title
list | over_18
list | post_content
stringlengths 0
9.37k
⌀ | C1
list | C2
list | C3
list |
|---|---|---|---|---|---|
[
"How have scientists been able to \"store' anti-matter?"
] |
[
false
] |
[deleted]
|
[
"Using electromagnetic fields to keep the atoms suspended and not touching the walls of the chamber they're in. An example is a ",
"Penning trap",
"."
] |
[
"It's the most energetic reaction in the universe, but antimatter is produced in such vanishingly small quantities that it's not the primary issue. They use the traps mostly so they can keep it around for use in experiments, which you can't do if it's constantly vanishing into thin air."
] |
[
"Extremely energetic. I am not sure if all of the matter is converted to energy, but if it is, a 500 gram apple would react with 500 g if normal matter to produce around 9x 10",
" joules, or about the same amount of energy as a 40 megaton nuclear bomb."
] |
[
"What is the difference between these equations involving Strangeness Conservation that makes one permissible and one not?"
] |
[
false
] |
Why is Λ -> p + π permissible, but K + p -> n + π not? uds -> uud + d and s + uud -> udd + u Both involve a change of quark flavour - the s quark of the Λ and K both becoming down quarks - implying the Weak Interaction and hence Strangeness not necessarily being conserved. So what is different between the two interactions that means the second is a no-no? The two interactions have been pulled from different sources. The first is from Hyperphysics and the second is from a CGP textbook which claims it is not allowed. Admittedly the textbook does not even approach the idea that Strangeness is not always conserved... EDIT: got the sources mixed up.
|
[
"The channel by which a strange quark decays into a down quark has additional byproducts, which show up in the decay channel of Λ",
" that you mentioned, but not in the second reaction.",
"The second section on this ",
"hyperphysics page",
" shows the channels in which the various quarks can decay. There are 3 particles (technically, fields, but we're just going to treat them as particles) that facilitate the weak interaction: The electrically and flavor-neutral Z boson, and the W+ and W- bosons. The Z boson can't alter flavor, so it can't turn quarks into different quarks. The W± bosons can, and that's what leads to violations of flavor conservation, of which strangeness is one example.",
"Strange quarks have -1/3 electric charge, and the W± bosons have ±1 electric charge. If a strange quark decayed into a W+ particle, it would have to also produce another particle with -4/3 charge; and no such elementary particle exists, so that can't happen (total electric charge is always conserved). On the other hand, a strange quark can decay into a W- and an up quark, which has +2/3 charge, because the total charge is -1/3 -> -1 + 2/3 = -1/3. The W- boson has a half life of about 10",
" seconds, so it practically instantly decays. W- bosons can decay into many different pairs of particles, such as any one of the leptons and their corresponding neutrino, or into ",
"d or ",
"s. This means that when a strange quark decays, it ",
" decay into an up quark, plus any one of those additional pairs of particles from the decay of the W- boson.",
"The reason why the first one is allowed is because you start out with ud+s, and end up with ud + du",
", where the second term is the decay products of the strange quark (an up quark, plus a d",
" pair). ",
"In the second example, you start with ",
"uud+s and end up with ",
"uud+d, indicating that the strange quark decayed into a down quark with no other products, which is impossible!"
] |
[
"There is no way for the second reaction to occur without additional particles in the initial or final state. There is no known process in the Standard Model that can turn a strange quark into a down quark with no other byproducts. Even hypothetical flavor-changing neutral currents above tree level would at least produce photons; and even if such a process is possible, it would be so highly suppressed that it would still be reasonable to say that the reaction (+ a photon) doesn't happen.",
"The problem of timescales is a good point, but the bigger issue is that it is fundamentally impossible, and not just unlikely due to mismatched timescales."
] |
[
"Yeah you're right. I was so focused on how an individual quark can decay, I completely overlooked the fact that we have interacting particles here. So yeah, as you said, it's not impossible, but we're unlikely to ever observe it. ",
"In that case, the textbook saying that \"it's not allowed\" is either an oversight like my own, bad wording, or it was using the term in the sense of forbidden processes, like in spectroscopy. Probably the latter."
] |
[
"Is there a foolproof way of drawing angles with relative precision without a protractor or compass?"
] |
[
false
] |
I've always wondered this. Protractors are lovely, but sometimes you need to draw that 65° angle and you don't have one on you. It seems logical to me that using a ruler there should be some way that I am missing to be able to do this.
|
[
"Folding paper can be handy. Fold a piece of paper diagonally from one corner for a 45º angle. Fold it in thirds for a 30º angle. Fold in half twice for a 22.5º angle. Fold in thirds, then in half for 15º, and so on. I use this to make reference angles for drawing from time to time.",
"edit: If you do thirds, then half, then flatten it out, you have a piece of paper with a 90º corner and creases marking each 15º wedge. If you hold your ruler perpendicular to the centerline of each 15º wedge in turn, and subdivide each of these wedges into 15 subdivisions using the millimeter or 16th inch lines on your ruler, you can make yourself a pretty accurate protractor. It won't be perfect, but it'll be close enough to get you within a degree or so. "
] |
[
"If you've got a table of trigonometry values...",
"Or a calculator?"
] |
[
"If you've got a table of trigonometry values and a very accurately marked ruler, it's possible. Just find from a lookup that tan(65) is about 2.1445, draw a line segment of length one and a perpendicular one with length 2.1445. Connect the dots.",
"If you'll allow a compass to be used as well, there's quite a bit ",
"more that you can do",
"."
] |
[
"How does the quantum tunneling effect limit development of micro processors and how do we overcome that?"
] |
[
false
] | null |
[
"The issue is not that the material is thin but rather that the distance between parts of different voltages (base and collector for example) is small enough that tunneling can occur through otherwise nonconductive material. The energy of the electrons is not the problem its the voltage gradient. Copper transistors would allow for smaller more efficient faster transistors if the technology to fabricate them existed but such technology would also make many other types of transistors possible and its extremely likely that copper would not be the optimal material. Metal oxide barriers can prevent tunneling between components but that will only help a little bit as it cant stop tunneling within transistors which is the main problem. 3D layouts allow for more transistors but makes thermal management harder so it is unlikely to be a solution to the problem. Graphene transistors can be made much smaller than silicon transistors without tunneling issues. I have no idea what you mean by \"take advantage of the ease with which carbon atoms can be arranged on the nanometer scale.\""
] |
[
"When transistors get to a certain size, they can’t always hold electrons back, which can cause false signals and errors in computations. As far as I know, we can’t directly overcome tunneling, we can only work around it."
] |
[
"As technology miniaturized within transistors, the materials became so thin that the energy of electrons flowing through them was enough to cause the electrons to tunnel through to the substrate to other transistors or be lost to the substrate of the processor.",
"As a professor of mine described it, back in 2007, there was talk of returning to using copper as a means of resolving the issue of voltage loss due to tunneling, but the technology didn't exist at the time to manufacture copper-based transistors at that scale, to the exacting specifications needed. Too many defects. The hope was that nanomaterial construction might resolve this issue and allow copper transistors to take the lead in continuing miniaturization, but multi-core processing instead became the new standard and effectively sidestep the issue for a period of time.",
"Moving forward, there are a few different options:"
] |
[
"Is there a succinct unit name for liters per meter squared per hour?"
] |
[
false
] |
Question above. The hypothetical situation would be in relation to absorption / dispersion of fluids. Example how much one individual sweats out of a particular area of their body.
|
[
"Well liters are a measure of volume, in fact 1L is 0.001m",
" so",
"1 L / m^2 . hour\n= 0.001m^3/m^2 . h\n= 0.001m / h\n"
] |
[
"In case someone is similarly weirded out by this unit of measurement:",
"This is how rainfall can be stated. Length per time unit."
] |
[
"Another weird/fun unit is using square metres for fuel consumption of cars.",
"For example, 6 litres per 100km is 0.006 m",
" / 100000 m = 6x10",
" m",
" = 0.06mm",
"This area is actually the cross-section of a very thin pipe or noodle of fuel, that when laid on the road would let you drive without a fuel tank if you slurp it up as you move over it. More efficient cars need a thinner noodle to keep them going."
] |
[
"Apparently the sun's magnetic field is about to \"flip.\" What causes this to occur and why does it do so every 11 years?"
] |
[
false
] |
Why 11? Does it have something to do with the way the sun spins? The Solar System? Some type of Galactic process?
|
[
"I'd like to elaborate in that we know a lot more than noott's post gives credit for.",
"It is very hard to pin down exactly what we know though, with physics as complex as plasma physics in a body the size of a star we will never fully be able to recreate it and it is unlikely we will every analytically be able to point at an equation and say \"this causes the magnetic field reversals\". We have made a lot of progress in explaining the Sun's magnetic field though there is even more still to do.",
"I'll give you a taste of the story so far though. The Sun's magnetic field comes from a physical process known as the Solar Dynamo, which is an example of an MHD dynamo. The sun is made of a conducting fluid, plasma, similar to the iron that makes up the Earth's core. A current in a conducting fluid produces a magnetic field, any conducting fluid you flow through a magnetic field will naturally produce currents that will reinforce the magnetic field. ",
"From this small starting current we can create the Sun's dipole, the general shape of the magnetic field, due to a giant ring of current under the surface going the entire way around the star. The region of high interest here is the Tachocline (border between convective and radiative regimes of the interior) where we think this current is generated due to the large change in rotation speed there (shear flows automatically = current).",
"So yeh I'd say we know how the magnetic field got there. As for the solar cycle, that's trickier...",
"...unlike a solid body different parts of the Sun rotate at different rates, the equator rotates fastest and as you get nearer the poles it lags behind. This wouldn't mean anything if not for a neat law of plasma physics which is known as the \"frozen in flux\" theorem, which basically means where the conducting plasma goes, the magnetic field follows.",
"Our fast rotating equatorial region drags our dipole field around and around, each time round the magnetic field becomes wound tighter and tighter, stronger and stronger. Magnetic fields resist being wound up and so this whole time energy is being stored up and it begins to leak out. This first manifests itself as active regions (and how those actually emerge from the interior is an entire posts worth itself), initally found nearer the poles when the dipole field is still dominant. As time passes and the magnetic field becomes more contorted, and these active regions form closer and closer to the equator. Around this time the magnetic field is very messed up. The field at the poles is shaky and the field nearer the equatorial regions is all over the place. Then a reversal happens.",
"This is where my expertise is light, I have seen the talks and papers but can't say I understand them. This is further exacerbated by the fact that although the reversals are very recreatable, it is not reliable. It requires much tweaking by the modellers in models that are already not the full story (i.e. missing physics).",
"TL;DR we do (mostly) know what is going on, how the field forms, how it gets twisted, how this leads to the sunpsot cycle and that this eventually causes a reversal but you are right there is more we need to find out!"
] |
[
"We do not know!",
"That's basically all. ",
"It's an active field of research within solar physics.",
" We do not understand the details of the generation of the sun's magnetic field, let alone what causes the flips and 11-year cycles. ",
"I can say with certainty that it is not a galactic process, though. The generation of the sun's magnetic field occurs beneath the surface, and it is processes there that cause all of its observed properties. The basic answer is that currents beneath the surface generate the magnetic field, but we cannot explain the shapes and distribution of magnetic fields at present (coronal holes, active regions, quiet sun regions, X-ray bright points, etc.). "
] |
[
"Meant to reply but made a top-level post by mistake, I think you are bang on that we don't know what is going on with the Sun's magnetic field but the things you highlight ",
" mostly understood, we do know what causes the dipole field as well as roughly what causes the flips, we also definitely know a lot about what causes coronal holes, active regions quiet sun, bright points etc. and where to find them.",
"We know an awful lot about the Sun, still plenty to learn though."
] |
[
"How do trains carrying liquid freight not have their cargo freeze in the winter?"
] |
[
false
] |
I have to imagine they don't want whatever is in those tanks to freeze, but I can't figure out how they might prevent it...
|
[
"Sometimes trains actually let the liquids freeze when transporting. Here's a fun fact:",
"Liquid Na metal that is refined from the ",
"Downs process",
" is directly put into ordinary railroad tank cars. That liquid sodium then freezes while it is inside the tank. When the car arrives at the destination, the entire car is heated up, most of the time with simple flames, until the Na re-melts. Then it is pumped or drained in liquid form into the delivery tank off the rail car. Pretty awesome."
] |
[
"Well, we don't transport water on trains. Hydrocarbons and oils typically freeze at a lot lower. Remember that water is also unusual in that it expands on freezing; most substances contract, so there's no risk of damaging the container. So there's relatively little need for heating unless it's a substance that would be damaged by freezing/thawing, which isn't true for most bulk chemicals either.",
"Heating a tanker car isn't such a big deal, mostly they cool them, though - when transporting liquefied gases, for instance."
] |
[
"Insulation."
] |
[
"How does aging/curing meats avoid contamination from harmful bacteria such as c-botulinum?"
] |
[
false
] | null |
[
"Depends on the method. Dry aging for instance is usually done at colder temps, which halts bacterial growth. Most “curing” relies on salt to prevent the growth of harmful bacteria, meanwhile fermentation relies on either salt or lactic acid, or a combination of both.",
"In all of these situations though it all boils down to “create an environment that isn’t hostile to humans, but is hostile to bad bacteria."
] |
[
"To add, pH plays a role, too. Especially in fermented sausages like salami, where lactic acid fermentation shifts the pH out of the viable range of C. botulinum. Also, addition of nitrite in non-fermented sausages."
] |
[
"Drying makes it harder for most bacteria to grow. Smoked meat uses the chemicals in wood smoke to kill/inhibit a lot of microorganisms. Biltong uses the acid from vinegar to make the meat inhospitable to pathogens. Spices like mustard, garlic, and pepper have antimicrobial properties.",
"Humans have figured out quite a lot of different ways to preserve food."
] |
[
"What are the benefits of copper or aluminium layers in pans and pots?"
] |
[
false
] |
What I think I understand are some copper clad frying pans I've seen that have sort of half cylinders of copper on their bottom transferring heat to a larger surface area of steel on the curve of the half cylinder of copper. What I don't understand are the very popular 3 ply steel pans that just seem to have a layer of aluminium sandwiched between steel. Is it textured at all? It seems like if it was flat there would be no surface area benefit, so the transfer would not improve.
|
[
"Al and Cu have much higher thermal conductivities than steel, meaning they transfer heat faster than steel. However, they are not suitable for the daily abuse pots and pans see. They scratch easily, dent, oxidize, etc. So, the manufacturer compromises by using them in the core of the pot to do the bulk of the heat transfer while leaving steel to handle the rough stuff. Table of thermal conductivities below.",
"http://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html"
] |
[
"If you are asking why they just don't make really thin steel pans, to maximize heat transfer, the reason is because fast heat transfer isn't the only goal of good cookware. The goal is also ",
" heat distribution across the surface of the pan. The thicker the pan, the more uniform the temperature on the surface of it. Less conductive materials also create more uniform temperature gradients. ",
"So idealy you want a pan that can heat up fast (or respond quickly to heat changes), can distribute that heat evenly, and is durable.This is done by striking a balance between thickness and different layers of different metals."
] |
[
"Yes, but the heat still has to be conducted through the aluminium. Does a simple flat sheet improve the conductivity at all? While copper is more conductive, what's even more conductive than copper is nothing at all, because copper doesn't conduct heat perfectly. So isn't it just a layer of inefficiency?",
"I understand how a 3d layer that has a higher surface area in contact with the aluminium can improve conductivity, but with a 2d surface with no increased surface area just seems like to introduces inefficiency."
] |
[
"How does it happen that an entire volume of data can become corrupt when the writing of only a small portion of it is interrupted?"
] |
[
false
] |
The other day, the electricity went out while I was saving a folder of word documents to a flash drive. When I powered it back up again, Windows told me that the drive was corrupt and needed a reformat. The flash drive is 16GB, and my documents were only a few megs. What happened that the entire drive became corrupt?
|
[
"In your case it seems was damaged the File Allocation Table (FAT). This file system can be read and written by a wide range of devices, but it is very fragile because of how data is arranged logically. In most cases you can fix the problem with a specialized application, like chkdsk under Windows.",
"If you are using it only on computers, you can format it with NTFS file system. This file system is more robust."
] |
[
"NTFS does not write 'less logically' but certainly it is more robust than FAT. ",
"You have journaling features in NTFS that can keep 'records'/journals of changes to the filesystem before being committed to writing which can make data-recovery far more likely, as well as less likely loss of data due to corruption. These are features that FAT lacks. ",
"What happens (in a simple explanation) is that the system logs actions to be taken on the disk; if you ask to write data to a disk the journaling file-system will mark the action down first then execute. If an interruption in operation happens (power failure, system crash, etc.) the journaling file-system needs to simply examine its journal to know where the step failed for it to be able to proceed on how to recover the operation. This does assume the action was fully written to the journal before the crash. If the action was not properly written before the crash you'll run into an unreplayable/unrecoverable operation. ",
"That's why Windows programs like CHKDSK or FDISK are still around. They can further search out inconsistencies with the filesystem and attempt to repair the file-system if any issues of corruption or other issues arise. ",
"http://en.wikipedia.org/wiki/Journaling_file_system",
"I hope that helps clear up how NTFS can mitigate data corruption due to a power failure/crash when writing data to a drive and still be 'logical' when organizing data as compared to FAT 16/32. "
] |
[
"Ah, cool! So the NTFS file system arranges data...less logically, but also in a less fragile manner? "
] |
[
"Do our eyeballs grow after we are born?"
] |
[
false
] | null |
[
"I love how the guy asked his question on reddit, reddit searched google, google found a yahoo answers post, reddit copied and pasted it back on reddit and ended up wasting a lot more time than needed. But with that being said, I find a lot of interesting posts on reddit that I would have never thought about in a million years, such as this one. Interesting. Also, redditredditreddit."
] |
[
"I love how the guy asked his question on reddit, reddit searched google, google found a yahoo answers post, reddit copied and pasted it back on reddit and ended up wasting a lot more time than needed. But with that being said, I find a lot of interesting posts on reddit that I would have never thought about in a million years, such as this one. Interesting. Also, redditredditreddit."
] |
[
"Short-sightedness (myopia) is usually a result of the eyeballs growing too long along the horizontal axis. So, yes, they do grow for some time after birth."
] |
[
"If a quantum wave function has 3 state would making a measurement on only 1 state collapse it?"
] |
[
false
] |
for example if a particle is superpose in position A B and C with an amplitude of sqr(1/3) each, would testing if the particle is at C collapse the wave function assuming we find nothing or change it to 50% for A and B keeping it superposed.
|
[
"Excellent question. I'm going to imagine this as a ",
"triple slit experiment.",
"So basically the wave function is as predicted by QM.",
"Now we put a detector beside one of the slits say the left slit, \"collapsing\" electrons that pass through this slit.",
"These results are the same as a double slit through slit left and centre, and single slit on right. So basically it acts as two seperate experiments.",
"The best way to think of wave function \"collapse\" is to imagine the \"real\" macroscopic world as being one big quantum mechanical system and collapse is jsut the particle becoming entangled with this system and since the system is so big that the particle can not become unentangled."
] |
[
"It would remain in a superposition between the A and B states.",
"An analogue of this measurement is to have a particle with three states A, B, C. Now, suppose states A and B are degenerate (same energy). We proceed to measure the energy of the particle. If we measure the particle to be at that shared energy, then what we have done is projected the particle's state onto this reduced basis."
] |
[
"It's not necessarily a single eigenstate. A measurement will collapse the wavefunction into a subspace of the original eigenspace."
] |
[
"Why didn't natural selection eliminate poor eyesight before glasses were invented?"
] |
[
false
] |
My train of thought with this is that poor eyesight seems like it would've been a big enough hindrance to things like hunting and gathering and generally not running into stuff and dying back in those times that eventually people with poor eyesight wouldn't have been able to reproduce. Am I just confused on how poor eyesight works?
|
[
"Most \"poor eyesight\" is heavily influenced by environment, not solely genetics. That is, you may inherit the tendency to become myopic, but you don't actually develop myopia unless you add in the environmental triggers:",
"Even though the tendency to develop myopia may be inherited, its actual development may be affected by how a person uses his or her eyes. Individuals who spend considerable time reading, working at a computer, or doing other intense close visual work may be more likely to develop myopia.",
"--",
"American Optometric Association",
"In the days when hunting and gathering was important, most of the people weren't spending considerable time reading or working at computers, so even if they had a genetic predisposition, they wouldn't develop serious eyesight problems.",
"(Alternatively, the people who had a genetic predisposition that didn't need environmental triggers -- or the people who had a genetic predisposition that was triggered by the components of a savannah environment -- did die, and those tendencies were not passed on.)"
] |
[
"Of those, only tool-making seems like it's really comparable to the modern environmental triggers. Watching TV or reading can easily leave your eyes focused on the same spot for hours at a time, day after day; whereas starting fires might take a few minutes or a half hour; cutting wood with a stone axe better not be right in front of your face, and would involve constantly changing focus; and so on. ",
"Making stone tools could, conceivably, lead to myopia if someone was a specialist in making them and did little else, day after day. (I don't know if we know enough about ancient specialization to say if that kind of thing happened.) But then, so what? Now your social group has a specialist who doesn't need to hunt or gather, because she is making knives for you instead. ",
"In other words, if ancient humans did something so much that they affected their eyesight, they probably weren't hunter-gatherers, even if their overall group was.",
"(But this is speculation, and the more likely explanation is that nothing in their routine did trigger myopia.)"
] |
[
"For something like poor eyesight to be weeded out of a population, it would have to kill a portion of that population before that portion had a chance to reproduce. Since a lot of young creatures have parents to look after them, they have a fairly good chance of living to a sexually mature age. And since you don't need good eyesight to reproduce, there's no pressure to select those with better eyesight over those with poorer.",
"No doubt someone with better credentials than mine can elaborate."
