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|---|---|---|---|---|---|
[
"What is happening with this chemical reaction?"
] |
[
false
] |
Thunderf00t is a smart guy and he doesn't know
|
[
"Product should be one K2SO4, no? "
] |
[
"\" the hot H2 would burn. In an truly inert atmosphere, it wouldn't, but if that clown used CO2 the carbon would be reduced\"",
"Maybe you should stick to what you do know rather than calling people names. The inert gas used in this case was argon.",
"The tenuous string of reactions you propose is also very unlikely."
] |
[
"The reaction is certainly NOT 2K + H2SO4 = K2SO4 + H2.",
"What is probably happening is potassium is 'burning' the oxygen n the sulfate. For certain there is residual elemental sulfur left at the end of this reaction and you can see it precipitating as clouds during the reaction.",
"The wired thing is this is a heterogenious reaction of two immiscible liquids, and they very rarely produced flames like this. Flames are usually associated with two gases mixing. Sulfuric acid doesnt boil till ~400C and potassium ~800. I have no sensible explanation for what causes the flames.",
"The reaction does also smell of H2S afterwards, however given the detection limits on H2S, this might be nothing more than a minor side reaction."
] |
[
"How realistic were TV show Babylon 5's Omega-class destroyer's rotating middle sections in simulating gravity? Would the rotating section be able to provide gravity for the entire ship or just the rotating part?"
] |
[
false
] |
One of my favorite shows of my younger days was Babylon 5. Someone in posted from a sourcebook a few days ago, and it got me thinking about the Omega-class destroyers and how they rotated just about one-third of the vessel to provide gravity, and it got me wondering if they provided gravity throughout the vessel. The bridge definitely had gravity, as they showed multiple vessels' bridges with gravity. The image in that post isn't the clearest, and I can't tell whether it is in the rotating section or right next to it. This is the best YouTube clip I could find to give you an idea of what the Omega-class destroyers looked like. So, how realistic were TV show Babylon 5's Omega-class destroyer's rotating middle sections in simulating gravity? Would the rotating section be able to provide gravity for the entire ship or just the rotating part?
|
[
"It is definitely possible to mimic the effect of gravity inside a rotating object. You can try this for yourself by partially filling a bucket with water, lifting it up by the handle and spinning around. The water will be pushed against the bottom of the bucket. The same principle can be applied to spaceships and this is commonly used in SciFi works.",
"But lets look at some calculations. The magnitude of the centripetal force (which gives rise to the fictitious / inertial force called the centrifugal force) is given by:",
"F = m * r * w",
" ",
"m is the mass of the object rotating, but since we're interested in acceleration rather than mass, we can divide it out (since F = m * a, you can replace F in the equation above by m * a and divide out the mass term). r is the radius, or the distance from the axis of rotation. Finally, w is the angular velocity. ",
"From the video, the rotating section of the spaceship has a rotation period of about 30 seconds. That means the angular velocity is 2 * pi / 30 = pi / 15 radians per second. Since pi is approximately 3, that's roughly 0.2 radians / second.",
"According to the B5 Wikia, the length of the Omega-class destroyer is about 1700 meter. From the crosssection image you linked, I estimate that the radius of the rotating section is about 500-600 meter. Lets go with 500.",
"Putting it all together gives us an acceleration at the far end of the rotating section of:",
"a = 500 * 0.2",
" = 20 m/s",
"That's more than twice the standard Earth gravity (9.81 m/s",
" ). I'm not sure what the effects are of long-term exposure to high gravity (simulated or otherwise) on humans and their ability to operate a ship, but the Earth Alliance would be well advised to only employ Vulcans on the high decks (pardon the cross-universe pun).",
"Going further down towards the central axis, the \"gravity\" goes down proportionally with distance. Close to the central axis, it will be almost zero.",
"So in reality, this setup wouldn't really work too well. Crew would feel significantly different effects depending on the level they work at. Moving between decks would require considerable time to readjust to the new \"gravity\" level if the move was large. And only the areas roughly halfway up the rotating section would feel Earth-like.",
"This is why many SciFi works that use a rotating spaceship to simulate gravity, use a rotating ring where all crew is at the same distance from the center. To approximate Earth-like gravity, the specs would have to be such that the square of the rotation period is roughly equal to 4 times the radius."
] |
[
"This difference in \"gravity\" depending on the distance from the axis of rotation is a plot point in ",
"Schlock Mercenary",
"."
] |
[
"I thought it was fairly comical in the movie. There was a point in the trip where a warning would sound and then they would seem to instantly shift from normal gravity to null gravity, and then moments later another instant shift to normal gravity in the other direction."
] |
[
"Why wasn't the SpaceX Dragon capsule full?"
] |
[
false
] |
As you can see in , there's plenty of room in the capsule on delivery? Why isn't it full of equipment and supplies? Is volume not the limiting factor?
|
[
"I think that when it comes to sending ships into space weight is the limiting factor, not volume."
] |
[
"I think the cargo was limited since this was the first flight."
] |
[
"The Dragon Capsule is rated for a certain amount of cargo by weight, not volume. Think of it the same way as a truck trailer, they're allowed to carry a certain weight so they're rarely, if ever, completely filled (in some cases, hardly at all)."
] |
[
"Does having a small head make you less intelligent?"
] |
[
false
] |
Do people with a small head (like myself) have a lower amount of intelligence?
|
[
"There's a very weak, spurious correlation. This correlation is probably due to the effect of childhood malnutrition on both intelligence and general body (including head) size. Based on this, I'd say the answer to OP's question is \"no\", but this really is just quibbling about language usage. "
] |
[
"Statistically speaking, yes. Head size does correlate with intelligence. The effect, however, is on the order of a few percent of one standard deviation, so don't think it carries that much predictive value.",
"\nEdit: See refs below for more accurate assessment of the magnitude of the effect. "
] |
[
"No, that is not true on either count. ",
"Count 1). It is a spurious correlation. ",
"Truth: The correlation explains 16% of the variance when MRI measures of brain volume are compared to IQ. It is substantial. ",
"Count 2) It is correctable by body size",
"Truth: Only a small portion of the correlation is accounted for by body size. The effect is real, the mechanisms still unknown. ",
"Refererences:",
"\n",
"http://www.sciencedirect.com/science/article/pii/S016028960200137X",
"\n",
"http://www.sciencedirect.com/science/article/pii/0191886994902275",
"\n",
"http://www.sciencedirect.com/science/article/pii/0191886995000436",
" "
] |
[
"How does transformation occur in eukaryotic cells?"
] |
[
false
] |
If I were to insert a DNA molecule into a eukaryotic cell via microinjection, for example, what is the mechanism by which the cell incorporates the foreign DNA into its own genome? Maybe I'm not googling correctly but I haven't found an answer, this question has been bugging me for days.
|
[
"Hi. As far as i know, P-elements which are a type of transposable elements have been used to insert fragments of DNA. This type of insertion is non specific and the mechanism is somewhat like the P-element from your insert (flanked to your gene of interest) finds transposable elements in the genome of the host and inserts itself at that site. This is not site specific.",
"For site specific insertion, CRISPR/CAS9, which is a relatively new method, is being used. It is basically a ribonucleoprotein (protein RNA complex) the protein is the CAS 9 or an engineered nuclease which cuts at specific sites which are complementary to the RNA in the complex. So the RNA guides your nuclease to the site where you want to insert. The host's DNA repair mechanisms are exploited to re-synthesise the broken strand but with an insert that is your gene of interest which is introduced to the cell along with your ribonucleoprotein. So basically the repair mechanism has to use the homology directed repair (HDR) pathway (your introduced gene is flanked with homologous sequences). ",
"If you are microinjecting one cell then it is a matter of chance probably whether the cell uses HDR or NHEJ (non homologous end joining).",
"Try googling transformation of yeast cells, CRISPR/CAS9, TALENS, germline transformation.\nhope that was helpful.. experts please comment and correct!"
] |
[
"It's complicated. You are basically asking how to make a DNA-based retrovirus. You need to accomplish the following:",
"Have a piece of DNA on hand. It can be single-stranded template strand DNA, single-stranded coding strand DNA, or double-stranded DNA. How this works depends on what type of DNA you have.",
"Next, get the DNA inside the nucleus.",
"If your DNA was already double-stranded and made to look like a chromosome, you would be all set, but chromosomes are too big to cross the nuclear membrane, so you can't do that. Therefore, your DNA will not survive the next replication cycle, and it will degrade rapidly during transcription cycles (because it doesn't have things like centromere and telomeres). So, you have to get your DNA into an existing chromosome.",
"The cell does not just copy-and-paste random DNA it has lying around into its chromosomes, though. You need an enzyme (integrase) to do it for you. So, you have to bring your own integrase. This is where the type of DNA you have comes into play.",
"If you have double-stranded DNA, you can keep integrase code in there. You might undergo one transcription cycle, produce some integrase RNA, which will then go off to the ribosomes to make integrase, which might come back and integrate your DNA into the cell genome. ",
"If you have single-stranded template strand DNA, you are in a good shape. Bring integrase code in your DNA. You will be transcribed by host transcriptase, and the same thing happens as before.",
"If you have single-stranded coding DNA, bring integrase code. You will need the host synthase to make you double-stranded, after which you can get transcribed by the host transcriptase, after which your integrase RNA can go off and make integrase, which can insert you into the genome.",
"In short: everything goes through template strand DNA, so you best bet is to have template strand DNA to begin with — and you need integrase in there somewhere.",
"Another way is to come in with RNA, rather than DNA, because then you can go directly to ribosomes, rather than having to deal with the nucleus transcription machinery. You bring your genome in RNA form, and also RNA for a reverse transcriptase and integrase. The reverse transcriptase gets synthesized, turns your genome into DNA, which the integrase then dumps into the cell genome.",
"Mainly, the bonus here is that you don't require host transcriptase at all, so you don't have to get into the nucleus before you can start your work.",
"Unsurprisingly, DNA-based retroviruses are much less common than RNA-based retroviruses.",
"Edit: it's the template strand, not the coding strand that is transcribed."
] |
[
"Thank you. I completely forgot about viral vectors. Don't the P-elements code for similar proteins like transposase? \nI do not the exact mechanism of insertion using P-elements or retroviruses so your explaination was very helpful."
] |
[
"Is this a meteorite ?"
] |
[
false
] |
i have found a strange stone. it is very heavy (680grams approx 18.3cubic inch) and on one side is a strange material, which seems to be glass. the brownish aeras are magnetic.
|
[
"I do not think it is. You can look around this internet for more \"Identifying meteorite\" but ",
"this one",
" is fairly helpful with a step by step guide.",
"Edit- the reason I don't think it is a meteorite is because of the white colored minerals on it, and the bubbles. However you may want a second opinion because I have never seen a meteorite (but I have seen many that people have claimed to be)."
] |
[
"the crust can be washed away, but one of the ko arguments are the bubbles. "
] |
[
"I'm not a geologist, but 680g/18.3in",
" is not very dense -- only about 2.25 g/cc. How did you estimate the volume? ",
"The brownish areas look like some kind of oxide; since you mentioned they are magnetic, it probably contains iron in some form or another. All of the cavities present suggest to me that it was exposed to some kind of selectively corrosive environment in which the materials in those pockets was eaten away much faster than the rest of it. It's also possible that those were created when the rock was made, but that would imply some kind of rapid cooling process (like a volcano?).",
"Looks cool either way. There are a lot of actual geologists on here, so I'm sure you'll get a better answer soon."
] |
[
"Is there a limit to the size of the wavelength of an electromagnetic wave, both large and small?"
] |
[
false
] |
And if so, what creates this limit?
|
[
"At very long wavelengths, the interstellar medium ends up damping electromagnetic waves, effectively making them die out pretty quickly. There is, however, some theoretical evidence which suggests that you could get wavelengths of light which are so long that they would be able to bypass this damping effect. This is discussed in ",
"this paper",
", which has the fantastic title, \"The End of the Rainbow.\"",
"As for small wavelengths, there's no limit in principle, as long as you have enough energy to generate the photon. You do run into one problem in space, however. High energy photons will interact with photons in the cosmic microwave background and will reduce the energy of the photon. (This process is known as inverse Compton scattering.)"
] |
[
"Although there's no theoretical limit, if you have an EM wave with a short enough wavelength, general relativity suggests that a single photon of that radiation would collapse into a black hole. If I remember correctly, the critical wavelength is the Planck length (about 10",
" m). Of course, we don't know how well GR applies at that scale (if at all), and in any case nothing actually emits radiation at that frequency."
] |
[
"Photons produced at the LHC can have energies in the GeV range, corresponding to wavelengths around 1 fm (roughly the size of a proton)."
] |
[
"Is there any possibility or any suggestion, at all, that life could have been seeded on a fertile, but lifeless Earth?"
] |
[
false
] | null |
[
"Is this what you were looking for? ",
"Panspermia"
] |
[
"Actually, I feel that if we are made of the same KIND of genetic material (DNA, RNA and protein in water-based lipid enclosed cellular systems), such life would resemble quite closely to what we have in our planet. Definitely not in the macroscopic anatomical level, but at least in the cellular level. The problems that need to be overcome by life for increasing in complexity are quite universal (surface-are/volume ratios that drove prokaryotes to become endosymbiotic with one another, forming eukaryotes; the mechanisms of gene regulation, etc). So, even if life is not exactly the same as what we have, we will have a good number of similarities (perhaps not to the level seen in star-trek or anything though).",
"Add to that some more constraints on the anatomy: any complex moving-life would need something like a central nervous system, some eyes and other sensory organs. The main sensory organs need to be near the CNS, and they have to be a little above ground. Whoa, that sounds quite similar to what we have in our bodies!",
"Though evolution is random, the ",
"evolutionary landscape",
" generally isn't a wide open field. It generally tends to be mired with valleys that lead to quite a finite number of lakes.",
"Though \"Evolutionary convergence\" is generally taken to be within parts of a kingdom or at least ",
" related species, even panspermia has potential to actually produce some amazing examples of convergence.",
"http://en.wikipedia.org/wiki/Convergent_evolution"
] |
[
"Possible, but highly unlikely. Unless they underwent the same forms of selective pressure in the same sequence, they wouldn't resemble each other."
] |
[
"How does light exist? (please correct me)"
] |
[
false
] |
Relativity says that for something to travel at the speed of light, either it has to have no mass or, if it has mass, it must have infinite energy to reach the speed of light. However, E=mc says that for something to have energy, it must have mass. Doesn't this mean that light has infinitely small mass (because it must have some mass, since 0 times anything is 0), but it must also have infinitely large energy for this mass to reach the speed of light? I get confused when I apply both relativity and e=mc please unconfuse me!
|
[
"E=mc",
" is for stationary mass. For things moving relative to an observer you need E",
" = p",
" c",
" + m",
" c",
" With p, as the momentum.",
"If momentum is 0, this reduces to E=mc",
" However, if mass is 0 this reduces to E=pc. So photons get their energy not from mass but from their momentum.",
"Or at least that's my understanding of it.",
"Edit: fixed the formula with the new subscript feature."
] |
[
"Momentum is actually not equal simply to mv, as you learn in a first newtonian physics class. This turns out to be an approximation that is valid for small v. When v is large, you have to use the full form; This gives the result that EtherCJ mentioned. You can also write his relation as E = gmc",
" where g is called the 'gamma factor' which starts at 1 for zero velocity and increases towards infinity as v->c. The relativistic momentum is p=gmv. If m=0, but v=c, the formula technically reads E= infinity 0 c",
" which is undefined. However, as Ether mentioned, if you go back to the basic relation you can see that E = pc, so an object can have zero mass and finite (nonzero) momentum and energy."
] |
[
"However, E=mc",
" says that for something to have energy, it must have mass.",
"No, that is not what this equation means. This equation relates mass to energy, but it doesn't say that all kinds of energy have an equivalent rest mass.",
"Photons possess momentum, but only because of their energy, not because of their rest mass -- they have no rest mass."
] |
[
"What happens in the body after a polymer like HDPE is consumed?"
] |
[
false
] | null |
[
"HDPE does not break down in HCl",
", nor does it react much (if at all) to salts. The temperature/mechanical action along the way isn't enough to break it down either. ",
"Basically, if it is small enough, it'll pass right through you. However, since it doesn't break down, eating large pieces can become stuck inside you (seek medical attention). Sharp pieces also are not a good idea.",
"tl;dr you might want to seek medical attention, depending on what you ate."
] |
[
"You really should not ask for medical advice on ",
"r/askscience",
". Not only for your own sake, but also because you open up the community and individuals in the community (particularly if they are in the medical profession) to lawsuits and all kinds of nastiness."
] |
[
"You really should not ask for medical advice on ",
"r/askscience",
". Not only for your own sake, but also because you open up the community and individuals in the community (particularly if they are in the medical profession) to lawsuits and all kinds of nastiness."
] |
[
"How does dark energy result in the expansion of the universe?"
] |
[
false
] |
[deleted]
|
[
"Let's consider an analogy from classical mechanics. We can modify Newtonian mechanics by giving everything an additional acceleration of c r where r is the position vector in some coordinate system. It doesn't matter where we put the origin of this coordinate system - every pair of objects will see a relative acceleration of c(r1-r2) which just depends on their distance. With positive c we get a uniform expansion, with negative c we get a uniform contraction.",
"General relativity has a similar freedom: You can just add a term that leads to an overall expansion or contraction of space over time. As far as we know it doesn't have to be there but you can put it in as a constant without breaking anything. You can just interpret it as that - as a cosmological constant.",
"The expansion (or contraction) of space is also influenced by energy and pressure - both slow the expansion if positive. If you want you can try to interpret the cosmological constant as mixture of energy and pressure, then you don't need a separate constant any more. Well, energy should be positive, but overall you want the expansion to accelerate, so pressure needs to be negative."
] |
[
"To confirm my understanding of your response, even if we express the cosmological constant as a function of energy and pressure, the pressure component's need to be negative is phenomenological since that's what we observe and that's what GR allows.",
"Right.",
"Secondary question, do we know how a material could exhibit negative pressure on a more micro level? Say if I have a unobtainium box with vacuum (and dark energy), would there be anything for me to measure?",
"If you make the box out of extremely small independent masses far away from any other masses they will move apart over time."
] |
[
"To confirm my understanding of your response, even if we express the cosmological constant as a function of energy and pressure, the pressure component's need to be negative is phenomenological since that's what we observe and that's what GR allows.",
"Right.",
"Secondary question, do we know how a material could exhibit negative pressure on a more micro level? Say if I have a unobtainium box with vacuum (and dark energy), would there be anything for me to measure?",
"If you make the box out of extremely small independent masses far away from any other masses they will move apart over time."
] |
[
"What are the stages in brain development of a newborn until its brain can be considered fully-developed, save for later major changes like puberty? Or can a newborn's brain be considered developed but without experience?"
] |
[
false
] | null |
[
"http://en.wikipedia.org/wiki/Infant_vision",
" -- a strong example of neural development after birth.",
"What you're looking for... a solid theoretical transition from 'developing' to 'developed' doesn't really exist. It's gradual and works for different systems at different rates at different times. You could pick a set of criteria and get something of an answer, but it really isn't scientifically useful, setting arbitrary lines to make broad statements."
] |
[
"A good way of looking at it is with Jean Piaget's cognitive development framework. There are 4 stages (hence the name 'stage theory'):"
] |
[
"A neonate's brain lacks an awful lot of myelination. This is why, for instance, tonic-clonic seizures don't really occur in newborns--epileptic discharges can't spread throughout both hemispheres very easily.",
"The frontal lobes aren't even completely myelinated until the early 20s. So it is a gradual process. And this is not looking at neuron count and synapse count, both of which vary throughout life.",
"The brain, more than anything, is a very dynamic organ."
] |
[
"Why do racecars have wide tires?"
] |
[
false
] |
I recently learned that surface area does not influence the friction so why are the tires so wide?
|
[
"Two things:",
"First, the claim that surface area doesn't affect friction is a good approximation, but it isn't exact. It comes from. The fact that the force per surface area of contact is roughly linearly proportional to the pressure, while pressure is inversely proportional to area. These two terms cancel out as long as \"roughly linearly proportional\" can be taken to mean \"linearly proportional.\"",
"The second thing is that making a tire grip well often requires a relatively soft rubber. The grip that these tires can get is so high that when the car slips it's not the rubber/pavement interface that fails. It's the rubber/rubber interface inside the tire. You see this as a black streak of rubber that's been sheared off of the tire and left on the pavement. ",
"The total shear strength of the contact patch is proportional to the size of that contact patch. A wide tire can therefore take more force before failing. "
] |
[
"The tires are also \"sticky\" and increase ground friction via some adhesive forces. More contact area for adhesion certainly does increase the grip in that case. "
] |
[
"In open wheel racing (not sure about NASCAR), the downforces produced by the aerodynamic surfaces of the vehicle also play a role here, increasing the size of the contact patch, and/or also increasing the force applied on that contact patch. IIRC, a Formula 1 race car generates enough down-force at around 80kph that it could theoretically drive upside down on the roof of a tunnel. Of course, the engine wouldn't like that, but details details... "
] |
[
"What would happen if a scientist received a huge grant and just decided to keep the money?"
