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[
"When someone says \"x is titrated against y\" which is in the burette and which is in the flask?"
] |
[
false
] | null |
[
"The titrand (or analyte) is the stuff in the flask.",
"The titrant is the stuff in the buret."
] |
[
"No, no: The titrand is what is being titrated, and it's in the flask. ",
"X is in the flask. Y is in the buret."
] |
[
"Thanks, I was still confused."
] |
[
"From a quantum mechanics perspective, why is a lower energy state more stable?"
] |
[
false
] |
Hi guys, Thus far my understanding (superficial though) comes from in which one user states that 'systems go to lower energy states when they share their energy with some other system' which in-turn has so many degrees of freedom that 'energy is divided into indiscernible small portions.'
|
[
"Without going into the math - as it doesn't really help drive the intuition here...",
"A higher energy state will always have a non zero probability of spontaneously shedding some energy to drop into a lower energy state (eg. an electron in an excited state can emit a photon). The reverse will only happen if you supply some external energy (the electron will not excite unless it absorbs a photon).",
"Over time, the electron will shed all the energy it possibly can, and find itself in the ground state with no possible way of raising its energy (unless it somehow gets the photons back)."
] |
[
"This is true but the thing is the total state (electron + photon) still conserves the total energy. It’s just that we regard as being a “stable” state of the electron actually isn’t the electron. And we are ignoring the coupling of the atom’s Hamiltonian to the external background field. If you look at this entire combined system there is no “dropping to lower energy”. There is just one energy, but with the probability of photon number increasing becoming more likely due to spontaneous emission."
] |
[
"Sure, but it's possible to answer the question without going into qft. ",
"And it's nice to define your system such that the energy is constant, but if you're talking about the stability of the electron state, then it only really makes sense to talk about the energy of the electron state (although I admit this was an arbitrary example, chosen for the purpose of answering the question). ",
"A completely isolated box of electrons should appear \"stable\" from the outside, even if none of the electrons within the box are in a \"stable\" state.",
"Edit to add: I suppose you could instead talk about the complete system (atom, electrons, photons and all associated fields), then give an answer to the question in terms of entropy - but I just felt that the way I initially described it offered some intuition that OP was after (even if not 100% complete in a QFT picture)"
] |
[
"What are trichomic prokaryotes, exactly?"
] |
[
false
] |
A google search doesn't reveal much at all.
|
[
"Trichomes are hair-like, or filamentous structures. Prokaryotes are single-celled organisms without a cell nucleus, i.e. bacteria. ",
"So trichomic prokaryotes are bacteria with a filamentous morphology."
] |
[
"Would be so much easier to just call them \"Hairy Prokaryotes\" :)"
] |
[
"Prokhairyotes."
] |
[
"Is there any theoretical/empirical support for the idea that creativity is \"noise\" (or chaos) among the otherwise orderly neural impulses in our brain? (X-post from /r/cogsci)"
] |
[
false
] | null |
[
"I haven't read iorg's link and this isn't really my specialty, but I can tell you that ",
"low latent inhibition",
" is correlated with creativity. In other words, individuals who are unable to ignore random sensory input (noise) tend to be more creative, especially when paired with high intelligence.",
"On that note, it's also worth mentioning that there seems to be a link between creativity and schizophrenia/psychosis, which one model describes as a reduction in top-down modulation, one consequence of which is an inability to properly process incoming stimuli, thereby generating things like hallucinations and delusions—or, put crudely, too much neural noise—which also supports that connection.",
"Again, not my specialty, so maybe someone else can chime in, but these were the first things that came to mind."
] |
[
"There are some psychology-based theories that creativity comes from \"freeing up associations\". That is, you create a sort of chaos in the brain, and then when everything cools down, you're left with new links between things that hadn't been linked before. But you have to break from \"normalcy\" in order to create those unusual, and sometimes useful, links."
] |
[
"This might help: ",
"http://www.on-rampis.ca/xyz-strengths/creativity/crea007.pdf"
] |
[
"How dense is the plasma inside fusion reactors?"
] |
[
false
] | null |
[
"Either (1) very not dense, or (2) not very dense. There are two main types of fusion reactors.",
"Magnetic confinement fusion reactors (such as JET and ITER) contain hot plasma (around 10",
" K) with magnetic fields and generate continuous fusion. Typical plasma densities in MCF are about 10",
" g/cm",
", or about a millionth the density of the atmosphere (but temperature is different, so not the same as an atmosphere of pressure).",
"Inertial confinement fusion reactors (such as NIF) take a pellet of mixed deuterium and tritium and implode it from all directions using extremely powerful (~petawatt scale) but brief pulsed lasers and generate fusion in pulses. Peak temperatures are about the same as MCF but densities are much greater at around 10",
" g/cm",
", or about 10",
" times the density of the atmosphere.",
"Source for densities: ",
"[1]",
"EDIT: corrected density comparisons to atmosphere"
] |
[
"The standard density of the atmosphere is 1.2 * 10",
" grams per cm",
" so 10",
" g/cm",
" is actually 100,000 times as dense as the atmosphere."
] |
[
"Ah, evidently I can google but I can't read"
] |
[
"If one were to fuse two hydrogen atoms together, would the resulting release of energy be visible to the naked eye?"
] |
[
false
] | null |
[
"Just the energy from two atoms fusing - no, you would never know it happened.",
"Let's calculate the energy in one hydrogen atom (doing this for the sake of simplicity, the actual energy released would always be less than the energy in two hydrogen atoms).",
"E = mc",
" ",
"= 1.6605402 x10",
" * 299 792 458 m/s ^ 2 ",
"~ 2 * 10",
" * 3 * 10",
" ",
"~ 6 * 10",
" j",
"Let's take a 60W lightbulb:\n60W = 60 j/s = 6 * 10",
" j/ms",
"So you would need the energy in 10",
" or 1 billion hydrogen atoms to power up a lightbulb for 1 millisecond.\nAlso keep in mind that is the entire energy of the hydrogen atoms, which is significantly less than the energy that would be released when fusing that many atoms."
] |
[
"Thank you very much!"
] |
[
"Energy as such is a concept, it cannot be seen.",
"Fusion of two hydrogen atoms releases helium, gamma rays and neutrinos, and energy in the form of kinetic energy. Those particules will likely interact with other particles and emit some photons at some point, which you will likely be able to see if you are in the right position.",
"The sun is a good example of a fusion reaction which you can indirectly \"see the energy\"."
] |
[
"Is it possible to view the moon landing sites through a powerful enough telescope?"
] |
[
false
] |
And if so, what can we make out? Can we see the lander? The buggy? I doubt we could make out the flag, though.
|
[
"Not through a telescope we currently have, no. (Well, not from Earth at least, the LRO did, but it was in orbit around the Moon). ",
"Here is an",
" interesting discussion we have about a week ago on this topic. "
] |
[
"This",
" is about as much detail as we have so far....will we ever make a powerful enough telescope to see this? maybe..who knows. ",
"The problem is that the objects are quite small, and the moon is a ",
" way away. This heavily reduces the amount of photons being reflected by these objects, back to earth and into the telescope."
] |
[
"You don't have permission to access /squidoo/moon-landing-photos/photo-of-lunar-lander.jpg on this server."
] |
[
"Why is it that animals can eat rotten meat and drink filthy water, but people can't?"
] |
[
false
] | null |
[
"Some animals, such as vultures, have adapted to be able to eat rotting meat, or carrion. It looks like ",
"some people propose",
" that the vulture's extremely low pH aids in killing most pathogens, although it seems to be a hypothesis and more research needs to be done to look for other possible mechanisms.",
"The thing people often don't realize is that wild animals often do get sick and die from things that affect us too. There have been quite a few recalls recently for Salmonella contamination in dog foods, and for ",
"aflatoxin",
", which is produced by mold on corn. (More info on pet food recalls ",
"here.",
") Antoher example is ",
"leptospirosis",
", a bacterium that affects many species, including wildlife, rodents, livestock, dogs and people. It infects the kidneys and bacteria is spread through the urine of animals, usually through a contaminated water source. Infected dogs get liver and kidney failure. Without treatment, and sometimes despite treatment, it is often fatal. If not directly fatal, it can leave an animal debilitated, thus making it easy prey for a predator.",
"Many wild animals die at a relatively young age. They live long enough to breed, but something usually gets them before they reach old age. For example, your average house cat can live to be 15-20 years old. You average feral cat ",
"lives less than 2 years",
" if they are not cared for by any people. Since cats can go into heat as young as 4 months and have 2 to 3 litters a year, they can still produce plenty of offspring before dying due to illness or injury that a house cat would likely receive treatment for."
] |
[
"When I worked in a clinic (in vet school now), I would hear this myth frequently that dogs GI tracts can handle anything. And maybe it's true that they are somewhat more robust. However, a huge part of the practice was devoted to GI upset. Many many parasites were found in fecals. Sure, they are exposed to more and may get immunity through exposure. In my animal nutrition class we learn that if the water looks like you wouldn't drink it, don't give it to your animals. This is said with livestock too. In parasitology we learn about tons of stuff infecting animals. Anyway, animals get sick from contaminated food, some species more, some less and with different degrees. Some things are species specific. This applies to other diseases as well. (I can't give my dog the flu for example and they can't give me parvo.)"
] |
[
"Can I post a followup question?\nWould humans living as hunter gatherers thousands of years ago have been more resistant towards bacteria in rotting meat and dirty water?\nAre we simply more susceptible (assuming we are) towards these kinds of bacteria because we generally don't encounter them as much today?"
] |
[
"Is it possible to raise the temperature of a half-filled Thermos of water by vigorously shaking it for an extended period of time?"
] |
[
false
] | null |
[
"Yes it is! See ",
"mechanical equivalent of heat",
".",
"The SI unit of energy, Joule, is named after ",
"James Prescott Joule",
", who did some pioneering work on that topic."
] |
[
"In a thermos the container wall is mirrored and surrounded by vacuum to minimize heat transfer to the outside (aka why it can keep you drink hot or cold for so long and why it can keep lN2 from evaporating rapidly.) ",
"So its still going to take a good bit of shaking to heat it (I certainly don't care to test measure or try it). But its easily possible and thats almost ideal conditions."
] |
[
"Even though this is actually true (that if you shake something, you add energy and it will eventually get warmer by this), you need to consider that since shaking raises the heattransfer of the water to the containerwall, it is not likely you can get the water really warmer than the surrounding fluid (e.g. air) just by shaking it around a bit.",
"Firstly, the added energy is not really a lot",
"Secondly, water needs a lot of energy to get warmer (4,18 kJ/(kg*K))",
"Thirdly, the shaking will also heat up the containerwall and as I said the heat transfer is raised by shaking the container.",
"Not only on the inside but also on the outside."
] |
[
"Why we don't see solar emission lines when we put sunlight through a prism but a continuous spectrum?"
] |
[
false
] |
When you put prism in the path of the sunlight you get a nice spectrum spread from red to violet (and beyond the visible part). Why do we get that spectrum? Why don't we see emission lines from hydrogen or helium? Where do other wavelengths come from? How do you "take out all those photons to see say He lines (how helium was first discovered)? I think I read somewhere you can sometimes see absorption lines from atmosphere gases, but why not original source bands? How does it work that we can determine composition of other stars or even exoplanets from their light if everything we get is "white"? Sorry for lots of questions, it just popped in my head.
|
[
"Keep in mind, when we are looking at the solar spectrum, we are really looking for absorption lines, not emission lines. The sun is very very hot, and so mainly acts as a black body emitter, which has a continuous spectrum. However, after the black body radiation is emitted, the gases and materials in the solar atmosphere will absorb light at their resonant wavelengths, leading to ",
"Fraunhofer lines",
" in the light that reaches the surface of the earth.",
"Anyway, the reason is that your typical prism setup has a pretty low spectral resolution, so the absorption lines get blurred to the point that you can't distinguish them.",
"If you look at the setup of a ",
"typical grating spectrometer",
", you can see that an essential function is an entrance slit and collimating lens. This makes it so that the light is more or less parallel to itself, which lets you focus each wavelength very sharply. Without that slit, the different colors will blend into each other to some extent, erasing any thin absorption lines."
] |
[
"These features were originally discovered in prism spectra, so they do occur there. However, the lines are quite narrow, which means that it's important to avoid as much blurring of the spectrum as possible.",
"The sun is not a point of light, but is extended with a diameter of half a degree. Light from each point on the Sun's surface passes through your prism and produces a spectrum, but each of those spectra will be slightly offset from each other because the light is entering the prism at a slightly different angle. What you observe is the sum of all those little spectra, producing a single blurry spectrum. The same thing happens with rainbows etc. I suspect that this is the main problem you're having in observing spectral lines.",
"To avoid this problem, don't let all the light from the Sun enter the prism. Instead put the prism in a dark room with a small slit through which sunlight can enter, and pass that thin beam of sunlight through the prism. The thinner this beam is, the less blurring there will be, and the darker the room is the easier it will be to see faint details in the spectrum."
] |
[
"It's useful to think about this in terms of ",
"optical depth",
". Think about looking something through some fog, for example. If there's not much fog between you and the object you can see it clearly, but as the amount of fog increases the chance of a photon making its way to you without being scattered away gets smaller and smaller, and eventually what you see is the fog itself rather than the object. The optical depth is a measure for the cumulative scattering opportunity whatever the light is passing through has had, and a typical photon will be stopped by the time the optical depth reaches 1.",
"What does this mean for the Sun? Spectral lines are frequencies where light interact strongly with atoms, both in emission and absorption. That means that at a spectral line light can travel a shorter distance before it scatters off something than outside a spectral line. Hence the optical depth is higher in a spectral line and lower between them. And so you can see deeper into the atmosphere of the sun when you don't look at a spectral line, and if you look at a strong spectral line you will only see the very top of the atmosphere.",
"If the whole atmosphere of the sun had constant temperature, then it wouldn't matter where the photons at each frequency were last scattered, they would all have the same intensity distribution and you would see a perfect blackbody (ignoring the effect of the ",
" atmosphere here). However, in practice the Sun's atmosphere's temperature varies with height. Typically (but not always) the temperature increases with increasing depth, so the deeper into the Sun you see, the brighter it appears. That means that at a spectral line, where the optical depth is higher, and you therefore can't see as far, you see less brightness than between the spectral lines. The spectral line therefore appears as a dark feature in the spectrum.",
"If you go far enough out in the Sun's atmosphere it eventually starts getting hotter again, so for every strong spectral lines you can actually end up with the spectral line looking like a thing positive peak inside a slightly broader negative peak. Effectively what you see as you move from the outside of a line to the center of a line is a sort of projection of the Sun's atmosphere's temperature profile. I wish I had a nice figure to illustrate this - I guess this might be confusing from my description.",
"If you don't look directly at the middle of the face of the sun, but towards the edge of the sun, then the rays of light are passing through the Sun's atmosphere at an angle, meaning that they have to pass through more gas before they reach a given depth. This means that you can't see as deep into the Sun, just like if you were looking at a spectral line. At most frequencies this means that you see colder, less bright gas that's higher up in the Sun's atmosphere, giving making the Sun look ",
"darker towards the edge",
". But if you look at the sun in a strong spectral line, then you could be looking high enough into the atmosphere that the temperature is increasing with height instead of decreasing, in which case you instead see the edge looking brighter.",
": Lines appear in the solar spectrum because light interacts more stronly with matter at those lines, and when it interacts more strongly light can't go as far before bouncing off something. Therefore you see deeper into the sun outside lines than in them. The temperature is typically higher deeper in, so outside a line the Sun looks brighter than in a line, making the line look like a dark feature in the spectrum."
] |
[
"Why can't we cure cancer by combining 3-d Imaging and a robot controlled moveable sphere of low powered lasers that destroy only kill cells at their intersection, leaving tissues they pass through undamaged?"
] |
[
false
] |
So, you've got existing technology that seems to me could target any cancer in any shape in the body by combining Put that tumor model into a computer that remembers all the x,y,z points of tumor and non-tumor tissue Build a sphere/half sphere of low powered lasers targeting at one point, kinda like the so that neighboring tissues go undamaged. Sedate and tie down patient so they don't move. Cauterize any major veins to tumor areas, add clotting agents to blood for small veins Using a robot controlled by the computer, move the laser sphere around to zap all tumor tissues Body digests organic dead cells of tumor ?????? Profit?
|
[
"That is basically how radiation therapy works now. But your idea assumes that we can do all of those things precisely (i.e. with no uncertainty). There are several things that we are, in general, uncertain of:",
"1) We can identify gross tumor tissue to within a few mm. But we can't identify the exact microscopic extend of disease. We also can't identify metastases that are still microscopic.",
"2) We can only localize things in the body to within a few mm (depending on where they are). If you were to do an MRI or CT scan, you would find that on the day of treatment everything has moved around.",
"3) We can only shoot radiation to within about a mm of accuracy. Some specialized treatments boast sub-mm accuracy, but there is always some uncertainty.",
"The main drawback, though, is that there are always structures around the cancer that you don't want to treat. For instance, if you are too aggressive with prostate cancer, you may permanently damage the patient's rectum or bladder. Your lasers would burn the tissue overlying the tumor, since visible/IR light lasers attenuate very quickly in tissue."
] |
[
"Could you do the MRI like 20 minutes before the procedure to reduce changes?",
"There is something called ",
"real-time MRI",
" now, but its limitation is in the fact that it's 2D. Three-dimensional imaging requires reconstruction from many 2D slices, and the collection process itself is one limiting factor - it simply takes ",
" to collect multiple slices. I'm not sure about the rendering requirements, so someone else more proficient in the algorithm side of things can better comment on this.",
"What would it cost to find a way localize tissue closer than ~2-3mm?",
"For an MRI to increase in spatial resolution, one must either increase the ",
" of the signal, or the magnetic field. Most MRI (and NMR) machines now go about it the latter way - increasing the magnetic field, thus the spectral width of the signal, can give better resolution while maintaining the same frequency resolution. (It's like blowing up a blueprint - your eyes has the same spatial resolution, but you can see more details by projecting it to a bigger page). So we have limitations in both ",
" and how strong a magnetic field we can safely put a human in."
] |
[
"Ah, ok, as a video editor i know the hassle of waiting for renders. If it is at it's core a matter of computing horsepower, i guess the fact that computers get faster exponentially is hopeful.",
"As far as spatial resolution, i did find this ",
"article",
" on some new technology that can do this stuff on a molecular level, a quote: ",
"\"Improved electronics and signal processing systems have increased the detail visible in MRI images and even allowed researchers to collect microscopic images of insects and other tiny samples.\"",
"could this help? too acute perhaps?"
] |
[
"If a computer monitor refreshes at 60hz, and you are displaying 24fps video, how is this mismatch resolved?"
] |
[
false
] |
I've tried searching about this issue, and I saw people referring to 3:2 pulldown, but I'm not sure exactly what they mean. I'm just wondering how mismatched frame rates are resolved on a display. 30fps video on a 60hz monitor could obviously mesh well by simply playing the same frame twice, but what about when the refresh rate is not evenly divisible by the FPS? Thanks.
|
[
"Your monitor's internals refresh the screen at 60 times per second based on the signal on the input, your computer/video-card updates the signal on that input at, say, 24 times per second. The mismatch is not resolved.",
"Sometimes, when the monitor refreshes while the video card is updating,.. half (or part) of the screen will show one frame - while the lower part of the screen will show the next frame. This is visible to people while playing some games and is known as \"tearing.\"",
"Video cards have a feature called \"VSync,\" which will slow down the video card to 60, or to the closest even divisor - so that the two are always in sync.",
"*Edit: grammar."
] |
[
"3:2 pulldown is explained here ",
"http://en.wikipedia.org/wiki/3:2_pulldown#23pulldown",
". Basically, remember that for a television (conventional NTSC american TV), the vertical refresh rate for the video signal is just below 60hz (59.94hz). Originally, it was exactly 60hz, but since the powerlines run at 60hz, one could often see power-line noise on the picture. So a value just slightly out-of sync with it was chosen. But this was just the refresh frequency. An entire image wasn't rendered at once. Instead, half the horizontal \"rows\" were rendered each refresh (the even rows on one cycle and the odd rows on the other). This resulted in a 59.94 \"fields\" being rendered every second (each field contained the info for half a frame) and the real frame rate was 29.97 fps. This technique was called interlacing - that's the \"i\" in 480i comes from.",
"If you have 24 frames per second and you need to convert it to roughly 30 frames per second, you basically need some way of blending frames such that 4 input frames turn into 5 output frames (or 10 output fields). The ratio of 4:10 is also 2:5. How do you convert 2 frames into 5 fields? Well, you use one frame to produce 2 fields and the other frame to produce 3 fields. It works slightly differently than that (you can see the precise layout in the wikipedia article), but as you can see, it's \"make 2 copies, then make 3 copies, then make 2 copies, then make 3 copies\". This is what a 2:3 pulldown means.",
"But this still gives you a 30fps signal. Your TV want's a 29.97 fps signal. So you have an additional concept called a drop frame. ",
"http://documentation.apple.com/en/finalcutpro/usermanual/index.html#chapter=D%26section=6%26tasks=true",
" Here, we subtly drop a frame or 2 ever minute or so to accommodate this slight drift in frame rate to make sure that our audio and video are in sync.",
"Now, when it comes to an LCD, you have a device that actually honestly does 60 full frames a second. No interlacing, no magic. Just 60 progressive frames every second. Bahahaha! I lie! Of course there is magic and lulz when it comes to computer video display technology but let's pretend for a minute that that isn't the case. In this case you actually get to convert 24 frames to 60 frames (2:5 ratio). You can do this in a very rough/naive manner and just perform a 2:3 frame pulldown (using full frames instead of fields). This can create a jumpy video which you sometimes see from really old DVD players with bad codecs. Newer decoders do something called frame interpolation. How well they do this depends on the codec, and the CPU/GPU available to the decoder. To compute the intermediate frames they use complex math usually involving detecting motion through Fourier transforms and analyzing frequency components. They then use this motion information to produce a whole bunch of synthetic intermediate frames that look realistic. Of course, how realistic the output is depends very much on the algorithm being used. If you over do it, it looks very unrealistic and fake. That's why a lot of new HDTVs that feature 120hz refresh rates for 24fps input signals look really weird because not only are you not used to seeing such little judder, you are also seeing mostly synthetic frames - most of the frames generated are purely fake interpolation frames based on what the decoder \"guesses\" the world looks like in between two known frames. The higher quality TV decoders obviously do a much better job at this than the ones found in cheaper quality TVs."
] |
[
"Tearing is a phenomenon that occurs when your video buffer is displayed half-way through a frame update. This rarely occurs when we're talking about video because the frame-rates are well-known and the decoder performance is well understood and optimized for a specific code-path. Video decoders use a specific pattern to convert between frame-rate and the technique they use depends on the particular input/output standard they are converting to."
] |
[
"Why don't we dream under a general anaesthesia?"
] |
[
false
] |
Why can't we dream under a general anaesthesia?
|
[
"We do dream under general anesthesia. Some patients report dreaming and, although it is not fully understood, some experience something called emergence delirium. If the sedation used is one that leaves the system rapidly, the patient can regain consciousness and be agitated, confused, and sometimes even violent. The incidence of this delirium is more common in children, but again, the reason is not understood. Also, some sedatives, ketamine comes to mind, tend to cause this more frequently especially at certain dose ranges. ",
"So, we do have some patients who report dreaming during sedation. The exact rate of patients having dreams is hard to exactly pinpoint because many patients after surgery are too loopy to really discuss their dream or they could be unable to remember it after some time in recovery and they could report having a dream during while under sedation when they were actually sleeping while recovering from being deeply sedated. "
] |
[
"It actually varies a lot with the drugs we use. Propofol, which is the most commonly used anaesthetic drug in \"the west\" often gives quite vivid, normally pleasant dreams. This indicates that it not just gives sort of a controlled GABA overload, but actually triggers the intrinsic sleep pathways in some way.",
"Dexmedetomidine is an interesting drug. We don't use it for general anaesthesia for humans, but vets use it like that on animals. It triggers the orexinergic nerve system to switch from awake to sleep, like the brain normally does. It is used for ICU sedation, and increasingly for preoperative and procedural sedation for children. This gives dreams similar to normal sleep.",
"Sedation/anaesthesia with propofol and dexmedetomidine actually make up for sleep deprivation, unlike other anaesthetics.",
"Ketamine gives ",
" \"dreams\", but those are more like hallucinations, and unlike propofol and dex the patients are generally more sleepy after a procedure using ketamine.",
"For barbiturates and anaesthetic gasses it seems like it is an unconsciousness more akin to passing out from drinking too much alcohol. The brain is in a state of general suppression of activity from a powerful GABA stimulation. As dreaming is an active state of the brain, this is suppressed along with wakefulness."
