Wikipedia:Reference desk/Archives/Science/2017 January 19

= January 19 =

Matter is liquid... at zero temperature?
Have a read of this link: http://www.aleph.se/Trans/Global/Omega/dyson.txt

at section "F. Matter is liquid at zero temperature"

It basically explains how quantum tunnelling will result in the rearranging of atoms and molecules in a solid over a time scale on the order of 10^65 years, so that the solid behaves like a liquid.

What I'm puzzled by is why he qualifies it with "at zero temperature". Why does the logic depend on the object being at zero temperature?

101.178.204.3 (talk) 00:39, 19 January 2017 (UTC)
 * Because at any temperature above absolute zero, the atoms of a solid would slowly have a non-zero change to rearrange on their own due to the random motions associated with thermal energy. At absolute zero, the atoms of the solid should remain where they are put.  See also Third law of thermodynamics for some discussion of the importance of absolute zero to a problem such as this.  What he is saying is that even AT absolute zero, quantum effects like tunneling will cause atoms to move over very long time periods.  The calculations show a non-zero likelihood of atoms remaining in motion at zero temperature, however the effect is practically insignificant.  After all, the universe is only 10^10 years old, so we're talking about a time scale which is older than 10^55 times older than the universe itself.  In other words, if the entire universe were only a second old, instead of 14 billion years, you would still have to wait like a billion billion billion billion billion billion years to see it.  Or in other words, don't hold your breath.  -- Jayron 32 02:19, 19 January 2017 (UTC)


 * Is Bose-Einstein condensate relevant ? StuRat (talk) 02:21, 19 January 2017 (UTC)
 * Not in any way. The main article is mostly about the behavior of normal matter.  The article is mainly discussing the issue the heat death of the universe, the idea that the Ultimate fate of the universe is an asymptotic condition and not a state we would ever reach.  -- Jayron 32 02:27, 19 January 2017 (UTC)


 * (ec) Dyson applies the analysis assuming tunneling of atoms when each atom is in the ground state of the local potential. If this was happening at finite (nonzero) temperature, he'd have to also consider the actual distribution of atom energies (or, equivalently, the phonon mode populations), which may substantially increase the rate of defect creation and shorten the estimated time-scale far below 10^65 years. Actually, even at zero temperature there still are null-oscillations; indeed, atoms cannot stay perfectly still at their exact minima of potential, as that violates the Heisenberg principle. Null-oscillations can make this time-scale much, much shorter if the atoms are very light or the density is very high (a so-called Kirzhnits liquid). See also liquid Helium, for some very interesting physics of an actual absolute-zero temperature liquid. To your question, StuRat: Bose-Einstein condensation occurs when particles (or particle pairs, e.g. Cooper pairs) are indistinguishable bosons. Dyson does not consider BEC in his analysis (it is irrelevant for his purpose), but both Helium-4 and Helium-3 do undergo it.  Helium-3 atoms are fermions, but their pairs are bosons. Dr Dima (talk) 02:31, 19 January 2017 (UTC)

What does it mean "generation number" in histology?
I saw this diagram and one of the column is about so called "generation number". What does it mean? 05:10, 19 January 2017 (UTC)


 * This is described in Respiratory_tract. Each branch point (going down) is declared to start a new generation. Someguy1221 (talk) 05:19, 19 January 2017 (UTC)


 * Sounds related to the term fractal dimension in fractal geometry. 14:47, 19 January 2017 (UTC)

Why is the benzyl group a phenyl group with atoms added?
Phenol is benzene with atom added. Shouldn't the phenyl group be called the benzyl group and the C6H5CH2 group be called toluyl or methylbenzyl? Sagittarian Milky Way (talk) 06:26, 19 January 2017 (UTC)


 * There is no "should" in chemical naming. They are lucky enough to make it consistent at all, and actually last time IUPAC put out a carefully checked nomenclature manual they had to publish a list of hundreds of errata afterward.  If we called things what we "should" call them, maybe we would invent a beautiful musical system that permits us to sing SMILES structures euphoniously, and finally understand some of those things the stars have been beaming at us.  Or not.  Anyway, phene is another name for benzene.  Phene was a compound obtained from some illumination based process, while benzene was the result of decarboxylation of benzoic acid, which is named because it is the acid gotten from gum benzoin.  The people giving these names were not NMR experts, or they would not have been making up names as they went along.  Anyway, benzoic acid can also be conjugated to things as a benzoate, i.e. benzoyl.  I assume but have not actually looked up that at some point this permitted someone to make a carbon-carbon bond to the acyl halide or something and they decided to call that "benzyl"?  And phenol and phenyl are to phene as ethanol and ethyl are to ethane, so no apologies needed there.  Anyway, there's a kind of sense to it, but it's more a historical sense than a regular kind of sense. Wnt (talk) 12:08, 19 January 2017 (UTC)