] |
[
false
] |
...I'm not even in the science field so that's not why I'm asking. For example, Bill Gates has apparently given out grants before. What if a scientist just decided to retire and keep the money? Do they even give the money to the researchers? Or do the researchers have to fill out paperwork to have stuff paid for? Would that be the same as keeping money from a place like the NSF (National Science Foundation)? What would happen? Has anything like this happened before, and what happened?
|
[
"Grants are not paid into researchers bank accounts. They are usually administered by the university or other research institution and they will pay invoices that are submitted to them by the researcher.\nFraud can still occur of course.",
"(I'm a grant holder on a 3 million dollar grant. I don't get to see the money)"
] |
[
"Embezzlement is embezzlement. I don't know why you'd think it'd work any differently with scientific grants. "
] |
[
"Well, I'm just getting into graduate study, so I'm not exactly an expert, but they'd go to jail for fraud. A \"grant\" isn't just a check for a huge amount of money written in their name. It's a contractual agreement to provide such-and-such budget for such-and-such amount of time performing research with such-and-such ostensible goal."
] |
[
"How Fast Would Something Need to Move in Order for it to Visibly Blueshift?"
] |
[
false
] |
Pretty much as it says. How fast would something need to move before it would Blueshift to the naked eye?
|
[
"The relativistic Doppler shift equation for when the source (s) is moving straight toward the observer (o) is:",
"(λo/λs)",
" = (1+v/c)/(1-v/c)",
"Solving this equation for the velocity v gives:",
"v = c (1-(λo/λs)",
" )/(1+(λo/λs)",
" )",
"Let's assume that a clearly visible blueshift would be if a solidly green color (λs = 530 nanometers) becomes a solidly blue color (λo = 470 nanometers). Plugging these numbers in we get:",
"v = 0.12 c = 36 million meters per second (81 million miles per hour)",
"For comparison the fastest manned vehicle relative to the Earth (Apollo 10) traveled at 11 thousand meters per second (25 thousand miles per hour)."
] |
[
"That is correct. A group in MIT actually made a game on this premise that way you can see what the world would look like if you could travel at a significant fraction of the speed of light. ",
"Check it out."
] |
[
"About what I expected. Thanks for the quick answer!"
] |
[
"Why does the angle of incidence change when light refracts?"
] |
[
false
] | null |
[
"When considering refraction keep in mind that the speed of light in the two materials is different. ",
"The easiest explanation comes from the idea that light travels in such a way as to minimize the travel time between two points. In free space it's a straight line. (Near a massive object like a black hole, it is a curve - gravitational lensing, if you've ever heard of it). When traversing a boundary though, the path that takes the least time is one that changes angles at the surface.",
"This same sort of thing applies to a lifeguard trying to save someone at the beach. The fastest path to the drowning swimmer is not a straight line, since the lifeguard can run on the beach faster than he can swim. It's best for the lifeguard to run further along the beach (since it's faster), and swim a shorter distance in the water (since it's slower). This concept results in the same bent path that light takes between two materials.",
"The mathematics addressing boundary conditions between two materials is extremely challenging. This question is usually addressed in an upper division physics course in college (after a year's worth of work using Maxwell's equations)."
] |
[
"When light waves pass into a denser material, they slow down. No new waves are created, so now the wave crests are closer together.",
"At one point (optical axis), the waves strike perpendicularly and must continue on a straight line into the new medium. Everywhere else the wavefront strikes obliquely, which means not exactly simultaneous. This explains the change in direction. The parts of the wave reaching the interface first will enter and slow down before the parts of the wave reaching the interface later. ",
"Passing from a lighter medium (air) to a denser medium (glass) the wavefront changes direction toward the axis (central ray). ",
"Passing from glass to air, the wavefront changes direction away from the axis.",
"So oblique rays passing throug a flat windowpane will continue in the same direction but offset by a factor dependent on thickness, refractive index and angle of incidence. ",
"Source - Fincham & Freeman IIRC"
] |
[
"I wish i could contribute to this discussion more that just saying that you are correct and i'm also a 'fan' of this lifeguard example when it comes to explaining things with refraction."
] |
[
"Slugs. Where are they?"
] |
[
false
] |
I can find slugs around my house in the shade (when there is shade) and they are in great numbers. they don't seem to bother me and I don't seem to bother them. Here's my question: Where do slugs go when we don't see them? Is there some sort of million-slug orgy going on somewhere that I don't see? related: where do butterflies go when it rains?
|
[
"I went on a butterfly walk with an entomologist, and that's exactly what she said the butterflies did - hide under leaves.",
"By the way, if you ever hear of an opportunity to go on a hike or walk with an entomologist, take the opportunity. They'll show you all kinds of fascinating little things you'd never ordinarily see, notice, or know were special."
] |
[
"They are probably outside dispersed under rocks and things. Check places that are damp. And there's always a bunch in the hosta :(",
"Butterflies shelter from rain under leaves and in other little nooks and crannies I believe."
] |
[
"I'm an avid gardener. I've learned a lot about slugs as I live in on a very damp island in the Pacific South West of Canada.",
"You can find slugs at any time under dense foliage, debris (Plastic sheeting, leaves from last fall, weeded plants, lumber/plywood boards, and anything else that will retain moisture throughout a warm day. Under flower pots, for example.",
"They can be a pain in the ass. If you ever take up gardening, save yourself a lot of grief and remove unnecessary debris, throw down diatomaceous earth when it will be dry at night, and take it upon yourself to kill ruthlessly at night when they slugs come out to eat your plants. After dark, slugs seem to stop hiding and you'll find them everywhere."
] |
[
"what does testing negative to COVID-19 mean after being positive? Does it mean the virus has completely left the body or the body's immunity has improved enough to keep a check on the virus? Can an infected person once cured can still infect others?"
] |
[
false
] |
I'm asking this because a well-known celebrity in my country has tested negative only after sixth time. And she has been discharged from the hospital. Can she still infect others?
|
[
"s tested negative only after sixth time. And she has been discharged",
"EEEEHHH. The test has a really high false negative rate. You're supposed to get two negative tests in a row at least 24 hours apart before leaving isolation.",
"According to the CDC",
" you can safely leave isolation and not be contagious if you either meet the above criteria or all 3 of the below criteria in the absence of testing.",
"1) At least 72 hours after the fever ends.",
"2) All symptoms must have ended",
"3) At least 1 week must have passed since the onset of symptoms.",
"the whole two weeks thing is nonsense based off the false assumption that the incubation period (which quarantine times are based off of) is somehow related to how long you're infectious once you show symptoms. They're two different things."
] |
[
"Here's the first article I could find on it",
". It looks like you have about a 15-25% chance of getting a false negative when tested.",
"If you get 6 tests in a row there's a pretty high chance you'll get a false negative on one of them just by random chance."
] |
[
"There's a handful of studies on it. ",
"Here's an early one",
" that shows a pretty dire sensitivity. Some others find the sensitivity to be 60 or 70%. The point is a lot of clinicians are using CT instead of the swab test for initial diagnosis."
] |
[
"Do species with a shorter lifespan evolve faster than those with a longer lifespan?"
] |
[
false
] |
[deleted]
|
[
"The answer to your main question is that yes, shorter generation times (lifespan itself doesn't really matter) allow faster evolution, more or less by definition since evolution is a change in allele frequencies from one generation to the next.",
"But the underlying premise is mistaken:",
"insects and the other species with really short life spans and noticed that they almost always have more complex eye systems, nervous systems, and more limbs",
"It sounds like you're imagining insects as somehow \"more evolved\" than longer-lived animals. Do you suppose lions and tigers are on their way to developing an extra pair of legs if you just wait for them to reach the same number of generations? Will mice get there before elephants?",
"The \"design space\" of eyes or brains or body plans is vast, and different lineages have gone down different routes that lead to different places. After billions of years of evolution, we can generally assume anything is pretty stably adapted to the environment it's been living in all this time, and the rate of adaptation only becomes important when the environment itself is changing."
] |
[
"Organisms with shorter lifespans will have more generations in the same period of time, so more mutations will occur in the population in the same period of time, meaning evolution will happen faster. We often use insects in labs when we want to study the evolution of traits partially for this reason; they also tend to breed in large numbers which helps too, though that's not related to their lifespan (plenty of organisms which breed in large numbers have long lives)."
] |
[
"Sure, you always hear the example of the peppered moth, where the dominant color of moths changed between white and black of a couple decades because of industrial pollution levels. The only way a human population could change so dramatically in the same time is for nearly everyone but those of a single race failing to reproduce in the 1 generation of time we'd have.",
"On the other hand, environments that change rarely, if ever, lead to the disfavoring of big changes to which organisms are already adapted. An initial biological plan that enters an environment will settle into an optimal solution over millions of years; descendant species will look like ",
". The bottom of the ocean, for instance, has the widely known examples of horseshoe crabs and coelacanths."
] |
[
"Is Electron Exchange a Physical Effect or a Mathematical Trick?"
] |
[
false
] |
I have been studying DFT (Density Functional Theory) recently, and it is pretty common to see electron exchange referred to as an independent term of the Hamiltonian. I have seen some authors simply refer to the Kohn-Sham hamiltonian (including the exchange term) as "the hamiltonian." However, my understanding is that the exchange effect only appears because DFT uses an approximation to the Coulomb potential that includes electron self-interaction, which must be cancelled out using the exchange functional. From this perspective it looks like the electron-exchange effect is just a mathematical "trick" to correct for approximation errors. To what extent is electron exchange understood as a real, physical phenomenon as opposed to a mathematical convenience?
|
[
"Exchange effects are not an approximation, and they are very real. They are caused by the fact that electrons are fundamentally identical to each other, and thus indistinguishable from each other. There is no experiment you can possibly perform to tell the difference between \"Electron 1\" at position 1 interacting with \"Electron 2\" at position 2, and \"Electron 2\" at position 1 interacting with \"Electron 1\" at position 2.",
"So both terms must contribute to probability amplitudes and expectation values."
] |
[
"Thanks for your explanations.",
"Unless you have an example of such a formalism, I'm not aware of any.",
"I think this is where I get tripped up. Some texts call formalisms like Slater determinant approximations (at least, when you use a finite set of Slater determinants) which makes it sound like the expressions for Slater determinants are just methods for calculation, not a true representation of reality.",
"It is possible that I am just getting too tripped up with language here.",
"A second quantized state vector doesn't explicitly show exchange terms, but when you calculate expectation values of N-body operators (for N > 1), they appear.",
"That sounds interesting. I don't know anything about second quantization, I will try to learn more about it.",
"Thanks for your explanations!"
] |
[
"Well, the Pauli principle and spin-statistics theorem are real things, and just generally the difference between how Fermions vs Bosons work. On the macro scale you have ",
"Electron degeneracy pressure",
" as a another way of looking at it. (Feel free to compare the derivation of the degeneracy pressure to the derivation of the LDA functional - they're largely the same thing)",
"You can calculate the energy difference between a mean-field bosonic system and a fermionic one; it's the difference between doing a calculation with a Hartree product wave function and an antisymmetrized Slater determinant wave function (or the Hartree vs Hartree-Fock methods). So that gives you the exact exchange energy for these mean-field-interacting particles but it doesn't include the ",
", which is basically by definition the difference in energy between the exact solution and the Hartree-Fock one. We call the difference 'correlation' because its due to the instantaneous correlation of electron motion, that electrons don't behave like they're only seeing he others' averaged fields.",
"The Hartree-Fock exchange energy is not exact for the real system once correlation is introduced, though. Because the stuff is defined this way, there's no clean separation between exchange and correlation energies or how to account for them. Although DFT often assumes separability; Exc = Ex + Ec, there's no reason to think that's anything but an approximation; they're a bit intermingled here, but that doesn't mean exchange energy doesn't exist. The ",
" exchange energy does exist, in the difference in energy between the exact fermionic and bosonic solutions. However that definition of the exchange energy isn't so useful in building a model for the purpose arriving at those solutions!",
"The problem with DFT is that while it's easy to figure out the potential energy in terms of the density, everything else is very difficult. Kohn-Sham solves this by constructing a ",
" system of electrons that don't interact, under the influence of an effective potential. These make up the density and the kinetic energy is minimized.",
"The solution to that gives you the correct density and kinetic energy, provided you can express the missing bits in terms of this effective potential v. (if this is possible; see ",
") And what is missing here is the exchange and correlation energy, and indeed the energy that comes from double-counting the electrons with the coulomb term, but this is not at all the reason for the XC term. ",
"You could, in principle, make your non-interacting electrons mean-field-interacting by removing the Coulomb term from the external potential and insert it as an internal potential. You would still need to include exchange and correlation but wouldn't have any double-counting. (it would also complicate matters more than its worth) ",
"In short: Exchange is a real thing but the exact quantity can be defined differently in different contexts depending on which approximation you're starting from. The main function of the exchange-correlation term is to account for exchange and correlation, not the self-interaction error introduced by the Coulomb term, although an accurate Exchange-Correlation functional must indeed cancel out the latter, and functionals like LDA do not do a good job of that."
] |
[
"a couple questions about food chemistry"
] |
[
false
] | null |
[
"Texture is partially a function of the molecules present, but mainly a function of how they're arranged. Layers, fibers, random granules, etc, have a much stronger effect on how a food feels to your mouth than what those structures are made from. We can use this to our advantage when designing food substitutes; microcrystalline cellulose has a close mouthfeel to fat, despite being an entirely indigestible carbohydrate, because the crystal structure is similar to the way solid fat is arranged.",
"Mozzarella cheese is kneaded during production. This aligns the casein proteins in long internal fibers, as opposed to most cheeses where the curd is cut up and squeezed together. These proteins stay aligned, and the high moisture content of mozzarella helps keep them pliable when melted, leading to awesome stretchiness.",
"Almost all grains have three kinds of complex carbohydrates, all of which are built from long collections of glucose molecules. If the molecule is linear, that is long chains of glucose, it is either amylose or cellulose (depending on the specific links between glucose units). Amylose is digestible starch, cellulose is nondigestible fiber. If the glucose molecules form a branched structure, the molecule is known as amylopectin. The specific ratio of cellulose/amylose/amylopectin determines the texture, nutrition, and cooking characteristics of any given grain. Grains higher in cellulose are thought of as high-fiber, and a diet high in fiber has shown some health benefits."
] |
[
"Ok, emulsions:",
"Emulsions are any two fluid substances that do not normally mix, forced into intimate mingling through massive shear forces. Basically, little tiny drops of one substance (discontinuous phase) surrounded by a thin layer of the other (continuous phase). We describe emulsions based on the two phases; for example, mayonnaise is an oil-in-water (o/w) emulsion, where the droplets of oil are suspended in the vinegar. Whipped cream is an air-in-water emulsion, and ice cream is a air-in-water-in-oil emulsion (ice cream is awesome).",
"Now as you'd expect emulsions aren't stable normally, unless the particle size for the discontinuous phase is ridiculously small, as in homogenized milk, or if emulsifiers are used. Emulsifiers are chemicals that can mix with both the continuous and discontinuous phase, and help stabilize the interface between the two. In mayonnaise, it's the lecithin found in egg yolks."
] |
[
"Mayonnaise. seriously, how the ...?",
"Can you elaborate?"
] |
[
"Why do the colors dark purple and deep red \"feel\" visually very related, but they are on opposite ends of the visual spectrum?"
] |
[
false
] |
[deleted]
|
[
"This article has many factual errors and is very misleading. Only a very small subset of the colors you perceive have a wavelength. That is, if you split up white light using a prism, only a small subset of colors would be part of the resulting rainbow. Like most colors, magenta does not have a wavelength. ",
"Take a look at the diagram at the top of this article: ",
"http://en.wikipedia.org/wiki/Planckian_locus",
"The numbers on the edge of the diagram (from 380 to 700) correspond to where the pure wavelengths of light (the ones in the rainbow) would lie in a specific projection of a specific color space. [Aside: really, this diagram shouldn't be colored in, since your computer monitor can't display it properly].",
"Most of the colors in this diagram (except for the ones on the edge) have the same \"problem\" as magenta: there is no single wavelength that has that color.",
"Does this mean that these other colors, the ones in the middle of the diagram, \"don't exist\"? NO. All colors you perceive exist. Colors are created by your brain. The perception of red is created in your brain just as much as the perception of magenta or the perception of white. "
] |
[
"This article has many factual errors and is very misleading. Only a very small subset of the colors you perceive have a wavelength. That is, if you split up white light using a prism, only a small subset of colors would be part of the resulting rainbow. Like most colors, magenta does not have a wavelength. ",
"Take a look at the diagram at the top of this article: ",
"http://en.wikipedia.org/wiki/Planckian_locus",
"The numbers on the edge of the diagram (from 380 to 700) correspond to where the pure wavelengths of light (the ones in the rainbow) would lie in a specific projection of a specific color space. [Aside: really, this diagram shouldn't be colored in, since your computer monitor can't display it properly].",
"Most of the colors in this diagram (except for the ones on the edge) have the same \"problem\" as magenta: there is no single wavelength that has that color.",
"Does this mean that these other colors, the ones in the middle of the diagram, \"don't exist\"? NO. All colors you perceive exist. Colors are created by your brain. The perception of red is created in your brain just as much as the perception of magenta or the perception of white. "
] |
[
"So anyway, no light source can produce and nothing interacting with light can interact with only one wavelength selectively. You can't even only produce/interact with light that's strictly in between two wavelengths - there's always a smeared out continuous range of wavelengths. This is actually kinda deep and connects to any sort of signal and Heisenberg's Uncertainty Principle, but that's way too much digression. Things like a laser do a pretty good job of keeping most of their output in a small range, so we call them monochromatic. Other things like the sun produce a lot of power all over a great range of wavelengths. We perceive a mix of all wavelengths in the visible range as white.",
"So humans have 3 receptors to cover the visible range, and the receptors for adjacent wavelengths have significant overlaps in their response. These are roughly the red, green, and blue receptors, though I think scientifically they are referred to as Long, Medium, and Short (wavelengths) receptors just because they don't exactly correspond to these three colors.",
"A beam of yellow light has a wavelength between the peak wavelengths the red and green receptors respond to. It doesn't activate either red or green at full strength, but it activates both at moderate strength. Now imagine two beams, half as powerful, one red, and one green. These two beams activate the red and green receptors at moderate strength. The blue receptor hasn't particularly responded to either. So you see, there's no way to tell the difference once you're looking at the R,G,B receptor signals."
] |
[
"Why are images displayed in inverse colors when you've looked at an image for a long time, then look away?"
] |
[
false
] | null |
[
"Because your eye and brain adapt to sensory input. This is a feature to reduce noise in the visual signal.",
"Here's an example. Your retina has ",
"many blood vessels",
". They sit in front of the photoreceptors in your eye. So why don't you see big red blood vessels all the time? Your eye and brain constantly get the visual input from these and have learned to ignore it.",
"So, if you stare at something long enough, your eyes will start to neutralize the image. This is done by artificially 'adding' a negative image. And then you end up seeing the inverse."
] |
[
"Thank you very much, I had no idea how to google this question so I really appreciate the answer; makes a lot of sense."
] |
[
"Psychologists call this ",
"the afterimage effect",
"."
] |
[
"I tried to make double slit experiment but it didnt work. Where did I make a mistake?"
] |
[
false
] |
So I wanted to make my own double slit experiment and see light as a wave but I ended up seeing it as a particle I guess :( I carved two slits on a thick non translucent paper and used my phone LED light behind that paper ij order to make interference pattern occur on the wall in the dark room but that did not happen. All I got was the two bars illuminated on my wall. Basically tue light just went through rectangular slits and made two rectangular light bars on the wall and there was no interference pattern. I really want to see that pattern for myself. Please help me. Where did I go wrong? Is LED bad light source? Thank you everyone who shares their opinion on this
|
[
"In order to see diffraction, the slits should be on the order of the wavelength of the light used. For visible light, that means the slits should be on the order of less than 1 micrometer which is like 1/20 of human hair."
] |
[
"For the 6 years I was a TA in 1st year physics lab, I'll have you know that not once was a student struck with PERMANENT blindness..."
] |
[
"Just take a laser pointer and shine it on a CD/DVD. Should give you a nice diffraction grating demonstrating interference. You can even look at the pattern and try and calculate the relative data density of CDs vs. DVDs given the diffraction pattern you get."
] |
[
"What's in the space between the electron orbitals and the nucleus?"
] |
[
false
] |
Is it simply a vacuum? Or does the 1s orbital occupy that space?
|
[
"The most probable spot for the 1S state is called the Bohr radius ",
"Nope. (Although I don't blame you, this is probably the most misunderstood plot in all of chemistry) The most probable ",
" (1 dimension) is the Bohr radius, which isn't the same thing as the most probable ",
" (3 dimensions). ",
"The ",
" is not the same thing as the ",
". For a 1s orbital, the probability of finding the electron at any spot (infinitesimal volume element) is |exp(-r)|",
" = exp(-2r). It thus has a ",
" at the nucleus, and drops off smoothly with distance.",
"The radial probability distribution shows the probability that the electron will be at that particular radius. So it's the probability at radius ",
", integrated over the surface area of a sphere of radius ",
". (which increases as 4πr",
" ) Since a sphere of zero radius has zero surface, the value at r=0 is always zero, purely as a matter of geometry. If the density was constant and the electron equally likely to be everywhere in space, the radial distribution function would be a parabola, not a straight line. So it can be rather misleading.",
"So there's no actual 'space' between the electrons and the nucleus. For a 1s electron, that's actually the most likely volume of space for it to be in. "
] |
[
"The orbitals ",
" like little planet orbits or hallow spheres in which the electrons buzz around wildly.",
"The electron's behavior is governed by what is called the wave-function or the Greek letter Psi. ",
"Here",
" is an example of the first few probability distributions of the Hydrogen atom at the first few energy states.",
"The complex conjugate of Psi multiplied by Psi is the probability distribution of the wave-function.",
"As you can see, the electron can be found at multiple locations at various distances at various likelihoods. The most probable ",
" radius for the 1S state is called the Bohr radius and is the distance that corresponded to the outdated Bohr model, where the electron was represented by a standing wave at a fixed distance.",
"The Bohr radius is roughly 0.53 Angstroms or 0.053 nanometers.",
"The fun thing about Quantum mechanics is that there is a small by non-zero probability of the electron residing inside the traditional radius of the proton or inside the nucleus. There is also a small, but non zero probability that the electron is between ten and twenty feet away from the nucleus.",
"If you draw these probability distributions in 3D space, you see that they make interesting shapes, in some cases, the shapes of donuts, lobes and hallow spheres. I guess you can say that it's all empty space (as electrons have no known volume), but a region in space that you're likely to find the electron if you chose to go looking for it."
