Wikipedia:Reference desk/Archives/Science/2014 August 9

= August 9 =

Beckman's Internal Medicine
In searching for answers for another editor's question, I repeatedly came across references to the book "Beckman's Internal Medicine", but have been unable to find mentions of this work except in the context of Eva Saxl. Anyone familiar with the book, or know if it's been archived online, had another title etc? Or who Beckman was? DuncanHill (talk) 00:28, 9 August 2014 (UTC)
 * This forum post is the best I can find - not a reliable source by any means. Apparently, the book exists, but it's not in "English, German, or Spanish" (French, possibly?).  Our source for the statement in the Eva Saxl article isn't particularly wonderful (a blog posting from 2002), and I think it's likely that all the other references are derived from our article.  I'll keep looking. Tevildo (talk) 20:45, 9 August 2014 (UTC)
 * Beckman _may_ be Arnold O. Beckman, but he was primarily a chemist. The timescale is right, though, and there aren't any other plausible Beckman's at Beckman (surname). Tevildo (talk) 20:50, 9 August 2014 (UTC)
 * Incidentally, I don't like that apostrophe, but "Beckmans" looks equally wrong. Apostrophe doesn't really help. But it's not critical. Tevildo (talk) 21:21, 9 August 2014 (UTC)
 * Got it! Gustav von Bergmann, Handbuch der inneren Medizin, Volume 2, Berlin, Springer, 1930. See this bibliography.  It presumably is in German, after all. Tevildo (talk) 21:00, 9 August 2014 (UTC)


 * Thanks! Beckman is a plausible typo/misremembering for Bergmann. DuncanHill (talk) 21:34, 9 August 2014 (UTC)
 * No problem. Now, we need to fix the Saxl article.  Just changing the text would be little more than a guess (not even WP:OR), so we need to find something that says Saxl used Bergmann's book... Tevildo (talk) 22:28, 9 August 2014 (UTC)
 * There's a copy in the Wellcome Library in London, (and in the British Library) if anyone fancies taking a look. See here. Tevildo (talk) 10:52, 10 August 2014 (UTC)
 * The Saxl article appears to be plagiarized from a website, unless the website is a mirror of Wikipedia. (Unfortunately I do not have a link to the website i found while looking for more info on Axl and the Beckman textbook. I will look for it again. Edison (talk) 03:28, 12 August 2014 (UTC)
 * The Saxl article, created in 2007, contains many sentences in exact or closely paraphrased form from the 2002 [ which was cited as a ref from the first version. I tagged it for excessively close paraphrasing. Not sure if it should be shut down as a copyvio; we really ought to have an article on the remarkable achievement, and it seems to be of some external interest. [[User:Edison|Edison]] (talk) 04:06, 12 August 2014 (UTC)

WARF testing lab
From the article SUPERthrive: "SUPERthrive is a WARF testing lab–certified nontoxic liquid concentrated growth enhancer product for plants".

Does the name "WARF testing lab" mean anything to anyone on the Science Reference desk?

Thanks, C7nel (talk) 00:34, 9 August 2014 (UTC)
 * You can check out their website here: http://www.warf.org/  —71.20.250.51 (talk) 00:45, 9 August 2014 (UTC)

Sabaton (not the band)
Did the long pointed toe of the sabaton (the shoe, not the band) have any functional purpose? Or was it only a decorative badge of rank? 24.5.122.13 (talk) 02:05, 9 August 2014 (UTC)
 * I can't imagine they didn't have some intended functional purpose. I've done some searches to find an answer for your question, and couldn't find anything substantial, but remember that medieval armor was absolutely functional; if your armor didn't work for you efficiently, you died.  I suspect the idea was to have a weapon to kick with from horseback, a well-planted kick to the ear would ruin someone's day.  Notably, the pointed toe did not stay in fashion for long; it may have had too many disadvantages in battle to counter any advantage of being able to use it as a weapon.  -- Jayron  32  02:16, 9 August 2014 (UTC)
 * The article does point out one disadvantage -- it would greatly hinder the knight when fighting on foot (which is why when dismounting to fight as infantry, they would change their sabatons for sturdy leather boots). As for possible advantages, could the pointed toe have helped the knight stay in the saddle when hit? 24.5.122.13 (talk) 02:22, 9 August 2014 (UTC)
 * The downward slope of some of them could conceivably provide some protection from a sword blow, deflecting it downward and away from the wearer, instead of upwards and into the ankle and shin. DuncanHill (talk) 02:42, 9 August 2014 (UTC)

Light reflection
My question relates to an older question I found in the archive.

https://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2007_September_12#Stupid_question_about_light_reflection.