 * Because linguistics develops to some extent at random. There was, at the time, no plan forward.  Things were named along the way as they discovered things, and the names are retained because they are familiar and well used, not because they follow strict rules.  That we later discovered that we could have done things better had we known before we started naming things is immaterial, and really not useful, because if we did suddenly start renaming things according to some perfect system, then we confuse the hell out of people who are working from established forms; the literature has always used certain terms, and if we up and changed them, that creates more problems than it fixed.  That's why even though IUPAC system would mandate things like "ethanoate" as a proper name, no chemist or major chemistry publication uses that word over acetate.  -- Jayron 32 13:35, 19 January 2017 (UTC)

Why aren't carboxylic acids COOH molecules literally COOH?
H | H-C-H | H-C-H | O  | O | H

Instead it's

H | H-C-H | O=C | O  | H

Sagittarian Milky Way (talk) 07:12, 19 January 2017 (UTC)
 * Not sure what you mean "why". Your first molecule, CH3CH2OOH, looks like it would literally be called "ethyl peroxide", and probably isn't much of an acid.  Searches for "ethyl peroxide" don't come up with that molecule, though; it seems to be a slightly inaccurate common name for diethyl peroxide, that is CH3CH2OOCH2CH3, which Wikipedia doesn't seem to have an article on either.
 * As to why it doesn't come up, my guess would be that it quickly disproportionates into acetaldehyde and water: CH3CH2OOH&rarr;CH3CHO + H2O.
 * --Trovatore (talk) 07:37, 19 January 2017 (UTC)


 * The first molecule is ethyl hydroperoxide. We don't have an article for it (!) but google scholar returns a ton of refs . Dr Dima (talk) 08:03, 19 January 2017 (UTC)

You can think of –COOH as a shorthand for the more detailed –C(=O)OH. Double sharp (talk) 10:50, 19 January 2017 (UTC)

Note in Trovatore's answer that there is no "COOH" in the peroxide. By convention the hydrogens (or even fluorines) are written first, so the only way the C is followed by O is if it doesn't have a bond to anything but the one atom behind it. That implies (sort of) that it has to have the =O structure. Wnt (talk) 12:16, 19 January 2017 (UTC)


 * The answer is that:

║  ¨ - C - O - H       ¨
 * is the correct lewis diagram of a carboxylic acid. To simplfy that so it can be written on a single line, the convention is to represent THAT lewis structure as COOH.  The correct way to understand the development of these conventions is --> --> .  That is, the structure of the molecule is the most important thing, lewis diagrams and condensed structural formulas are second- and third-order representations, several steps removed from the actual structure of the molecule.  As a chemist, the conventions for writing molecules need to be understood knowing how the actual atoms work.  -- Jayron 32 13:22, 19 January 2017 (UTC)
 * Yes, but the OP posted two correct structures, so I give him credit for knowing that. :) Wnt (talk) 15:21, 19 January 2017 (UTC)
 * So maybe it was history like the benzyls? Importance certainly piles on. If there was a planet where the relative importances to biology were reversed (so 5% ethyl hydro xy peroxide is common vinegar) then maybe our chemists would have different "CH3COOH"s? Sagittarian Milky Way (talk) 17:18, 19 January 2017 (UTC)
 * As Trovatore said (and your formula indicates), ethyl hydroperoxide is CH3CH2OOH - note that extra H2 in there.  I suppose you could reorder the basic rules on how you put substituents with carbons to bring "COOH" back into the formula, but it seems strained.  As for the hydroperoxide, it is going to be very reactive; it would be an heroic biology that devises a way to tame it.  (I looked up hydroxyperoxide and found some uses like, but I remain skeptical... I'm thinking hydroxyperoxide ought to be -O-O-OH, something so unstable that merely drawing the formula on a blackboard could cause an explosion) Wnt (talk) 19:36, 19 January 2017 (UTC)
 * But then nature says "ooh look, we can have trioxidane, and even as a controllable intermediate in biochemical systems". DMacks (talk) 19:59, 19 January 2017 (UTC)
 * And ascorbylperoxide (2,3-diketo-4-hydoxyperoxyl-5,6-dihydroxyhexanoic acid) is known and seems somewhat stable in solution.(10.1016/j.freeradbiomed.2015.06.040) DMacks (talk) 20:06, 19 January 2017 (UTC)
 * I stand pleasantly corrected. I didn't realize substituted ozones were actually relevant; I had the impression that in ozonolysis they rearranged too rapidly to be isolated.  Our article says trioxidane can actually last 16 minutes in organic solvent. Wnt (talk) 15:10, 20 January 2017 (UTC)
 * If one just wants to see COOH representing something other than a carboxylic acid, I suppose in principle there could be such a thing as ethinyl peroxide, that is HC≡C–OOH. Maybe no one would shoot you for leaving out the single bond and writing HC≡COOH.  Anyone want to start a pool on its half-life at STP?  I'll start off by guessing 100 microseconds.  Just a pure guess. --Trovatore (talk) 20:08, 19 January 2017 (UTC)
 * For HC≡C–OOH, 10.1021/jp031067m lists ΔHf=42.25 kcal/mol at 298 K. I think that value is calculated not measured. It's "only" about 100 kcal/mol less stable than its isomer, glyoxal (10.1016/j.theochem.2004.11.012). DMacks (talk) 20:46, 19 January 2017 (UTC)

Why is the 5.56×45mm NATO called like this, when it is neither 5.56 nor 45mm in dimensions?
Came to this, which seems correct, by a look at File:5.56x45mm_NATO.svg. Tigraan Click here to contact me 15:04, 19 January 2017 (UTC)


 * 5,56 mm is the bore (barrel) diameter, measured between the lands of the rifling. 45 mm (actually 44.70) is the case length, marked as L3 in the drawing. Rgds ✦  hugarheimur 16:02, 19 January 2017 (UTC)
 * ...and read internal ballistics for an overview to explain why those parameters, out of all the other measures and dimensions and tolerances, are the ones of interest to the people who design and specify the standard cartridge. Nimur (talk) 16:17, 19 January 2017 (UTC)
 * The dubious tag is correct, since the cartridge is far from 5.56 mm.Sjö (talk) 11:17, 20 January 2017 (UTC)
 * I meant that the tagging seemed correct in my OP, though in retrospect it is ambiguous. Tigraan Click here to contact me 11:31, 20 January 2017 (UTC)
 * Bullets, like arrows and most artillery shells need a twist to rotate in a predefined way during their ballistic trajectory since they changed them from the ball form to cylinder form. Otherwise they would start to tumble uncontrolled in flight, which doesnt matter for a ball but does for a cylinder, and lose allot of energy tru higher air friction and lose trajectory precision aswell as reach on top. So bullets are actually slightly oversized and forced into the (undersized) barrel when shot. The barrel itself has like a very minimal, very long streched screw nut thread that forms a twist over its length and forces the projectile to "spin out". Bigger artillery shells actually only have a small ring section that is oversized. So the number 5.56 does not describe the bullet but the barrel type this ammunition is meant for. Somehow it became common to call them "5.56mm rounds", maybe because everyone felt to lazy to call them "rounds for 5.56mm barrels". --Kharon (talk) 00:29, 21 January 2017 (UTC)

Psychopathy and Monoamine oxidase
Amongst mental disorders, my impression is that Psychopathy is considered the closest thing to an untreatable condition. And psychopaths can tend to be very destructive to society, commonly, though by no means always, ending up in prison. And even those who do not get in trouble with the law can wreak a trail of destruction. So given that therapy pretty much does not work, there is a need to look at other approaches.