] |
[
"I'm aware of these 3D representations, which is why I asked whether the 1s orbital wholly occupies that space. That electrons have no known volume is a fair point; I was initially picturing a point mass whizzing around a larger central mass, but it would perhaps be fairer to say that there's a point charge exhibiting wave-like behavior appearing with some probability near the nucleus? In which case that 'empty space' would actually be occupied, though technically it's a vacuum.",
"*Edit for grammar and clarity"
] |
[
"Why are pigs and humans so alike?"
] |
[
false
] |
I know that pigs are close enough to humans that we can transplant organs from pigs. My question isn't about the big picture of evolution, but rather why are pigs so close to us compared to other mammals? Do we have a common ancestor with pigs that is closer than whales or dogs? Do we know anything about that common ancestor?
|
[
"I believe they're not any closer to us than other non-primate animals like whales or dogs, but I'm not an evolutionary biologist, so I could be wrong. People who receive grafts from foreign species (xenographs) are on heavy immunosuppressives. ",
"Pigs are, however, close to our size, and have similar cardiac anatomy such that we can use their aortic valves in human transplantation. Pericardial sac tissue of horses and cows can also be fashioned into a valve for a human. We are obviously more similar to primates like baboons, but on a practical standpoint, pigs are easier to keep and breed than large primates. There's also a smaller chance of disease transmission when you use a non-primate, since there are fewer things we can catch from them.",
"Also, whole organ xenotransplants have never really been successful as far as I know, besides as a temporary solution tiding people over until they can get a human organ. Transplantation from animals is certainly not common and mostly discussed in hypotheticals at this point."
] |
[
"Humans and pigs are not even closely related. In fact, phylogenetically, humans are more closely related to rabbits and rats than they are to pigs. The last common ancestor to both pigs and humans existed around 90 million years ago, towards the end of the Cretaceous.",
"I was trying to find one grand article that would help you out, but the best I could do was the ",
"Wikipedia article on evolution of mammals",
", but it is long and complicated, and you have to get through about ¾ of the article to actually answer your question.",
"Whales, dogs, and most other mammals are not very closely related to humans and primates. So, strictly speaking, your best animal friend should be a rabbit. Or a rat. ",
"As for transplants, those are usually temporary (thought there are some genetically manipulated pigs that may give a bit longer times). The genetic difference between pigs and humans are huge, and there's just too many immune issues for that.",
"So, in summary, pigs aren't closely related to humans. But remember to be nice to your cousin the rat."
] |
[
"Partially true. Their stomach is relatively similar as are the small intestines. Their colon is actually very different, though, as it is arranged in a spiral. Our colon looks more like that of a dog."
] |
[
"If the Hubble or Webb telescopes were focused on a region of space and exposed for a very long time, is it likely that the resulting image would be so dense with galaxies that they could not be differentiated."
] |
[
false
] |
The Hubble Deep Field image was an exposure of 10 days. The length was required in order to capture enough photons from the dimmer distant galaxies to be detected by the telescope's CCD. If you arranged for an exposure which is on order of magnitudes longer in duration, would so many photons eventually interact with the CCD to create an image that would essentially just be a bright rectangle with every pixel illuminated? IE instead of having black background with a few pixels illuminated by distant galaxies, have the entire field of view illuminated by the billions of galaxies which would fill in the gaps between other brighter galaxies?
|
[
"The problem is that you can't really expose for longer than one hour with the hubble, as after that it disappears behind the earth for a while. You could make a bunch of pictures of that timeframe and overlap them, but this would not give you an infinite integration effect if you would have infinitely many so to speak. ",
"The same goes for the telescope if it would be able to look at one point for one month, its pixels have a maximum light collection limit, after that you just get a white dot with no information at all. So yea, if you would let it integrate for a really long time, then it would get you a white picture, but it would not be intelligible. "
] |
[
"Hubble turns off much more frequently than that due to passages through the South Atlantic Anomaly. All Hubble images are made by coadding a series of shorter exposures."
] |
[
"So I suppose in those shorter exposures, the CCD would never reach the critical threshold where it does become activated by the faintest and farthest away galaxies."
] |
[
"Earth has been through several mass extinction events. How quickly did these occur? Did most life die out in months and years or centuries, etc?"
] |
[
false
] | null |
[
"Earth has gone through 5 mass extinction events throughout its 4.7 billion years of existence. While the exact duration of a mass extinction event is always disputed by scientists we have a general idea of their longevity. To give you an idea, the Permian-Triassic extinction event (also called the Great Dying) lasted about 60,000 years, give or take 48,000 years. ",
"This was discovered when ",
"geologists studied ash layers",
" (emitted from the eruption of the Siberian traps) in between limestone layers in China. This mass extinction occurred about 252 million years ago and many organisms started to disappear slowly after the eruption and the sudden warming and climate change that followed. Most of the life lost was from the oceans which experienced a calcification crisis (animials couldn't form shells because the oceans were too acidic). The Cretacious-Paleogene mass extinction that ended the reign of the dinosaurs lasted about 32,000 years. We are currently in the beginning of Earth's 6th mass extinction."
] |
[
"The rate of the disappearance of species is as big as the last mass extinctions.",
"Biodiversity is rapidly decreasing everywhere in the world due to us, humans."
] |
[
"It's really not that bold at all considering the impact humanity has on the biosphere, see ",
"https://en.wikipedia.org/wiki/Holocene_extinction",
"You could argue that it's not a 'true' extinction event because...no wait, you can't really.",
"Take a look at the makeup of mammalian biomass that's near the top of the page (or this xkcd ",
"https://xkcd.com/1338/",
"). We outweigh wild mammals by an order of magnitude. Our livestock outnumbers wild mammals by an order of magnitude and outweighs us as well."
] |
[
"Can somebody please help me understand, at least on some fundamental level, the expansion of the universe?"
] |
[
false
] |
I want to understand this subject to some degree, but any research I do is either above my head, or very inconclusive (I realize that there is no definitive answer yet). I'm trying to wrap my head around the whole 'what is it expanding into?' thing. From what I understand (or don't understand, not sure), expansion means that distances between objects in the universe are growing? Or is there a defined end to the universe that is actually growing larger and filling another void? I'm sorry if I sound like a complete dumbass here. I just want to understand this better.
|
[
"Yes, distances between galaxies are increasing even though they aren't moving through space - the space itself is expanding.",
"Imagine a raisin cake rising in the oven. The cake represents the Universe and the raisins are galaxies. As the cake rises the raisins all spread apart, even though they aren't moving through the cake mixture - it is the cake mixture itself that is expanding, and carrying the raisins apart.",
"As for 'what is the Universe expanding into', as best as we know, space and time are properties ",
" our Universe, not some 'box' that our Universe is in. So as it expands, it ",
" space, it isn't expanding ",
" space in any real way. Or at least, 'space' as the normal three-dimensional thing that we understand; there is no reason that the Universe doesn't exist within some higher-dimensional parameter 'space', but the dimensions of this won't be anything like the normal 'space you can move about in' as we understand it. That kind of space is a property of our Universe."
] |
[
"But the raisins in the cake are moving. The cake moves them by expanding. Is this where the analogy fails?",
"Jimmycorpse got it spot on, but just to re-iterate:",
"The raisins are moving with respect to one another, but they are not moving through the cake. With respect to the cake mix they are stationary. If you made a co-ordinate 'grid' system to measure position that expanded along with the cake expansion, the raisins would appear to be stationary. "
] |
[
"I probably don't belong in this subreddit, because I was a philosophy major and not a physicist. That being said, what is the difference between the emptiness that exists between objects in our universe and the emptiness that lies beyond. Further, what is to say that in the non-space void outside our universe that there is not another universe a gazillion light years away ... if there was, does the theoretical non-space become space. To me, space is the void where there is not stuff (including energy) be it between the stuff within 'the' universe or beyond it. Is there no concept of nothingness/void as something that lives within 'the' universe? Is it different than what apparently has no definition outside of our energy stuff? "
] |
[
"Does the Higgs boson itself have mass?"
] |
[
false
] | null |
[
"It comes from Higgs bosons interacting with the Higgs field that fills space.",
"Things that get mass get their mass from interacting with the Higgs field that fills space, NOT from interacting with Higgs bosons."
] |
[
"No, the Higgs fields are not made of Higgs bosons.",
"Rather, there is a Higgs field. It manifests in various ways. It has a non-zero value that fills space. That is what produces mass for various particles.",
"The Higgs boson is a different entity that the Higgs field can form.",
"In fact, the Higgs boson is a kind of deviation the Higgs field from this background value.",
"Put it another way: In processes that are too low energy for Higgs bosons to be made, electrons still have a non-zero mass. This is because the mass doesn't come from Higgs bosons."
] |
[
"Yes. It has a mass of around 125 GeV/"
] |
[
"How are amino acid residues on proteins numbered?"
] |
[
false
] |
Being of limited formal education in molecular biology, this has me stumped. For example, bovine serum albumin has 35 cysteine residues. Cys-34 is free. Why is it numbered 34? From the , the free residue resides in position 58 of the ~ 582 amino acids, and is the cysteine in the sequence. I'll accept Cys-1 or Cys-35, but 34?
|
[
"You're right about general numbering schemes.",
"In the case of bovine serum albumin, residues 1-18 make up a signal peptide that is cleaved off, and 19-22 are also apparently only present in the propeptide, not the final version (not clear where 23-24 go). The functional protein, then, consists of resides 25-607 (583aa total), as shown ",
"here",
". If you count from the beginning, and call amino acid #25 \"1,\" then the first cysteine is present at position 34, hence Cys-34. It's just a coincidence that there are 35 total cysteines in BSA. ",
"It seems to me that this should still be called Cys-58 and not Cys-34, but I'm not in charge :) It often depends on how much research has been done and how much has been published at the time of annotation. If the whole field has described the residues one way, they're not going to go back and change it. Maybe the initial residues were not appreciated until later. ",
"Hope this helps!"
] |
[
"This may be influenced by the fully processed or pre-processed variations of the same protein. Many proteins were discovered in certain states and numbered without knowledge of other primary structures. For example, in structure A this residue is cysteine in the 34th position, but the protein only takes on structure A in a certain disease state so more appropriately it could be cysteine 58. Though it is good practice to refer to the sequence number when numbering residues. :D"
] |
[
"Generally, they're numbered exactly as you would expect, from the first amino acid starting with \"1\" to the last in the sequence. However, there can be quite a lot of modification in the sequence, such as when introns are excised during ",
"splicing",
". This can throw off the numbering, and there's no absolute rule on it, so it's largely a matter of custom.",
"In your specific case, you are showing a precursor sequence, so I would assume the numbering occurred in a different, processed sequence (maybe even from another species as the one you gave is from a cow), and since the cysteine has a functional purpose, the numbering is carried over."
] |
[
"If the universe's rate of expansion is increasing can we assume this will happen infinitely? If so, what would happen when it is expanding near the speed of light?"
] |
[
false
] | null |
[
"Yes. The speed of light is not a limit on the expansion of the universe, since the speed of light, c, is, to quote wikipedia :'the upper limit of for the speeds of objects with positive rest mass'. In other words, it is the maximum rate at which information (and energy) can be propagated. This speed does not apply to the expansion of the universe, since the expansion of the universe is the expansion of space itself - it does ",
" mean that the objects in the universe are moving away from each other at some speed, but that the space between the objects in the universe (galaxies, galaxy clusters, etc.) is getting bigger."
] |
[
"According to Kraus's \"A Universe From Nothing\" lecture, once the universe's expansion rate exceeds the speed of light, we will no longer be able to see other galaxies because as fast as their information (light) is moving towards us, the space between us and the information source is moving away faster. ",
"Imagine two people flying away from each other extremely fast. Lets say faster than a bullet. Now one shoots a bullet at the other. The bullet (light & information) is still traveling towards the other person, but will never reach him, as the people are flying away from each other faster than the bullet."
] |
[
"To answer the first part of your question, the jury is still out. That, despite the Nobel prize in physics this year concerning the universe's accelerated expansion. What these scientists observed was an unexpected acceleration of the expansion as of many billions of years ago. It remains to be seen if that expansion has continued to the present. Further, we also don't know what's causing this expansion. So, I'd say it's premature to guess what will happen at some point in the future. Keep in mind - Einstein's relativity is ",
" getting tested to this day, and that's 100 years after first getting the idea out! "
] |
[
"Do we know how far earth is from the \"location\" of the big bang, or how far the farthest objects have traveled since the big bang?"
] |
[
false
] | null |
[
"I know that the concept of \"distance from the location of the big bang\" is tricky...",
"It's not just tricky. It's not meaningful. See the FAQ:",
"http://www.reddit.com/r/askscience/wiki/astronomy#wiki_big_bang"
] |
[
"I'm not sure what you're referring to with the link.",
"There's a post that states there isn't a center to the universe: ",
"http://www.reddit.com/r/askscience/wiki/astronomy/shapeoftheuniverse",
"Maybe I'm overlooking something, but I also don't see any references to distances traveled since the big bang. Other references?"
] |
[
"First link under the subsection on the Big Bang:",
"Did the universe start as a single point?",
"It doesn't make sense to speak of distance from some \"location\" of the big bang, because it didn't occur at one single point."
] |
[
"If water is at a certain salinity, could you drink an unlimited amount free of consequence?"
] |
[
false
] |
(I'm excluding dry drowning/ water going into the lungs.) So, if you drink purified water, the water goes into you cells via osmosis, popping them. If you drink seawater, water leaves the cell, causing the cell to die. Is there a value of salinity where neither of these would happen?
|
[
"If you drink water that is isotonic (~9 g NaCl per Litre of water) then you can drink an awful lot of it without any acute adverse consequences.",
"You can still drink too much of this solution but it just pushes the problem down the line, so to speak. I'm not sure what medical problem would manifest most acutely. Probably something to do with calcium, since that is the next most used ion."
] |
[
"Isotonic saline is used widely in hospitals for fluid replacement. As it has been correctly stated, 9g of NaCl per liter is isotonic to normal plasma. The major fluid compartments of the body are the intracellular and extracellular spaces. The extracellular space is further broken down into the vascular space and the interstitial space. The majority of your total body water (TBW) is in the intracellular space (2/3). The The interstitial space has about 1/4 of your TBW and the vascular space has about 1/12 of the TBW. When giving isotonic saline intravenously, it actually stays in the extracellular space because sodium is the major osmole of that space and potassium is the main osmole of the intracellular space). So for every liter of normal (isotonic) saline we give, 750cc goes to the interstitium and 250cc stays in the vasculature. We cannot give any amount of fluid we want because a person can go into fluid overload, which can lead to unsafe blood pressure, or pulmonary edema. You are correct in that the normal saline will not cause any major fluid shifts, since the tonicity is the same as the plasma.",
"So, to answer your question, the tonicity of water required would be 9%, and having too much would likely cause an unsafe elevation in blood pressure or pulmonary edema. As a point of clarification, hypotonic and hypertonic solutions do cause fluid shifts, but the body is incredibly resilient and will bring itself back to homeostasis. So, while pure water might cause some cells to swell (and could even lead to water intoxication if you really had too much), cells wouldn't really lyse from the osmotic pressure (and vice versa in the case of seawater). For that to happen, your electrolyte balance would be so off kilter you'd probably have bigger problems (like your heart stopping)."
] |
[
"I wanted to add a bit about the homeostasis resisting the change, particularly against hypotonic solutions, but couldn't eloquently sum it up. So thanks for adding a lot of detail!",
"To those reading on, this effect is the entire reason we have advanced life: Cell walls and transport mechanisms that allow effective compartmentalisation of ",
", working against Osmotic forces, is the very essence of cellular life."
] |
[
"Are optimally aerodynamic and hydrodynamic shapes the same?"
] |
[
false
] |
I understand that air and water are different fluids, with different reynolds numbers and the like, but given that the goals are usually the same - reducing turbulent flow, dynamic pressure, etc., are the differences we see in these types of shapes due to their different applications, or the different properties of the fluids? i.e. propulsion and materials aside, would an air- submarine and an underwater airplane look like their real world counterparts?
|
[
"In short, yes and no. You are correct that both air and water are fluids with different properties. The most important property is that both are governed by the physics of Newtonian fluids. That is, the viscosities of air and water increase linearly with added shear stress. Since both are Newtonian fluids, the dimensionless Reynolds number given as (density",
"length/viscosity) applies to both.",
"The magic thing about these dimensionless numbers is that they can be used to scale down systems through the manipulation of the variables. For example, it is common practice to test airplane wings in water to visualize aerodynamics using dye streams in water. This is because the aerodynamics and hydrodynamics are the same. However, the wing can not be at 1:1 scale and the water can not flow past it at the velocity the plane would travel through air. In order to properly model the aerodynamics of the system as hydrodynamics, the Reynolds number, as calculated for the wing in air, must be the same as that of the wing in water. In order to do this, the velocity of the water and the size of the wing must be decreased as compared to the speed of the air and the wing in reality. This counteracts the much higher viscosity of water as compared to air. ",
"Give the fact that you can manipulate the variables in the Reynolds number equation, yes the hydrodynamics of your underwater airplane and the aerodynamics of your flying submarine (think blimp) would do just fine, they would just move slower or faster according to the laws of fluid dynamics. ",
"Sources: \nundergrad fluid mechanics course for chemical engineers\nFluid Mechanics for Chemical Engineers, J.O. Wilkes",
"TL;DR Due to laws of fluid dynamics, a flying submarine (think blimp) and an underwater airplane would have appropriate aerodynamics/hydrodynamics given that sizes and speeds of the objects were changed. "
] |
[
"The dimensional analysis using the Reynolds number is correct up to some limit, and in the case you mention (air-submarine and underwater airplane) it can be valid for a big range of parameters. One big difference is that the Mach number is very different between air and water. More than that in the case of a normal airplane and a normal boat there is also a very big difference as the airplane is submerged in air while the boat is at the surface of water and energy has to be spent in order to create surface waves."
] |
[
"No. Wings are designed to efficiently generate lift and since planes are heavy, you need big wings. Submarines don't need to generate lift, so they don't need wings. The ideal aircraft shape is a wing with no fuselage (e.g. the B-2), while for a submarine, it's all fuselage (which it is). Don't be surprised if you're flying on a flying wing aircraft (no fuselage) in 30-40 years.",
"And yes, I'm an aerospace engineer."
] |
[
"Thumb vs Toe Prints"
] |
[
false
] |
Today I had quite a large piece of skin come off of my foot which looked eerily exactly like a thumbprint(bball in vans. bad idea). Which gave me the idea that you could use pieces of skin off of your feet(because they resemble thumbprints so much) glue them onto your thumbs/other fingers and go about committing crimes and getting away with them. However, i compared my toe print(actually the pad below it not the toe itself) to my thumb and they seemed identical. Q:Is your thumb print them same as your foot/toe print or am I a shitty forensics expert? if they are not, what would prevent people from getting away with crimes by faking their finger prints in this manner? if they are indeed the same, has anyone ever been arrested by toeprinting instead of fingerprinting?
|
[
"Your finger and toe prints won't always be identical. Fingerprints are formed during foetal development due to differential pressure experienced by the growing skin (known as the volar pad), and is not entirely due to genetics - that's how identical twins end up with different prints.",
"You ",
" glue your toe skin to your fingers, but to a trained fingerprint identification expert, the size discrepancy will be too obvious. Also, if the latent print of where the two pieces join is left on a scene, they'll have reason to suspect something is up.",
"Also, while your toe (or the pad below) print may ",
" identical to your thumb, it probably has the same overall pattern - that is, whorls, loops and arches - but not the same ",
", which is what forensic experts look for and match. These are points of bifurcation, ridge endings, islands, etc. that they look for, and their spatial relation is how they determine if two prints have the same source. It is also what they enter into a database such as ",
"IAFIS",
"."
] |
[
"is there any way to make your fingerprint indiscernable?",
"Well, you can sandpaper your fingerprints off if you want, but then you'll be leaving other bodily fluids (blood, etc.) instead. Even if you let it heal a little, most forensic identification experts will tell you that a scar or injury makes it ",
" for someone to match a print. Of course, if sufficient time elapsed in between the crime and when you were arrested, you'll have your normal print back since your fingers would've healed. Do keep in mind though that fingerprints is never the only evidence one uses to convict a person.",
"If you really want to hide your prints... Why not just wear a damn glove? Better yet, wear ",
". Dispose of the outside glove at a discrete place, to prevent bringing home anything from the crime scene. The second pair would contain your own prints on the inside, so you won't be leaving anything directly identifiable on the outside pair. But again, this is way too much trouble for ",
". There is almost something you forget - shoe prints, bodily fluids, hair, fibres are just a fraction of physical evidence that can link you to a crime scene - either by you leaving something behind or you picking up something from the crime scene: ",
"Locard's principle",
"."