If a flashlight beam is shining on a good mirror and another flashlight beam of equal intensity and color is shining on a white surface, which surface will return more light?

I realize that with the mirror the direction of the light leaving the surface largely follows the rule that the angle of reflection is the same as the angle of incidence while with the white surface the angle of reflection is much more variable. My question is, IGNORING differences in the direction the light is reflected, which surface will reflect more light in total?

I'm not asking about something that might be called "a perfect mirror" or "an ideal white surface". I'm thinking about a good quality mirror compared to a piece of hardboard painted with a good quality white paint.

Thanks, C7nel (talk) 06:02, 9 August 2014 (UTC)


 * As surprising as it may sound, a surface painted white with the best-quality white paint (titanium dioxide) will reflect more light in total than a mirror made from the best reflective material (silver). 24.5.122.13 (talk) 08:04, 9 August 2014 (UTC)
 * Agree with that. Ordinary titanium white paint can reflect 90% of the light and good quality stuff may only lose a few percent. An ordinary mirror you buy in a shop may lose nearly half the light. The article on titanium dioxide unfortunately doesn't give figures about paint but it describe its use in dielectric mirrors which only lose a tiny fraction of a percentage of thee light. Dmcq (talk) 08:11, 9 August 2014 (UTC)


 * There are optical coatings that produce mirrors with 99.9% or better reflectivity, depending on the range of wavelengths desired. See Dielectric mirror. --Mark viking (talk) 08:21, 9 August 2014 (UTC)




 * It's close...and the devil is in the details. The exotic aluminium/molybdenum mirrors we use on our laser cutter are 98% reflective in IR and about the same in the visible light spectrum...but the chart at right shows how good a gold, silver and aluminium mirrors can be - and it depends where in the spectrum you are.
 * For paint, some manufacturers actually print the reflectivity for visible light next to the paint samples you see in the store. So if you see "LVR=90" (or sometimes "LRV=90") would mean that this paint (on a smooth surface) is 90% reflective - averaged over the entire visible spectrum.  Titanium white (which I think is the whitest white you can get in paint) is generally claimed to be about 93% reflective - so it's comparable to a silver or aluminium mirror.
 * The decision as to which is best depends on the frequencies of light present in your light source - and (critically) on how smooth the surface is. SteveBaker (talk) 14:10, 9 August 2014 (UTC)


 * "and (critically) on how smooth the surface is"  So if they were both painted with the same paint, a smooth sheet metal surface would return more light than a piece of hardboard?   C7nel (talk) 18:11, 9 August 2014 (UTC)


 * They would reflect the same amount of light (if the light is incoming perfectly equally from all directions, including within the nooks of the material itself), but the reflected light would be diffuse and/or bounce around in the cracks, losing energy, in the uneven surface. In a home experiment, grab some aluminium foil straight from the roll and carefully keep it completely smooth over a flat surface. You should just barely see the reflection of your hand when you put it close. Now start curving the foil (no need to fold or crumple), and you'll see the image fall apart. The amount of light reflected is the same, but it's no longer aligned with what went in. That's also why water must be quite still to give you a reflection - even tiny slow long waves will diffuse the image (small narrow ripples are local enough to preserve it in places, though). SamuelRiv (talk) 00:50, 10 August 2014 (UTC)


 * If each time light hits the surface, (say) 90% of it gets scattered away in all directions and 10% if it is absorbed into the material - then a perfectly smooth surface will cleanly reflect 90% of the light and turn the remaining 10% into heat within the material. But if the surface is slightly rippled - then when the material scatters the light everywhere, some amount of it will hit the surface again.  This produces a second reflection - also with 90% reflectance - but now it's 90% of the 90% that escaped the first reflection...so only 80% of the light that must undergo two bounces will emerge.  The rougher the surface is, the higher the amount of light that will undergo double, triple, quadruple reflections.  A matt painted wood surface is likely to be a lambertian reflector - but gloss paint is capable of 'specular' (mirror-like) reflection - and now you have something that's a hybrid of a mirror and a matt painted surface.  At this point, the explanations get complicated.  Some surfaces (notably copper) reflect light in different colors depending on the angle...diamonds are incredibly reflective even though they're transparent because they have a crazy refractive index and reflect by 'total internal reflection' and easily cleave into near-perfectly flat surfaces. SteveBaker (talk) 02:32, 10 August 2014 (UTC)
 * 81%. - ¡Ouch! (hurt me / more pain) 05:56, 12 August 2014 (UTC)
 * I was working to one significant digit...80% was OK. SteveBaker (talk) 14:25, 12 August 2014 (UTC)
 * In the sense of "85% to 95%" – OK. I read it as an example with an exact value, i.e. infinitely precise. - ¡Ouch! (hurt me / more pain) 06:06, 13 August 2014 (UTC)