In Psychopathy, there are discussions of research into the role of Monoamine oxidase A. Good things is, we already have drugs which can modify levels of this. We have plenty of "inhibitors", but what I gather that we're looking for is an "inducer". The only current drug of this sort listed in our article is Valproate. I don't know if this particular drug would work, as it wasn't really designed for this purpose - but then again, it just might. But my question is a broader one: If MAO-A has a role in antisocial and psychopathic behaviour, have there been any studies into modifying levels of it as an attempt at reducing this destructive behaviour? If this turns out to work, it could have a MASSIVE impact on reducing our prison population, similar to the way anti-psychotics helped us empty the old "lunatic asylums". (No, we will not be able to abolish prisons, but we could significantly reduce prison populations). Valproate may well be a start, as it is readily available, and commonly used for other conditions, so has it been tried? And if not, why not? Our article states that it has been used off-label for Impulse control disorders, so any research into its efficiency in this regard, including, by extension, psychopathy?

Please put ethical issues to the side here - I'm well aware that your average psychopath is extremely unlikely to agree to take medication aimed at making him less of one. Though frankly, the alternative in some cases may well be more or less permanent imprisonment. And no doubt some ethicists would argue that this sort of disorder warrants compulsory treatment to protect society. But please put these ethical dilemmas aside.

My question is, has there been any research on this area? Or any experts suggesting that it might be a promising area for research? Or am I missing something, which would make such research useless or impossible? Eliyohub (talk) 15:41, 19 January 2017 (UTC)


 * Valproate is mentioned as a concurrent med in this 2014 research . But they focused on Clozapine, and showed it to be effective treatment for psychopathy, and that is already mentioned in our article.
 * As fore the general idea, I'd recommend reading this 2015 article "The neurobiology of psychopathy: recent developments and new directions in research and treatment" . I can send a copy via email if any interested parties cannot get access through other means. SemanticMantis (talk) 16:12, 19 January 2017 (UTC)


 * I think substantial skepticism is called for here. There are entire societies where I am tempted to say psychopathy is the norm (e.g. ISIS, or even America's chickenhawk adventures).  Psychopathy seems like a basic requirement for a Cabinet-level position recently.  As the section on criticism notes, the diagnosis is not really falsifiable.  Now to go from the association with a trait associated with some traditionally used but widely criticized copyrighted profile of characteristics is dicey, but to go from the association of a common genetic polymorphism with "coldness" of some sort... well, that's even more problematic.  And all in order to make the genuinely psychopathic decision of which of two different kinds of largely unscientific and legendarily ineffective systems of locked cells to put the defendant in?  The mind rebels! Wnt (talk) 20:09, 19 January 2017 (UTC)


 * It is important to note that "psychopathy" is not even a well-defined entity. It does not appear in the DSM, for example.  Much of the time it is used essentially to mean "evil", or more cynically, "behavior that I don't approve of".  There are grave issues with trying to treat such a thing. Looie496 (talk) 14:01, 20 January 2017 (UTC)

Feynman Lectures. Lecture 44. Ch.44-6 Entropy
Can you please explain this part in simple words? I don't understand why if the substance will be returned to point a on figure 44–10, it will be a reversible engine. Where are the borders of this engine? Why " there would be no loss of heat to the reservoir at unit temperature" (this engine can work between arbitrary two temperatures, including unit temperature)? I see on fig. 44–10 6 small engines. Even if we will come back from b to a the same path, the substance will liberate again dQ-s. Username160611000000 (talk) 17:53, 19 January 2017 (UTC)


 * Without looking at this (or understanding it) in detail, my assumption is that the reversible engine part comes in if the path from a to b is different than the path taken back from b to a, so some area is enclosed on the graph. Wnt (talk) 19:53, 19 January 2017 (UTC)
 * Yes, some area is enclosed and the area is the work. So some heat is converted to work and the remainder is delivered to the cold reservoir (including possibility that its temperature can be 1 degree). According to Ch. 44-3 in reversible engine the cycle can be inverted, so it's not necessary that area must be >0 . So I don't understand, if it's reversible engine, why can't it deliver some heat at unit temperature. Second: are those 6 little reservoirs and engines a part of the engine with substance? If yes, then at backward path 6 reservoirs will again absorb some heat (while temp. of the substance > little reservoir temp.), and again 6 engines will run in straight  direction, not backwards.Username160611000000 (talk) 05:56, 20 January 2017 (UTC)