] |
[
"Bizarre, The lights go out and you wake up again, but there was nothing in the middle, it's as if you didn't exist with no self awareness to yourself."
] |
[
"How are there still radioactive elements such as uranium on earth when they've had billions of years to decay?"
] |
[
false
] | null |
[
"Really simple answer: because they take more than billions of years to decay.",
"Uranium-238 has a half-life of about 4.5 billion years, which is (very roughly) the age of the Earth. That means we've only used up half of it."
] |
[
"Long half lives of elements like uranium is only part of the answer. Shorter lived radionuclides such as carbon 14 or beryllium 10 are produced all the time when cosmic rays, often in the form of protons or neutrons, collide with other atoms and cause neutron activation or spallation reactions. This process can also happen in rocks exposed to cosmic rays as well. An example of cosmogenic production is carbon 14 in which a neutron simply replaces a proton in a nitrogen 14 nucleus in the atmosphere. Another way to produce short lived isotopes is through spontaneous fission. This is a type of decay when a nucleus splits itself apart. One part forms the nucleus of a new isotope while the other part forms the nucleus of another. This process often occurs in soil and rocks and explains why we can find short lived isotopes deep underground in places that have been isolated for millions of years."
] |
[
"This seems like a semantics argument. How do you define the age?",
"If when the planet formed, was it there? Almost all, yes (small addition due to celestial impacts). ",
"If the uranium was formed in supernova before earth was formed? Yes, but so was the rest of the material considered earth. "
] |
[
"By what mechanism does male circumcision inhibit HIV infections?"
] |
[
false
] |
I've read in a number of places, and just recently heard Bono say on The Daily Show, that male circumcision can inhibit HIV infection in men. says infections can be reduced by more than 57%. Why would that be? Circumcision and exchange of fluids seem more or less unrelated to my layman's perspective. How could a circumcision be effective at preventing transmission of HIV? Thank you in advance, askscience!
|
[
"from ",
"http://en.wikipedia.org/wiki/Foreskin#Langerhans_cells",
"Langerhans cells are immature dendritic cells that are found in all areas of the penile epithelium,[52] but are most superficial in the inner surface of the foreskin.[52] A study by Szabo and Short (2000) targets Langerhans cells as receptors of HIV, and states that these cells \"must be regarded as the most probable sites for viral entry in primary HIV infection in men.\"[53] Langerhans cells are also known to express the c-type lectin langerin, which may play a role in transmission of HIV to nearby lymph nodes.[52] However, de Witte et al. (2007) reported that langerin, produced by Langerhans cells, blocks the transmission of HIV to T cells.[54]",
"Basically the HIV (Human Immune Virus) targets the immune system, and the foreskin contains special immune system tissue, much like the gut mucosa or the tonsils. usually these tissues are helpful because they allow early detection (and subsequent timely immune response) for pathogens entering the body, but in the case of HIV it (supposedly) just makes the kind of cells that the virus likes to infect more available. so chopping off the part of your dick with the most exposed immune tissue could reduce your chances of that immune tissue being infected with HIV. "
] |
[
"I just wanted to lend support to this answer, as current literature theorizes this is the reason. This has been shown in a number of studies, such as ",
"here",
"."
] |
[
"In regards to the OP, it doesn't. Not really. It is a contested subject but I personally have seen no evidence to indicate it prevents infection of HIV.",
"A foreskin that has not been cleaned can allow bacteria to grow, but HIV is not a bacteria. The V stands for Virus, and those don't \"grow\" like bacteria."
] |
[
"If a photon has no mass, why doesn't it pass through objects like other forms of radiation?"
] |
[
false
] |
If a photon has no mass, why doesn't it pass through objects like other forms of radiation? Why is it reflected by a mirror instead of just passing through? Why can't we use a mirror to reflect Gamma radiation for instance?
|
[
"Mass has nothing to do with it.",
"Visible light is reflected by matter because it interacts with the electrons in the species that make up that material.",
"The frequency of the light determines how it will interact with various materials.",
"Radiowaves (photons) rarely interact with most materials, except for metals.",
"Some objects ",
" reflective to, say, gamma radiation, but those are not necessarily reflective to visible light."
] |
[
"Well in fact gamma radiation is composed of photons. It is electromagnetic radiation that is simply more energetic than the visible light photons. The reason that certain wavelength photons do not pass through certain objects is because their energies are just right to interact with the electrons in the matter.",
"I believe that mirrors reflect light by absorption and re-emission of the photons. Not entirely sure on this but I think that gamma rays are usually too powerful and simply eject the electrons out of the atom (photoelectric effect) or interact with the nucleus via Compton effect where they are scattered but not reflected."
] |
[
"You're getting into some seriously complicated physics with this question. If you'd really like to begin to grasp this subject, QED [Quantum Electrodynamics] is a series of lectures by Richard Feynman in book format. It's pretty popular because it's designed to explain this challenging subject to a layman, and it's incredibly well done. It's rather short and I would recommend that you read it.",
"Edit: It's ten bucks and will revolutionize the way you look at the universe. Buy it.\n",
"http://www.amazon.com/QED-Strange-Princeton-Science-Library/dp/0691125759/ref=sr_1_1?ie=UTF8&qid=1340836137&sr=8-1&keywords=QED"
] |
[
"Why does food perish and how can suppliers so accurately put a date on when this will occur?"
] |
[
false
] |
[deleted]
|
[
"The date is actually rarely accurate. Most foods are good for several days (in some cases weeks or months) after the best before dates. If storage was not ideal, or the packing faulty things can also go off before their stamped date."
] |
[
"There is also a difference between \"best before\" and \"use by\" dates. ",
"http://en.wikipedia.org/wiki/Shelf_life#Best_before"
] |
[
"The dates listed on food are simply a required thing the FDA does. ",
"Lots of factors go into how long it takes food to spoil so there is no way to put a spoilage date on the food. ",
"The date is a \"sell by\" date most of the time which I think has more to do with covering the stores ass than it does with spoilage. ",
"Food actually soils from either bacterial growth like when that chicken you left out gets funky looking and slimy. Bacteria eat the food and leave behind their waste. Thus why it is bad to eat and also why it smells bad. ",
"Mold and fungus can also potentially start growing however I know little about molds and slides. Maybe a biologist can elaborate. ",
"The other thing is that Oxygen in the air is extremely corrosive, believe it or not. That is why you want to keep things sealed up and out of the air. ",
"Perhaps a biologist can give you a better answer and go more in-depth for you. "
] |
[
"About the 7 base SI units: \"7 is the number of physical quantities that are independent from one another. It comes from the fact that the number of quantities in physics is higher by 7 to the number of determination equations (definition equations and natural laws).\" -- Can someone explain this?"
] |
[
false
] |
Side question: How is the Candela not , since it is a measure of luminous power ( ) per solid angle (dimensionless)? Edit: Thank for the insightful inputs. I took that course for granted, but after going through the lectures again, and having passed the exam, I am now fully convinced that these guys have absolutely no idea what they are talking about. I did learn a lot from you guys so thanks again.
|
[
"Where did you read that statement? It sounds made up and not based on reality."
] |
[
"To answer the side question, the candela is defined to be a unit of PERCEPTUAL intensity of light based on the sensitivity of the human eye. The definition takes into account the perceptual brightness of different wavelengths.",
"That's the difference between luminous intensity (candela) and radiant intensity (W/steradian)"
] |
[
"I have never heard such a statement. On the surface, it seems to suggest that there are, say, N+7 equations that \"determine physics\" with only N unknowns. Yes, that would mean there are 7 free parameters in that system of equations, but I'm not sure what equations the statements refers to or why they think 7 such free parameters implies 7 base units."
] |
[
"If I replaced the bones in my body with metal replicas what problems would occur?"
] |
[
false
] | null |
[
"Ok, so even if you get it done that all the tissue will grow onto it and it won't poison you etc...",
"Your ",
"bone marrow",
" would be missing.",
"As you see in the article, you would be pretty screwed without it.",
"So yeah. You would die."
] |
[
"White blood cells are generated in your bone marrow, you'd probably miss those. "
] |
[
"People already explained the lack of white blood cells and red blood cells because of the missing bone marrow. Another thing that can happen is the inability to heal. Bones can regenerate if a fracture occurs. If you get some type of crack within your metallic bone, you're screwed because eventually the crack will become a stress concentration and begin to propagate through the material. Eventually a full fracture will occur and bad things happen."
] |
[
"Detecting Observers - Is This Possible?"
] |
[
false
] |
I am by no means an expert on Quantum Physics, but I've been reading about the and the experiments, and I had an idea which I fully expect to be (gently) disabused of. Let's say you have a room that has a display device which consistently outputs information regarding which-path and/or which-slit photon results from repeated automated dual-slit experiments. This is the only place where the results can be obtained. Let's also say that there is a device which discerns interference Pattern-Present, or Pattern-Not-Present, and triggers an alarm when it changes between experiments. <-- [Problem is most likely here.] A researcher walks into the room... Essentially, regardless of my feeble attempt at creating the suitable conditions above, I am trying to determine if a device can be created where a human would be tricked into "observing" the measurement data and thus destroy the interference pattern, which could be detected and trigger an alarm. Yes, a sentient being detector--ridiculous I am sure, but I would like to know why at the very least. Just as interesting, I think. :)
|
[
"I don't know if this really answers your question, but...\nFrom the observer effect wiki you linked:",
"This thought experiment was proved correct experimentally. The people conducting the experiment found that when the sensor was turned off, an interference pattern developed, but when it was turned on, the interference pattern was destroyed. It was even found that the level of detection could affect the result.",
"Even if the display is wiped after each experiment, its use is what disrupts the interference pattern. To register an interference pattern, you would have to not use any device to determine which slit the photon traveled through."
] |
[
"From what I know about the observer effect, it does not have to be a human observing the experiment to interfere with the results.",
"Let's say you have a room that has a display device which consistently outputs information regarding which-path and/or which-slit photon results from repeated automated dual-slit experiments. This is the only place where the results can be obtained.",
"This device outputting data regarding which slit the photon travels through is considered observation, so an interference pattern would not be registered based on that device's use. "
] |
[
"Yes, this is the case. Observation really doesn't describe what is happening here. Observation means any sort of interaction, it has nothing to do with whether a human sees it or not. "
] |
[
"How are there still unsolved math problems from the '50s?"
] |
[
false
] |
[deleted]
|
[
"We understand the laws (we made them, after all), but not all the consequences of them. Fermat's last theorem is a great example of an extremely difficult problem which is nevertheless very simple to understand.",
"Could it ever be true, for some integers ",
" ≥ 3 and ",
", ",
", ",
" ≥ 1, that ",
" + ",
" = c",
"?",
"Here, you understand all the laws. You know what integers are, you know how to exponentiate them and add them. Could you answer this question? Chances are that you (like me) don't even know where to begin.",
"This conjecture was famously cracked in the 90s, 350 years after it was posed. It is not possible. It required an absurd foray into some ",
"hideously complicated material",
", highly contrasting with the simple statement of the problem.",
"How do mathematicians go about solving unsolved problems, are there teams of people who show up everyday and stare at a chalkboard covered in numbers and scratch their heads?",
"The chalkboards are unlikely to be covered in numbers… more like symbols and abstractions that nobody would be able to understand unless they wrote them. But there's perhaps less chalkboard-staring involved than you think.",
"Most mathematicians would not attempt to solve a famous unsolved problem. It's much more fruitful to deal with stuff they know they can do. You're more likely to ruin your academic career, because chances are that you're going to make zero headway and have nothing to publish.",
"You're more likely to see established and famous experts do this. They know everything about all the failed attempts, they know the layout of the problem and they most importantly have an idea of some intermediate step that could prove useful, but which is tractable by a handful of people. And they also have the stability required to risk it.",
"Many of these problems (Fermat's last theorem among them) end up being solved by the accumulated efforts of many generations. Very often, the last person does nothing but fit the last piece of the puzzle in. That's what Andrew Wiles did in the PDF I linked to above. There's very little in there about the actual Fermat's, he ‘merely’ solved a problem that had been posed by Taniyama and Shimura, and which had been determined by Frey would imply Fermat's. These building blocks go back 50+ years, to when Wiles himself was a toddler, and they were all important."
] |
[
"How are there still unsolved math problems from the '50s?",
"It's worse than that ! There are problem that are thousands of years old. For example, the existence or non-existence of odd perfect number (we found evidence that this problem was considered by Nicomachus of Gerasa around 100 AD, and it's still an open problem to this day). ",
"So how are there still maths problems that remain unsolved decades or centuries after they were initially advanced?",
"Because it's not your usual \"math problem\" where you have a known method to give an answer. Most of the unsolved math problem are not computational problems. ",
"How do mathematicians go about solving unsolved problems,",
"Let's look at a concrete problem. The one I stated about the existence of odd perfect number. A number is ",
" if the sum of its divisors is equal to itself. For example 28 is a perfect number because it is divisible by 1, 2, 4, 7, 14 and 28 = 1+2+4+7+14. These number are studied since ancient Greece. ",
"We know many perfect numbers. But, as of now, all the ones we found are even (like 6, 28, 496, ...). So a natural question to ask is \"is there a perfect odd number ?\". ",
"So how would you try to answer the question ? ",
"A naive approach would be to search among all odd numbers to see if we can find a perfect number. So you take all the odd numbers one after the other, you compute the sum of their divisors and then you compare it to the number. Easy to program on a computer. So if there is indeed a perfect odd number, your computer program will find it after some time. But may be a very very long time. As of now, we couldn't find any perfect odd number using this method. (there are none that are smaller than 10",
"So what if there is none ? Then your computer program is of no help, it will never find one, but it will not be able to tell you if the next number that it didn't try is a perfect number or not. So you have to find other ways to tackle the problem. And with this overwhelming evidence, you should perhaps try to find a proof that there are no perfect odd numbers.",
"But this is difficult. Extraordinarily difficult. (The reason why some problems are difficult while others are easy is not really well understood at all)",
"So mathematicians go step by step. Instead of looking at all odd numbers at once, they can for example first study the odd numbers that are a product of two primes (like 15 which is 3x5 or like 49 which is 7x7). And it's very easy to see, that there can be no odd perfect numbers of this form (because 1+p+q is always strictly smaller than pxq, when p and q are greater than 3). So what about numbers that are products of three primes (like 105 = 3x5x7) ? A little bit more difficult but relatively simple. And so on, for products of four primes, etc ... ",
"As of now, mathematicians have proved that there are no odd perfect number which is a product of less than 100 primes. They actually proved ",
"many other things",
" but they still can't exclude the possibility that there is a perfect odd number somewhere, in a case they did not consider yet (like a product of 102 primes).",
"So in practice, a mathematician will not try to find the final solution at first sight. He will split the problem in many smaller subproblems. Probably some of the subproblems are easy, but some are hard and need to be split again, and so on ...",
"The point is to split your problem in pieces that you can solve.",
"are there teams of people who show up everyday and stare at a chalkboard covered in numbers and scratch their heads?",
"That's actually very close to what is happening, except that it's probably not numbers. In fact, I have already seen mathematicians scratch their heads and stare at chalkboards for hours with only a small drawing on the board (like a drawing of two triangles).",
"And most importantly, most mathematicians do not focus on only one problem. Usually you are a specialist of a particular domain which have a lot of open questions. And while discussing with other people that have a slightly different background (or reading their work), you can sometimes understand how to solve a given question from your domain using their result, or perhaps use a method you know to solve a problem in their domain, or even ask new questions that people did not think about before.",
"That's how mathematics is done."
] |
[
"That really depends on the problem. Fermat's has negligible impact outside of mathematics. Something like ",
"P vs NP",
" could potentially be huge for just about the whole of humanity if answered in the affirmative (although, it's likely that the right answer is ",
" affirmative)."
] |
[
"Is there a material that can block magnetic fields?"
] |
[
false
] |
I understand that magnetic objects create a field, and that a magnetic field can cause damage to certain electrical equipment or data storage. (Bonus Question: Why is that?) But is there a material that can create a barrier for magnetic fields, So if you put a paperclip on one side and a magnet on the other, the paperclip would be affected?
|
[
"Any ",
"closed ferromagnetic surface will block magnetic fields",
". Basically, a hollow iron shell will divert magnetic field lines away from the inside of the shell and force it to travel around the outside of the shell.",
"In addition, ",
"superchilled superconductors will expel a magnetic field by automatically creating its own counter-magnetic field",
". This is why superconductors can ",
"levitate a magnet in mid-air",
"."
] |
[
"Type 1 superconductors will completely expel magnetic fields, type 2 allow magnetic flux through vortices with a quantized amount of magnetic flux in each vortex.",
"1"
] |
[
"Over-unity or perpetual motion/free energy machines are impossible. Just putting that out there.",
"That being said, there are materials which can change the shape of a magnetic field. Check out mu-metals.",
"Bonus: Many forms of data storage today is achieved in the form of very delicate magnetic orientations on a magnetized platter. A strong magnetic field will scramble and wipe the data. Other electronics are susceptible to current induced by a moving magnetic field causing surges of power in unexpected ways, causing damage. "
] |
[
"When animal mothers \"adopt\" a newborn animal from another species, does the mother know that they are not the same species?"
] |
[
false
] |
[deleted]
|
[
"Probably not. Unless there is evidence that the animal is self aware, it would do well to think of what is going on as automated processes rather than a decision and that the adopted animal is hijacking this.",
"A good way to illustrate this is with parasitic cuckoos.The common cuckoo lays its eggs in the nests of other birds. When the chicks hatch, they push the eggs / other chicks out of the nest. The 'mother' continues to raise the bird that killed its real offspring, despite the chick growing larger than the mother herself. ",
"A picture of this",
". The mother continues to feed the imposter because it is able to 'code break' the prompts a chick normally gives its mother. In this case, the gaping red mouth is a sign to the mother that something is hungry. The response to a gaping red mouth is to drop food into it. ",
"When you see a story something along the lines of a dog adopting tiger cubs, keep in mind that a dog isn't going to be producing milk unless it's recently given birth anyway. Here, ",
" intervene to hijack the dog's instincts. "
] |
[
"While we are on the subject of parasitic cuckoos, interesting side note: ",
"here",
" is an article about the Eurasian magpie which evantually figures out how to identify these eggs and deprive them. These birds do demonstrate some self recognition though so makes sense."
] |
[
"So the cuckoo triggers an instinct in the parent bird to feed it. Wouldn't this be similar in some ways to humans adopting \"cute\" pets -- isn't the perception of something as \"cute\" part of our instinctive behavior to care for children?"
] |
[
"Does a Martian night sky look similar to our own? Do stars appear brighter since the atmosphere is thinner with less light, or are they substantially dimmer because of the dust?"
] |
[
false
] |
I'm riding the curiosity hype train right now. I know the colors in the pictures we get from Mars usually have false colors, but it always seems that visibility is usually slightly less than earth's, even though the atmosphere is thinner.
|
[
"Isn't it also clearer because there is 0 light pollution as opposed to earth's bright cities which block out some stars?"
] |
[
"Isn't it also clearer because there is 0 light pollution as opposed to earth's bright cities which block out some stars?"
] |
[
"\"Think\" is supposed to be \"thin,\" not \"thick.\" It confused me the first time I read it, too."
] |
[
"Is there a limit to how big a Rocky Planet like Earth can get?"
] |
[
false
] |
Like can there be a Earth type planet the size of the Sun.
|
[
"Others have described the physical limits. But there's another problem that will kick in earlier: such a planet will not even form. The protoplanetary disk that planets are born from contains mostly hydrogen and helium, with a small percentage of rocky material. Smaller, rocky planets like Earth don't have enough gravity to hold a hydrogen-helium atmosphere, so most if these gases escaped during Earth's formation. Bigger planets like Neptune have enough gravity to hold them. The reasoning here is that if a planet is big enough to hold this gases, it will become a gas giant and not be rocky anymore, simply because these gases are so abundant. The limit is at about 5-10 Earth masses."
] |
[
"This answer is correct, but realistically, such a large rocky planet would be incredibly rare or maybe impossible, as any body that size would have enough gravity to begin drawing hydrogen and helium in from the proto-disk during its formation and inevitably become a gas giant of some sort."
] |
[
"This answer is correct, but realistically, such a large rocky planet would be incredibly rare or maybe impossible, as any body that size would have enough gravity to begin drawing hydrogen and helium in from the proto-disk during its formation and inevitably become a gas giant of some sort."
] |
[
"How and why does a brain-dead body decompose?"
] |
[
false
] |
In the news lately, there have been several cases of brain-dead people being kept alive artificially, even though they are medically dead. In reading these articles, I've learned that an artificially animated body cannot stay that way indefinitely and will begin to decompose. How and why does this happen? If a person still receives sustenance and oxygen, why does the body decompose? In what ways is it different from regular corpse decomp?
|
[
"A person can be \"brain dead\" and the rest of the body \"alive\" as long as the body is kept breathing (ventilator) and given sustenance (gastric feeds). The body will waste away due to inactivity, but it won't \"decompose\" as such. The muscles will atrophy with misuse, the skin will break down from pressure sores (unless very good care taken)."
] |
[
"Without knowing all the details, it sounds like they were saying \"decomposing\" instead of \"wasting away\" (which is what I would say)."
] |
[
"So my question is this, the woman from Texas was stated to be decomposing. Was that an exaggeration? I believe it was said by a family member. Or was it just her body not being in use, so to speak? Would it be easy to confuse the two? ",
"Sorry if this isn't appropriate, I'll delete if need be. "
] |
[
"If snow is just sub 0 precipitation, then why does it snow more in the winter than it rains in the spring/fall?"
] |
[
false
] |
[deleted]
|
[
"2 reasons. First, the colder it is the harder it is for the air to retain moisture. Second, snow takes up more volume because there is a lot of air in snow on the ground. 1 foot of snow is about as much as 1 inch of rain."
] |
[
"Oh wow, that's cool to know, thank you!"
] |
[
"There are also a lot of seasonal patterns at work. There are some places where this isn't the case. In a lot of places though, it's about the seasonal winds and how they interact. Sometimes you just tend to get more precipitation in the winter because of wind patterns"
] |
[
"[Astronomy]Is it possible to have an orbit around the earth and the moon?"
] |
[
false
] | null |
[
"Because the center of mass of the Earth-Moon system is inside the Earth, this would just be a distant orbit around the Earth. So in a sense, any orbit around the Earth farther than the moon would satisfy this. The upper limit of this, the region where the Earth's gravity dominates over the sun's, is called the Hill Sphere, and extends about three to four times as far as the moon."
] |
[
"So, just outside of the Hill Sphere, would an object then start to orbit around the Sun?"
] |
[
"There important thing here is the word \"just\". The Hill sphere doesn't have sharp edges, instead, the stability of orbits around objects depends on whether it is prograde or retrograde and the masses of all involved bodies. Retrograde orbits are more stable than prograde orbits (about 70% and 50% of the Hill sphere respectively), orbits above that are unstable and will eventually settle into one or the other stable orbit over time. ",
"Otherwise your assumption is correct, objects with orbits sgnificantly exceeding the earth's Hill sphere will orbit the sun. ",
"Visualization"
] |
[
"What happens to all the electrons that are flowing through an insulated power cord when you unplug it?"