Bird id


Can anyone identify this chap for me? He's our neighbour in Bratislava, Slovakia. Possibly a kestrel? HenryFlower 06:14, 9 August 2014 (UTC)
 * His name is "Charles", but his friends call him "Karl" . -But, seriously... We have a List of birds of Slovakia; this fellow looks as if he belongs in  Caracaras and falcons section, but I've checked all 8 listed, and he doesn't seem to be there.  The closest is the Red-footed falcon (see female) —but without the red feet.  I suspect he might be a she ("Charlene", a.k.a. "Karla"), due the the somewhat demure appearance.  It would be useful to see the bird from the front.  You might check the 23 species listed in the 'Hawks, kites and eagles' section, but none seem likely.  ~Sorry that I couldn't be more helpful, ~E: 71.20.250.51 (talk) 09:52, 9 August 2014 (UTC) 
 * Thanks for trying. :) I have some more angles on my blog, but I suspect they don't reveal anything more. I had thought of the red-footed falcon, but they seem to be rare in Slovakia; I do like the idea of having discovered the even rarer non-red-footed-red-footed falcon, however.


 * If I may add a bonus question, is this second bird a Eurasian Buzzard? There were five of them, also near Bratislava. HenryFlower 13:36, 9 August 2014 (UTC)
 * Almost certainly - it's not a Honey Buzzard according to this identification page . Mikenorton (talk) 15:31, 10 August 2014 (UTC)


 * Have you considered asking at WT:BIRDS? Some of the guys there are really good at this. There is usually (but not at the moment) a long 'birds for identification' thread on there... --Kurt Shaped Box (talk) 15:34, 10 August 2014 (UTC)


 * Thanks again (both). I'm hesitant to try there, as it seems to be for actual wikipedia-related discussions; I ask out of interest, not because I'm working on any related articles. HenryFlower 18:25, 11 August 2014 (UTC)


 * It's absolutely fine to ask there - if you look back through the archives, they've been identifying bird photos for years. Besides, identifying your birds is certainly WP-related! Once the birds' identities are confirmed, the images can then be uploaded to commons, appropriately categorized and thus made available for use in articles! --Kurt Shaped Box (talk) 18:45, 11 August 2014 (UTC)

Vortex rotational profile
Could an expert have a look at this issue?--Almuhammedi (talk) 08:53, 9 August 2014 (UTC)
 * Thanks, I think it is done.--Almuhammedi (talk) 11:47, 10 August 2014 (UTC)

