, so I'm just curious which type this is."
] |
[
"What OP described is a bomb calorimeter. He did leave off the standardization step... we burn a known mass of a reference material (most commonly benzoic acid) to determine the heat capacity of the system."
] |
[
"What does a model of the universe with no start and no end mean for the second law of thermodynamics?"
] |
[
false
] |
[deleted]
|
[
"This is a legit theory (the \"",
"Big Crunch",
"\"), and a really intuitive idea, but recent observations indicate that it's pretty unlikely to happen. The expansion of the universe is actually accelerating, the opposite of what the Big Crunch would predict (there's a lot more technical information in the Wikipedia article I linked to, if you're interested)."
] |
[
"I think the OP is referring to ",
"this submission to /r/space about a cosmology without the big bang"
] |
[
"I did misunderstand the question. Gave the OP a lot less credit than he deserved. It sounded more like \"since our universe has obviously been going on forever\" than \"if this theory without a big bang is correct\" to me. My bad."
] |
[
"Are these white streaks caused by cosmic rays?"
] |
[
false
] |
I recently launched a weather balloon and I'm looking through the footage. At high altitude I keep seeing white streaks/dots on the screen. The footage was filmed using a Raspberry Pi camera. At 34-36 seconds in on the video below, you can see an example of these flashes if you watch it in high definition.
|
[
"They certainly look like cosmic rays - however, noting that the balloon burst only a second or two later, I'm wondering if they might have been some very fast bits of debris. The delay between the streaks and the rest of the debris seems a bit too long for that, though."
] |
[
"They occurred some considerable time before and after the balloon popped, and only appeared above a certain altitude."
] |
[
"I tried finding some away from 34-36 seconds, but got bored of watching it spin rather quickly! It sounds like they're cosmic rays then - congratulations, I guess? :)"
] |
[
"askscience, I need some help"
] |
[
false
] | null |
[
"A de-ionized water rinse, followed by several days in rice might be able to clean off the stains behind the screen. I would recommend ",
"doing a breakdown of the device",
" rather than just rinsing it off. ",
"Make sure you do deionized water, and not distilled or tap. I've saved an old-school Archos player this way."
] |
[
"Usually liquid damage to electronic devices comes from conducting electricity and shorting something out (which is why you remove the battery or power source immediately) or from corrosion. Some devices like keyboards are often safe to wash with water when disconnected and thoroughly dried before use.",
"That being said I cannot comment on your device specifically. However the fact that the stains changed when put into the rice for the second time implies there was still moisture present. I would keep trying to dry it before resorting to more aggressive measures."
] |
[
"Thank you, I hadn't heard of de-ionized water. It's gonna take some motivation but I really should take the device apart"
] |
[
"How much did the age of the universe change, if at all, when the expansion of the universe was discovered to be accelerating?"
] |
[
false
] |
I'm assuming the age of the universe was predicted by estimating when all the matter was at a single point, and I am not exactly sure that is how it is done. But I was thinking that the discovery of an accelerating expansion would cause a huge impact on the estimation of the age of the universe over 14 billion years.
|
[
"If you look at the original paper from Perlmutter et al, section 6, paragraph 2, they find that the age of the universe (relative to Hubble's constant, which isn't exactly known), was 14.5 (+- 1) billion years, compared to the estimate for a flat universe which was 14.9 (+1.4-1.1) billion years, scaled by .63/H where H is Hubble's constant in appropriate units. Now we know H is closer to 0.7, so these are multiplied by 0.9.",
"So to answer your question, not a whole lot."
] |
[
"Your referring to the correction factor F from the equation ",
"age = F * 1/Ho.",
"http://en.wikipedia.org/wiki/Age_of_the_universe#Cosmological_parameters"
] |
[
"You can also calculate the estimated age of the Universe (+/- about 360,000 years) by measuring the temperature of the CMB. The temperature of the CMB would already have encoded in it any sort of non-constant expansion, since the temperature is inversely proportional to the scale factor of the Universe."
] |
[
"How could super heavy elements have an \"island of stability\" ?"
] |
[
false
] |
Wouldn't the sheer size (volume) of the element exceed the range of the strong nuclear force? Isn't this why elements of a higher number than lead decay? Also wouldn't the repelling force of the electrostatic force, according to Coulomb's law, push the protons apart and cause alpha decay?
|
[
"I think I can clear this up. It could more feasibly be call the island of relative stability, because these theoretical elements will still be radioactive just like every other element heavier than lead. You're right about why atoms decay, but it is theorized that nucleons (protons and neutrons) occupy \"shells\" or energy states just like the electrons around all atoms. There are what are called \"magic numbers\" of nucleons that form relatively more stable states, allowing these superheavy elements to last significantly longer than we would have originally expected."
] |
[
"OK. I get nucleons have different energy states (the cause of gamma radioactivity is the falling from a higher to a lower) but how does this affect the battle between strong nuclear force and the electromagnetic force?"
] |
[
"The extent of my knowledge is that the \"magic numbers\" of nucleons constitute a relatively stable configuration that the electrostatic repulsion has to overcome for the atom to start decaying. ",
"From what I've read, it seems pretty analogous to how electrons behave in an atom. When the valence is full, this is a pretty stable state. But, atoms with a single electron in the valence are prone to letting that extra electron attach to a different atom.",
"It's just a bit different because atoms can exist as ions for as long as is needed, so more/fewer electrons than the most stable state doesn't affect the integrity of the atom itself. Also, electrons are simply balanced between the electrostatic attraction to the nucleus and the repulsion by other electrons, which are both caused by the same force. However, in the nucleus, it is a balance between the electrostatic repulsion and the strong force (which is stronger). So, an unstable state in the nucleus will cause the atom to decay."
] |
[
"What would the Earth look like now if life had never formed here? Would there be any major geological differences?"
] |
[
false
] | null |
[
"The main thing that comes to mind is that various erosional processes would function quite differently without land plants. Plants have important roles as both weathering and erosional agents. Plant roots are quite efficient at exploiting cracks in rocks and essentially breaking up rocks into smaller bits (i.e. weathering). Processes like tree throw, where a large tree falls over and brings up big mats of sediment with its roots, represent important mechanisms for transport of sediment down hill, both tree throw and weathering accomplished by plants are ",
"discussed here",
". Plant roots also represent important ",
"stabilizing forces for hillslopes and river banks",
", so plants can both increase or decrease erosion depending on the process you're considering.",
"Plants also do a fair bit of work in terms of regulating soil moisture, which in turn is important in determining the amount of water that makes it from rain that falls on a landscape to a river. In a very simple sense, if plants are able to extract a lot of water then more water will infiltrate the soil as opposed to just run downhill and into a river. So, without plants, you might expect that you'd quickly \"fill\" up the sediment layer with water promoting more runoff and thus larger stream flows. Here is a ",
"quite technical take on the role of plants in influencing runoff (might be behind a paywall, sorry!)",
". ",
"Additionally, plants (along with microbes, fungi, etc) are extremely ",
"important in soil forming processes",
", so without them, the Earth wouldn't have soil, it would just have sediment, so it might look more like what we see on Mars.",
"To a lesser extent, the absence of various animals would also change certain aspect of erosion. Burrowing animals and insects can also make large impacts on erosion of hillslopes, ",
"e.g. gophers",
". ",
"Beavers also have a surprisingly large effect on the behavior of rivers",
".",
"Off the subject of erosion, without life, our atmosphere would likely not have any, or very much, dioxygen floating around ",
"e.g. the Great Oxygenation Event",
". Thus, in general, the chemistry of our atmosphere and oceans would be very different and weird deposits that were common before this event, like ",
"Banded Iron Formations",
" would likely be forming.",
"I'm sure there are other things I'm missing, but these are the big ones that spring to mind for me."
] |
[
"There would also be huge changes in the composition of deep ocean sediments. Calcium carbonate and opal would be largely absent from the geologic record. The White Cliffs of Dover and the Great Bahama Bank wouldn't exist. Sediment accumulation rates in the ocean would be way lower. The sediments might look something like in the pelagic South Pacific, mostly iron-rich clays that accumulate really slowly. "
] |
[
"The atmosphere wouldn't be oxygenated, but reducing. Gases like methane and hydrogen sulfide would be in the atmosphere. With carbon more in the atmosphere and less in carbonates, the atmosphere would be denser, with mostly carbon dioxide and nitrogen. Since iron would exist as reduced Fe(II), it would dissolve in the ocean. The sulfur would also appear as sulfuric acid, so oceans would be acidic and shores greenish-reddish colored from mixed Fe(II) and Fe(III). Sort of a planet-sized Rio Tinto. Due to CO2, temperatures would be much higher. The troposphere would be a wet, warm blanket full of toxic mist. But, also, organic compounds would float freely around like on Titan. Red polymeric sludge would be common.",
"This was the state of the Earth before life evolved, and when the Sun goes red giant, also for Titan."
] |
[
"How do scientists tell each other that a hypothesis is wrong, if at all?"
] |
[
false
] |
It's my understanding that articles and papers are typically written and published about findings from experiments that worked. However, sometimes the experiment doesn't work and can potentially be a large waste of resources, and to my knowledge, articles and papers usually don't have these kinds of things as their subject matter. Is there some way that scientists can communicate to each other that a certain hypothesis is incorrect so that others don't have to spend time and resources testing it themselves?
|
[
"They usually publish those results, too.",
"The abstracts tend to look something like:",
"\"We studied an interesting thing to determine if stuff we'd like to be true about it was true....After careful analysis, we're pretty sure that stuff isn't true. You might try looking for this other stuff that could be true instead.\""
] |
[
"Instead of publishing negative results it is far more likely that someone will show a hypothesis to be wrong in theory. Science is to a large part done on paper before it ever makes it to a laboratory. "
] |
[
"I would expect this to depend heavily on the field of study however. "
] |
[
"Can liquid oxygen (LOX) be used alone as rocket fuel?"
] |
[
false
] | null |
[
"No, to burn something you need a fuel and an oxydizer. You could also use products that react spontaneously with itself (hydrazine, hydrogen peroxyde, ...). Oxygen alone doesn't release any energy."
] |
[
"No, oxygen is not really interesting for nuclear reactions.",
"The general idea for making rocket fuel in the Moon is to mine water in the north or south pole and split it into oxygen and hydrogen."
] |
[
"It might be possible in theory but running hot oxygen gas into a rocket engine is incredibly difficult without destroying it. Oxygen will burn the metals."
] |
[
"Is consciousness independent or dependent on sensory input?"