] |
[
false
] |
Wouldn't that cause the atomic structure of the atoms to become unstable?
|
[
"A few reasons this isn't an issue:",
"1) Remember that you're starting with a material which is approximately neutral (i.e. same number of electrons as protons) and that electronic motion is in a circuit. So any electrons that exit are approximately compensated for by electrons that reenter; on average, you're not changing the number of electrons in the material at all. ",
"2) Electrons in typical circuits actually don't move very far. For direct current, they typically move at ",
"drift velocities",
" of order a meter per hour. And for alternating current, the electrons just wiggle back and forth very small distances, of order 10",
" m (see same wiki page). So it's not as if electrons are flying in and out of your material very quickly.",
"3) Even if there's a little bit of charge imbalance, i.e. a few electrons got left out, that doesn't actually change much in the material. You see, in a metal, even one without any voltage across it, there is a set of electrons which are free to move about. These are the ones that carry the current when you plug your wire into the wall, and they have already detached themselves from any particular atom. Since they can move around freely, they will typically rearrange themselves to make almost every place in the material neutral. If there are extra or missing electrons, that net charge lives on the surface of the material, but it will be spread out across the whole surface. So even if there is a small change in the number of electrons in the system, the average number of electrons near any particular atom changes very, very little."
] |
[
"Thanks for the detailed explanation. It's always intrigued me how current flows when the electrons barely move at all (especially in AC). How exactly does this happen? I have a vague notion of energy being passed down the line like a wave (or something), not really sure to be honest."
] |
[
"It's just that there are a whole lot of electrons moving slowly, so the total current is actually pretty large.",
"In fact, the total current is the electron charge times drift velocity times electron density times the cross-sectional area of the wire. If you plug in numbers, say for a 1 mm diameter copper wire, you find that even with a drift velocity of 1 meter per hour, you get a total current of 3 amperes, which is a huge amount of current!",
"As for AC, the current is constantly switching directions, but at any given point in time there's almost always a net current."
] |
[
"If the human egg is the largest cell, and human sperm is the smallest... How big would a sperm cell be if the egg was the size of a basketball?"
] |
[
false
] |
It is just a question to which I have always wanted an answer.
|
[
"Egg Cell\n Diameter = 130 um\n Volume = (4 * pi * (130 um / 2)",
" = 1150000 um",
"Sperm Cell\n Diameter = 5 um\n Volume = (4 * pi * (5 um / 2)",
" = 65.4 um",
"Ratio of Diameters = 130 um / 5 um = 26\nRatio of Volumes = 1150000 um",
" / 65.4 um",
" = 17500",
"So, what's 1/26th the diameter of a basketball? A basketball is roughly 765 mm in circumference.",
"Diameter of Basketball = 765 mm / pi = 244 mm\n",
"By comparison of diameter, the sperm cell analogue of a basketball-sized egg cell would be of diameter equal to 244 mm / 26 = 9 mm, so marble-ish sized.",
"The same result is obtained by comparing volumes. (Edited for math derp)."
] |
[
"You are sort of missing the point."
] |
[
"I'm sure he meant the largest cell in a human"
] |
[
"How exactly does White Blood Cell detect bacteria ?"
] |
[
false
] |
[deleted]
|
[
"The main white blood cells chasing bacteria are neutrophils. They have receptors on their surfaces that recognize what are called formylated peptides produced by bacteria. Formylated peptides are breakdown products of proteins made by bacteria, since bacteria begin all of their protein sequences with the modified amino acid N-formyl methionine (",
"https://en.wikipedia.org/wiki/N-Formylmethionine",
"). ",
"The most potent of these formylated peptides is three amino acids long and has the sequence N-formylmethionine-leucine-phenylalanine. This is abbreviated fMLP. fMLP and other formylated peptides are constantly being released by bacteria in small quantities. They are not made by human cells. Since the neutrophils are attracted to fMLP and other formylated peptides, they will follow the bacteria around that are making them.",
"I'm not sure how or even whether bacteria sense white blood cells."
] |
[
"I did some googling to pin down the source of this gif, and it was apparently made by",
" David Rogers at Vanderbilt University sometime in the 1950s",
". The bacteria here is ",
" and the white blood cell is specifically a neutrophil. ",
"As you suggest, the neutrophil is following some sort of biochemical trail of a chemoattractant substance. In this case, the substance could be either complement component 5a (C5a) or it could be ",
"-formylmethionine-leucyl-phenylalanine (FMLP, essentially a three amino acid chain). Or it could be both chemicals. ",
"FMLP is a substance that is produced by the bacteria and binds to Formyl Peptide receptor 1. C5a is a bit different in that the source for this chemical comes from the body, not the bacteria. What the video is showing is a sample of blood. Blood contains a set of proteins called the complement system, which help defend the body against pathogens by binding to those pathogens. C5 is one such component, and as part of its activation, it becomes split into C5b and C5a. C5b remains bound to the surface of the bacteria, whereas C5a defuses out and attracts immune cells like neutrophils, binding to C5a receptors on the surface of the cell.",
"As for the mechanism that allows the cell to go after the bacteria, its thought that the signal generated by the receptor (either of them) causes the underlying actin skeleton of the cell to rearrange such that it pushes the membrane and cytoplasm in the direction of the signal, thus generating the movement. ",
"As for the bacteria, I don't think it is actually trying to avoid the neutrophil. ",
" is generally regarded as a non-motile bacteria ",
"(except for some new research that suggests it does have some form of movement as a colony)",
". What we're likely seeing here is brownian movement, or movement of the fluid on the slide, which is causing the bacteria to tumble about but isn't actually deliberately moving. "
] |
[
"This is severely incorrect.",
"B cells ",
" neutrophils or macrophages. Neutrophils and macrophages are leukocytes (white blood cells) that are in the myeloid lineage. B cells are completely different cell type which are also considered leukocytes, but they are in the lymphoid lineage. B cells are part of the adaptive immune system while neutrophils and macrophages are part of the innate immune system. Neutrophils and macrophages have pattern recognition receptors (covered elsewhere in this thread) which recognize various features of pathogens. B cells also have pattern recognition receptors but these are largely for stimulating activation and not active direct attack of pathogens. Innate immune cells can be thought of first responders that regularly patrol an area for infection. Adaptive immune cells can be thought of as a tactical assault team called in by innate cells to perform a specialized immune response.",
"B cells do not natively initiate an immune response. That is the role of innate immune cells like the neutrophils and macrophages. B cells are part of the adaptive immune system, which is initiated upon signals from innate cells. B cells are generated with random B cell receptors which indeed recognize antigens. However, this recognition occurs largely in secondary lymphoid tissue (lymph nodes) where B cells screen antigens and form germinal centers where they try and improve the receptor. B cells do not actively seek out bacteria during an initial immune response. Additionally, antibodies do not have a ",
", they ",
" the soluble form of the B cell receptor. Antibodies get produced by B cells after they have recognized their antigen and differentiate into plasma cells. Again, part of the adaptive immune response which occurs after an initial innate response. These antibodies enter the blood where they can partake in their functions. Some effector B cells/plasma cells will migrate to the site of infection and produce antibody locally, however this is after the initial immune response."
] |
[
"If I kept my finger on an object for long enough, would the skin start growing to it?"
] |
[
false
] |
I am just wondering if skin can/will start growing to inanimate objects if it you kept it there for a long time? This may be stupid.
|
[
"No it will not. Think about piercings, I've had an earring in my tragus for over a year and skin has never grown onto it. ",
"This",
" is about how bodies reject piercings, but it talking about how the body rejects foreign objects is the same reason why your skin won't begin to grow on an inanimate object. It doesn't recognize the object as part of your body, therefore will not treat it as such."
] |
[
"Your piercing healed. If you have exposed wounds and it is in constant contact with a foreign surface, it would grow over it, would it not? Isn't this essentially how a graft works?"
] |
[
"A (skin) graft is basically a bandage. It does not heal into new skin."
] |
[
"Why do nuclear bombs form this typical mushroom cloud and not just a ball shaped cloud?"
] |
[
false
] | null |
[
"The cloud is called a pyrocumulus and it is not specific to nuclear blasts - large forest fires and volcanos can cause it too - with a slightly different shape. It is very similar to the classical storm cloud - the cumulonimbus. Hot air rises and expands. Expansion cools it down. Eventually it reaches an equilibrium level and starts expanding horizontally until it starts falling down (oversimplification).",
"https://en.wikipedia.org/wiki/Flammagenitus_cloud"
] |
[
"Mushroom clouds form in any case where there is a very hot central feature, causing a strong updraft shaft where that heated air rises. This rising against the stagnant colder air around the circular rising center \"stem\" causes swirling currents in the form of a muffin top or mushroom cap."
] |
[
"Another reason not yet mentioned is that the shockwave may reflect off the ground, pushing the fireball into a donut shape, which helps catalyze the other effects mentioned. ",
"Here",
" is a slow motion video of this happening. If you think this is weird looking, I suggest reading about the ",
"rope trick effect",
"."
] |
[
"Why do you get a runny nose from eating spicy foods?"
] |
[
false
] |
Also, why do you get a runny nose from the cold?
|
[
"It's the same mechanism behind both instances, just triggered by different things.",
"The reason you get runny noses is increased blood flow to mucosal tissue that makes all that snot in the first place. More blood means more fluid being supplied to the mucosa meaning more fluid oozing out of your nose.",
"In the case of spicy foods, spice acts as a mild irritant to the mucous membranes, causing increased blood flow mediated by the ",
"TRPV1 receptor",
", also known as the capsaicin receptor. ",
"In the case of a cold, your immune system is triggered by the foreign virus (or less commonly, bacteria), causing the release of cytokines (chemical signals) that make blood vessels dilate (bringing more blood) and leaky (more fluid leaks out). Aside from the standard benefits that this has for the immune response from increased blood flow, it's of extra benefit to have a runny nose because of the presence of Immunoglobulin A in mucus. It's an antibody that hangs out in mucus all over the respiratory system and the gut and helps target the pathogens that like to infect via those routes. "
] |
[
"Can you comment on why \"the\" cold [temperature] (as opposed to \"a\" cold [virus]) would trigger a runny nose? It seems contrary to the idea that increased blood flow triggers it. I would think cold weather would cause arteries to restrict, causing less blood flow.",
"Edit: Punctuation"
] |
[
"Yes actually! I'm glad you brought that up. That's a couple of other factors that are in play. One of the purposes of the nose is to filter and humidify the air that reaches the lungs. Cold air is much dryer than warm air as it's less capable of holding moisture, as anyone that's suffered dry skin in the winter can attest. The nose increases fluid production more as a response to the lack of humidity in the air than as a function of temperature constraints on blood supply in that case.",
"The other one is a bit of physics. Ever breathed onto cold glass? Notice all the condensation that happens? There's a lot of fluid loss that occurs via the lungs (which is why the nose wants to moisten the air going down there). Because all that air is now warm and moist, when it hits the much cooler, dryer nasal passage on the way out, condensation occurs. This has the effect of making any mucus being produced much more runny and liquid. "
] |
[
"Do insects perceive time differently than us humans since their reaction time and reflexes seem far superior than our brain can process?"
] |
[
false
] | null |
[
"This ",
"source",
" indicates that neural impulses travel at speeds of 2-200 mph. ",
"Neurons don’t transmit electrically like you would think; they use waves of ions chemically moving through the cell."
] |
[
"Yes, not only insects but different species in general perceive time differently. We measure it by checking in what frequency of flickering light a species perceives it as a continuous (non flickering) light. For example the value is greater for dogs than humans so they will see every frame of a movie on a TV with low refresh rate, when we perceive it as a continuous moving picture.",
"https://www.google.com/amp/s/www.dailymail.co.uk/sciencetech/article-2685860/amp/Dogs-SLOW-MOTION-Animals-brain-processes-visual-information-faster-humans-study-finds.html"
] |
[
"This is a fantastic answer for so many reasons. Distance that electrical synapses have to traverse matters: it’s a 2-way street. Upstream then downstream. The reason their reflexes are superior is precisely bc of that AND the fact that they’re not processing like higher order life does. Much of their response is dictated by instinct, which is famous for being action without thought. So you have these organisms that respond a certain way under certain circumstances, unhindered by the need to process anything, and with a tiny travel path for information. And that HAS to be the case right? Bc they’re so small, they are lower on the food chain so survival is literally a fight or flight event for them: thinking isn’t something they do. Yeah what a great question and answer. Kudos to you both"
] |
[
"When a network \"loses a packet\", where does it go?"
] |
[
false
] |
[deleted]
|
[
"The packet is not lost in that we don't know where it went. Don't think of it as two cars colliding. It would be more accurate to say that the signal of the original packet is changed or becomes combined with the other and is then unreadable by the receiver. Packet loss can also be associated with attenuation(signal degrades) and cross talk(Signal mixes with other pairs of wire in the same cable). Same thing happens, signal just gets corrupt and the receiver doesn't know how to interpret it. Most protocols can detect the packet loss and resend the data, some just send it and forget it(UDP and half duplex anybody?). Like Colechristensen said the network equipment will just disregard any of the packets that it knows to be bad.",
"Same goes for fiber-optic cable, although you have to account for other factors like dispersion due to faults in the glass. The signal will still travel along the medium, but the equipment/protocols usually account for that and disregard bad data."
] |
[
"http://i.imgur.com/lq88U.jpg"
] |
[
"http://i.imgur.com/lq88U.jpg"
] |
[
"In the context of nuclear reactions, what is the difference between prompt criticality and delayed criticality? Why does one result in a nuclear explosion while the other does not despite them both being supercritical reactions?"
] |
[
false
] |
I already understand the process by which nuclear reactors work to a fundamental level with the ideas of moderators and control rods sorted, but what I cannot seem to understand is the difference between prompt and delayed criticality. Like, both are supercritical so both should in theory result in exponential growth of neutron emissions, right? Rephrasing the question, why is there a sudden difference between 1<k<1/(1-beta) and k>1/(1-beta) despite both resulting in exponential growth of neutrons? I looked up the Wikipedia article on this and it did not help. Thank you!
|
[
"For each fission reaction, you have ",
" neutrons and ",
" neutrons. Prompt neutrons are emitted during the fission, so they come out on a timescale or effectively zero for engineering purposes. The delayed neutrons are the result of beta-delayed neutron emission, so they’re limited by the half-lives of the betas decay of the fission fragments (~ milliseconds to minutes).",
"β represents the fraction of neutrons that are delayed (often they’ll be broken into groups based on lifetime, since such a large range is spanned).",
"If the system is supercritical but the prompt neutrons alone are not enough to reach criticality, then it’s the timescale of the delayed neutrons that sets the scale for the exponential growth. This is delayed supercritical. So you get exponential increase, but with a relatively large time constant (greater than ~ 1 second).",
"If the prompt neutrons alone are enough to make the system supercritical, then the delay from beta decay is irrelevant, and you can have a very fast exponential runaway of the neutron population."
] |
[
"It's about timescale. Delayed criticality ramps up slowly enough that operators can control it, and also slowly enough that even if it isn't actively controlled it will be self limiting, as the increase in temperature disrupts the reaction.",
"By contrast, prompt criticality occurs fast enough that it releases the intended yield quicker than mechanical change can occur to the reactant mass. Thus it can't disrupt itself (if this does occur, it's called a fizzle). Obviously this is also far faster than an operator could react to the ongoing criticality. "
] |
[
"It all depends on the equation you use and the assumptions with it. ",
"The period equation has some assumptions built in which are true when you are close to k=1, but results in this large step change in calculated period at prompt criticality. One of the assumptions is that k is near 1 and that you can change the ln of certain values to the value itself. You end up with oddly behaving terms. ",
"Using different methods gets you more realistic results. ",
"A write up of this is on the web page below. I think the author passed away some time ago but he has some great articles on reactor physics and behavior. ",
"http://www.pipeline.com/~rstater/nuke1tttt.html",
"If you develop the reactor period equation you’ll find yourself stuck with natural logs in the equation you can’t get rid of. And the assumptions for getting rid of them are only valid close to criticality hence the apparent prompt critical change in reactor behavior. That’s not to say that being prompt critical doesn’t change the reactor behavior, rather it’s not this extreme jump. "
] |
[
"Can pi be expressed rationally in a non base 10 number system?"
] |
[
false
] |
[deleted]
|
[
"No.",
"The definition of a rational number is that it can be expressed as the ratio of two integers. So a number X is rational if there exists integers A and B such that X = A / B.",
"Note that this definition is completely independent of the number system. "
] |
[
"Regardless of the base, PI will always be irrational and transcendental. Properties of numbers hold regardless of base because the base is just a way of representing the number, not defining it. You could put 3 in any base you felt like and it would still be a prime odd number. "
] |
[
"No, it doesn't. At its heart, pi is a ",
" having to do with the geometry of a circle, not a literal count. ",
"For example, 17 is a prime number in any base. Changing the base doesn't change the fundamentals of mathematics, it merely changes how you express it. If you try to use pi as the base, all you've really done is make it extremely hard to express the number 1 on paper. ",
"EDIT: Gold? Gosh thanks!"
] |
[
"How exactly does radiation mutate DNA? Is there a specific mechanism behind it?"
] |
[
false
] | null |
[
"Directly, they cause faulty bonding within DNA strands",
". ",
"Indirectly, they ",
"create free radicals through homolysis",
"; the free radicals can then damage DNA."
] |
[
"Radiation is often described as either ",
" or ",
" radiation, depending on whether its individual particles have enough energy to knock an electron off an atom. Ionizing radiation has enough energy to break molecular bonds and cause the chemical changes that ",
"/u/airbornemist",
" describes, and by doing that, cause mutations and cancer. Non-ionizing radiation doesn't. (It's not a perfectly sharp distinction but it's still useful.)",
"An example of non-ionizing radiation is microwaves (as found in ovens, radars, wifi, etc.); going up the energy scale, infrared and visual light are still non-ionizing, but somewhere in the ultraviolet it crosses the threshold (skin cancer, germicidal lamps), and of course x-rays and gamma-rays are ionizing. They're all electromagnetic waves, but at different energies they can cause qualitatively different effects.",
"Non-ionizing radiation can still be harmful (microwaves can cook you) but they don't cause DNA mutations like UV/gammas do."
] |
[
"Pyrimidine dimers induced by radiation as mentioned above will produce mutations if not repaired by specific cellular mechanisms that exist to recognize these bulky bases and excise them. Pyrimidine dimers are suspected to be the main cause of UV-induced mutagensis leading up to melanoma.",
"Other forms of radiation can cause double-strand breaks, which if repaired by ",
"non-homologous end joining",
" will end up excising a number of bases before putting the ends back together. A double-strand breaks in an exon for a specific gene could cause loss of function mutations. "
] |
[
"In \"The Martian\", a stranded astronaut lives off a daily multivitamin and 1200 calories worth of potato. Is this actually feasible?"
] |
[
false
] |
In the book, it's explained that he uses potatoes for energy, vitamins for his actual "nutrition", and supplements with a meager amount of protein from his nasa-supplied rations. Is this viable in the short term? In the long? What would prolonged subsistence on that diet do to a human over the course of a year or longer?
|
[
"This would not be viable for more than a year or so, as the astronaut would run the risk of protein deficiency and phosphate deficiency. ",
"Although potatoes contain acceptable amounts of phosphate, it's in a form that the human body is unable to digest. Specifically Inositol PolyPhosphates also known as Phytic Acid.",
"Incidentally, potatoes also do not contain a form of niacin that is able to be absorbed by the human body. However the vitamin supplements would take care of Niacin.",
"Furthermore, potatoes do not contain complete protein, meaning they have a low level of certain essential amino acids that the human body can't synthesize.",
"Finally, potatoes do not contain vitamin B-12, except for trace amounts. The only reliable source of B-12 is meat from other animals. Or multivitamins in this specific case. B-12 is produced to a certain extent by bacteria in the small intestines of all animals, but in humans our intestines simply aren't large enough that we can absorb enough b-12 from this source. Therefore we're arguably forced to eat some meat in our diet. ",
"A way around the problem with phosphate and niacin is to ferment some of the potatoes, and make sourdough potato pancakes, which are pretty tasty if you ask me. Yeasts generate enzymes which can break down inositol polyphosphates, and others which convert niacin into absorbable forms. these are enzymes which humans don't have the genes for.",
"Also, the multivitamins would eventually run out. The danger at that point would be B-12 deficiency and niacin deficiency, as well as protein deficiency. At that point anemia from B-12 would become increasingly debilitating for our accidental martian colonist. ",
"This scenario would be a bit more realistic if the astronaut also brought dried uncooked beans with them, and possibly some kind of cereal grains such as corn. This would result in a diet with complete protein."
] |
[
"There are a lot of breakfast cereals and other prepackaged foods, that have added B-12. ",
"Also, as I mentioned a certain amount of B-12 is produced by gut bacteria. But anyone on a vegan diet should be aware of B-12 intake. ",
"Vitamin D deficiency can also be a problem for vegans, if they avoid milk products which are commonly fortified with D.",
"Aside from eating seafood, meat, liver, (or fortified milk,) the only way to get vitamin D is from sun exposure, and many people in modern countries simply don't spend enough time outdoors. ",
"I should have also mentioned iodine.",
"I'm not certain what the iodine level in martian soil is. But this could potential be a problem for someone depending on food farmed on mars. Plants generally require much less iodine than vertebrate animals do. I don't know much about the role iodine plays in plant metabolism, compared to that of animals. I do know that many food crops don't contain dietarily significant amounts of iodine. I don't know if this is the case with potatoes or not."
] |
[
"There are a lot of breakfast cereals and other prepackaged foods, that have added B-12. ",
"Also, as I mentioned a certain amount of B-12 is produced by gut bacteria. But anyone on a vegan diet should be aware of B-12 intake. ",
"Vitamin D deficiency can also be a problem for vegans, if they avoid milk products which are commonly fortified with D.",
"Aside from eating seafood, meat, liver, (or fortified milk,) the only way to get vitamin D is from sun exposure, and many people in modern countries simply don't spend enough time outdoors. ",
"I should have also mentioned iodine.",
"I'm not certain what the iodine level in martian soil is. But this could potential be a problem for someone depending on food farmed on mars. Plants generally require much less iodine than vertebrate animals do. I don't know much about the role iodine plays in plant metabolism, compared to that of animals. I do know that many food crops don't contain dietarily significant amounts of iodine. I don't know if this is the case with potatoes or not."
] |
[
"Is it possible to make a region of space absent of atoms?"
] |
[
false
] |
I know that a perfect vacuum cannot be formed because of quantum fluctuations. But also when a compressible fluid is shaken with enough force, vacuum-bubbles can form. Are these completely empty? Can they suck in extra matter momentarily? Are they truly absent of any atoms? Edit: Because of the inertia of the fluid, the atoms of the fluid cannot enter the vacuum, or at least so I'm told. But could they suck in lower density fluids before the higher density fluids?
|
[
"I know that a perfect vacuum cannot be formed because of quantum fluctuations.",
"Not necessarily, but that's perhaps a bit off-topic. ",
"But also when a compressible fluid is shaken with enough force, vacuum-bubbles can form. Are these completely empty? ",
"Nope. Caviation bubbles can have a quite low pressure, but they're not a perfect vacuum."
] |
[
"The best laboratory vacuum is about 1 nPa (nanopascal). Anything below 10 nPa is already quite state of the art."
] |
[
"Thank you. This is the answer I wanted."
] |
[
"How likely is it that digital data we have right now (of music, movies, pictures, etc) are preserved and recoverable thousands of years into the future?"
] |
[
false
] | null |
[
"Such a question is difficult if not impossible to answer. There are too many variables; what country are you talking about for example, as a war or conflict could destroy data storage infrastructure. Which company because of how likely they would be to preserve data through acquisition or financial ruin. What is the nature of the data; financial data is virtually assured of destruction within the next few decades simply for legal reasons. Ultimately the web of supposition becomes too thick for any level of reasonable conclusions to be drawn."
] |
[
"Probably not for very long. The lifespan of commercially produced DVDs and CDs is 25 years \"or more\" at best; those you can write at home are only 5 to 10 years. The problem is that the plastic in the disk will deteriorate and become impossible to read through. Perhaps we can imagine a future where the backing is recovered and transferred to a new medium, but speculating about such advanced recovery techniques is pointless. For our purposes the disks would almost certainly be unreadable after 100 years, much less thousands.",
"Flash drives seem to fare even more poorly. While they have no moving parts and are more reliable during their operational lifespan, their shelf life is only about 10 years for most types. Hard drives suffer from similar issues; the generally agreed upon conservative shelf life of a platter is 5 years so lets say 10 years the same as the flash drives.",
"Now none of this is to say that there won't be any data whatsoever recoverable after those estimates. However if we are talking about extreme time periods it would probably be completely unrecoverable. The best methods of data preservation is the active transfer to newer media by humans, perhaps by a company dedicated to the preservation of historical data (Google would be a good fit for that right now, who knows in 200 years). If we assume a hands-off approach then a particular storage system would have to be designed for the purpose. A quartz crystal etched DVD platter might be interesting."