] |
[
"The two gloves is something people often don't think of, then when they, say, get the victim's blood on the gloves, and their fingerprints are on the inside of the glove, that's one piece of evidence connecting you to the victim.",
"One of the resident physicists will help you a lot more - I know next to nothing about the strong and weak forces. The only relation I have with that area is that my field has the word nuclear in it, but it's really there just to denote a difference from electron paramagnetic resonance. It's sufficient in my field (as with most of chemistry) to treat the nucleus as one entity."
] |
[
"Do different kinds of dairy affect lactose intolerant people differently?"
] |
[
false
] | null |
[
"ie. Will some yogurt make me poop more than milk? ",
"Real yogurt is lactose free."
] |
[
"... All my life.... All my life i have been avoiding yogurt. Whyyyyy"
] |
[
"I've honestly never understood why people don't like yogurt. I've always loved it. Kefir is really good too.",
"Note that I mentioned \"real\" yogurt. That would be yogurt cultured by bacteria. They process the lactose in the culturing process leaving you with a lot of healthy stuff. There are some \"yogurts\" out there that are thickened with chemical thickeners... essentially you are talking about pudding at that point."
] |
[
"Can anyone tell me the name of this rock?"
] |
[
false
] |
[deleted]
|
[
"Can you provide any more information? Where did you find it, how heavy is it, etc? More pictures might also help.",
"That being said it reminds me of a rock I once found which appeared to be porous obsidian. That had a similar structure but was more of medium-dark gray rather than the deep black the stone in your picture is. I really can't say for sure based just on the picture. Maybe someone with a better geology background will recognize it but more info would definitely help."
] |
[
"I'm stumped. Obsidian that formed foamy rather than dense is still my best guess (gas bubbling through it as it cooled perhaps). Hopefully someone my knowlegable than I can help you out. This ",
"rock identification guide",
" may help though it seems some of the questions are tough to answer... "
] |
[
"As entropy2057 said, we need some more info.",
"I have seen graphite look like that, so if it doesnt mark paper I can't help you without maybe some more angles, where its from, etc. "
] |
[
"Why prescribe different antibiotics for different illnesses?"
] |
[
false
] |
[deleted]
|
[
"Broad spectrum antibiotics work against many types of bacteria including both gram positive and gram negative. But different bacteria have different strengths and vulnerabilities, so if they can identify the specific bacteria it can be targeted by a narrow (specific) spectrum antibiotic.",
"The main reason for wanting narrow spectrum antibiotics is to avoid killing the good bacteria (in your gut for example).",
"Wikipedia",
" explains it pretty well.",
"\"Antibiotics are often grouped by their ability to act on different bacterial groups. Although bacteria are biologically classified using taxonomy, disease-causing bacteria have historically been classified by their microscopic appearance and chemical function. The morphology ",
" of the organism may be classified as cocci, diplococci, bacilli (also known as \"rods\"), spiral-shaped or pleomorphic. Additional classification occurs through the organism's ability to take up the Gram stain and counter-stain; bacteria that take up the crystal violet dye stain are referred to as ",
" those that take up the counterstain only are ",
" and those that remain unstained are referred to as \"atypical.\" Further classification includes their ",
" (ie, aerobic or anaerobic), patterns of hemolysis, or other chemical properties. The most commonly encountered groupings of bacteria include gram-positive cocci, gram-negative bacilli, atypical bacteria, and anaerobic bacteria.[4] Antibiotics are often grouped by their ability to act on different bacterial groups. ",
"For example, 1st-generation cephalosporins are primarily effective against gram-positive bacteria, while 4th-generation cephalosporins are generally effective against gram-negative bacteria.\""
] |
[
"The effectiveness of individual antibiotics varies with the location of the infection, the ability of the antibiotic to reach the site of infection, and the ability of the bacteria to resist or inactivate the antibiotic. ",
"Antibiotic susceptibility testing (AST) is an in vitro (laboratory) measure to assess the likelihood that a particular antimicrobial agent will treat an infection caused by a particular organism.",
"In clinical practice, antibiotics are most frequently prescribed on the basis of general guidelines and knowledge about sensitivity. ",
"For example, uncomplicated urinary tract infections can be treated with a first generation quinolone, because E. coli is the most likely causative pathogen, and it is known to be sensitive to quinoline treatment."
] |
[
"Not all antibiotics work on all types of bacteria. There are broad spectrum antibiotics, narrow spectrum antibiotics, etc. Certain infections are more likely to contain certain types of bacteria and therefore are more likely to respond to certain antibiotics. When possible testing is done to see which type of bacteria you have and which antibiotic it will be sensitive to. "
] |
[
"Does light make a sound (in a medium)?"
] |
[
false
] |
In my very surface level understanding of the physics of photons, I know that photons have no mass but still have momentum that can be transferred to other particles. If enough photons interact with a medium, could it generate compression waves and be registered as sound? To be specific, I don't mean signal transformations like radio waves where signals are coded and decoded. And would there be any correlation between the frequency of the light and the frequency of the sound? (If any part of this post sounds weird, it could be because the question came to me as I was trying to go to sleep. But, it could also be that I have no idea what the hell I'm talking about.) Anyway, thank you and I'm looking forward to the responses! :D
|
[
"Sonoluminescence",
" is the production of light from acoustically excited bubbles in a liquid. The time-reversed equivalent of this process would be light producing sound, so it’s certainly possible in theory. ",
"A more straightforward approach to produce sound from light is to focus a laser beam to a small waist, then modulate the beam at audible frequencies. This will periodically heat and cool the air near the focal point, resulting in a buzzing sound commonly heard in many laser physics labs."
] |
[
"Well, within a solid (and actually in liquids and gases too), the \"quantum\" of sound is called a phonon, and photons can absolutely be absorbed by a material to create said phonons. So on the face of it, that's your question answered. However, if we think sound, like from a speaker, there's actually more going on than an excited bath of phonons. For one, sound waves from a speaker are coherent, all the phonons have the same starting phase and are \"in step\" with one another, like a laser. This because they're all being excited \"in step\" by the compressive push of a diaphram, which gives them sort of the same \"starting line\". Phonon excitation by a laser is a random process. You might wanna call this \"random sound\", but actually it's just heat loss (the two are the same).",
"And would there be any correlation between the frequency of the light and the frequency of the sound?",
"Yes, definitely. As you said, light has both energy and momentum, and each photon->phonon event (what's calldd phonon emission) must conserve energy and momentum. For acoustic phonons, which are responsible for sound (there're other types of phonons) their energy vs. momentum relationship is actually very light-like and there'd thus be a pretty direct mapping of wavelength of light absorbed to a mapped/shifted wavelength of phonon. Regardless, frequency must match exactly (for single absorption/emission processes)."
] |
[
"Optical phonons are gapped, acoustic phonons are not and can have arbitrarily long wavelengths, that's the main distinction. ",
"Regardless, if we're willing to get a little hand-wavy, look at a typical optical phonon gap, take for example that in our favorite material silicon; the energy cost to excite an optical phonon in Si is ~60meV, which corresponds to a frequency of (60meV)/hbar = 90 THz (which is 90,000,000,000 kHz). Humans hear sound in the range of like 20 Hz to 20kHz. Frequency is matched at an interface (speed and wavelength are not), so if you blast the highest pitch human-audible noise possible (20kHz), it's still a billion times less than where you're going to need to start exciting optical phonons."
] |
[
"What causes that we sometimes \"feel\" a presence whenever we are afraid?"
] |
[
false
] |
I am intrigued by what kind of brain signal is involved on this: For example, we watch a horror movie. Later we go to the kitchen to have a glass of water, and when coming back to the bedroom through the dark corridor, we falsely feel the presence of someone. Has science documented this kind of reaction? Thank you very much and sorry for any mistakes when writing. I am not native english.
|
[
"We're scared so our brain is in panic mode. We perceive the slightest movements to be a crazy axe murderer, because hey, better safe than sorry."
] |
[
"Sound, vibrations from walking, seeing them, etc. It's just that it is very subtle so you don't really notice it."
] |
[
"Sound, vibrations from walking, seeing them, etc. It's just that it is very subtle so you don't really notice it."
] |
[
"The instructions say \"microwave for 60 seconds at 800w\", but my oven only goes up to 750w. How do I calculate the appropriate time for my oven?"
] |
[
false
] | null |
[
"60 * 800 / 750"
] |
[
"Can plain inverse proportion really be applied here though? I feel like cooking 64 seconds at 750w wouldn't be the same as cooking 60 at 800.."
] |
[
"In terms of pure energy delivered by the microwave oven: Yes. The unit \"Watt\" is defined as Joule/second, so if you multiply the amount of Watt by the number of seconds, you get the total energy.",
"Since microwave ovens don't deliver the energy in the same way to all areas of the interior (which is why the bottom plate in the oven often rotates), longer runs at a lower power setting tend to heat up the contents more evenly. On the other hand, it also gives the contents more time to lose their heat to the air inside the oven. However, at the time scale you're looking at of about a minute, this effect is negligible."
] |
[
"What is fire?"
] |
[
false
] |
What is fire itself made from? and what causes it to form the shape it does? I hope this question doesn't seem stupid or anything...
|
[
"The visible flame is composed of hot gases (carbon dioxide, water etc.) and particles of soot (unburnt carbon and other particulates). Its shape is mainly formed because the hot gas is rising while being cooled down by the surrounding air. The gas nearest the edge cools fastest and so stops glowing sooner whereas the gas at the centre can rise further before this happens, creating the tapered shape. There are many other aerodynamic factors that influence the flame but this is a basic overview"
] |
[
"Fire is essentially a chemical reaction of some object with another, usually oxygen. In typical combustion, a hydrocarbon, like methane CH4 reacts with 2 O2 moleclues to form CO2 and H2O. Because the potential energy of CH4 and 2 O2 is higher than CO2 and H2O heat is released, the CO2 and the H2O leave the reaction with a lot of energy. Essentially the same thing is occurring for essentially anything you burn. The products might be different, but the concept is the same.",
"The reason you see the color you see is because the gas near the flame is hot enough to radiate, in the same way that metal can be heated up enough so that it is glowing hot. The reason it tends to rise upwards is because hot gases tend to be less dense than cold gases. The hot CO2 and H2O leave, and tend to go upwards, until they cool off enough that you can no longer see their radiation. Fire in zero gravity looks very different than what you see on earth, because there isn't a tendency for the gases to rise upwards."
] |
[
"Link"
] |
[
"Boil, Microwave, or Wait for Tap? Which way is more efficient to get a cup of hot tea?"
] |
[
false
] |
[deleted]
|
[
"This depends solely on the efficiency of your microwave/water kettle.",
"And what do you consider efficient? The water from the tap has to be heated, too. It's not like it's naturally hot. From your description it seems that you have a serious amout of time before hot water comes out of your tap. In my house i get the hot water in like 5 seconds, so waiting + boil (in a water kettle, not on a stove) would be the fastest option."
] |
[
"I know the water from the tap has to be heated. So does the water in the pipe. There are losses from the pipe filling with hot water, which is never used as hot. That's one area of waste. ",
"So...my question is, which method takes less energy to get a 250 ml cup of 100 C water? Included in that calculation, is the losses associated with each method..."
] |
[
"You should not ever cook with or consume hot tap water, it leeches undesirable things from the walls of your water heater and piping."
] |
[
"How can atoms that are bosons occupy the same state in a Bose-Einstein condensate if they are made of fermions which can't be in the same states as each other?"
] |
[
false
] | null |
[
"If you have a Bose-Einstein condensate of Cooper-paired fermions (I'll just call this a superconductor), and write the wave function in terms of the underlying fermions, you'll find the the Pauli exclusion principle still holds for all of them.",
"I'll come up with an example of how this works using the simplest model of a superconductor. Let's say the original fermions form a circular Fermi surface, ",
"like this picture",
". In the standard Cooper pairing theory, the Cooper pair bosons are made up of electrons with equal/opposite momentum +/-",
" (a vector) like the picture, and the resulting boson will have ",
" = 0. However, you can do this for any ",
" on the Fermi surface, so you can have many bosons in the same state (",
" = 0) by combining electrons with different states (",
" lying on distinct points on the Fermi surface). So we don't violate the Pauli principle.",
"I realize your question is also germane to any BEC, not just those from Cooper pairs. A similar construction can be made for other BECs, like ",
"He atoms made up of electrons and nucleons full of quarks. The individual fermion momenta will add up so that the bosons are at zero momentum, but no individual fermion needs to be in the same state as another."
] |
[
"Since you seem to be an expert on this topic (especially given your tag), follow up question, if you don't mind. ",
"We're often taught in physic classes than ",
"He will act as a boson because it can have a zero or integer spin nucleus, which is why it behaves as a superfluid at cold, but not extremely cold temperatures. But what is special about ",
"He. Why doesn't this apply to any atom with an even number of nucleoids?"
] |
[
"Tangential topic but what condensed matter textbook do you recommend? "
] |
[
"[META] April Fool's Day is Over: The Demise of Sponsored Content"
] |
[
false
] |
Many of you saw the new plans on AskScience. We introduced it at about 6:30 AM in the timezone of New Zealand, and have kept it going through sometime this afternoon (though it got more and more ridiculous as time went on!). We progressed from shilling oil to shilling homeopathy and quantum healing. We broke our own rules (non-scientific content on AskScience). We also broke with the time-honored convention of assuming every redditor is in an American time zone. Many of you were not amused by our clear abandonment of the preferred time zones and unsubscribed in protest (though bizarrely we have more subscribers now than when we started). Those of you who fell for it shouldn't feel too bad: some of our own panelists who missed the memo were even angrier than you. We can all be somewhat proud that some of them resigned in protest, at least until we pointed out the date. Our modmail and PM volume was much higher than normal - both people who were extremely amused, and people who were extremely angry. Over the day, the mods got called every name in the book, and got called on to resign (more than once). And while we don't like getting angry mail, we like seeing how much everyone cares about this corner of the web, and rest assured that we care about it too. Everyone pulled together to make sure the crappy sellouts who mod this place didn't get their way, and we thought it was awesome that so many people were so defensive of AskScience's integrity. But rest assured: no one is going to be putting any Sponsored Content in, we haven't hired an , and the guidelines of the subreddit are firmly in place. There was no Grand Design or pedantic lesson behind this joke (we just thought it would be fun!), but two things should be made clear: Scientists aren't humorless robots And industrial science isn't inherently bad (many of our panelists work in industry, and are great scientists). The intent wasn't to mock industry, it was to mock transparent PR, and to have fun pretending like we were blatant sellouts. So, it's back to business as normal on AskScience. Sound off below if you have something you want to say about the April Fool's prank, or if you have to say about AskScience. To further the joke, we had been removing everything that mentioned April Fool's. We're going back and undeleting those, so you can see how many of those posts there were.
|
[
"I can't believe people actually got angry over this. It was obvious that it was an April Fool's joke and I for one thought it was hilarious. "
] |
[
"I can't believe this was all a joke. I was patiently waiting for my money to arrive and now I found out that there wasn't any. This is completely unacceptable."
] |
[
"Sure, why not."
] |
[
"Why did the big bang not result in the creation of a black hole?"
] |
[
false
] |
So, if moments after the big bang, the universe existed in an immensely hot and dense state, and given that E=mc (mass and energy are equivalent), why didn't the immense density result in a gravitational collapse, and the creation of a black hole?
|
[
"This question has already been answered here.",
"http://www.reddit.com/r/askscience/comments/qxyne/why_didnt_the_big_bang_turn_into_a_black_hole/",
"http://www.reddit.com/r/askscience/comments/ovxel/mass_of_the_universe_right_after_the_big_bang_why/",
"http://www.reddit.com/r/askscience/comments/i3vyg/why_didnt_the_universe_collapse_into_a_black_hole/"
] |
[
"regions of space that the Universe have yet to have expanded into",
"There are no such regions; the universe is ",
" of space, and (as far as we can tell) there are galaxies throughout all of it."
] |
[
"E=mc2 is an incomplete formula, describing only stationary objects. It's truly written with an additional + mv for mass x velocity aka momentum.",
"You're right E=mc",
" is somewhat incomplete if m is the rest mass but adding mv does not fix the problem. That does not even have the same dimension. The complete formula is E",
"-p",
"c",
"=m",
"c",
". Where p is the spatial component of the 4-momentum (so basically p is the generalization of the Newtonian momentum that makes sense in Relativity). If the particle is at rest (p=0), we get the popular formula E=mc",
".",
"One can define a relativistic mass m_rel so that E=m_rel c",
" always holds but we don't really do that anymore."
] |
[
"How did researchers avoid triggering a severe immune reaction with mRNA vaccines?"
] |
[
false
] |
Just read this great article ( ) detailing the development of the COVID vaccine, had my mind blown by how crazy/smart the development of this vaccine was. It mentioned one of the big obstacles in animal trials was avoiding a dangerous immune response, but the solution was glossed over. The article mentioned the vaccines developed just coded for the spikes, so was the solution to use just a tiny amount of mRNA so it would fly under the radar and avoid triggering an allergic reaction?
|
[
"Freely floating mRNA causes immune reactions but also blood clotting and other dangerous stuff.\nAs far as I understand the vaccines use specific coatings for the mRNA, which liberate the mRNA when it enters muscle cells and thus prevent it to float freely."
] |
[
"The mRNA is encapsulated in a lipid ‘shell’ kinda like the sugar coat on an ",
"m@m",
"... this shell does two things-",
"1) hides the mRNA from the host immune system so it doesn’t trigger a response and",
"2) the shell fuses with the membranes of muscle cells and releases the now naked mRNA into the cell cytoplasm where it is transported to the correct location for processing into the protein the mRNA message codes for."
] |
[
"A liposome, liberal definition would be a ball of fat that houses the mRNA in its center.",
"Also, the general idea would be for dendritic cells to take up the mRNA."
] |
[
"How are we able to tell how animals sees things? e.g. a snake seeing in infrared or a mantis shrimp seeing more colors than a human can"
] |
[
false
] | null |
[
"Researchers do this by analyzing the pigments contained in the light sensing organs of animals. Specific pigments react to certain wavelengths of light, so presence or lack of those pigments determine the light wavelengths that are detectable by an animal."
] |
[
"There's two basic lines of approach here. First is looking at the structures that do the seeing and determining what kinds of light they respond to. You can look at the light absorbing chemicals in the eye and see what wavelengths they respond to. You can also measure the electrical response of the nerves connected to the eye and see what colors of light they react to. ",
"Second is training the animals to react to light of a specific color in order to get a reward. Nearly all animals can be trained to do this sort of thing, and if you can train them to, eg, push the green button to get food and they can tell the difference between green and red buttons, then you know they can see the difference between green and red. ",
"A few notes on your specific examples: Snakes don't exactly ",
" in infrared, they sense it with the pits on their face rather than their eyes but this is good enough to let them hunt rodents in the dark, although the image is pretty blurry. Here's a paper, which also discusses ",
" they estimated the nature of the image by studying the physics of the pits [link]",
"https://jeb.biologists.org/content/210/16/2801",
").",
"Mantis shrimp are actually ",
" capable of seeing nearly as many colors as they have a popular reputation for. If you just look at the eyes of the shrimp you find a ton of color-receptive pigments. So if shrimp eyes work like human eyes (we have three color receptive pigments, and combine their inputs to produce all the colors we see) then they would be seeing an enormous number of colors. But when behaviorally tested, mantis shrimp are actually pretty bad at distinguishing colors...worse than humans. When people looked at the neural wiring of mantis shrimp eyes, they found that they don't work like human eyes. Instead of combining inputs from different pigments to determine color, each pigment operates independently looking for one color at a time"
] |
[
"Nobody really knows, but rats and new world monkeys have been successfully taken from 2 to 3 color sensing pigments and developed the ability to see more colors as a result, so I suspect it would work. ",
"Medical regulators frown on doing experiments on people because it's really awesome instead of because it would cure some sort of disease, so no one has tried it in humans. I suspect it would work, possibly even as gene therapy in adults."
] |
[
"Does light exhert a backward force to propel itself forward?"
] |
[
false
] | null |
[
"Physics"
] |
[
"Physics"
] |
[
"Does light exhert a backward force to propel itself forward?",
"Yes. Light carries momentum."
] |
[
"Is there feasible, truly green energy?"