] |
[
false
] |
Does our sensory input feed into consciousness (independent), or is consciousness a product of sensory input (dependent)?
|
[
"Consciousness evolved as a way to improve an animals chances of surviving their environment. To have a properly working mind your senses have to be stimulated from birth. We've seen what it looks like when a child has been deprived of sensory imput. locked away unloved, untouched, and unable to interact with the world. Consciousness is dependent on environment. "
] |
[
"Can you link some of those studies on babies who were deprived of stimulation?"
] |
[
"This is like a round about way of asking \"does free will exist?\". Personally I would assume that the input is fed into the consciousness, because let's say you see two boxes, the sensory input has been fed to your brain that the boxes are there, but your consciousness must determine whether or not you open one, and which one."
] |
[
"Scientific basis for thinking that old stuff was better? Ex: every generation seems to think snl was best when they grew up with it. Is there an \"old stuff was better\" syndrome or something?"
] |
[
false
] |
I think it has something to do with just remembering the good and not the bad, making music etc. from earlier times seem better than what we have now. Is there a scientific explanation and name for this?
|
[
"Your brain has all these little rules which enable you to make complex decisions quickly. These produce cognitive distortions, which tend to make people overgeneralize or be dichotomous in their thinking. This means that they tend to take a few instances of an event, and give it more weight than it realistically should have. Dichotomous thinking is where you tend to see things as being all negative or all positive. There's other types of distortions, but yes, it's the factory programming that gets us every time."
] |
[
"Hi all,",
"Incidentally I happen to be a researcher who specializes in this exact topic (small world isn't it). I otherwise never post on reddit, especially ask science because I generally find it sort of intimidating, so forgive me if I make some rookie mistake here. ",
"As someone else in the thread helpfully pointed out, nostalgia is still a relatively young area of research, so a lot of this stuff is fairly new and quite a hot issue as of late. In terms of nostalgic preferences in the consumer sphere, there are a couple of researchers named Robert Schindler and Morris Holbrook who have done some really interesting work looking at this exact thing. What they find is that individuals seem to develop \"age related preference peaks\" - Preferences towards certain styles of clothing, music, all sorts of things - Based on what is popular / preferred in a critical period of life. This \"window\" is generally early adulthood, something like 20 years old give or take a few years. ",
"There is still some speculation as to why this particular window seems to be most critical for developing lifelong preferences for so many products, but I think the most compelling argument is that we generally see it as a really important period of our lives in which we fully \"come into our own\" as adults. As such, we hold onto these preferences because we associate them with a period in our lives which we see as particularly important in retrospect.",
"As far as the appeal of general nostalgic experiences - I believe this ties with the idea of nostalgia as a \"rosy view\" (originally suggested by Mitchell if I recall correctly...do we have to formally cite these in here or what?). The basic idea is that when it comes to nostalgic memories, the affective signature attached to that experience becomes a bit more generalized across the whole experience. So, if we remember an experience that was largely positive, but had a whole bunch of annoying little issues, our brain sort of puts a positive sheen over the whole thing in retrospect and we forget all of the little details we found annoying while remembering all of the nice things about it. ",
"There is, of course, much more to this and I apologize for running long but I've written so many hundreds of pages on the topic that I find it difficult to pick out the most important few paragraphs. As FriendlyEgoBooster in the thread already mentioned, nostalgia is a very social emotion - In addition to helping us find more meaning in life and fight existential threat / death anxiety, it helps us feel closer to others, makes us happier in general, and improves our self-esteem among other things. This whole cocktail of positivity in nostalgic experiences has a way of rubbing off on things we relate to those experiences, like movies we watched together with friends, games we spent all night playing together, and so on. They are sort of woven into these important social experiences. ",
"I hope this meandering explanation is useful to someone! :) ",
"EDIT for references:\nHere are a few links to papers that might be of interest.\n",
"Schindler and Holbrook on nostalgia and consumer preferences",
"\nThis one talks a fair bit about age related preference peaks related to nostalgia and references many other papers that explore the same thing",
"Nostalgia: Content, Triggers, and Functions",
"\nAn excellent paper by Tim Wildschut and colleagues exploring several of the key findings in social psychology research in nostalgia",
"Other users have linked to some articles about Rosy Retrospection, I would recommend looking up Terence Mitchell for anyone interested in learning more about that. "
] |
[
"It is why I think things like logical fallacies and cognitive biases should be taught in school because it is extremely important that people recognise the negative effects of these \"factory settings\". Reading the wikipedia page on both concepts illuminates you as to how flawed our brains really are. It is scary how much our minds can play tricks on us."
] |
[
"How do human postures work? Why is it so difficult to consciously correct a bad posture ?"
] |
[
false
] |
What is the science behind posture correction? Is it possible to successfully correct one's posture? I came across some "exercises" for correcting posture. Do they really help? Is it possible to manipulate muscle strength and flexibility? If so, how does it work? EDIT:Sorry. I meant why is it so difficult to to be conscious of bad posture?
|
[
"I'm going to chime in although reddit hates chiropractors.\nTL:DR - Yes postural correction is possible but it takes a change in your daily lifestyle to be effective and long term. Don't sit.",
"Posture is something that is created through habit. Good posture is referred to as \"neutral spine\" with a cervical lordosis, thoracic kyphosis, and lumbar lordosis. The cervical and lumbar curves develop as we grow up and become bipedal. They are there to allow the body to withstand stresses in multiple directions (",
"http://umm.edu/programs/spine/health/guides/anatomy-and-function",
"). Wanna see something cool? Look at a child before they start school. They will have near perfect posture, their squat will be near perfect, ankle mobility will be awesome, few if any muscle imbalances. Once we start school, things go downhill. \nNow you go to school. Instead of climbing, running, and playing for 8 hours a day, you sit in a desk. Humans (and almost all mammals) are meant to stay active. Our bodies evolved to rise to our demands and adapt to meet them. Well if we are spending 8 hours a day hunched over a computer in the seated position, then sit on the couch and watch TV or browse reddit when we get home, our body adapts to this.",
"\nThe adaptations are not what we \"want\" but are made in demands to what we do. Although poor posture is detrimental to our health, both physically and mentally (",
"http://psycnet.apa.org/psycinfo/2014-37739-001/",
"), our bodies don't necessarily make the corrections because we continue to keep the same bad habits.\nLets look at sitting:\nNeck/upper back: Trapezius muscles are stretched from a forward head, deep neck flexors are in a shortened position. Shoulder retractors (rhomboids) are stretched and the pecs are shortened and tight. \nLow back: Hip flexors (psoas) are shortened and tight, hip extensors are stretched and weakened. These patterns are more commonly referred to as Upper and Lower Crossed Syndromes (",
"http://www.jandaapproach.com/the-janda-approach/jandas-syndromes/",
").\nYes they can be corrected, sometimes moderately through strengthening the weak stretched muscles (rhomboids, glutes) and stretching tight shortened muscles (pecs, psoas). But they will return if your daily habits do not change. Sitting is the new smoking, so you need to get up and move. All of the postural strengthening will help, but how you spend most of your waking hours will decide how your posture will be. "
] |
[
"Muscles are weak in a shortened and lengthened position. The greater amount of sacromeres in series allows more contraction, until you venture to a point where neurologically you have limited function. Posture is adaptation, your body gets stronger in positions it spends more time in. This is not bad posture as it is the most efficient setting for your body to conduct itself. Even as years go by bone tissue will continue to reappropriate itself based on the forces applied to it. Take a look at the tuberosities of a baseball pitcher."
] |
[
"Muscles are weak in a shortened and lengthened position. The greater amount of sacromeres in series allows more contraction, until you venture to a point where neurologically you have limited function. Posture is adaptation, your body gets stronger in positions it spends more time in. This is not bad posture as it is the most efficient setting for your body to conduct itself. Even as years go by bone tissue will continue to reappropriate itself based on the forces applied to it. Take a look at the tuberosities of a baseball pitcher."
] |
[
"Why can we not always remember everything immediately, or off the top of our head?"
] |
[
false
] |
What is the reasoning behind some pieces of information taking longer to recall (or needing an associated piece of information to help recall) than others, regardless of whether they are committed to long term memory or short term memory?
|
[
"Arguments stemming from the information-capacity of the brain have been out of favor for quite a while. The information capacity is so large that it's not really clear that it ",
" be reasonable to expect it to have the capacity to store everything.",
"That said, models of more distributed memory function so unlike computer memory that the analogy isn't really useful anymore. Cued retrieval with interference becomes a much better explanation for performance on memory tasks."
] |
[
"Think about how it would work out if your computer saved every file, web page, video, application, and keystroke ever entered, opened, or utilized, ",
" than those that you deem important enough to save.",
"Like that, but with your brain. ",
"The human brain has a finite (though impressive) capacity, so it only dedicates pathways and neurons to those items that are most used (and therefore, most useful) within the brain. Things like your address, how to drive, or whatever skills or knowledge you access and implement on a regular basis are strengthened, while what you had for dinner last month or the name of that one kid in your 5th grade class are not bothered with.",
"It's a simplified version of the truth, but it's the only one I can adequately explain, due to the fact that I am not a neuroscientist as of the last time I checked.",
"Hopefully I could be of assistance. "
] |
[
"The process of memory formation, as some have already stated, has to do with the strengthening of connections between neurons, or long-term potentiation (LTP.) Different molecular cascades have to occur to activate genes which create proteins, such as neurotransmitter receptors or cross-synaptic bridges, which operate to strengthen the synapse. However, your bodily resources are not infinite and these processes do take time and repetition. This entire process is dynamic and requires the use of the attentional cortices of the brain to determine what needs to be remembered.",
"Source: B.S. in Psychology and currently working as a laboratory technician for a neuroscience lab."
] |
[
"Why did Chernobyl's control rods have graphite on the tips?"
] |
[
false
] | null |
[
"Hello,",
"Some good answers ",
"here",
" and ",
"here",
".",
"Best."
] |
[
"The disaster occurred because of a cascade of slightly wrong things which all ended up being wrong in the right way to create a big reactivity excursion.",
"There were design flaws, like the positive void coefficient and the effects of the graphite tips of the control rods. There was also human error, like operating the reactor in an unstable low-power state. No ",
" thing is to blame for what happened."
] |
[
"The disaster occurred because of a cascade of slightly wrong things which all ended up being wrong in the right way to create a big reactivity excursion.",
"There were design flaws, like the positive void coefficient and the effects of the graphite tips of the control rods. There was also human error, like operating the reactor in an unstable low-power state. No ",
" thing is to blame for what happened."
] |
[
"Are dry ice \"snow flakes\" possible and what shapes would they be?"