] |
[
"I disagree; I don't interpret this as a question about the course of history, but rather about the aging properties of digitial media. I cannot claim to be comprehensively knowledgable about this subject, however, I was able to hunt down this link,",
"http://www.auldworks.com/awserv/archcdr.htm",
"which says:",
"The organic dye used in the recording layer of CDR's is not perfectly stable, especially when exposed to light. ",
" Since the format has only been around for about a dozen years, we don't yet know how well this corresponds to reality. Of the various dyes used, phthalocyanine (as opposed to cyanine) seems to be more resistant to deterioration from exposure to light. CDR's made with that particular dye may be a better bet for archival use.",
"(Emphasis mine)",
"This is but one example, however, it seems that it is almost 10 years out of date- so it may not be the most accurate information available. However, it nonetheless demonstrates a scientific answer to the question at hand; According to these estimates, CDRs will not last for thousands of years."
] |
[
"How does time dilation work in this scenario?"
] |
[
false
] |
The classic example is that Alice leaves earth on a spaceship traveling at some speed comparable to the speed of light. Bob however stays on earth. Special Relativity tells us that Bob will observe a clock traveling with Alice to tell time at a slower rate than one that Bob himself has. If Alice makes a round time and returns to earth, her clock will have shown less time to elaspe than Bobs. However, from Alice's point of view, could not the math be adjusted such that she is a stationary point and earth itself is moving away from her at some speed close to 'c', thus leading to Bob's clock moving slower than her's. Then, once they rejoin, bob's clock being the one the is behind? I've done an undergraduate degree in Physics, and im ashamed to say this is one question ive never known the answer too. It has been bothering me for years. I'm sure there is some error in my logic and I'm hoping that someone can put my mind to rest by pointing it out.
|
[
"Alice turns around and stops. She feels this acceleration. Accelerated and rest frames are not equivalent; Bob does not feel this. This breaks the symmetry."
] |
[
"Any inertial frame (one that is at a constant velocity) is equivalent to a rest frame. Alice saying Bob is moving one way is the same as Bob saying Alice moving the other way. However, accelerated frames are not equivalent to rest frames. Alice saying Bob is accelerating is not the same as Bob saying Alice is accelerating the other way, because one feels a force and one doesn't. You can't do an experiment to see who is moving and who is standing still, but you can do an experiment that determines who is accelerating and who isn't (for example, both Alice and Bob could have a pendulum and only the non-accelerating one would be vertical).",
"The exception is a falling frame, which is equivalent to rest."
] |
[
"There isn't a first or a second. Simultaneity doesn't exist when you compare between different reference frames. Differently moving observers don't necessarily agree on the order of events."
] |
[
"If you chip your bone from an injury, what happens to it?"
] |
[
false
] | null |
[
"This is also how moving teeth with braces works. Tugging on the tooth causes bone to break down in front of the tooth and new bone to be created behind it. So the tooth \"flows\" through your jaw to its new location. "
] |
[
"If the cells in the chipped piece are still healthy and it's in the right place, I suppose so. Broken bones will reconnect when set after all. "
] |
[
"Injury or no injury, bones constantly undergo breakdown and regeneration. There are two types of bone cells: osteoclasts break down old bone while osteoblasts create new bones. Hormones keep the actions of these cells in balance so that bones are kept in healthy density. Based on this, I would imagine that an injury will trigger wound repair mechanism, signal the osteoblasts to regenerate. With healthy diet, calcium and other minerals would also fill in, restoring the hardness of the bone tissue.",
"Edit: forgot a word."
] |
[
"How long has Earth had conditions that would be suitable for the survival of a 21st century person?"
] |
[
false
] |
Ignoring the complexities of getting to a said time period and ignoring the issues of surviving contact with the predators that exist in the time period, how far back can one go and survive? And I know that humans would probably succumb to any foreign illness, bacteria or virus eventually so for the purposes of this question, being able to live, move and observe for a couple of days is defined to be survival. So I presume humans could easily survive back to the Roman era, the Egyptian era and even pre-history because well humans lived then. How about back to when the common ancestor of primates and humans were alive? Jurassic? Triassic? Carboniferous? I know that some era, for example during Carboniferous, the atmosphere was rich in oxygen (thus the large insects and the large amounts of trees giving us coal today), but humans survive in high (and low) oxygen areas today with no major issues. Would they be able to make it to pre-cambrian and witness life leaving the oceans? Also as an additional question, what are all the issues one would face if he or she did go to some era many millions or billions of years ago?
|
[
"First, as for the illness factor, 21st century humans have the most advanced diseases",
"What expertise/evidence do you have that it's useful to think about a linear progression of 'advancement' for infectious diseases over history? It's perfectly plausible that the host/parasite battle was simply at a qualitatively different equilibrium at various points in the past, without a notion of better or stronger. Evolution is not an advancement ratchet."
] |
[
"First, as for the illness factor, 21st century humans have the most advanced diseases. Any contact a 21st century time traveller would make with ancient humans would almost assuredly kill them off.",
"Looking at geological timescales, the first place to look would be atmospheric oxygen levels. This lecture seems like a good place to start: ",
"http://www.globalchange.umich.edu/globalchange1/current/lectures/Perry_Samson_lectures/evolution_atm/index.html",
". From the link, it looks like the earliest atmospheric oxygen was around 2 billion years ago. There's a ratio between breathable air based on O2 vs. CO2 levels. I don't recall what. I suspect, then, that humans could at least in principle survive sometime before 2 billion years and onwards."
] |
[
"I think this would also depend if you would consider the \"snowball earth\" ice ago 650 million years ago survivable. Though the theory does now include the probability of an equatorial \"belt\" that was not covered in ice allowing for life "
] |
[
"Would it be possible to make a bio-luminescent tattoo?"
] |
[
false
] |
Over in it was noted that are pretty cool, followed by discussion of how to make them. One idea was an implanted LED system, another more interesting one was injecting bio-luminescent fungi or bacteria. Would it be possible to do this?
|
[
"I couldn't find anything that had already been done the way you're looking for (UV tattoos have already been mentioned and I think you're looking for something that glows under any amount of light). So, I went about trying to figure out how this could be accomplished. The idea of bio-luminescent organisms was interesting but it brings up the question of a \"food\" source for the organisms. If you aren't having to constantly feed them by putting some sort of media on the tattoo, then one would presume they would have to take it from your skin in some way, and I imagine it would probably cause some long term damage to the skin. Then you run into problems of keeping the culture exactly in the shape of the tattoo and I'm sure countless other issues that I can't think of.",
"The solution that seemed easiest (or at least the most effective and practical) would be a light emitting and heat sensitive nanoparticle. ",
"This article",
" (which you can't access the full version without subscribing to the service, but the abstract and highlights are available to anyone) seems to be looking at a way to construct chains of nanoparticles that will absorb heat and emit light. I'm not suggesting using this particle, in particular, because I also can't access the full findings, but with the ever increasing number of nanoparticles being able to be synthesized and all of the different properties coming from them, I would imagine there are others with similar properties already being produced or at least in the making. ",
"These particles could then be put into the \"ink\" and injected into the surface of the skin to produce the desired result. I will point out, though, that not a lot is known about most of the nanoparticles being developed today in relation to their toxicology. Nanoparticles tend to be very reactive with such a high surface area to volume ratio and this leads to many having unexpected dangers that have to be dealt with.",
"The conclusion I've reached, then, is that these tattoos are very possible but also most likely several to many years in the future, provided someone with the know-how and the resources decides to endeavor into tattoos.",
"P.S. Please correct me if I've misrepresented anything and I'll make adjustments as necessary, it's 4 am my time."
] |
[
"I presume this person would like something that glows in natural light versus UV light like the one you linked."
] |
[
"I presume this person would like something that glows in natural light versus UV light like the one you linked."
] |
[
"How are the polytropic constant, adiabatic index, and central density chosen for simple TOV neutron star models?"
] |
[
false
] |
I am a bit confused (and getting a little frustrated) trying to get a straight answer about these. For a bit of background, I'm a 3rd year grad student in physics but my program has not had a course offering in GR available since a year before I started. I have been trying to self teach basic GR, and have successfully derived the TOV equation in between all the normal grad student/TA duties. But, there are three things I am still quite unclear about: the polytropic constant used for neutron star models (I've seen 0.25, 100, 5.38x10 and 1), the adiabatic index gamma (I've seen 4/3, 5/3, and 2), and central density (I've seen 1 used, but generally is unstated). I'm just hoping to get clarity on these choices, in the hope of putting together a simple TOV numerical solver in Python that gives the 1.44 solar mass, approx 10 km result to see it with my own work, if possible. Cheers, stay safe and healthy, and thanks for any elucidation!
|
[
"But, there are three things I am still quite unclear about: the polytropic constant used for neutron star models (I've seen 0.25, 100, 5.38x109 and 1), ",
"Those are going to depend on the units you're working in. Someone might might use a dimensionless density (normalized by nuclear saturation), they might be using more 'physical' units like cgs, or they might have some other normalization that's convenient. It all depends on what units they want. ",
"the adiabatic index gamma (I've seen 4/3, 5/3, and 2), and central density (I've seen 1 used, but generally is unstated).",
"Depends on what you're modeling. Ultrarelavistic EoS will take 4/3, nonrelativistic will take 5/3. Ultimately we don't know the equation of state of neutron stars so when we use polytropes we're just trying to build a sorta realistic model that we can do something useful with. If you want to put some kind of exotic core in with hyperons or quark matter then you'll probably play with that polytropic index to try to reproduce energy densities which might come from other calculations. Piecewise polytropes are super common, see for example Lattimer or Steiner's papers. ",
"and central density (I've seen 1 used, but generally is unstated).",
"Central densities are boundary conditions. More massive stars will have greater central densities. You have to take your EoS and integrate all possible to central densities outward to get an MR curve. We don't actually know the central density, or the central pressure, or the EoS there. ",
"I'm just hoping to get clarity on these choices, in the hope of putting together a simple TOV numerical solver in Python that gives the 1.44 solar mass, approx 10 km result to see it with my own work, if possible.",
"I highly recommend starting with a code to do a WD MR using the classical equations of stellar structure if you haven't already. You'll learn a lot in the process which will help you when you try to do a NS with TOV."
] |
[
"Oh dang, thanks for the in depth reply!",
"I'll take a look at white dwarf modeling today!",
"Cheers!"
] |
[
"I was under the impression we do not really have a good understanding of the equation of state for neutron star and hence the polytropic index was not really well known. Not really my area though."
] |
[
"Why do elliptical galaxies have a low abundance of heavy elements?"
] |
[
false
] | null |
[
"There are a couple of moving parts here to keep track of to understand their observation: the age of the galaxy, the shape of the galaxy, and the chemical composition. I'll try to circle around them all.",
"The distribution of elements in a galaxy, sometimes called its metallicity, is a function of its age. This is because elements heavier than helium are created by stars -- either in their interior or when they explode. If you think in terms of generations, you have to have one generation of stars come into being, run through their life, and then explode to release heavier elements.",
"Elliptical galaxies are typically old, have lower mass stars, don't have a lot of stars forming in them, and are more 3-dimensional. Spiral galaxies are planar and the spirals are optically apparent because that's where new star formation is going on, and tend to be bright, massive stars. The brightness and mass of a star are negatively correlated to its lifespan, so they burn bright and go out fast. It's this process that releases the heavy elements that make Earth-like planets, so bars areas around the bars of galaxies are a good place to look.",
"As to the question of \"full size\" planets, I'm a little hazy. It seems plausible that the small stars that are typical of elliptical galaxies likewise come from the accretion of small stellar nebula, which would mean there's not a lot of mass left over for planet formation, even of gas planets. This would be consistent with the relatively low density of the interstellar medium in elliptical galaxies. It's also consistent with the theory that density waves are responsible for the spirals in spiral galaxies; the localized density increases that occur at the bars are simply a better place to make planets because in a universe that's mostly empty space, the bars are a notable exception."
] |
[
"Assuming you meant to say ",
" and not ",
" galaxies in your description I don't see the soundness of their reasoning. A typical elliptical galaxy has significantly ",
" metallicity than a spiral galaxy such as our own."
] |
[
"Older stars. In general, the earlier a star formed, the less metals (in the astronomy sense, meaning anything heavier than hydrogen) it will contain. The interstellar medium hadn't been \"polluted\" with as much of the heavier elements as it has been today.",
"I don't know what they mean by full sized planets though. IIRC, metallicity does affect the size of protoplanetary disks, but a terrestrial planet shouldn't have an issue forming.",
"I'm only an astronomy undergrad though, so perhaps someone more well informed will answer. "
] |
[
"Would the length of a Sidereal day change if the Earth had an orbital period of 6 months? How about a Solar Day?"
] |
[
false
] |
This is for an assignment in my astronomy class, as a heads up. This is not the direct question however (since I want to figure it out on my own, at least as much as I can). I know (or, think I know) that a Sidereal day is 23 hr 56 min, and is determined by the apparent location of the fixed stars. A solar day (I think) is simply 24 hours, and refers to the amount of time it takes for the sun to appear in the same spot. So, if the Earth had the same rotational speed, and the revolution took 6 months, would we have the same lengths of solar and sidereal days? Thanks!
|
[
"The sidereal day would remain the same. The relationship to the stars would not change.",
"The solar day would be shorter, because the earth would not have to rotate as far as at present to put the sun at the same position in the sky."
] |
[
"Currently, the solar day is about 4 minutes longer than the sidereal day.",
"If the earth traveled twice as fast along its orbit, it would only have to spend have as much extra to make up the distance. So the solar day would be about 2 minutes shorter than it is now. "
] |
[
"Thanks! To ask a follow-up, is there any way to tell how much shorter the solar day would be than the sidereal day? ",
"The way I understand it now, a year with the Earth moving around the Sun in 6 months would only be 182.625 solar days, is that correct? If so, does that make a solar day half of what it is during a 1-year orbit?"
] |
[
"BiologyDoes the CAS9 enzyme associate and disassociate with it's guide RNA like a regular enzyme with some known kd? or once it's formed the complex does the guide RNA just stay bound."
] |
[
false
] |
Also what are the ramifications of this for delivery of the enzyme to every cell in the body simultaneously.
|
[
"There's no such thing just \"staying bound\" if two things are not physically connected in any way. Guide RNA is a ligand like any other, so Cas9 will have some particular affinity for it, with accompanying on and off rates. ",
"This",
" paper states a Kd (so the lower the number, the higher affinity Cas9 has for the guide RNA) of about 10 picomolar. In the protein-nucleic acid field, we would call this a \"really fucking strong\" interaction. "
] |
[
"Thanks for the reply. I just wasn't sure about the structure of the complex since they look quite intertwined - it must be strong. 10 picomolar is crazy strong, getting into biotin/avidin territory. ",
"There's no such thing just \"staying bound\" if two things are not physically connected in any way. ",
"Do you mean that literally? It's surely possible for some case to exist..."
] |
[
"I think avidin/biotin is something like 1fM, but 10pM is nothing to sneeze at. One of the DNA-binding proteins I used to work on bound its highest-affinity sequence with a Kd of 200pM, which in our lab was extremely high affinity. "
] |
[
"What animal is this skull of?"
] |
[
false
] |
My family and I have found a skull during a hike in Germany, in the "Sächsische Schweiz" (near Dresden) to be exact. We're puzzled about what animal it could be. These are the pictures: Our first thought was a wolf or fox or something similar, but the molars aren't matching, and if it weren't for the fangs we would have thought it was some kind of deer. Can you figure it out?
|
[
"Some kind of ",
"wild boar",
"? That was my first thought on seeing those lower teeth at the front. Seems kind of long and narrow, but there's at least one like that in a google pic search."
] |
[
"That looks very much like it, indeed. The teeth and the size of the skull fit. Stupid that it didn't cross our minds, hehe. Thank you very much."
] |
[
"I was thinking about this a little more, and it occurred to me that it might be a female boar skull. That might explain why a lot of those google images look more robust and fierce, because hunters go for males, which would have bigger tusks and a head built for fighting other males."
] |
[
"How/where do we find naturally occurring mercury?"
] |
[
false
] |
Does it appear in pools on the surface of the earth? Inside geodes? Especially in large quantities, it just seems strange that it is naturally occurring... Curiosity piqued by the frontpage post with the cannonball floating in mercury. Thanks in advance! Edit: Thanks, Science! Great answers. Some pokemon related. You're the best.
|
[
"The liquid metal doesn't occur naturally, we extract it out of ores like ",
"cinnabar",
"."
] |
[
"Native mercury can sometimes be found with cinnabar, actually. "
] |
[
"Native mercury will locally be found as tiny blebs within pockets of cinnabar within mercury deposits or mercury-bearing polymetallic deposits. I've never encountered sufficient volume for pools to form, only the tiniest little balls clinging to a fracture surface or clogging some porosity. ",
"My understanding is that due to it's very high mobility (both physical and chemical), native mercury is at most a transient product of low-temperature alteration or decomposition or weathering of mercury minerals in shallow rocks, which is rapidly dispersed in the environment but replenished by ongoing reactions."
] |
[
"Aren't all intercalating nucleic acid dyes (such as GelRed, SYBR Green) mutagenic to some extent?"
] |
[
false
] |
If so, how are they "less" carcinogenic as compared to EtBr ?
|
[
"Sorry to be linking bitesizebio (I like them a lot, and they do list sources), they are not exactly peer reviewed, but they had a really good article - OK it was in 2007, but still relevant:\n",
"Ethidium bromide - a reality check",
". Apparently EtBr isnt really even that bad, and may even be less toxic in mice than SYBR safe.\nThere is also an excellent article ",
"on the alternatives to EtBr",
" (again bitesizebio) and their safeness is mentioned too. "
] |
[
"Clinical molecular technologist here, I think a fume hood is completely unnecessary. As a powder, EtBr is unlikely to become airborne and once it is in solution...well. And as for the correlation to cattle, I understand where you are coming from, but I think this is a logical correlation. The mechanics of DNA are similar in all life. Cattle are relatively very similar to ",
". If the molecule in question were a neurotoxin then I would say, \"Yes. There might be different toxic effects between even closely related species because of possible variances in binding site due to evolution.\" However, since the mechanics of DNA function and what types of cell barriers are in place are similar between cattle and us, I would say that this is useful information."
] |
[
"Being better safe than sorry is one thing, but it's also important to have a realistic understanding of a chemical's real hazard level. People go insane about EtBr in particular like it is some sort of superweapon. In some labs there are huge signs with skulls and bones wherever people use EtBr and everything that goes near has to be labeled accordingly. All this while some other known (and worse) mutagens, carcinogens, and toxins in general are kept at the bench and treated as if they were water. Having one specific chemical singled out as the one major threat and given disproportionate attention only hurts general safety. "
] |
[
"Suggestions for cheap fluorescent substance, to be used in science projects involving ultraviolet?"
] |
[
false
] |
I've plenty of projects to keep myself busy for a while, but I'm also adding to the queue. So right now I'm researching the N2 (nitrogen) laser, which appears to be one of the easiest DIY lasers to get lasing, but of course it's ultraviolet. So I need something to make UV visible. Some people recommend thick ultrawhite paper like the one used for business cards. There's some pigment in it that fluoresces. Also, some (permanent or erasable) marker inks appear to fluoresce. An idea (possibly dumb) I had a few days ago is to use riboflavin (vitamin B2) - the stuff that makes your pee neon yellow when you take multivitamins. It's fluorescent and it's cheap. So dissolving a B2 pill in water may just provide a cheap fluorescent liquid. Or not. Other ideas?
|
[
"Tonic water. The quinine in tonic water is fluorescent."
] |
[
"Buy a bunch of highlighters, pull out the spongy bit, drop it in water. Classic college party \"decoration\" trick"
] |
[
"Search glow in the dark or blacklight paints, or just go to a craft supplies store. There are plenty of products for a few bucks that are designed to fluoresce and in different colors too."
] |
[
"Do viruses stay in the human body forever after symptoms stop?"
] |
[
false
] |
I don’t remember where I had heard it from but I thought that it was impossible to get rid of all individual viruses from a body even after symptoms have stopped and the person has recovered. Is it true that all viruses stick around in the body for the rest of our lives after we’ve contracted it, just in very small, undetectable quantities?
|
[
"Some viruses become immortalized in the body, but not all. The biggest example is the Herpes family (Herpes 1&2, Chicken Pox/VZ, CMV, EB). They basically \"live\" forever inside a certain cell type and go through cycles of dormancy and activity usually mediated in part by the host's immune system and hormonal/stress/homeostasis. ",
"This is not true for all viruses. HIV can \"write\" itself into the production line of immune cells. Rhinovirus causes a bit of a cold then is usually cleared. Parvo (humans not dogs) is so bland most human hosts do not even realize they had it and cleared it. ",
"In these \"cleared\" versions, the host cell dies during the viral release. The body clears the dead cell and kills the viral particles stopping infection of another cell. An infected cell can get tagged and gets eliminated. ",
"Hypothetically, could there be a host cell not killed by a virus that is not being killed, but waiting for a break in the immune system? Yes. But, in this scenario, it is also possible for the cell to become cancerous in part due to viral infection -- CMV, EBV, Hep B (especially with concurrent Hep D), HPV...etc are associated with oncogenesis."
] |
[
"The vast majority of viruses are permanently eliminated from the body by the immune system. There are a handful of exceptions, but they are way in the minority. ",
"Examples of viruses that are permanently eliminated within days to weeks of infection: measles, mumps, rubella, yellow fever, polio, noroviruses, influenza viruses, rhinoviruses, coronaviruses, dengue, smallpox, chikungunya, Zika, West Nile virus .... and so on and so on. ",
"Some of these viruses can cause long-term damage even after they’re eliminated (Zika causes permanent damage to the developing brain of a fetus, for example), and some of them have very rare scenarios where they’re not fully cleared (immune deficient people infected with polio). But typically in normal people, if you survive long enough you will get rid of all of the virus. ",
"There are viruses that do set up long-term or lifelong infections. Most of the herpesvirus family such as chicken pox, Epstein-Barr virus, cytomegalovirus do this. HIV, hepatitis B, often hepatitis C. Some of the human papilloma viruses persist, at least as defective versions of the virus. A handful of others. ",
"You’ll notice that while the list of persistent viruses is short, few of them have effective treatments or vaccines (with exceptions, of course - hepatitis B, chicken pox). Their life style makes them harder to vaccinate against because they’ve evolved to deal with immune responses. That might be why people think it’s common for viruses to persist - people in the First World have simply forgotten about the long list of viruses that vaccines have effectively eliminated, and only remember the ones that are not yet controlled."
] |
[
"I agree with everything ",
"u/Grauzevn8",
" said! A good example of how retroviruses can stick with you forever is endogenous retroviruses in humans. Our genome actually carries the DNA of ancient, now extinct retroviruses that infected our ancestors and then after cleared the infection, their DNA stayed. The genes are most not transcribed but everyone has them! They have even been implicated in some diseases. Read more here",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1187282/"
] |
[
"Why does an epsom salt/magnesium sulfate bath help remove a splinter or other foreign body?"
] |
[
false
] |
My dog occasionally gets spear grass and splinters in his paws. If I don't check him after walkies and they become embedded under his skin, a little oozing bump appears and I have to go hunting with tweezers. A few times now I've had to go to the vet to get the things removed, and every vet I've talked to suggests giving an epsom salt (magnesium sulfate) bath to the affected paw in order to "help draw the foreign body out" and "enlarge the opening". I've done it and it seems to be effective, but it's hard to judge since I don't have a control to compare to. So I ask: Does giving an epsom salt bath actually help draw out splinters/foreign bodies? If so, how?
|
[
"Here are some guesses on my part:",
"the magnesium sulfate solution has a lower water potential than your body, so water flows outwards into it. This would necessarily be through the hole created by the splinter and might help to dislodge it.",
"Magnesium ions reduce swelling and are used by muscles so that could possibly help loosen the area around the splinter",
"warm water itself will help to dilate the wound around the splinter",
"That said I don't really know."