] |
[
"Just as \"red\" in hundred meant \"count\", does the \"ter\" in close relatives (like father, mother, brother, sister) mean anything in Old(er) English?"
] |
[
false
] | null |
[
"Yes! Father, mother, and brother are derived almost unchanged from proto-Indo-European (PIE), where -ter is a kinship suffix. Sister is from PIE *swesor, probably from PIE roots *swe- \"one's own\" + *ser- \"woman.\"",
"If you like this sort of thing, etymonline.com is fantastic. "
] |
[
"There are four r-stem kinship terms in proto-Indo-European (pronounce h₂ like the sound you make in the back of your throat when you blow on your glasses to fog them up):",
"*-tḗr does seem to be an agentive suffix elsewhere in the PIE corpus (e.g. *h₂eh₁s- (burn) + -tḗr (-er) > *h₂stḗr (burner) > \"star\"), but it's hard to say what (if any) role it plays in the kinship vocabulary, because we don't find e.g. *ph₂-, *méh₂-, *bʰréh₂-, and *dʰugh₂- as separate morphemes, either free or in combination with other suffixes.",
"\"Sister\", a fifth term that seems to fit the paradigm in English, has a different etymology: *swé (self) + *h₁ésh₂r̥ (blood) > *swésōr (sister). It was probably brought into line with the four terms above through analogical leveling in proto-Germanic."
] |
[
"I'm curious why we don't have this in other relatives like son, uncle, aunt, nephew, and niece.\nIt seems that they had multiple words that meant \"man\" and \"woman\" to be able to just add \"ter\" or \"ther\" to mean a specific relative.\nI haven't been to etymonline, thanks for mentioning it."
] |
[
"Can the Stern–Gerlach experiment measure spin direction?"
] |
[
false
] |
[deleted]
|
[
"it can also measure the spin of individual particles, but not the direction of them",
"What do you mean by this? How do you define measuring the spin vs measuring the direction of spin?",
"The apparatus measures the direction of spin along any arbitrary axis. "
] |
[
"In the SG experiment, each of the final two beams are uniquely polarized along the axis of the inhomogeneous magnetic field. In a sense, you can use a series of inhomogenous magnets in the same way you can use a series of polarizers for light beams. However you've only isolated the ",
" of spin along a certain axis. The spin-vector itself isn't sharply defined as only a single component of spin can ever be specified at a time, but you can say that it lines somewhere on the cone centered on the axis of polarization ",
"as seen in this picture here.",
" More technically the SG experiment only directly measures magnetic moment and not spin, but the two are intimately related. There is an often unspoken convention for whether the magnetic moment lies parallel or anti-parallel with spin for a particular particle.",
"The positron and electron spins are opposite for the same magnetic moment polarization which is due to the signs of their charge. So if a SG experiment produces an upper beam of spin down electrons, then the equivalent positron beam would be spin up.",
"As a further note the SG experiment actually hasn't been done for electrons or positrons - only for neutral systems. The reason is that magnetic fields deflect charged particles (Lorentz Force) and mixtures of angular momentum states and spin states (called Landau levels) have very nearly the same energy and thus amount of deflection ruining the SG result seen for things like silver atoms. Technically the degeneracy (multiple states with same energy) is broken by the very small anomalous magnetic moment of the electron, but as far as I know this fact hasn't yet been exploited in making a direct measurement of the Stern-Gerlach effect for electrons or positrons.",
"As the anomalous moment of protons is much larger, they would make an ideal candidate to test this out, but the nuclear magnetic moment is much smaller than the electrons (as moment ~ e/m) so the deflections would be tiny. "
] |
[
"a further note the SG experiment actually hasn't been done for electrons or positrons",
"I came across this a while ago, not sure if it has been repeated, but claims to do just that:",
"https://www.researchgate.net/publication/292985810_Picture_and_Result_of_Experiment_-_Stern-Gerlach_experiment_by_free_electron"
] |
[
"Watching things at the speed of light"
] |
[
false
] |
I read that the Pillars of Creation were destroyed about 6000 years ago, but due to the light speed limits we will not see the event for another 1000 years. If I travel towards the pillars at, let's say, 99% velocity of speed will I see the events fast-forwarded? And if away - slow-mo? Is it that simple or is there a catch to it? Does that mean that there's a natural fast-forward/ slow-mo limit?
|
[
"If I travel towards the pillars at, let's say, 99% velocity of speed will I see the events fast-forwarded? And if away - slow-mo? Is it that simple or is there a catch to it? Does that mean that there's a natural fast-forward/ slow-mo limit?",
"There is a distinction that can be very subtle here and many people - even those with degrees in physics - often miss. The distinction is between what you see and what is actually happening in your frame of reference. What you see is determined by the light that reaches you at any given time.",
"When you fly towards the pillars at nearly the speed of light, or better understood as the pillars flying towards you, the pillars are time dilated - they are ",
" running in slow motion, and the destruction event happened millions of years ago.",
"The space between you and the pillars is contracted to just a few light years or less, and all of the light which relays what happened to the pillars over all those millions of years since they were destroyed is scrunched into that space. So you will effectively see these events unfold in fast motion, even though the pillars are really going in slow motion. Additionally, the light will be blueshifted - i.e. it will appear as a higher frequency.",
"There is no fast-forward limit. As you go get closer and closer to the speed of light, the space between you and the pillars continues to get smaller, and so you will see the destruction arbitrarily quickly.",
"I have to think about the moving away case."
] |
[
"Then there is also the relativity of simultaneity, which means that after you boost, the destruction of the pillars now happened way longer than 7000 years ago, and what is currently happening for the pillars is an event that was in the (spacelike) future is now in the present.",
"The only good way that I have found to easily understand what one sees is to draw the Minkowski diagram from the frame of reference of the light emitting object, and trace out the light being emitted and when it hits the moving observer. Then calculate the proper time of the moving observer so one can actually figure out if he sees the emitter as fast-motion or slow-motion. "
] |
[
"You will see it faster. You just discovered relativity, and are travelling forward in time. ",
"I'd read up on it. RobotRollCall likely has a relating post."
] |
[
"If a billiard ball was fired into someone at close to the speed of light, would it vaporize him or leave a clean circular hole?"
] |
[
false
] | null |
[
"There is a fantastic explanation by an MIT professor depicting of the kinds of effects you might encounter with fractional C projectiles hitting another mass (in this case, a can of ravioli hitting a starship): ",
"http://www.mit.edu/people/daveg/Humor/ravioli_as_gas",
"actually, let me just post the entire text for you.... ",
"What am I, the neighborhood blast physicist??? Well, maybe... :-)",
"It all depends on speed of impact versus the speed of sound in the target\n(what is called the Mach number, where Mach 1 means the speed of sound,\nMach 2 is twice the speed of sound, etc), and the speed of the ravioli\nversus the speed of light in the target (which I'll call the Cerenkov\nnumber, where Cerenkov 1 is the speed of light in anything; Cerenkov 1.3\nis the speed of high-energy protons in a water-cooled reactor (that's why\nyou get that nifty blue glow), and you can get up to Cerenkov 2.4 using\ndiamonds and nuclear accellerators. In the late 40's people used to talk\nabout Cerenkov numbers, but they don't anymore. Pity.). Lastly, there's\nthe ravioli velocity expressed as a fraction of the speed of light in a\nvacuum (that is, as a fraction of \"c\"). \"C\" velocities are always between\n0 and 1.",
"At low speeds (REAL low) the ravioli will simply flow over the surface,\nyielding a space-cruiser with a distinctly Italian paint job.",
"Faster (still well below speed-of-sound in the target) the metal of the\nspace-cruiser's skin will distort downward, making what we Boston drivers\ncall a \"small dent\".",
"Faster still, you may have a \"big dent\" or maybe even a \"big dent with a\nhole in the middle\", caused by the ravioli having enough energy to push\nthe dent through, stretching and thinning the hull metal till the metal\nfinally tears in the middle of the dent.",
"Getting up past Mach 1 (say, 5000 feet/sec for steel), you start to get\npunch-a-hole-shaped-like-the-object effects, because the metal is being\nasked to move faster than the binding forces in the object can propagate\nthe \"HEY! MOVE!\" information. (After all, sound is just the binding\nforces between atoms in a material moving the adjacent atoms -- and the\nspeed of sound is how fast the message to \"move\" can propagate.) From\nthis, we see that WileE Coyote often reached far-supersonic speeds because\nhe often punched silhouette-type holes in rocks, cliffs, trucks, etc.",
"Around Mach 4 or so, another phenomenon starts -- compressive heating.\nThis is where the leading edge of the ravioli actually starts being heated\nby compression (remember PV=nRT, the ideal gas law?) Well, ravioli isn't\na gas, but under enough pressure, ravioli behaves as a gas. It is\ncompressed at the instant of impact and gets hot -- very hot. Likewise,\nthe impact point on the hull is compressed and gets hot. Both turn to\ngasses -- real gasses, glowing-white-hot gasses. The gasses expand\nspherically, causing crater-like effects, including a raised rim and a\nbasically parabolic shape. In the center of the crater, some material is\nvaporized, then there's a melt zone, then a larger \"bent\" zone, and the\nraised rim is caused because the gas expansion bubble center point (the\nbending force) is actually ",
" the hull plate. If the hull plate\nisn't thick enough, then the gas-expansion bubble pushes through to the\nother side, and you get a structural breach event (technically speaking,\na \"big hole\") in the side of the space-cruiser.",
"Compressive heating really hits the stride up around 20,000 feet/sec (Mach\n4 in steel, Mach 15 in air) and continues as a major factor all the way\nup to the high fractional Cerenkov speeds, where nuclear forces begin to\ntake effect.",
"Aside: the \"re-entry friction heating\" that spacecraft endure when the\nreenter the atmosphere is NOT friction. It's really compressive heating\nof the air in the path. As long as the spacecraft is faster than Mach 1,\nthe air can't know to get out of the way, so it bunches up in front of\nthe spacecraft. When you squeeze any gas, it gets hot. So, the glowing\n\"reentry gas\" is really just squeezed air, which heats the spacecraft heat\nshield by conduction and infrared. The hypersonic ravioli can be expected\nto behave similarly.",
"As we increase speed from the high Mach numbers (about 10 miles/sec) all\nthe way up to about 150,000 miles/sec, not much different happens except\nthat the amount of kinetic energy (which turns into compressive heat)\nincreases. This is a huge range of velocity, but it's uninteresting\nvelocity.",
"At high fractional Cerenkov speeds, the ravioli is now beginning to travel\nat relativistic velocities. Among other things, this means that the\nravioli is aging more slowly than usual, and the ravioli can looks\ncompressed in the direction of travel. But that's really not important\nright now.",
"As we pass Cerenkov 1.0 in the target, we get a new phenomenon -- Cerenkov\nradiation. This is that distinctive blue glow seen around water-cooled\nreactors. It's just (relatively) harmless light (harmless compared to\nthe other blast effects, that is). I mention it only because it's so\nnifty...",
"At around .9 c (Cerenkov 1.1) , the ravioli starts to perceptibly weigh\nmore. It's just a relativistic mass increase -- all the additional weight\nis actually energy, available to do compressive heating upon impact. The\nextra weight is converted to heat energy according to the equation E=mc",
"\nit looks like compressive heating but it's not.",
"[Here's where I'm a little hazy on the numbers; I'm at work and\n don't have time to rederive the Lorentz transformations.]",
"At around .985 c (Cerenkov 1.2 or so), the ravioli now weighs twice what\nit used to weigh. For a one pound can, that's two pounds... or about sixty\nmegatons of excess energy. All of it turns to heat on impact. Probably\nvery little is left of the space-cruiser.",
"At around .998 c, the impacting ravioli begins to behave less like ravioli\nand more like an extremely intense radiation beam. Protons in the water\nof the ravioli begin to successfully penetrate the nuclei of the hull\nmetal. Thermonuclear interactions, such as hydrogen fusion, may take\nplace in the tomato sauce.",
"At around .9998 c, the ravioli radiation beam is still wimpy as far as\nnuclear accellerator energy is concerned, but because there is so much of\nit, we can expect a truly powerful blast of mixed radiation coming out of\nthe impact site. Radiation, not mechanical blast, may become the largest\nhazard to any surviving crew members.",
"At around .9999999 c, the ravioli radiation may begin to produce\n\"interesting\" nuclear particles and events (heavy, short-lived particles).",
"At around .999999999999 c, the ravioli impact site may begin to resemble\nconditions in the original \"big bang\"; equilibrium between matter and\nenergy; free pair production; antimatter and matter coexisting in\nequilibrium with a very intense gamma-ray flux, etc.[1]",
"Past that, who knows? It may be possible to generate quantum black holes\ngiven a sufficiently high velocity can of ravioli.",
" --Bill\n",
"Ninja edit - not sure if this \"Bill\" was a professor or not"
] |
[
"Assuming that the billiard ball survives reaching the guy, the guy would be vaporized by the huge fireball accompanying the billiard ball.",
"Already movement at high Mach numbers heats the atmosphere around the object up ",
". ",
"http://www.youtube.com/watch?v=RChlt5wdqBs",
"Now if you look to meteorites which produce intense glares in the sky due to atmospheric heating, the still only go at less than a hundred kilometres a second. That is they are not even at 1% of light speed.",
"Now extrapolate to even 30% of light speed and you get very interesting effect which would pulverize anything in the way of the high speed object, and surely also the object itself.",
"Edit: Even without atmosphere a high speed impact is unlikely to just leave a hole (",
"http://www.toomanyparsecs.info/2012/05/hypervelocity-impacts.html",
"), however, if all the displaced material can exit through the back of the guy and does not have to go elsewhere, i am not sure what would happen. I guess that frictional heating would still pulverize anything."
] |
[
"Anatoli Bugorsk",
" put his head in an accelerator beam accidentally. About the only thing close to that speed and body interaction."
] |
[
"Do animals ever think of \"why\" something happens? If they can only think visually, do they ever reach that level of asking \"why\"?"
] |
[
false
] |
I always wonder what my cat thinks me and a friend are doing when we're sitting on the couch talking. I wonder he thinks we're making sounds and looking at each other. But a recent discussion about animal thinking talked about how, without language, you can only think visually. People that didn't get taught language until they were well into their teens describe that period as very fuzzy and emotional, akin to what it was like being a baby. So, do they ever even to that level of questioning?
|
[
"People that didn't get taught language until they were well into their teens describe that period as very fuzzy and emotional, akin to what it was like being a baby.",
"What's your source on that? My impression is that the number of cases of this sort of thing happening that have been well documented is so small, and the conditions in each case so variable, that the results aren't generalizable (i.e. you can't really draw conclusions from them).",
"Your question--whether dogs wonder why things happen--gets philosophical very quickly when you start to analyze it. By that I mean, it gets harder to give any sort of sensible scientific answer. ",
"What does it mean for someone to ask a question, if you ignore the linguistic form that marks questions in human speech? ",
"You might say \"trying to gain a particular piece of information\". Really, questions are just tools for trying to learn something (i.e. to create a model of \"reality\" in your brain, however crude).",
"Does a dog try to gain information about the world? In a technical sense, of course! Get out a treat, and a dog will very clearly ",
" how to get you to give it to him. You might as well call that \"asking himself the question: How can I get that treat?\", if you're working with the definition above. ",
"But you might at this point say \"Wait, sure, technically it's seeking information, but that's just instinct, not actual conscious questioning!\"",
"And that's where you'll say goodbye to science, and enter the realm of philosophy. You'll have to decide how to define consciousness, for starters, and, well, good luck with that.",
": As for whether, by the same logic as above, dogs ask \"why\" questions about things that don't directly concern their interests... that's a more difficult question. How would you recognize/measure that? Can you think of an experiment that would prove/disprove the hypothesis that dogs \"wonder\" why people are making sounds with their mouths all the time? I'm not sure I can."
] |
[
"Hahaha reproduction deviates from your field."
] |
[
"That deviates too far from my field for me to say anything even remotely authoritative about it. ",
"I would speculate (as a layman) that no, they don't understand the connection between mating and birth the way they understand the connection between rolling over and getting a treat. That's something they would have to somehow learn from their environment, and I just don't see how they would have enough clear-cut evidence (especially pet dogs) to recognize a correlation. ",
"Again, though, you could surely argue that they technically \"know\" the connection, because they will have all sorts of behaviors that are evolutionarily fine-tuned according to the connection. But whether you call that \"understanding\" is up to you. "
] |
[
"How does the flash from a camera \"ruin\" a painting?"
] |
[
false
] |
In some museum galleries, they have signs that say "no flash photography" because it'll supposedly mess up the paintings over time. However, in other galleries flash is completely fine. Does it have to do with the type of paint or the age of the painting? What exactly happens?
|
[
"It's not a single flash, I don't think, but the prospect of hundreds of strobing flashes every day---if they were exposed to sunlight, they would bleach and fade, and I'm positive the same would happen under enough flashes. "
] |
[
"Wikipedia is your friend",
"."
] |
[
"Not in this case. The photobleaching being discussed in the Wiki article is ",
" about the photochemical destruction of a fluorophore - a fluorescing molecule. Sadly, photobleaching is a more general term than that and can be used within the context of this discussion - the loss of color agents (dyes and/or pigments) upon expose to light over time."
] |
[
"What is the relationship (if any) of the electric field in EM radiation to the Force (perpendicual)/ Force (centripetal) and the electric field of a point charge?"
] |
[
false
] |
In lecture today, I was told that E(rad) = kqa/(c r). Playing around with the equation I found another way to write this equation, however my professor quickly brushed this off as just a coincidence. I would like a second opinion. Here is what I found: given E(point charge) = kq/r , F(perpendicular) = ma(perpendicular), and when v=c F(centripetal) = mc /r, I saw that E(rad) = kqa/(c r) = E(point charge)F(perpendicular)/F(centripetal). Is there some reason behind this, or is it just like my professor said, a coincidence?
|
[
"I'm not sure I understand your notation.",
"An electromagnetic wave incident on a surface exerts a pressure that is equation to the average Poynting vector divided by c. The Poynting vector is proportional to the square of the electric field.",
"Also, no charged object moves at the speed of light."
] |
[
"My understanding in this area is fairly limited, as I'm sure you could tell from the question, however hearing that no charged particle moves at the speed of light makes my relation void. Thank you for your response."
] |
[
"What you're talking about sounds sort of like the ",
"classical electron radius",
" which is the radius at which the electrical potential energy equals the rest energy (mc",
" )."
] |
[
"Is there a way to tell if a product is made from BPA plastics over say, another plastic like ABS, if there are no recycling symbols given?"
] |
[
false
] |
Asking because I have fake nails I glue onto my fingers, which means I’m in skin contact with plastic for weeks and through hot showers. I’ve heard BPA is a big issue in both these situations so I’m a bit concerned. Is there any way to test this at home? For example, through their reaction with alcohol or through testing their melting point?
|
[
"You can’t tell on products like that unless the manufacturer tells you the material. You can ask them usually. I will say, the main way we’re exposed to these chemicals is through ingestion. I personally don’t think absorption through the nail would be very large. As a person who studies microplastics for my PhD, I wouldn’t be too worried about it"
] |
[
"BPA isn’t a type of plastic, it is a plastic additive which makes plastic more flexible, clear, and durable. ",
"It is generally safe to say that unless a plastic product specifically indicates that it is BPA free, then it contains BPA."
] |
[
"Ingesting foods with fat that are packed in plastic, scented products (some microplastics stabilize fragrances and are labeled as “parfum”), and then receipt paper. But honestly we need a systemic change, where countries fully ban these products for you to fully avoid them. Most of these microplastic chemicals have ubiquitous exposure and are virtually unavoidable."
] |
[
"How much of the water we drink stays in the body?"
] |
[
false
] |
[deleted]
|
[
"None of it. The water you drink passes ",
" your body. Along the way, your body uses it to transport various water-soluable chemicals around in useful ways, until finally the last thing your body does with it is use it to carry water-soluble waste products ",
" of your body as urine.",
"Assuming you're a generally healthy person in other respects, all the water you drink ends up leaving your body as urine, sweat, lacrimal secretions or any of the various ways your body excretes it."
] |
[
"What is the cause of the times in which the body discharges the fluids..",
"Some days its normal ever few hours others you may not go for extended periods threw out the day. While odd days it seems like every 40m. Referring to urination and considering roughly same amount of water is taken in daily. Healthy of course"
] |
[
"It depends on different factors. The following are notes from my Anatomy and Physiology class. You'll see that water output SHOULD equal water input, but perhaps if you spend a hot and dry day outside, you'll expel more through diffusion or sweat and less through urination. In your question's case though, perhaps an increased consumption of moist foods or a rapid consumption of water within a short period of time would increase frequency of urination",
"A. Water Intake\n Water intake should equal water output each day.\n Average intake is about 2500ml/day.\n Water is obtained through:\no 60% drinking\no 30% moist food\no 10% internal metabolic pathways.",
"B. Water Output\n Water output should equal water intake, about 2500ml/day\n Water is lost through:\no Urine = 60%\no Feces = 4%\no Sweat = 8%\no Skin diffusion and lung evaporation (heat) = 28%"
] |
[
"What kind of unethical/illegal/immoral experiments would advance your field?"
] |
[
false
] | null |
[
"Being allowed to try any cancer treatment we can come up with (with plausible reason to think it would work) \"just to see\".",
"but that's an easy/obvious one."
] |
[
"Can't you already do this with rats?"
] |
[
"You think everything happens identically in rats as it does in humans? you would be sorely mistaken."