] |
[
false
] |
For instance on a planet with a mostly CO2 atmosphere and a mean temperature around the -56.4°C freezing point of CO2.
|
[
"I can't see any reason why this wouldn't be possible given the right temperatures and pressures. ",
"The shape of snowflakes comes from the ",
"hexagonal structure of ice Ih",
". Hexagonal structures have one unique axis in the unit cell and 2 which are identical. This leads to a snowflake being flat - two dimentions are similar with the third being unique (as an aside, the shape of a needle is equally valid for two similar dimensions with one unique.)",
"Now on to CO2... This ",
"crystal structure",
" is ",
"cubic",
" and has three identical axes. You would have to expect a three dimensional shape for these imaginary dry-ice snowflakes. Their macroscale symmetry would likely be a variation on cubic symmetry like cubic, tetrahedral, or octahedral. I would not care to speculate whether the fine structure (lacey patterns) of snowflakes might or might not be found in such a system."
] |
[
"Here is a page about ",
"snow on Mars",
" that describes experimental work on CO2 snow. Apparently the experiments produced cuboctahedral grains of frozen CO2."
] |
[
"So, it looks like the answer is cuboctahedral miniature sleet rather than 3-D lacey snowflakes. - Cool!"
] |
[
"How do behavioral adaptations get passed on genetically?"
] |
[
false
] |
I think I understand how physical adaptations can be passed onto offspring through DNA, but how do behavioral adaptations become instinct, (assuming a species does not teach their offspring)? Does this happen and does science understand it?
|
[
"Many behavioral traits can have a genetic basis, and so are passed on to offspring the same way any other trait is. Any trait without a genetic basis (which means it has no heritability, in geneticist speak) will not be passed on to offspring without teaching. ",
"There's an entire field devoted to studying the genetic basis of behavior, which you can read about ",
"here",
".",
"My own favorite example of behavior with a genetic basis is mating preference in female fruit flies, which creates a barrier between two species (",
"cite/pdf",
" )"
] |
[
"So it sounds like I had the cart before the horse, so to speak. I was thinking of an animal that learns a behaviour which is advantageous to its survival, which then somehow becomes encoded in its DNA (which was what I didn't understand). It sounds like its more like a random mutation in its DNA causes a behaviour which, because it is an advantage, then gets passed onto more offspring. Like you said, its just like any other trait. I just didn't make the connection that a behaviour could stem from a random mutation rather than being learnt."
] |
[
"No worries -- it actually seems crazy to me that something as complex as behavior can stem from random genetic mutations. One of the reasons I love my field :)"
] |
[
"Are there instances when an infection with one pathogen 'miraculously' cures another pathology or condition within the same patient?"
] |
[
false
] |
I feel as though I've heard anecdotal evidence that patient X had, for example, a pathology (acquired or intrinsic) and after becoming infected with another illness found their other malady disappeared. Any validity to this or evidenced-based studies you are aware of?
|
[
"Yes, malaria was used to treat terminal neurosyphilis before the advent of penicillin ('malariotherapy'). The evidence isn't great due to the time period, but it certainly did benefit many patients.",
"http://jama.jamanetwork.com/article.aspx?articleid=398731",
"It's been proposed for HIV, and Lyme disease more recently, but this is considered pretty fringe."
] |
[
"there are case reports of cancer patients who become concurrently sick with some sort of viral or bacterial infection, and if they fight it off, their cancer would go into full remission post-infection. scientists think the infection may have retrained potent components of the immune system to recognize cancer cells that would otherwise slip by disguised as 'non-foreign' material. the immune system could then successfully eliminate the cancer from the body and there is the thought that the immune system in this case would even provide powerful immunity against that specific cancer in future. this is where a lot of ideas for immunotherapy and t cell editing comes from in todays cutting edge of cancer treatment"
] |
[
"C. Difficile infection. This is a bacteria with a large toxin that lives in the large intestine. The toxin is related to the tetanus toxin. If the C. Difficile gets too prominent in the gut, you will die. Treatment is with a strong antibiotic (vanco), but it often recurs. If that happens, you take a sample from a different person, and apply it to the large intestine. The sample has a different population of bacteria, which outcompete the C. Difficile and cure the problem. ",
"Yes, a poop transplant. "
] |
[
"Is it possible to forget?"
] |
[
false
] | null |
[
"I'm aware of the fact that the brain works very differently; thanks for the clarification though. My answer was meant in reference to OPs question, where they imply that the brain undergoing physical changes (what alternative is there?) makes erasing information impossible.",
"EDIT: I understand now that there might be a difference between general physical changes and changes with respect to the neuron-level structure."
] |
[
"I'm aware of the fact that the brain works very differently; thanks for the clarification though. My answer was meant in reference to OPs question, where they imply that the brain undergoing physical changes (what alternative is there?) makes erasing information impossible.",
"EDIT: I understand now that there might be a difference between general physical changes and changes with respect to the neuron-level structure."
] |
[
"More or less, yes, that is my question. I was lead to believe that information was all neatly stored within our brains in one place or another. If this was the case, and our brains did not receive any trauma or large shock that would change this information, why ever would we forget it? Do we as humans \"overwrite\" within our own brains, or is there another process which applies?"
] |
[
"Why Am I Not Getting Cauliflower or Broccoli Heads?"
] |
[
false
] | null |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):",
"/r/AskScience",
"For more information regarding this and similar issues, please see our ",
"guidelines.",
"If you disagree with this decision, please send a ",
"message to the moderators."
] |
[
"Ok, thank you."
] |
[
"You might want to try ",
"/r/gardening",
" or other garden related subreddits. "
] |
[
"When we rip paper, are we breaking covalent bonds or just hydrogen bonding between molecules?"
] |
[
false
] |
Other examples would be cutting plastic with scissors, snapping a strand of hair, or smashing a rock. A carbon-oxygen bond (C=O) requires 745 kJ/mol to break the bonds. An AK47 bullet can impart 2kJ, and a human punch weighs in at a measly 100 joules. So when you tear, rip, punch, or shoot something, are you actually breaking bonds or just separating individual molecules?
|
[
"The forces that hold together the fibers in the paper are van der Waals forces (forces of attraction and repulsion between molecules, AKA friction). So you're rubbing fibers against each other and possibly breaking a few that were held together by van der Waals forces, hence the force you feel when you rip it, but you're breaking very few molecular bonds if any."
] |
[
"No. ",
"Breaking a bond is a chemical change, but ripping a piece of paper is physical. When you rip paper, you are tearing fibers, that latch onto each other to hold the paper together, apart but no bonds are broken. "
] |
[
"Ok thank you, i kinda had that feeling but was not sure."
] |
[
"How different would modern man be if we evolved as herbivores?"
] |
[
false
] | null |
[
"Of course, the question is speculative, the true answer is \"we don't know.",
"However, in this case, we had a relative which may provide some insight: ",
" aka \"Nutcracker man\"",
". These guys had specialized teeth adapted to eating coarse vegetable matter, tough roots and possibly cracking hard shelled nuts. Huge molars, thick jaws, powerful jaw muscles and a sagittal crest at the top of the skull to anchor those jaw muscles and provide leverage.",
"So .... Imagine those guys lineage following in our footprints, and you'll have a pretty good idea."
] |
[
"Gorillas are herbivores but aren't what I'd call a prey species; the two don't necessarily go hand-in-hand."
] |
[
"Gorillas are herbivores but aren't what I'd call a prey species; the two don't necessarily go hand-in-hand."
] |
[
"Is nipple confusion real?"
] |
[
false
] |
According to some, if a newborn is given a bottle rather than breastfed, they will often prefer the bottle over the nipple, and thus not latch onto the breast. The top result on Google indicate that this is not the case - that this is only the case if the child already has trouble latching (and thus the problems were already there).
|
[
"Even most hardcore breastfeeding advocates will admit that the idea of \"nipple confusion\" is not real.",
"However, for some infants switching back and forth can change feeding patterns. The reason being the hard false nipple of a bottle (or a pacifier for that matter) engages a feeding reflex when the roof of the mouth is stimulated. As you can imagine, a real woman's nipple is far softer than a plastic one. The hard nipple can overstimulate this reflex especially if the infant is used to breastfeeding only. Think of it in evolutionary terms, other than occasionally sucking it's own thumb (which is still tiny and soft) the reflex was developed only while sucking on the mother's soft nipple, not a hard plastic object.",
"As a result, the physiologic signals the baby is used to receiving that help it recognize when it has \"fed enough\" or needs to \"feed more\" can become muddled. ",
"I have research on this on my work computer, I'll update with it when I'm able to."
] |
[
"Because nipple confusion, as the name somewhat implies, is that the infant is confused going back and forth because the mechanism of action - the latch and the suckle - are different. This is juxtaposed instead with newer ideas that it is a different satiation response rather than a mechanical issue."
] |
[
"How is what you wrote consistent with saying it is not real?"
] |
[
"Why do nuclear power plants have those distinct concave-shaped smoke stacks?"
] |
[
false
] | null |
[
"Those are cooling towers (",
"https://en.m.wikipedia.org/wiki/Cooling_tower",
"). That particular design is apparently really good for stability, air flow, and minimal material use. They aren't just for nuclear plants, I know of coal fired plants that have them as well."
] |
[
"They are natural draft towers so the long term operating costs are less than a mechanical draft tower ( one that uses fans and motors). However the initial installation cost is higher with a natural draft tower. "
] |
[
"The “smokestacks” are ",
"cooling towers",
" – essentially giant radiators that are used to cool clean water. A common design uses a water spray that’s directly exposed to the air, resulting in the cloud of condensation.",
"The distinctive shape is a hyperboloid. It’s stronger than a cylinder, but can still be built with straight beams."
] |
[
"Why do electric motors have maximum torque at low rpms, while combustion engines have to be at high rpms to get maximum torque?"
] |
[
false
] |
I know that combustion engines get their torque from piston firings, and at low rpms, there simply isn't enough firings to get any torque. Why are electric motors able to have torque at very low rpms?
|
[
"Not all electric motors have maximum torque at low RPM's. Some AC induction motors don't, for example. But let's assume we are talking about a permanent magnet DC motor. ",
"In a DC motor, when the armature rotates it causes a back-EMF (coils in the armature rotating in the magnetic field of the field magnets causes the back-EMF). Essentially this just means that a voltage is generated that is opposite in polarity to the supply voltage. This voltage is proportional to speed, so at high speeds the voltage is big and at low speeds, the voltage is small. ",
"What this means is that when you are at your highest speed your voltage from your back-EMF is going to be almost as big as your supply voltage. Because the 2 voltages are opposite in sign this means the effective voltage across the coils in the motor's armature is going to be the difference between the two and that means it will be small. And because the effective voltage is small, this means that the current in the armature is going to be small (I = V/R). And because torque in a DC motor is directly proportional to current in the armature, the torque is also going to be small. ",
"Now, as the speed decreases, the voltage due to the back-EMF is going to decrease and the effective voltage across the armature coil is going to increase. This means the current in the armature will increase and the torque will increase. Therefore, the lower the speed the higher the torque."