 * His discussing the fact that entropy is a state function, like other thermodynamic quantities like enthalpy and free energy. He's discussing the entropy analogue of Hess's law; our article does as well from a different perspective.  Some of this gets covered in the Wikipedia article titled Entropy (classical thermodynamics). -- Jayron 32 21:20, 19 January 2017 (UTC)
 * Thank you. But in the article Hess's law the enthalpy is used and in Entropy (classical thermodynamics) the line integral properties are used (from the phrase "That means the line integral $$\int_L \frac{\delta Q}{T}$$ is path independent." I'm not sure). Feynman did not give the materials on the enthalpy and line integrals. (except casual mention in Ch. 14-1). Username160611000000 (talk) 05:56, 20 January 2017 (UTC)
 * Yes, strictly speaking, Hess's law applies only to Enthalpy. Practically, the same conceptual framework can be applied to other thermodynamic functions of state, such as entropy and free energy.  -- Jayron 32 12:19, 20 January 2017 (UTC)

Mechanism of curly hair
What makes curly hair curly? I recently edited the straightener section of Hair iron and was puzzled by the material about loosening bonds through heat. My science education was severely lacking. Our biology teacher told us that curly hair is curly because the shaft the hair grows out of is kinky, which makes sense in light of the problem using electric wire depilatories on curly hair, but I find it hard to reconcile this with chemical bonds, and I'm not sure I can see how the shape of the channel the hair grows out of would produce Afro-type hair. I checked our article section to which Curly hair redirects, but it's all about the arguments over why curly hair might have arisen through natural selection, and has nothing about the mechanism. Are they both true and I'm just not envisaging well how it would work, or was it another error on the part of my biology teacher? Yngvadottir (talk) 21:43, 19 January 2017 (UTC)
 * Which part are you having trouble reconciling? You can force straight hair to be curly by using a hair iron, just like you can force curly hair to be straight straight. But straight hair has a round shaft which over time will return to it's straight state, and curly hair has a "flattened" shaft which over time will return to it's curly state. I don't see any contradiction. It's not the atomic bonds that are being changed by the heat, it's probably the hydrogen bonds, or some other weak bonds between hair molecules. Vespine (talk) 22:51, 19 January 2017 (UTC)
 * My mother always said eating your crusts made your hair curly? :) I used to eat my crusts. --TrogWoolley (talk) 10:19, 20 January 2017 (UTC)
 * I'm having trouble reconciling my biology teacher's statement that it's curly because it grows out of a bent hole in the scalp with the statement that it's curly because the hydrogen bonds in the hair shaft hold it in that shape. (And I also can't see how Afro-type hair could be produced by a bent hole in the scalp.) Since our article has absolutely nothing on the mechanics, I wondered whether I was missing some action of growing out of a bent hole on the hydrogen bonds - which is way, way, way beyond the amount of science I was taught - or whether it's another example of my teacher being wrong :-( (I also think it would be useful if the article spent less time on the evolutionary/racial aspects and explained the physical reality - the flattened shaft you refer to would be a start - but for all I know the published sources assume it's basic and obvious. Hence my first query here (I have no shame, it wasn't my fault I got a very poor science education.) Yngvadottir (talk) 17:00, 20 January 2017 (UTC)
 * Without recourse to the quality of your own education, since the invention of the internet there is really no excuse for lacking in self education. This from Northwestern University (a well respected research institution) notes that your teacher was, in part, correct.  The real answer is that the curl or shape of hair is a complex mixture of factors, from the shape of the follicle (round, oval, or flat) the angle of the follicle (shallower angles to the scalp produce more curls than more perpendicular angles), the chemistry of the hair material itself also plays a big role, as mentioned in that article (specifically disulfide bonds from cysteine-cysteine cross linkages (see Cysteine).  So your teacher wasn't actually wrong, and neither were the other people.  There are several different factors that all play roles in the shape and texture of hair.  -- Jayron 32 17:11, 20 January 2017 (UTC)
 * Thanks! That helped a lot; the writer even explained the cysteine bonds bit in a way I could kind of understand. I had wondered about the possibility of the curve in the hairs accentuating itself through attraction, since bonds were mentioned regarding hair straightener, but I really didn't get to learn enough for self-educating to be safe. My teacher was talking about a curved shaft through the scalp, rather than one at an angle to the perpendicular, so all in all I think we were taught wrong on the topic. And now I must reiterate that Nuffield Science Project should be checked for neutrality. Yngvadottir (talk) 17:36, 20 January 2017 (UTC)
 * For those interested and in the area, there is an exhibition running at the Marcus Garvey Library featuring African combs and hair.  These are apparently treated by the police as offensive weapons, although they have great cultural significance.   The display discusses the reason for the curliness, mentioning anomalies in the distribution of keratin in the fibre as one cause. 92.2.72.206 (talk) 18:47, 20 January 2017 (UTC)
 * Looking around I found something similar here . That site talks about a red and green one with metal teeth that was viewed as a weapon but I don't think it shows it.  No doubt the message of some of the combs alone would have been enough to get certain cops to look for something to accuse people of. Wnt (talk) 21:48, 20 January 2017 (UTC)
 * Sharpened metal combs are quite dangerous, that's why one should not have a metal comb in one's hand luggage when flying and they are banned from many events along with screwdrivers and other potentially lethal weapons. Dmcq (talk) 09:53, 21 January 2017 (UTC)
 * LOL, a web search makes it sound like whenever white folks get hold of Afro combs something different happens.  But apparently the metal teeth work better.   When police mistake them for a gun, well...   Nonetheless, yes, sounds like they have found some nasty uses even by black folks. .  An entertaining subject to search on. Wnt (talk) 11:46, 21 January 2017 (UTC)