] |
[
"Osmosis",
"By submerging the paw in a bath solution that is saltier than conditions within the cells of said paw, you encourage the cells to flush out water (in an attempt to reach equilibrium between the saltiness of the water and the saltiness inside the cell). The excess deluge of fluid, flowing from inside the cells to outside, helps to dislodge the splinter. "
] |
[
"I don't know either, but I could only guess it would dehydrate your skin, causing it to recede from the splinter."
] |
[
"If deep water is colder than surface water, then why does the water at the bottom of a lake not freeze before the surface water?"
] |
[
false
] | null |
[
"A few key things to keep in mind:",
"- Nearly all heat transfer to/from a lake occurs at the surface\n- It takes increasingly more energy to mix water masses of increasingly different temperatures\n- The maximum density of water occurs at 4º C\n- Less dense masses float on top of more dense masses\n",
"So, for a typical temperate, dimictic lake during warm summer months, the surface waters of the lake are much warmer and less dense than deep water. The difference is usually so extreme that lake water becomes stratified such that the surface waters (\"epilimnion\") do not mix at all with the bottom water (\"hypolimnion\"). Moving into autumn, the epilimnion water begins to cool until its temperature equals that of the hypolimion. When this happens, the lake water begins to fully mix and lake stratification disappears. As atmospheric temperature continues to fall, moving into winter, the surface water of the lake continues to cool. However, since water is most dense at 4º C, water at this temperature sinks to the bottom while ",
" water, which is less dense, floats to the top. Lake stratification becomes reestablished with the coldest water in the epilimnion and warmer (4º C) water in the hypolimnion. The surface of the lake eventually cools to below 0º C and freezes into a solid; however, water has a unique property in that its solid phase is less dense than its liquid phase. Thus, ice floats on liquid water. When the lake begins to warm throughout the spring, the epilimnion reaches 4º C and again mixes with the same-density water of the hypolimnion. Upon continued warming, the surface waters -- which are now less dense than the 4º bottom water -- become once again isolated by lake stratification throughout the summer.",
"This is generally the case of most lakes, especially in temperate latitudes. These lakes are named \"dimictic\" because they mix twice during the year. There are many other mixing regimes for lakes and it all depends on their climatic setting and, sometimes, the chemical composition of their water. Some lakes mix once (\"monomictic\"), some mix many times (\"polymictic\"), and some don't mix at all (\"meromictic\")."
] |
[
"One of the things other people didn't mention pressure hinders water crystallization (One of the few exceptions. Most materials behave oppositely) and deep water tends to be under fairly large pressures. "
] |
[
"Short version: same reason ice floats. ",
"Most things, the colder they get the denser they get, so the cold things sink to the bottom. With water, the solid form [near standard temperature and pressure] is less dense than the liquid so if the water starts solidifying it rises. The maximum density is at 4 deg. C, which I think is the temperature of the entire ocean except for a thin layer on top. "
] |
[
"Why are soap bubbles different colors?"
] |
[
false
] |
We have a soap bubble machine, and the bubbles come out in a range of colors. With no immediately discernible system of size, position etc to explain why some are orange, som bluish etc
|
[
"This is actually a physics question. Light gets reflected by both the inside and outside surfaces of the bubble and because these surfaces are so close together the light waves from each at able to interfere with each other depending on the thickness of the bubble each wavelength (color) of light may interfere constructively or destructively causes us to see certain colors more than others. The angle of the light relative to the surface also effects the interference, which is why the colors change over the surface."
] |
[
"It might not be discernible to the naked eye but there is probably slight size differences. This slight difference causes the white light that enters the bubble to refract at different angles in each bubble. Here’s the cool part, different wavelengths of light within the white light will refract at different because their speeds will change in the bubble and this causes the white light to separate. This is how rainbows and Pink Floyd’s ",
" cover are made. What’s even neater is that one of these now separated light waves will hit the other side of the bubble at a special angle that will cause it to bounce back inside the bubble while the others pass through. This is called total internal reflection. This is how fiber optic cables work. The light that is bouncing around inside is what will give the bubble its color. There’s some more subtle things going on but this is the gist of it."
] |
[
"This phenomenon is also visible when liquids that don't mix with water are spilled into water (eg, gas, oils, etc). When you see a rainbow on a puddle, it's because the oil is very thin and not of a constant depth and so as its depth varies, you'll see a variety of rainbow colors."
] |
[
"Seeing as blackholes don't let light escape, how can we have quasars ?"
] |
[
false
] |
[deleted]
|
[
"The light escapes before it reaches the event horizon.",
"If too much matter is trying to fall in the black hole, it starts to bunch up in the accretion disk and to heat up.",
"When things get hot they emit light."
] |
[
"To add, you actually get a lot of the emission you're seeing as synchrotron (spinning electrons in a magnetic field) on top of the thermal. The magnetic fields around the black hole also create collimated jets, and emission can come from these regions as well. You can see a model of quasar and the various areas of emission ",
"here",
"."
] |
[
"This is a common question here - a search for something like \"light quasar\" would probably give you some high-quality answers.",
"I'll give a brief explanation though. The light from quasars isn't being emitted from the central supermassive black hole itself - it's actually being generated from an accretion disc orbiting it. Since the disc behaves as a fluid body rather than a solid one, it experiences ",
"differential rotation",
" (allowing for material to \"rub\" together). Since the black hole's gravity can accelerate this material to near-light speeds, a ",
" amount of friction is generated in the disc, causing it to glow incredibly hot. Also, since this heat generates radiation pressure, which acts against the inward gravitational pressure, you get a push-and-pull effect that ultimately balances out, allowing the disc to maintain a fairly steady luminosity and temperature instead of just having the black hole consume or eject the material. This is called \"thermal equilibrium\", and it's actually the same basic mechanism that allows main sequence (hydrogen-fusing) stars like the Sun to maintain a steady (more-or-less) radius and temperature over time.",
"Someone else might be able to expand on this (I didn't cover the relativistic jets, which requires some technical knowledge of magnetic fields that I don't remember too well), but that's the basic idea behind why they glow."
] |
[
"Possible to decompose function into non-sine waves?"
] |
[
false
] |
Fourier decomposition transforms a function into a superposition of sine functions with different amplitudes and phases. But is it also possible to decompose a function into other types of waves, for example triangular or square waves?
|
[
"/u/thephoton",
" is correct, and I'd like to mention a few details. If we have a set X, maybe a subset of the real line or ",
" or the complex plane or ",
", then there is a special set of functions called ",
"L",
"(X)",
". These are the functions f(x) from X to ",
" (or ",
") so that the integral of |f(x)|",
"dx across all of X is finite. So L",
"(",
") is all functions such that the integral of |f(x)|",
" from -infinity to infinity is finite. ",
"L",
"(X) is an infinite dimensional vector space, but unlike typical vector spaces you find in physics, engineering and computer science, it's elements are not things with \"magnitude and direction\" and we usually don't look at the vectors in L",
"(X) as arrays of numbers. For instance, e",
" is a vector in L",
"(",
") and x",
" is a vector in L",
"([0,1]), because these are functions whose square integral on their domain is finite.",
"But L",
"(X) is a special infinite dimensional vector space, it is an \"",
"Inner Product Space",
"\". In finite dimensional vector spaces over ",
" or ",
", it is often taken for granted that there is a dot product. This allows us to do a lot of things, like measure distances in the vector space, talk about orthogonality, use transposes and talk about things like ",
"Unitary Matrix",
" or ",
"Orthogonal Matrix",
", lots of nice things that are used all the time. Many vector spaces do not have a dot product like this and it is, in general, hard to find one. Pretty much the only vector spaces with dot products that are used are ",
", ",
" and L",
"(X). Almost every other infinite dimensional vector space does not have a dot product and have to figure out more abstract ways to talk about some of these things. ",
"In ",
", you can get the inner product of two vectors v and w by multiplying all the entries pairwise and them adding it all up. That is, if v=(v",
",v",
",...,v",
") and w=(w",
",w",
",...,w",
"), then an inner product is given by v·w = v",
"w",
"+v",
"w",
"+···+v",
"w",
". If we're in ",
" instead, then you just complex conjugate the entries of v and you're good. In L",
"(X) we can get an inner product in a similar way. If I have two functions f(x) and g(x) in L",
"(X), then clearly I can't multiply all the f(a)g(a) for a in X and add it all up, because that would almost surely diverge. But there's a sneaky thing you can do in math (most of the time). You can view many general kinds of summation formulas as integral formulas, just over a finite or discrete set rather than a continuous one. That is sums are just discrete integrals or, equivalently, integrals are just continuous sums. Sums and integrals are the same thing, it just depends on what set you're \"summing\" over. If you're ambitious, check out ",
"Summation by Parts",
" which is a summation version of Integration by Parts. Measure Theory is the field of math that views integrals, areas, sums, counting and probabilities as the same thing. So instead of multiplying and summing all things of the form f(a)g(a), we'll just integrate instead. That is, the \"dot product\" of f(x) and g(x) is the integral of f(x)g(x)dx over all x in X. We'll say that <f,g> is this dot product. If we have complex valued functions, the we complex conjugate the f(x) in this integral.",
"One important thing that you can easily in Inner Product Spaces that you can't do easily in others is write your vectors in terms of a given basis. For instance, if e",
", e",
",...,e",
" make an orthogonal basis in ",
", then every vector v in ",
" can be written as v=v",
"e",
"+v",
"e",
"+···+v",
"e",
", which is where we get the \"list of numbers\" v=(v",
",v",
",...,v",
") from, since this is a unique representation of the vector. The thing is, that the dot product allows us to easily find the coefficients v",
" as v",
"=v·e",
". If I have a set of functions {e",
"(x)} in L",
"(X) that are all mutually orthogonal, where t is some infinite index that can be discrete or continuous, then I can find coefficients in exact same way. If f(x) is a vector in L",
"(X), then I can get coefficients through the inner product F",
"=<f, e*_t_* >. Just as in the ordinary case, f(x) can be written as the \"sum\" of all F",
" e",
"(x). If t is a discrete index, then this is just the sum, if t is a continuous index, then f(x) will equal the integral of F",
"e",
"(x)dt over all ",
" in our index.",
"Let's talk about ",
"Fourier Series",
". If I'm in L",
"([0,1]), then the functions e",
"(x)=e",
" form an orthonormal basis for the space L",
"([0,1]). Note that any periodic function of any period can be seen as an element of L",
"([0,1]) after appropriate squeezing/stretching. The Fourier Coefficients of a function f(x) in L",
"([0,1]) are then just the coefficients F",
" = <e*_n_*,f> of this basis. The Fourier Series is then just us writing f(x) in this basis as the ",
" of F",
"e",
"(x) over n (it's a sum because the n is a discrete index over the integers). Write out the formula for the complex Fourier coefficients (from which the sine/cosine ones can be obtained) and write out the formula for the inner product <f,e*_n_*> and you'll see these as the same. You can view the formula for Fourier Coefficients as a glorified dot-product. Looking at the values F",
" is like looking at the \"list of numbers\" for the vector v=(1,3,5).",
"Let's talk about ",
"Fourier Transforms",
". Now we are in L",
"(",
"). In this vector space, the functions e",
"(x) = e",
" ",
" act like an orthonormal basis (their nature is more subtle, see ",
"Pontryagin Duality",
"). In the same way as above, we can get the coefficients to write a function f(x) in this basis by looking at the inner product F",
"=<e*_r_*,f> (note that the complex conjugation makes the the function e",
"). This is the Fourier Transform. The Fourier Transform of f(x) evaluated at r is just the coefficient of e",
"(x) in the basis expansion given by these exponentials. Then to write f(x) in this basis, we need to \"sum\" over all the F",
"e",
"(x) over r. Of course, r is continuous so this will be an integral and we have that F(x) is equal to the integral of F",
"e",
"(x)dr, which is the Inverse Fourier Transform. So the values of a Fourier transform are to the vector f(x) in L",
"(",
") as the numbers 1,2,4 are to the vector v=(1,2,4).",
"An important thing is then trying to find orthonormal bases for L",
"(X). This is where Differential Equations come into play. A fundamental theorem about Inner Product spaces is that the eigenvectors of a sufficiently nice linear operator will be an orthonormal basis (after some scaling). This is the ",
"Spectral Theorem",
". In L",
"-spaces a good source of nice linear operators are differential equations. Anything from Quantum Mechanics to Fluid Mechanics to Probability can offer nice enough differential equations on L",
"(X), and from their eigenvectors, we can find nice bases. ",
"Bessel Functions",
", ",
"Legendre Polynomials",
", ",
"Hermite Polynomials",
", the ",
"Spherical Harmonics",
" even the exponential functions all come from being eigenvectors to differential equations. All of these are non-sine wave decompositions. This is why Quantum Mechanics is basically just finding eigenvectors, we want to write out quantum states in terms of a basis that is behaves well under the given Schrodinger Equation. "
] |
[
"Yes it is possible. There are many sets of functions that form a ",
". Any of these sets of functions can be used as the basis into which you can decompose any well-behaved function. ",
"One such set that is closely related to square waves is the ",
"Walsh basis",
".",
"I don't know of any such set that resembles triangular waves.",
"It's more common, though, to pick a basis set that simplifies solving some type of differential equation. For example the ",
"Bessel functions",
" are often used when solving certain equations in a cylindrical coordinate system."
] |
[
"Conversely those sine waves can be decomposed back in to square waves so..."
] |
[
"Which greenhouse gas is worse? Methane or the fumes from burning it?"
] |
[
false
] | null |
[
"It burns into carbon dioxide and water. Methane is a worse ghg than carbon dioxide by itself. The epa's global warming potential numbers take the gases' lifetimes and decays into account.",
"But overall the question isn't burning it vs not, but retrieving it from underground storage and releasing it into the atmosphere in either state, raw or decomposed into carbon dioxide. "
] |
[
"The product of pure methane combustion is 1 CO2 and 2 H2O. for every methane molecule burned (with 2 Oxygens). Although water vapor is a greenhouse gas of it's own I will ignore it as the amount of methane burned should not really make a dent in the amount of water vapor in the atmosphere. So we can compare methane to CO2 directly (Oxygen does not seem to be a greenhouse gas per se, although it does form ozone which is also a small greenhouse contributor, but I will let someone else try to calculate the impact of that).",
"From this ",
"Wikipedia source",
" CO2 is a much lower contributor to the greenhouse effect than methane, especially on shorter timescales since CO2 lasts much longer in the atmosphere."
] |
[
"Luckily this touches upon my area of expertise, as one of the things I am doing right now is genetically engineering methanotrophs for precisely the purpose of removing the need to vent methane into the air and to produce value added chemicals from the relatively cheap methane.",
"Now, onto your question, why is an atmospheric increase in methane not contributing to methanotroph blooms. Disclaimer, my knowledge of methanotroph diversity is somewhat limited, so some things I say may not be 100% correct in terms of all methanotrophs. First, methanotrophs live in water, and consume their methane from said water. Because methane does not dissolve as well as CO2 or O2 in water, an increase in methane in the atmoshpere does not really increase the levels of methane in lakes and wetland soils, where most of methanotrophs live. In fact, ",
"it's thought",
" that most methanotroph methane oxidation occurs on methane produced nearby and not of atmospheric methane. Secondly, methanotrophs are not usually limited by methane. Many methanotrophs are competing for other resources with organisms that have faster utilized carbon sources. Take oxygen for example. Methanotrophs use oxygen to oxidize methane to form methanol, while sugar consuming bacteria oxidize hydrogen to form water. Because of the thermodynamic and kinetic differences, the oxidation of hydrogen occurs a lot faster, thus in places where oxygen is abundant, methanotrophs cannot compete with bacteria that are consuming sugars. So the move to areas where O2 levels are lower and aerobic respiration cannot move at fast levels, where they can compete. This happens for other nutrients as well, with the net result being that methanotrophs are more limited in those resources than methane for which they don't have to compete. So with no other changes, an increase in methane does not really contribute to extra growth."
] |
[
"Would it be possible to put an Atmosphere on Mars?"
] |
[
false
] |
I started asking this question on the TIL about the giant water quasar. The people there answered that it is most likely possible but never provided ways that we could do it. They said that Mars doesn't have a magnetic field preventing it from retaining an Atmosphere. Would thawing out the core give it a magnetic field? Would moving the planet closer to the sun (using directed asteroids or something) help melt the core? Would it be possible to have Mars in the same orbit of the sun as Earth, just on opposite ends of the sun? Sorry for the sporadic questions and shoddy grammar I at tired and intrigued.
|
[
"Magnetic fields on planets are generated internally. Yes, if some method of remelting Mars' core could be devised, and the different layers would rotate, then yes Mars could start generating a planetary magnetic field. Of course there is no way known to do this.",
"Moving the planet would not help with the core. Honestly most of the heat from the sun will just warm the surface and reradiate back out into space. That heat will never get below the crust.",
"If you moved Mars into the same orbit as the Earth I think you would need it to be 60 degrees in front of or behind the Earth's orbit to be stable."
] |
[
"If you moved Mars into the same orbit as the Earth I think you would need it to be 60 degrees in front of or behind the Earth's orbit to be stable.",
"You'd also need it to be much less massive than it is. L4/L5 orbits are only stable when one body is less than 1/25 the mass of the other."
] |
[
"Doing anything to the planet's core isn't really possible. There isn't any way to make Mars have a magnetic field or more gravity, so any atmosphere you give it will be stripped away on the order of 100 million years. But Earth itself will become uninhabitable in a few hundred million years too, and that's obviously a much longer timescale than we have any reason to care about. So it's not an issue.",
"If you make Mars a few degrees warmer, the frozen CO2 will sublimate into the atmosphere, thickening it and warming the planet further. Moving the planet closer to the Sun would accomplish this, but that's not practical. Even if you could move an entire planet, it could not be moved to Earth's orbit without destabilizing the orbits of both planets. You could much more easily warm the planet by producing greenhouse gases on the surface."
] |
[
"How would humans adapt to life in space?"
] |
[
false
] |
If humans had to colonize space, without ever finding a planet able to sustain life like Earth, how would humans evolve and adapt? What would be the most challenging obstacles? What kind of resources would become critical and how could they be created/supplied (I'm thinking oxygen, water; but what else?). How would humans' physiology evolve? As you can see, I'm just trying to wrap my head around all the changes and challenges such a scenario would entail for the human race on a scientific level.
|
[
"It would take millions of years to evolve and adapt to space life on another planet. It all truly depends on the planet we go to. If we go to a planet that is in the goldilock zone then theoretically we can go and live there, but if the planet is not then we would have to live there with special suits and other equipment. But as of right now, we cannot predict the outcome because is evolution. There are many ways for creatures to adapt to their environment and any guess we would make would just be a guess."
] |
[
"The answer is really complicated. For example, microgravity exposure causes calcium to leech from the bones of astronauts."
] |
[
"It's been shown that astronauts lose muscle mass and bone density just by being up in space a few months. If humans were to colonize space without having a planet to call home, the primary long-term goal would be some way to simulate gravity. ",
"However, if humans could indeed survive in zero gravity long term, I would suspect that the human physiology would evolve such that we would lose physical traits that were once important in a gravitational environment, eg endoskeletal structure. As far as adaptation, the most difficult thing I can foresee is getting used to moving and operating in zero gravity."
] |
[
"How does Antivirus software work?"
] |
[
false
] |
I mean, there are ton of script around. How does antivirus detect if a file is a virus or not?
|
[
"While there are many different styles of viruses and attacks, a lot of antivirus software deployed relies on a currently known threats or vulnerabilities. It is hard to defend against an unknown vector of attack (I use virus here generically), but some basic attacks/detections are as follows:",
"An easy way to detect if a file has been altered is the size of the file. Some viruses like to tack on their malicious code at the end of the file, and that is a dead giveaway when an antivirus scanner scans it. It compares the before and after sizes, and if there has been no modification by the user, it suspects some malicious activity.",
" ",
"Viruses often have a telltale signature that they use to infect your computer. It could be couple lines of assembly code that overwrite the stack pointer and then jump to a new line of code, it could be a certain series of commands that throw an error in a common application, or it could be using an unchecked overflow or memory leak to grab an exception thrown. Regardless, a lot of infectious software uses an reproducible exploit that is found on the target operating system or application, and those tell tale signs (because they have been spotted before) go into a huge database of known exploits and vulnerabilities. When your antivirus scans through it checks your programs for these malicious activities.",
"Since viruses like to use these known exploits, malware writers sometimes like to inject code into pre existing programs, like when you 'accidentally' installed that malicous program. These kinds of attacks typically inject code into dead regions of documents or files, and use a jump to go to the malicious code. To explain further, since blocks of memory are allocated to files, sometimes the very end of the memory block does not get used up, or in some cases, there are certain exploits within certain types of files that have legacy sections that are no longer used. This legacy section is a perfect spot to hide malicious code, since it does not increase the size of your program or file. An injection attack uses the initial startup code to 'jump' to the malicious code, and then 'jump' back, making it seem like nothing was ever wrong, and your program boots up perfectly. There are many many variations of this attack, but an antivirus program typically looks for those strange 'jumps' and code that looks like it doesnt belong in certain sections.",
"Some antivirus programs analyze the programs/files byte for byte, and literally compute the sha-1 hash of the item it is detecting. It stores every single hash for everything on your system, and if the program has been modified it will not compute the same hash (that is the whole point of a hash, it changes drastically if only a tiny bit of the program/file changes). This detection is flawed, because if the virus discovers where all the hashes are stored or the algorithm used, it can overwrite the 'secure' hash with the malicious one and the antivirus will never know.",
"Whenever you start your computer, or plug an external device into it (hard drive, cd, usb, there are core drivers or 'code' that runs to setup the connections from your computer to the external device. Some viruses exploit this when the connection is being established, and could either execute arbitrary code (instead of the connection code) or can become a man in the middle, where everything acts fine but the virus is actually the one creating the connection, as well as inserting its own code where ever it feels like. Since these threats can work themselves deep within the operating system and core functions, these are extremely hard to detect. If the deeper OS calls are not compromised, like the antivirus calls to the OS, then these attacks can be detected. If the whole system is compromised, then the virus is embedded so deep that you some times have no choice but to wipe it and hopefully do a fresh install. If the code that starts up your operating system is compromised, you have even bigger problems because wiping will not get rid of it.",
"Hopefully this is in layman enough terms for anyone to understand, I didnt rely on any references so please leave a comment correcting me (I will probably be asleep). Hopefully I will wake up tomorrow morning and everyone will understand the basics of computer infections and detections.",
"EDIT: Thank you for reddit gold, and bestof! My life is now complete!"
] |
[
"Excellent summary. You neglected to mention detection methods for encrypted viruses and metamorphic viruses though. As this expands upon your post, I'm not sure if I should add it as a reply to your post, or as a general reply to the original poster. Oh well.",
"Before going further, if you ware really interested in how virus detection works I would recommend \"The Art of Computer Research and Defence\" by Peter Szor. I found it to be an enjoyable and easy to understand read on the subject. Though a large portion of the book is just the collection of various papers he has published (and you can most likely find those for free).",
"There are ways to strip some basic encryptions, though the easy method to detect encrypted viruses is to let the virus do the work for you. This may be done by allowing the virus to run in a safe, emulated environment to decrypt itself. When it does so, it can be scanned for signatures. This is especially useful for oligomorphic and polymorphic viruses whose encryption changes from generation to generation.",
"Metamorphic viruses, or viruses that can change their form, are a bit more difficult to detect as they basically rewrite their code. Even if they are not encrypted, their signature can change. To detect these kinds of viruses other methods are necessary. Hashing and size measurements can be useful in narrowing down suspect files, but ultimately different techniques may be needed.",
"Such techniques may involves trying to strip junk instructions from the virus to attempt to get a leaner representation that may be able to be matched to a signature. It may attempt to track the suspect file's behavior to see if it acts like a specific virus. It may opt to see if the file contains information that would discount it as a virus (a negative signature if you will)."
] |
[
"I realize I missed this, and could not have better explained it myself. Very nice job covering the aspects I did not, and Szor's writings are definitely an excellent source if anyone would like to dive deeper into the subject of computer security. Up vote for you!"
] |
[
"Is there a cumulative benefits of flu shots?"