Rest & relativistic masses
Lately I had a long debate with a quantum physics professor regarding those terms as he was insisting to forget about rest mass & relativistic mass (I don't know if there is a dedicated article/section for this historic argument). He was addressing those who still use such terminologies as non educated or basic level learners. I understand the argument among physicists about this but do we seriously need to get rid of such terms and if we do so, what could be the alternative accounting for mass–energy equivalence principle? On the other hand, does special relativity directly affect the mass of the proton if we know that its individual mass of quarks is much less than that whole mass of the proton?--Almuhammedi (talk) 09:10, 9 August 2014 (UTC)
 * I'm not exactly sure what the question is about. I would consider the idea of relativistic mass as very useful in many ways. How else does one go around explaining that if you heat up a box of gas it will get heavier? That effect is not observable at normal temperatures but it is most definitely what the physics says will happen. If they are saying you should not use the terms then they should be saying what you should be doing instead, so what have they said? Have you read the relativistic mass article where it explains that relativistic mass is not a property of the object but relates to the frame of reference it is in. Really it is not all that different from a collision between cars. If they are both going in the same direction at the same speed ten they just bump each other, if they are going in opposite directions and crash then it is major incident. Dmcq (talk) 12:10, 9 August 2014 (UTC)
 * The statement "relativistic mass is not a property of the object but relates to the frame of reference it is in" explains why it's such a dubious and dangerous concept. Dubious because mass really is a property of an object and should therefore be frame-independent; energy and momentum are used to describe an object's state of motion."Dangerous" because it conveys wrong ideas about special relativity as it suggests for instance that the impossibility to accelerate things beyond the speed of light might be due to an increase of mass of those things rather than the fundamental structure of space-time. The mass of a box of gas does indeed increase when it is heated; that's because it's internal structure is changed, energy is pumped into it, and the box's mass includes both the rest masses of the gas particles and their kinetic energies. The mass of the box does not change when it is moved around (no physical change of the box there, only a change of reference frame!), only its energy and momentum change. --Wrongfilter (talk) 17:37, 9 August 2014 (UTC)
 * A question that comes up with this is whether rest mass is a property of an object, or its interaction with the Higgs mechanism. Related question: if individual Higgs particles have a position and momentum in space, why does the Higgs mechanism give the same rest mass to a particle regardless of its frame of reference? Wnt (talk) 20:11, 9 August 2014 (UTC)
 * The Higgs mechanism actually accounts for only a small part (~1%) of the rest mass/energy of ordinary baryonic matter. Most of the rest is from the strong force (and kinetic energy of the valence quarks).
 * The vacuum isn't filled with real Higgs particles (which would indeed have a state of motion, breaking Lorentz symmetry), but with a uniform Higgs field. -- BenRG (talk) 22:00, 9 August 2014 (UTC)
 * Yeah, come to think of it I suppose it's not so mysterious with an electric field that is the same regardless of frame of reference, and this is sort of the same idea, apart from the whole mysterious uniform throughout space yet prone to change over time part. As for the 99% of rest mass that is "kinetic energy of valence quarks", you do mean the relativistic mass of the valence quarks, right? :) Wnt (talk) 01:13, 10 August 2014 (UTC)
 * Alright, suppose I have an electron sitting in a conductor at -10,000 volts. It therefore has 10,000 electron volts of energy it would like to get rid of, which could be used to run a transformer or electrocute a hapless captive.  This potential energy has mass, which I assume we call rest mass because the electron has it at rest.  So the electron's mass is whatever the physical constant is, plus 10,000 eV of mass.  Is the way that the electric field has given the electron extra mass the same as the way the Higgs field gives a particle mass? Wnt (talk) 01:24, 10 August 2014 (UTC)


 * Relativistic mass is a useless concept, because it is synonymous with the totale energy of an object. The mass-energy equivalence says that the inertia of an object at rest is equal to its total energy content at rest. So, the whole concept of mass is redundant. But the rest energy of a particle is an invariant property of that particle which is useful in physics. It's conventional to use the word "mass" instead of "rest energy". Count Iblis (talk) 20:24, 9 August 2014 (UTC)


 * Do I detect a certain prejudice/judgementalness? Just to put a cat amongst the pigeons: it is essentially impossible to define rest mass in a consistent way that is not equivalent to "relativistic mass as measure in a center of momentum frame. And since such a frame is only well-defined for an isolated system (even in special relativity), it certainly puts a question mark over whether the concept of rest mass is in any sense fundamental. BTW: "Relativistic mass is a useless concept, because it is synonymous with the total energy of an object" implies that the total energy of an object is a useless concept ;-)  —Quondum 21:14, 9 August 2014 (UTC)