] |
[
"I heard that after sex, men have a biological impulse to get away from the woman and women have the impulse to keep the man close. Is this true?"
] |
[
false
] | null |
[
"The real, honest answer, that you won't actually see much on reddit: Nobody knows.",
" ",
"We don't know much about our behaviour and evolution and \"Why do humans..\".",
"We can make guesses based on neurotransmitter levels. (Oxytocin especially in this case would be relevant).",
"The statement \"Humans have evolved to...\" seems almost entirely irrelevant in todays human. Our behaviours are unpredictable and very largely based on enviromental factors like culture and parents. ",
" ",
"Off tangent. But I hate seeing people try to guess \"why we evolved...\" Like \"Why did humans evolve to enjoy music so much\" You will have countless postulations that go on patterns and seem plausible but in reality have ",
" to a scientific look at the question. The answer may actually be \"We didn't..\" or \"no\". Similar to your question. "
] |
[
"This (and a myriad of other gender-related behavior explanations) are very popular in evolutionary psychology. However, there is hardly any supporting research to back-up these claims. Take it with a grain of salt."
] |
[
"I'm an evolutionary psychologist, and this is gibberish. It assumes that there are tons of women just waiting for a man to come along and have sex with them, so that the guy could just go find another mate immediately, which is just ridiculous. Furthermore, if this were true, then presumably some other male would come and have sex with the woman the guy just ran away from, which would completely obviate the supposed goal of the behavior.",
"Contrary to what everyone in this thread seems to think evolutionary hypotheses are very testable (they're just like any other psychological hypothesis, except they are derived from evolutionary theory, rather than say the researcher's imagination). For example there is some very interesting work on ",
"semen displacement and the refractory period",
". It seems likely that the refractory period males experience after orgasm is designed to prevent them from displacing their own semen. If this is true, it would suggest that something closer to the opposite of your hypothesis is true, namely that males should stick around and mate guard after having sex. I don't know of any evidence for that off the top of my head, but it is a plausible and testable hypothesis."
] |
[
"What is quantum tunneling? Under what conditions does it occur?"
] |
[
false
] |
If we model an electron as a particle in a box, I understand that the box represents an orbital, its opaqueness models uncertainty, and the length of the box models the idea of different orbitals having discrete energy levels, sort of like standing waves of a string. How does quantum tunneling fit into this picture? BONUS: How do scanning tunneling electron microscopes work? How about atomic force microscopes? EDIT:Thanks for all the replies so far...I actually posted before searching the subreddit and your replies combined with some past questions about quantum tunneling have helped my understanding of the phenomenon. In particular, has been extremely insightful. Now I'm more curious as to why this is so. Mathematically or experimentally, how do we know that the probability distributions overlap at higher energy levels?
|
[
"I only have a moment now, so let me answer what tunneling is, and leave the microscopy for another person or another time:",
"In classical physics, we know that kinetic energy is always positive, and as a consequence, the total energy is always greater than the potential energy. That means it is impossible for an object to be in a region of space in which the potential energy is greater than the total energy; such a region is therefore classically inaccessible.",
"In addition, classically, if you have two regions A and B in which the potential energy is less than the total energy separated by a region M in which the potential is greater than the total energy, because that middle region is inaccessible, an object in region A will never be able to get to region B, not because region B is intrinsically disallowed, but because the inaccessible region M is in the way.",
"In quantum mechanics, it turns out that the classically inaccessible region M is no longer forbidden. Tunneling is the process by which an object that starts in region A can make it to region B; the region M decreases the probability that an object will make it from A to B, but it can and will happen."
] |
[
"Lol, I think we both are a little confused. Wikipedia article on Schrodinger's equation:",
"The time-dependent Schrödinger equation predicts that wavefunctions can form standing waves, called stationary states (also called \"orbitals\", as in atomic orbitals or molecular orbitals).",
"I thought only electrons need quantum numbers to accurately portray their behavior in the context of an atom. Aren't nuclei too \"heavy\" for us to worry about uncertainty in most cases? I'm talking about at low kinetic energy, where the proton would (according to my understanding) behave more or less classically. ",
"And yes, since this is a one-dimensional approximation I thought each n could represent an \"orbital,\" or some simplified approximation of one, (even more so than the Bohr model) where we don't have to deal with the nucleus, but still conceptually gain some insight into the quantum behavior of electrons in orbitals, since their behavior is modeled by wave functions as well. "
] |
[
"Lol, I think we both are a little confused. Wikipedia article on Schrodinger's equation:",
"The time-dependent Schrödinger equation predicts that wavefunctions can form standing waves, called stationary states (also called \"orbitals\", as in atomic orbitals or molecular orbitals).",
"I thought only electrons need quantum numbers to accurately portray their behavior in the context of an atom. Aren't nuclei too \"heavy\" for us to worry about uncertainty in most cases? I'm talking about at low kinetic energy, where the proton would (according to my understanding) behave more or less classically. ",
"And yes, since this is a one-dimensional approximation I thought each n could represent an \"orbital,\" or some simplified approximation of one, (even more so than the Bohr model) where we don't have to deal with the nucleus, but still conceptually gain some insight into the quantum behavior of electrons in orbitals, since their behavior is modeled by wave functions as well. "
] |
[
"Why does U-Pb dating have a limit of 500,000 years ago?"
] |
[
false
] |
[deleted]
|
[
"You got the direction wrong. It is largely useless for samples newer than 500,000 years because the amount of lead that accumulated in that time is small. Uranium-lead dating is better for older samples. The absolute uncertainty is still large, but \"635 +- 1 million years\" is a good measurement while \"0 +- 500,000 years\" is not. For more recent samples other dating methods are better."
] |
[
"Thanks a lot for this response. I think my book contradicts what you're saying, it states that this dating method is only usable for 500-500,000 years BP. But you say that it's useless at short time scales, I think that your explanation is way more logical but I'm not sure what I have to remember now."
] |
[
"Thanks a lot for this response. I think my book contradicts what you're saying, it states that this dating method is only usable for 500-500,000 years BP. But you say that it's useless at short time scales, I think that your explanation is way more logical but I'm not sure what I have to remember now."
] |
[
"Why does sound travel farther on water?"
] |
[
false
] |
For example, when I go fishing. I can hear a couple on a kayak at least 100 yards or so away, and still be able to make out what they're saying. While another person is only 20-30 feet down shore and I can't make out a word they're saying.
|
[
"Two reasons: First, on the water there are much fewer objects to get in the way of the sound. Other than a few waves (and if you're fishing, I'm assuming there aren't many of those) there is nothing to block the sound getting to you, so you can hear it from further away. On the shore, you have rocks, trees, and other people that block the sound.",
"Second, sound travels well through liquids (and solids), but it doesn't move from one medium to another (air -> water; air -> solid) very well, which means that your surroundings are not absorbing much of the sound either, which allows the vibrations to travel further. "
] |
[
"This is correct, but there's more to it. Sound also reflects off of water when the water is calm. "
] |
[
"You're right, that's what I meant when I mentioned the surroundings (I.E. water) not absorbing much sound, but I should have made that clearer."
] |
[
"Would a plant housed in darkness with mirrors directing light at the leaves photosynthesise at the same rate as if it were outside?"
] |
[
false
] |
Are the light waves reflected from a mirror identical to those hitting the mirror or is the mirror absorbing some of the energy? Does it depend on the mirror material (eg glass vs polished metal)? If so would this significantly impact a plant's capacity to grow under these conditions?
|
[
"There is no such thing as a perfect mirror, so in some ways the answer is no. All mirrors transmit some percentage of light (transmit being the opposite of reflect), and it varies across the spectrum. That said a typical silver mirror reflects about 85-90% of visible spectrum light that hits it. Some specialized mirrors can reflect well upwards of 95%. So in practical terms the intensity of the light would be negligibly different. Using a parabolic mirror to focus light you could increase the intensity of the light hitting the plant beyond direct sunlight levels (and easily damage the plant). In terms of the \"quality\" of the light, the light reflecting off of a mirror isn't changed in any way. The mirror may reflect more or less light at certain frequencies, but there is nothing different about a photon of light that has reflected versus one that hasn't. Hope that helps."
] |
[
"that does help thanks"
] |
[
"Reflected light isn't precisely identical to the incident light, as mirrors only imperfectly reflect light, doing so in a frequency-dependent way (meaning that different colors of light are absorbed slightly differently by the mirror). Also, light reflected from mirrors is ",
"polarized",
", whereas the incident sunlight is not, generally. Mirrors typically reflect around 90% of visible light over a broad range of frequencies. ",
"For the most part, plants don't seem to care much about polarization, so the main thing is the frequency and number of photons hitting the plant. "
] |
[
"What exactly happens when we \"startle\"?"
] |
[
false
] |
Kind of curious about this. What happens physiologically when we startle or jump from for example a sudden sound?
|
[
"What do you mean by \"psychologically\"? Do you mean \"physiologically\"?"
] |
[
"That could very well be. I was thinking in regard to why we react as we do. Jumping, screaming, etc. Are there any benefits from such a reaction, both chemical and physiological?"
] |
[
"Thank you for a detailed and informative response! "
] |
[
"How does putting more metal stuff around it correct a ship's compass?"
] |
[
false
] |
I visited a maritime museum over the weekend, and there was a part of the exhibit explaining that when ships started to be made of metal, it was found that it caused inaccuracies in the compasses when travelling east-west. This is corrected by a steel bar called a Flinders Bar, and maybe Kelvin's Balls - which are unmagnetised iron, all of which pretty much surround the compass. But how does this all work? Surely adding a lot more metal around a compass means it will be more attracted to that than to the earth's magnetic field?
|
[
"The earth's magnetic field induces a magnetic field in the ship's metal, which alters the magnetic field seen by the compass. The point of the Flinders Bar is to exactly cancel the field generated in the ship's metal so that the total field seen by the compass is again the earth's field."
] |
[
"Interesting related fact, realizing that the metal hulls of ships took on magnetic properties, the Germans actually developed mines to be triggered by magnetic fields, so they would go off when any ship came near them. If I recall correctly, they started wrapping metal cables around the hull to demagnetize the entire ship in order to counter this.",
"EDIT: Related link if anyone is interested in reading up on it.",
"http://en.wikipedia.org/wiki/Degaussing#Ships.27_hulls"
] |
[
"Sounds like a pain to set up, but in a pinch it is clever. Of course, GPS is becoming a little more common; my tank in the military didn't seem to have any such thing to keep its hull demagnetized, my compass didn't work, but the GPS sure did."
] |
[
"Did Baumgartner ever actually break the speed of sound, measured at ambient air density, at any point in his descent?"
] |
[
false
] | null |
[
"The speed of sound in a gas depends on the composition of the gas and the ambient temperature. This relationship is c=sqrt(gamma * R * T) where c is the speed of sound, gamma is a thermodynamic property known as the ratio of specific heats, R is the gas constant for the gas, and T is the temperature. Gamma and R won't vary by much over the 39 km Baumgartner traveled, leaving only a temperature dependence. That's why the speed of sound varies with altitude.",
"Mach number is always relative to local conditions, so when he was reported to have gone Mach 1.24, that means he was going 1.24 times the local speed of sound. And since temperature tends to decrease the higher up you go (although only to a point, then it starts increasing again, and then things get kind of ",
"silly",
"), the speed of sound tends to decrease.",
"In short, relative to the local speed of sound (the only frame of reference that matters for aerodynamics), Baumgartner was supersonic."
] |
[
"No problem.",
"As an interesting side note, if CNN's reporting is accurate, and Baumgartner reached Mach 1.24 and 834 MPH (372 m/s), that corresponds to an outside temperature of 224 Kelvin, or 50 degrees Celsius below zero. ",
" this temperature matched the Standard Atmosphere model (big if), that means Baumgartner hit his max speed at 27.6 km high, or about 11.4 km lower than his jump altitude. Had he not had any drag on him, he would have reached a speed of about 472 m/s, or Mach 1.57 (1056 MPH) over that same distance.",
"It's interesting to see just how much drag he felt up there, even with the ",
" low air density."
] |
[
"The speed of sound is dependent on temperature, but not altitude (in the context of pressure). And the speed of sound is lower at low temperatures than high ones, so he did break the sound barrier of the air in which he was travelling."
] |
[
"Is this a reasonable simulation of tension across a hanging rope?"
] |
[
false
] |
Hi! I was debugging some game physics/kinematics and noticed something I found interesting: the tension across my hanging rope was equal across its entirety, which goes against one of the few things I remember from my physics classes a decade ago. However, the simulation seems to match most of the rope characteristics that I can think of having observed in real life (it's actually uncanny; I hate it), so I'm reexamining my understanding of physics. Attached is a screen grab; the red circles represent the tension at each point (the radius of each circle is equal to "tension", which is a variable in the engine with a magnitude and no real units). Is reasonably representative of what I would expect to see if I were to measure across a real rope with a... tension...ometer? Note: I haven't seen any posts like this, before, so I'm not sure if I'm allowed to do this; I notice that media posts are disabled, but I'm not sure if that means that they're not allowed, or if they're disabled to weed out low-effort spam. I'm assuming the latter (and also assuming that this isn't a low-effort question); please let me know if I'm wrong! Edit: suggested that the tension should be equal across the horizontal component, but I didn't even write code to that, let alone have it . So I wrote a little script to draw components of vectors (and to pass the direction component of the tension vector to the script) and !! Aaaaaahhhhhhhh I am BESIDE MYSELF RIGHT NOW HOW DOES CODE I WROTE KNOW MORE PHYSICS THAN I DO THIS IS AMAZING!!! I love all of you.
|
[
"Tension is a vector. It has a magnitude and direction.",
"The horizontal component of the tension should be equal everywhere.",
"The vertical component will be max at the highest point and zero at the minimum point.",
"This is assuming you have gravity.",
"So, you probably have it right, but it would be easier to understand if you could separate out the two components of the tension vector."
] |
[
"That is ",
"a beautiful catenary",
". That is exactly what it looks like.",
"In intro physics, you dealt with ",
" ropes, which are massless. Try changing the density of the rope to zero to see what happens to the tension."
] |
[
"Well, now I'm curious. I'm going to separate out the components; this'll be fun.",
"Edit: Wait, I forgot that I already updated the sim to (try to) replicate rigid motion, instead (which is somehow ",
" harder than stretchy motion, go figure). I'm totally going to do this ",
", though!",
"More edit: Well, to be fair, it already ",
" a vector in the sim, sort of: it always pulls directly toward the adjacent nodes. However, that's due to the nature of the function that the \"tension\" variable is called in; the vector isn't stored ",
" the tension variable. Splitting them up wouldn't be hard; I just didn't ",
" it because drawing circles was one line of code and drawing specific vectors takes three.",
"YET MORE EDIT: I WROTE A SCRIPT TO DRAW VECTORS AND THEIR COMPONENTS AND ",
"LOOK",
"!!! GUYS I ACCIDENTALLY PHYSICSED!!!!"
] |
[
"\"An implication of Albert Einstein's theory of general relativity is that physical space itself is not Euclidean, and Euclidean space is a good approximation for it only where the gravitational field is weak.\" What does this mean?"
] |
[
false
] | null |
[
"Euclidean space is space described the Euclidean geometry, which is derived from a set of postulates originally published by the Greek mathematician Euclid. Euclidean geometry is geometry as you expect it in our every day life. Where the sum of the angles in a triangle is always 180 degrees, where two parallel lines will always be at constant distance from eachother, etc...",
"If you change the basic axioms, however, you can come up with geometries with different properties. For example on a sphere, the sum of the angles in a triangle will be more than 180 degrees and two parallel lines will eventually converge. We call these geometries \"curved\", because when we visualize them, they appear that way. In contrast, plain old Euclidean geometry is called \"flat\".",
"An important implication of the theory of relativity is that our physical space that we live in is not flat, but curved. Mass (and energy) cause spacetime to curve and this curvature is experienced by us as gravity.",
"So in areas where gravity is weak, the curvature of spacetime is also rather small and it becomes reasonable to approximate it and say that it's flat, Euclidean space. This makes some calculations much simpler. For many purposes, we can make this approximation for the situation here on Earth. When civil engineers design a bridge or large building, they do not need to take into account the curvature of spacetime and they can simply assume that we live in a flat spacetime. However, when an astrophysicist models the behaviour of matter near a black hole, the curvature becomes very important."
] |
[
"Euclidean geometry only works on a flat plane",
"Err, no - Euclidean geometry works perfectly well in 3 dimensions. In fact it's applicable to any number of dimensions, but is most often associated with 3 for obvious reasons.",
"where real world measurements have the Z of X,Y,Z axis to complicate the math because of Pi.",
"...what?"
] |
[
"Euclidean geometry only works on a flat plane",
"Err, no - Euclidean geometry works perfectly well in 3 dimensions. In fact it's applicable to any number of dimensions, but is most often associated with 3 for obvious reasons.",
"where real world measurements have the Z of X,Y,Z axis to complicate the math because of Pi.",
"...what?"
] |
[
"Can typical consumer Hydrogen Peroxide solution (3%) be turned into safe drinking water?"
] |
[
false
] | null |
[
"Heating water doesn't release oxygen from the oxygen atoms in H₂O. All it does is release any dissolved oxygen. This process does not involve the breaking of any chemical bonds.",
"Hydrogen peroxide ",
"naturally decomposes into water and oxygen",
", and heat will increase the rate of decomposition."
] |
[
"Yea, I meant O2 molecules in solution with the water.",
"OK, but how do you know when the Hydrogen Peroxide solution has decomposed enough to be safe drinking water?",
"Is there a way to accelerate it besides heating it?"
] |
[
"I don't think it is in the place of anyone on an internet forum to provide procedures on making commercial hydrogen peroxide solutions safe to drink. It's the reason I provided the relevant information about decomposition, and that's it."
] |
[
"Which berry and fruit was the first in the world?"
] |
[
false
] |
How did berries evolve? And where did they grow? I would like to learn all about this! (Posted this in , but mods told me to try instead, so here I am)
|
[
"Fruits in the strictest botanical sense are produced by Flowering Plants, only. ",
"Fruits of flowering plants as we recognize them today, probably originated in the late cretaceous or the early paleocene, possibly after the K-T extinction event that killed most of the dinosaurs as well a large amounts of other life forms. Because fleshy fruits of course decompose very rapidly, there aren't many fossils of them and we can only compare to what we know about living families of plants, to fossils of leaves and stems.",
"There are quite a few gymnosperms that use enlarged, oily or starchy seeds to encourage small animals to disperse them. Pine nuts for example. The idea being that while the majority will simply be eaten, some will be buried and forgotten about. ",
"This may have been the case with Cycads which are some of the most ancient woody plants and most primitive conifers. Modern cycads feature enlarged conspicuous seeds that may have been dispersed by ancient megafauna. ",
"Certain conifers such as those in the yew family, ginkgoes, and junipers produce specialized female cones with only a singe, or sometimes two seeds. The seeds are surrounded by a single, enlarged, fleshy scsle called an ",
". ",
"While these may resemble fruits or berries produced by flowering they are in fact cones and are an example of ",
" ",
"Ginkgoes are represented by only a single species today, but were much more common in the mesozoic era first appearing around 290 M.y.a., it's likely that many of those featured fleshy arils like modem ginkgoes, and used various animals to help disperse their seeds.",
"Likewise yews were quite common in the middle jurrasic and cretaceous period."
] |
[
"True fruits in the botanical sense grow from the ovary of a flower. True flowers are found on angiosperm plants, which are about 130 million years old depending on who you ask and how old your botany textbook is.",
"The murky origins of angiosperms make it hard to speculate what the very first fruit would have tasted like. ",
"Juniper berries (conifer, really just fleshy cones like a pine tree) and sago palm fruit (cycad, not really a palm) are not technically fruits, but they are ancient unrelated fruit-like structures that can be easily sampled today."
] |
[
"Thanks!"
] |
[
"Total Energy in the Universe?"
] |
[
false
] |
Has anyone ever attempted to determine the sum of all the energy in the universe? Would it be expressible as a number of Joules? Is this even a meaningful question, considering that dark matter counterbalances the energy of matter (maybe my question is about the sum of the absolute values of all energy in the universe)?
|
[
"Depends on what kind of energy you are talking about.",
"\nIn general, cosmologists talk about the energy density of the universe (the ammount of energy per unit volume), because this is what actually matters in the evolution of the universe, not the actual total, this is because the total volume of the universe is not known and, depending on your cosmological model, is not always a meaningful concept (of course, one could measure the ammount of photon energy in the visible universe).",
"\nEdit: Now that I'm more awake, there's a paper that deals with exactly how energy is spread in the universe, ",
"http://arxiv.org/pdf/astro-ph/0406095v2.pdf",
".",
"\nA nice graphic of this: ",
"http://www.mpia.de/home/poessel/UT2012/cosmic_inventory.svg",
" ",
"Now take notice, we know the how energy is spread in the universe, but we don't know the total, because we don't know the volume of the universe! Luckily, the universe seems to be isotropic and homogeneus, so we can mesure the energy coming from different sources, add them up and see what fraction of the energy is coming from which source."
] |
[
"What? Your link has absolutely nothing to that effect (only a throwaway line about how large the energy contained in a uranium nucleus is), and \"power of a uranium packed nucleus\" is nonsense anyway! Not to mention that all uranium nuclei are contained within the universe meaning that the energy of the universe must necessarily be larger than the energy of not ",
" uranium nucleus, but ",
" uranium nuclei combined!"
] |
[
"What? Your link has absolutely nothing to that effect (only a throwaway line about how large the energy contained in a uranium nucleus is), and \"power of a uranium packed nucleus\" is nonsense anyway! Not to mention that all uranium nuclei are contained within the universe meaning that the energy of the universe must necessarily be larger than the energy of not ",
" uranium nucleus, but ",
" uranium nuclei combined!"