] |
[
false
] |
Is there any cumulative benefit to getting flu shots every year, beyond the current year? (ie if I get a flu shot every year for 20 years, am I any better protected than if I get a flu shot this year without any prior shots?) Maybe almost the same question, but also, what effectiviy, if any, does a flu shot from a previous year continue to have? It's not easy to find this kind of info on the intereebs, what with all the back and forth on anti vaxxers.
|
[
"Stay with me, this is kind of like Cliff notes of immunology 101.",
"There is only benefit when receiving the same particular shot. ",
"The influenza virus is what the flu shot is attempting to protect you against. The trick is that the influenza virus mutates itself in order to outsmart your immune system, it does this by changing proteins on the outside of its coat. ",
"By changing these proteins, the antibodies produced by your adaptive immune system aren't as effective at attaching to the virus and slowing its passage in the blood stream. ",
"Note that your adaptive immune response is exponentially more effective than your innate immune response. Your innate immune response wipes out 99% of infections you would get, including cancers, but if a virus evades it and enters your cells then you are going to suffer the ill effects of illness. The virus will then replicate by commandeering your cells and destroying them unleashing many copies of itself.",
"The body will catch wind of these proteins and develop antibodies. Proper antibodies mean that we stand a good chance at stopping viral movement in the blood stream. Particularly IgG antibodies. ",
"TAKE HOME: a vaccine gives you the surface proteins or it crippled the infectious part of a viruses so you get a strong adaptive immune response. Each time you get a vaccine your adaptive immune cells (B and T cells) create stronger antibodies to better attack a virus by identifying its surface receptors. This response improves with each pass, however influenza mutates it's surface proteins, e.g, H1N1 becomes H1N2",
"WHY IS IT HIT OR MISS? \nEpidemiologists try to predict every year what influenza virus will exist based on migratory patterns of birds since they are responsible for the movement of the virus. Occasionally they fail to predict the mutation and thus a vaccine confers little advantage. ",
"Note: influenza kills something on the order of 10,000 people in the US every year. When people say they got \"the flu\" chances are they got a viral infection, but not influenza. Influenza sends folks to the hospital. "
] |
[
"Are there any statistics known on this hit or miss? How often do we 'guess' correctly the new version of the mutated virus?"
] |
[
"https://www.cdc.gov/flu/professionals/vaccination/effectiveness-studies.htm",
"It varies from year to year.\nThe most recent failure in memory would have been the H1N1 outbreak within the past decade. \nIn all matters of public health I would defer to the data collected by the CDC. "
] |
[
"Could a gun fire in the vacuum of space?"
] |
[
false
] |
Google seemed to almost unanimously say yes, but nothing was sourced and I don't know how reliable the guesses were. So, askscience, could you shoot someone in space with a standard firearm?
|
[
"Yes, the CCCP did it once, the space station Almaz had a cannon on it and they test fired it.",
" http://en.wikipedia.org/wiki/Almaz",
" "
] |
[
"Yes, the majority of guns have the oxidizer built into the cartridge so don't need oxygen to fire. Some can also fire under water."
] |
[
"Some poor bastard 50 million lightyears away will get a bullet in the back a very long time from now."
] |
[
"Is there a delay between a star beginning fusion and light becoming visible?"
] |
[
false
] |
So I've heard on various science programs that a photon of light takes thousands, even millions of years to reach the surface of the sun because it bangs into so many particles along the way. If I were an observer one AU away from a star the size of our sun, looking at it just as fusion begins would it take millions of years for light to be visible to me?
|
[
"Aren't protostars usually surrounded by gas clouds and hard to see before fusion begins?"
] |
[
"Aren't protostars usually surrounded by gas clouds and hard to see before fusion begins?"
] |
[
"Fusion is reponsible for ",
" stars manage to remain hot and bright",
"There was an interesting calculation before people knew about fusion: If the Sun would get its energy purely from gravitational collapse it would be ~100 million years old. Short compared to its lifetime (and so short that it was in disagreement with age of Earth estimates), but far from instantaneous."
] |
[
"How do scientists know where the mutation in a gene originated and how long ago the mutation occured?"
] |
[
false
] |
Title
|
[
"The way they track these mutations in time is by creating a huge family tree and puzzling the way back up. If a group of people is known to have split up e.g. 5000 years ago and none of them have that mutation, it likely occured less than 5000 years ago in the main population.\nIt's not perfect but with enough samples and meta info we can create accurate philogenetic trees.",
"It also helps a lot that we have found a couple mummified people with intact DNA from different places and points in time."
] |
[
"Without getting into excessive detail: today we genotype or sequence DNA from many many people. We can cluster people into groups that have a higher than expected number of shared alleles (genetic variations). This happens when people share common ancestry because at some point their ancestors were isolated to an area, or had moved to a new area and founded a new settlement that grew, establishing a large group with common genetic history. This is effectively what the ancestry DNA companies do. You can plot that information on a map and to show where those people groups are most prevalent.",
"That same information can be used to make something like a pedigree of people groups, tracing the divergence and convergence of populations in time and geography.",
"Given that information, you can then ask where in that tree is the allele most common? Can you trace a line between people groups that carry the specific mutation? Sometimes you can, and you can estimate when and where the mutation first occurred. In some cases, you simply can’t — perhaps because the mutation is very common and doesn’t seem to have a strong correlation with any people group."
] |
[
"What do you mean where it originated? A mutation is stationary in a gene and doesnt move. Should a mutation occur in another location it is a seperate mutation. You cannot really time a mutation, however you can determine the homology between genes and their differences (could be referred to as mutations) and somewhat give an estimate when the genes went from one to become two seperate similar genes. If you are taking somatic mutations like those given rise to cancer, they always appear mostly within a decade at the most before the cancer becomes apparent to the individual."
] |
[
"How can we talk about vapor pressure before boiling point?"
] |
[
false
] |
In thermodynamics chapter we used to say in change of state for ex: h2o liquid at 50 degrees>h20 liquid at 100 degrees .then h20 liquid at 100>h2o gaz at 100 The 2 reactions are at 1 atm pressure, and temperature are in degrees celcius But now we're taking vapor pressure, hence there could be gaz before achievieng the boiling pt (regardless of the combination of pressure and temperature at wich boiling can occur). I understood the concept of vapor pressure, but here is my question if in a room there is a pressure of 1 atm, hence boiling will occur ony at 100 degree celcius, could there be vapor in the first place to talk about vapor pressure? If yes, then why we used to say that h2o l at 50 degrees>h20 liquid at 100 degrees then in turns then h20 liquid at 100>h2o gaz at 100( i am taking water in this conditions as an exemple), and vaporization can occur at specific conditions, because at any instant there could be vapotization when water turn into vapor or gaz. Any help would be appreciated
|
[
"The water in the liquid phase is always in equilibrium with water in the vapor phase. This is why water can evaporate even when it's not boiling. ",
"The pressure of the water vapor in equilibrium with liquid water is the vapor pressure, and its value depends on temperature. The boiling point is the temperature at which the equilibrium pressure equals atmospheric pressure."
] |
[
"I'm not sure of your question. Are you asking about what goes on at the horizontal line?... You are clearly adding energy and yet the temperature does not change. Its weird. The full description why is kind of complicated and requires Physical Chemistry concepts, not just general chemistry. This website kind of touches on it... ",
"https://web.stanford.edu/~peastman/statmech/phasetransitions.html",
" . Sorry if I'm not answering your question"
] |
[
"Evaporation also breaks hydrogen bonds, as those exist all over the place in the liquid and molecules are leaving the liquid in evaporation.",
"Vapor pressure is just the pressure (of a pure gas) at which the same number of molecules on average are leaving the liquid phase as entering it again from the gas. There are always molecules breaking off from the surface of a liquid, but also some in the gas phase that strike the liquid and stick back to it. The pressure of gas that allows the rates of leaving vs. returning to be equal is the vapor pressure.",
"Without belaboring the details, the difference between evaporation and boiling is that once the vapor pressure is equal to the ambient pressure, a tiny bubble of gas that forms in the liquid will ",
" rather than ",
".",
"Below the boiling point (for a given pressure), if a small bubble forms in a liquid, then it will be full of pure gas at approximately the ambient pressure. As the vapor pressure is lower than the ambient pressure, more gas molecules that hit the walls of the bubble stick to it and return to the liquid phase than liquid molecules leave the walls of the bubble and join the gas phase, and the bubble shrinks.",
"Below the boiling point, molecules still leave the surface of the liquid, so long as the ",
" of the pure gas over the liquid is less than the vapor pressure. When the partial pressure is equal to the vapor pressure, evaporation stops. We call this \"100% relative humidity\". If the partial pressure is greater than the vapor pressure, we get condensation instead of evaporation!",
"At the boiling point, the vapor pressure is now equal to the ambient pressure, so a small bubble will neither shrink nor grow, as rate of molecules leaving and entering the gas phase is equal.",
" you add more energy at this point, then it will force ",
" molecules into the gas phase, driving the bubbles larger. The temperature doesn't increase, as that extra energy simply drives the equilibrium towards more particles leaving the gas phase. As breaking those hydrogen bonds takes energy to do, that extra energy you've added has now gone into converting more liquid into gas and the temperature remains unchanged.",
"To summarize, below the boiling point:",
"Once you're at the boiling point, it looks kind of like this:",
"The full details are ",
" complicated (surface dynamics, the pressure in the bubble is radius-dependent, but so is the surface energy, ",
" the vapor pressure is what it is, etc.), but this is the basic gist of it.",
"TL;DR: the rate at which molecules leave the liquid phase for the gas phase vs. the rate at which they return from the gas phase to the liquid phase is what gives us a boiling point."
] |
[
"Two plant-life questions about Mars."
] |
[
false
] |
[deleted]
|
[
"Genetic engineer specializing in plants here; this is my life career-goal. Although I previously believed that genetically modified microbial life was the best way to generate an atmosphere for Mars, I began taking astronomy courses to become better educated regarding potential hurdles.",
"I'm not certain (I'm never certain, it's a job requirement) but I now believe that deliberately impacting the planet with large asteroids would be the quickest, easiest, and most permanent way to make Mars habitable. If the effects of these impacts would be too unpredictable, I expect a more \"surgical\" effect could be implemented with the careful placement of nukes. Basically the planet needs a large influx of kinetic energy to reignite the core if extended and immediate change was desired (e.g. re-emergence of oceans, habitable atmosphere within decades but lasting for millions of years). We'd need to make sure also that whatever force was used directed resultant debris not captured by Mars' gravity toward Jupiter for clean-up instead of raining doom upon Earth.",
"The lack of atmosphere would definitely affect plant life--this is visibly true when considering mountain tree lines on Earth. Extended exposure (to wind, sun, etc) without adequate humidity makes it damn near impossible for life-forms to survive. Pressure-wise, I'm not sure what to tell you because I know that plants can handle denser atmosphere, and I think the primary problem with \"less dense\" would be \"can they breathe\". If the atmosphere was \"less dense\" but was nearly all CO2, plants would definitely get by. CO2 is just a fraction of a fraction of our atmosphere, and we're clearly not hurting for plant life. In fact, many recent experiments have involved funneling excess CO2 into valleys or other relatively closed systems to see if plant growth rate increases. Some scientists hypothesize that plants were able to achieve such massive size in prehistoric times because of higher atmospheric CO2. The nitrogen, however, would concern me. ",
"Plants don't use atmospheric nitrogen as a general rule, only fixed (in the soil) nitrogen sources. However, as I'm unsure of soil nutrient content, I think your best \"starter\" would be legume species inoculated with their bacterial symbiotes and vascular arbuscular mycorrhizal fungi. A super-efficient strain of the rhizobacteria would work wonders. An example of a leguminous plant that tolerates alkaline soil (off the top of my head) is the locust shrub/tree, so I don't believe the pH would be a problem (and pH is one of the easiest things to amend, in any case). I am certain many good legume candidates exist--life that exploits niches wins the evolutionary game.",
"As for sunlight, many plants deliberately build in mechanisms to prevent \"too much\" sun causing tissue or DNA damage. These could merely be bred out or excised (for the simpler traits). I think the vast majority of plants could adjust (or be adjusted) to low-light conditions. Safe bets are most forest plants, which are already equipped for survival in a shaded understory. You'll notice many understory plants have \"green top-side, purple under-side\" coloration to help achieve maximum efficiency from low light--introducing this pigmentation in crop species could certainly help. Because Mars' day length is very similar to ours, and plants usually flower based on dark period rather than light, I expect we would have no trouble with flowering/fruiting plants producing correctly.",
"The ",
" concern, as far as I can tell, is liquid water. Very few of our most extreme microbes can survive and multiply in ice. Virtually all life on our planet ",
" liquid water. Remember that on our planet, water and warmth preceded life (possibly several times.) Thus, I believe terraforming operations which plan to \"seed\" a frozen planet with GM microbes and wait for atmosphere to appear are terraforming operations devoid of biologists. This is why I now believe we really need to reset Mars' core to get things going--life on any scale which will not take millions or billions of years to colonize the planet will require liquid water. I genuinely do not believe that algae or other microbes will do the job until this step has been achieved.",
"Sorry for the rant, I get excited. I am obviously more of a biologist than a geologist or astronomer, but I am actively attempting to rectify those gaps in my knowledge. If you've already got water in 2243, you're set--grow ",
"."
] |
[
"So, can I ask you to be a little bit more clear about what we ",
" for certain here?",
"I mean, stuff like this",
"The lack of atmosphere would definitely affect plant life--this is visibly true when considering mountain tree lines on Earth.",
"-",
"I think the primary problem with \"less dense\" would be \"can they breathe\". If the atmosphere was \"less dense\" but was nearly all CO2, plants would definitely get by. CO2 is just a fraction of a fraction of our atmosphere, and we're clearly not hurting for plant life.",
"Safe bets are most forest plants, which are already equipped for survival in a shaded understory",
"I wouldn't really expect you to cite, because they're pretty straightforward and unextraodrinary claims.",
"But this:",
"I now believe that deliberately impacting the planet with large asteroids would be the quickest, easiest, and most permanent way to make Mars habitable. If the effects of these impacts would be too unpredictable, I expect a more \"surgical\" effect could be implemented with the careful placement of nukes. Basically the planet needs a large influx of kinetic energy to reignite the core if extended and immediate change was desired (e.g. re-emergence of oceans, habitable atmosphere within decades but lasting for millions of years). We'd need to make sure also that whatever force was used directed resultant debris not captured by Mars' gravity toward Jupiter for clean-up instead of raining doom upon Earth.",
"Thus, I believe terraforming operations which plan to \"seed\" a frozen planet with GM microbes and wait for atmosphere to appear are terraforming operations devoid of biologists. This is why I now believe we really need to reset Mars' core to get things going--life on any scale which will not take millions or billions of years to colonize the planet will require liquid water. I genuinely do not believe that algae or other microbes will do the job until this step has been achieved.",
"with all due respect, sounds like something you thought up on your free Saturday afternoon. Is there any science out there to suggest anything like this can be done? Any citations? Otherwise it's just speculation, which we'd really like to avoid in top level comments."
] |
[
" This is reply post 1 of 2.",
"I split this into two posts because of length. For the second post, I thought people in this thread might like the snip of information without paying $140 each.",
"This paper is nearly twenty years old, but sadly still a good reference for terraforming theories because we've focused on it so little in the mean time. (",
"Abstract",
",",
"Full text online",
"). You'll note that while the paper is primarily on ",
" an atmosphere, the section on starting the change states:",
"Activating the Martian hydrosphere in a timely fashion will require doing some violence to the planet, and , as discussed above, one way this can be done is with targeted asteroidal impacts. Each such impact releases the energy equivalent of 10 TW-yrs. If Plowshare methods of shock treatment for Mars are desired, then the use of such projectiles is certainly to be preferred to the alternative option [4] of detonation of hundreds of thousands of thermonuclear explosives. After all, even if so much explosive could be manufactured, its use would leave the planet unacceptably radioactive.",
"The use of orbiting mirrors provides an alternative method for hydrosphere activation. For example, if the 125 km radius reflector discussed earlier for use in vaporizing the pole were to concentrate its power on a smaller region, 27 TW would be available to melt lakes or volatilize nitrate beds. This is triple the power available from the impact of 1 10 billion tonne asteroid per year, and in all probability would be far more controllable. A single such mirror could drive vast amounts of water out of the permafrost and into the nascent Martian ecosystem very quickly. Thus while the engineering of such mirrors may be somewhat grandiose, the benefits to terraforming of being able to wield tens of TW of power in a controllable way can hardly be overstated.",
"Additionally, Mars doesn’t have much of a magnetic field to protect the planet from solar winds. According to the dynamo theory of planetary magnetism, it has enough rotation to generate a magnetic field, and because of its inner planet “rocky” composition, it should have the right metallic core (I mean, the planet is covered in rust, we know it should have plenty of iron.) The missing piece, according to dynamo theory, is interior heat to restart convection. This is another reason I think an impact would help make the planet more habitable—it should generate a magnetic field (or disprove a popular theory.) This in turn should prevent stripping of any atmosphere we’re able to generate.",
"Citation",
" and quote:",
"The dynamo theory of planetary magnetism indicates that Mars may have had a dipole moment of about one-tenth of Earth's when it was first formed (Schubert and Spohn, 1990). The rotation rate Of Mars is approximately that of Earth and is thus sufficient for the operation of this initial dynamo. The other necessary ingredient of a convection driver in the core was supplied by heat left over from the accretion of the planet, which may have been effective for up to a few billion years. If such a field did indeed exist, evidence of it may still be present on the surface in the form of magnetized rocks and crustal regions like those observed on the Moon. No observations indicating the presence of such fields have been reported other than the aforementioned SNC meteorites' magnetization.",
"From the same paper:",
"Phobos 2 did detect significant fluxes of planetary ions (mainly O+, as at Venus) that had been scavenged from Mars by the passing solar wind (e.g. see the Naturespecial issue mentioned above). The details of the acceleration of these ions are not completely understood, but the electric field in the solar wind is expected to remove ions formed in the upper atmosphere that extends above the 'obstacle' boundary into the magnetosheath and undisturbed solar wind. The observed rates of escape for the oxygen suggest that the solar wind scavenging process has the potential to remove all of Mars' present inventory of atmospheric oxygen over the next 108 years. These observations also suggest that the solar wind interaction must have played some role in the Martian atmosphere's evolution over the past 4.5 billion years, or at least after the thermally driven planetary dynamo ceased to operate."
] |
[
"What revolves around what in the universe?"
] |
[
false
] |
So, satellites revolve around planets, planets revolve around a star, the star revolves around what? The center of the galaxy? If so, then what does the galaxy do?
|
[
"Moons orbit planets.",
"Planets orbit stars.",
"Stars orbit the galactic center.",
"Galaxies orbit other galaxies in their local group (if they have one).",
"Note that technically everything orbits a common center of mass which is not the actual center of the object. So, for instance, the earth and moon orbit a point that is about 1,000 miles beneath the earth's surface (about 25% of the way to the earth's center). ",
"This happens to the sun too and is how we find planets orbiting other stars. Since we cannot see those planets directly we can infer their presence because they make their star wobble a bit as they both orbit the common center of mass.",
"So too for galaxies although with a bunch in our local group I am not sure how that complex dance plays out."
] |
[
"thank you sir"
] |
[
"yes, the stars rotate around the center of the galaxy.",
"galaxy can be organized into clusters and they all affect one another. sometimes they are denser in the center and in that case they rotate around that, in other cases they all kinda just rotate around each other "
] |
[
"Why is Newtonian mechanics the most widely taught version of classical mechanics in the school room?"
] |
[
false
] |
[deleted]
|
[
"The other formulations of classical mechanics need a somewhat higher level of mathematical background (calculus of variations, PDEs), so it’s easiest to teach Newtonian mechanics at the introductory level.",
"And while Lagrangians and Hamiltonian are very aesthetically pleasing and convenient, they have some ugly disadvantages, like not being able to handle dissipative forces very well. Newtonian mechanics will work for a broader class of forces, which can’t necessarily be written as potential energy functions.",
"Newton gets a lot of credit compared to Lagrange or Hamilton, for example, because Newton came up with his formulation of classical mechanics about 100 years before they did."
] |
[
"Just to add emphasis to your post, Hamiltonian and Lagrangian mechanics are ",
" of Newtonian mechanics. They say the same thing in different ways."
] |
[
"Those other scientists did not independently \"come up with the same thing\" (putting Leibniz aside). They after-the-fact reformulated the \"fundamental\" physical theory of Newton, built from simple, empirical principles, into other mathematically useful forms. The physical theory is really Newtonian, although there are several equivalent ways to express it."
] |
[
"\"Breakfast the most important meal of the day\"?"