] |
[
false
] |
Lead by a comment in a below thread, in how dams are heavily impacting the nature in and along the subjected river, I wonder, is there truly green energy that is feasible to produce? Green energy here being energy that doesn't impact nature (in a negative way). So, what am I missing here? Can we, in a scifi mindset, produce truly green (and cheap, or at least economic) energy for the world? Or do we always have to alter (up to) massive parts of nature to satisfy our ever growing hunger for energy, given that we are not willing to pay endless amounts for the power generated?
|
[
"I think your outlook is far more bleak than is warranted by the technologies which are already available and the rapid ongoing progress in their advancement. At the moment, the key barrier to the transition from an economy mostly reliant on fossil fuels to one primarily powered by renewable energy sources is not as much the feasibility as the cost. If were to face the imminent prospect of running our of oil and gas, we could realistically switch to renewables within a reasonable amount of time. In fact, in certain countries renewable energy sources already play a dominant role, e.g. as in Iceland, where ",
"85% of the primary energy stems from renewable energy sources",
" such as geothermal, Norway, ",
"where most electricity comes from hydroelectric power",
", or Denmark, where ",
"half of the electricity is generated by wind power",
". ",
"In reality, any practical switch to renewable energies will rely on a package of renewable energy sources optimally suited to the region in question (i.e. a greater reliance on solar energy in deserts, more reliance on hydroelectric power in the cold North). Now when it comes to environmental concerns, I would argue that virtually all of these sources are far, far, less harmful than burning fossil fuels, especially when you consider the ",
"uncertain consequences that may result from fossil-fuel driven climate change",
". However, I want to focus on one technology namely solar power, which is arguably one of the most environmentally benign sources of energy and also one of the most abundant sources of energy, as described in through detail in ",
"this excellent comprehensive report from MIT",
". ",
"The first thing to emphasize about solar power is that it is really abundant, as you can see from a map of the annual ",
"solar energy incident on Europe",
" To get a better feel for these numbers, we can consider how large of a surface area we would need to collect solar energy to fulfill our electricity or total energy needs. A good general guideline to keep in mind is that in developed countries electricity consumption is usually about one fifth of total energy consumption. In Serbia for example, the per capita electricity and energy consumption in one year are roughly 4,000kWh/year and 20,000kW/year respectively. So if the insolation is roughly 1,600kWh/m2, this means that each person would need to collect solar energy over about 3m",
" to secure his energy needs or about 13m",
" to secure all of his energy consumption.",
"Now of course as you said we can't collect solar energy with 100% efficiency. However, the best silicon PV modules now approach 20% power conversion efficiencies, which really isn't all that shabby, especially when you consider that the thermodynamic limit for such cells is about 30%. When we need to multiply the two areas by 5 to get 15m2 and 65m2 respectively. Now these numbers may sound high, but to put them in context, Serbia has a population density of 93people/km",
" , which means that each person gets roughly 11,000m",
". Therefore, if 0.15% of Serbia's land was covered by 20% efficient PV panels, then it could secure its electricity needs, while coverage of less than 1% of the land by PV panels would secure all of its primary energy needs."
] |
[
"I think what you're really asking is, \"is there a way to power a civilization without affecting the natural world in any way\", and the answer is plainly \"no\". There's no free lunch for energy. It has to come from something, and inevitably that means something has to change.",
"One of the biggest failings of \"green\" thought, is to fail to recognize the difference in scale in the negative consequences of different forms of power. In other words yes, hydro or solar or wind or nuclear have negative impacts associated with their construction and/or operation, but these impacts are local/regional. The fact that they let us burn less fossil fuels is ",
" beneficial."
] |
[
"Why have you chosen such a strict definition of the word 'green?' ",
"All human activities have an environmental impact. It's all just pushing stuff around from point A to point B. ",
"Environmentalism shouldn't necessarily be about 0 impact, but mitigating the impact to the point that we can 'live with it.' "
] |
[
"Are there any natural processes in which something grows in order to contain a toxin?"
] |
[
false
] | null |
[
"Could you explain a bit more? Do you mean a growth to surround a toxin and prevent it causing harm? I guess you could say antibodies 'grow' around toxins by recognising their structure and binding to them. This creates a shield to prevent the toxin interacting with the body. ",
"http://en.wikipedia.org/wiki/Neutralizing_antibody"
] |
[
"There are structures in many cells that contain substances that would be toxic if they weren't contained within the structure. Lysosomes for example contain digestive enzymes in an acidic solution. These structures grow for the sole purpose of isolating otherwise toxic substances from the rest of the cell, keeping the cell safe and providing a distinct location for hazardous biochemical processes to take place."
] |
[
"I can't give you an example of a structure built to contain (I think you're implying sequestration) a toxin, but I can give you evidence of acquiring a way to be resistant to a toxin. Many many bacteria produce toxins to kill other bacteria for environmental competition. These bacteria develop an anti-toxin or immunity protein to render the toxin harmless in them but active (toxic) in cells that don't have the immunity proteins. So, these bacteria have to evolve to have the immunity protein in order for the toxin to be made (or else it would kill them too!). Some of this comes from random mutagenesis in the genome to make a gene encode an immunity protein, some times the systems get picked up from the DNA of other dead bacteria in their environment.",
"In bacteria like E. coli, myxococcus xanthus these are toxin/antitoxin systems: ",
"http://en.wikipedia.org/wiki/Toxin-antitoxin_system",
"E.coli (and Burkholderia) these are Contact-Dependent Inhibition (CDI) systems. (The articles on this are all scientific, a google search will give you a lot of abstracts to read, but I haven't found a good layman article)."
] |
[
"How do driverless cars detect and deal with ice and snow, especially when it comes to braking?"
] |
[
false
] |
As a seasoned winter driver, I often have to anticipate stopping distance and turn speed based on the road conditions ahead. You could be on a plowed patch of a main road, and then turn onto a snowy/icy side street. How do driverless cars deal with these conditions? Do they also “see” or is it based on the tire traction?
|
[
"Snow is a “really interesting problem” for self-driving vehicles, says Carl Wellington, a senior engineer at Uber’s autonomous research centre in Pittsburgh. No company has quite yet claimed their cars have mastered the ability to drive through snowy conditions. ",
"1"
] |
[
"I think OP is asking more regarding anticipation based on visual perception rather than real-time adjustments. For instance, living in New England, I drive in snow all the time. One technique is, heading uphill, you notice some snow 2/3rds the way up the hill. In anticipation of getting to the snow I will gather some speed so the momentum carries me through and I am not relying on adjusting my speed once I get there. If you wait until you reach the snow, coupled with the incline, often you won't have enough traction to pick up your speed and you will get stuck. This is the tactical knowledge that may be hard for a computer to anticipate. "
] |
[
"If you would not add some artificially produced effects on the steering wheel, so you can feel the condition, the driving experience would be worse.",
"This was actually a problem they discovered when power steering became popular. They originally made it so there was nearly no resistance, for the optimally smooth ride. However, the disconnect between the steering wheel and the motion of the tires was too much, and they ended up emulating fly-by-wire."
] |
[
"How do we know scientific studies are true and not fake?"
] |
[
false
] |
[deleted]
|
[
"When publishing a study in a journal, there is a peer-review and editorial-review process.",
"Authors are required to indicate their funding sources. This tells you who paid for the study and why. If you fail to acknowledge sources of funding (or even income) which might indicate any bias — you may have to retract your paper or even lose your funding. More serious actions include not being able to publish in a specific (or any) journal for a certain number of years — a forcible perish in the \"publish or perish\" model of academia.",
"Additionally, every journal/stylistic approach (APA, MLA, IEEE, etc...) require particular reporting mechanisms of your statistics. It's easy to build back numbers from there to find out if numbers don't match/make sense. ",
"In the major peer-reviewed journals, trust isn't much of an issue. Studies aren't going to be fake. They might not be experimentally perfect, but they won't be fake."
] |
[
"There is no guarantee. ",
"However, the beautiful thing about science is that for it to be accurate - it must be replicable by an independent party. So peer review and verification usually helps root out the evildoers :)"
] |
[
"There's a more pernicious kind of bias than outright fakery.",
"Let's say you've got 2 scientists. Both of them are honest and set up their experiments. But they each have very slightly different methodology, and honest differences of opinion about what the \"right\" method is, and the arguments are subtle and detailed and only understood by a few. ",
"Well, suppose one scientist gets an answer that happens to be more favorable to a company with deep pockets. The company responds by funding that one alone to do more research, while the other one receives less funding, and can do fewer experiments. As a result, the overall \"body\" of studies ends up biased towards the favorable set of results, even though everyone was \"honest\". Add to that the fact that the favored scientist now has both financial and scientific reputation to defend in arguing for the chosen method, and you can see how it can get entangled.",
"Replication is great, but ultimately limited by the funds to perform the replicates."
] |
[
"Am I reading this Pubmed article about accuracy of pregnancy ultrasounds correctly?"
] |
[
false
] |
(It isn't long) As I understand it, they took 280 pregnancies. They used IVF to determine exactly when conception occurred. In the 1st trimester, CRL was accurate inside of 7 days about 99% of the time. In the 2nd trimester, BPD was accurate within 7 days about 8% of the time. Is that correct? Also, what exactly does this mean: In singletons there was a high correlation in the gestational age at birth assessed from the time of IVF and from CRL, from the time of IVF and from BPD. If I've posted this in the wrong place, please forgive me.
|
[
"It looks like CRL was accurate within 7 days ~99% of the time (3/208 were off by more than this) and BPD was within 7 days ~87% of the time (27/208 were off by more). I got that info from the results section between Tables 1 and 2. ",
"As for the sentence \"In singletons there was a high correlation in the gestational age at birth assessed from the time of IVF and from CRL, from the time of IVF and from BPD,\" the grammar is strange but I'm guessing they mean that the gestation ages predicted by CRL, IVF, and BPD are highly correlated. They also say that more clearly in the results section: \"There was a significant correlation between gestational age at birth assessed from the time of IVF and from CRL (R = 0.992, p < 0.001), between gestational age at birth assessed from IVF and from BPD (R = 0.975, P < 0.001) and between gestational age at birth assessed from CRL and from BPD (R = 0.975, P < 0.001).\" "
] |
[
"Thanks.",
"I know the percentages go up if you only count the singleton pregnancies, but did the study say anywhere that they weren't counting the twins?"
] |
[
"For that first set of accurate within 7 days statistics both singletons and twins are definitely included (\"In three pregnancies there was a difference of more than 7 days between the gestational age estimated from the IVF and from the CRL and in 27 pregnancies between the gestational age estimated from the IVF and the BPD; in the latter group there were five twin pregnancies (10 infants).\")",
"The age correlation statistics don't explicitly say that they are singleton-only so it seems they include twins. If the twins were taken out, the correlation coefficient would be stronger. "
] |
[
"How can TSA/Airport security workers stand next to X and T ray machines all day everyday without any ill effects?"
] |
[
false
] |
I know the people walking through the machines have nothing to worry about, but are there any precautions in place to stop the workers absorbing these rays? Do the machines focus the radiation into one area? Thanks in advance.
|
[
"Like any radiation worker, they apply ALARA. That means that you should take steps to make your radiation exposure \"As Low As Reasonably Achievable\". The ways to do this are the maximize distance from the source, minimize time near it, and use shielding when possible.",
"If you pay close attention when passing through security, you'll see that they rotate between positions throughout the day. So the people operating the x-ray machines rotate around to other positions as well.",
"You may also notice that some employees are wearing badge dosimeters. These are little badges that you wear on your body. Over time they will accumulate on average the same exposure density to radiation that your body does. Every few months you send them in for testing to see if you had an abnormally high exposure within that time.",
"I don't know much about the manufacture of their machines (I'd guess it's not something they want the public to know much about), but it's not hard to add some shielding to strongly attenuate x-rays."
] |
[
"You can use anything as long as the mass is sufficient. Lead is used because it's relatively easy to make and use a lead apron, for instance, and since lead has a high atomic number it will block a lot of radiation per unit of volume. ",
"But really in theory you could construct a wall of french toast to protect you from gamma rays, it would just depend on how much radiation we're talking about (and of course how much french toast). "
] |
[
"I didn't think I would ever need to know how thick a wall made out of french toast needed to be to protect me from gamma rays, but now I do.",
"How much french toast would I need to protect myself from an xray machine?"
] |
[
"Why does smoke from incense or cigarettes not (usually) set off smoke alarms?"
] |
[
false
] |
[deleted]
|
[
"Fire Alarm / Smoke Detector Manufacturer here.",
"Household smoke detectors (excluding Heat and CO) fall into two catagories, Ionisation and Photoelectric.",
"Ionisation smoke detectors (the detectors with the radioactive symbol on their label) are designed to detect sub-micron particals of combustion (different to thick smoke). Examples of sub-micron particles are cooking vapours (released from opening a hot oven, for example), vehicle exhaust, birthday candles, lighters, etc. Basically smoke that is the result of something burning, but you can't see. Ionisation smoke detectors in the house react well to fast, flaming fires (flash flame from a pot of oil on the stove, for example). ",
"Photoelectric detectors simple use an infrared transmitter and receiver in a chamber in the detector. As thicker, visible smoke enters the chamber, it blocks the light and the detector registers this change. When the smoke obscures the light to a certain level, the detector goes into alarm. Photoelectric detectors react well to visible smoke (cigarette, burning meat, smouldering natural fibres, etc).",
"To answer your question, it's very likely you have an Ionisation type installed, and it is not detecting sub-micron particulate in the air (as cigarette smoke is greater than one mircon and the ionisation detector cannot see it).",
"Please read this: The majority of household fires are caused by smouldering fires (burnt out motors, rats chewing electrical wires, cigatette left on bed or couch). These fires do not immediately show flame, but smoulder away and generate smoke for minutes (sometimes hours) before flaming. In this case, Ionisation detectors are not effective in providing early warning. Photoelectric detectors are far more suitable for this purpose. Please, when you check your battery next, check it's type. If it is a ionisation detector, please consider purchasing a photoelectric as a replacement.",
"TL;DR - Ion detectors (ones with radioactive symbol) can't see visible smoke. Photoelectric type detectors (stated on product label) do."
] |
[
"The what and why of smoke detector installation has been debated for many years, to the point where manufacturers are either too vague in their installation guidelines, or cover themselves from litigation (and drive up sales) by suggesting that smoke detectors of different types are installed EVERYWHERE. Both methods are dangerous.",
"The most common (and, in my professional opinion, incorrect) guideline is to have ionisation types installed in the dining area, adjacent the kitchen, and photo electric types installed inside bedrooms and hallways.",
"The first part of this is not good for a simple reason. Ionisation in the kitchen is supposed to catch flaming kitchen fires (pot of oil, curtains touching gas stove, etc). However, kitchens generate a lot of sub-micron particulate from common cooking activities. This results in many nuisance alarms, and eventually occupants of the household remove the detector or it's battery to stop these alarms. Smoke detectors don't save lives very well when sitting in a drawer without a battery. Also, it is assumed that if a flaming pot of oil ignites in your kitchen, you're in attendance at the time. If you leave a pot of oil on the stove and walk away from it... you're an idiot. Fire blankets and fire extinguishers in the kitchen are a must.",
"Personally, I have three quality photoelectric smoke detectors installed throughout my single-story house. One at the back in the hall covering two bedrooms, one in the dining area covering the middle of the house, and one at the front entry covering the lounge and master bedroom. These are mains voltage powered with battery backup, and are wirelessly linked, so an activation of the detector at the front of the house, will set the other two into alarm.",
"It's a matter of being informed. Contact your local fire department / brigade, research on the internet, check guidelines from multiple manufacturers against each other.",
"Edit: Typo's. Everywhere."
] |
[
"Would it be beneficial to have both types installed?"
] |
[
"What is the travel time to the Saturn's moon Titan?"
] |
[
false
] | null |
[
"While there's no definite maximum speed that modern technology can go, it took the ",
"Voyager 1",
" spacecraft about 3 years and 2 months to get to Saturn from Earth, using a ",
"gravity assist",
" maneuver at Jupiter."
] |
[
"What speed are you going at?"
] |
[
"The maximum speed modern technology can go."
] |
[
"Why do antigen rapid tests not work after 15 minutes?"
] |
[
false
] |
I've used two different types of antigen rapid tests. Both say that the results aren't valid if more than fifteen minutes have past since testing (dropping the solution onto the test kit.). Why is this so? Do the coloring/colored molecules that do the binding no longer work, or weaken, after 15 minutes? Or does a positive turn into a negative?
|
[
"These tests work by detecting the virus with manufactured antibodies that recognize some specific feature of the virus, like the infamous spike protein. There are two antibodies that together form a sandwich around the antigen (the feature the antibodies are looking for). The sandwich is formed when all the parts come together like a lock and key, or ball and cup. It’s very similar to an enzymatic reaction in that everything matches both physically (like the grooves in a key and the pins of the lock), but also electromagnetically - sometimes there are concentrations of positive charges that expect to meet negative charges (and vice versa) in these components that fit together. So the better the fit, the more obvious and fast the reaction. Where the fit is good, the test can be considered positive because all those antigen-antibody sandwiches are present in a high concentration (and therefore visible as a line on the test). ",
"However, over time, these sandwiches will still continue to form even in the absence of one of the components (even in the absence of the antigen), because positive and negative charges are so drawn to each other. They make a bad, but detectable, reaction. Because this isn’t an ideal sandwich, it happens slowly, very slowly, compared to a true positive. That’s why they give a cut-off time to the test - eventually it’s going to turn positive, but the speed at which it does is the telling part."
] |
[
"Yes, absolutely. Generally you’re supposed to read it after a specific number of minutes has passed (the exact time differs slightly from brand to brand.) Wait any longer than that and it could show a false positive."
] |
[
"Another redditor commented that these types of tests are the same for pregnancy tests. Does that mean that if someone who was not pregnant were to take a test, receive their negative test, and throw that test in the trash. That after some time, that negative test could then show positive?"
] |
[
"Cell tower triangulation - can they track data usage and location with that too?"
] |
[
false
] |
[deleted]
|
[
"Data/device 'usage' is accomplished completely separately from tower triangulation--regardless of where/which towers the cell was connected to, if successfully connected to ANY tower, the provider will have a log of the in/out call data for the phone. Voice calls, text messages, and if applicable, cellular data sessions--how much data and when it was transferred, but not necessarily the content of all of the cellular data because of security/encryption/etc.",
"Location can be tracked via cell tower triangulation (triangulation may be a misnomer--you can theoretically have a halo of location using 1, 2, 3 or more towers), but can be inaccurate--it depends on how dense the towers are in the given area, and how many towers the device is near. In an optimal situation, in proximity to 3 towers, you could get accuracy down to about 3/4 of a mile (this gives a nice description: ",
"http://wrongfulconvictionsblog.org/2012/06/01/cell-tower-triangulation-how-it-works/",
")",
"Slightly off-topic, also been listening to Serial--it's a great story, and I'm continually dumb-founded that he ever got convicted!"
] |
[
"Even without triangulation, there are countless readily available ways to track device data and activity. Triangulation is probably one of the more difficult methods to achieve at this point, and yields limited information about a target. Remote access tools, key loggers, device malware, and software/device security vulnerabilities will yield almost everything about a person's device activity. Also consider sophisticated technology, like Stingray, and what its capabilities are (as an example that is external to a device). If someone wants to stalk an individual's location, there's an entire tool belt available for relatively inexpensive cost. It's actually quite terrible and there's little or no regulation of many of these methods. "
] |
[
"Essentially Cell triangulation is done based on beacon signals sent by the cell phone to nearby towers. This can occur without a call being made or data being used, so long as the cellphone is searching for a tower to talk to. The triangulation is done based on relative signal power between each tower and the cellphone, which is known once the cell phone attempts to use the tower in any fashion. This happens at a level lower than a phone call, SMS, data transmission, etc. due to how a cell phone interacts with the tower before it is able to make a call or send any data. One technique used is multilateration",
"1",
". Other techniques exist depending on cell network, technologies in place, and the operational status of the cell phone device."
] |
[
"Can ants understand the chemical signals left by another species of ant?"
] |
[
false
] |
It seems that it would be evolutionarily advantageous for ants to be able to understand some of the markers left by other types of ant (eg - don't go this way or you will die, FOOD!, etc.). Is there a basic chemical language common to most ants?
|
[
"I can't speak too far on the premise of your question, but it would appear that there is at least ",
" ability for cross-communication between species:",
"Ants use pheromones for more than just making trails. A crushed ant emits an alarm pheromone that sends nearby ants into an attack frenzy and attracts more ants from further away. Several ant species even use \"propaganda pheromones\" to confuse enemy ants and make them fight among themselves. ",
"source"
] |
[
"So if you want to get rid of ants, killing them with force is actually counter productive?"
] |
[
"Squishing them wouldn't be an effective way to get rid of them even without the alarm pheromones since you're only killing a few of the worker ants, and the rest of the colony is still bustling. Ant baits (those little plastic things) usually work well because the worker ants don't ingest the poison, but collect it because it's tasty, take it back to the colony and feed it to the larvae, which secrete a couple types of liquid, one of which is used to feed the queen(s). The idea is to slowly poison the queen(s), the only ants capable of reproducing in a colony, eventually causing the collapse of the entire colony. ",
"So it's technically counter-productive to squish them, but the colony is largely unaffected by it either way."
] |
[
"How much could we tell about our solar system from X light-years away?"
] |
[
false
] |
With things like and those other "look at how big X is compared to our sun!" charts, it seems like we have a very precise idea of exactly how large planets farfarfar away are and even an idea of how they look. The only photo I've seen of Earth from a distance is , and at the edge of our solar system it's just that. I've never seen a full image of our solar system from beyond it, and now I'm curious. Say an alien civilisation made and analysed images of our solar system from just beyond it, one light-year away, five light-years away, ten light-years away, and 100 light-years away. How much would they be able to tell, from a Hubble-quality telescope, about our solar system's composition, the nature of our sun, and even individual planets? From the edge of the solar system, would you know that Earth is populated?
|
[
"but acids are still \"water-based.\" Water is almost a necessitating condition for life imo. Reactions in solids take place ",
" too slowly, and gasses are extremely diffuse. So we need a liquid. But we need a liquid that supports all kinds of elements and molecules in solution, so it can't be super hot like molten metal. And super cold liquids like liquid methane probably also have prohibitively slow chemical reactions.",
"So what's a liquid, that dissolves a lot of things (but not everything so you can actually make a cell), is neither too hot, nor too cold, and is made of a fairly simple molecule from very abundant elements in our universe (so that it too is sufficiently abundant for life to form in it)? ",
"Water. "
] |
[
"the only candidate in our solar system to possibly have life on it.",
"What about Europa? ",
"http://en.wikipedia.org/wiki/Habitable_zone#Criticism"
] |
[
"not much more than a couple of light years actually. It gets so weak it's swamped by the cosmic background."