] |
[
"Is it true that my wife can't get pregnant when not in her fertile tone of the month?"
] |
[
false
] |
I just got married last week. Someone in another thread mentioned natural family planning and the idea that it is impossible for a woman to get pregnant if she's not ovulating. Is this true?
|
[
"Close. You can learn enough about her cycle to ensure she doesn't get pregnant, but the window is wider than just her partial week of ovulation. Sperm can fertilize an egg for about 3-5 days after it has been deposited. So pre ovulation sex can cause pregnancy. ",
"If you are serious about avoiding pregnancy through understanding her cycle, there are two outward signs I'm aware of that indicate ovulation. Body temperature, and the viscosity of her feminine fluid. For the temperature part, one should take their temperature every day, ideally right when waking at a regular time, and chart the increase that indicates ovulation. There is a bit more detail that an expert may know, but these are places to start.",
"And I'm sure I'll get laughed out of the park for stating this~ but the catholic church, which is very anti contraception, has done a bit of research into this topic, and has classes on how to avoid pregnancy without using it by understanding her cycle. If you can get over the stigma of a church embracing science, then the title of their program you'd want to research is \"natural family planning.\" "
] |
[
"This is known as 'the rhythm method'.",
"It is well known for producing parents.",
"The underlying reasoning is sound - if you avoid sperm finding an ovulated egg, yes it is impossible to become pregnant - but unfortunately as with all things human, it is very hard to be that precise and any number of events can confound the rhythm method and lead to poorly-timed intercourse (if attempting to avoid pregnancy).",
"Humans are surprisingly infertile, compared to other species, but there is still a significant window each month, and it can be very difficult to be sure you've got your timing right."
] |
[
"no this is not known as the Rhythm method. It is known as Natural Family Planning/ Fertility Awareness Method, of which Rhythm is the oldest (developed in 1930). Rhythm corrected the medical advise doctors were giving couples. Prior to that, it was believed a woman was least fertile in the middle of her cycle (full cycle, not just menses). The problem with the reliability of Rhythm was twofold. First it was based on women having regular cycles. People overall were unreliable in abstaining consistently during the perceived period of fertility.",
"in the 50's and sixties, the Billings ovulation method and the sympto thermo method were being developed. These developments were glossed over by the success of the pill, and initial studies found that while the method effectiveness significantly improved, the user effectiveness remained about the same as Rhythm. The biggest factor is that people don't consistently abstain. Charting is also more complicated when a woman struggles with PCOS or when getting off of birth control or having a baby and waiting for cycles to resumes. This is because the body produces mucus that protects the sperm when the hormones to cause ovulation are present, but if ovulation does not occur, it goes back to trying, and the mucus persists. The mucus is one of the primary signs of fertility (in fact even if a woman is ovulating, if she has mucus free cycles, she will have great difficulty becoming pregnant).",
"The most user effective methods of birth control are the ones that demand the least out of individuals. ",
"Now in answer to the OP, if a woman is truly in her Luteal phase, it is impossible to get pregnant. This phase lasts about 14 days. It can be shorter due to hormone problems. if it gets shorter than 10 days, she would be considered infertile as her endometrium lining would prematurely be shedding. If her luteal phase is fine, than in a 28 day cycle, she would have approx four days after her period that were semi safe. the big thing here is the presence of mucus in the crypts of the cervix. when there is a lot, it will show up at the vulva and she can observe it. Gravity will work it down. But there is the chance that you end up having sex before it show up at the vulva. Still it's reasonable safe as even in fertile sperm the sperm typically die within 3 days, but I have read that in rare cases, they can survive up to a week. Stress can also delay ovulation and cause a longer patch of mucus or a patch that is shorter, dries up and then returns in a few days. This is called a double peak. Her body failed to ovulate the first time. Usually you can be taught to recognize this, but an uneducated person may assume they are in the lutael phase when they are not. In such a cycle, the period will be delayed.",
"In cases like PCOS, mucus can be almost constantly present with ovulation and thus menses occurring after long periods of time. Usually the consistency of the mucus will change, so you can chart a basic infertile pattern to reduce lengths of abstinence, but unless a woman is consistantly cycling, NFP is a pain in the butt.",
"But if a woman is cycling, than the period of abstaining tends to be about a week and apparently in recent studies, using a barrier method during the fertile time only slightly increases the odds of pregnancy (as opposed to abstaining. If you have unprotected intercourse on a fertile day, you have about a 76% chance of getting pregnant that cycle unless you are struggling with a fertility problem.)"
] |
[
"Am I losing hearing in my left ear?"
] |
[
false
] | null |
[
"We can't comment on personal anecdotes / isolated incidents without resorting to speculation which we try to avoid."
] |
[
"Ah im sorry. This is an area of concern for me and this sub was the first to enter my mind when I started asking questions about it"
] |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):",
"medical or safety advice",
"/r/AskScience",
"guidelines",
"If you disagree with this decision, please send a ",
"message to the moderators."
] |
[
"If earth's core suddenly stopped being nuclear and would drop to surface temperature, would earth's surface get any colder?"
] |
[
false
] | null |
[
"Other than what the other comment said, the earth's core is definitely not nuclear. "
] |
[
"Absolutely. It would turn into mars. Without a heat gradient, the electromagnetic rotation in the core would cease, removing the magne field around the earth. The solar wind, which the earth fends off with its magnetic field, would then carry away our atmosphere, and without the greenhouse effect, the temperature would plummet. As well as the oceans drying up and life pretty much ceasing. "
] |
[
"Also thought so. While earth still being fully liquid, the heavier elemts (which most radioactive are) sunk toward the core, leaving them as heat source."
] |
[
"What IS the placebo effect?"
] |
[
false
] |
I don't understand it at all, any help?
|
[
"Basically, sometimes people get better when you give them a fake treatment ",
"Its thought to be caused by a variety of reasons, the \"power of positive thinking\" , the relief of anxiety in knowing that youre now on a tablet for your condition, self delusion etc",
"In testing drugs , you always divide up the patients , and give half of them a drug, and half a fake drug that is just a sugar pill which looks identical. This fake pill is a placebo.",
"Time and time again, some people on the fake drug ( the placebo ) will get better. These people have experienced the placebo effect. In order for our new drug to actually be useful, it needs to be better than a placebo.",
"It is very important to compare everything to a placebo, because if I brand some sugar pills and give them to patients with headache, you will see a few of them improve (even though its just sugar ), and I could claim that I have a treatment for headache. Which would of course be wrong. ",
"This is why if a treatment or drug trial isnt placebo controlled, you would be very VERY skeptical of any results."
] |
[
"Something not mentioned yet, is that the placebo effect can work even if people know that they are getting a placebo.",
"http://www.guardian.co.uk/science/2010/dec/22/placebo-effect-patients-sham-drug"
] |
[
"It's when there's an improvement that's not attributable to the pharmacological effect of the treatment. In a classical example, when sugar pills are given to treat pain or anxiety - small amounts of sugar don't affect either - but for many people, taking pills for pain does affect pain and taking pills for anxiety does affect anxiety, even if all they contain is ineffective sugar.",
"This might help, it explains some of the weird stuff that goes on with placebos.",
"http://diseaseoftheweek.wordpress.com/2010/04/17/the-placebo-effect/"
] |
[
"If the solar system travels thru a nebula or a big dense cloud of matter, can our solar system grow? Can Jupiter or the sun grow in size?"
] |
[
false
] | null |
[
"No. The solar wind creates a \"bubble\" of plasma that prevents gas and dust in the interstellar medium from touching planets or the sun. And that plasma doesn't touch most planets since it is deflected by their magnetic field. In the few cases when it does, like Mars, it actually takes away more atmospheric matter than it deposits.",
"Actually the sun is traversing through a region of interstellar gas and dust (though not dense enough to call it a nebula). Even though density is very low, molecules actually collide with each other over very long times (at a much larger scale than a human lifetime). It is possible to define sound and speed of sound, even though it's not human hearable. The sun is travelling supersonic in that medium, so it creates a shockwave. Voyager 2 felt it with its magnetometer when it crossed the shockwave and entered interstellar medium, confirming scientific predictions.",
"EDIT: corrected the \"nebula\" term. Thanks for the corrections."
] |
[
"Every year, Earth gains about the weight of two aircraft carriers landing on it – two HMS Ark Royals, or about 40,000 tonnes-worth of debris, which lands on Earth from space. You can demonstrate this for yourself. If you put a big plastic sheet or a white sheet on your grass in the garden on a nice day, leave it for a few hours and then run a magnet over it. You can often find specks have just fallen down from outer space and landed on your magnet. Debris, dust and other stuff raining in from space contributes a huge amount of weight to the Earth every single year. "
] |
[
"That's interesting, forgot about the bubble the sun creates... So are we leaving a trail of matter as we travel? Like footprints"
] |
[
"How do nations communicate with each other?"
] |
[
false
] | null |
[
"Formal day to day communication happens between embassies and (typically) state departement of the host country. They send very official hand delivered letters to each other (called Diplomatic Notes). They can also have meetings with government officials and ambassadors. ",
"Highest form of diplomatic contact are state visits were big bosses talk to each other. Bigger issues are settled in conferences and meetings between state heads during these visits and they ofthen end with signed agreements or statements. Usually they involve lots of negotiation beforehand between lower lever officials (it can take years). ",
"Of course there are informal phone discussions and emails whenever the need arisies. "
] |
[
"There is also both Track 1 and Track 2 means of communication with countries. Track 2 is what you don't hear about, i.e informal talks between academics on both sides. Track one is as listed already, state departments communicating through embassies, and even phone conversations set up between executives."
] |
[
"This may not be a popular or even legal response in this subreddit, but I recommend referring to the West Wing. Despite having a political science degree, it gave me some understanding of the different methods.",
"As far as translation goes, most treaties are formed in multiple languages and verbal communication is usually in dual translator format. (I speak English, their interpretor translates it to Chinese. They speak Chinese, my interpretor translates to English.)"
] |
[
"Can you trap light in a vacuum?"
] |
[
false
] |
[deleted]
|
[
"I'm an electrical engineer, though there's really not much of a distinction between us and physicists these days, especially when dealing with nanoscale devices and effects.",
"If you take a slightly leaky 'light-trapping box' and replace the vacuum inside it with a material that can amplify light (given some sort of energy input), you've just created a laser. I basically design and create really small lasers -- since they're really small, they don't use a lot of energy, and they're really easy to turn on and off quickly (so you can send information quickly with them -- basically, think morse code, with dots/dashes each lasting only several picoseconds)."
] |
[
"I'm an electrical engineer, though there's really not much of a distinction between us and physicists these days, especially when dealing with nanoscale devices and effects.",
"If you take a slightly leaky 'light-trapping box' and replace the vacuum inside it with a material that can amplify light (given some sort of energy input), you've just created a laser. I basically design and create really small lasers -- since they're really small, they don't use a lot of energy, and they're really easy to turn on and off quickly (so you can send information quickly with them -- basically, think morse code, with dots/dashes each lasting only several picoseconds)."
] |
[
"Total internal reflection happens at an interface between materials; in this case, it would be vacuum and some non-vacuum material. The resulting device would be functionally equivalent to a vacuum-filled box with mirrored walls, in which case light can most certainly be trapped inside the box, just like it could if the box were filled (eg. with air or water)."
] |
[
"How exactly does cutting things work at the molecular/atomic level?"
] |
[
false
] |
Sharp object cut things. I understand that. How does that work at the molecular/atomic level? Does it interfere with the covalent, ionic, metallic, or whatever other types of bonds the object being cut has? Can someone explain?
|
[
"Check out ",
"this big thread",
" on this topic.",
"In short, it depends on what you're cutting. At the end of the day, you are disrupting intermolecular and/or chemical bonds when you physically separate something with a knife."
] |
[
"It has a higher hardness than the things you're cutting usually. "
] |
[
"It does a bit - which is how a knife gets chipped or dulled.",
"But the knife's bonds are mostly stronger than the bonds it's mostly cutting."
] |
[
"Do large impacts cause nuclear reactions?"
] |
[
false
] |
For example, in the impact that wiped out the dinosaurs, or the impact that created the moon, are the conditions during impact sufficient to cause nuclear reactions within the impact material?
|
[
"I'm going to go ahead and say that I think it's not possible. ",
"The Chicxulub impactor (the one that we think killed the dinosaurs) was 10 km across, and going 20 km/s. A little Googling tells me people have done lots of simulations of ",
"what this might look like,",
" and seem to believe that 10,000 K sounds like a reasonable upperbound for the peak temperature. ",
"This is also way way way smaller than temperatuers where thermonuclear fusion takes place. Think millions or even billions of Kelvin for that. "
] |
[
"To get nuclear reactions with ordinary material you need to focus the energy ",
". There are current experiments trying to do it with very powerful laser bursts being focused to very small points. Even though there's a lot of total energy in the impact, it won't be concentrated enough to get any atoms to fuse."
] |
[
"I don't think that is necessarily true. There are a lot of nuclear reactions that occur at lower energies (photoexcitation and inelastic scattering). The impact releases large amounts of energy over a wide area. There will be hot spots. Especially from the plasma formation. The lowest nuclear excitation is 7.6 eV and the second lowest is 77 eV. Both can be excited from the plasma fairly easily. "
] |
[
"In movies/tv they say there are always a percentage of people who are naturally immune to new diseases, is this true and are there people naturally immune to COVID-19 specifically?"
] |
[
false
] |
I'm watching The Last Ship right now (lol), and i realized while they're discussing the Immune, we've not really heard anything about any people being naturally immune to COVID-19. why? is that not a real thing?
|
[
"Very few people are going to be spontaneously immune, in the formal sense of \"immunity\", to a novel pathogen.",
"What can happen is that people can be genetically resistant to infection or disease caused by a new pathogen, due to random or not-so-random mutations and variations. The classic example is CCR5 deletion -- CCR5 is a receptor for HIV, and people with a particular mutation in CCR5 are highly resistant to HIV (",
"Genetics of resistance to HIV infection: Role of co-receptors and co-receptor ligands",
"). (Someone is about to jump in and claim this was a mutation in response to the Black Plague. Nope; that was never more than a wild idea with no evidence that someone threw out in the Discussion section, and there are multiple lines of evidence against it.)",
"Does such a thing occur with COVID? We don't know. One of the ways such genes are identified is through genome-wide association studies (GWAS), and several of those have been conducted with COVID, but so far as I know no slam-dunk powerful resistance loci have been identified. A number of loci have been linked to relative resistance to severe COVID, but none of them are very clear or convincing links. ",
"(This is typical of GWAS -- I think the early hopes that GWAS would quickly identify genes very important for infection, or other disease associations, have pretty much faded, and it's generally accepted that most loci identified this way tend to be relatively weak and more important as cumulative contributors.) ",
"Someone lacking the ACE2 receptor for SARS-CoV-2 would probably be very resistant to COVID, but ACE2 is an important receptor for many important functions, and people lacking ACE2 would probably be severely affected. ",
"There may well be other genes that are important for SARS-CoV-2 infection or replication, and there may be people who have variants in such genes and are therefore resistant, but we don't know of them yet.",
"Further reading: ",
"Why Do Some People Develop Serious COVID-19 Disease After Infection, While Others Only Exhibit Mild Symptoms?"
] |
[
"A few months ago I read about this man: John Hollis didn’t realise he’d been infected with Covid-19. But he later discovered he’s one of the rare people who has developed “super antibodies”. The research claims that you could dilute the antibody in his blood 1 to 10,000 times and it would still kill 90% of the virus. Reading your response, I thought immediately in this man, and how incredible would be if researchers could understand what's the science behind it... What do you think about this specific case? Here is the link to the BBC News video:",
"Covid: The man with 'super antibodies'",
"Thanks!"
] |
[
"Scientists have found \"super antibodies\" (e.g. broadly cross-reacting, very high affinity antibodies) against several diseases in a handful of individuals, including to HIV and influenza. In some cases (HIV especially -- the very few people who have these antibodies seem to be extremely resistant to HIV infection) these have been known for a long time, and there's been extensive work trying to specifically elicit these in all people. ",
"It's been very hard to get this to work. In some cases, the antibodies seem to depend on the individual having a specific rare allele in their B cell repertoire, and without that rare allele the specific antibodies can't form. ",
"More recently, new techniques for mining antibody repertoire have shown that a surprisingly large number of people may have very low levels of potentially very potent antibodies, and the problem has become how to elicit these antibodies instead of the usual, less-potent ones. (And also, making sure that these potent antibodies aren't also, say, autoimmune or otherwise harmful -- mainly a theoretical concern so far, but no one wants to miss it if it's real). ",
"Examples of broadly cross-reactive antibodies against SARS-CoV-2 and other coronaviruses:",
"Cross-reactive antibodies against human coronaviruses and the animal coronavirome suggest diagnostics for future zoonotic spillovers",
"A protective broadly cross-reactive human antibody defines a conserved site of vulnerability on beta-coronavirus spikes",
"Cross-reactive coronavirus antibodies with diverse epitope specificities and extra-neutralization functions",
"Broad cross-reactivity across sarbecoviruses exhibited by a subset of COVID-19 donor-derived neutralizing antibodies",
"Approaches mainly involve modifying the vaccine antigen in various ways so that the appropriate antibodies are more efficiently made. (This is already done to some extent - e.g. the 2-proline modification of the spike protein in many COVID vaccines drives a much more efficient antibody response than to the natural protein, which is probably one reason the vaccines give better immunity than infection.) For example, ",
"A chimeric hemagglutinin-based universal influenza virus vaccine approach induces broad and long-lasting immunity in a randomized, placebo-controlled phase I trial",
" and many more. ",
"It's very early days for these approaches, but combining protein structure understanding and prediction with antibody repertoire mining is a very interesting and potentially powerful approach to designing much more potent vaccines."
] |
[
"Why is it possible to ground an electrical device by connecting it to the Éarth?"
] |
[
false
] |
Where does the electrical energy and charge go? Is it possible for the Earth to become saturated at some point in the future?
|
[
"Remember that net charge is always conserved so to charge the earth you would have to either remove a lot of its electrons and shoot them off into space, or get a whole bunch from space and shoot them into the ground.",
"In addition to this, even if you did manage to change the net charge of the Earth, there is a constant supply of charged particles in solar wind that would return it to neutral. (e.g. if the Earth had a net positive charge, it would attract negative particles and even out)."
] |
[
"An object with charge has an excess (or lack) of electrons. These electrons repel each other and so will flow as far away from each other as possible. ",
"By connecting a charged object to the ground, you give a path for these electrons to flow so they are even further from each other. The earth is really big and neutral, so the electrons will spread out freely until they no longer feel the repulsion. ",
"You can saturate the ground in small spots, but really all you’re doing is pumping in charge more quickly than the electrons “spread” so to speak. Remember that net charge is always conserved so to charge the earth you would have to either remove a lot of its electrons and shoot them off into space, or get a whole bunch from space and shoot them into the ground. "
] |
[
"Total charge is conserved. If an object gets an excess charge, this charge came from the Earth before - it just returns when the object is grounded. Earth is huge, it doesn't care about the temporary motion of a little bit of charge on its surface. It is only relevant for electronic devices if they build up too much charge."
] |
[
"What would the proposed DC-AC-DC converters do for the U.S. national power grid?"
] |
[
false
] |
If they're there to regulate flow from one side of the country to the other, why use these converters and not something else? Thanks!
|
[
"They aren't there so much to regulate power flow as they are to allow for different chunks of the grid to run asynchronously. Each colored block you see there is a proposed region with a constant phase and frequency. Technically, this means that if you plot a voltage somewhere in one section the transmission system of a colored portion of the grid, and plot the voltage somewhere else in the transmission system of the same colored portion of the grid, they should be more or less the same wave (neglecting small power factor differences and magnitudes). In practice, this means a whole lot of communication must take place between transmission companies, generation companies, distributors, and a whole lot else. If you make the \"interface\" point between the grids a DC connection, you have no frequency or phase to worry about. ",
"The disadvantage is costs and efficiency losses."
] |
[
"It's more about balancing power flow between many different loads, while keeping the frequency at roughly 60 Hz. The power output of a coal plant takes hours to change, and a nuke plant is even longer. Hydro and gas plants can change output relatively quickly, so they are used as 'topping' reserve that keeps everything balanced.",
"For all this to work, all the utilities and customers have to work together, and it can get complicated. Another problem is sending power through lines that have limited capacity. If you send too much power through, they could be damaged. Instead, automatic systems would break the connection. This affects the whole system and could result in a cascading failure."
] |
[
"You're misunderstanding the nature of a grid intertie. This isn't a transmission line subject to DC or AC line losses, it's a facility. Addition of a DC interconnect will always make the network less efficient because it's not replacing any line, it's just a node in the network. "
] |
[
"Are film clips still \"moving pictures\" when recorded and stored digitally, or does the recording of a digital video work differently from analogue recording?"
] |
[
false
] |
I put computing as flair, but I'm honestly not sure in which category this belongs. Feel free to mark it with more appropriate flair, admins.
|
[
"The basis of digital video formats is still a sequence of still images, just like analogue film.",
"However, for efficiency purposes, various optimizations are made, because storing a full resolution still image for every single frame would require a large amount of storage space (and a large amount of bandwidth to transfer).",
"The main way that digital video optimizes storage requirements is by not storing each frame as a full still image. Instead, a frame will only contain the differences between that frame and the previous. For most video clips large parts of the scene remain unchanged between two consecutive frames, which allows the next frame to be constructed using a relatively small amount of data.",
"In order to facilitate actions like forwarding and rewinding through a video, a \"key frame\" is inserted at regular intervals. Key frames contain the full image rather than only the differences between two frames. That way it's possible to start playback at a different point than the start of the video without having to first reconstruct the entire set of frames leading up to the selected starting point.",
"There are various techniques that further optimize the tradeoff between storage, quality and processing power needed, but the basic idea remains the same: Just like with analogue video, digital video still consists of individual frames that are recorded, stored and played sequentially."