] |
[
false
] |
I'm wondering if this is indeed the case? Does it matter (contribute to weight gain) if I have my first meal in the early afternoon, for example? Also, will eating a big dinner before sleep contribute to weight gain "because I am not burning off the calories"? I realise there's fitness subreddits but I am after a scientific answer. Thanks!
|
[
"This thread has 79 comments right now, and I see few references to actual scientific papers. For this reason, I feel like I need to describe the process of answering your question (which most people didn't attempt) well, while actually trying to answer it. Try using this outlined procedure on your own.",
"Step 1: Go to ",
"PubMed",
"Step 2: Search for ",
"\"breakfast\"",
"...ok... 5288 results. No way of getting a comprehensive idea of the literature myself, so...",
"Step 3: Go to the right, near the top, where it says \"Filter your results:\", and click on \"Review\"",
"Step 4: Ctrl+F \"Free\" until you find an appropriately-titled article",
"\"A consensus document on the role of breakfast in the attainment and maintenance of health and wellness\"",
" sounds pretty useful.",
".",
"Note that this process isn't foolproof, but it's at least as good as having 15 (the number of authors on the paper I linked to) ",
"/r/askscience",
" experts answer you the same way, and provide 50 references for further reading.",
".",
"Anyways, when reading a review, my eye always scans for claims of experimental evidence. (I'm not knocking epidemiological/observational evidence and here... it's just that, funding to do an intervention/experiment in humans usually isn't there unless the \"cheaper\" studies already support the hypothesis.)",
"However, the most supportive evidence comes from observational studies, whereas only few randomized intervention trials have been published on this topic, mostly lasting for short periods and performed on limited population groups. The vast majority of the intervention studies available thus far is, anyway, consistent with the positive effects of breakfast (particularly if rich in grains and low glycemic index carbohydrates) on the reduction of risk factors for cardiovascular diseases and diabetes, namely glycaemia, insulin levels, cholesterolemia, and overweight (12, 33- 35).",
"OK. Still with me? ",
"FYI: From here on out, I'm pretty much just reading papers' abstracts.",
"One reference is \"The effect of breakfast cereal on diet and serum cholesterol: a randomized trial in North Karelia, Finland\" Well, serum cholesterol correlates with, and has a causal role in, some aspects of good health (e.g. risk of cardiovascular disease) but is a bit indirect -- you'd rather know about e.g. weight loss itself, rather than a correlate of weight loss. Two other studies sound better, given your interests...",
"\"Weight loss is greater with consumption of large morning meals and fat-free mass is preserved with large evening meals in women on a controlled weight reduction regimen\" looks useful for answering your first question. Now that I know exactly what I want, I hop on ",
"GoogleScholar",
" and do a ",
"search",
". Good! The paper is available for free ",
"(pdf)",
"!",
"...just reading the abstract, it uses a powerful experimental design, but is a very small study and isn't as clean as I'd like. When having people eat all their calories in only two meals a day (in either in the AM or PM... i.e. skipping dinner or breakfast, respectively)...",
"[...] ingestion of larger AM meals resulted in slightly greater weight loss, but ingestion of larger PM meals resulted in better maintenance of fat-free mass. Thus, incorporation of larger PM meals in a weight loss regimen may be important in minimizing the loss of fat-free mass.",
"Next up is \"Low glycemic index breakfasts and reduced food intake in preadolescent children\" which, again, is available via GoogleScholar ",
"(pdf).",
" They feed kids different breakfasts, having e.g. high vs. low glycemic indexes (\"GI\"), then monitor how much they eat at a buffet lunch. Here's the conclusion to possibly the longest abstract I've ever seen:",
"These results suggest that low-GI foods eaten at breakfast have a significant impact on food intake at lunch. This is the first study to observe such an effect in a group of normal and overweight children and adds to the growing body of evidence that low-GI foods may have an important role in weight control and obesity management. The potentially confounding effect of differences in the macronutrient and dietary fiber content of the test breakfasts warrants additional study. In addition, the impact of GI on food intake and body weight regulation in the long term needs to be investigated.",
"~~~~~~~~",
"At this point, I'm not impressed by the experimental data. So I try another strategy: Going to PubMed, and doing a filtered search for clinical trials. I found this free paper, \"An increase of cereal intake as an approach to weight reduction in children is effective only when accompanied by nutrition education: a randomized controlled trial\" ",
"(pdf).",
"Here are the highlights:",
"Children were randomly assigned to one of four different treatments. They were stratified into 4 groups with similar age, height and BMI percentile and same gender, in order to create groups with similar baseline characteristics. ",
"\"RTEC\" is ready to eat cereal.",
"Children in group 1 consumed one serving of 33 ± 7 g of RTEC (Kellogg's de Mexico, Querétaro, Mexico) at breakfast. Children in group 2 consumed two servings of 33 ± 7 g of RTEC, one at breakfast and another serving at dinner. Children in group 3 consumed one serving of 33 ± 7 grams of RTEC and in addition, both children and mothers received a nutrition education guide that contained recommendations for healthy eating. Children in group 4 were involved in the study and had no treatment. ",
"and this Table",
"Basically, the nutrition education does something. Adding breakfast cereal to their diet does nothing (good or bad).",
"~~~~~~~~",
"At this point, I'm still not impressed by the experimental data. Basically, if you regularly eat breakfast anyways, just make sure it's a \"good\" one (in the sense of low glycemic index, \"eat food, not too much, mostly plants\" sense). If you ",
" regularly eat breakfast anyways, you may want to give it a try, but I see no compelling evidence to force yourself to do it: I'd sooner attribute any benefits of breakfast that are seen in the epidemiology (see abw's and CindyMcHinklehanky's posts) to e.g. feeling less hungry at lunch, when you're away from home and maybe have less control over your food (e.g. eating fast/restaurant food or something you packed yourself but traded \"healthy\" for \"still tasts good, even hours after I made it sitting at room temperature\" vs. something leaner you freshly make yourself), than any change in e.g. the body's catabolic/anabolic rhythms or some such.",
"Notes/Disclaimers: (1) ",
", (2) If a dietitian (",
" \"nutritionist\") chimes in with superior evidence & links below, I happily defer to their expertise"
] |
[
"I'm in school for dietetics. I might be able to shed a little light on this. Eating breakfast doesn't necessarily \"boost\" your metabolism in a way that would increase weight loss or anything like that, but it may help to curb appetites later. I grabbed this from the American Dietetic Associations 'Evidence Analysis Library'",
"\"Cross-sectional studies and epidemiological data from the USDA Nationwide Food Consumption Survey, NHANES III and the SEASONS study report that the prevalence of breakfast skipping ranges between 3.6 and 25%. Skipping breakfast is associated with a higher BMI and increased obesity risk, despite lower reported daily energy intakes. Two randomized controlled trials show that breakfast eaters had a greater reduction in impulsive snacking and ate less at later meals.\"",
"As for eating a large meal before dinner, it really depends. You should calculate the number of calories you will burn for the duration of your sleep time. Any meal or snack within the hours before bed time shouldn't exceed the calories you'll burn while sleeping. Your body will use circulating carbohydrates preferentially over breaking down glycogen (from your muscle), so an appropriately sized meal/snack is actually beneficial to your muscle mass. "
] |
[
"Well, I don't know about the health benefits, but eating breakfast at the exact same time every morning is an excellent way to wake up at the exact same time every day."
] |
[
"Can you actually hear your thoughts?"
] |
[
false
] | null |
[
"So no, you are not actually hearing your thoughts in the sense that it is not going through the same sensory pathways as a normal auditory stimulus. "
] |
[
"Yes! That's what I'm talking about, thank you :). So, what im wondering is when you are thinking or reading, is the temporal lobe showing similar activity?"
] |
[
"Yes! That's what I'm talking about, thank you :). So, what im wondering is when you are thinking or reading, is the temporal lobe showing similar activity?"
] |
[
"What is the strongest muscle in relation to it's size?"
] |
[
false
] | null |
[
"It is the masseter in particular. The other masculatory muscles are accessory in one way or another (Medial Pterygoid, Lateral Pterygoid, and Temporalis). ",
"Some may argue that the myometrial layer of the uterus (seen in females) is stronger, but I exclude this since it is not present in both genders. "
] |
[
"It is the masseter in particular. The other masculatory muscles are accessory in one way or another (Medial Pterygoid, Lateral Pterygoid, and Temporalis). ",
"Some may argue that the myometrial layer of the uterus (seen in females) is stronger, but I exclude this since it is not present in both genders. "
] |
[
"It is the masseter in particular. The other masculatory muscles are accessory in one way or another (Medial Pterygoid, Lateral Pterygoid, and Temporalis). ",
"Some may argue that the myometrial layer of the uterus (seen in females) is stronger, but I exclude this since it is not present in both genders. "
] |
[
"How did scientists in the 1800's know that they isolated an element and that it wasn't a novel chemical compound?"
] |
[
false
] |
I feel like it'd be very difficult to first isolate an element, then to know that you have gotten to the element, rather than a new compound. It doesn't seem like there's any test you can do determine that without a lot of supporting evidence or advanced equipment. Were people mistaking compounds for elements as they made discoveries?
|
[
"Trying a whole battery of reactions, basically. One of the most important is whether anything significant happened if you heated the substance up in a vacuum, as many compounds would tend to decompose on heating. But you can heat a chunk of carbon or phosphorus or metal and so on in a vacuum all day and nothing will happen. That's the thing about elements, they're unchanging while compounds are not.",
"Other tests were more specific; if the substance was a suspected metal, you would try to create the corresponding salts - oxides; sulfates, nitrates and so on. You could then crystallize and examine those properties. They would test for various compounds by using reactions that would react with specific functional groups (even before they knew what a 'functional group' was), for instance decarboxylation reactions to reveal carboxylic acids. (This is also how early organic chemical structural analysis was done in the late 19th century)",
"I (as generations of chemistry students before me) have tried my hand at classical 19th century qualitative analysis; If you have an unknown metal ion in solution, you can more or less follow a procedure of sorts: Does it form a precipitate at high, low or medium pH? Does it form an insoluble salt with nitrate? sulfate? carbonate? sulfide? sulfite? nitrite? At which pHs? What solubility does the salt have? What melting point does it have?",
"In the early 19th century you ahd ",
"bead tests",
" but by the late 19th century you had spectroscopy (in the visible region) and we'd realized each element had its own distinct spectrum. That lead to the discovery of elements we'd not even found in the laboratory yet, such as helium - named after the sun because it was observed in the sun's spectrum before it was found on earth."
] |
[
"It's also worth noting that once all known elements were ordered and indexed, the gaps in the periodic table gave scientists a pretty good idea of what to expect out of unknown elements which aided in their eventual identification. "
] |
[
"Yea this all is good but you didn’t answer op’s most interesting question - how could Nikolay know that he has pure gas element atoms in a jar?"
] |
[
"Are the umbilical cord part of the mother or the child? Whose genes does it contain?"
] |
[
false
] | null |
[
"The placenta consists of a layer that contains maternal cells and a layer that contains fetal cells. The fetal layer of the placenta is connected to the umbilical cord. The umbilical cord consists of fetal cells."
] |
[
"The umbilical cord is made out of fetal cells and that is why you can store the umbilical cord to be used as a source of stem cells should the person later need them!"
] |
[
"No, these are not embryonic stem cells in that they are not totipotent and cannot differentiate into any cell type of the body. But they are very useful when needing to do research on blood or specific genetic disorders. ",
"EDIT: I'm just going to type this out here for help:",
"Totipotent: Embryonic stem cells. They can turn into any cell type of the body AND the placental/trophoblast cells. ",
"Pluripotent: They can turn into any cell from the 3 germ layers, but not placental cells: ",
"ectoderm: examples neural/brain and skin. ",
"endoderm: examples include your digestive tract and lungs. ",
"mesoderm: examples include the heart, blood, bone, muscle and ",
" everything else I haven't mentioned with the other two germ layers. 5-6 days after fertilization, you are this little ball of cells called a ",
"blastocyst",
" and the 3 germ layers begin to arise from this point with the mesodermal layer forming last.",
"Multipotent: These cells can differentiate into multiple cell types WITHIN its own germ layer. A blood cell cannot be turned into a skin cell, for example. This does not mean it can do ANY cell within the germ layer, but it's a good starting point. Blood cells are a great example of being multipotent. "
] |
[
"Why cant we transmit wireless electricity for long range ?"
] |
[
false
] |
I know that tesla tried to do something like that back in his time. But its 2015 and still we depend on cable transmission of electricity. Are there any risks for doing this ?
|
[
"There are a lot of issues with transferring electricity through the air.",
"At the basic level, electricity is the movement of electrons. To get an electron to move through air you have to discharge it into the air. Electrons are pretty uncontrollable and they like to react in air - hence the giant electric sparks called lightning. That lightning and those sparks dissipate the power quickly. Not to mention, this discharge is potentially dangerous to any living thing around.",
"So we can't transmit electrons through the air without safety problems and the length of these sparks are very short - maybe a few hundred feet tops for man-made stuff.",
"We can convert electricity to other stuff though. We could generate a huge magnetic field that moves back and forth which could cause induced magnetic fields in electronic items, powering those things. That works. However, magnetic fields reduce in strength inverse to the distance squared. At if you double the distance from the transmitter, you reduce power to a fourth of what it was. This make mag fields quickly weaken. Plus, it would cause inductive heating of anything metal nearby which might be unpleasant. Furthermore, magnetic fields go in all directions (more or less spherical) and are power-intensive. In other words, it would be really inefficient to do it this way. ",
"We could convert the electricity to light, like a laser, and direct it at a properly tuned photocell. Lasers are really inefficient too, though. Plus, the atmosphere would interact with the light, absorbing some of it, reflecting some of it, and generally causing more inefficiencies. It wouldn't be worth it to produce several mega watts to power a single house.",
"We could maybe convert it to microwaves and direct those, via dishes, to other receivers. Similar to the laser. This is probably a bit more efficient, but you still run into atmospheric affects. Not to mention, microwaves heat water really well - and most living things are made with lots of water. If a bird flew into the beam it might die. If a person walked in they might die. Lots of safety problems, and again, not very efficient. ",
"So what's left? Copper wires. Copper is relatively abundant, easy to get, and (by comparison) significantly cheaper than air-transmitted electricity. So we do what we've always done, hang copper wires from the power plant to the houses and buildings. They're capable of carrying the electricity much further than any of the above mentioned methods and more efficiently. ",
"Edit: We could convert it to a different power source, move that instead, and convert it back to power. This might be something like use the electricity to split water, creating oxygen and hydrogen. Release the oxygen to the atmosphere and move the hydrogen through pipes. Unlike copper wires, the hydrogen wouldn't lose energy over a given distance. However, the hydrogen is tougher to move. It's also more dangerous since the pipeline could explode. So, although over EXTREMELY long distances (probably 1000's of miles) this might be better than sending electrons over copper wires, it would still be more cost effective and safer to move the power plant to the customer.",
"Additionally, a better question than why do we still use copper wires might be, \"Why do we still have centralized power grids?\" We could put solar panels and wind turbines on every building and throw some very small natural gas plants into subdivisions which could provide power for almost everything. That would reduce our thermal losses along those long stretches of transmission wires. It would also make the electricity at your house more reliable since you wouldn't need to worry about a power pole getting knocked down from wind."
] |
[
"The concept has been proven time and time again the problem, however, lies in the efficiency of the system. Tesla was in fact successful in his quest to power small lights a distance away but this required an extremely large amount of power input. There is a company today that is trying to make a system that will transmit energy wirelessly but it uses ultrasound and converts it to electricity at the end location. If you want I can post a link when I get to my computer (on mobile now)"
] |
[
"The problem is that the energy radiated follows the inverse square law. The energy decreases with distance far too quickly to be of any use. You'd need an incredibly powerful transmitter, or transmitters every few hundred feet."
] |
[
"Could one interpretation of super symmetry be that part of the very early big bag/inflation consisted of a split where two universes were originally created, one based on antimatter and the other \"normal\" matter?"
] |
[
false
] | null |
[
"No, this is not what supersymmetry is. Supersymmetry is a very precisely defined property, not something with differing interpretations, and it is not a symmetry that relates matter and antimatter as duals. Supersymmetry pairs bosons and fermions, and does so in a way that is integrated into the existing spacetime symmetries (translational and rotational and Lorentz invariances)."
] |
[
"I suppose I was asking if it, at all fit our current scientific understanding, and established models, of such things. "
] |
[
"I suppose I was asking if it, at all fit our current scientific understanding, and established models, of such things. "
] |
[
"How does light from an object in space update itself to us?"
] |
[
false
] |
For example, if a star is 200 light years away, it will take 200 years for that light to arrive here. When we see it in the future, it might look different. What determines the exact light we see once those 200 years are up? Also, once that light appeared, would the object just suddenly change, or would it be gradual process? (I am really not too knowledgeable in how light works, which is why I might sound confusing)
|
[
"Let's bring it to human scale.",
"Suppose that back in the 1600s (no fast communication), a king wanted to keep citizens in a distant city updated with the latest proclamations on a minute by minute basis. (Assume the king is crazy.) Obviously, being the 1600s, there is no form of communication faster than a fast horse (except maybe a pigeon). The king might dispatch a rider to that city each minute, bearing the proclamations as they stood at that moment.",
"Assuming the riders all ride at the same speed, they'll arrive at the city at a rate of one per minute, bearing the proclamations as they stood when they departed. However, they have no information about updates to the proclamations that have been made since they left. The city won't learn about those changes until the riders dispatched at that time arrive.",
"Photons from the star in your question are akin to those riders. When they arrive here after 200 years, they show us what the star looked like at the moment they left, 200 years ago. The star has continued to age in the meantime, however. We just won't know about it until the new riders (photons) arrive here.",
"Does this make any sense?"
] |
[
"Basically, you would see what the object looked liked 200 years ago. If a star were to form quickly enough for a human to observe it within a lifetime, they would be able to see its formation gradually. "
] |
[
"You are actually looking into the past when you look up at the stars. The light you see is actually 200 years old and the light you see the next second is 200 years and 1 second old. That star could already be destroyed but you wouldn't know until 200 years after it happened (ignoring all other ways of detection). It is not intuitive for our brains to think of light having a set velocity since it moves soooo fast, 300,000,000 meters/second."
] |
[
"How do plants \"know\" which way the sun is facing?"
] |
[
false
] |
Most of us have seen the timelapse video of plants moving with the sun as it moves across the sky throughout the day. Those of us with house plants see the same thing, except plants facing the windows. Plants in my apartment are even tilting their leaves to get the maximum surface area possible. How do they know which way the sun is facing? There's no brain controlling their movements, so how do they do it?
|
[
"A chemical called auxin elongates cells furthest from the light.",
"http://en.wikipedia.org/wiki/Phototropism"
] |
[
"These things in the cells called phototropins are responsible for allowing plants to detect light. UV light hitting them causes them to turn 'on', and blue light causes them to turn off.",
"As corey-- said, Auxin is the real workhorse in phototropism. Phototropins that are facing the sun will be more turned on, due to the larger amount of UV light hitting them, and will downregulate the expression of auxin on their side of the plant. This means that the side of the plant away from the sun will be producing more auxin, which will elongate cells on that side of the plant, bending it towards the sun. "
] |
[
"I'll try an analogy, hope it works: \n",
"Eyes have cells specialized in being excited by light stimuli. If you lay this cells in a straight line, then turn on a light source at the edge of the line the nearest cells would be more excited than the last cells in the line.",
"In a similar fashion, plants have a protein, called ",
"phototropin",
", which works just like the example of the eyes: the phototropin nearest sunlight would be more excited, it would send feedback to the growth component of the plant, resulting in a directional growth towards the sun."
] |
[
"Why is steam formed, when water is not at boiling point?"
] |
[
false
] |
Like when you make a hot drink, steam comes off the top, but the drink itself is not 100 degrees Celsius.
|
[
"What you see coming from the top of your hot drink is condensating water vapour which is NOT steam. Steam is 100 per cent pure water vapour and is invisible.",
"\nYou can see clouds in the sky (condensating water vapour) but can not see water vapour in the atmosphere until it condensates (forms clouds) and the air or water temperatures do not need to reach anywhere near waters boiling teperature for evaporation to occur which increases the airs humidity by vapourisation.\nWhat this means of course is not all water vapour is steam. Steam is 100 per cent water vapour which can be differentiated from condensating water vapour as the area that appears to be nothing above a boiling kettle which is just below the cloud of visible condensating water vapour.",
"Water can be be vapourised at as low a temperature as 0.01°C ",
"https://en.wikipedia.org/wiki/Water_vapor",
"https://en.wikipedia.org/wiki/Triple_point#Triple_point_of_water",
"This is just one example of what many people refer to in everyday life as simply being incorrect or at least not absolutely accurate. i.e. the stuff you see coming off hot drinks is NOT steam."
] |
[
"Just to add to this: the molecules of water inside your drink do not all have the same amount of kinetic energy. The distribution of energy levels is determined by statistical mechanics, and is a direct function of temperature. So while the ",
" kinetic energy of the entire container of water may be at a temperature below the boiling point, there are some molecules which do have enough energy to leave the container and become vapor. If you keep heating the container, the average energy goes up, more and more molecules can enough energy to turn into vapor, and you get steam.",
"This energy distribution is the reason why the human body sweats to cool down. Inside the droplets of sweat, those molecules which have sufficient energy to vaporize will do so (assuming that the surrounding atmosphere is not at 100% humidity), ",
" all those molecules without sufficient energy (that is, lower energy). The average temperature of the droplet then drops. The remaining low-energy molecules will continue to extract heat from your skin, vaporize, and leave the system. Your body cools as a result."
] |
[
" NO. Steam is 100 per cent pure H ",
" O vapour. For pure H<sub>2</sub>O vapor to be produced the water has to be boiling - which is steam.",
"\nCondensation, water vapour and steam are three entirely different entities which is why they have different words to describe and define them.",
"Steam is pure H2O vapour and needs boiling water to be produced and is absolutely invisible.",
"Water vapour can be produced from 0.01°C up to boiling point and above. ",
"Water vapour created from water that is not boiling (i.e. by evaporation) is NOT steam.",
"Atmospheric water vapour is a mixture of atmospheric gases and water vapour which is NOT steam.",
"The stuff that nstanding972 is asking about which they see coming off the top of their hot drink is NOT steam as it is visible and the water is not boiling so what they are seeing is more accurately described as condensation or condensating water vapour. "
] |
[
"Is computation without energy possible?"
] |
[
false
] |
Is computation without an expenditure of energy possible? If not, why exactly?
|
[
"This question is related to information theory and statistical mechanics.",
"First, ",
"Landauer's principle",
" states that any ",
" computation has to transform energy into waste heat. All ordinary computers are based around irreversible operations, e.g. taking two numbers and adding them (from just the sum you cannot tell what the original two numbers were), or taking a memory location and overwriting it (afterwards you can't tell what the memory originally contained). ",
"The general idea is that the state of the computer memory (in bits) is a high-level description of the state of the molecules of the computer. Since molecules are reversible, if the high-level state of the computer changes irreversibly, the amount of low-level state that is not deducible from the high-level must increase --- this is the thermodynamic idea of waste heat. The principle says that a computer which is at temperature T must use up kT log(2) joules of energy for each bit it \"forgets\".",
"Actual computers dissipate trillions more heat than the limit, so it's not currently a limiting factor. But it has led to interest in ",
"Reversible computing",
" as a way of avoiding the limit. The idea here is to write programs using only reversible operations---it has been shown that ordinary programs can always be rewritten to be reversible with only a constant-factor slowdown in time and memory.",
"If a computer is reversible, there is no fundamental thermodynamic reason that it must use up energy. But it is still not clear if it is possible to build one in practice.",
"One theoretical example is the ",
"Billiard ball computer",
": one sets up a system of billard balls to start moving towards each other in exactly the right way (encoding the program), and from the way they bounce off each other (assuming perfect Newtonian mechanics) one can read off the answer. This example shows that there is nothing in the laws of physics that forbids computation that is done without using any energy at all (since an idealized billiard ball collition conserves energy). However, such a computer can not be built in practice, because small errors when positioning the balls would quickly get magnified when they bounce around. (One ",
" build them using ",
"swarms of live soldier crabs",
", but this has no energy saving advantages).",
"So far, the best known reversible computer scheme that ",
" be built are \"brownian computers\" or \"adiabatic circuits\". Here the computer is always almost at a thermal equilibrium, and random thermal noise causes it to take random steps forward or backward. By applying some external force one can cause it to drift in the \"forward\" direction; this doesn't cut energy use to zero, but the total energy for the computation can be made arbitrarily low at the cost of waiting longer for the answer. This could be realized chemically, using DNA, or physically using electrical components that store energy between computer steps. "
] |
[
"Understood. But I don't see how computation can be performed at absolute zero."
] |
[
"No, even if you had a mechanical computer, or even used pen and paper to compute, you would still move. Even with though, energy is required to think."
] |
[
"Can you be born with an allergy?"
] |
[
false
] | null |
[
"The simple answer is no. You cannot be born with allergy. You have to have a first exposure to the allergen in order to develop the allergy.",
"The real answer is more complicated....you can be born with a predisposition to becoming allergic, and you can (in rare cases) be born with a predisposition to become allergic to a specific thing."
] |
[
"This is difficult to answer. Allergy is a broad term that covers many pathological immune processes. IMO the Gell and Coombs classification of hypersensitivity (type I-IV) doesn't cover every manifestation of allergy. Traditional teaching is that anaphylaxis-type hypersensitivity requires exposure to the antigen to develop sensitisation. However \"allergy\" includes more things than just anaphylaxis. ",
"For instance the ",
" of asthma has a genetic component. In population based twin studies the contribution of genetics is estimated to be 35-70%. People are working on isolating the specific genes but I don't know what progress has been made. Atopy also has an hereditary component.",
"So I think the answer is that you can be born with an allergy but that you have to interact with the environment to cause the allergy to manifest.",
"Addit: remember also that the newborn has passive immunity due to the transfer of maternal antibodies across the placenta into the fetus. even before any exposure to environmental antigen I'm not sure any of these antibodies have been shown to be specifically involved in allergic type reactions, but it just means the the newborn is (passively) immunocompetent at birth."
] |
[
"Allergy is specifically type 1 hypersensitivity, and they are IgE mediated. This is how it's ",
"defined",
". I wish I could give you a better reference than Wikipedia...but the only good reference I know of is Janeway.",
"Type 1 is not limited to anaphylaxis. Pollen allergies are (edit : TYPICALLY) type 1. "
] |
[
"Weird reflection?"
] |
[
false
] |
So I installed a window film on my home windows with up to 70% UV blocking capacity so then the film reflects rainbow alike colors on the floor is the film working?
|
[
"Assuming this is a thin film, meaning in the order of micrometres and not some millimetre thick plastic thing you stuck on, it is probably ",
"thin film interference",
". It's the same reason soap bubbles and oil spills on puddles in the road have rainbow reflections. ",
"When light hits the thin film some is reflected and some is transmitted (goes through). Then when light hits the main surface the same thing happens, some is transmitted and some is transmitted. So now you have light being reflected from two different layers coming back. If these two reflections are out of phase (the light has to travel a multiple of a quarter wavelength further to pass through the film) they cancel each other out so there is no reflections. This is how anti-reflection coatings work. If the light has to travel a multiple of a half wavelength further, both reflections are in phase and they add, the reflectivity is high. Any thing in-between will reflect somewhere in-between. ",
"Now each colour has it's own wavelength and bends differently in the film, so depending on angle you view it at and film depth different colours are going to be constructively or destructively interfering. You'll see a rainbow. ",
"Is the film working? Yes, this effect won't have any impact on UV absorbance of the film. Though I'm pretty sure most glass windows are already somewhat UV opaque, so I'm not sure how necessary the film is. "
] |
[
"Could be that too. I'm not sure exactly what OP has given his description. "
] |
[
"Thank you for the information very much appreciated! I live in a very hot country and the window is from like 2007-2000? It's old one."