] |
[
"The torque that an internal combustion engine produces at a given RPM is the result of many, many variables. First, let us not forget ",
" torque is. Torque is the ",
" force acting to rotate the crankshaft/flywheel (or wheels, when measured on a chassis dyno). So the highest torque is going to be essentially when the most force is pushing one of the pistons downwards in the cylinder after the fuel in the cylinder is ignited and expanding. ",
"Now, the first, and most obvious thought, is that this is going to be when the most fuel and air are in the combustion chamber. Bigger boom, more force, right? Well, it's a big part of it, but it's not the whole story. As you probably know, getting air into an engine is a pretty tough thing to do. As the RPMs go up, the harder it is to draw sufficient air into the engine. Things like camshafts, intake ports, intake manifolds, airbox designs, and even aerodynamics all play a role in exactly what RPM the engine takes air in most efficiently. ",
"The other big part is the actual rotating mass of the engine's internals. As you get that heavy crankshaft, flywheel, valvetrain, and drivetrain moving, they're going to want to keep moving until friction slows them down. Think of when you spin something that's free to rotate. If you spin it faster, it's going to take longer to come to a stop. Since the engine's rotation isn't being slowed as much, each little bit of fuel burned will find better use in actually speeding up the movement of the engine (which will manifest itself as a higher reading of torque), rather than overcoming the friction, since the rotating mass of the engine does a lot more of that work. ",
"There's more to it, surely. But when these two things align the best, that's where you'll produce the most torque. It doesn't always have to be at a very high RPM. Diesel engines produce a ton of torque at fairly low RPMs. Especially large diesel engines. ",
"Turbocharged and supercharged cars are a bit different, as those systems themselves add a third dynamic in both the airflow and the parasitic loss of the engine. ",
"Note: I'm far from an expert here, but I am a huge gearhead and things like this have always interested me, so I'm basing this on knowledge I've gained over the years. I may not be using 100% correct terminology but this should give you a basic idea of the concept of how the torque characteristics of an engine can change. ",
"Edited: Said \"instantaneous torque\" instead of \"instantaneous force\" in the second line"
] |
[
"Also, steam engines generate maximum torque at zero RPM. "
] |
[
"Is the super-massive black hole at the center of the Milky Way spinning in the plane of the rest of our galaxy? Would this just be a coincidence, or does one or the other have ability to influence the other into matching it?"
] |
[
false
] |
In all the animations I've seen of the SMBH at the center of the Milky Way, it's axis of rotation always seems to be the same as that of the surrounding galaxy. Is that just an assumption we've made, or is there some evidence to back that up? And if it does hold true, then how did that happen? Is the SMBH really massive enough to gravitationally pull the rest of the galaxy to rotate around it? Or as it migrated (I'm assuming that's what happened?) to the center, did the galaxy impart the rotational direction?
|
[
"A black hole's spin is determined by the matter which has fallen into it (and by its initial spin, for example when a stellar core collapses to form a BH, it has some spin). Since most of the matter which has fallen in came from the plane of the galaxy, we tend to assume that the SMBH's axis of spin is roughly aligned with the axis of spin of the Milky Way. The SMBH's spin does ",
", however, determine the spin of the rest of the galaxy."
] |
[
"Is the SMBH really massive enough to gravitationally pull the rest of the galaxy to rotate around it? Or as it migrated (I'm assuming that's what happened?) to the center, did the galaxy impart the rotational direction?",
"The BH is part of the galaxy, it appeared and grew as a part of normal processes that exist in most galaxies.",
"The galaxy as a whole has a spin. When galactic matter falls into the BH, spin is conserved. The BH grows by feeding on galactic stuff, in the center of the galaxy, more or less. Ergo, the spin of the BH should follow the direction of the overall galactic spin pretty closely."
] |
[
"It's likely that our black hole spins on the plane of our galaxy disk as most other galaxies do. Evidence is in visible polar jets coming from the center of some galaxies.",
"Read about polar jets here:",
"http://en.wikipedia.org/wiki/Polar_jet"
] |
[
"In a gridded ion engine, what mechanism pulls the positive ions through the plasma sheath at the rear of the engine?"
] |
[
false
] |
[deleted]
|
[
"I think you may have a bad representation on how the electrical potential looks like in an ion thruster. The whole plasma inside the chamber is at high potential with respect to the plasma outside the thruster. The screen grid (the first one) is usually either left floating (and self polarize at the floating potential of chamber plasma) or, if it's a DC plasma generator, tied to the cathode potential. If it's cathode tied it is a bit lower potential than the plasma and thus better screen the electrons while not accelerating the ions toward the screen grid structure too much. ",
"Here is a a figure",
" showing what the potential looks like [1]. The ions go through the grid like in a traditional sheath with a Bohm current at the sheath border. The whole electrical setup ",
"looks like this",
" [2].",
"[1] S. Mazouffre, “Electric propulsion for satellites and spacecraft: established technologies and novel approaches,” Plasma Sources Sci. Technol., vol. 25, no. 3, p. 33002, Jun. 2016.",
"[2] D. M. Goebel and I. Katz, Fundamentals of Electric Propulsion. Hoboken, NJ, USA: John Wiley & Sons, Inc., 2008."
] |
[
"Thanks a lot. This really clears up a lot. So based on the circuit diagram, the screen grid pulls the plasma to a lower potential then once through, the ions then are accelerated by a very low potential on the accelerating grid. Then once through that, the higher potential on the decelerating grid acts upon the ions. What is the purpose of the deceleration? Thanks again. "
] |
[
"The screen grid doesn't really need to be at a lower potential. The ions can just diffuse naturally through it. Its real purpose is to control the sheath at the acceleration grid and help with ion optics. In theory you could make a thruster with only one grid.",
"The deceleration grid is optional. It helps screen the electrons in the outside plasma so that they are not accelerated back into the thruster and cause havoc there. "
] |
[
"If you had to pick the 3 most important vitamins, minerals etc. that one should take on a daily basis, which would they be?"
] |
[
true
] |
[deleted]
|
[
"It might be worth pointing out that, ",
", all vitamins are necessary, though as you say present in a balanced diet. So the correct answer might be \"all of them but none at all.\""
] |
[
"Not a damn one. Eat how you're supposed to eat, that's the answer. Even if you ",
" eat how you're supposed to eat you ",
" don't need any."
] |
[
"Nutrition dilettante here,",
"Three micronutrients that are 1) cheap, 2) lacking in the modern diet/lifestyle, 3) likely to be slightly benificial, 4) have a very low risk of problems/accidental overdose:",
"Magnesium. According to Wikipedia, \"Human magnesium deficiency (including conditions that show few overt symptoms) is relatively common, with only 32% of the United States meeting the RDA-DR\". Dietary sources are various vegetables and hard water.",
"Vitamin K2. Found in fermented foods and organ meats, both of which are rarely eaten these days. It's not stored in the body, and high intakes have multiple beneficial effects in clinical trials and observational studies.",
"Vitamin D from ",
" in healthy people. Do not get enough to cause burns. I'm personally skeptical of dietary Vitamin D supplements, but that's definitely a minority opinion. Not sure if it counts as a vitamin though, it's more like a (pro)hormone."
] |
[
"How come our Wisdom teeth grow against the other ones?"
] |
[
false
] | null |
[
"tl;dr Our snouts shortened faster than our teeth were reduced during our evolution, so they ran out of room.",
"If you look at the heads of other great apes in profile, including the oldest ones closer to us than to chimps and bonobos, you'll notice that their mouths stick out much farther than ours and are still jam-packed with teeth. Since our mouths got shorter and shorter, we needed to reduce our teeth somehow to make them all fit in there nice and neatly. There are two ways vertebrates go about doing that when there's already no space between the teeth: reduce the number of teeth, or reduce the size of the teeth.",
"Humans have the same number of teeth as other apes, so that's not what our lineage did. Our species does have some teeth—incisors and canines—that are noticeably smaller from just looking at pictures, no measurements needed, but it's just not quite enough compared to how much our mouths shrank in length.",
"That all adds up to mean that when wisdom teeth grow in, for most people they're trying to push themselves into the spot where the molar ahead of them is already at least partially occupying, so they end up pushing against it as they grow and erupt (a.k.a., becoming impacted)."
] |
[
"That goes back even deeper. ",
"This paper",
" says that a sequence of two sets of teeth erupting from the front of the mouth to the back first shows up in advanced cynodonts, but before crown mammals. They don't make it clear whether the front-to-back thing starts earlier, though...just that earlier species had continual replacement, but not whether completely new teeth (not replacements) were added, or where in the jaw they erupted if they were. But what does happen at that point is that everything in front of the molars gets both a baby tooth and an adult tooth, while each molars only erupts once throughout the cynodont's life. I assume (but don't know of any studies demonstrating it; just my assumption) the back teeth erupt later simply because the snout grows in such as way that that's where room is made for new teeth as it lengthens.",
"The paper postulates that the decreased number of tooth replacements is related to the onset of determinate growth (the animal stops getting bigger at some point in its adult life, rather than continuing to grow until it dies). Teeth got replaced in the first place partially to replace ones that were too damaged, but moreso to keep the size of the teeth appropriate for the size of the mouth (and the food being put in it). When teeth erupt, they stop growing. Enamel doesn't keep growing like bone does. New layers only get added while the surface is surrounded by flesh. So if you want a bigger tooth to fit in your bigger mouth you ",
" to replace the old one. Since mammals stop growing early in their lives, there's no need to constantly replace those teeth. And enamel's a costly thing to produce, so a mutation that shuts down the replacement sequence is more favorable than a wasteful one that keeps replacing teeth that don't need replacing."
] |
[
"How come wisdom teeth usually start growing much later than normal teeth?"
] |
[
"Archeologist, and those who study Native Americans: Do we know of any large areas of North America that were never explored by native Americans?"