] |
[
"Yes. If you look at ",
"http://wp.xin.at/archives/3465",
" which compare two different ways of encoding a video, more precisely these screenshots : ",
"http://wp.xin.at/wp-content/uploads/2016/02/H.264-AVC-1mbit-00000006-proc.png",
"http://wp.xin.at/wp-content/uploads/2016/02/H.265-HEVC-1mbit-00000006-proc.png",
"You see that one contains a blurry sky but on the other one you can see the rain.",
"To get the second video (where rain is still visible) you need to produce a bigger video file (or as with this particular example, a video file of same size but made with a more advanced encoder, therefore capable of doing more within same size constraints)"
] |
[
"So would a video of constantly changing static take more storage than a typical movie of the same resolution?"
] |
[
"Do plants ever feel \"full\" if they absorb enough sunlight?"
] |
[
false
] |
For humans, we have hormones that tell us we feel full when we eat enough food. Do plants have a similar system that tells them when they absorb enough sunlight? What do they do with the excess energy they absorb?
|
[
"Roughly speaking, photosynthesis turns carbon dioxide + water + light into sugar + oxygen. The sugar production is not evenly distributed through the plant, so the plant must transport sugar from where it is produced to where it is consumed. It has things called phloems (like blood vessels) to do this.",
"There does not seem to evidence of a satiety mechanism like humans have, which (probably) means the entire process in the plant is mediated by availability of inputs and products from a long chain of chemical reactions. So, simply put, if the plant does not have enough tissue to consume produced sugars, it will eventually get \"backed up\", and prevent the formation of further sugar (for example, the concentration of sugar may be too high for it to be moved from the phloem to the tissues, leading to the concentration being too high in the phloem to move it into the phloem, and excess sugar at the site of production will prevent the chemical reaction from producing more).",
"As an analogy, it's like how you never feel \"full\" or \"hungry\" for vitamin B12. If you can't produce it for some reason you'll have problems, but your body will not make you hungry for it, or stop you from eating when you already have enough.",
"There is some separate process within the plant that does long-term storage of the sugar by converting it into starch. It seems the details of that process are not entirely understood yet, despite how it is what fundamentally supports human agriculture.",
"Sources:",
"[1] ",
"https://askabiologist.asu.edu/moving-sugars-plants",
"[2] ",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4919380/",
"[3] ",
"https://en.wikipedia.org/wiki/Photosynthesis"
] |
[
" is (as far as we know) a concept limited to organisms with a nervous system. However, the sensations we experience serve biological functions, as you alluded to, and these biological needs can be met in ways other than with a nervous system. Therefore, plants of course don't actually ",
" anything the way we conceive of feeling, but they have cellular-level mechanisms which regulate their metabolism. While in animals, the nervous system serves as a centralized regulator, compiling sensory information from across the body and then instructing the entire body to perform various actions, plants are much more decentralized. (Plants have hormones too, but these are likewise decentralized in production, whereas animals have an endocrine system that produces hormones in specific locations.)",
"Excessive sunlight is indeed an issue that plants are adapted to deal with. Photosynthesis actually requires very little energy compared to the amount available in typical sunlight, and much of the photosynthetic machinery is devoted to dissipating some of the energy from high-energy light, leaving only just enough for photosynthesis to occur. This is important because excess energy would unleash uncontrolled electrons from chlorophyll, and free electrons are bad news for living cells. Plants have a whole host of methods for avoiding excess light, including producing various chemicals that absorb and harmlessly dissipate light energy, or simply moving chloroplasts around within the cell so that they're physically shaded.",
"And like all organisms, plants have to keep their metabolism in balance. Excessive nutrient availability isn't really an issue for plants (except for things like metal micronutrients which are toxic in high concentrations), but biomolecules must be produced in the proper ratios. Producing lots of cellulose for cell walls is a waste of energy and resources if you don't also produce lipids for cell membranes, for instance. Many photosynthetic enzymes are down-regulated by sucrose (one of the main products of photosynthesis), and many are also regulated by redox state. Basically, photosynthesis produces strong reducing agents (chemicals which readily donate electrons to other molecules), and these can become hazardous if over-produced - or similarly, cells can run out of their precursors, in which case electrons start spilling out of photosynthesis and produce all sorts of nasty compounds. When the generation of reducing agents exceeds the capacity of carbon fixation (turning carbon dioxide into usage biomolecules), plants divert energy to other activities, such as reducing nitrate to ammonia in order to produce amino acids and then proteins. This sort of molecular-level regulation is by no means unique to plants; merely the specifics of ",
" metabolism are."
] |
[
"Disclaimer, I'm not basing this off of anything I've read, but logically I doubt this would be needed. ",
"Hunger and fullness exist in some animals because locomotive species have to go acquire food when we need to eat. We need intrinsic motivation not to starve to death. Don't think of fullness so much as prevention from eating more, but rather as a respite from the hunger that keeps an animal alive. ",
"Plants having some kind of impulse that they are \"full\" would be evolutionarily pointless. Like, they can't really move so what would they even do about it? For the same reason, plants can't really feel \"hunger\" either. The best they can really do is bend towards the light when there's not enough, or out of it when it's too intense (this is more about tissue damage than \"making too much energy.\") Also, after a certain point of light intensity, a plant can't produce any more energy. There's only a certain amount of photons a plant can absorb at any given time, so shining more intense light on the plant once it's reached that point will have no real impact on its energy production. ",
"To answer this question:",
"What do they do with the excess energy they absorb?",
"They can use it to grow faster, or produce more energy stores (akin to how animals produce fat when they eat more than necessary)."
] |
[
"How do computers know which wireless signals are meant for them?"
] |
[
false
] |
Additionally, how do they know which signals the individual signal fragments belong to? With all the information being sent wirelessly across the globe, it seems like it'd be extremely difficult to know what constitutes any individual signal as opposed to fragments of other messages. How on earth do our computers manage to know exactly what messages are meant for them and the exact content of those messages with all the "noise" they're surrounded by?
|
[
"Messages which are carried through networks contain meta data as well as payload. The payload might be part of a web page or image. The meta data would be addressing information (sending and receiving IP addresses), and sequence numbers which identify that message (or packet) as a part of a stream. The reieving node looks at the IP address (and MAC address, which is used on ethernet networks) to check that the recipient is correct. It then reassembles the complete message and passes it to the application layer."
] |
[
"Extending the scope of the question and answer, I'd also like to add to this that because the packets don't have any extra encryption that is only readable by the target computer as opposed to another (just like snail mail has a TO: address, but isn't locked; you just hope it goes unopened to its proper destination), this opens up wireless connections to be interfered with and read by computers it's not intended for.",
"This is a reason you don't want to wirelessly connect to unprotected networks--anybody who can connect to that network can intercept and look at your packets.",
"edit: I accidentally typed the wrong word. \"and\" -> \"can\""
] |
[
"Anybody else also connected to a secured network can also see the \"plaintext\" packets. \nEven somebody not attached to that network can capture them and later decode them."
] |
[
"Realistically, what would motivate humanity to colonize our moon or other planets?"
] |
[
false
] |
[deleted]
|
[
"There is probably no non-scientific reason to colonize the moon because it can't be terraformed, has no magnetic field and lacks a number of other qualities you'd want. If we found a planet that could support our life reasons to colonize would include ",
"It's like saying \"why would we colonize land if there's another part of land across the water?\" before the Americas were discovered. "
] |
[
"Commerce."
] |
[
"Simply, a lack of natural resources, the inefficiency of creating artificial resources, and the rise of pollution to the point of no return. "
] |
[
"What is the relation between photons and electromagnetic fields?"
] |
[
false
] | null |
[
"Photons are the ",
" of the electromagnetic field. This means that electromagnetic fields come in discrete 'chunks' called photons. But this is a quantum-mechanical description, not a classical one, so you shouldn't think of photons as particles with definite properties--the photons act like waves, too, just as the EM field did, it's only that they come in discrete chunks of energy instead of continuously variable energies."
] |
[
"Yes. Well, QED is specifically the theory of photons and electrons, but it includes photons. (And you also generally want something around that can interact with the photons.)"
] |
[
"Thank you for your answer.",
"Yes, but not in terms of a classical field--you need a quantum field theory to describe photons.",
"Are you having specifically quantum electrodynamics in mind here? If so, I'll look into that. Will I perhaps get all of these questions explained to me in QED?"
] |
[
"What mental illness needs the most recognition in 2018, and why?"
] |
[
false
] | null |
[
"Hello,",
"Open-ended questions are more appropriate for ",
"/r/AskScienceDiscussion",
".",
"Cheers."
] |
[
"Hello,",
"Will this be allowed then? Or removed?"
] |
[
"It is removed, but you are encouraged to post it to ",
"/r/AskScienceDiscussion",
" instead."
] |
[
"Does water have a limit to how much soluble matter it can dissolve? If so, how is this measured?"
] |
[
false
] | null |
[
"Fellow coffee lover here! But definitely not sophisticated enough to be a snob- I just consume large quantities of it. :3",
"This is actually a very complex question because coffee is not a uniform substance. It contains thousands of different chemical compounds, each with its own unique properties and varying solubilities. Different compounds will be saturated before others, some compounds are more soluble than others, some are present in greater concentration to begin with, some are desirable and others are not. For example, some less desirable compounds dissolve more readily than more desirable ones. This is the main reason that brewing method and time can make such a difference in flavor, and why it usually tastes worse when you make a tiny amount rather than a decent sized batch (higher proportion of \"yucky\" compounds than \"yummy\" ones due to variable solubility). Some of the compounds important for flavor are actually non-polar (don't dissolve well in water but instead in fat), which is why coffee strains with higher fat content are linked with better flavor. And the proportions of different compounds present differ depending on which variety of coffee you're drinking and how it was prepared/roasted. What it all comes down to is that unfortunately there is no set way to determine X amount of coffee will saturate Y amount of water. ",
"I think the best you can do under the circumstances is to experiment! Maintain the same amount of water, temperature, grind, brew method and time that you normally use, but vary the amount of coffee. Start with a smaller amount than usual and gradually increase the amount of coffee. At a certain point, it will not be stronger/taste better than the one before. That's when you'll know the one you made before is the right/maximum amount you want to use. ",
"By the way if you'd like to learn more about the science of coffee you might find ",
"this",
" interesting. "
] |
[
"The limit to how much solute can be dissolved in a solvent is dependent on a lot of different factors, such as the properties of the solute and solvent, temperature, pressure, and pH. We don't assign a \"maximum solute dissolving ability\" to the solvents, like water in this case. We measure the concentration of a ",
" when the solution is fully saturated. This property is called solubility, and it can be measured/expressed in units of concentration, molality, mol ratio, etc. So for example, we can discuss the solubility of sucrose in water (at specific temp, pressure, etc) but we can't talk about just the solubility of sucrose or the water by itself.",
"Certain substances, like ethanol, are fully miscible (infinitely soluble) in water and will never be saturated. Other solutes dissolve very poorly in water and the solution will become saturated at very low proportions. It just depends on the properties of the substances. It's also possible to super-saturate a solution under specific conditions and this can lead to ",
"neat results."
] |
[
"It's actually a tremendously complicated question. You're not so much \"dissolving\" the coffee as doing an extraction. The various compounds in the coffee beans (and there are quite a few of them) are trapped inside the matrix of the bean. Some of them leave that matrix (are extracted) more or less easily under different conditions. Grind size, temperature, brewing time, the specific composition of each coffee bean, etc all determine the blend of compounds and their concentrations in the final cup.",
"Incidentally, this is one reason cold brew coffee can taste very different than hot brewed. Different chemicals are extracted preferentially at low temperatures. ",
"The saturation point for caffeine in water, fyi, is 66 g per 100 mL of water. That would kill you. I'd be a bit surprised if any of the compounds in coffee were actually saturated, although that's somewhat of a guess."
] |
[
"Apart from AIDS, is there any other syndrome/disease the HIV causes? Or is it only AIDS?"
] |
[
false
] | null |
[
"A short time after initial infection, many people experience a brief flu-like illness. After this subsides the virus becomes latent for a period of up to a number of years before causing immune deficiency.",
"The virus also can cause neurocognitive changes independently of opportunistic infection, ranging from mild thinking and memory problems to dementia."
] |
[
"In addition, though these are not ",
" by HIV, there are a number of infections/symptoms that are indicative of HIV due to the immune suppression:",
"• Candidiasis (esophagus)\n• Cytomegalovirus of retina, brain, spinal cord, gastrointestinal tract\n• Lymphoma, brain or non-Hodgkin's (B-cell or immunoblastic)\n• Mycobacterium avium complex or kansasii (disseminated)",
"This is a great article on the matter:",
"http://www.ashm.org.au/images/publications/monographs/hiv_viral_hepatitis_and_stis_a_guide_for_primary_care/hiv_viral_hep_chapter_6.pdf"
] |
[
"HIV-Associated Neurodegenerative Disorder (HAND) is actually believed to be caused by the virus itself, or at least indirectly through proteins it encodes. HIV is believed to activate the infected macrophages, as well as microglia and astrocytes, which then damage their surrounding neurons.",
"An article on HAND and some of its putative causes: ",
"http://www.nature.com/nature/journal/v410/n6831/full/410988a0.html"
] |
[
"I've read that living an inactive lifestyle is on par with being a smoker. Is one particularly worse than the other?"
] |
[
false
] | null |
[
"It could just mean many more people are inactive than smokers."
] |
[
"It could just mean many more people are inactive than smokers."
] |
[
"I would think that if one made a Venn Diagram with two circles representing both groups, there would be considerable overlap."
] |
[
"If i let a bottle of let's say vodka sit for a while, will the alcohol rise to the top of the bottle?"
] |
[
false
] |
Since alcohol has a lower density than water? Additionally, if i freeze the liquid, could i then cut a piece of the top, which would then contain more alcohol than the average of the overall bottle at a specific abs.vol (40%)?
|
[
"Ethanol has a lower density than water, so if the two didn't mix, you'd have ethanol on the top. Ethanol and water are fully miscible however, so you don't get a concentration of ethanol on the top. The two liquids act as one and don't retain the density properties of the original liquids.",
"Vodka also doesn't typically freeze in a standard freezer. For alcoholic beverages that do, like wine and beer, the part that starts freezing will indeed have a lower alcohol content. ",
"Fun facts from my freezer: freezing wine tends to precipitate the tartaric acid. Freezing vodka makes for good cocktails and requires no defrosting.",
"ETA: The idea that two liquids mix into something with new properties is also why volumes aren't additive (100 mL of water + 100 mL of EtOH does not equal 200 mL of mixture) and why the freezing point depresses but not to the point that only water freezes, leaving pure alcohol behind."
] |
[
"I was wondering how these mixtures evaporated. If you left a 50/50 EtOH/H₂O container uncorked at 25°C for a week, would more EtOH evaporate by virtue of its higher vapor pressure? I think yes.",
"So I went off to read about the behavior of vapor pressures of miscible liquids and discovered ",
". Cool stuff."
] |
[
"Funnily enough if you slowly lower the temperature of an an alcohol water mixture, the exact opposite happens. The water actually freezes first and the liquid you leave behind is enriched in alcohol, this is because alcohol water mixtures have a lower freezing point than either pure water or pure alcohol and the composition of the liquid phase will shift towards the Eutectic composition of the mixture.",
"Brewers use this trick to make 'beer' that is ",
" stronger than is possible by simple fermentation."
] |
[
"So our brains release dopamine when we listen to moving music, but why?"
] |
[
false
] |
What's the point of that? how did it come about?
|
[
"The first step in answering the \"why\" questions is to look at any evolutionary benefits it might have had - if any activity is evolutionarily beneficial, the body will want to promote it, and one way to do so is to have it be pleasurable so that the individual and eventually the species at large engages in said activity more. There are different takes on the role of music in evolution, Steven Pinker famously claims it's just \"auditory cheesecake\" and any pleasure we derive from it is a side effect of the development of language. Others have suggested various potential benefits that could have resulted in music being promoted by pleasure circuits in the brain, such as promoting mother-infant bonding, courtship and/or group bonding and synchronization (Daniel Levitin's book \"The World in Six Songs\" may be a good read if you're interested in that). I'm on mobile right now and can't provide a huge array of sources, but this is a decent in-depth take on different accounts on music, mind and evolution - ",
"http://www.mus.cam.ac.uk/~ic108/MMS/",
"."
] |
[
"Great answer! I've always liked to think human's affinity for music also stems from the sheer mathematics of the music. Is there any science behind that, or just my wishful thinking?"
] |
[
"It is entirely possible that it is the math. I study the relationship between various mathematical aspects of music and how we behave when we listen to them. There's something called fractal structure which is present in nature, art, your body's rhythms, etc... This is also found in music (distribution of pitches, rhythm, tempo fluctuations, loudness). May of us think that the reason it is found in music is b/c we like this kind of structure. Why do we like it? Well, that's more of a post hoc argument, but it may be b/c we are exposed to it all the time. Basically, music walks the line between being completely predictable (and boring) and completely unpredictable (and impossible to follow). This is sort of what I mean when I say that the statistical structure of performed music tends to be fractal (or 1/f). I'm currently looking to see if there are neural correlates for varying amounts of this structure in the pitches and rhythms presented to people (fMRI). I know that animals have neurons (in auditory coretx) that fire optimally when you give them fractal stim., so we probably do too."
] |
[
"How much is too much coffee?"
] |
[
false
] | null |
[
"TIL I'd have to drink about 174 cups of coffee in short succession to die.",
"Thank goodness my 5ish cups a day comes no where near that number!"
] |
[
"There have been tons of studies based on coffee and it seems that every week the verdict is out whether it is good for you/bad for you. For the most part though, most of the effects of coffee are tied to the caffeine content of it.",
"Everybody has a different tolerance for caffeine based on their weight, height, age and overall wellness. Some people can handle it very well and do 5 pots of coffee a day, whereas some people (such as myself) can't do a single cup without feeling sick. To have a true overdose, it takes a lot of caffeine, more than any human could consume in liquid form (about 6 gallons of McDonalds coffee). Usually when a death does occur, it is due to messing with the body's chemistry in a negative way such as people with heart problems, or due to pill forms of caffeine that a person ODs on.",
"Caffeine can also cause issues with blood pressure. read: ",
"http://www.mayoclinic.com/health/blood-pressure/AN00792",
"Going back to your question, \"how much is too much?\", it's hard to say, everybody is different, do what feels right, don't make yourself sick, some people can have a lot of it, some can't have any. If it's making you feel weird, don't have it. Don't become dependent on it either, but again, everybody is different. ",
"Sources:\n",
"http://www.popsci.com/science/article/2012-10/fyi-how-much-caffeine-would-it-take-kill-you",
"\n",
"http://www.nlm.nih.gov/medlineplus/ency/article/002579.htm"
] |
[
"Your complement, the 5 pot/day guy, checking in. I don't have any studies to back myself up, but I feel fine. It definitely varies from person to person."
] |
[
"How are you supposed to picture complex wave functions?"
] |
[
false
] |
intro quantum mechanics.
|
[
"It's not easy when you have a wavefunction that depends on three spatial coordinates and time, and it's complex-valued. But there are little tricks you can use. For example you can often decouple the radial and angular parts of the energy eigenstates. The angular parts are usually just given by the spherical harmonics, which give you those probability density diagrams you often see in chemistry textbooks. Then the radial part just becomes a function of one variable.",
"You can often choose a convention that the radial wavefunctions are all real-valued as well. So now you just have a real-valued function of a single variable. You can plot this very easily.",
"Another way to avoid all of this is to simply plot probability densities. Probability densities are just the squared moduli of wavefunctions, and these are always real-valued. You still have the problem that they depend on three coordinates, but you can plot projections onto 2D planes, or surfaces of constant probability density, something like that."
] |
[
"There are some ",
"cool applets here",
" (the quantum mechanics section) that let you visualize wave functions. The main idea is to use the color wheel as a stand in for the phase of the wavefunction. Then a wavefunction like e",
" that has constantly varying phase will have a rainbow color pattern where the colors are evenly spaced. The molecular orbital simulation uses transparent colors for small magnitude and opaque colors for large magnitude."
] |
[
"Physically, and not mathematically, what does the radial part represent? Many thanks for your answer,"
] |
[
"Is it possible to have red lightning?"
] |
[
false
] |
if so how or why does this work?
|
[
"Might be a camera artifact, color-balance issues. Perhaps it shows up in a photo, but nobody saw any red color during the lightning strike. Odd colors are associated with ",
"lightning inside ash clouds."
] |
[
"I suspected this to be a time lapse because the clouds move quickly, and tin eye says I was correct.",
"http://bocaberta.org/2009/09/tempestade-em-timelapse.html",
"The third image there is the same location and cloud formation, and has the red lightning's shape shown in white. The video shows more of the same and all the same lightning formations. No colors though. That said, in your link the bright teal bolt is clearly photoshopped. Notice how it's upper right branch is very segmented and squarish. My bet for this .gif is somebody took a portion of that time lapse, color corrected it very red, then chroma keyed out white from a few of the other strikes in the video and made them different colors.",
"Having made my little theory, I should say that the site is Portugese, which implies Brazil, and IIRC there was strange lightning going on around volcanos in Chile not too long ago, so it's possible that wbeaty's theory is correct, and the colored lightning is from the same photoshoot by Brazilians on a trip to Chile? Hell if I know. Probably photoshopped.",
"edit: Worth noting, red lightning ",
" appear to exist, continuing my trend of tineye superpowers. The picture linked by wbeaty is, as far as i can tell, real. The largest and most changed results for it are all equally red, so I suspect it hasn't been color corrected."