] |
[
"Are certain types of people more or less influenced by ads?"
] |
[
false
] |
Some of the discussion on the question about reading and its influence on the brain, made me wonder if some people are more influenced by ads than others. For example is intelligence a factor? education, ethnic background, cultural identity, social ties? Maybe, loners are less influenced than social butterflies? I'm interested in any factor that might make a difference. And, on the same subject, but a different question, how much of the influence are we aware of? Can we feel entirely non affected, yet still be affected? EDIT - Finally saw glaring spelling mistakes. EDIT2 - Thank you for all of the thoughtful answers. You've given me a lot to ponder.
|
[
"Ooh! This one's right up my alley. Having studied advertising and worked in the advertising industry, I can say that this is a huge point of interest for advertisers as well.",
"As others have said, it has less to do with the ",
" of person you are (educated, versus uneducated, ethnic background, etc.) and way more to do with the message being sent and the target for that message.",
"A stupid ad isn't stupid if it works exactly the way it was meant to. Odds are if you find it offensive, annoying or dumb, you probably aren't the target audience. There are exceptions, of course, but advertisers do a tremendous amount of market research and focus groups to determine what types of messaging resonates with the audience they are trying to reach.",
"A social butterfly will be influenced by an ad that has to do with his or her interests - chiefly social stuff. A loner will be influenced by ads that appeal to a loner (a new game perhaps, or something that appeals to his or her more introverted nature - a new MP3 player with music from his or her favorite band might be right up that persons alley). ",
"The thing is, the labels you have chosen to define people are insufficient. People are complex with lots of different interests and needs. A person who is responsible for keeping the house clean will be more interested in a new cleaning product (Mr. Clean Magic Erasers) than someone who is not, or someone who isn't particularly concerned with keeping their bathroom sparkling. That's really all advertising is. ",
"Branding, however, which I think is more what you were getting at, is much more about creating a feeling or a lifestyle. Nike is a lifestyle brand in many ways, because you aren't buying a pair of sneakers, you're buying a pair of nikes. In that respect, I think it has MUCH more to do with how a person defines themselves and the lifestyle they wish to create or emulate. ",
"Advertising and finding that message that resonates as a lifestyle brand is this magical holy grail for advertisers. We all want to tap into it, but none of us are really sure what's going to catch on and what isn't. We hire \"cool hunters\" which is exactly what it sounds like: people with their fingers on the pulse of trends who have insights into what types of trends are picking up and can recommend how a new product is marketed (and to whom). ",
"It's less about appealing to that type of person, and much more about finding out what kind of lifestyle brand will resonate with you. It's our job to identify what that lifestyle message is, not to figure out some magic formula of demographic information that is more susceptible to branding (because as far as we can tell, there isn't one). We know that cool hunters can be useful. We also know that when it comes to setting trends, ",
" most powerful and influential demographic group are young females, usually teen and preteen. They play a major role in purchasing decisions for the entire household, much more so than we previously thought. ",
"But they aren't easily influenced. In fact, they're pretty savvy. Understanding what they like and why they like it is really hard to do.",
"Edit: A really fascinating case study is Apple. You can see how they evolved from conventional \"this is a product that you need because it does this,\" to \"here's an abstract idea, and we are the brand behind that idea.\" ",
"Check out the history of their marketing",
" as well as one of the ",
"major turning points in 1998",
", the \"leave your mark\" campaign. It was one of the fewer lifestyle ads that Apple had made up to that point, but it was an interesting break from their otherwise pithy-yet-technical ads. Shortly thereafter, you see the lifestyle ads for imacs, which were all about color and something else, something ",
". They were stylized computers, fashion statements for your desk. Then came the ads for ipods, an extension of that theme. It's color and silhouettes - there's ",
" technical about the actual product. This is a huge change from the ads they had been doing before, where they appealed to a ",
". ",
"No one ",
" an ipod. Everyone ",
" one.",
"While tech specs are still present in some of their advertising, and need advertisements are still made and can be found, they are less popular, less recognizable and less influential.",
"Edit 2: To be clear, the Apple edit is to show the evolution of advertising from standard (here's how our product meets your need) to lifestyle (we're ",
", you want us). I'm not pushing Apple products or saying they're great, so much as highlighting the contrast between the two advertising approaches. So let's try to avoid the love/hate Apple comments, please. That's not the point."
] |
[
"As a matter of fact, most people believe everybody else is strongly affected by ads and propaganda, while they also believe they themselves are pretty much immune. It's called the Third Person Effect. Doesn't answer your question, but I thought it'd be interesting to add. I don't have any citations right now, but you can easily find some - it's a well known thing."
] |
[
"The antihipster ethos is just regular douchey enough, I take it?"
] |
[
"Why doesn't DNA get tangled inside the nucleus?"
] |
[
false
] |
We've got about 1.8m of DNA in our nuclei and for most of the time it's not condensed so why isn't it just a single hypercomplex knot like my headphones when I put them into my pocket?
|
[
"for most of the time it's not condensed",
"Your assumption here is wrong.",
"Most of the DNA in each nucleus is condensed to chromatin, although it is not in the highly discrete chromosome confirmation you see in meiosis/mitosis. Typically only the portions of DNA undergoing active transcription are uncondensed and \"free\"",
"The condensed portions of nuclear DNA are called heterochromatin and the uncondensed portions, ready for transcription, are called euchromatin.",
"DNA doesn't just knot for a couple of reason. Firstly nuclear packing is incredibly dense so there isn't the space for the ends of the chromatin bundles to move past one another and knot (which is a requirement for headphone knotting).",
"Secondly we have sparse evidence that the interior of the nucleus is not random. Specific portions of euchromatin are localised to specific regions of the nucleus. Which again suggests that the DNA is not free to move and is likely tethered in some manner",
"https://www.ncbi.nlm.nih.gov/pubmed/24804566",
"https://www.ncbi.nlm.nih.gov/pubmed/18493085",
"https://www.ncbi.nlm.nih.gov/pubmed/20811620"
] |
[
"We actually have enzymes called topoisomerases that prevent our DNA from getting too knotted, by passing strands through each other. Viruses don't have these enzymes, so occasionally when they splort out their DNA, it's knotted."
] |
[
"Also, it's not just 1.8m of free-floating strands. The DNA is wound op on proteins called Histones, resulting in 90 micrometers of chromatin."
] |
[
"Why do Lightning strikes on my iPhone 6s appear like drops of water?"
] |
[
false
] | null |
[
"Hi RyanJT324 thank you for submitting to ",
"/r/Askscience",
".",
" Please add flair to your post. ",
"Your post will be removed permanently if flair is not added within one hour. You can flair this post by replying to this message with your flair choice. It must be an exact match to one of the following flair categories and contain no other text:",
"'Computing', 'Economics', 'Human Body', 'Engineering', 'Planetary Sci.', 'Archaeology', 'Neuroscience', 'Biology', 'Chemistry', 'Medicine', 'Linguistics', 'Mathematics', 'Astronomy', 'Psychology', 'Paleontology', 'Political Science', 'Social Science', 'Earth Sciences', 'Anthropology', 'Physics'",
"Your post is not yet visible on the forum and is awaiting review from the moderator team. Your question may be denied for the following reasons, ",
"/r/AskScienceDiscussion",
"There are more restrictions on what kind of questions are suitable for ",
"/r/AskScience",
", the above are just some of the most common. While you wait, check out the forum \n",
" on asking questions as well as our ",
". Please wait several hours before messaging us if there is an issue, moderator mail concerning recent submissions will be ignored.",
" ",
" "
] |
[
"I thought hurricane season was over?"
] |
[
"I thought hurricane season was over?"
] |
[
"If I could exert a large force on the entirety of the earth, what direction would cause the earth to fall into the Sun first?"
] |
[
false
] |
This is ignoring the fact that the Sun is expanding, obviously. My ideas were a force pushing the earth directly towards the sun and a force directly against the direction of its orbit, slowing it.
|
[
"Actually, if you wanted to push the earth into the sun, you'd want to exert the force against the earth's direction of motion. ",
"Pushing any other direction increases the earth's kinetic energy at its current point in the orbit - the best way to 'lower' the earth into the sun is to remove kinetic energy until the earth's orbit contracts to an ellipse which intersects with the sun. "
] |
[
"The direct approach (integrating the acceleration of the earth due to the sun's gravity) is a pain in the ass. ",
"Fortunately, it's easy to just use Kepler's third law. If the earth comes to a dead stop relative to the sun, then it is now on a highly elliptical orbit. In fact, the eccentricity is 1 so the ellipse has collapsed to a line. The square of the orbital period is equal to the cube of the semi-major axis. The semi major axis is half the distance to the sun, so Kepler's Third gives us an orbital period (the length of the orbital 'year') of 1.12x10",
" seconds - that's about 129 days. Since that's an orbital period, that number actually tells us long it takes us to get back to where we started, so we want to cut it in half. ",
"It'll take about 65 days for the earth to hit the sun. "
] |
[
"There is already a force pushing towards the sun- gravity. Increasing this force would make our orbital period shorter, and our year quicker. ",
"Pushing in the opposite direction of our orbit is the way to drop us in to the sun- this direction is \"retrograde\". You'd have to push hard in order to drop us into the sun- the earth's kinetic energy in its orbit is 4.8*10",
" joules, and almost all of that would need to be applied as work in order to stop us. That's 7 hundred trillion hiroshima bombs, or hundreds of millions of tons of antimatter. Please do not attempt this. "
] |
[
"Has there a type of amnesia that affects semantic memory?"
] |
[
false
] |
I've always heard of retrograde and anterograde amnesia, but never procedural or semantic. Has amnesia caused people to forget that "the sky is blue?"
|
[
"While there are many types of aphasia (difficulty with language), what you're talking about is different. In Broca's aphasia, for instance, one \"knows\" what one wants to say, but simply can't say it. A loss of semantic memory would mean the loss of the underlying concept.",
"The answer is yes, and it's called semantic dementia. It's a variant of frontotemporal dementia (aka Pick's disease.) Not only could a sufferer forget that the sky is blue, they might forget what the sky is, or what blue is. ",
"https://en.wikipedia.org/wiki/Semantic_dementia",
"\n",
"http://memory.ucsf.edu/ftd/overview/ftd/forms/multiple/sd"
] |
[
"There are acquired language disorders, such as Broca's aphasia (difficulty in fluent sentence production) and Wernicke's aphasia (difficulty in producing sensical language utterances-- with no fluency issues). There is also anomia, difficulty in remembering nouns and agrammatism, difficulty in producing 'functional' morphemes, like ",
", ",
", ",
", etc. You may wish to look into neurolinguistics for more on this."
] |
[
"Amnesia affects episodic memory. Although episodic and semantic memory are both part of declarative memory, only episodic is affected by amnesia. So by definition, no. It cannot cause semantic impairment. ",
"There are disorders that can result in impairments to both episodic (ie personal knowledge and memories) as well as semantic memory. The most notable is amnestic dementia (dementia that involves memory). These include Alzheimer's disease, MCI, and Binswanger's disease (among many more). "
] |
[
"Is there some scientific way to measure smell and attraction as it relates to humans?"
] |
[
false
] | null |
[
"And every piece of ",
" says that they don't. That one piece of research doesn't prove anything or come close to proving they do."
] |
[
"They don't ",
" anything. Until they do, it's safe to assume that pheromones don't exist in mammals."
] |
[
"So, let me get this straight. They don't prove anything because TitsKnobs says they don't. And, I should believe you over scientific studies because, well, just because?"
] |
[
"What screen resolution would be equivalent to the resolution of the average person's eyes?"
] |
[
false
] |
In other words, if we were to say what resolution humans saw in would it be 100000x100000 or something
|
[
"Human eyes (and probably all other eyes, including eagle) only see detail in a small area focused at the center of vision. So a display the size of a house might have smartphone-density pixels for the central 1m",
" area, gradually becoming less dense as you go further out, until the \"pixels\" in the periphery are perhaps each the size of your hand.",
"If you allow the viewer of the screen to physically move his eyes or head, then the question falls apart because you'll need high pixel density everywhere for as far as you allow movement."
] |
[
"The resolution of the human eye is measured as angular resolution. ",
"Foveal vision",
" (the subsection of your visual field that is actually high-resolution) is about the central two degrees of the visual field. With a typical angular resolution of 1 arcminute = 1/60 of a degree, that would mean high-resolution vision is a circle 120 angular \"pixels\" across. This tiny area of high-resolution vision moves very rapidly to whatever you're actively looking at. ",
"Angular resolution means the absolute size of the smallest distinguishable features depends directly on how far away they are from your face. ",
"This blog post",
" has good explanations as well as some calculations. However, this isn't quite what you asked, as it is more relevant to the resolution required for printing or a display screen to make the pixels indistinguishable to a human. It turns out that 300 dots per inch is sufficient for an object held one foot away from your face for a human with average vision. "
] |
[
"It is less about resolution and more about pixel size and density."
] |
[
"How small a space could the mass of the milkyway be compressed into?"
] |
[
false
] |
[deleted]
|
[
"If you were to compress all the mass in the milky way into a black hole it would have a radius of about 0.2 lightyears."
] |
[
"I thought the radius of the mass would be 0 or infinitely small but the radius in which light could not escape would be .2 light years. "
] |
[
"All that is known about the actual mass distribution inside a black hole is that current physical theory fails to accurately describe it. The conclusion of \"infinitely small\" only occurs if you naively apply both the 'large object' physical theory (general relativity) and the 'low mass' physical theory (the standard model) together to a system which is both small and high mass. In short, current physics cannot be used to predict what occurs near the 'singularity' the black hole, so the radius of the event horizon is the best value of size you can come up with."
] |
[
"Which is larger, the largest known star in the Universe or the largest known black hole in the Universe?"
] |
[
false
] | null |
[
"In terms of mass, the largest known black hole is like 20 billion times as massive as the sun and the largest stars are a few hundred times the mass of the sun.",
"In terms of radius, it's a bit closer, but the biggest supermassive black holes have a Schwarzschild radius on the order of light-days whereas for the biggest stars it's light-hours."
] |
[
"Black hole."
] |
[
"No"
] |
[
"If a piano, a violin and someone's voice can each reach the same frequency (note), what is it about the sound they make that allows you to differentiate them?"
] |
[
false
] | null |
[
"The term for that difference is the ",
" of the sound. It is caused by different relative amounts of overtones and harmonics. Say you play a standard A (440Hz). The string is vibrating at 440 cycles per second. This is what is called the ",
". But that string is also vibrating (less intensely) at its ",
"harmonic frequencies",
".",
"The untrained human ear typically does not perceive harmonics as separate notes. Rather, a musical note composed of many harmonically related frequencies is perceived as one sound, the quality, or timbre of that sound being a result of the relative strengths of the individual harmonic frequencies.",
"When talking about musical instruments ",
"resonance",
" also plays a significant role. I'm not exactly sure of the mechanisms in a wind instrument, but can explain stringed instruments. That plucked string at 440Hz is transferring some of those vibrations into the body of the instrument, which in turn is causing other strings on the instrument to undergo ",
", vibrating at their own fundamental and harmonic frequencies, which in turn feedback into the instrument. On a piano you are also hitting 3 strings at once for every key stroke, so there is a subtle chorus effect caused by minute differences in tuning between the strings.",
"*edit- Since this made it to the top I'll add mention of the envelope of the sound, as others have pointed out. The Attack, Sustain, Decay, and Release characteristics of an instrument (acoustic or synthetic) have a role in the specific sound. Those aspects are easily controlled on a synthesizer, adding to it's range of tones, and can be manipulated to some extent on acoustic instruments by technique (pick vs. fingers, piano pedals, etc.)."
] |
[
"This is a good start to the explanation. It's true that differences in sound are caused by sympathetic resonances and chorus effects not only in the instrument itself, but also the room, and even sometimes inside the human ear itself(!), but there's a little deeper explanation rather than just \"it changes the timbre of the sound\".",
"The waveform of the sound wave is the actual pattern that the wave takes on when it leaves the instrument. Various methods of sound production cause different waveforms. However, we can generate simple waveforms with synthesizers, which will shortly be used to illustrate my point. Check out the ",
"buttons on this page, which are labeled \"square\" \"sine\" \"sawtooth\" and \"triangle\".",
" For an idea of what these waveforms look like, ",
"check out this link I blatantly stole from the Wikipedia page.",
" Neither the frequency nor the amplitude change, only the waveform, but notice how different the waves sound! Every sound an instrument produces has its own unique waveform that can be measured with an oscilloscope (a scientific instrument that doesn't produce sound, but can be used to look at the defining characteristics of a sound wave, such as frequency, amplitude, and yes, waveform).",
"As ",
"/u/Pyroshock",
" points out below, all of these waves can be represented as infinite sums of sine waves. This is known as a ",
"Fourier Series",
". As Fossafossa says above, all of these characteristics of the instrument compound, producing a slew of different sine waves, which then sum together to create a unique waveform, which we recognize as timbre.",
"Edit: Many people have pointed out that what ",
"/u/Fossafossa",
" said above is basically the same thing that I said; that the characteristics of the instrument producing the sound change the waveform through the methods he/she describes. ",
" I have edited my post to better reflect this."
] |
[
"Isn't a waveform just the sum of the fundamental frequency and the harmonics and overtones, like Fossafossa said above?"
] |
[
"[Physics] If strong force is strong enough to keep protons together, why do atoms need neutrons to keep the protons from tearing apart?"
] |
[
false
] |
I was watching Crash Course Chemistry and watched the lateral SciShow video about how quarks are held together with strong force, which keeps protons together and also ties together the whole atom through nuclear force. When I got back to the video on the nucleus, however, Hank stated that neutrons are important because without them the protons would tear each other apart. Wouldn't the strong force be more likely to pull the protons in close? What force would be pushing the protons apart and creating a need for neutrons?
|
[
"You might be tempted to say that the Coulomb force is what prevents all-proton nuclei from existing (except hydrogen-1 obviously). But that's not the full story.",
"In fact, if you replace all the protons with neutrons, the system is still not bound.",
"So the ",
" simply cannot bind a system of multiple protons (neutrons) without any neutrons (protons). Then of course Coulomb only makes things worse in the case of protons.",
"The nuclear force between two nucleons can be attractive or repulsive depending on their distance, spin orientations, etc.",
"If you try to put a bunch of ",
" kind of nucleon (either kind) together, they just don't form bound states.",
"You need protons and neutrons (for A > 1) to have any hope of forming bound states."
] |
[
"Why is it that no amount of neutrons can form a bound state?",
"The nuclear force is spin- and isospin-dependent. If you try to form a system of nn or pp (both of which have isospin-1), Pauli exclusion forces you to anti-align their spins. The spin-spin part of the nuclear force is repulsive for spins pointing in opposite directions, and apparently enough so to disallow bound states.",
"And if you can't even bind ",
" together, a system of A protons or A neutrons doesn't have much hope of being bound."
] |
[
"The short explanation is that two neutrons feel a Pauli repulsion, meaning they both contain udd quarks in the same spin state, so some quarks would need to go to a higher-spin state (which costs energy) to get close to each other. This repulsion is stronger than their residual strong force attraction, so they are unbound.\nThe same goes for a pair of protons, except they feel an electric repulsion in addition to their Pauli repulsion. A neutron and proton feel neither of these repulsive forces.",
"That being said ",
"the dineutron has been observed",
", although it is very short-lived."
] |
[
"Is carbon fiber a heat conductor or a heat insulator?"
] |
[
false
] |
No matter how much I google, I keep getting constantly contradicting results on the thermal conductivity of carbon fiber. Some say that since it has low thermal conductivity that it's an insulator, but I keep constantly seeing other contradictory remarks about it having a lot of potential for heat conductivity, so if anyone can clarify I'd appreciate that.
|
[
"Carbon fiber itself, the literal fibers, are fairly thermally conductive (generally less than most metals but far more than conventional insulators). The actual value of conductivity depends on the method of production.",
"Probably part of the problem you're having finding information is that carbon fiber is most frequently used as a reinforcement in a composite material and the thermal properties of the composite materials depend both on the properties of the fiber and of the matrix."