 * What was the argument for getting rid of rest mass? I don't think I've heard that one.
 * The best argument for getting rid of relativistic mass is that you should write everything in a manifestly covariant form, with tensors (and/or spinors). Rest mass stays because it's a scalar (= rank-0 tensor), relativistic mass goes because it isn't. You still have the four-momentum, which is a rank-1 tensor whose time component is the relativistic mass, so you haven't really lost anything, you're just writing everything in a consistent notation where it's easier to avoid mistakes (such as forgetting to apply the correct transformation law between reference frames). -- BenRG (talk) 22:00, 9 August 2014 (UTC)
 * That is no argument at all, if it is intended to support the abolishment of the term "relativistic mass". It would have us get rid of names for any non-tensor quantity that you care to mention, such as momentum, energy, velocity, the stress tensor, charge density, current density, electric field... It is clear that it is simply how we describe one component of the energy–momentum tensor in a given frame, scaled by c2 to give a meaningful mass, which also translates as inertia, etc.  At least it is always well-defined in special relativity, which is not the case for rest mass.  There is another quantity associated with a field that might rightfully claim to be a scalar: e.g. the m in the Dirac equation. But I strongly suspect that it will be very different numerically from what we normally call the rest mass of an electron. And the rest mass of a quark probably needs very special interpretation, since a free quark probably has infinite rest mass... All I'm trying to point out is that the claim that I've seen periodically to the effect that rest mass is king, and relativistic mass is a term reserved for the confused is not as obviously true as some seem to think.  I've even seen WP articles being written to say that E=mc2 refers to rest mass and that's how Einstein intended it since "mass" means "rest mass", but then, obviously he only meant it to apply in the comoving frame of the particle, because otherwise it's invalid. Or perhaps he meant the rest energy too... You tell me who's getting confused in this scenario?
 * We are side-tracking from the OP's question, so I'll add this: the rest mass of a proton is considered to be made up by: the rest mass of the quarks (which constitute only a tiny percentage), the mass contribution of their kinetic energy (which, if I remember correctly, makes up the bulk of the total energy –or mass– of the proton due to the relativistic velocities of the quarks), plus a contribution from the gluon fields. —Quondum 00:25, 10 August 2014 (UTC)
 * Rest mass/energy is the length of the momentum four-vector, so it's defined whenever relativistic mass/energy is (the t component of the same four-vector).
 * I think we are better off without the things you listed when doing relativistic physics, but they're useful in the Newtonian limit. Relativistic mass isn't, and that could be why it's singled out for special dislike.
 * Relativistic mass is not straightforwardly interpretable as inertia (see also transverse and longitudinal mass) and it doesn't gravitate either (fast-moving objects don't collapse into black holes). -- BenRG (talk) 05:10, 11 August 2014 (UTC)
 * Yes, tensors are to be preferred. So, I take it you believe that the general consensus is with Okun on this? And that the COM frame is unique for distributed, nonisolated systems? —Quondum 06:40, 11 August 2014 (UTC)
 * Forget reference frames. The definability or not of a reference frame in some situation is not physically interesting. What you're really talking about, I assume, is that a nonisolated system doesn't have a constant four-momentum, and you can't covariantly make the four-momentum a function of time; you have to make it a function of spacetime position, which gives you a field description. There's no global invariant rest mass in this case. There's also no global relativistic mass—that is, people don't use the term "relativistic mass" in this sort of situation, to my knowledge.
 * I don't think I've read Okun's article before, but I skimmed it just now. He starts out by saying that Einstein showed E0 = mc2, where E0 is the "rest energy" and m is the unqualified "mass", which he distinguishes from the "rest mass" m0. Later on that page he says that the "mass" m is also called the "relativistic mass". On the next page he says mass (unqualified) is a relativistic invariant. So I have no idea what he's talking about. According to his own poll, the majority of physicists disagreed with him, saying that Einstein showed E = mc2 or E0 = m0c2, and I imagine that would still be true today (it's certainly what I'd say if I were polled). -- BenRG (talk) 22:38, 11 August 2014 (UTC)
 * Of course one can take the perspective that if you can describe something via invariants, why would one want to confuse things by doing otherwise? With which I agree, but in the pedagogical setting, this approach is somewhat limiting. As far as Okun is concerned, my own impression is that he is a bit muddled, and in particular is arguing circularly, so should be ignored. —Quondum 04:01, 12 August 2014 (UTC)

Thank you all Wiki-contributors. Just to have a root cause analysis to my question, please refer to Okun's criticism:

Of course there were also some critics replying to his article at the time but I thought physicists should have resolved this argument since then.--Almuhammedi (talk) 11:05, 10 August 2014 (UTC)
 * Scientists don't tend to resolve matters like this and come to a unified conclusion about how they should think about something. What normally happens is that the old guard just eventually gets replaced. Dmcq (talk) 11:30, 10 August 2014 (UTC)
 * I'd rather describe it as that there is nothing to resolve in this instance, since every discipline tends to define its use of terms. —Quondum 16:04, 10 August 2014 (UTC)