] |
[
"I believe what ",
"this website",
" tells me on what causes lightning to have color"
] |
[
"Why are certain people able to eat and digest spicier foods better than others?"
] |
[
false
] |
[deleted]
|
[
"TRPV1",
" is a receptor that responds to capsaicin. People who lack this or are deficient in this would theoretically have little or no response to spicy foods. So yes, technically, by mutation, you can avoid the spicy sensation.",
"On the other hand, what you are probably referring to is why Indians can eat such spicy food, and why a German probably can't. This is something that people simply become used to. You may try something that is bland to one person but seems incredibly spicy to you. This is mostly due to the fact that they are desensitized to the response. Think about pain response in general.",
"If you attempted to break a cinder block with your arm, you would likely fail and end up screaming...a lot. A taekwando master on the otherhand wouldn't have such a difficult time. Strength differences aside, his hand is still impacting the surface at the same speed (probably greater), but he perceives less pain because of habitually destroying hard objects with his hands. The concept can be applied to firewalking. ",
"This is a vast oversimplification, but it demonstrates the power of our minds to be desensitized by various things."
] |
[
"FWIK, digestion is irrelevant. Capsacin only affects the mouth and anus, it does absolutely nothing to the digestive system. i still dont understand why some people fly to the toilet after spicy food."
] |
[
"It is a mixture of both genetics and environment, with a larger proportion of it being environment. When one is exposed to certain spices and ingredients in their diet from a very young age, his/her digestive and immune systems adapt to digesting and metabolizing those substances very well. That being said, there is probably some genetic component to taste bud sensitivity as well as levels of mucous and enzymes in the digestive tract that provide slightly more protection from and digestion of these particular foods (although it would be hard to say which came first, the physical or cultural development of the tolerance). "
] |
[
"Would you still be able to donate your organs if you've gone though chemo therapy?"
] |
[
false
] | null |
[
"In practice, the answer is no - but it's not because of the chemo, it's because of the cancer you presumably had which the chemo was treating. If you for some reason received chemotherapy but did not have cancer, I'm not sure if the chemo on its own would preclude organ donation."
] |
[
"Thank you! Thats what I was trying to figure out as well. "
] |
[
"In my country, depending on the cancer and the treatments and so on people might still be able to donate organs. Obviously not during therapy, but if somebody had a potentially curable cancer, received the adequate therapy and remains cancer free after X years, they may be able to donate certain organs. Usually, they'll be scanned thoroughly before donating. "
] |
[
"A time travel/astronomy question: could I look at the stars and tell the date at any time in the past?"
] |
[
false
] |
Let's say my time machine has malfunctioned and I know where I am, but not when. What can I do to determine my location within time?
|
[
"With careful measurements of the positions of the stars and moons you could work out a range of time in which you found yourself. ",
"You could narrow it down further by quizzing locals about comet sightings (assuming after building a time machine you also built a cochlear implant to translate language to understand extinct languages and dialects.)",
"You might be able to get the same estimate of temporal location by observing the species that are alive (or not yet evolved)."
] |
[
"If your time machine had a computer you could indeed analyze the position and drift of the constellations and stars to tell when you are, It would be very difficult to do this manually though. "
] |
[
"How would that help the traveller? The location of the constellations would not necessarily have been recorded, or even necessarily in a uniquely different location, and it would be so difficult to calculate any of this, even with the aid of a computer, that it would be practically impossible. "
] |
[
"Does lightning make a sound when it strikes small bodies of water?"
] |
[
false
] |
such as lakes, or Tampa Bay?
|
[
"Yes. Pretty much all of the sound comes from the air the lightning goes through, not from the target itself. By far most lightning goes only through the storm cloud itself and we can hear these aswell. Only reason they do not seem as loud is because they are further away than if lightning strikes near you."
] |
[
"The sound comes from the expansion of the super heated gas where the lightning travels through the air, making a pressure wave from the expanding gas (thus sound). So where the lightning connects to the earth does not matter."
] |
[
"The sound lightning makes comes from the air expanding. Air has a huge electrical resistance, so when current passes through it, energy is deposited in the form of heat according to the formula E = t * R * I",
" where E is energy, t is time, R is the resistance, and I is electrical current. When gasses are heated they expand, and the resulting pressure wave is what your ears perceive as sound."
] |
[
"Why is it when i dry clothes in a dryer they're soft and fluffy, but air dried in a heated bathroom they come out crispy?"
] |
[
false
] |
My dryer is broken so I've been hanging up some clothes and it's been bugging me.
|
[
"The answer for this is simpler than you'd expect. When you dry them on a rack inside your house, they stay in virtually the same position for the entire drying period. The clothes kind of 'set' in the position that they're in. When using a dryer, clothes are tumbled around throughout the cycle and don't have a chance to 'set' as in the above scenario. Thus they lack stiffness.",
"You should notice that when putting them on a line say outside, the stiffness will depend heavily on the amount of wind. They'll be stiffer if there's no wind, and a bit softer if it's windy out!",
"You can always make sure to use a bit of fabric softener in your laundry to make them softer, although keep in mind some materials and clothes strongly discourage the use of softener."
] |
[
"there's also a waxy softening agent that makes things feel softer. Counter-intuitively though, this stuff makes towels slightly LESS absorbent."
] |
[
"So should i patent the tumbling outdoor solar clothes dryer?"
] |
[
"Has an industrial accident ever caused volcanic lightning?"
] |
[
false
] |
If not, what would it require? Would it be possible?
|
[
"Volcanic eruptions typically involve quantities of energy and matter that are very large by human standards. (It would have to be a ",
" of an industrial accident to be in the same ballpark.)",
"But something that might be of interest are pyrocumulonimbus (pyroCb) clouds that can result from forest fires, and can produce lightning. ",
"- \n",
"http://news.nationalgeographic.com/news/2010/08/100816-fires-thunderstorms-thunder-clouds-volcanoes-science-weather-russia/",
" ",
"- ",
"https://en.wikipedia.org/wiki/Pyrocumulonimbus_cloud",
" ",
"Human activity can cause forest fires, so under the right conditions human-produced clouds can cause lightning."
] |
[
"as ",
"/u/charlaron",
" said - its unlikely that an ",
" has ever led to a pyrocumulonimbus-like cloud or lightning. However, it is well ",
"studied",
" that nuclear bomb tests have induced lightning, as can be seen ",
"here."
] |
[
"ah thanks",
"I actually had no idea that forest fires could produce them."
] |
[
"Eye color relating to sight perspective question"
] |
[
false
] |
I've wondered this for a long time, and I hope one of you can answer this for me. Is it possible for people with different eye colors or any other reason to see colors different? We've all been taught to call blue blue, but what if I see blue as your green? Reasoning(in my head) that this could be: different eye colors/the way its communicated to the brain in different people.
|
[
"the color of your iris isn't going to affect your photoreceptors. "
] |
[
"Eye colors do not effect vision whatsoever. Light does not pass through the iris to reach the retina.",
"It is nonsense to ask what we perceive colors as.",
"I perceive it as a set of electrical impulses passing chemical gaps.",
"So do you.",
"To try to tell if they're the \"same\" is meaningless. Perception is complicated and on a cellular level rather poorly understood."
] |
[
"Very informative, thank you for your time. "
] |
[
"How to classify and differentiate of sub-species and breeds."
] |
[
false
] |
Hi simple question How do we classify and differentiate if its a sub-species or a breed of something? There are times when a subspecies look alot like the other, but breeds can appear to look very different. (e.g Borneo Pygmy Elephant to Asiatic Elephants = subspecies, Toy poodle to Huskies = Dog Breed) Please provide academic link, google is only giving me forum links. Thanks
|
[
"A major distinction is that the term \"breed\" is really only used in animal husbandry and not in taxonomy (classification of species) or in biology. The problem with breeds is that they are often based on shifting sets of physical characteristics both in time and in space. One kennel club may describe a breed a certain way, and another in a different way. Moreover these descriptions often change over time. Describing domestic animals using breeds does not clearly reflect what is actually going on from a natural selection (or artificial selection) perspective. A breed is a specific group of domestic animals or plants having homogeneous appearance, homogeneous behavior, and other characteristics that distinguish it from other animals or plants of the same species and that were arrived at through selective breeding. Despite the centrality of the idea of \"breeds\" to animal husbandry, no scientifically accepted definition of the term exists. A breed is therefore not an objective or biologically verifiable classification but is instead a term of art amongst groups of breeders who share a consensus around what qualities make some members of a given species members of a nameable subset.\"",
"Whereas subspecies are typically a more valid classification scheme which take into consideration many different factors. Factors such as: genetics, time since divergence, morphological and behavioural differences, degree of gene flow between populations etc. ",
"But before I get into how we classify subspecies it would be best to consider the species classification first. The most popular definition for a species is the \"the biological species concept\" -- basically a \"species\" is defined as a population of organisms that are able to reproduce with each other. If two populations can't interbreed, they are two different species. This is typically used for sexually breeding animals, and is not usually applied to other lifeforms, like bacteria, which reproduce asexually. So for the purposes of your question I will simplify my response and concentrate on animals and how we classify one species from another.",
"When a single population of organisms diverges into two separate populations to the point that they can no longer interbreed, then you would say that you have the emergence of a new species. At this point, the two populations are forever separated, and they may follow very different evolutionary paths. As long as two populations can interbreed, there will be some amount of \"gene flow\" between the populations, and they will never be able to adapt to very different ecological niches. There are many ways for populations to ",
"speciate",
": allopatric, peripatric, parapatric, and sympatric (see the article for details). Usually there is some starting mechanism that begins to pull two populations apart. We can look a many barriers to gene flow external and internal (I can go over these in more detail if you like).",
"Of course populations may be in the process of speciating, it does not happen over night and it can even take a few million years...if it happens at all. This complicates trying to classify populations into one species or another. Some times it is very obvious, like a leatherback turtle and a crow. Other times it is more subtle like humans and Neanderthals, or amur tigers and south-china tigers. Often when we try and classify populations that are speciating we may call them \"subspecies\". ",
"Some factors which may be considered for classifying populations into the \"subspecies\" catergory:",
"They are in the process of speciation but have not fully speciated yet.",
"Two populations of a species living in two different areas where gene flow between them is very very low, or becoming non-exsistant.",
"It is obvious that sexual and behavioural barriers to reproduction are being produced.",
"That hybrids between the two populations are have less-vigour, not suited to their environment and/or are dying.",
"Genetics will be done on the populations to determine the amount of gene-flow, time since divergence etc.",
"We may also consider human disturbance, in that the population may have been continuous at one point in the not so distant past, but due to hunting or habitat destruction isolated populations have been created. Limited gene flow between these newly isolated populations may lead to speciation.",
"There may also be a human a goal involved in classifying subspecies. From a conservation perspective it is better to split a species up into subspecies because subspecies populations tend to be smaller and more threatened than if all the populations were considered together. When population numbers are low and isolated it is easier to get funding, or to get support to save some track of land. This is not just some \"scheme\" in the malevolent sense of the word, it makes a lot of sense biologically. Some populations of species are indeed very isolated from other populations to such a degree that they can no longer interbreed. If these isolated populations were not protected they would go extinct, and while the species would remain alive a key component of its genetic diversity would be lost forever. In order to do this, subspecies classification can make all the difference in terms of funding. To give a real example with hypothetical numbers we can consider the tiger. On the whole there may be many tigers in the wild (say 100,000). But these tigers do not all live in the same place, moreover these places are quite isolated from one another. So that a tiger in Russia is not likely to every interbreed with a tiger from India. So by classifying subspecies based on gene flow (i.e. population lines are drawn where gene flow stops) we get a better sense for how tigers really are doing - in that the population in India has 10,000 tigers but the population in Russia has 80. When we consider this, the Russian subspecies is much more threatened then the Indian subspecies. Thus the Russian subspecies likely to receive more funding when it is considered separately from the whole population (i.e. 100,000 tigers)",
" \"Breed\" is used in animal husbandry, \"Subspecies\" is used in taxonomy and biology - they are applied differently and the criteria used to distinguish breeds and subspecies are very different."
] |
[
"Thanks a lot :D"
] |
[
"So you're asking \"when do we say 'x' and 'y' varieties of a living being are breeds vs being subspecies?\"?"
] |
[
"Is photosynthesis more efficient than combustion of oil?"
] |
[
false
] |
Hey askscience, could you guys help me determine if plant photosynthesis is more efficient at energy production than other forms we are using for energy (combusting oil, moving water, decaying radioactive elements)? What does photosynthesis do, that we can't mimic or utilize the process? Thanks! Edit: grammar
|
[
"I'll let somebody else tackle the comparison to petroleum-- but compared to currently-available ",
"photovoltaics",
" (6-40% efficiency, with common panels around 15%), photosynthesis is ",
"not very efficient at all.",
" (0.1%-8%, with most plants around 0.1% and most crops around 1.5%).",
"Put another way, generic off-the-shelf consumer solar panels are about 10 times as efficient as crops, which are themselves about 10x as efficient as normal plants. "
] |
[
"But the way that the efficiency are being calculated here doesn't lend itself to a fair comparison. For photovoltaics you are looking at the amount of electricity generated to calculate the efficiency, and for photosynthesis you are looking at the amount of biomass. Looking at biomass to calculate efficiency is like measuring the energy released when you burn down a solar panel factory and calling that the 'efficiency' of a solar panel. Which is another way of saying that the number isn't an efficiency at all.",
"You can, however, compare the quantum efficiency of photosynthesis to the quantum efficiency of a solar cell. The quantum efficiency expresses the probability that a photon produces a usable electron (for a photocell this is an electron pushed across the n-p junction, for photosynthesis this is an electron pushed across a lipid bi-layer). Photosynthesis typically has an internal quantum efficiency around 75% and an external quantum efficiency around 25%. These numbers are similar to the quantum efficiency of photovoltaics, which can hit external quantum efficiency around 50%. "
] |
[
"You can, however, compare the quantum efficiency of photosynthesis to the quantum efficiency of a solar cell.",
"When comparing the efficiency of two things, it is crucial to know what we're comparing-- but that doesn't make comparing the useful energy obtained with both systems an \"unfair comparison.\" ",
"Which is another way of saying that the number isn't an efficiency at all.",
"It is as valid, and generally more useful when comparing things as energy sources, to compare the total energy output to the total energy input. Efficiency means a lot of different things and must be carefully defined, but I was clear in what I was comparing and not attempting to mislead. Quantum efficiency is also a valid definition of efficiency-- but I would argue it is substantially less useful when comparing the effectiveness of two things as energy sources."
] |
[
"Do black holes really have infinite density? (And other questions about black holes)"
] |
[
false
] |
: I've heard black holes described as having infinite density a bunch of times. Density=Mass/Volume, and black holes definitely have a non-infinite mass (otherwise its gravity would be infinitely attractive). If density is infinite and mass is not, the volume of the black hole would have to be infinitely small, right? : Now, to my understanding, black holes can be formed when a big enough star runs out of fuel. (Are there any other ways to form black holes?). Stars need to have some sort of fusion reaction going on in its center in order to counteract the massive force of gravity, and when fuel for this reaction runs out, the force of gravity overcomes the (pauli exclusion forces? is there some kind of name for this force?). The star then collapses in on itself. But how does this make for a black hole with infinite density? Wouldn't all the matter in the star occupy the space of a single neutron? How can the black hole get smaller than that? Thanks :D
|
[
"neutron degeneracy pressure is actually only minimally responsible in supporting a neutron star from collapse. At densities greater than the nuclear saturation density (~ density of an atomic nucleus), the strong force actually becomes ",
" trying to restore the nucleons back to a nuclear equilibrium. In fact, if you model a neutron star without considering strong force interactions, you'd come up with a maximum possible mass of just ~0.7 solar masses, while most neutron stars are at least twice as massive.",
"If you want lots more detail on this subject, check out",
", Norman Glendenning, 2nd Ed., 2000. ",
"Ch.4 has a good overview to nuclear field theory, for someone with a background in undergrad physics. ",
"but of course, i'm just a lowly first year grad student, maybe you have some more insight on this subject?"
] |
[
"question 1... it depends. The most conventional understanding is a point mass. It's kind of improper to specifically call it infinite density, but yeah the mass divided by zero volume is \"infinite\". More contemporary interpretations that aren't really necessarily a part of science proper skirt this definition.",
"question 2: Pauli Exclusion ",
" usually, but yeah. If the gravitational pressure is greater than the remaining Pauli Exclusion Pressure, there's nothing left to prevent the mass from occupying the same position."
] |
[
"I've heard black holes described as having infinite density a bunch of times. ",
"I'm not an astrophysicist so someone correct me if I'm wrong, but my understanding is that there are two conflicting schools of thought. Based on just General Relativity, there should be a true singularity, which would have \"infinite\" density. On the other hand, quantum mechanics predicts otherwise and if I'm not mistaken a true singularity is impossible under quantum mechanics.",
"is there some kind of name for this force?",
"In order for a neutron star to form, the force of gravity must overcome what is known as electron degeneracy pressure, which is based on the Pauli exclusion principle. Another force, called neutron degeneracy pressure (also based on the Pauli exclusion principle), keeps stars that aren't sufficiently massive to form a black hole from further compression, which is where we get neutron stars from. If gravity overcomes neutron degeneracy, then a black hole can form."
] |
[
"Did the early universe have an event horizon and behave as a black hole?"
] |
[
false
] |
I was watching a lecture about calculating the Schwarzschild radius/event horizon for a black hole, and I wondered what the Schwarzschild radius of something with the mass of the observable universe would be so I ran the numbers on it using an estimate for the mass of the universe from . Where r = 2GM/c r = [(2)(6.67E-11)(3E+52)]/8.99E+16 r = 4.45E+25m = 4.7 billion light years If all of the observable matter in the universe existed within a radius of less than 4.7 billion light years, which it must have during the inflation of space-time, would that not mean that it would have qualified as a black hole? Would it only have appeared as one to an outside observer, but because the density inside the radius would have been so much lower, would it not have appeared as one to a contained observer? And what would have happened after inflation drove the matter outside of its own Schwarzschild radius? Or have I just gone horribly wrong in my math somewhere?
|
[
"The equation r = 2GM/c",
" is really only valid for a system that can be reasonably approximated by a point mass at the center of the coordinate system and very little mass outside. When you have such a setup of a large concentrated mass one can solve the Einstein Field Equations to get the ",
"Schwarzschild metric",
" which explains the curvature of spacetime and hence gravitational influence of the large mass at the center.",
"The observable universe is better approximated as a roughly homogenous concentration of mass everywhere in space - even near the beginning when there was far less space!. This setup leads to a different solution of the Einstein Equations called the ",
"Friedmann–Lemaître–Robertson–Walker metric",
". This solution is often referred to as the \"Standard Model\" of cosmology and from it one can look at the curvature and expansion of the Universe as a whole."
] |
[
"Very cool. Thanks for the reply"
] |
[
"The Schwarzschild metric and associated equations are roughly valid as long as the mass is all inside some finite radius R, it is spherically symmetric, it isn't spinning too fast, and it doesn't have much charge. ",
"For example, I can treat the sun like a black hole with all of its mass at the center, and the equations for the Schwarzschild metric are roughly valid as long as I only apply them in the area of space outside of the Sun's surface. One can, for example, predict the path of a photon bent by the Sun's gravity to a good approximation by treating the sun as a black hole and using the Schwarzschild geometry.",
"My statement that the system has to be \"reasonably approximated by a point mass\" is probably too strong. You have a good point, and it might have been better to say that all the mass can be enclosed in some radius R, then one can approximate the system as a black hole as long as one makes calculations outside of R.",
"The expanding universe is the worst possible violation of this assumption, since the mass is assumed to be spread out everywhere in space at an equal density, and can therefore not be enclosed in any sphere."
] |
[
"Have there been any unbiased studies on the long-term effects of cannabis use and what are the effects on lung capacity, memory intelligence, ect."
] |
[
false
] |
[deleted]
|
[
"I just went through a bunch of MJ stuff on a ",
"bestof",
" thread and on another thread ",
"here",
".",
"There haven't been great studies on MJ and lung function. There are some earlier studies from New Zealand, including ",
"this one",
". Then there was a recent paper published in JAMA ",
"here",
". These both measure spirometric (airflow, which is the most important measure of lung function but not the only measure of lung function) parameters in chronic marijuana users and do pretty good jobs at quantitating the amount of marijuana used.",
"These can be summarized as saying that light marijuana use has no detectable effect on lung function and may actually have a bit of an improvement in some spirometric measurements. Heavy use seems to decrease it. However, there are several other studies showing that marijuana has effect as a bronchodilator. This could be due to a direct effect, or it could be due to the breath hold pattern that most MJ users use -- a deep breath and hold could lead to smooth muscle stretching and some bronchodilatory activity. Therefore, I think we should take these pulmonary function studies with a HUGE grain of salt. It is quite possibly, maybe even likely in my book, that light marijuana users are just covering over decreasing lung function with the bronchodilator effect. That starts to be overcome in heavy users, who do more damage to their lungs with more smoke. And so both the JAMA and the New Zealand study note that the highest users have falls in lung function.",
"No study that I have seen has looked at other parts of lung function -- volumes and diffusion as opposed to just airflow. As I have mentioned in the other thread, I have seen a few cases of serious lung disease directly attributable to marijuana -- two cases of smoking-associated lung fibrosis (DIP and RB-ILD for those curious), a few cases of bullous apical disease, with one leading to repeated pneumothorax and requiring extensive surgical repair. I've also seen several cases of heavy fungal pneumonia in immunosuppressed patients and those with other severe lung disease who are using medical marijuana and MJ is known to be coated with mold. But that is personal anecdote and doesn't fly in this forum... :)"
] |
[
"You're welcome to read the citations in the wiki if you don't think they're good sources... ",
"http://www.cmaj.ca/content/166/7/887.long"
] |
[
"I read the ",
"wiki",
" section on the long term use of marijuana's effect on memory and intelligence and I find it pretty accurate... that moderate use has little or no effect, but heavy use does."