Time dilation and getting there faster by travelling at a lower speed
The following was stated in a Buddhism & Science course I attended this evening:

A spaceship travelling from Earth to Alpha Centuri (lets say for simplicity it's 4 light years away) travels at 50% the speed of light. The round trip would take 16 year from the point of view of those on the spaceship but 18 years and 5 months from the point of view of observers on Earth.

If a spaceship travels at 99% of the speed of light the round trip would take 8 years and 1 month for the traveller, but 56.6 years would have elapsed on Earth.

So if both spaceships left Earth at the same time, the slower ship would arrive back sooner. Is that correct? --Stanstaple (talk) 22:53, 19 January 2017 (UTC)


 * Less than 16 years on board (but not loads less), 16 years on Earth. @99%c it'd take 8 years 1 month on Earth and and less than that on board. That's if they had magic inertial dampers or something. It takes half a year just to reach 0.5c at high sustainable acceleration. Sagittarian Milky Way (talk) 23:06, 19 January 2017 (UTC)
 * I think they've got something backwards. A craft travelling at 99% the speed of light will take 4 years to make the trip according to an observer on earth. The ship will experience the time dilation, not the earth. Vespine (talk) 23:11, 19 January 2017 (UTC)


 * (multiple e/c's) Something's wrong. Actual photons travel at 100% the speed of light, and appear to us to take 4 years one way (I'm not sure how long the trip takes from the point of view of the photon). I think the time dilation affects the spaceship, not the observers on earth. Either you misheard, or they misspoke. --Floquenbeam (talk) 23:12, 19 January 2017 (UTC)
 * I was about to suggest twin paradox and in fact an almost identical explanation exists right here Twin_paradox. A rocket making a return journey to a star 4 light years away and travelling at 80% the speed of light, the earth (and everyone on it) will have aged 10 years, while the "traveler" who went there and back only aged 6 years. Vespine (talk) 23:17, 19 January 2017 (UTC)


 * So I'm thinking that the teacher made an error here. If the ship is travelling at .5c (avg) the journey will take 16 years from the pov of Earth and less time for the travellers. If the ship is travelling at .99c, then from Earth's pov the trip takes approx 8 years and even less time still for the astronauts. Is that right? — Preceding unsigned comment added by Stanstaple (talk • contribs) 23:33, 19 January 2017 (UTC)
 * That's how I understand it. Vespine (talk) 23:39, 19 January 2017 (UTC)
 * Correct. Your original numbers are the correct answer to a different problem, so either you or your teacher were confused. At 99% the speed of light, one year on the spaceship is 50 years on Earth, but the round trip only takes a couple of months. Someguy1221 (talk) 23:41, 19 January 2017 (UTC)
 * So, if I'm understanding you correctly, you're saying that we could travel to somewhere 4 light years from Earth in a few months if we could travel at .99c?--Stanstaple (talk) 00:03, 20 January 2017 (UTC)
 * In a few months to you, on the craft, yes. To observers in other reference frames, like those staying on Earth, it would take longer. You experience time dilation and length contraction due to your speed. Yes, it seems crazy to our ape brains, but it's been extensively verified by experiment. Here's one video on the topic. Here's a video series from PBS Space Time that I highly recommend; it goes into general relativity as well, but the first video in particular covers special relativity (what we're talking about now) as well. --47.138.163.230 (talk) 00:23, 20 January 2017 (UTC)
 * As a result: the ladder paradox. Sagittarian Milky Way (talk) 02:44, 20 January 2017 (UTC)
 * I would take issue with the sentence "You experience time dilation and length contraction due to your speed." The observation effects of special relativity are symmetric. Time appears perfectly normal to observers within the fast-moving craft, and it is the rest of the universe that is observed to be length-contracted.    D b f i r s   16:40, 20 January 2017 (UTC)
 * You're completely right. I was just being bad at stating things. I'm not an expert. --47.138.163.230 (talk) 20:42, 20 January 2017 (UTC)
 * "(I'm not sure how long the trip takes from the point of view of the photon)". Floquenbeam, Photon does not have inertial frame of reference. Photon reaches destination instantaneously. It only travels in space, not in time. manya (talk) 05:24, 20 January 2017 (UTC)