 * To give another simple example comparable to the quarks, what is the rest mass of the Earth-Moon system? You take the frame (center of gravity) where the relativistic mass is lowest, add the rest mass of the Earth plus the rest mass of the Moon minus the mass-energy released by their decrease in potential to their current positions plus the relativistic mass of their current motion, right?
 * But the goofiest case of this was a thread we had here (I think) about black holes here a while back. To the limited degree I understand it, the matter falling into a black hole can potentially release all its mass value of energy on the way down.  Leaving the hole full of, what, massless particles and the ghost of a gravitational field?  (I guess the stuff inside the event horizon couldn't emit gravity anyway...)  There was some physicist arguing on arxiv about whether the black hole really had twice its mass or 1x but I didn't know if he was a crank or not. Wnt (talk) 20:35, 10 August 2014 (UTC)


 * I should emphasize that this debate is purely semantic, and has absolutely nothing to do with how the universe works. Every physicist agrees on what happens to a proton when it's accelerated to 99.99% the speed of light.  If you ask any competent physicist what a certain instrument would measure, they would give you the same answer.  The universe doesn't care what we define "mass" to be; we define mass in whatever way appears convenient.
 * An analogous situation is the "debate" over whether Pluto is a planet. Pluto couldn't care less whether humans call it a planet or not. Whether Pluto is a planet or not doesn't increase or decrease our knowledge of it by one iota.  It's purely a linguistic issue, just like the definition of "mass".  --Bowlhover (talk) 04:16, 11 August 2014 (UTC)
 * Yep, I agree. Names are purely a convenient short-hand for some underlying phenomena.  Everyone agrees about what's "real" - the nomenclature simply represents mathematical convenience.  I'm reminded of XKCD 123. SteveBaker (talk) 19:29, 11 August 2014 (UTC)
 * Yeah, that's why when talking about relativity to a broad audience like this, I generally try to avoid using the word "mass" by itself at all, and instead always use the phrase "rest mass" or "relativistic mass" as appropriate. There will always be some subset of the people who read those phrases who are going to sneer a little at one phrase or the other or both for one reason or another, but everyone will at least understand exactly what I mean, and a semantic argument will be avoided.  The same cannot be said about using the word "mass" by itself.  I personally would rather be sneered at than have a communication problem.  Successfully communicating with people about physics is fun; arguing about semantics is not.  Red Act (talk) 20:58, 11 August 2014 (UTC)
 * Above I argued that the term "relativistic mass" conveys wrong ideas about the physics. What do you make of that? (I like "purely semantic" about as much as "just a theory") --Wrongfilter (talk) 06:50, 12 August 2014 (UTC)
 * If you have a box with a ball bouncing around inside it weighs more than one where the ball inside is still. You would say one box had more mass than another if you couldn't see inside. What is your wording for saying the extra mass is just energy after you look inside? Mass of the energy? But where is the energy? Dmcq (talk) 11:40, 12 August 2014 (UTC)
 * Have you read my contribution above? Yes, the box with the bouncing ball has a larger mass than a box with a ball at rest (I prefer boxes with gases, the box itself bounces less...). But that is due to a change in internal structure, whereas "relativistic mass" is usually used to refer to some sort of increase in mass due to a change of reference frame. The terms are well defined in the usual formulation of the theory (they have to be - semantics is important!) and I don't see why we should let go of that when talking to a broader audience. --Wrongfilter (talk) 14:18, 12 August 2014 (UTC)
 * 'Internal structure' is far less well defined or meaningful than energy or relativistic mass. Dmcq (talk) 18:03, 12 August 2014 (UTC)
 * Only if you don't want to understand what I mean by that. You may prefer "heating the box", "increasing the kinetic energy of particles in the box". The point is that the box (or any composite particle) has internal degrees of freedom which can carry energy. A point particle has no internal structure, and when people assign it an increased "relativistic mass" this is only due to change of reference frame, a change of coordinates. The four-momentum of the box is the vector sum of the four-momenta of the particles in the box, and as usual with vectors, its length (mass is defined as the invariant length of the four-momentum) can be larger than the sum of the lengths of the particles' four-momenta, i.e. the sum of their masses. --Wrongfilter (talk) 19:55, 12 August 2014 (UTC)