] |
[
"How do you calculate the limits of functions using the exponential one?"
] |
[
false
] |
Something has been bugging me for a while. Let's say there's a function such as . How would you get the limits when x reaches infinity? It sounds impossible to me. Moreover, how did we find out about the way to solve this problem (if we ever did)?
|
[
"You can use ",
"L'Hopital's Rule",
". If you write g(x)=x and h(x)=e",
", then f(x)=g(x)/h(x) and the limit of g(x) at infinity is infinity, and the limit of h(x) at infinity is also infinity. It then looks like the limit of f(x) at infinity should be infinity/infinity, but since this is undefined, we can't break up the limit like that. L'Hopital's rule says that the limit of f(x) will equal the limit of g'(x)/h'(x)=1/e",
" and the limit will be 0."
] |
[
"L'Hopital's rule is super cool and essentially tells you which infinity is growing or shrinking faster.",
"So, you can intuitively determine that e",
" approaches 0 much much faster than x approaches infinity and will win out in the end."
] |
[
"L'hopitals is usually the answer. Sometimes you need repeated applications for it to resolve things. Another under utilized tool you can use though is the logarithmic derivative, which can be used to determine convergence. (If you're actually interested hit me up and I can elaborate) ",
"The example I use is 2",
" versus 3",
". "
] |
[
"I think randomness doesn't actually exist, but is only used to fill in gaps of understanding. My colleagues think I'm being ridiculous. What are your thoughts?"
] |
[
false
] |
Let me explain what I mean first. I am a scientist and always come across statistics used to explain phenomena as best we can, but are ultimately deemed "random" usually because they have far too many nonlinear factors involved in any possible explanation. An example would be quantum physics where we cannot determine where an electron might be at any one time. However, I find randomness to be a stopgap only. While events may "randomly" occur in the view of current scientific understanding, this does not mean the system follows laws of randomness, whatever those might be. Instead, there are laws governing why that electron might be in one location versus another, but we lack the necessary tools to prove that scientifically. Overall, I believe that there is only a "perceived randomness" that we use as a tool to approximate the behavior of the system, no different than when we use finite difference methods to solve numerical simulations. We know that the output of a simulation is only approximate, but we can gain insight from using the output. I feel we should use the same approach towards "randomness" where it is only a tool of understanding and not necessarily a final classification of the behavior of a system since statistics is only a tool we use to gain insight into complex systems, and not a definitive solution. Does any one else think along these same lines? If not, please feel free to explain why. I'm always open to learning new things that I have not considered when it comes to what I believe. Edit: Thanks everyone for responding. I'm glad there are still places on Reddit to have logical, serious discussions, instead of just mindless bashing. I'm learning a lot from each of you! Thank you!
|
[
"The first thing that came to my mind to address your question was something that I learned from some of the physics panelists' posts: ",
"Bell's theorem",
", which seems to refute what you've suggested."
] |
[
"It's been a little while since I got my physics degree, but what you're suggesting is a well-known but physically refuted set of notions about quantum randomness, termed ",
"hidden variable theories",
".",
"Basically, the idea behind hidden variable theories are exactly what you suggest, ie, that seemingly-random quantum phenomena are based on deterministic hidden variables that we just don't know how to model or don't care to model because they're too complicated, but that a perfectly patient all-knowing scientist would be able to determine. However, hidden variable theories are demonstrably false: ",
"Bell's Theorem",
" proves that hidden variable theories imply certain experimental outcomes that do not occur.",
"Now, this only applies to quantum mechanical randomness -- it's physically real according to everything we know. However, in ",
", classical physics, complex systems theory, and all sorts of other fields that make use of randomness, approximate randomness is used to represent all kinds of predictable-but-too-darn-complex phenomena all the time! For example, weather phenomena: totally predictable if you had a powerful enough simulator; quantum effects don't matter at that scale and are dwarfed by classical interactions, but it's so darn complicated we can just throw in random numbers to simulate effects we can't compute. ",
"As you correctly observe, when using statistics in much of science (ie, psych, etc) we are required to do this much more often, for instance, assuming observations are drawn from a random distribution when in fact they actually occur completely deterministically. This isn't always an approximation: it's an appropriate use of randomness as a modeling tool to help us understand a system or how likely our experiments are representative of what's actually going on."
] |
[
"I wouldn't call that proof, more like a lack of evidence for determinism. We can apply statistics to highly chaotic systems like human behaviour and our brains (without getting into QM, although some scientists are trying to bring QM into neuroscience); that doesn't make us inherently random. Tossing dice or a coin is also described probabilistically, but unless you resort to QM, it's just very chaotic but still deterministic. ",
"Logically, randomness isn't ",
", because we could always be missing something (i.e. nonlocal hidden variables, retrocausality, and other speculative concepts , some of which we may not be able to conceive). The search can be futile, but there's no way of knowing that it is, only of knowing that it isn't, and, of course, this comes down to an argument from ignorance. Physics, for the most part, has moved on, and I'd say, is justified in doing so. However, I still think it's good that some physicists are still working on the interpretations, whether or not they're getting very far. "
] |
[
"What science related podcasts do you recommend?"
] |
[
false
] |
I like listening to general science based podcasts on my commute, but can't seem to find any more good ones on iTunes. I really like Skeptiod, but have listened to the entire back catalog. I also like Science Fantastic with Micheo Kaku, but they are only clips on iTunes. Any recommendations?
|
[
"Radiolab"
] |
[
"Neil deGrasse Tyson's StarTalk Radio Show is interesting and often humorous as his guest hosts are usually comedians."
] |
[
"This Week in Virology",
". It's a podcast by a virology professor with a few other co-hosts, including a science writer, other professors, and special guests. The discussion is much more in depth then a Radiolab-type podcast and might be initially daunting for a layperson, but they make a real effort to make the material accessible without sacrificing detail - if you hang in there you'll end up knowing more about the cutting edge of virology then you ever thought possible."
] |
[
"When marine mammals have open cuts or wounds does the salt water sting for them?"
] |
[
false
] |
I asked this in last night and got no answers so I decided to put it up here; I don't think I broke any rules...
|
[
"Yes, almost certainly. But they are to a degree desensitised to it after a life of living and getting injured in salt water. It’s difficult to measure long term discomfort from a wound in marine mammals because, as with most animals, they ignore the pain and just get on with things.",
"But yes, there is no reason why open wounds wouldn’t sting in the salt water for a while."
] |
[
"That is wrong, since the salts are dissolved, right? I'm pretty sure the pain is due to osmotic stress."
] |
[
"That is wrong, since the salts are dissolved, right? I'm pretty sure the pain is due to osmotic stress."
] |
[
"How does the stomach let through food and water, but not the acid?"
] |
[
false
] | null |
[
"The stomach doesn’t hold the acid back. First, the stomach makes a small amount of acid when you aren’t eating. When you do eat the acid production increases. However, it gets diluted buy the food/liquids you ingest along with your saliva and mucus. Second, your body neutralizes the acid as fast as possible once it hits the small intestine by secretions from the pancreas and liver. This is partially necessary because the enzymes that you use to digest food are destroyed or don’t work in an acid environment. Third, despite all these mechanisms the acid environment is still quite high in the first part of the small intestine (duodenal bulb) and this is a common place to get ulcers. ",
"The intestinal tract is actually very complex and amazing. The diseases of the intestinal tracts are also very diverse and complex. This is why I love being a gastroenterologist so much. "
] |
[
"Abundant Brunner glands in the duodenum also secrete bicarbonate to buffer the acid. There’s plenty of checks to prevent acidic damage. The GI tract is truly remarkable for its Neuroendocrine regulation. "
] |
[
"Abundant Brunner glands in the duodenum also secrete bicarbonate to buffer the acid. There’s plenty of checks to prevent acidic damage. The GI tract is truly remarkable for its Neuroendocrine regulation. "
] |
[
"Is it possible to not \"see\" a tangible object?"
] |
[
false
] |
Hallucinations are defined as "an experience involving the perception of something not present", so I was wondering if it were possible for a human to not "register" something that is physically present?
|
[
"It is absolutely possible. Attention gates our consciousness of visual phenomena - if our attention is directed elsewhere, our conscious mind often misses things. For demonstrations, check out this ",
"selective attention experiment",
" and this ",
"change blindness experiment",
"."
] |
[
"A local scotoma or hemianopia perhaps? Blindsight and spatial neglect are another two possibilities."
] |
[
"My pleasure! Many of the problems with eyewitness testimony are caused by memory issues induced by ",
"misinformation",
", but a weak initial memory trace due to inattention would certain compound them."
] |
[
"What does string theory does differently that current theories do not when dealing with singularities?"
] |
[
false
] |
From what I gather, in string theory we naturally unify QFT and GR, so does it solve the problems that we encounter at a singularity? If so, what explanation does it offer when particles are broken down to their fundamental bits inside a black hole. Please shed light on how our understanding of black holes, and singularities would further if string theory is indeed true?
|
[
"In string theory, black holes are actually p-branes. Normal particles are instead strings. When the strings are very close to the brane, it becomes possible for them to interact in a way that leads to no infinities. The interaction is the result of the contribution of all possible histories (as it's normal in a quantum mechanical theory) which can involve strings splitting or joining, string endpoints getting attached to the brane or detaching, some other things and all combination of this building blocks. The outcome is always finite because outcomes in string theory always are.",
"To give an intuition, the \"regulator\" that prevents the singularity from being truly singular is the finite size of strings which cuts the divergences short - intuitively the string cannot see things smaller than the string length. Instead a particle theory (such as local quantum field theories like the standard model) has point particles and will lead to infinities when placed near a singularity. "
] |
[
"So can you expand on it a little bit maybe? Like in QFT, you have quarks as fundamental particles, so what exactly would happen to the vibrating strings of a quark once it gets sucked into the black hole (or p-branes as you say)",
"They would join the brane and could generate one or more quanta living on the brane (that is, open strings with endpoints fixed to the brane) or also other free strings bouncing away. There is no single simple answer but it is possible to compute the finite probability for all possible outcomes.",
"Note that this is one possible picture of a black hole, in which spacetime isn't curved and the gravitational interaction is mediated by virtual gravitons (i.e. closed strings). You could also give a completely equivalent picture which is similar to what GR does for \"far-away observers\" in which there is curvature and a horizon - in this picture string theory predicts there is no inner region, or better the inner region is encoded holographically on a membrane above the horizon. Not crucial now but it is important to understand that things in string theory can have wildly differing interpretation while remaining the same physics.",
"Also, are you aware of any other mathematical solutions that prevent singularities, or have renormalized the equations other than string theory?",
"No. Such a theory would be a satisfying quantum gravity proposal, of which the only one known is string theory."
] |
[
"To give an intuition, the \"regulator\" that prevents the singularity from being truly singular is the finite size of strings which cuts the divergences short - intuitively the string cannot see things smaller than the string length.",
"So can you expand on it a little bit maybe? Like in QFT, you have quarks as fundamental particles, so what exactly would happen to the vibrating strings of a quark once it gets sucked into the black hole (or p-branes as you say)",
"Also, are you aware of any other mathematical solutions that prevent singularities, or have renormalized the equations other than string theory?",
"Lastly, thank you for your reply to my query, it was very helpful."
] |
[
"When I'm scuba-diving, I'm breathing pressurized air. Why don't breaths last longer when I'm deeper ? Don't they contain more oxygen ?"
] |
[
false
] | null |
[
"The urge to breathe is controlled by rising levels of carbon dioxide, not falling levels of oxygen. This is why you can pass out if you hyperventilate - you're lowering the CO2 levels to such a low level that you can run out of oxygen before you feel the need to breathe.",
"So while it's true that more oxygen molecules are entering your lungs, you are still producing and needing to get rid of carbon dioxide.",
"I am not a doctor, but I do know quite a bit about diving physiology."
] |
[
"The urge to breathe is regulated by both CO2 and O2 levels. The body responds much more rapidly to high CO2 levels than to low O2 levels.",
"Otherwise, you are right on the money. The CO2 builds up, which can cause a big problem. This is the principle behind ",
"rebreathers",
", which 'scrub' CO2 from the exhaled gas and recycle it."
] |
[
"http://en.wikipedia.org/wiki/Hypoxic_drive"
] |
[
"Why doesn’t diesel have different grade levels at gas stations like gasoline?"
] |
[
false
] | null |
[
"Gasoline engines compress a mixture of vaporized gasoline and air, and ignite that mixture with a spark plug in an explosion that pushes the piston back down. Now, compressing a gas heats it. That can lead to the mixture detonating prematurely, before the piston has reached the top, in which case explosion will try to push the piston back down in the wrong direction. That's called engine 'knocking'. Obviously that's not helping you move forward, at best, and can destroy the engine at worst. ",
"What the octane number measures, is how much compression the gasoline can handle before it causes knocking. (relative a kind of fictional mixture of octane, hence the name. It's a weird unit) Higher compression ratios means a more efficient engine, but not all engines can handle it. (so you can use high-octane fuel in a \"low-octane\" engine but not vice-versa)",
"Diesel engines don't work the same way. As many know, diesel fuel is much less volatile. It doesn't need to be volatile, because the fuel is not vaporized before going into a diesel engine cylinder. A diesel compresses the air and then injects an aerosol of tiny fuel droplets, which then combust from the compression heat (no spark plug needed), so you don't have the issue of knocking in the same way. Diesel engines can knock, but then it is caused by the fuel igniting too long after injection."
] |
[
"By way of explanation: diesels spray a precise amount of fuel directly into the cylinder at very high pressures. In modern electronic controlled diesels there may be as many as 20 injections in one combustion cycle. They don't use a spark for ignition, instead, rely on having a much higher compression ratio to preheat the intake air.",
"The high pressure and temperature makes the fuel spontaneously combust.... usually.",
"Diesel fuel does typically have what's called a \"Cetane\" rating. This is an inverse of the measurement of the ignition delay. Specifically, that's the time between the first injection and when pressure and temp in the cylinder actually starts to rise. ",
"Having a higher cetane means a shorter ignition delay, and a lower cetane means a longer delay.",
"In a way, cetane is analogous to the opposite of an Octane rating which measures the ability of gasoline to ",
" spontaneously combusting at elevated temperature and pressure.",
"In most US states, the minimum Cetane number is 40, and typical fuels sold will be around 42-46. I don't think most state laws requires stations to display the Cetane on a sticker on the pump. They just buy what's cheap. ",
"Having a low cetane only tends to cause problems when starting in freezing weather, or at rather high engine speed. ",
"In the former, the cold cylinder walls and very slow cylinder speed cool the air charge while it's being compressed. Thus the temperature at Top Dead Center is significantly reduced to the point of failing to cause ignition.",
"In engines operating a high speed, there may not be sufficient time for low cetane fuel to burn completely before the exhaust stroke begins. This results in loss of power, high exhaust temperature and excess soot."
] |
[
"Thank you for explaining this!"
] |
[
"Why is ADD and ADHD so common now?"
] |
[
false
] |
I have read it affects somewhere from 5-9% of the population. I myself have ADHD, and have been prescribed Ritalin. However, my diagnosis wasn't based on cognitive ability/other factors, but rather on questions such as "Do you feel so and so when you do so and so?". Is it just overdiagnosed, it is because of television and the internet, or what? Also, is there any way to cure AD(H)D? Edit: Wow, this question got more attention than I thought it would O__O. Don't bother answering the second question if you're going to contribute. Also, could anyone explain the causes behind ADD/ADHD?
|
[
"[Master's in Clinical Psychology, working towards Doctorate]",
"Go, go, EBSCOHost!",
"QUESTION 1",
"Sciutto, M. J., & Eisenberg, M. (2007). Evaluating the evidence for and against the overdiagnosis of ADHD. Journal of Attention Disorders, 11(2), 106-113. doi:10.1177/1087054707300094",
"LeFever, G. B., Arcona, A. P., & Antonuccio, D. O. (2003). ADHD among American Schoolchildren: Evidence of Overdiagnosis and Overuse of Medication. The Scientific Review of Mental Health Practice: Objective Investigations of Controversial and Unorthodox Claims in Clinical Psychology, Psychiatry, and Social Work, 2(1), 49-60. Retrieved from EBSCOhost.",
"Chilakamarri, J. K., Filkowski, M. M., & Ghaemi, S. (2011). Misdiagnosis of bipolar disorder in children and adolescents: A comparison with ADHD and major depressive disorder. Annals of Clinical Psychiatry, 23(1), 25-29. Retrieved from EBSCOhost.",
"QUESTION 2",
"No, there is no cure. Symptoms can be most effectively managed with a combination of psychotherapy and psychiatric medication."
] |
[
"Don't forget about the chance of misdiagnosis as ADD/ADHD when the child is actually intellectually gifted/talented. The ",
"list of characteristics for each",
" has a lot of overlap. Also, sorry I don't have a better link than that - most of my knowledge comes from books like ",
"Webb's",
" on Misdiagnosis & dual diagnoses."
] |
[
"From an epidemiological perspective, one of three things could be going on... ",
"The actual incidence of true ADD/ADHD may be increasing. If this is the case, it would be helpful to identify what causes ADD/ADHD. Unfortunately, there is probably no one cause, nor is the exposure to one of those 'causes' guaranteed to result in ADD/ADHD, so research will usually focus on exposures that increase the RISK of ADD/ADHD. I'm not an ADD/ADHD expert, so maybe someone else can speak to risk factors.",
"Another possibility is that the rate of ADD/ADHD has always been what it is now, we just didn't have sensitive enough tools OR wide enough screening programs to detect it. This would be a false negative or simply a failure to test.",
"The third option is what you suggested... that it is simply becoming more and more incorrectly diagnosed. These are what we call false positives. Given the medical culture today's tendency to issue prescription medication to solve mental/behavioral problems rather than refer to the appropriate specialist / address the underlying issues, I'd tend to think that this option is the most likely culprit. (in America, at least... I can't speak for internationally) "
] |
[
"Why aren't quadricopters more common in full-scale?"
] |
[
false
] |
As common as quadricopters seem to be for drones, why haven't we seen a larger adoption of that format in full-scale? The advantages of the platform (easy to fly, stable, maneuverability) would seem to be applicable to some roles that helicopters or airplanes serve now. Is there an inherit disadvantage to the 4 rotor setup in large scale, or do the advantages just not outweigh the drawbacks for most usage?
|
[
"Quadcopters are great in the hobby scale since they are incredibly maneuverable and can be relatively inexpensive.",
"The maneuverability does not scale up. Just as the surface area to volume ratio of a cube will drop as the cube gets bigger, the ratio of rolling/pitching/yawing power to moment of inertia goes down as well. Also, the weight tends to scale up with length",
", while lift tends to scale with length",
". This makes larger quad copters less maneuverable than their tiny counterparts, and explains why a lot of the leading work on quad copters is done with very tiny ones—they have the best maneuverability.",
"As for cost, a lot of that comes from the fact that you can use brushless DC motors directly driving rigid propellers, then vary the speed of the motors for maneuverability. By contrast, a conventional helicopter has to change the pitch of its blades as they sweep out each arc of rotation. The latter configuration is fairly expensive to create for a hobby craft (although there are certainly RC conventional helicopters with that kind of mechanism; I should note that the ",
"cheap tiny RC helicopters",
" are using something similar, but it's passively controlled by a weighted bar instead of by pilot input). When you scale up to full-sized aircraft you're going to be spending a lot on the rotors anyway, so it can be cheaper to use one complicated rotor instead of four rigid ones. "
] |
[
"Single rotor aircraft can already lift incredibly heavily loads and for even heavier loads we have dual rotor craft such as the Chinook. There are few needs for an aircraft with 4 rotors. The extra weight and extra fuel makes such a large aircraft less efficient. If you research the history of the V-22 you'll find there are many difficulties with multiple rotor aircraft. The multiple rotors can cause certain air patters that cause large aircraft to fall out of the sky, these do not influence small drones significantly if at all."
] |
[
"Koooooj got the answer right here, but I'd like to say something regarding your assumptions. Quadcopters are most certainly NOT easy to fly, nor stable. They only seem that way due to a generous amount of gyro's and stabilization software. Aircraft in contrast can be (and usually are, except for fighter jets and the like) inherently stable, and will simply fly on if you release the controls. From what I've heard from heli pilots a helicopter is quite stable when flying with a decent forward speed, but very hard to keep still while hovering."
] |
[
"What are physicists' best ideas for solving the proton radius puzzle?"