] |
[
"Carbon fiber itself has a thermal conductivity around 24.0 W/(m·K) while epoxy often used as the matrix in carbon composites has a thermal conductivity of 0.17 - 0.79 W/(m·K). So if you are asking for the fiber itself, the conductivity is as above. If you are asking about a composite, it depends on the density of fiber, orientation, layering, etc. of the specific layup. The answer should be somewhere between the bounds 0.17 ~ 24 W/(m·K) (yeah, pretty broad, but maybe good enough for your application?). A paper showing performance of several layup samples can be found here: ",
"https://www.researchgate.net/profile/Ronald-Joven/publication/288102626_Thermal_properties_of_carbon_fiber-epoxy_composites_with_different_fabric_weaves/links/56be27e408aee5caccf2f5d3/Thermal-properties-of-carbon-fiber-epoxy-composites-with-different-fabric-weaves.pdf"
] |
[
"The heat-resistant tiles you're thinking of were reinforced carbon-carbon composite and the insulation was provided by the voids inside the carbon matrix, not the carbon itself.",
"Beyond that, the RCC was used for its high temperature tolerance, not its low thermal conductivity. In fact there had to be insulation below it to protect the remaining structure of the orbiter.",
"Since carbon is a good thermal conductor, the adjacent aluminum and the metallic attachments must be protected from exceeding temperature limits by internal insulation. Inconel 718 and A-286 fittings are bolted to flanges on the RCC components and are attached to the aluminum wing spars and nose bulkhead. Inconel-covered cerachrome insulation protects the metallic attach fittings and spar from the heat radiated from the inside surface of the RCC wing panels.",
"The nose cap thermal insulation ues a blanket made from ceramic fibers and filled with silica fibers. HRSI or FRCI tiles are used to protect the forward fuselage from the heat radiated from the hot inside surface of the RCC.",
"https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_sys.html#sts-rcc"
] |
[
"On a molecular level, why is sugar sticky when wet?"
] |
[
false
] | null |
[
"It’s kinda a weird mix between the hydrogen bonds in the sugar being broken and made again with water meaning that the sugar will be more likely to H bond with say the sweat on your hand. Why this would happen I would say is due to the sugar molecules being relatively big and each -OH group will be sterically hindered so the intermolecular bonding between two sugar molecules can be quite difficult but water being very small can easily access the -OH groups. This is also why sugar when dry is not sticky as it is happy with the H bonds between each sugar molecule. ",
"It also is to do with having a supersaturated solution. Since sugar will dissolve in water an abundance of water will not make sugar sticky but a small volume of water won’t be able to dissolve all the sugar giving it its sticky nature. "
] |
[
"Stickyness is really bonding? Wow I've never really though about that before"
] |
[
"Put a different slant on it. When sugar is very concentrated in solution the solution is viscous (thick) like honey. This means that is acts a little like a fluid as well as a solid (viscoelastic). As a fluid it wets any surface it touches and sticks to it (if it didnt want to stick then it wouldnt wet it) The solid part of the viscolasticity means than it resists pulling apart, and hence feels sticky. So it works a bit like the pressure sensitive adhesives. Where the other explanations have come in is that the sugar forms extended structures in concentrated solution due to the hydrogen bonding, which gives some elasticity"
] |
[
"Are modern occurances of appendix ruptures more prevalent than in the past, and if so could it be an evolutionary mechanism to select out a vestigial organ?"
] |
[
false
] | null |
[
"Regardless of the actual answer, I'm pretty sure it wouldn't be an evolutionary mechanism because most people reproduce regardless of whether their appendix ruptures or not so the genes that cause appendicitis would get passed to the next generation regardless. "
] |
[
"You are also \"cheating the system\" by taking any modern medicine for any ailment so to speak. What's considered routinely preventable today were once deadly diseases. This is a wrong way to think about any medical intervention. \nThere need be no grand system to cheat and no grand plan... And it is ok.",
"Also evolution is not a directed process with a laser-sharp aim at a goal. The arc of evolution is huge and on a timescale way larger than a single human lifespan by orders of magnitude.",
"I often advise people who have trouble understanding evolution, to take up basic probability lessons; an introductory undergraduate course should suffice. What probability teaches you, through rigorous logic, is how often human intuition can be plain wrong when reasoning about random systems."
] |
[
"It is not really mutation, rather recombination. Though mutations may play a role also. "
] |
[
"Why doesn't deuterium fusion occur in main sequence stars?"
] |
[
false
] |
What makes the proton-proton chain reaction the preferred method of fusion in main sequence stars if deuterium fusion occurs at lower temperatures? Following that, why does the carbon-nitrogen-oxygen cycle become the preferred method of fusion in stars starting around 1.3 solar masses?
|
[
"Deuterium fusion is too easy. There's only a small percentage of deuterium to start with, and it (along with lithium isotopes) are fused early on, during the transition from protostar to main-sequence star. Hydrogen fusion is dramatically slower, which is why stars live so long, they ",
" burn their fuel any faster."
] |
[
"Its not that simple. Deuterium fusion has a relatively high reaction rate, so it all burns off pretty much immedately as soon as the star gets above a million degrees or so. Hydrogen fusion then begins, once the small amount of easily fusible isotopes are gone. Which route occurs depends on the mass and composition of the star. Lighter mass stars have relatively cool cores and fuse hydrogen via the proton-proton chain. Heaver stars (if they have a trace of heavy elements in them) can reach higher temperatures and catalytically fuse hydrogen via the CNO cycle."
] |
[
"Thank you. So there isn't that much deuterium to begin with, and once the star becomes hot enough, it simply just starts fusing hydrogen?",
"I'm assuming that once you increase the temperature further, then CNO fusion starts to occur, correct?"
] |
[
"How/Why did the first organism develop its need for a heart/lungs/kidneys/etc?"
] |
[
false
] |
[deleted]
|
[
"\"It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g. lungfish, gar, and bichir) and into the swim bladders of the ray-finned fish.[5] \""
] |
[
"\"It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g. lungfish, gar, and bichir) and into the swim bladders of the ray-finned fish.[5] \""
] |
[
"There really isn't a \"first organism\" to develop each specific organ, because there will be lots of organisms over time with varying pieces and types and sizes of different versions of these and over time minutes changes to those eventually proved to be better for survival. They are intricate in design because they were designed very, very slowly and the ones that didn't work as well died off. ",
"If you want to explain it to someone, I'd start much smaller than an organ to understand first. For example, pigment. A fish with a mutation for a darker color survives better, passes on that gene, and more darker colored fish are born each generation. Perhaps another mutation adds on to that to give the fish a mottled appearance, then those survive better. Then, maybe there's a mutation that causes the bones in the fin to be separate instead of having skin stretched across them. That fish wouldn't survive in open water, but might survive better by digging into the mud in a shallow area. Now there's a population of fish that can dig into the mud. Over a long, long time these itty bitty changes add up to an intricate functioning organ."
] |
[
"Most types of life I can think of seem to have originated in the ocean but I've never seen an \"ocean mushroom\". Did the fungi kingdom originate there too then move onto land later? What was the world like then?"
] |
[
false
] | null |
[
"Single-celled fungi, like yeast, can live in water and the ocean. There are even varieties of multi-cellular fungi exist in the ocean but they tend to be rather small.",
"P.S. ",
"http://deepseanews.com/2011/08/marine-fungi-are-totally-badass/"
] |
[
"I too have trouble thinking of an 'ocean mushroom'. Let's remember however that mushrooms do not occupy the entire of the fungi kingdom. Fungi can exist as single celled organisms, and there are quite a few of these that live in aquatic environments. ",
"For example, many fungi of the ",
"Chytrid",
" division are aquatic. ",
"Have a read of the ",
"Diversity section of the Wiki's Fungi",
" page. I wish I could tell you more, but I'm just an undergraduate still learning about all this stuff."
] |
[
"I found that article too! It's worth noting that fungi of all types really are apparently less common in marine environments. They didn't state it outright, but it seems like this could be because most fungi live on decaying matter and detritus, and the ocean is lacking in decaying plant matter (and fungi are indeed found in decaying driftwood and mangroves, 2 major exceptions to that). Fungi eating decaying animal matter have to deal with a much higher diversity of animal life (many phyla live only in marine environments) which may put them at more of a disadvantage there. Finally, it's entirely possible we've just not found most of the marine fungi.",
"Freshwater fungi, now, are very common. But they don't make mushrooms (with, I think, a single known exception) for much the same reason you rarely see flowers underwater (even though there are lots of aquatic plants)...both are specialized reproductive structures for reproducing in the air, and are less necessary/useful underwater."
] |
[
"Why are some human internal organs asymmetric, or located on one side of the body?"
] |
[
false
] |
Doesn't nature strive for symmetry? Also, are there any animals that are naturally asymmetric externally? Like they look different on one side compared to another. Thanks!
|
[
"Evolution is trial and error, symmetry works well for things like eyes (depth perception, field of view, etc.), ears (identifying the directions of sounds), and limbs (motion by legs/feet work well in even pairs). Internally it's a different story for mammals like you pointed out. ",
"It has little to do with what looks nice or what you could use more of (I want two livers... purely for science) it's just what works with what space and resources you have. Externally human's are only symmetrical in one plane. Some animals are more symmetric (like a jellyfish) and others much less symmetric (like a fiddler crab).",
"Evolution isn't like the creature creator from Spore, every subtle change is a tiny accident, most of the time the adaptation is negligible, useless, or harmful.",
"TL;DR Nature doesn't strive for symmetry, it strives for reproductive advantage and goes with what works, not what looks nice."
] |
[
"On your assumption on symmetry: Symmetry ",
" not be what is striven for—it's could just be that it's easier to encode for. Just a thought."
] |
[
"My favorite theory about this is that at some point the ancestor of chordates basically fell over, with one side eventually becoming the top and the other side becoming the bottom. The external features rearranged to become bilaterally symmetrical again, but the internal organs stayed jumbled up. However, this theory is kind of off the wall (which is why I like it). A more straightforward explanation is that internal asymmetry allows for more complicated organs. For instance, your intestines can be longer because they curl all over the place instead of running straight down the middle. ",
"This page details all these ideas and some others: ",
"http://www.palaeos.org/The_Fall_and_Rise_of_Orders_of_Symmetry"
] |
[
"People who contract Covid 19 report losing their sense of taste and smell. Is this temporary while the virus runs its course or permanent?"
] |
[
false
] | null |
[
"Friend and his wife had it, both in their 50s, they lost their sense of taste and smell as one of the last symptoms. Started to come back after about a week also. They still have some lingering issues after 3 weeks but sense of taste and smell has returned."
] |
[
"Friend and his wife had it, both in their 50s, they lost their sense of taste and smell as one of the last symptoms. Started to come back after about a week also. They still have some lingering issues after 3 weeks but sense of taste and smell has returned."
] |
[
"Did anything happen to your lung capacity?"
] |
[
"burning wood to create energy does not create global warming?"
] |
[
false
] |
I was watching a Feynman video on youtube where he was talking about fire.. he explains that trees take CO2 and break it into C and O2 with energy provided by the sun (by this point am assuming this is and Endothermic reaction). Then he later states that when you collide O2 with C again with enough energy they will reattach themselve creating CO2, and releasing the "light" (flames) or energy previously provided by the sun to break them appart in the first place. So if trees take the CO2 in the air to grow, and then when they die (burn) they release that CO2 that was originally in the air back to the air again, burning woods to create energy would not create any surplus of CO2 in the atmosphere, thus not creating any global warming (I know am taking big leaps here). Its all a balance of some sort I guess. If we (the planet) are a closed space (theres no CO2 getting in or out of the planet) and everything is balancing out, why do we have global warming? is the CO2 generated by burning oil not initially in the air? Sorry if this sounds stupid!, I know am making a mistake in here, I just wanna know where..
|
[
"Our planet has not always been the same as it was now. It took a very long time for photosynthetic life to store all of the carbon currently sequestered. (It's also worth noting that not all carbon in the world is either in plants or the atmosphere.) The problem is that when people burn wood for fuel, we have the capacity to burn wood far more quickly than plants can fix CO2 from the atmosphere. Though the net amount of carbon on Earth is pretty fixed, the percentage in the atmosphere is currently increasing."
] |
[
"So basically we are decreasing the amount of carbon stored in land (oil, wood, etc) and increasing the amount in the atmosphere..",
"thanks!.."
] |
[
"We dig up coal, gas, and oil to burn carbon from eons ago, faster than the CO2 is consumed by today's trees. In addition, there are other greenhouse gases that contribute to global warming."
] |
[
"Where did the fish in volcanic crater lakes come from?"
] |
[
false
] |
I've seen two volcanic crater lakes in Southeast Asia - Lake Toba and Lake Taal. Both have fish, but both are isolated and cut off from other bodies of water. Since they're volcanic crater lakes wouldn't the eruption would have killed all the fish? So where did the fish in the lakes come from?
|
[
"Mostly from migratory Birds. A duck that goes from Lake to Lake will often carry eggs on its feet. All it takes is a few and with them traveling so much it's about the only way that you can have a species of fish that spans multiple areas. Otherwise every Lake would end up with a unique species due to a lack of genetic mixing. Some are also intentionally introduced by fisherman or government projects."
] |
[
"The general answer is that either humans transported the fish there (intentionally or accidentally) or that the fish arrived as eggs clinging to the legs of waterfowl. ",
"That said, although experts generally accept that dispersal by waterfowl is the most likely reason, it hasn't been formally proven - in other words, though it's probably right, no one has shown that it's true, as opposed to being plausible and probable.",
"Dispersal of fish eggs by water birds was overall the most frequent explanation online and in the questionnaire. In the scientific literature, however, we found hardly any empirical research on passive fish egg dispersal. ",
"--",
"Colonizing Islands of water on dry land—on the passive dispersal of fish eggs by birds"
] |
[
"Thank you! "
] |
[
"The creation and destruction of matter and energy?"
] |
[
false
] |
Does the law of "matter and energy cannot be created nor destroyed" still hold today? And if it does, has there been any advancement of knowledge and theory on where current matter and energy comes from? Like GOD!!! (j/k, scientific theory/fact only please)
|
[
"Matter isn't a precisely defined term in science. What we do know is that ",
" is a kind of energy along side energy of motion: E",
" = (pc)",
" +(mc",
" )",
" . And we know that energy is conserved for \"time translation invariant systems\"",
"The law of conservation of energy stems from a fundamental aspect of reality known as Noether's theorem. Suppose I represent a physical system mathematically, perhaps as a \"Lagrangian.\" Then I do something to that Lagrangian that I can apply in a \"continuous\" manner. Shift everything by some arbitrary distance, rotate it, etc. If the continuous \"change\" creates a bunch of terms that cancel out, and you have the same thing that you've started with, you've found a \"continuous symmetry\" of the system. Well Noether's theorem states that for every continuous symmetry there's an associated conservation law.\nSo if you shift everything an arbitrary space, you get conservation of momentum. If you rotate everything an arbitrary angle, you get conservation of angular momentum. If you are doing electromagnetism, you can perform a transformation of the \"gauge\" of the system, and if you find that the system is \"gauge invariant\" that system has conservation of charge. Well if you find that a system doesn't change by shifting it forward or backward in time, you get conservation of energy."
] |
[
"Unsure if I understand. So if that's the case, in a mathematical representation of a physical system could you ever do something that doesn't create a bunch of terms that cancel out? Or would any change applied to the \"Lagrangian\" always result in a continuous symmetry?"
] |
[
"yeah there certainly can be cases. For instance if we construct a Lagrangian with some potential energy that changes as a function of time, that system won't conserve energy ",
". When we get down to the nitty-gritty levels of reality, the fundamental particles, yes energy and momentum are conserved because the lagrangian is all just particle interaction terms (more technically, field interaction terms). But again, on the largest scales, scales much larger than galaxies, we must consider the expansion of the universe itself. And metric expansion ",
" time-translation symmetry, meaning that on very large scales energy conservation is broken by the expansion of the universe in a very specific manner."
] |
[
"What is it about water or bismuth that makes it expand when it freezes?"
] |
[
false
] |
Water expands when it turns into ice, and that's how ice weathering works. But what is it about water and other substances (bismuth, gallium, germanium, etc.) that makes it do that?
|
[
"Water has a lot of odd properties that mainly stem from it's tendency to form hydrogen bonds with itself. Water can form up to four hydrogen bonds with neighboring water molecules. These bonds are pretty strong compared to most interactions between molecules, but not strong enough to be essentially permanent like a covalent bond. Importantly, hydrogen bonds are directional, meaning that the bond between two water molecules is strongest when they're both oriented correctly relative to each other.",
"In liquid water, each molecule is bouncing around, rotating, and vibrating constantly. This energy is enough to prevent water molecules in the liquid phase from ever staying fixed in exactly the right spot to form four hydrogen bonds. Instead, on average, each water molecule is bonded with ",
"somewhere between 3 and 4",
" other individual waters at any given point in time, due to all of this constant movement. The \"empty\" spots have enough room to accommodate more than just one extra molecule, so each water will be surrounded by a little more than four other molecules on average.",
"When some of this energy is removed and water freezes into a solid, the best way to minimize energy is for each molecule to form the most number of hydrogen bonds possible. Now each water has ",
"exactly four nearest neighbors, instead of the slightly more than four",
" it had in the liquid state. Because all of the molecules are on average slightly farther apart now, the overall density decreases, meaning that the newly-formed ice has to expand its volume in order to accommodate the same amount of mass.",
"So, the expansion of water upon freezing is a direct result of its directional hydrogen bonding. I will freely admit that I don't know much about Bismuth/Gallium/etc and can't say for certain whether this effect in metals is due to similar considerations with directional bonding. Maybe someone else with more experience in metallic chemistry can clarify."
] |
[
"Does this hydrogen bonding add to the heat of fusion?"
] |
[
"In a way, the hydrogen bonding exhibited by water ",
" the energy of its enthalpy of fusion. The enthalpy of fusion is defined as the change in enthalpy to change its state from solid to liquid (or vice versa). So since water as a solid is held together by hydrogen bonds, the enthalpy of fusion when melting goes to breaking those bonds."
] |
[
"Is there a symmetry associated with the conservation of information?"
] |
[
false
] |
I have a mediocre understanding of Noether's theorem, and I've gathered that conservation of information also appears to be a fundamental property of the universe. Is there a correlated symmetry or does the conservation of information come from a completely different place/idea?
|
[
"In quantum mechanics conservation of information is phrased as \"unitary evolution\" or you might say the conservation of probability. I think typically you'd consider this a postulate of the theory rather than a consequence of some symmetry, but it can be related to (for example) the \"phase symmetry\" of the wave function: only the magnitude of the state vector matters, not its phase in the complex plane. "
] |
[
"In QM, systems either evolve via unitary evolution (according to the Schroedinger equation) or, when measured, via collapse.",
"If that were true this would be an enormous self-inconsistency in QM. This clear if you consider a situation in which my total quantum system contains elements that make \"measurements\" on one another, thus causing non-unitary evolution. At the same time certainly for me the complete system undergoes unitary evolution, hence the inconsistency.",
"In fact what we think of as state collapse is a phenomenon that shows up only when considering the relative information between sub-parts of a global quantum system. It can be explained and understood using purely unitary evolution and proper understanding of what physically happens when we \"measure\" something."
] |
[
"In QM, systems either evolve via unitary evolution (according to the Schroedinger equation) or, when measured, via collapse.",
"If that were true this would be an enormous self-inconsistency in QM. This clear if you consider a situation in which my total quantum system contains elements that make \"measurements\" on one another, thus causing non-unitary evolution. At the same time certainly for me the complete system undergoes unitary evolution, hence the inconsistency.",
"In fact what we think of as state collapse is a phenomenon that shows up only when considering the relative information between sub-parts of a global quantum system. It can be explained and understood using purely unitary evolution and proper understanding of what physically happens when we \"measure\" something."
] |
[
"What's the difference between organic fruits and vegetables and regular fruits and vegetables?"
] |
[
false
] |
Isn't everything organic? I don't understand. If I have a banana and an organic banana aren't they the same? I mean if a regular banana has pesticides on it, I'm not eating the peel. Can someone clear this up for me? Thanks !
|
[
"The requirements for organic certification vary depending on the country, but usually require that synthetic chemicals including pesticides, fertilisers, antibiotics and additives are not used, and have not been used where the food is grown for a number of years. There are usually other requirements too. You can look up the specific requirements for certification in your particular country pretty easily.",
"I'm not qualified to speculate on whether there are measurable differences, in chemical levels or otherwise, between a peeled organic banana and a peeled regular banana. It does mean enormous quantities of pesticides and fertilisers have not been released into the environment in order to grow it, which may or may not give you a warm fuzzy feeling when you eat it."
] |
[
"No anecdotes in ",
"r/AskScience",
" please.",
"Organic food uses pestisides, uses fertilisers, and additives. They are just \"organic\" in the fact they were not synthetically made. In many cases they can be worse for the enviroment. It depends on the crop grown; some organic food production may not use fertilisers or pestisides but also yield amounts way below normal crop production.",
"Organic food has restrictions; such as not using food that has been modified by artifical selection(Natural Select done by humans) or by genetic modifications.",
"It is also found that organic food does not have any more or less nutrients; taste is the same. Pestisides are used in both; and in both cases neither has negative impacts on the growth or toxity of said crop.",
"http://en.wikipedia.org/wiki/Organic_food",
"http://en.wikipedia.org/wiki/Organic_food#Environmental_impact"
] |
[
"I specified synthetic. I didn't go into specifics since the exact requirements vary so much between countries and certifications. (The requirements for Soil Association certification in the UK, the one I'm familiar with, are listed ",
"here",
" in excruciating detail. Note, for example, that there are allowances for specific plant extracts and bacteria as pesticides, and some other pesticides in exceptional circumstances subject to approval and controls, but no general non-synthetic pesticide allowance.)",
"The article you linked notes \"crop yields were 20% lower in organic systems while fertilizer plus energy input was 34% to 53% lower. However, pesticide input was reduced by 97% in organic farm systems.\"",
"I also do not see how any of what I said was an anecdote. If you'd like a source for people feeling good because they believe that organic food is better for the environment, ",
"this book",
" has a chapter on consumer perceptions of organic food (you should be able to see most of it on the preview) which cites a number of studies showing this is a common perception."