If two metals contact in space, do they blend, simply as that?
Does the oxygen in the air prevent metals from melting together - or some layer of oxygen on the metal? Can we simulate the process on Earth? (in vacuum) OsmanRF34 (talk) 14:51, 9 August 2014 (UTC)
 * Are you thinking of Cold welding? Nil Einne (talk) 14:57, 9 August 2014 (UTC)
 * Right, this is the article I needed.OsmanRF34 (talk) 15:33, 9 August 2014 (UTC)
 * Explosion welding is an extreme example of "push metals in contact and they blend". Maybe not quite the same process at heart because it sounds more like the two pieces deforming into each other rather than just contacting. DMacks (talk) 15:57, 9 August 2014 (UTC)


 * If you could make two perfectly smooth, perfectly clean, metal surfaces and bring them together in a vacuum - then you'd immediately have one piece of metal - you don't need pressure or anything. The atoms don't know that they 'belong' to one piece or the other.  So it's the microscopic roughness and the presence of thin layers of impurities that prevent that from happening in our normal experience. SteveBaker (talk) 02:11, 10 August 2014 (UTC)
 * So, if a rub two metals together in the vacuum, I should expect that they will 'click' at some point and weld together, specially softer metals? OsmanRF34 (talk) 15:54, 10 August 2014 (UTC)
 * You don't even need to rub them. If a) it is a true vacuum and b) the metals are scrupulously smooth, clean, and free of oxidation or other impurities, ANY contact will cause the metals to fuse together.  This has been demonstrated to work between like metals (two pieces of the same metal) and dissimilar metals (with, say, one piece of gold and another of silver).  This has to do with the nature of metallic bonding, which features highly delocalized electrons.  In non-metallic substances, all electrons have a high degree of "association" with a specific geometric region around a specific atom or molecule (this is called an orbital).  In quantum mechanical terms, we say that each electron has a region of space where it has a VERY high probability of being found (see almost surely for how probable this is...)  What makes a metal a metal, at the quantum mechanical level, is this lack of association between electrons and specific geometric regions.  When I take, say, two pieces of plastic into vacuum, and let them touch, nothing particularly interesting happens: the electrons stay in their orbitals around the molecules they always have been in, because of this high degree of association.  The plastics retain their individual identity BECAUSE the electrons retain their association with SPECIFIC orbitals.  In metals, which LACK this association, once two metals touch (and TRULY touch, without any gas or impurities between them), the electrons from piece "A" don't know the difference between the nuclei in piece "A" and the nuclei in piece "B", since they weren't really tied to any one part of piece "A" anyways, so they just sort-of all jumble up and become one piece.  -- Jayron  32  17:46, 10 August 2014 (UTC)
 * Isn't sintering related to this? Think of it as a very large number of microcrystalline metallic particles in contact with one another. At low temperature, there is a high ratio of surface area to volume, but the void ratio is also high, so instead of bonding, the particles can only clump together in a weak association. When the temperature is raised sufficiently, the particles soften, allowing the voids to be filled in. As a consequence, more surface area of individual particles in contact with each other, thus allowing them to weld together into a superstructure. Plasmic Physics (talk) 00:33, 11 August 2014 (UTC)
 * See Friction welding for joining 2 pieces of metal by rubbing them together.--Wikimedes (talk) 02:42, 11 August 2014 (UTC)

Earth gravity and animal speeds
I started to think that, for example, the peregrine falcon in his 300+km/h hunting stoop is significantly assisted by the gravity of Earth (essentially enjoying a free fall), which imparts him the acceleration and affects the bird's own speed. Cheetah, to the contrary, generates his hunting speed on his own, without g acceleration. So perhaps it's incorrect to declare the peregrine falcon the fastest animal among all, since birds, aquatic and terrestrial animals face different conditions when generating their speeds? Now if a hungry peregrine falcon is put into weightlessness and then sees his prey, what theoretical speed he would show? Brandmeistertalk  16:21, 9 August 2014 (UTC)
 * The article List of birds by flight speed lists another bird that's capable of straight-line [yes, I meant horizontal] speeds over 100 mph, i.e. faster than the cheetah. As to what's "correct", or what straight-line speed the birds of prey can attain, that would depend on what the sources have to say. ←Baseball Bugs What's up, Doc? carrots→ 16:39, 9 August 2014 (UTC)
 * Keep in mind that "straight" doesn't imply "horizontal". With the right "straight" line, you can get extremely high speed figures.
 * Even for the cheat... I mean cheetah. - ¡Ouch! (hurt me / more pain) 07:01, 12 August 2014 (UTC)
 * I wonder whether the diving bird speed is controversial due to Earth acceleration. If yes, then looks like the Oscar goes to Golden Eagle, rather than peregrine falcon, according to that list. Brandmeistertalk  17:18, 9 August 2014 (UTC)
 * It doesn't appear so, just that it's "qualified". You could google the subject, perhaps "bird speed controversy" and see if it hits anything. ←Baseball Bugs What's up, Doc? carrots→ 21:10, 9 August 2014 (UTC)
 * A friend of mine made the same argument for dethroning the falcon in conversation with me not too long ago. I was not convinced. The cheetah wouldn't do too well in zero-gee either. Evan (talk&#124;contribs) 05:40, 12 August 2014 (UTC).