] |
[
false
] | null |
[
"While there are certainly areas where no archaeological remains have been found, it's nearly impossible to know if no native ever explored an area. If all they left behind is footprints and all they took with them were stories that have been lost in time...",
"One might think the best bet for unexplored terrain would be the extreme far north, such as Ellesmere island. However, there is evidence that this area was inhabited both by the Dorset (who were probably related to other native american populations but are now extinct) and the Thule (present day inhabitants who came from Asia in the middle ages). There were also likely some viking explorers in the area during the period when Greenland was settled and vikings traded with the Dorset and Thule. ",
"There are areas in Northern B.C. that remain remote and difficult to travel to today, even with modern technology. For example, it is estimated that more people have walked on the moon than have been through some parts of the stikine river valley, which has some of the most difficult rapids in the world inside of a deep canyon. It is unlikely that natives could have successfully explored such areas as they wouldn't have had durable modern kayaks or, likely, the deathwish necessary to paddle into that canyon. However, it's not impossible. ",
"Perhaps the only sure bet for unexplored terrain is any area recently uncovered by retreating glaciers. If a place was under a kilometer or two of ice until the last few decades, it was not accessible to native americans during prehistory.",
"As remote as some places are today, it is important to remember that 99% of Archaeology is done no more than a couple hours drive from the nearest pub. It's not that archaeologists are lazy, it's just that it's incredibly difficult and expensive to survey remote areas and more difficult still to dig there. There have been significant finds in the last few years thanks to the availability of satellite imagery of remote areas. However, such imagery, no matter how well massaged by algorithms, can only reveal relatively large features, such as buildings. With only a few exceptions (e.g. the mound builders of Cahokia), large structures were not built by natives in North America. There are, no doubt, many archaeological sites we have yet to discover, especially in remote areas where humans seldom tread even today. "
] |
[
"The basic motivations were probably the same. You go where you can make a living, but curiosity and ambition probably paved the way for more practically motivated folk. ",
"The Thule are descended from northern coastal hunter-gatherers from what is now Siberia, who migrated towards Alaska along the Bering straight around 1000 AD. They were a coastal culture adapted to fishing and hunting whales, seals, etc.. They spread across the Canadian North, driving the Dorset out as they went. They succeeded because they were already adapted to the conditions and were more aggressive than the Dorset. Inuit oral traditions do have some accounts of people thought to be the Dorset, and there is the suggestion from these accounts that the Dorset were neither friendly nor successful in offering resistance. ",
"The vikings, Dorset, and Thule were probably all attracted to the far North by the milder conditions during the ",
"Medieval Warm Period",
". Only in the last few decades has the climate in the far North begun to approach what it was then, which is probably why today's Northern most settlements (e.g. Alert) are military/research outposts. Although the Thule did not possess the ability to mine or refine metal, they were adapted to use metal obtained via trade in Asia. They did use meteorites they found in Canada, but some have speculated that one reason why they expanded so rapidly Eastwards across Canada was to find new trading partners to supply them with metals. ",
"The Norse were in the far North for trade with the \"skraelings\" and also to farm. Narwhal tusks could fetch princely sums of money if sold as unicorn horns, and farm land was always at a premium for Europeans. Once the medieval warm period ended farms in Greenland became less productive and failed. A contributing factor may have been Thule raids, although the evidence for this is not very concrete. The Thule were certainly more aggressive and better equipped for conflict than the Dorset were. For example, the Thule used bows while the Dorset did not. ",
"In short, the far North was somewhat more hospitable a thousand years ago than it is today, but the peoples who lived there did so because they came from cultures already well adapted to the climate. After the medieval warm period ended the far North was largely abandoned until the last century or so. "
] |
[
"What was the motivation for people to go so far as to Ellesmere Island?",
"Today people do so for recoverable resources, for pure exploration, etc.",
"But I would assume more ancient people wouldn't have those same motivations, and it would be more of a matter of attempting to settle in a cold wasteland?"
] |
[
"Can I catch a disease from myself?"
] |
[
false
] |
For example, say I eat my own feces. Will that cause problems? Whatever nasty thing I ate has already been inside me, I'm hardly introducing anything new. What if I got some other body fluid from one part of me and brought it into contact with a different part, is there any way that can cause disease? Basically, I think what I'm asking (but I'm not sure) is: are some otherwise harmful viruses/bacteria/parasites/gremlins that are confined to particular areas in the body and thus harmless, or can they easily travel around wherever they want?
|
[
"Oh that's definitely a bad idea!",
"Broadly speaking we can categorise the body into bits that are inside (across a membrane) or outside (like your GI tract which is essentially just a hole which runs through us). What you're asking is can I extract the bacterial/viral content of your feces by some method and inject it IV and then hurt myself? Definitely!",
"We have a huge amount of gut flora which largely cause us no problems. They are either beneficial (high dose antibiotics have unstable bowels as a common side effect) or cause us no harm. Autoimmunity can present when the body incorrectly decides to attack the gut flora, but that's another story. However, we do make very sure to keep the gut flora in the gut, partially mediated by a thick mucosal layer (also present in the airways). ",
"You could also think of it from another angle - taking cancer cells from one area of you and placing them in another - that's metastasis and technically falls under your definition of \"a disease causing agent translocated\". Certain diseases are also very much more dangerous in some locations than others - see TB meningitis vs TB in the lungs.",
"But largely I see your point - taking bacteria from your blood and injecting them somewhere else probably won't make much of a difference.. "
] |
[
"Don't forget good old Staphylococcus aureus. Harmless on the skin, folds, orifices etc. But a huge problem if it gets into oxygen poor areas."
] |
[
"Everyone's favourite necrotising bacterium. Top marks if you combine it with gangrene."
] |
[
"Is -1 considered a prime number?"
] |
[
false
] |
Alright so I know the definition of a prime numer is a number only divisible by one and itself, and that 1 isn't considered a prime number. However is -1 a prime number, as it is only divisible by 1 and itsel?
|
[
"In a general commutative ring (a set with multiplication and addition that are compatible, e.g., distributivity), an element ",
" is said to be prime if all of the following hold:",
"If we take our ring to be the integers (",
"), then the prime elements are the usual prime numbers and their negatives (i.e., ±2, ±3, ±5, ...). The numbers 1 and -1 are units in ",
", hence are not prime by definition.",
"The reason we don't want to include the units as prime numbers is so we can talk about ",
", which is a commutative ring in which all non-zero, non-unit elements can be written ",
" as a product of primes. If we allowed units to be primes, then we couldn't have unique factorization because if ",
" is a unit, then ",
" and ",
" are two distinct factorizations of the same element.",
"u/functor7",
" can add more details, but that's the gist."
] |
[
"-1 is not prime in the traditional sense because the definition of prime numbers is restricted to natural numbers."
] |
[
"There are three answers to your question:",
"By the usual definition, primes are defined on the integers greater than 1.",
"We could define primality for negative integers. ",
"-n",
" divides ",
"m",
" whenever ",
"n",
" does, because indeed -1 divides 1 (and is called a ",
"). Two numbers ",
"a",
" and ",
"b",
", for which ",
"a = b × Unit",
", are called ",
". So ",
"n",
" and ",
"-n",
" are ",
". We can go on and show that whenever one of ",
"n",
" and ",
"-n",
" is prime, so is its associate.",
"Defining primality for negative numbers does not give us any new insights into the primes.",
"In ring theory, however, ",
"prime ideals",
" generalise this notion; ",
"-n",
" and ",
"n",
" are represented by the same ",
"."
] |
[
"What stops us from being able to see SUSY particles if we are able to see standard particles just fine?"
] |
[
false
] |
As far as my understanding goes we are completely able to see standard particles. What is stopping us from seeing SUSY particles (being that they do exist?)? Is it an equation we have yet to discover? Or am I just completely misunderstanding Supersymmetry?
|
[
"SUSY, if it exists in nature, is what is known as a broken symmetry. Exact SUSY would, as you probably know, give exact copies of all Standard Model (SM) particles but with spin differing by one half. We don't see this though, which means, if it exists at all, SUSY must be broken by some effect. The result is that the SUSY partners must be much heavier than the SM equivalent - current constraints mean that they must be ~1TeV or heavier. ",
"A caveat of this is that we don't expect them to be much heavier than say ~10TeV, because at higher masses the symmetry is less appealing. This is because a big draw of SUSY is that it fixes the hierarchy problem - this basically refers to a problem with the mass of the Higgs boson and it's higher order corrections. For slightly broken SUSY, the superpartners more or less fix the problem with the Higgs mass. However if the SUSY partners are really really heavy then the problem isn't fixed at all... this would make SUSY less appealing. That said, if we saw a stupidly heavy SUSY particle at any energy it would be interesting, it just means there is less motivation to believe they exist. ",
"tl;dr - SUSY particles must be heavy, which means they don't interact as much with the SM particles, so we can't detect them easily."
] |
[
"They are (if they exist), apart from maybe one type, unstable so they aren't just there to be found. Therefore you have to produce them and because they are heavy that is hard."
] |
[
"As far as my understanding goes we are completely able to see standard particles.",
"It's not as simple as you make it sound, or else it would not have taken about 50 years from when its existence was first posited to discover the Higgs boson. In light of that, what right have we to believe it will be easy to discover superparticles? ",
"Furthermore, there's still a plenty good chance they just don't exist. "
] |
[
"How come the moons of other planets in our solar system are all named but we just call our moon \"the moon\"?"
] |
[
false
] |
as the title states. always wondered this.
|
[
"I believe the moon is also known as \"Luna\"."
] |
[
"This is a question suitable for ",
"/r/askhistorians",
" rather than ",
"/r/askscience",
"."
] |
[
"so what you're saying is that we shouldn't call anything \"a moon\", we should call them \"natural satellites\" and that the only \"moon\" is \"The Moon\"."
] |
[
"Surface area to speed ratio for O'Neill Cylinder to replicate earth's gravity"
] |
[
false
] | null |
[
"There is no demonstrable difference between the gravity due to a massive body and the force felt by an accelerating body, this is known as the ",
"equivalence principle",
" and is actually a rather deep point that serves, in a sense, as the starting point to the general theory of relativity. (This is actually not true in the sense that a resident on an O'Neill cylinder would notice the Coriolis force, but in a very general sense, we cannot distinguish between the force felt on a body due to gravity, and the force felt by an accelerating body.)",
"As for the surface area to rotation speed...well, centripetal acceleration is actually a function of radius and angular velocity (rotation speed). Using the period of revolution rather than angular velocity, since I feel this leads to more easily visualisable results, the formula is a = 4pi",
" r/T",
" Since we require 'a' to be approximately 9.8m/s",
" that gives 4r = T",
" ",
"(That's easily verifiable by the way, just take a metre long or so piece of string and tie something on to the end of it, and swing it vertically, you'll see that if it's going around roughly once every two seconds, then that's the minimum speed needed to keep it taut all the way through the rotation).",
"So if we have a O'Neill of radius 2.5km, then 4R = 10,000m so it would need to rotate once every 100 seconds to maintain earth gravity. "
] |
[
"Thanks, Antic_hay. That seems pretty fast to me. I believe the guy in the video made the assertion that the speed of the cylinder wouldn't have to be very high. ",
"Is this then inaccurate? Is there evidence there is any nation with a program even a few decades from such a station? What other obstacles are to be figured out aside from the obvious construction?"