] |
[
"AskScience AMA Series: I'm Mike Parker Pearson, Archaeologist and Professor of British Later Prehistory at University College London, here to talk about my research around the world and on Stonehenge, AMA!"
] |
[
false
] |
Hi, Reddit! I've worked on archaeological sites around the world in Denmark, Germany, Greece, Syria, the United States, Madagascar, Easter Island (Rapanui) and the Outer Hebrides. I have been UK Archaeologist of the Year and am a Fellow of the British Academy. My research on Stonehenge over nearly 20 years has helped to transform our understanding of this enigmatic stone circle, including the discovery of a new henge, a settlement where Stonehenge's builders may have lived, and the quarries for Stonehenge's bluestones in the Preseli hills of west Wales. I've published 24 books on a wide variety of archaeological topics, but I really love being out doing fieldwork. You can follow more of my recent work on PBS' , where my team and I painstakingly searched for the evidence that would fill in a 400-year gap in our knowledge of the site's bluestones. The episode reveals the original stones of Europe's most iconic Neolithic monument had a previous life before they were moved almost 155 miles from Wales to Salisbury Plain. I'll be ready to go at 3:00pm EST (20:00/8:00pm GMT), AMA! Username:
|
[
"What do we know about the social organization of the people who built Stonehenge?"
] |
[
"Mummified and fitted together as composite bodies? What do you mean?"
] |
[
"What part of your job is spent kneeling in the dirt, and what part of it is spent using lab tools to analyze what you found in said dirt? I always picture archaeologists in holes or ditches but I figure you guys must use a lot of \"new\" tech as well!"
] |
[
"What are computer architectures and how are they different?"
] |
[
false
] |
I'm a Computer Science student and I feel like I should know this. I've looked it up and I have a basic understanding but I'd like to get a better understanding of this. What are some common types of computer architecture and how do they differ from one another? How does this impact development of software for these computers? I recently overheard a conversation where someone claimed that Sony's PlayStation 3 had a different architecture than the Xbox 360 or even the Playstation 4 which made it difficult to develop games for the PS3. Why and how does a system's architecture make it difficult to do things on a certain system versus another, and why would companies make the decision to support an architecture that could make development of software for that system difficult?
|
[
"In the question you asked, computer architecture refers to the logical structure of, and the interoperability between processors and memory. Take XBOX 360 and PS3 as an example. While both have processors derived from the PowerPC ISA (instruction set architecture), they are very different in their computer architecture:",
"The X360 has a tricore Xenon processor and a uniform memory architecture. The three cores are the same general purpose processors, they see the memory as the same piece - and what makes it even better, the GPU shares the same memory, too!",
"The PS3, on the other hand is quite a different beast. Its Cell processor has one general purpose PowerPC core, along with 7 weird ass Synergistic Processing Elements (SPEs). The SPEs are highly dedicated in floating point processing, which is good for games, and that makes PS3's FPP several times more powerful than X360 in the book. However, these SPEs are very nasty to deal with. They have their own instruction set, their own memory, and they are unable to access the system memory directly, but with clumsy asynchronous DMA operations.",
"So now you should see why people say it's much harder to make games for PS3 than X360. Of course, if you are making a sketch level game and you don't need much processing power, both architectures could be almost the same to you; but if you are making a 3A game and want to get the most out of these boxes, it'll be much more challenging to deal with PS3's heterogeneous architecture - and much more rewarding if you managed to release all its horsepowers."
] |
[
"I'll try to simplify, as computer architecture is a large area in Computer Science and you might want to find a course on it if you like it. ",
"The basic difference between different architectures that affect development is the assembly instructions set.",
"Intel and AMD have slightly different architectures/instructions, for example. They both are from the ",
"CISC",
" type, that have a large instruction set, where the ",
"RISC",
" type has only a small set.",
"To the specific question about the consoles, as far as I know the have different instructions set as well. Being difficult to develop game is never something they want, but they want their consoles to be ",
". ",
". And sometimes they can only achieve this with tricky complex instructions for some situations they find to be frequent, and therefore it is harder to better optimize the software for these more complex sets. Consoles from different companies have different assembly instruction sets, and consoles from different generation also have their instruction sets evolved, so they differ and is hard to make PS3 games run on PS4 without emulation (even though they say they are trying to make it easier for the next generation.",
"That's not my main area, but that's what I know on this topic. Hope I've helped you understanding this topic! "
] |
[
"To answer your direct question about the popular architectures out there and hopefully show the difference. Here's a very simplistic look at the difference between a RISC vs CISC (reduced vs complex set)",
"the x86 arch",
"http://www.azillionmonkeys.com/qed/k7-architecture.gif",
"vs",
"ARM arch",
"https://image.slidesharecdn.com/armarch-170326092113/95/arm-architecture-13-638.jpg?cb=1490520088",
"",
"Let's look at the ARM first, data comes in, there's a instruction decode and control that tells the processor to make sense of the data. The data then flows through the architecture where data gets allocated to the register file where it can then be appropriately directed to each of the \"worker components\" like the barrel shifter which profits shift and rotate operations on the registered data (you can do things like logical tests or arithmetic operations by shifting the registers). The \"flow\" of data is very streamlined. Data in, follow a few serial steps, data out. 1 instruction processed per clock cycle.",
"The result then gets written out back to the user",
"",
"Compared to the x86 architecture. An instruction queue comes in on the instruction cache to the controller who decides how to process each of the instructions coming in onto the general registers, things like the FPU/Integer scheduler (to do the same useful calcs that you need for the user). Because the x86 allows multiple operations per clock cycle, that means you need to store the results somewhere. So you build an internal memory bank and store/access the data with a memory controller. The required results then feed onto the internal bus eventually to the user. ",
"",
"But please take this as also a very high level explain.",
"Now to explain how the PS3/PS4/XBOX might be \"harder\" to code. This is not necessarily an architecture issue but a huge combination of things besides architecture. It could be a compiler issue. It could be an APK issue. Different syntax, different ways of addressing memory, too much boiler plating code, different ways of allocating memory, different ways of when to expect data to be processed, etc. There are nuances to coding on different languages/compilers/platforms that it's not necessarily fair to say one is \"harder\" to code on. ",
"Imagine a bicycle, you have a seat for your butt, you sit up right in it and yer feet pedal your hands control the raised handle bar, you turn, go from a to b, it's familiar. ",
"Let's say you have this new bike, instead of sitting upright, you lay on your stomach. They got rid of the raised handle bar and instead stuck them on either side of the front wheels to faster turns. You have almost all of the same functionality, but they got rid of that familiar raised handle bar you've been using all this time. You're also now crouched much lower towards the ground. So it takes you a few times to practice, get used to this new bike, and realized you like riding the bike on your belly instead of an sitting position. It might feel \"harder\" int eh beginning, but eventually when you're accustomed you can go from a to b just the same."
] |
[
"Can stars spit out elements we've yet to discover?"
] |
[
false
] |
Okay. Well, I am a scifi buff. I play EVE Online. In EVE, we use minerals with scifi origins and and psuedoscience lore to explain their existence. We also use actual elemental stuff like technetium and polonium, to name only a couple. I understand that all of the elements we know of on the periodic table, the ones we encounter here on Earth, originate from our star as a byproduct of the sun's life cycle. Heavy elements created as a byproduct of our sun's fusion, spat out in solar flares, traveling through the void and eventually collecting and making new stuff. Obviously not all of our celestial material came from just our star, but all of the material came from a star at some point. What I want to know is, is there any scientific precedence, theoretical or known, that could explain the existence of elements that may be unique to a star system? Like, for instance, an extremely large, very hot, very old star. It's so old it's spitting out elements heavier than any we've ever encounters naturally or in a laboratory. Kryptonite, because why the hell not? Could these elements exist? Or are the elements we know of here on Earth and in our own galaxy consistent throughout the entire universe? Can modern science explain the existence of unique and strange elements we've yet to discover as a natural occurrence due to star emission?
|
[
"What sorts of things could stable super heavy elements be used for?"
] |
[
"Well atomic matter up to a point can be made in a particle a accelerator. Unless there is a mechanism that we're not understanding or are over looking in the processes creating atomic matter than I don't think stars are doing anything different. Maybe there is some strange quark neutron star which spits up stable super heavy elements bound through the strong force. Or maybe there's some stars producing non atomic matter that can't exist at lower temperatures in our universe? Its very wonderful to think about."
] |
[
"Like, for instance, an extremely large, very hot, very old star.",
"In our current understanding of star formation and evolution, very hot (or equivalently, very massive) stars don't live very long (still a couple of million years though). So the oldest stars will have very low masses. But this also implies it will be less efficient at forming heavier elements."
] |
[
"Can someone explain the stress-energy tensor to me and how it plays into general relativity?"
] |
[
false
] |
I know it has something to do with energy density, but not much more than that.
|
[
"For any conserved quantity there's a current, which is a vector quantity. When you put the density of that quantity together with its current, you get something called a four-vector, which has four components instead of three, and is essentially a vector in spacetime.",
"But energy and momentum already form a four-vector, and all four of them are conserved independently. The current for energy is the same as the momentum density, while the current for momentum is precisely what we normally think of as stress. As an example, in an object under pressure there's x-momentum moving in the x-direction, y-momentum moving in the y-direction and z-momentum moving in the z-direction. When you put the currents for these quantities together with their densities you get a four-vector of four-vectors, which is a tensor. This is what we refer to as the stress-energy tensor.",
"As to its role in general relativity, the stress-energy tensor is the source term in GR, just as the electric charge density and current are the source terms in E&M and the mass density is the source term in Newtonian Gravity."
] |
[
"You can calculate it for a rigid body, in which case it's a fairly straightforward combination of the object's energy and momentum as well as any stresses within it (I don't remember all of the formulas off the top of my head). More often, though, you're interested in the stress energy tensor of either dust (a collection of slow-moving matter fragments that don't interact), radiation, or some kind of fluid, such as the interstellar medium. There are standard formulas for all of these.",
"In the most general case, the stress energy tensor is derived from a value called the Lagrangian, which encodes all of the physics particular to a model. There's a theorem, called Noether's theorem, which states that for any symmetry of the Lagrangian, there's a conserved charge and current, and gives the means of calculating said current. The stress-energy tensor is the four-current associated with translational symmetry (the laws of physics don't change from point to point or moment to moment)."
] |
[
"Well, spacetime curvature is encoded in a 4-by-4 matrix as well, and the two are related by ",
"Einstein's equation",
"."
] |
[
"With all the recent \"super Earth\" discoveries, what is the heaviest gravity world a human could live on?"
] |
[
false
] |
The heaviest gravity a human could live in for a standard life span while being able to maintain a "normal" life. This means without adverse effects causing early death or deformities. You can also expand on what you think possible through the evolution of humans breeding and propagating on said planet. I would also be interested in things like atmospheric density and pressure, whether a thicker atmosphere might be present due to a stronger gravitational pull or magnetic field, and how that would affect the people living on the planet. I am mainly interested in this for colonization purposes and I believe we need to populate other planets to survive as a species. I know humans can live in places with lesser gravity but not without some health problems. (I looked to see if this question has been asked before but couldn't find anything, if it has please share the link!)
|
[
"I know you're probably mostly interested in the physiology of humans in high gravity environment. I can't really comment on that but I can tell you that Earth is pretty close to the limit where we are able to get to orbit using rockets. If you increase the surface gravity by about 50%, then we'd be unable to get to orbit. And even smaller increments are going to make it considerably harder. If you keep the density of the planet the same, then 50% increase in radius means also 50% increase in surface gravity. See ",
"this article",
" about the subject by Don Pettit, an astronaut who's been on ISS twice."
] |
[
"It's all explained in detail in the article I linked. But the short version is that we're already at the point where about 85 to 90% of a rocket is fuel. With a higher surface gravity this number would have to go up. You get to some point, say 99% of fuel, where you can't construct the needed structure to support all the fuel and the engines in the 1% you have left. Not to mention putting actual payload in there too."
] |
[
"Doesn't work like that. Huge rockets have to carry themselves as well, you just end up wasting fuel. The Saturn V is about as big a rocket as you could ever efficiently build, if it can't even get to space, you probably won't ever."
] |
[
"Why do foods that are unhealthy taste better than foods that are healthy?"
] |
[
false
] |
For example, foods that contain a lot of sugar or fat vs one that doesn't.
|
[
"The main reason is that our brains have learnt to have a higher pleasure response to energy-dense foods. This makes sense in a primitive environment, the advantage being you have a desire for rich foods with plenty of calories!",
"Since then, however, we do not have to seek out meals in a primitive sense anymore, but we have retained our desire for these foods."
] |
[
"why, not what."
] |
[
"Primitive humans:",
"What this means is, for them, calorie dense foods like sugar and fats were the healthiest foods they could possibly eat because those were the foods that were best at keeping them alive.",
"Then we invent agriculture and other technologies that let to a surplus of food, but retained our primitive appetites, which no longer match what is most healthy for us."
] |
[
"Does a star visibly change when it begins using a new fuel? And is the timescale observable?"
] |
[
false
] |
For example, if a star fusing hydrogen has enough mass to fuse helium when the hydrogen is depleted, will it visibly change? And if so, will it happen quick enough for us to see the change?
|
[
"Was confused as to why the earth had already been destroyed.",
"This is a good opportunity to use the Future perfect tense: \"The Earth will have already been destroyed several billion years earlier.\" ",
"(In the future I'm talking about, this has happened between now and then)",
"Otherwise, thanks for the explanation!"
] |
[
"Stars like our sun do an hours-long transition to helium burning called a ",
"helium flash.",
"The helium flash is guaranteed to not destroy the Earth because there is almost no immediate outward change, and because Earth was already destroyed several billion years ago much earlier in the ",
"red giant phase",
" before the sun fully ran out of hydrogen in its core. The huge release in energy from helium fusion switching on is quietly absorbed by a change in the density in the core.",
"The sun will continue to be a red giant for a while after helium fusion starts. Eventually the outer layers boil off, and the core that is left is a big dumb lump of carbon and oxygen made by fusing helium. The remnant core is called a white dwarf."
] |
[
"It changes a lot, but not notably (to the human eye) within a human life span. Supernovae are an exception.",
"https://en.wikipedia.org/wiki/Red-giant_branch",
"Some stars change their brightness a lot frequently while keeping their fuel type. ",
"Mira",
" is an extreme example. It varies by a factor of more than 1000 with a period of a few months. Sometimes it is easily visible to the naked eye, sometimes it is way too dim. The next maximum will be October this year.",
"There is also ",
"Algol",
" which appears to get dimmer every few days - but this is a binary star where the brighter star is partially eclipsed by the dimmer star."
] |
[
"Why is the universe cooling down?"
] |
[
false
] | null |
[
"The amount of heat in the universe is really really small compared to the amount of space in the universe. The sun is hot, and heats a big piece of out solar system pretty well, but if you divided that heat evenly across our solar system, it'd be too cold for any life. Same principle with the universe. It's really hot in a few spots, but really really cold in most. The average is cold."
] |
[
"The universe is expanding so volume is increasing."
] |
[
"I'd guess that the book tried to tell about ",
"http://en.wikipedia.org/wiki/Heat_death_of_the_universe",
" ."
] |
[
"A question about light."
] |
[
false
] |
My main question is; Can we see a star in the night sky whose light has been bent by gravitational pull? [ ] If that does happen, would it be possible to see the same star twice? [ ]
|
[
"Yes, but it's not as extreme as your diagrams suggest. A search on \"gravitational lensing\" turns up a bunch of ",
"pictures",
"."
] |
[
"That picture is awesome. "
] |
[
"See ",
"Gravitational lens",
".",
"I don't think there are cases of light bending as much as your diagram suggests, though."
] |
[
"When you have a cold, why do you feel worse in the mornings and evenings and better during the day?"
] |
[
false
] |
I've always noticed that when I have a cold, my nose is stuffier for a few hours after I first wake up, and when I'm going to sleep. It's not because I stop paying attention to it during the day, either - it actually clears up a lot more during the day. This happens with other symptoms too, for example, headache and sore throat, but mostly with a stuffy nose. Why does this happen? Thanks!
|
[
"As the other comment says, position probably has a lot to do with it. You're laying all night, the mucus drains and pools and runs down your throat, causing stuff nose, sore throat, headache from sinus pressures, etc.",
"Another factor is the body's circadian rhythm. Your body goes through daily cycles where all sorts of variables change. Body temperature, hormone levels, various other body chemical level fluctuations. One example is cortisol. Among other things, it acts as an anti-inflammatory. As its level rises, it reduces swelling in your sinuses, improving draining, reducing pressure, and contributing to you feeling better overall. Its level drops in the evening, however, so you start to feel worse."
] |
[
"You’re actually right: sleeping with a very big pillow under your back and head, half-sitting, is a great way to not feel as bad during the night."
] |
[
"Makes sense. Maybe I'll try sleeping sitting up next time haha."
] |
[
"Does the body have a single thermostat, such that if <part> gets cold, you shiver, and if <part> gets hot, you sweat?"
] |
[
false
] |
I'm just curious whether there's a single spot or set of spots that tell the brain what your body heat is, or whether it is distributed across your whole body?
|
[
"The hypothalamus, a structure of the lower brain, is responsible for thermoregulation in humans. The hypothalamus directly receives a variety of inputs from the blood, one of which is temperature. When blood temperature rises, the hypothalamus induces sweating and vasodilation, and when blood temperature drops we get shivering and vasoconstriction. Only the one \"thermostat\" is needed because one of the functions of blood is to maintain constant temperature by dissapating heat away from heat producing organs like the liver, brain, and skeletal muscle. So the temperature that the hypothalamus senses is an aggregate of the temperatures of all the other areas of the body.",
"The hypothalamus also integrates signals from thermoreceptors all over the body, which can play a role in thermoregulation (think of jumping into an icy river - almost immediately you may begin to shiver) but which most often serve to provide us with conscious perception of temperature."
] |
[
"Thanks for this answer.",
"Could you reconcile this with shut_it's response below regarding the localized sweat response your arm would experience if it alone was put outside on a hot day?"
] |
[
"I do not think that there would be a localized response. The arm cannot be thermally isolated from the rest of the body (blood flows through the arm and back to the body) so heating the arm would also cause a bump in core body temperature and should cause a full body response. If we think about thrusting an arm inside a freezer for a few seconds, the intensity of that localized external temperature change is not enough to trigger an immediate thermoregulatory response."
] |
[
"How would a hydrophobic coating effect the buoyancy of an object?"
] |
[
false
] |
[deleted]
|
[
"Not much; the displacement wouldn't really change though skin friction drag (which is only a tiny part of total drag) might decrease a bit.",
"Next time, use the search box on the right. I put in \"hydrophobic ship\" and the top result was this previous question:",
"If one were to coat the outside hull of a ship with a hydrophobic coating, would it decrease the displacement of said ship? Would it increase the ship's top speed? What would it do to a submarine?",
"http://www.reddit.com/r/askscience/comments/14dvb0/hydrophobic_substances_and_ships/"
] |
[
"If it's flexed inwards more water will be below the object and displaced, and the object will ride marginally higher. I didn't mention this because it's basically negligible at boat sizes."
] |
[
"By that logic the entire interior of the boat, which is air, not water, should lessen the buoyancy and cause it to sink. Think it through a little bit and you'll see what's going on. When you replace what would have been below the water level with something less dense, you get buoyancy. Replacing something above the water level with something denser (i.e. air with a steel superstructure) is extra weight that counteracts the buoyancy."
] |
[
"How do engineers check the structural integrity of a building, particularly after a damaging event like an earthquake?"
] |
[
false
] | null |
[
"I'm an aerospace engineer not civil but the same basic principles apply. We have to do something similar if we suffer a tail strike. It all boils down to Nondestructive Inspection (",
"NDI",
"). The simplest will be a visual inspection, if anything is cracking or deformed. A quick test people do (for composites at least) is called the coin tap test. It's exactly what it sounds like, you tap a coin on the structure. A solid structure will have a clear return, a damaged one (i.e. voids, AKA air pockets) will have a duller sound.",
"For real testing you could use fancier techniques like ultrasonic testing or x-ray testing. I would hazard that it would simpler for structures to just reinforce the areas that may have been damaged rather than actually check for structural integrity. Better safe than sorry and weight isn't as big of a factor in static structures."
] |
[
"The short answer is that if you know how the building was built, you know the weak points for an earthquake and you look for cracks. The direction and location of cracks will tell you how connection points may have held together (drywall is quite brittle so cracks easily).",
"A typical seismic assessment guide is here:",
"http://www.curee.org/projects/EDA/docs/CUREE-EDA02-2-public.pdf"
] |
[
"Every building type is unique in the way that it fails, but when it comes down to it, generally it is just looking for cracks. Damage is primarily caused by displacement of the building, or drift, which varies for different types of buildings.",
"Reinforced concrete, for example, is more stiff than wood, and will not tolerate the same horizontal displacement. This will cause cracks to develop at top and bottom of columns and walls, generally at an angle of 45 degrees (shear cracks). It is generally easy to visually identify the amount of damage on these structural elements, ranging from small cracks to spalling (outside layer of concrete falls off), and exposed reinforcing bars.",
"Another indicator would be failed connections, which are usually obvious, such as broken bolts / nails / screws etc.",
"I attended a seminar of this PhD student whose thesis was on automated damage assessment based on visual clues. Here is a link to the abstract / thesis pdf:",
"https://smartech.gatech.edu/handle/1853/47686",
"Damage assessment is simple enough that a computer aided analysis of photos can handle it (with a reasonable tolerance)."
] |
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