] |
[
"If we ever get to do brain transplants, what would happen? Would the person with the new brain have the new brain old memories, or would all memories be forgotten?"
] |
[
false
] |
Or... would he have he's old memories...?(I think thats impossible)
|
[
" it were ever possible, and that is a big if, you, and every conscious aspect of you, would be transported with your brain. Your mind is the product of the pink squish stuff between your ears. Move the squishy stuff around, and the mind follows.",
"Now, there would be some things that would probably not be transported (at least to some degree), like certain physical skills that involved precisely timed interactions between muscles and the nervous system. But conscious memories? They're in your brain."
] |
[
"It's not really a case \"Memories are in the brain\" being established by a single study. It's the kind of thing that has evidence accumulating for hundreds of years, but some super important findings over the years... just papers off the top of my head:",
"Scoville and Milner (1957). \"Loss of recent memory after bilateral hippocampal lesions\". Journal of Neurology, Neurosurgery and Psychiatry 20 (1): 11–21.",
"Liu X, Ramirez S, Pang PT, Puryear CB, Govindarajan A, Deisseroth K, Tonegawa S. Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature. 2012 Mar 22;484(7394):381-5. doi: 10.1038/nature11028.",
"Morris R, Anderson E, Lynch G, Baudry M (1986). \"Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5\". Nature 319 (6056): 774–6. ",
"Reijmers LG, Perkins BL, Matsuo N, Mayford M. Localization of a stable neural correlate of associative memory. Science. 2007 Aug 31;317(5842):1230-3.",
"Those are the ones that jump out of my memory (no play on words intended). But there are thousands of others, those are just some hyper-famous ones."
] |
[
"Oh, a couple thousand years of human endeavour.",
"But basically, like any good scientific theory, my basis is that there has never been any evidence to the contrary.",
"I'm not exactly sure which bit your picking at, but in general, if I make the statement \"the mind is a product of the brain\" the evidence to support this is based on the 10s of thousands of observations that would fit into the category of: things that change the behaviour of the brain change the behaviour of the mind, and things that change the behaviour of the mind change the behaviour of the brain.",
"And of course, you could then argue that the brain is just some receiver for the mind, rather than the producer of the mind. But you would have to be a big giant dick to do that."
] |
[
"Would pi be as difficult to perfectly describe using a different number system?"
] |
[
false
] |
Such as base-12 or base-8 or something? Or are irrational, infinitely extending numbers just inherently that way?
|
[
"If a number is irrational in one base, it is irrational in every (integer) base.",
"The notion of irrationality doesn't depend on base as can be seen from its definition: A number is irrational if it can't be written as the ratio of two integers. Pi is irrational because there are no integers ",
" and ",
" such that ",
" / ",
" = pi."
] |
[
"Then you would never be able to accurately say you have ",
" ",
" of something. Which is a hell of a trade off. All other irrational numbers which are not a function of pi would still be irrational."
] |
[
"One is 1 in every base since b",
" = 1. But you can't write any other integers than 0,1 and -1 accurately."
] |
[
"What is the difference in supernova types? Do they create a shockwave?"
] |
[
false
] |
If there is a shockwave, is it a big wall of fire? Or is it more of a compression wave?
|
[
"They wouldn't produce a shockwave so much because there is no speed of sound in space.",
"That's not correct. Space is not a perfect vacuum, which is why supernovae do produce shockwaves. Interestingly, these shockwaves play a vital role in star formation since they compress hydrogen gas in the interstellar medium and can trigger the gravitational collapse of \"nearby\" gas nebulas into protostars. See for instance ",
"https://www.space.com/22446-supernova-shockwave-speed.html"
] |
[
"They wouldn't produce a shockwave so much because there is no speed of sound in space.",
"That's not correct. Space is not a perfect vacuum, which is why supernovae do produce shockwaves. Interestingly, these shockwaves play a vital role in star formation since they compress hydrogen gas in the interstellar medium and can trigger the gravitational collapse of \"nearby\" gas nebulas into protostars. See for instance ",
"https://www.space.com/22446-supernova-shockwave-speed.html"
] |
[
"I'm sure you could read the wiki page of supernovae for details. Type 1a occurs when white dwarves explode. They siphon energy from nearby stars until they reach a limit and boom. Other types 2s are more normal explosions of big stars ending their lives with an implosion leading to an explosion. Their are other weirder supernovae that occur when certain stars collide. 2 white dwarves. 2 neutron stars and possibly other rare events. Yes they create a shockwave. ... of their own materials. As someone mentioned it helps in star formationnand gets the ball rolling for other astronomical events. "
] |
[
"What percentage of total world photosynthesis are plants responsible for?"
] |
[
false
] |
In learning about the global carbon cycle, I had always learned that plants are responsible for converting CO2 into O2 and organic compounds. What percentage of total photosynthesis are bacteria responsible for? Protists?
|
[
"http://en.wikipedia.org/wiki/Photosynthetic_efficiency",
"From a 2010 study by the University of Maryland, photosynthesizing Cyanobacteria have been shown to be a significant species in the global carbon cycle, accounting for 20–30% of Earth's photosynthetic productivity and convert solar energy into biomass-stored chemical energy at the rate of ~450 TW",
"That may shed some light, though I imagine algae and such also contribute a non negligible amount.",
"Maybe this link will shed some more light, but I haven't actually read it, sorry."
] |
[
"From that article:",
"Using satellite-derived estimates of the Normalized Difference Vegetation Index (NDVI) for terrestrial habitats and sea-surface chlorophyll for the oceans, it is estimated that the total (photoautotrophic) primary production for the Earth was 104.9 Gt C yr−1.[13] Of this, 56.4 Gt C yr−1 ",
", while the remaining 48.5 Gt C yr−1, was accounted for by oceanic production.",
"(Emphasis added.)"
] |
[
"And just to clarify, the terrestrial production is going to be almost all plants, while the oceanic production will be nearly all bacteria or protists"
] |
[
"What is transcriptional polarity?"
] |
[
false
] |
I've asked my prof a lot of questions recently and i'm kinda feel bad bogging him down with questions, especially over the weekend. However, since it is a short question I hope you the reader wont' mind answering it. In the context of virology, what is transcriptional polarity? image for reference by the way this image sharing website is awesome!
|
[
"It means that as the RNA polymerase is transcribing the viral 'operon' (I don't know the correct term), it has a chance of stopping transcription in between every gene. That means that genes closer to the beginning of the transcript are more likely to get transcribed whereas genes closer to the end of the transcript are less likely to get transcribed since the polymerase might stop transcribing before it reaches them. That means that, all other things equal, the genes near the beginning of the transcript will be expressed more."
] |
[
"Transcriptional polarity:\ngenes closest to the 3’ end of the genome are transcribed in greater abundance than those towards the 5’ end.",
"Happens in viral genomes at least"
] |
[
"OH!!! does it mean that some genes are expressed more than others?"
] |
[
"How to make \"tv\" time-travel possilble, geometrically?"
] |
[
false
] | null |
[
"Wait...what are you talking about?"
] |
[
"The fallacy of time travel is that no two points are physically in the same spot one moment to the next. Between universe expansion, earth rotation, universe rotation, earth's movement around the sun, and so on... if you moved forward or backwards a minute in time, you'll be quite a far distance away from where you began. Can we calculate that distance, and if so, how accurately?"
] |
[
"The notion of two positions being the same at different instants in time depends on the observer / coordinate system... there is a frame for which Earth is really still. It's the frame of the Earth. There is no absolute notion of how much we moved from one second to the next."
] |
[
"How is a ship's weight distributed when it enters a body of water?"
] |
[
false
] |
I saw this article ( ) and started wondering about how strong that bridge would need to be. But what about when a ship is travelling over it? How does the fluidity of water affect the weight distribution of the ship throughout the water? Technically, isn't the water displaced the minute the ship hits the water, so it's already "accounted for" by the time the ship crosses? Practically, is it localized to some area around the ship? Basically, would this bridge "feel" it when a ship crosses it?
|
[
"Think about it this way. If you balance a dish full of water on a point...say the top of a pencil eraser, and then place a piece of styrofoam or another object that floats at one of the extreme edges of the dish such that it is floating, what will happen to the dish?\nThink in terms of center of gravity (for the dish example) and newton's third law. The water exerts a bouyant force on the ship, which in turn exerts an equal gravitation-based force on the water. This downward force is not distributed throughout the channel, it is localized to where the ship is floating, and more specifically the center of buoyancy (assuming the ship is stable), thus the bridge will feel the ship cross and will have to counter-act the moments imparted by the ship as it crosses over, just like any land bridge.",
"Not really a good analogy. A dish full of water is confined, you are adding mass to a closed system. In the river bridge case, an equal amount of water (in terms of weight of the ship) would be displaced somewhere and would not remain \"above\" the bridge. The bridge would be supporting the same weight.\nThe determination if an object will float or not is whether or not it can displace a volume of water with an equal weight of the object. When you put a boat in the water it will continue to \"sink\" until the hull manages to displace enough water that is equal to the weight of the boat.\nAs long as the displaced water has a route to escape, the weight of the system does not change. A 5 ton ship has to displace 5 tons of water in order to float."
] |
[
"When an object is placed in a fluid (such as the image you posted) the water of water that is displaced is being replaced by the weight of the ball. No more no less. A way to visualize it is to imagine a balance scale with a glass of water filled to the brim on each side. If you were to add an object that floats to one of the glasses a certain amount of the water would pour out. The weight of the water that would pour out is exactly equal to the weight of the object you placed in it. The balance scale would not move."
] |
[
"The system would stay in equilibrium. I like your illustration, and can understand where the intuition disagrees. You mentioned reaction moment, lets run with that:",
"In this case, there is no force acting in the horizontal direction. ",
"Since there is no force in the horizontal, we are only concerned with vertical forces.",
"Since we are only concerned with vertical forces, we are only concerned with horizontal distances to the force from where we are evaluating the moment. This deems the height of the boat above the water irrelevant.",
"The weight of the boat is, by definition, exactly equal to the weight of the water it is displacing. ",
"By the transitive property, we can replace the boat with the correct volume of water and have no changes in eqilibrium",
" (Note that this is new water so we keep the mass of the system constant)",
"Hope this helps!"
] |
[
"How do we know that space/ the universe is expanding and not matter \"shrinking\" in relation to space?"
] |
[
false
] |
How do we know that the universe is expanding and not matter "shrinking"? I have no physics background so as far I can see the observable effects would be the same: As matter gets smaller/is shrinking, but space is'nt, it would seem like every galaxy is moving away from us. Also there would be no need for dark energy etc ... This might be a ridiculous question, but I just could not find an answer myself how we know for certain that this isn't the case.
|
[
"When observing very distant objects, we observe the phenomena of red-shift, where the light waves from the galaxies and quasars have be stretched as a result of the expansion of the universe. Objects which are further away appear to be moving away much faster than objects which are nearer--the red-shift is more stark. If matter were simply shrinking, the effects of red-shift would come solely from the actual movement of distant objects from us. Nearly everything in the universe is moving away from us and some of these objects are \"moving\" faster than the speed of light, which would be impossible with the shrinking matter theory."
] |
[
"Expanding space and shrinking matter actually have very different effects:",
"The interesting thing about expanding space is that the further something is away from you, the faster it appears to move away from you. Think about ants on a soap bubble, the more distant two ants are on the bubble as it expands the faster they will be moving away from each other. If they are right next to each other barely anything happens to them but if they are on the opposite side of the bubble the expansion will increase the distance between them along the surface quite rapidly.",
"Now shrinking: Imagine that you have two tennis balls right next to each other (no distance). Now imagine that these balls just shrunk to half their size - did the distance between them change? Yes, quite significantly so: before it was zero and now its one tennis ball radius. But now think about two tennis balls ten meters apart from each other - what happens when you shrink to half their size? Well, barely anything; the change is still one tennis ball radius and that's rather insignificant compared to the ten meters.",
"Conclusion: Expanding space means further objects move faster away from us whereas shrinking matter means all objects move away from us at the same rate (or possibly faster depending on their size). The only way that you can make the shrinking matter theory consistent with our observations is by saying that magically all ways for us to measure distance (e.g. strength of certain fields or the time it takes light or particles to travel) would also change which would lead to other observable changes. All in all, your theory would need extreme fine tuning to make it plausible which is an obvious flaw when you compare with the expansion theory that just works."
] |
[
"All light moves at light speed, so no - there's no such thing a \"slow\" light (for a given medium, that is, light goes at different speeds in different media). Red-shifted light has less energy, however, and light red-shifts (and blue shifts) in many of the same situations where matter would speed up or slow down. (Rising out of, or falling into a gravity well, for example.)"
] |
[
"[Biology] Do probiotics actually work?"
] |
[
false
] |
Full disclosure: I did study biochemistry and for that reason, I'm doubtful that probiotics actually work. I do have friends who have started a probiotic regimen and have seen results. Does the science back this up?
|
[
"They likely work for preventing C. Diff infection in patients who have been taking antibiotics, based on a ",
"meta analysis of 39 randomised controlled trials",
". ",
"The evidence for other benefits from probiotics is not as strong. There isn't a good standard for reporting such studies to disclose the strains used, how dosage is regulated, how controls are defined, how well those strains \"take\" within the GI tract. Mostly, studies give people a dose and then check for any beneficial effects, which means that they are susceptible to other factors which are not necessarily associated with probiotic colonisation of the gut.",
"For example, most probiotics are delivered in yoghurt. Let us say you have an experiment comparing bacteria + yoghurt, and plain yoghurt. People drinking the bacteria+ yoghurt are healthier, does that mean the \"good\" bacteria has colonised the gut ? No, because the bacteria can digest the yoghurt and produce nutrients which can produce health improvements. ",
"So I can still imagine that they are getting health benefits from the probiotic regimen. But it may not be due to \"good bacteria\" settling in their digestive tract, and it could be a number of other factors. ",
"Even if we discount the nutrient content of the probiotics themselves, other behavioural factors can play a role. If the commitment to be healthy is altering their decision making, then they may see positive results which they can attribute to the probiotics colonising the gut.",
"I'm not saying that probiotics can't work for improving health, but the evidence isn't clear yet due to a number of factors, including (but not limited to) the potential confounders I have mentioned above.",
"If your friends are seeing benefits, more power to them."
] |
[
"Microbiome bioinformaticist here.",
"It helps with very few diseases if any besides C diff mildy. Might help your GI tract a tad.",
"Pretty much all the leaders in the field from the microbiome conferences I've been to seem doubtful at best that it does much more than that. Those probiotic species rarely establish themselves in your gut long term. FMT(fecal transplant) is what the field seems interested in atm because it has helped some people immensely, but it needs more research to see if it could help more broadly.",
"Edit: More backing for this, a huge percentage of the top specis in the gut based on the Human microbiome project samples seems to be Bacteriodes, Faeculbacterium, Alistripes, akkermansia etc. Not lactobacillus and bifidobacterium that are commonly found in probiotics."
] |
[
"There's also the problem that if you create a perfect environment for the probiotic it becomes the only species, like a forest with nothing but pine trees. It's hard to fake biodiversity, especially when you don't know the desired population or the parameters they desire to be in. We were born with the assumption that our guy flora would remain unchanged through life, so we don't really have a backup plan."
] |
[
"What does radioactive waste look like?"
] |
[
false
] |
[deleted]
|
[
"Radioactive waste just looks like rods or pellets of metal. Reactor fuel comes in the form of rods of uranium or other nuclear fuels, and when they're used they look just the same. Also included in nuclear waste is irradiated equipment and more common waste, which just looks like normal except it's radioactive.",
"Nuclear waste isn't glowing green slime like it's portrayed in TV shows and the like."
] |
[
"Interestingly, if you put radioactive waste underwater it will look different from normal materials: the water will glow ",
"as in this picture",
".",
"Radioactive waste emits highly energetic particles which we normally can't see, so the material looks no different than typical materials. However, when these particles are charged (such as electrons as opposed to neutrons) and move through a medium where the phase velocity of light is less than their speed, they emit Cherenkov radiation.",
"Light propagates at about 0.75 x c in water (i.e. three quarters of the speed of light in vacuum). But electrons emitted by the waste will often travel at speeds very close to the speed of light (e.g. 0.9999999 x c) and thus will produce Cherekov radiation."
] |
[
"Hi Nuclear Engineer from Canada here. I worked in the nuclear waste department and in Ontario we store our waste in ",
"these.",
" The are called Dry Storage Waste Containers (DSCs) and are essentially made up of high density steel reinforced concrete and all the spent ",
"fuel rods",
" are stored in them along with water and the fuel is in these rods is made of ",
"natural uranium pellets",
".",
"Edit: Also there are other wastes such as mechanical components of the reactor core, resins and oils used to lubricate, and even protective clothing and cleaning equipment are all classified as different levels of waste."
] |
[
"Are neutrinos WIMPs, or a type of WIMP?"
] |
[
false
] |
They have (non-zero) mass, and don't interact electromagnetically. To this layman that sounds like a Weakly Interacting Massive Particle. In theory could dark matter be composed (at least partially) of neutrinos, or does the math just not work out? Heck, can we even do the math? Wikipedia says the neutrino's absolute mass is still awaiting experimental measurement. I apologize if I'm way off base here, feel free to point me to some good reading material or just list some phrases to google. Thanks.
|
[
"Current direct detection efforts searching for WIMPS have found some evidence for a 'low mass wimp' of around 5-10 GeV. A proton is almost 1GeV, for reference. I'm not well versed as to what astrophysical constraints say about a lower bound for the WIMP mass, but the point is that the mass has to be heavy enough that we would not expect the particles to be traveling relativistic velocities- this is what qualifies it as 'cold' dark matter. Many theories of electroweak unification tout some kind of stable, heavy particle that is a potential WIMP dark matter candidate- the masses of those particles are usually somewhat higher, heading up towards the TeV range. (1 TeV =1000 GeV)."
] |
[
"Glad to have helped. Viridian's post is correct. You can see ",
"here",
" the masses that have been ruled out by experiment. To give you some idea, neutrinos have masses on the order of eV. Wimps need to have mass on the order of a billion eV."
] |
[
"Excellent, good to know someone's already gone down that route and ruled it out. At least I'm not proposing zany ideas, just disproven ones. ",
"So how \"massive\" do these WIMPs have to be to make the math jive with the observations? Less than an electron, more than a proton, somewhere in between?",
"Also, thanks for the lightning quick reply. It took longer for me to write my question than it did for you to answer it. "
] |
[
"What would an observer, moving faster than the local speed of light in a medium (eg. water), see?"
] |
[
false
] | null |
[
"This is an amazing question and one that requires a ton more though and devotion than I can give it here. Let me just make an observation or two about what dynamics this medium would support, ",
"Here's a spacetime diagram of this situation, ",
"https://i.imgur.com/pHdP0of.png",
"We have an x,t plot with the true lightcone in black and a \"modified\" lightcone in red and a \"superluminal\" spaceship in green. Immediately notice that light signals generated anywhere behind the ship's direction of travel will never reach the ship. The ship will also ",
" light signals propagating away from the ship. The visuals would be... bizarre.",
"Also worth noting, since your spaceship is made of charged objects, most likely Cherenkov radiation will be emitted from your ship since a light shockwave will form as the medium's charged matter responds. ",
"https://en.wikipedia.org/wiki/Cherenkov_radiation",
"Lastly, your ship would have to essentially not bump into the medium for any of this to work. A real ship would explode from the extreme friction."
] |
[
"that going faster than light is not the same as travelling backwards in space time",
"No, you heard right the first time. Faster than light ",
" as time travel in special relativity. The distinction here is that the light in a medium travels less than c, but information and causality is still bounded by c even in a medium."
] |
[
"extrapolating the \"runs into own signals\", light emitting things far backward would appear to run in reverse as you are catching up on light emitted a while ago. So for example a dolphin would appear to swim in reverse."
] |
[
"Is parapsychology real? If so, then what's the evidence?"
] |
[
false
] | null |
[
"What is parapsychology?"
] |
[
"Abilities like telepathy, photographic memory and other \"super-human\" abilities.",
"Or according to the internet it is \"the study of mental phenomena that are excluded from or inexplicable by orthodox scientific psychology (such as hypnosis, telepathy, etc.)\""
] |
[
"Telepathy: no empirical evidence. Hypnosis and hypnotherapy are used and there are courses offered in mainstream clinical psych programs at large research universities. There are people who have photographic-like memory or are \"super-recognizers\" (very good memory for faces). I'm not sure what constitutes other \"super-human\" abilities so it's hard to answer generally."
] |
[
"Why is a kilo still a lump of metal in Paris? Why haven't scientists come up with a better kg, and what are proposed new kg?"
] |
[
false
] | null |
[
"We are building a new kilogram standard measurement that can be calculated rather than actually weighed so we will have a proper formula that will be constant. ",
"Look here",
"There are a few projects on in this area. "
] |
[
"It'd be easy to define kilogram exactly by tying it to some natural constant. We could have easily done this decades (or centuries?) ago if we wanted to. The problem is that we can't measure those natural constants very accurately, but of course we can just define one to some exact value and be done with it. When we later make better measurements of the constant, then what we're actually measuring is our definition of kilogram, not the constant.",
"But ultimately what we really want is an extremely accurate way to compare masses. The current way of defining kilogram with a prototype object has been the most accurate way of doing it, despite being very arbitrary. The Wikipedia gives the accuracy of the prototype as 20 ppb (parts per billion). ",
"Some ",
"other proposed methods",
" are measuring mass with watt balance or counting carbon atoms. The watt balance method gets you to 37 ppb (bigger is worse of course) and carbon counting to about 50 ppb. So these are both still worse than the existing method. These numbers are off Wikipedia so they may not reflect the very latest developments but the point is that they definitely haven't been better methods for long if they are at all. (Check the wiki page, there are other proposed methods too.)",
"Furthermore, it's fairly easy to produce accurate replicas of the prototype mass. These can then be shipped all around the world and you can have accurate, maybe not to 20 ppb but still, measurements anywhere. A sufficiently accurate watt balance is not a simple thing to build and you wouldn't have them in every laboratory. So you need to also consider the practicality of the possible new definition. We want to make accurate measurements of mass all around the world."
] |
[
"'We' as in 'human society', 'scientific community' or are you directly involved?"
] |
[
"How sparse are the rings of Saturn?"
] |
[
false
] |
I know that was poorly worded but I can't think of a better way to word it so I'll hopefully explain better here. I found out a long time ago that the asteroids making up the asteroid belt are actually incredibly sparse and nothing at all like I initially imagined them. Can the same be said for the rings of planets like Saturn?
|
[
"The Rings are Saturn are much more dense than the asteroid belt. The rings themselves are actually seven sets of concentric circles that circle the planet. There are gaps between each set of rings big enough that Cassini was able to fly between two sets of rings on its way closer to the planet. Flying THROUGH a ring would be similar to how you would probably have originally pictured the asteroid belt",
"Edit: spelling is hard on a phone"
] |
[
"Yeah, the most of the rings are fairly compact the the exception of the sparse rings further from the ones in all the famous pictures (G & E rings apparently), but the space between them is pretty large. Large enough, at least, for the ",
"shephard moons",
" which were discovered in the last 30 years or so.",
"The shepherd moons are these small, natural satellites that clear out debris from their respective orbits, and, due to their proximity to one another, help maintain the shape and consistency of the rings by contributing their own tidal forces. It's all pretty cool."
] |
[
"Voyager also passed through the rings. The answer to this question also varies depending on which ring set (and there are several of distinct composition) you're talking about. ",
"JPL's Voyager Saturn rings page",
" quotes several features: ",
"'The edges of the rings where the few gaps exist are so sharp that the ring must be less than about 200 meters (650 feet) thick there, and may be only 10 meters (33 feet) thick.'",
"and",
"'at the 2:1 resonant point with 1980S1, a series of outward-propagating density waves has about 60 grams of material per square centimeter of ring area, and the velocity of particles relative to one another is about one millimeter per second. '",
"The ",
"wiki",
"'s as good a place to start as any."
] |
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