] |
[
"It seemed pretty fast to me too, and I wrote it at 5 o clock in the morning, so bear with me as I recheck my figures :p"
] |
[
"When will most of the visible universe disappear from our sight?"
] |
[
false
] |
When will the known universe disappear from our sight due to the expansion it is experiencing due to these dark energy? I know the Milky Way will still be visible
|
[
"All objects within the Local Supercluster will coalesce on a timescale of about a trillion years, so those will always be visible. According to the timeline ",
"here",
", it will take about 2 trillion years for the last non-Local Supercluster objects to redshift to the point that we can no longer detect them even in principle due to the physics of photon interaction. (More distant objects will \"disappear from view\" in this way more quickly.)"
] |
[
"This is actually not true. Anything with a redshift greater than 1.5 or so is receding faster than the speed of light. We have observed objects with a redshift of ~8. Even professionals get this confused (see Appendix of my reference).",
"http://arxiv.org/abs/astro-ph/0310808"
] |
[
"Thanks so much to both of you, you each get an upvote "
] |
[
"What is the difference between cosmological inflation and expansion of the universe?"
] |
[
false
] |
I always thought they were the same thing until I watched a documentary today which seems to suggest they are different, but doesn't give a good definition of either.
|
[
"There is the standard expansion of the universe from GR, but then there's some \"bonus\" superfast expansion early on in the universe called \"inflation\"."
] |
[
"They're the same thing and they're different. Both involve the scale factor of the metric changing over time. But the mechanism of inflation is presumed to be different from the mechanism of expansion, because none of our models for expansion suggest that inflation should occur. Which simply means we need to add more terms to our models. Figuring out what those terms should be is a bit of a trick."
] |
[
"The main differences are twofold: first, the causes of these two phenomena are separate, and second, the effects on the universe we observe today are different. ",
"Inflation is hypothesized to have been driven by a so-called primordial inflationary field. In other words, a field that existed in the earliest stages of the universe is thought to have cause rapid (an understatement) expansion, bringing the universe to many times its original size in a matter of mere moments. Today however, the field that drove inflationary expansion in the early universe is no longer detectable, having settled into a nearly-zero vacuum state. Thus the inflationary model is one focused on the history of the universe and has less impact on its evolution from this point forward.",
"The expansion of the universe we observe today is thought to be due to a similar but different origin. Like inflation, the accelerating expansion of the universe is the result of a strange field (known as dark energy) permeating all of space that causes the geometry to expand uniformly. However while inflation lasted a very short time and occurred extremely rapidly, cosmic expansion is occurring continuously at a rate that doesn't affect our daily lives (if inflation occurred at this very instant, I suspect it would tear apart the atoms from which we are composed). Additionally, as inflation caused the universe to balloon in size, the field driving this motion became weaker and weaker, leading to the end of the inflationary period. With cosmic expansion, the field appears to be constant in time and space, meaning that it will continue to accelerate the expansion constantly and indefinitely. ",
"So hopefully this quick blurb was helpful. It's a very important and interesting question you asked, and I expect it's one that will continue to require new research as more is discovered about these two relatively poorly understood phenomena. "
] |
[
"If the spin of a proton is 1/2, why is it said that the combined spins of its three quarks do not add up to the proton's total spin?"
] |
[
false
] |
I have been trying to teach myself some of the basics of quantum mechanics and am running into the formation of the idea behind spin. I know photons and electrons can have spin, and since quarks are fermions, they carry spin in increments of . Since a proton is made up of three quarks, and the spin of a proton is 1/2, I assumed the three quarks' spins would add up in such a way that two of the quarks spins cancel each other out(1/2-1/2), and the third quark's spin (1/2) explains the measured spin of the proton, which is also 1/2. It has been discovered, however, that the spin of the quarks contributes very little to the spin of the proton (called the proton spin crisis) and that the proton gets the majority of its spin from somewhere else, but I am confused by this because the spin of a proton has been measured to be 1/2? If the net spin of the three quarks is 1/2, doesn't that completely account for the magnitude of spin in the proton? If quarks have spin 1/2 and a proton has a spin of 1/2, why is it said the quarks only contribute 5-25% of the total spin of the proton, which is equal?
|
[
"If the spin of a proton is 1/2, why is it said that the combined spins of its three quarks do not add up to the proton's total spin?",
"Because numerous experiments have attempted to measure the spins of individual quarks inside the proton, and all of those experiments have concluded that most of the proton's spin does not come from the quarks, but that at least some of the spin must come from other sources such as gluons, or orbital angular momentum of the quarks/gluons.",
"While all of the experiments are generally in agreement that the proton's spin cannot be completely explained by the spin of its quarks, those experiments are also in broad disagreement about exactly how much spin can be attributed to each of the possible sources.",
"This state of affairs is known as the ",
"proton spin criss",
", and we don't currently know what the correct resolution is. It is an unsolved problem in physics right now."
] |
[
"We have measured the spin of a proton to be 1/2. We have measured the spins of individual quarks to be 1/2 also. This spin is intrinsic, it can’t be added to or taken away, right?",
"Yes, that's all correct.",
"Are you saying that when a quark is confined within a proton, its spin is significantly less than 1/2?",
"No, actually, that's not what I'm saying. I'll clarify that for you in a moment, but first I want to point out that a proton isn't made up only of its three valence quarks. It's also made up of a great many gluons, which have an intrinsic spin of 1, and the interactions between all the gluons and the quarks also means that non-valence quarks and antiquarks may be created fleetingly and participate in these complex interactions. There is also orbital angular momentum of all the quarks and gluons around the proton's center-of-mass to consider — like spin (intrinsic angular momentum), orbital angular momentum also contributes to the total angular momentum of a bound system. There are also numerous tertiary factors like gluon polarization. So, right from the get-go, it's not nearly as simple as \"add up the three valence quarks' spins\" even in the most naive picture.",
"Experiments to determine how a proton's spin is distributed among its constituent particles can only be done on a statistical basis: you prepare a bunch of protons in the same state — with the same spin direction — and then you shoot other particles such as electrons or muons at the proton targets; these high-energy particles interact with random parts of the proton, and as I understand it depending on the details of the result you can determine whether an interaction was with a quark or a gluon, and work backwards from the result to determine details of the interaction such as what the quark or gluon's spin must have been.",
"If the proton's spins are all aligned in the same direction from the start, and naively you expect only the spins of the valence quarks to be significant, then statistically if you look at the distribution of valence quark spins one should expect to find that 1/3 of the quark spins were anti-aligned with the proton's spin, while 2/3 of the quark spins were aligned with it. However, this is not what the experiments established — the experimental results indicate that the distribution of quark spin directions is nearly equal.",
"It gets messier too, when you consider other experiments that have also attempted to measure the distribution of gluons' spins, and to narrow down how much the orbital angular momentum of all constituents contributes to the proton's final spin. Different experiments are not, in general, in agreement about all of these values — the various results are in conflict with each other, and the underlying reason for the conflict is not known.",
"Hope that helps clarify ...",
"TL;DR: It's way, way more complicated than you are thinking. That is to say, things are not nearly as simple as 1/2 = 1/2."
] |
[
"Just a follow up question, when you talk about a proton-bounded quark's orbital angular momentum, is this analogous to the orbital angular momentum of electrons bounded to the nucleus of an atom (and tells you if it's in the s, p, d, or f orbitals)?",
"Yep, you got it. Same basic concept, though I'm sure many of the details are different."
] |
[
"Why do we instinctively clutch a wounded area?"
] |
[
false
] |
i.e. if we are injured in a specific spot, right after the injury we'll "hold" it.
|
[
"it's called endogenous analgesia, basically you rub over the spot it hurts and helps to stimulate more 'touch receptors' and decrease the stimulation of the receptors of where the injury occurred. That way you don't feel the pain as much.\n Why your mommy would rub your head after you hurt it and it would feel better."
] |
[
"Touching our own wounds helps our bodies deal with pain as it allows our brains to better \"map out\" the part of our body the pain is coming from. The more information the brain gets about the actual location, the better it can process and effectively deal with the pain. So actually, grabbing the afflicted area with two hands is the best option if you want it to feel better faster. ",
"Source",
"Note: I took this from a Cracked article ",
"here",
" if you want to have a look at it. "
] |
[
"dailymail has time and time again proven to be an extremely unreliable source."
] |
[
"Can someone explain string theory and p branes?"
] |
[
false
] |
The subject came up earlier, and I know nothing about either, although I know a little bit about quantum physics, and I'm curious.
|
[
"Disclaimer: I cannot give a super-detailed explanation. My understanding probably falls above \"read a Brian Greene book\" but below \"string theory PhD student\" and well below \"Ed Witten.\"",
"String theory, as you may or may not know, is an attempt to both create a theory of quantum gravity and to unite all four physical forces into a single framework. On a basic level, it does this by imagining tiny pieces of string, either with endpoints or as a loop, and we demand that these strings obey special relativity and quantum mechanics. When we quantize the string, we find that they have vibrational modes which act like elementary particles.",
"One of the most well-known features of string theory is the fact that it requires extra dimensions. The reason for this is anomaly cancellation. An anomaly of a theory is some symmetry of the classical version of the theory that doesn't survive quantization. For gauge theories, these lead to fatal inconsistencies. It can be shown that string theory uniquely fixes necessary number of dimensions in which anomalies cancel out: 10 for bosonic string theory, and 26 for superstring theory. But we only see four dimensions in nature. This leads us to conclude that the extra dimensions are somehow hidden, and the standard way to do this is by compactification. The extra dimensions are curled up to a very small length. We can imagine this like a cylinder with a very small width, but really the curled up dimensions are generally chosen to belong to a very specific and complicated class of geometries: Calabi-Yau manifolds (complicated to explain, anyway, they have very nice properties in actuality which is why we use them). Zwiebach's string theory text has a very nice example of how small, curled-up extra dimensions can hide new physics at high energies, which is accessible to an undergraduate who has taken quantum mechanics. ",
"p-branes are like a generalization of strings to other dimensions, i.e. a string is a 1-brane, a 2-brane is like a sheet, a 3-brane takes up a volume, etc. An important class of these objects is D-branes. When we have open strings, we'd like to give boundary conditions that their endpoints must satisfy. A Neumann boundary condition means that endpoints are free to move around in space, whereas Dirichlet boundary conditions are requiring endpoints to move in some restricted space (e.g. tying one end of the string to a wall). D-branes are the things that these endpoints are attached to, but D-branes are also dynamical objects in their own right. I admit my understanding of D-branes is a bit lacking at this point, but it's my understanding that they allow us to incorporate gauge theories into string theory, as gauge theories naturally live on the volumes that D-branes sweep out as they travel through spacetime.",
"String theory has generated so much interest because it has some very attractive properties:",
"-When we quantize the closed string, we find that they have a massless spin-2 excitation. This acts like general relativity, and so gravity just kind of appears as a natural consequence of the theory. ",
"-String theory is UV finite, meaning it does not run into the sort of intractable infinities that attempting to incorporate gravity into QFT causes.",
"-Contrary to some popular claims, string theory is very restrictive. A big unsolved problem with string theory is that it has a \"landscape\" of something like 10",
" possible vacua, and we don't know how to pick one to get a physically reasonable theory. However, quantum field theory has (very) infinitely many possible models, and it's only hard work and ingenuity that allowed us to construct a physically reasonable one. String theory gives us a (large, but) finite number of possible models, and their low-energy descriptions happen to be of a very restricted class: anomaly-free Einstein-Yang-Mills-Dirac theories. Since this pins them down as having several features at least of the Standard Model, this is quite nice.",
"-While QFT has quite a few free parameters which must be tested experimentally and put into the theory by hand to make predictions, string theory has one free parameter: the string length (or, equivalently I think, the string tension).",
"For anyone who understands better than me, I appreciate any corrections, additions, or clarifications. If you'd like explanations of further topics, like why we string theory needs extra dimensions, I can add more when I have time."