] |
[
false
] |
When you measure the proton radius by firing electrons at it, you get a different value than if by firing muons. Currently, this is unexplained by the standard model and there is no widely accepted explanation as to why this should happen. Despite this, what believable interpretations have physicists postulated since?
|
[
"Yes, I beleive its about 5%, but the quoted ucertainties from the relevant measurements are ~1%, perhaps less. In otherwards the discrepancy of about 5% is significantly larger than the variation one would normally expect to see given the stated precision of the measurements. "
] |
[
"Of course you are completely correct that protons are not classical hard balls of matter, but that doesn't really address the issue that OP is asking about.",
"As far as I understand the proton radius puzzle comes from disagreements about the charge radius, specifically measurements which are used to infer the root-mean-square value of the charge distribution. From my limited knowledge the value as inferred from electron proton scattering vs from energy levels of muonic hydrogen differ by something similar to or north of 5 standard deviations. ",
"So far as I can see the rms of the charge distribution is a perfectly well defined quantitty. Nowithstanding the (extreme) difficulties of calculating anything in non-perturbative QCD, I see absolutely no reason why you shouldn't be able to tease such a radius out of the mathematics. The fact that it is not the same thing as the radius of a hard ball is (mostly) irrelevant.",
"That being said I have no particularly useful insight into the proton radius puzzle. If I had to guess I would guess that it will be resolved by improved modelling/experiments and not by fundamentally new physics. That shouldn't be taken as a criticism of anyone involved, nor is it a deeply held or researched position. It just stems from general observations of how these measurements tend to develop. The discrepancy is strong, but these types of experiments tend to have many potential sources of these discrepancies, and uncertainty estimates are not (and cannot be) as bullet proof as everyone would like. ",
"Hopefully someone with more knowledge of the problem than me can give a more insightful answer to OPs question."
] |
[
"Of course you are completely correct that protons are not classical hard balls of matter, but that doesn't really address the issue that OP is asking about.",
"As far as I understand the proton radius puzzle comes from disagreements about the charge radius, specifically measurements which are used to infer the root-mean-square value of the charge distribution. From my limited knowledge the value as inferred from electron proton scattering vs from energy levels of muonic hydrogen differ by something similar to or north of 5 standard deviations. ",
"So far as I can see the rms of the charge distribution is a perfectly well defined quantitty. Nowithstanding the (extreme) difficulties of calculating anything in non-perturbative QCD, I see absolutely no reason why you shouldn't be able to tease such a radius out of the mathematics. The fact that it is not the same thing as the radius of a hard ball is (mostly) irrelevant.",
"That being said I have no particularly useful insight into the proton radius puzzle. If I had to guess I would guess that it will be resolved by improved modelling/experiments and not by fundamentally new physics. That shouldn't be taken as a criticism of anyone involved, nor is it a deeply held or researched position. It just stems from general observations of how these measurements tend to develop. The discrepancy is strong, but these types of experiments tend to have many potential sources of these discrepancies, and uncertainty estimates are not (and cannot be) as bullet proof as everyone would like. ",
"Hopefully someone with more knowledge of the problem than me can give a more insightful answer to OPs question."
] |
[
"Why do sockeye salmon rot, while still alive, during their trip upriver to spawn?"
] |
[
false
] |
I'm watching an episode of Monster Fish, and they briefly touched on it, but didn't really give any explanation for it. Just that the salmon "undergo a dramatic change" during their trip upriver, and the flesh literally rots off of them by the time they're done. Why does this happen? And does it provide some kind of an evolutionary advantage?
|
[
"Pacific Salmon go on a one way trip to spawn. Once they start upstream they stop eating, but still need large amounts of energy to get up those turbulent cataracts. That energy has to come from somewhere, so their bodies strip off the body fat and starts to eat itself from within to supply the body with nutrients to complete the swim and spawning. Then, body spent, they die and add nutrients to the stream ecosystem, and bears.",
"The advantage is that it gives them enough energy to get to the safe spawning zones so they can spawn wee baby salmon, once theyve done that they have no further use."
] |
[
"Nature tends to take the most economic path in the end. If you have problem which requires a fish to spend immense effort to swim up a difficult stream as quickly as possible and spawn, after which the fish has no further purpose, then this is a quite efficient way of doing it.",
"Initially they all survived, but then those which spent the most effort and ate the least (time wasted) got to the head of the stream first and produced more offspring, even though they were then weaker and more prone to dying younger due to the effort. Over millions of generations their descendants pushed this solution to a problem to the extreme. Slow fat healthy fish who took the time to eat well along the way just got their too late."
] |
[
"Is this different from fasting?"
] |
[
"How can we experience such seemingly long dream sequences if REM Sleep cycles only last 10-15 minutes?"
] |
[
false
] |
I may be wrong, but it's my understanding that there is REM sleep during which dreaming occurs which lasts about 10-15 minutes, then we go through Stages 1-4 then 4-1 of the sleep cycle before we get back to REM sleep. Furthermore, we very rarely can recall our dreams unless we are woken up during REM sleep. I'm perhaps incorrectly presupposing that dream sequences are not continuous and related from one period of REM sleep to another because of the long period of time separating them. So, my question then is how can we experience "events" during dreams that we perceive to elapse over a significantly longer period of time than actually passes in the real world? Thanks in advance for anyone who takes the time to read and respond.
|
[
"There seem to be a few potential misconceptions here, so let me add some thoughts.",
"1) As has been pointed out, REM sleep bouts are short early in the night but become longer towards the end of the night.",
"2) It is not usually true that \"we go through Stages 1-4 then 4-1 of the sleep cycle before we get back to REM sleep\". Early in the night, we will usually get to stage 3 (note that there is no such thing as stage 4 anymore, as it had been combined with stage 3 now), but after the first couple of cycles, almost all of NREM sleep is spent in stage 2, with some time spent in stage 1.",
"3) Subjects report dreaming about 90% time of the time when woken from REM sleep. But it is not true that people only dream in REM sleep. Dreaming can also occur in NREM sleep (e.g., ",
"http://psycnet.apa.org/psycinfo/1993-20351-001",
"). In general, the more vivid and detailed dreams are associated with REM sleep, but it is definitely not the only stage in which dreaming can occur.",
"4) People tend to only remember dreams upon awakening, not because those dreams are necessarily special, but because short term memory does not function normally during sleep. People will only tend to remember a dream if they are awake for a while after it. Most people wake up briefly 10+ times per night, but tend not to remember these brief awakenings. People also often report not remembering things that happened just prior to falling asleep or during the night (e.g., getting up to answer the phone) for the same reasons."
] |
[
"Sorry for not originally posting sources, most of that was based on memories of college courses. I should have looked this up before posting, now I feel silly. For the real-time aspect of dreams, Daniel Erlacher & Michael Schredl conduct a study in which they state: ",
"Nowadays, the hypothesis is widely accepted that the subjec- tively experienced time in dreams corresponds with the actual time (overview: Schredl, 2000). This relationship was first experimentally demonstrated by Dement and Kleitman (1957). In this study, the participants were awakened in a random order either after 5 or 15 minutes of REM sleep. After awakening, participants were asked to estimate whether the elapsed sleep interval was 5 or 15 minutes. From 111 awakenings, 83 % judgments were correct. Furthermore, the elapsed time of the REM period correlated with the length of the dream report (from r=.40 to r=.71). The latter findings were re- plicated by Glaubman and Lewin (1977), as well as by Hobson and Stickgold (1995). Rosenlicht, Maloney, and Freiberg (1994) found only small differences between time of REM sleep and the reported length of dreams. ",
"I couldn't find that paper to view online, but here's a ",
"download link",
" for anyone interested. The quote above is from section 4.1. The section goes on to mention their own findings. ",
"I believe it stands to reason that if dream-time, for the most part, equates to both real-time and REM-time (as the quote above states) that the average length of a REM cycle and thus the dream would be five to thirty minutes, maybe closer to twenty five. Though I could be dead wrong here, that's just how I viewed it. I found a lot of claims that confirm that particular length of time for dreams, but I can't find seem to find the original study by Domhoff. Most studies prefer to measure length of dreams by the report size."
] |
[
"Your view of the sleep cycle is pretty close, here's a ",
"handy little chart",
" showing a breakdown of the different cycles over six hours. The first REM cycle lasts about ten minutes, with each successive REM cycle lasting longer. The longest REM cycles are close to an hour in length. ",
"To answer your question, dreams will last between five and thirty minutes and occur real-time. So a dream can exist in any single REM cycle, though the more memorable ones occur during the longer cycles at the end of the night. Concerning perceived length vs. actual length, time perception is very subjective. So perceiving a dream to last longer than it really does is common. I'm sure someone else could go into better detail about time perception. ",
"Also, you state you'll rarely remember a dream unless woken in REM sleep. Instead, it's more accurate to say your dreams will be much more vivid when woken up during REM."
] |
[
"Just read about denaturation. Question about fevers"
] |
[
false
] |
So I'm taking Biol 190 and just read about how proteins can undergo denaturation. As I have understood it, this is basically the proteins losing their shape and becoming useless. the book then mentions that high fevers are fatal because the proteins in the blood become denatured at the high temperatures. I know that ice packs are usually put on a person's body to help cool them off. I was wondering if injecting a cooled saline solution into the blood stream would also work or work quicker? Or if that would cause more damage?
|
[
"Perfusion would be the act of pumping blood into the organs. Vasoconstriction can lead to multitudes of issues - by definition, it would mean the \"tightening\" or \"narrowing\" of blood vessels. ",
"You have it right - if we vasoconstrict someone's vessels by using vasopressors, we pharmacologically increase someone's overall blood pressure. The reason why decreased perfusion is bad is because it means that blood is not reaching the organ well, and with that comes decreased organ function."
] |
[
"Thank you. I didn't think the body would react well to basically cold water being injected into. But there was some gray area in my head. Thank you for clearing that up. ",
"Follow up question. What is perfusion? Also I can gather what vasoconstriction means, but why exactly is it bad? I think its bad because it would decrease blood flow and increase blood pressure. But I would like some clarification on exactly why if you don't mind :D",
"EDIT: I realize that vaso probably refers to the vascular system. I know the circulatory system is vascular, but I'm not sure if anything else falls into that category as well. "
] |
[
"Thank you. I didn't think the body would react well to basically cold water being injected into. But there was some gray area in my head. Thank you for clearing that up. ",
"Follow up question. What is perfusion? Also I can gather what vasoconstriction means, but why exactly is it bad? I think its bad because it would decrease blood flow and increase blood pressure. But I would like some clarification on exactly why if you don't mind :D",
"EDIT: I realize that vaso probably refers to the vascular system. I know the circulatory system is vascular, but I'm not sure if anything else falls into that category as well. "
] |
[
"Does turning a box of icecream on its side with the top off make it melt faster?"
] |
[
false
] |
I have been pondering this with my grandpa. He usually does this to make it melt faster. It does seem logical; given that cold air is heavier than warm air, it would presumably "spill" out, or the air that has been cooled in the heat exchange will fall down and allow new warm air to exchange heat. But maybe the thermodynamics are more complicated than that.
|
[
"It's more likely just about contact area. Ice cream cartons sit on a ridge so the only contact between the cold ice cream and the countertop is a thin ridge of material and so heat is not transmitted into the ice cream very quickly. . When your grandpa turns it on its side then the cold carton is touching the countertop directly and so heat can be transmitted more quickly into the ice cream."
] |
[
"I agree with the argument saying that the contact with countertop increases temperature. ",
"Another reason could be that you’d increase the pressure applied on the ice cream towards the side of the box, since that side would be supporting more of the weight. As pressure decreases the melting point, this might also cause the ice cream close to that side to melt faster. Sure, if you didn’t turn the box on its side and kept it upright, the same would happen with the ice cream at the bottom. But, unless you’re already at the end of the box, they are not accessible, are they? Sides, on the other hand, are completely accessible and yours to enjoy."
] |
[
"I’d guess the convection properties of the cold air spilling over the edge and being replaced with ambient air probably make it a wash",
"Perhaps.",
"But ",
"this video",
" from Technology Connections makes a pretty good argument for why chest freezers are so efficient. In short: dense, cold air easily spills out of front-opening refrigerators and freezers, whereas the cold air simply sits on top of a top-opening freezer. And it stands to reason that an open box of ice cream would exhibit the same effects - turned on its side, an open box of ice cream allows the cold air to escape, which is replaced with warmer ambient air, which in turn allows the ice cream to warm faster.",
"Though I also like ",
"u/Dagkhi",
"'s theory. The ridge on the bottom lessens contact with the countertop; placing the box on its side maximizes contact. "
] |
[
"[Earth sciences] I have some trinitite. How would I check if it's legit?"
] |
[
false
] | null |
[
"Use a geiger counter. Or put it in a very dark room with no light and look at it with your cell phone camera. If you see little distinct white dots in the picture it is emitting beta/gamma radiation that can be picked up by the camera's CCD."
] |
[
"Thank you! I'll check this out!"
] |
[
"/r/whatsthisrock",
" is probably a good place to ask about identifying rocks."
] |
[
"Why isn't CO2 visible?"
] |
[
false
] |
I happened across a of CO2 which included the very end of the visible spectrum. It seems to show CO2 absorbs light in the 630-700 nm wavelength, at least somewhat. I'm curious why, if CO2 seems to absorb some visible light, high concentrations of it are not visible as bluish/cyan gas (white light removing the deep reds). Is there something I am missing here? What led me to this was an interest in replicating the sort of things shown or . These all seem to use mid wave IR and a narrow bandpass filter. I would imagine that if a narrow bandpass filter around 650 nm on a regular camera would let you see CO2, they would have done that instead. But I don't see why it wouldn't work. : As Shookfoot notes below, the units on the graph are wavenumber, not nanometers wavelength. As such, the absorption isn't in the visible spectrum at all.
|
[
"Hi there! The answer to this question is about the units of the absorbance spectrum you have shown us. The units are in wavenumbers, not nanometers. 600 wavenumbers is approximately 17,000 nm. Hope that clears things up!",
"Source: aspiring chemist. "
] |
[
"That makes perfect sense, thanks! The simplest answer is I just misread the units."
] |
[
"The unit cm",
" is commonly pronounced as \"wavenumber\", so it's not unreasonable for someone to write it out that way."
] |
[
"2 'prompt critical' reactions were caused by accident during experiments on the 'demon core' in 1945-1946."
] |
[
false
] |
[deleted]
|
[
"Nuclear chain reactions are full of negative feedback mechanisms. This is what makes building a nuclear bomb so difficult. In the case of the ",
"demon core",
" accidents, the system is very rapidly increasing its rate of energy release (doubling on a sub-millisecond timescale). All this heat and energy tends to disassemble the critical system.",
"So, had the experimenters involved in both accidents not disassembled the system themselves, the energy release would have stopped the chain reaction. ",
" As the critical system releases more energy, it tends to separate its own components through various mechanisms, and the end result is a cessation of the reaction.",
"However, if the reaction had proceeded longer, significantly more energy would have been released. This may have resulted in melting/vaporization of some of the components, and would have almost certainly lead to the deaths of the other exposed people who survived the accident (due to increased radiation)."
] |
[
"if I have a huge mound of gunpowder, and I light a little bit of it, only a small fraction of the gunpowder will detonate - the rest will be scattered by the energy release and not explode. ",
"As someone who's lit a pile of gunpowder, I can tell you from first hand experience that the gunpowder will all go up in a flash. Perhaps a better analogy might be a pile of firecrackers that are not bound together?"
] |
[
"Yeah, not the best analogy. I'll leave it to other people to think of something better."
] |
[
"In double-blind studies measuring effectiveness of birth control, does the control group receiving the placebo have affected pregnancy rates?"
] |
[
false
] | null |
[
"Depends what you mean by placebo affect. Medically it means all the non-specific effects of a trial. For example participants might be having regular check ups with doctors as part of trial, or be told not to drink during a trial, which would all be part of the 'placebo effect'. A lot of people use the term to just mean the effect that taking a placebo actually has on a person. However it appears this generally only has an effect on subjective reported outcomes. For example if you ask people whether they have fewer wrinkles after taking a placebo they will say yes, but if you record the number of wrinkles before and after there generally won't be a difference. I certainly know of no studies that show a 'placebo effect' on something as objective as getting pregnant."
] |
[
"Affected compared to what? If compared to the general population than it would typically be different since the study participants aren't a perfect sampling of the population. If compared to the pregnancy rate of the participants prior to starting the trial than it would also typically be affected since the behaviour of participants is generally different when they are in a clinical trial than not. If you are asking whether taking a sugar pill can in itself affect pregnancy rates I don't believe there is any evidence of that and it seems theoretically rather unlikely."
] |
[
"To be fair the placebo effect seems theoretically unlikely also, and yet there it is."
] |
[
"Are drops of liquid (say water) limited in possible size, or can they grow very large, depending on circumstances?"
] |
[
false
] |
[deleted]
|
[
"As drops grow there volume/mass increases by a cubed factor as the surface area increases by a squared factor. drops are held together by a combination of surface tension and adhesion to the surface they are fixed. As drops get bigger the stress per surface are increases until the tension is no longer supported by the surface tension value and the drop separates. as far as drops falling at similar sizes in the shower same effect just that the adhesion of water drops is a function of surface area contact which increases slower than the mass as the drop gets bigger until it starts sliding."
] |
[
"Although there would still be a limit to the size of a liquid planet. At some point the middle of it would be compressed into a crystalline structure."
] |
[
"Do you mean their?"
] |
[
"Stumped by my 1st grader's potato battery. Am I using the right kind of voltmeter?"
] |
[
false
] |
[deleted]
|
[
"if you can't get a battery to read (assuming it's not dead) then you're not using it properly. Can't get far without this, so break out the instructions, try a few things and get that reading. "
] |
[
"if you can't get a battery to read (assuming it's not dead) then you're not using it properly. Can't get far without this, so break out the instructions, try a few things and get that reading. "
] |
[
"Did you test the voltmeter against a battery to make sure it's working?"
] |
[
"Why are old buildings so good at blocking phone signals?"
] |
[
false
] | null |
[
"This is labeled a physics question, but I think it's just as much a structural engineering question. ",
"Ignoring ancient Roman buildings, modern concrete construction really began in the late nineteenth and early twentieth centuries. However, construction isn't a fixed thing. Like every kind of technology, it improves over time. Some things that have improved:",
"Stronger concrete: we've learned a lot about designing higher-strength concretes. A modern high strength concrete commonly used in large buildings will have a strength three times that of concrete used in the early twentieth century. Today, we can use less concrete to carry the same amount of load. Concrete construction today is significantly more slender than that of a century ago.",
"Stronger steel: steel has also increased in strength. Our metallurgical and manufacturing technologies have greatly improved, at least doubling the strength of steel reinforcement compared to the early twentieth century. Since the reinforcing bars are stronger, we don't need as many of them. As a conductor, steel will do a lot to block radio signals, so less rebar means less radio signal blockage.",
"More advanced design: when concrete construction was in its infancy, engineers didn't know as much about how concrete buildings perform and age as we do today. They also lacked modern computers to perform complex structural analysis calculations. They had to use large factors of safety to make up for this imprecision in design. There's an old civil engineering adage, \"when in doubt, make it stout!\" Those old designers took this quite literally.",
"All of these together mean that old concrete buildings are substantially heavier and have substantially more steel in them than current buildings of the same size. Thicker walls and floor slabs between your cell phone and the tower means lower quality cell service. "
] |
[
"Depending on how old we're talking, it could also be lead paint in the building."
] |
[
"What about brick? Does the iron oxide in brick have any effect?"
] |
[
"How do we know that Polio has been eradicated from North Korea? From uncontacted groups in the Amazon?"
] |
[
false
] |
, Polio was eradicated from Brazil around 1990 and North Korea in the mid-1990s. But how do we know this? Does North Korea allow disease surveillance by international organizations? And what about in the Amazon? How do we know that they don't harbor Polio?
|
[
"You can find a high-level overview of what is involved in polio-free certification for a region (it's region-based, not country-based) on ",
"this WHO page",
". The short answer is that, yes, North Korea does cooperate with the WHO, and the extent of that cooperation is documented in cooperation briefs also ",
"available from the WHO",
".",
"(I don't have an answer about the Amazon off the top of my head.)"
] |
[
"They're uncontacted by Europeans directly, but presumably they're in contact with other tribes that have been contacted."
] |
[
"I would think that uncontacted groups in the Amazon never were exposed to the polio virus in the first place."
] |
[
"Why is Dexamethasone used for severe covid patients if it has immunosuppressant effects?"
] |
[
false
] |
[deleted]
|
[
"It's not counterintuitive when you think about what corticosteroids actually do. They don't directly immunosuppress you. They dampen your immune response by preventing inflammatory mediator release, including cytokines. Your body responds to COVID by producing these inflammatory mediators - that's what's causing airway edema, free radical generation, and tissue damage as your body tries to kill it. Dexamethasone suppresses that."
] |
[
"To build on your comment, think about the long term effects of inflammation on highly vascular tissue, such as the lungs. When inflammation becomes sustained, eventually scar tissue builds up over time.",
"Dexamethasone is a powerful tool to help fight inflammation and it's downstream adverse effects."
] |
[
"I dont know about the nitric oxide, but COVID-19!is characterised by two phases: the viral phase and the hyper inflammation phase. The people who become severely ill usually fall into the latter. The immune system has become dysregulated. Cytokine storm is an extreme form of that and not all people get it. However, by the time you start requiring oxygen the body has likely crossed over into the hyper inflammation and at that point dexamethasone is beneficial by limiting the production of proinflammatory cytokines and preventing some complications. We are essentially accepting the immunosuppressive effects of the drug for its anti-inflammatory outcomes. Think of the immune response to SARSCoV2 as releasing a serial killer to kill the bogey man. Once he’s incapacitated, then the serial killer is the problem that remains. Dexamethasone helps contain the lung damage caused by a dysregulated immune response to the virus",
"Edit: for clarity purposes"
] |
[
"Did inflation of the early universe stop or did it just slow down?"
] |
[
false
] |
Is the current expansion of the universe a remnant of initial inflation phase or is dark energy a different mechanism? If they are the same mechanism does that mean that the rate of inflation is decreasing? If inflation stopped ... why?
|
[
"In the most accepted models, Inflation was caused by an excited exponential scalar field , and stopped when it rolled down to its ground state."
] |
[
"Like the Higgs field, or is there a different scalar field?"
] |
[
"A different field:",
"http://en.wikipedia.org/wiki/Inflaton"
] |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.