 * One might naively attempt to correct for the dive speed by subtracting, say, the terminal velocity of the diving falcon, which I calculate to be approximately 100m/s or 360kph. However, TV takes many seconds to reach, while the falcon's dive is from relatively low altitudes, so by the 3-second half-growth rule of thumb of TV, we'd be subtracting maybe 150-200kph from the record time. Again, I emphasize that this is a very naive back-of-the-envelope estimate, but it could be a method to compare dives to horizontal flight. SamuelRiv (talk) 01:57, 10 August 2014 (UTC)


 * I agree that handing the title fastest animal" to something that can fall really quickly is a flawed thing. Is the terminal velocity of a sperm whale greater than a bowl of petunias?  Does either of them beat a falcon?  Unless someone knows - or specifies "fastest animal when engaging in it's normal behavior"...all bets are off (and even then, humans have the falcon beat too).  Records are things that are mostly of interest as trivia quiz questions - they have little value beyond that - and just like most trivia questions, if you pull them apart and stick them under the microscope - they all have more than one answer.  Sure, it might be of practical interest to examine which birds can dive faster than others in order that we may learn something about their aerodynamics - but ranking a bird against a cheetah and arguing about which one is fastest is kinda silly.


 * I have the same problem with people who claim that jai alai is "the fastest sport in the world" (presumably because of the ball speed) - when it's clear that placing a ping pong ball inside a formula one race-car would have it beat - and claiming formula one as the fastest is kinda silly when you have air-racing listed as a sport - and even that's broken compared to rifle shooting. Come to think of it, playing flashlight tag entails speed-of-light constraints, so maybe it gets my vote for "fastest sport".


 * From a science perspective, you just need to be a lot more specific about what you're measuring. If you aren't then it's not apples-and-apples comparisons and all bets are off.


 * This is more a question about linguistics and human nature, not science.


 * SteveBaker (talk) 02:04, 10 August 2014 (UTC)


 * "A Yugo has impressive acceleration. But, of course, this is only the case when pushing one off a cliff, which is an excellent idea, BTW." StuRat (talk) 00:44, 11 August 2014 (UTC)

Speed of magnetism
Watching Donald and Pluto makes me wonder — what's the speed of magnetism? Not mentioned in magnetism, and all I found in Google was Q&A websites and passing mentions in things such as this book. Nyttend (talk) 20:36, 9 August 2014 (UTC)


 * The only sensible interpretation that I could give to it would be as a field. Since magnetism is part of electromagnetism, any disturbances in this field propagate at the speed of light. —Quondum 20:44, 9 August 2014 (UTC)


 * Your basic magnetic field is just a special relativistic effect of the electric field (speed of light). In field theory, it's a field with a vector potential, while the electric field is a scalar potential, but I don't think that could possibly make a difference to propagation speed. And just in case this other thought crossed your head, ordinary permanent magnets are approximately magnetic dipoles, but in spite of their curvy look and $1/r^{3}$ field strength, the force is still based on the charge carriers in the material itself and that still propagates by photons at the speed of light. SamuelRiv (talk) 02:07, 10 August 2014 (UTC)
 * Could you add something of this to the magnetism article? Nyttend (talk) 04:32, 10 August 2014 (UTC)
 * I've added a brief sentence to this effect. —Quondum 06:20, 10 August 2014 (UTC)
 * Thanks; I don't understand enough to write this sensibly. Now created a Speed of magnetism as a redirect to this section.  Nyttend (talk) 13:52, 10 August 2014 (UTC)