] |
[
"Yes.. anything you can add would be great. I love your high-level overview/description of string theory. If you can discuss why we need extra dimensions and also how forces like gravity work in string theory, for example, that would be great."
] |
[
"I added an extra paragraph on extra dimensions. That's all I have time for for now!"
] |
[
"Why are most animals organ systems so similar?"
] |
[
false
] |
Why are there not dozens or hundreds of completely different types of organ systems? Thanks
|
[
"Because we arose from common ancestors."
] |
[
"Descent from common ancestors is the most straightforward explanation. ",
"However, some organ systems seen in various types of animals (such as eyes in vertebrates, insects and molluscs) do not share common descent (i.e., the common ancestor of those three groups did not possess eyes) but the advantages of light sensing organs was sufficient for eye development in all three.",
"Eyes are believed to have developed independently 7 times in various animal lineages, indicating that some sort of advanced photoreception was rather beneficial in these lineages."
] |
[
"For the same reason you look like your biological parents."
] |
[
"Is it possible for Jupiter to get enough mass through asteroid impacts to ignite fusion? How much more mass would it need? Is there any evidence of binary systems forming this way?"
] |
[
false
] |
[deleted]
|
[
"Jupiter has a mass of ~ 1.9 x 10",
" kg, while the total asteroid belt's mass has an upper bound of 3.2 x 10",
" kg. Jupiter is literally around one or two million times heavier and wouldn't care a bit if the whole asteroid belt fell into it.",
"Jupiter is not even close to having the necessary mass to start fusion. If I recall correctly it's off by at least a couple of orders of magnitude."
] |
[
"It's a common false claim based on imperfect nuclear physics in the 80s. The modern calculation predicts ~80 Jupiter masses to start fusing."
] |
[
"The modern calculation predicts ~80 Jupiter masses to start fusing.",
"Well, to be clear, at 80 Jupiter-masses is where starts fusing regular old hydrogen. ",
"At just 13 Jupiter-masses, though, it can start fusing deuterium (heavy hydrogen), which is what defines the boundary between giant planets and brown dwarfs. There's still nowhere near enough material orbiting the Sun to reach that limit, either."
] |
[
"How do we know velociraptors hunted in packs and killed large prey?"
] |
[
false
] |
In response to this comment: they had 5 inch claws on their hind legs with which to tear you apart. Emphasis is mine. In that thread, some suggested that teeth/claw marks could indicate that; however, couldn't this also be due to velociraptors acting as scavengers like hyenas or vultures? Please include sources if possible. Edit: In regards to the fighting dinosaur fossil: How come the fossil is considered to be lifelike poses if the pair was possibly later scavenged? Burial must have been extremely fast, judging from the lifelike poses in which the animals were preserved. Both forelimbs and one hindlimb of the Protoceratops are missing, which has been seen as evidence of scavenging by other animals.[24] It seems odd to me that the creatures would have been buried very quickly, and yet scavenged later without disturbing their natural positions.
|
[
"It's all just hypothesis and conjecture. We only have limited evidence to the behaviors of dinosaurs, for instance certain dinosaurs may have been protective mothers as fossils have been found of both the parent and nest in close proximity.",
"We use modern day animals and compare them to these prehistoric ones. We can view trends in certain animal types and try to apply those to ones from the past. For instance you can take a survey of all the small carnivores which are alive today, you might see a trend that many of them are pack hunting. Assuming these behaviors weren't unfavorable back then, it isn't a completely bad assumption to try to label some of the smaller carnivorous dinosaurs as potential pack hunters.",
"However if you try the same trick from an evolutionary perspective, you do not find all that many instances of pack hunting among birds (though there are some, such as the Harris Hawk), but you do see community structures and flocking tendencies, in fact birds often display extremely complex organizational structures.",
"Birds evolved directly from the same branch that includes all the small therapods such as velociraptors. ",
"The Paraves",
".",
"In short, we don't, but we can make educated guesses based on the behaviors of descendants and characteristically similar non-relatives.",
"If you had a gun to my head, I'd call raptors and other medium to small therapods to operate in small family tribes consisting of only a few members, with strong parental behavior and would display opportunistic hunting and scavenging tendancies."
] |
[
"Yeah, I'm not a paleontologist. Years of feeding the 5 year old inside me with dinosaur books, have led to this. :P",
"The wiki provides all the information I know about it. It's inconclusive. Probably the only thing you could get people to agree on is that the raptor was buried almost instantly. The missing limbs seem to indicate the other animal was already dead. I haven't read into it."
] |
[
"Ah, thanks for the response. By your last statement alone, it seems you have some previous knowledge. You wouldn't happen to be a paleontologist, would you?",
"Also, if you could I would appreciate any further explanation about the fighting dinosaur fossils I would appreciate it.",
"Edit: Ah, I suppose you are most likely a chemical engineer."
] |
[
"How do you date rocks? Shouldn't every rock on Earth be 4.6 billion years old?"
] |
[
false
] | null |
[
"When we date a rock, we are typically actually dating a particular mineral in that rock. What the age of that mineral means depends on the mineral, the rock that hosts that mineral, and the way we are dating that mineral. ",
"Let's consider some examples. One of the most common minerals we use to date rocks are ",
"zircons",
" and we often use the ",
"U-Pb system",
" to date these zircons. Now, what we are doing is measuring the amount of specific isotopes of uranium, which are radioactive and decay at a specific rate and are incorporated into the crystal structure of zircon in small amounts, and the amount of the specific isotopes of lead, which are the decay products of the uranium and accumulate in the zircon AND are not otherwise incorporated into zricon when it forms, to determine how long lead has been accumulating in the zircon. For zircon and the U-Pb system, this is effectively dating when that zircon crystallized from a melt (i.e magma/lava). Before that, uranium was decaying into lead in the magma, but there was no zircon within which to accumulate that lead and the temperature at which U-Pb becomes a closed system (i.e. when lead is retained in zircon without any leaking out) is the same as the crystallization temperature of zircon. This is not true for all decay systems. In the process of decaying to Pb, U also decays to He (as does Th and Sm) but zircon 'leaks' He until it cools below a relatively low temperature of around 180 degrees C, so if you date a zircon via ",
"the U-Th/He method",
" then you're not dating the crystallization of the zircon, but rather you're dating when that zircon cooled to below ~180 degrees C. ",
"Now, returning to U-Pb in zircon, if we get an age of a particular zircon, this means that the age we have reflects the age that this particular zircon crystallized from a magma. If we took that zircon from an igneous rock (i.e. a rock that crystallized from a magma/lava) , the age of the zircon reflects the age of crystallization of that rock. If instead we took that zircon from a sedimentary rock (i.e. a rock formed from the deposition and lithification of sediment) the age of that zircon (1) implies that there was at some point an igneous rock of that age that was turned into sediment, (2) reflects a maximum age for the sedimentary rock (i.e. if a sedimentary rock contains a 10 million year old zircon, it can't be any older than 10 million years), but (3) the sedimentary rock could be much younger than the zircon it contains because zircon is a pretty robust mineral (i.e. it survives weathering and transport). This is actually the context for the oldest terrestrial material ever dated, i.e. ",
"4.4 billion year old zircons from within a sedimentary rock",
"."
] |
[
"So the age of rock is simply when it solidifies into a rock, from magma, on the Earth's surface?",
"What about rock left behind from asteroids? Can it be differentiated from normal 'crust' (is that a term?) rock?"
] |
[
"For igneous rocks, yes. ",
"For your question on asteroids, you'll have to be more specific. If you're talking about a literal piece of stone from space, then yes we can differentiate those. Most of the time they're hard to tell from just looking at them. Some of them will be magnetic (",
"iron meteorites",
") but others won't be (",
"chondrites",
"). The best way to know for sure is to get a rock saw and slice through them and do some chemistry. Asteroids are fun. ",
"There are also rocks that form from melts created by asteroid impacts. That is, rock that forms from lava created by the force of the impact itself. Asteroids literally melt a portion of the Earth when they strike. These rocks are called ",
"tektites",
" and they can also be distinguished. You can also look at ",
"shocked minerals",
" in impact craters and sometimes you can find little pockets of melt here that can be dated. Sometimes the presence of shocked minerals, breccia, and melt are found in the subsurface and, bingo, ",
"you've found a crater",
"."
] |
[
"Theoretically, if I were to cut, say, two of my fingers and position the wounds the right way, would they be able to heal and get stuck together?"
] |
[
false
] | null |
[
"short answer: yes",
"longer answer ",
"here"
] |
[
"My father shot his middle finger off with a .45 (no, I don't know how) and since it was completely unsalvageable considering parts of it were embedded in the walls and ceiling, they ended up cutting his hand open, cutting the carpals of his index and middle finger, and tranplanting the index finger onto the middle finger's carpal, then pinned it all in place, so that instead of having a big gap in his hand, he now just has a Simpsons hand. I have an xray of it somewhere, let me see if I can find it. ",
"Edit: ",
"Here is xray!"