Thank you all for your answers and refs. I learned something. :) --Stanstaple (talk) 20:27, 21 January 2017 (UTC)

Diff
Whats the diff tween hair and fur?--31.92.84.47 (talk) 23:57, 19 January 2017 (UTC)
 * Fur is a subset of Hair. ←Baseball Bugs What's up, Doc? carrots→ 00:04, 20 January 2017 (UTC)
 * We have articles on both hair and fur. There is really no physiological difference between hair and fur, that is actually consistent across all things called hair or fur. Many people will give ad hoc definitions anyway, and these do at least provide an easy way to talk about it in some circumstances. Someguy1221 (talk) 00:06, 20 January 2017 (UTC)
 * I always thought fur was a collection of hairs. I don't think I have ever seen fur as a singular (a fur) unless used as an adjective (a fur coat).  Hair can be either singular or plural. DrChrissy (talk) 00:22, 20 January 2017 (UTC)
 * According to the industrial and academic expert I've asked (WP:OR pers. comm), the conventional usage is that "fur" is reserved for finer textures of denser strands and "hair" is used for sparser densities of coarser strands. Thus we speak of hair on pigs, but fur on chinchillas. Something like an otter may be roughly in the middle. This is of course a fuzzy line, but one supported by expert usage. SemanticMantis (talk) 17:58, 20 January 2017 (UTC)


 * One distinction is that hair grows continuously, vs. fur that grows to a certain length and stops (which seems mysterious to me). 2606:A000:4C0C:E200:D52B:CE19:E1B3:FD5B (talk) 02:38, 20 January 2017 (UTC)
 * Fur doesn't "stop" growing. What happens is each follicle grows for a period of time and then briefly stops and the hair "dies" and falls out, then the hair starts growing again. The "period" between these stops is what determines a hair's (or furs') length. The longer the period, the longer the hair. ALL hair does this, eyebrows, eyelashes, pubic hair, even the hair on your head, some people have ridiculously long hair, but most people have a "length" that their hair will grow to and won't grow longer. Vespine (talk) 04:01, 20 January 2017 (UTC)


 * In the cases of people who simply have ludicrously long hair, it is often the case that no single strand is close to the length of the, I'll just call it the "hair mass". Instead, shorter strands, which can still be several feet long, are all just held together like dreadlocks. Someguy1221 (talk) 04:12, 20 January 2017 (UTC)


 * I personally had waist-length hair during the 60s. I could easily pull out a single strand and show that it was at least three feet long, if not longer. Therefore, it is necessary to define "ludicrously long". Waist length? Knee length? Floor length? Dragging on the ground 20 feet behind you length? 209.149.113.5 (talk) 13:57, 20 January 2017 (UTC)
 * Long hair has some info about the longest observed hair on humans. ←Baseball Bugs What's up, Doc? carrots→ 14:30, 20 January 2017 (UTC)
 * I am male and I had waist length hair in my late teens too, well maybe not quite, definitely to the small of my back. Apparently according to that article, waist length (abut 100cm) is the average "longest" most people can achieve. To get longer than waist length, you have to have unusually long anagen. Vespine (talk) 22:32, 23 January 2017 (UTC)