] |
[
"Yes, in fact, this is how most, if not all major surgery wounds are healed. This technique of bringing the two exposed surfaces of tissues together so that they heal naturally is called ",
". I think you might be interested also in ",
"wound healing",
".",
"What happens is that collagen, a long, fibrous protein in the extracellular matrix, gets laid down and then converted from type III collagen (a loose, first-response collagen) to type I collagen (a stronger collagen in scar tissue).",
"Wound healing occurs only in the presence of adequate oxygenation, blood supply, and lack of a foreign body/infection.",
"If you were talking about putting both of the cut fingers together, then they will not heal and get stuck together, because they don't have a blood supply. Instead, they will probably develop necrosis and rot away. Coagulation from the left over blood may cause them to stuck together, but it will be a loose connection based on fibrin, not collagen.",
"If you were talking about cutting the fingers and putting them back into the stumps, then they will heal as long as the blood supply is not disrupted and as long as there is no infection.",
"Edit: I imagined cutting the fingers in a cross-section, but if you were talking about cutting down the length of your finger, then they would be stuck together."
] |
[
"What makes the best drain cleaner? Acid or Base?"
] |
[
false
] |
So I have a nasty clogged drain which I think warrants the big gun drain cleaner. Which one is the best a typical standard pipe? I know most commercial cleaners contain NaOH and aluminum, but I have heard low pH cleaners might be better. Is there any scientific basis to as which may be better?
|
[
"If you've tried some drain cleaner with little to no result your best bet is probably a snake (drain augur). ",
"Any acid-based drain cleaner you can get your hands on at the local home improvement store is likely to be dilute garbage due to the inherent safety issue of concentrated acids."
] |
[
"Maybe some of the scientist type will comment on this concoction. I made a drain cleaner by pouring OxiClean (sodium percarbonate) (Na2CO3•H2O2) down a drain. The particles are heavier than water and I gave them time to settle to the bottom. I then poured Muratic acid(Hydrochloric acid)(HCL) in a container of heated water to end up with a ratio of about 1:10 HCL to H2O. Poured that into drain with the OxiClean and it worked great as a drain cleaner. It did some serious bubbling and cost few pennies."
] |
[
"It would depend on what they're trying to clean, really."
] |
[
"Does the human body spend more energy to keep itself warm in the winter (thermogenesis), or by trying to cool itself in the summer (perspiration)?"
] |
[
false
] |
Provided the human wears adequate clothing for both the hot and cold day, and does the same level of activity
|
[
"This argument was a friendly debate ",
"One shared an article which stated that they made tests on cyclists to exhaustion (on different temperatures), and the fastest ones to exhaustion were those cycling in 30°C",
"The others shot this argument down by saying \"putting activity in the argument complicates it, and physical activity is a whole another argument (maybe the cyclist exhausted first by getting dehydrated)\"",
"Who is in the right here? Did he indeed exhaust himself for another reason other than depleting his energy?",
"Thanks for your comments"
] |
[
"This argument was a friendly debate ",
"One shared an article which stated that they made tests on cyclists to exhaustion (on different temperatures), and the fastest ones to exhaustion were those cycling in 30°C",
"The others shot this argument down by saying \"putting activity in the argument complicates it, and physical activity is a whole another argument (maybe the cyclist exhausted first by getting dehydrated)\"",
"Who is in the right here? Did he indeed exhaust himself for another reason other than depleting his energy?",
"Thanks for your comments"
] |
[
"Perspiration is the main way in which we cool our bodies. The evaporation of sweat is a passive process that does not require energy.",
"Warming our bodies, on the other hand, involves burning stored fat and shivering.",
"Therefore the human body spends more energy to keep itself warm in the winter, rather than cool in the summer."
] |
[
"Do two objects ever actually touch on a molecular level?"
] |
[
false
] |
This is kind of long, and only know enough to recognize I don't understand it at all. Can two objects actually ever 'touch' each other? When you place your hand on a table? From my basic understanding, no atom actually ever touches another atom, therefor the atoms that create [the molecular chemical compound of] the skin of your hand never actually come in contact with the atoms that create [the molecular chemical compound of] the table. Is this a technically accurate statement? Does this mean your skin isn't ACTUALLY anymore solid than water, or any less solid than a table? Under the assumption that question 1 is true, why doesn't your hand 'merge', or 'pass through' with the table? I understand the density issues, but if you held your hand on the table with enough force, for long enough, would you become conjoined to the table? On objects of similar molecular composition and density, why don't they join together? If you pressed your hands together long enough, would they eventually melt together? If they would, what determines the amount of time it takes? If nothing ever actually touches anything else, why does tape stick to an object? Does it have something to do with the glue's ability to 'bond' with other molecules? Thanks for your time! The more technical the response the better <3 P.S. - I swear I'm not high, or on drugs. These concepts have been a reoccurring debate between my sister and I for literally a decade.
|
[
"Everything we experience as \"touch\" is a result of the electromagnetic forces. This force shows itself in several ways: magnets, static electricity... but also less obvious phenomenon like molecular bonding. Every atom is made of a nucleus of protons and neutrons and a large, spherical area of electrons. The electrons don't orbit the nucleus like planets, but instead exist as a kind of cloud that permeates the sphere. They are part of the atom: without the electrons, the nucleus is not happy. But the few outer-most electrons of atoms can be traded and stolen by other atoms, or even shared in the form of molecular bonds. All of this is a consequence of the electromagnetic force. ",
"Yes, objects can touch each other. Just because the atomic nuclei of your molecules aren't touching the nuclei of the table's molecules doesn't mean the atoms aren't touching, because atoms are made of more than just nuclei, they are made of electrons as well! Technically, the nuclei don't touch, but I don't think that translates into the atoms/molecules themselves don't \"touch\". If you want to get really real about it, \"touch\" doesn't really exist at that level, it's just electromagnetic interactions between atoms in your hand and atoms in the table. 2. is also answered by this answer.",
"Your hands are incredibly rough (even if they feel really smooth). Maybe your hands would join together, but it would be because the skin grew together, not really because of a similar chemical makeup. Metal is a different story. If you have two pieces of identical metal (and you have to do this in a vacuum because oxygen in the air forms a thin coat of \"oxidized metal\" on the surface of many metals that acts as a barrier) and you touch/push them together, they ",
" become one piece of metal.",
"Tape and glue stick because of electromagnetic forces! I'm not sure if I can elaborate at the moment, I'll be back. ",
" yep, these things work through covalent, ionic, or Van der Walls forces, all of which arise from the electromagnetic force!"
] |
[
"I think this is more of a physics question than a biology question. ",
"Basically the overly simplified version I recall is imagine pushing together two like poles of a magnet, it requires a fair amount of force. Electrons are going to repel each other the closer they are together so 'touching' doesn't necessarily happen at an atomic level without massive amounts of energy. "
] |
[
"This might be the answer you 're looking for: ",
"Veritasium: Can we actually touch anything?",
"\nActually, this one might be better: ",
"Minutephysics: Touch"
] |
[
"why is it that accidental cheek bites and other misc damage done inside the mouth do not get infected from the massive amounts of bacteria?"
] |
[
false
] | null |
[
"Some of these bacteria are a part of our normal flora when in the mouth but when they get aspirated or enter your lungs can and do cause issues like pneumonia and abscesses. Also, our saliva has natural anti-microbial properties as well. "
] |
[
"Some of these bacteria are a part of our normal flora when in the mouth but when they get aspirated or enter your lungs can and do cause issues like pneumonia and abscesses. Also, our saliva has natural anti-microbial properties as well. "
] |
[
"Saliva and tears contain the enzyme ",
"lysozyme",
", which break the bonds between the N-acetylmuramic acid and N-acetylglucosamine (NAM and NAG) which make up the cell wall structure of many bacteria. ",
"This video",
" has a good demonstration for the overall \"pacman\" like activity of lysozyme as well. "
] |
[
"Suspending a balloon in thin air?"
] |
[
false
] |
[deleted]
|
[
"Sure, it's very easy - deflating helium balloons do it by themselves all the time. All you have to do is attach a weight to the balloon such that the density of the balloon and weight together equals the density of the surrounding air. At sea level, that density is about ",
"1.225 kg/m",
"."
] |
[
"I do this all the time with mylar balloons. I usually use a few pennies then bits of duct tape until I've found neutral buoyancy.",
"If you try this you'll find that it sinks and rises and may even move from room to room as there temperature differentials in most houses."
] |
[
"Yeah, but each time you wan't to fly it, you need to spend an hour dropping pennies into the front and rear compartments, making tiny adjustments, sliding them around, it is a huge pain."
] |
[
"Why doesn't the universe have a 1:1 matter to antimatter ratio?"
] |
[
false
] | null |
[
"This is one of the major open questions in fundamental physics. You might be interested to learn more at the ",
"wikipedia article",
" on this subject."
] |
[
"We don't know."
] |
[
"This is a great question and touches on some of the unsolved problems in today's physics. Many people think: if the big bang was truly an expansion from the universe that observed pair production, then equal amounts of antimatter and matter should have formed; further, these equal amounts should have annhilated to produce a universe that is a sea of photons. In fact, we believe that there existed baryogenesis and leptogenesis asymmetries; somehow, for every 1 billion baryons (like protons and neutrons) and leptons (like electrons, muons) produced, 1 billion antipartices were produced AND 1 extra proton/neutron and electron/muon (giving an asymmetry of 1 to 1 billion). We are still struggling to explain this asymmetry; however, we believe that the answer lies in CP violation, which is currently being studied in the LHCb experiment. CP violation involves the asymmetric decay of neutral kaons and B-mesons into less massive particles. However, we are not yet sure if CP violation is sufficient to account for the baryogenesis/leptogenesis asymmetries. \nLook to:",
"http://en.wikipedia.org/wiki/Baryogenesis"
] |
[
"What does true reality look like?"
] |
[
false
] | null |
[
"What does it mean to expand your perception?"
] |
[
"Well..humans are only experiencing a slither of what's out there as we are constrained by our built in sensory receptors and our brains make sense of the results in a language we can understand...but what we are perceiving is a subjective reality...by hypothetically expanding our perception I entertain the question of what objective reality might be like and what actually lies beyond our spectrum !"
] |
[
"Ok so this should really be posted to a philosophy sub, but here are my quick comments: on the one hand, maybe Kant was right and we cannot help but experience the world through our sensory systems and minds so we can never gain true access to the \"thing-in-itself\". This is a common view in perception / psychology at the moment. See Hoffman's current ideas on \"the interface theory of perception\" which is a modern take on this. On the other hand, if we didn't experience the world as it really is to a certain extent, we would be liable to make all sorts of mistakes and survival would be difficult. In fact, there is plenty of information that we can get about the world directly from information at the level of the retina. This is the direct view of perception advocated by JJ Gibson in the 50s and 60s and has many important insights and merits.",
"Also, I think you mean sliver, not slither."
] |
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