Wikipedia talk:WikiProject Physics/Archive February 2008

Homi J. Bhabha

Could someone review the first two paragraphs of Homi J. Bhabha#Work? I was a bit confused by Bhabha's work on cosmic rays and I am not sure if everything I wrote is accurate. I would appreciate it if someone could look into this matter. Thanks, Nishkid64 (talk) 20:02, 2 February 2008 (UTC)

Atom / Magnetic Moment

The atom article seems to be shaping up fairly nicely, I think, but there is some unhapiness with the Atom's magnetic moment section. Would it be possible for you physics experts to look it over and fix it where necessary? I'd like it to be comprehensible to non-experts, but still correct. (My head's rather disfunctional because of a lingering cold, so I'm not doing the best job of it right now.) Thank you!—RJH (talk) 22:47, 7 February 2008 (UTC)

Not much seems to be said about orbital angular momentum, which certainly contributes to magnetic moment. Depending on applied field strength spin orbit coupling might be important may or may not be important, so the magnetic moment obtains contribution from both L and S, and there might be precession. Also, I'm not sure if the ferromagnetism is as important, as it isn't as much an atomic phenomenon as a property of many particle systems (see also exchange interaction), but it probably doesn't hurt to leave it in. I've tried to improve the section a bit, but without using too much quantum mechanics it's a little hard.--Lionelbrits (talk) 05:16, 8 February 2008 (UTC)

Thank you!—RJH (talk) 16:37, 8 February 2008 (UTC)

Quantum Archeology

Given that the above article was placed in Category:Quantum Theory(sic), I thought it might be appropriate to inform this project of its AFD. --Sturm 19:20, 8 February 2008 (UTC)

Astronomy versus Astrophysics

Please come join the discussion about Astronomy vs Astrophysics articles here. WilliamKF (talk) 16:55, 9 February 2008 (UTC)

No Kinetics article on Wikipedia

This was shocking: dynamics (physics) disambigs to Kinematics and Kinetics, while Kinetics disambigs to Kinematics and back to dynamics. Result: there's no Kinetics article on this here wiki! --jwanders^Talk 08:04, 15 February 2008 (UTC)

Caloron

The stub Caloron is currently on AfD Wikipedia:Articles for deletion/Caloron. There seem to a a fair few papers on it, but I don't know enough to understand what its all about. Perhaps someone here could take a look and turn it into something more than a dicdef. --Salix alba (talk) 19:59, 22 February 2008 (UTC)

Harteck Process

Hello.

When possible, could anyone give some input at Harteck Process? We think it's a bogus article, so some input or reliable references would be appreciated. Oberiko (talk) 21:17, 25 February 2008 (UTC)

Nonlocality complete rewrite

I just rewrite this article mroe or less from scratch. It needs a bit of a copy-edit, but hopefully it is now a more rigorous discussion of non-locality in a physical sense. Needs some expansion in the applications section, but I just couldn't keep typing. I also feel that the lead may need expanding, but I can't see how. Can editors please go and take a look and begin editing if necessary? Happy to receive any and all constructive criticisms! - Fritzpoll (talk) 23:29, 25 February 2008 (UTC)

Edit war over postulates of special relativity

There is an edit war at Special relativity over the Postulates of special relativity. One side claim that the second postulate is redundant, and that Einstein said as much. The other side not only (correctly) denies that, but also will not allow the dispute to even be mentioned in the article. Check it out please. JRSpriggs (talk) 07:48, 20 February 2008 (UTC)

Here is the disputed text that is being deleted by Denveron (talk · contribs):

Einstein dropped the second postulate in his second 1905 relativity paper "Does the Inertia of a Body depend on its Energy-Content?" noting that "The principle of the constancy of the velocity of light is of course contained in Maxwell's equations."

--Michael C. Price ^talk 00:32, 23 February 2008 (UTC)

Suppose we reword this to say "In his second 1905 relativity paper, "Does the Inertia of a Body depend on its Energy-Content?" Einstein relegated the second postulate to a footnote, saying 'The principle of the constancy of the velocity of light is of course contained in Maxwell's equations.'."? JRSpriggs (talk) 02:00, 23 February 2008 (UTC)

But it is not a postulate if it is derived from Maxwell's equations (which was how Einstein arrived at it), whether relegated to a footnote or not.--Michael C. Price ^talk 21:51, 23 February 2008 (UTC)

If we drop the constancy of c, how is special relativity different from Galilean relativity? Even if you understand time to be a transformable coordinate, you would still need to explain what an inertial coordinate transformation is for time. You might mistakenly choose the metric to be (1,1,1,1) and end up with an infinite speed of light. There is a minimal set of postulates that give you relativity, which might include homogeneity and isotropy of spacetime Palash B. Pal. "Nothing but Relativity". Eur. J. Phys. 24: 315–319. doi:10.1088/0143-0807/24/3/312. {{cite journal}}: Unknown parameter |arXiv= ignored (|arxiv= suggested) (help) or causality Zeeman, E. C. Causality Implies the Lorentz Group, Journal of Mathematical Physics 5 (4): 490-493; (1964). But the first postulate isn't enough.--Lionelbrits (talk) 16:21, 23 February 2008 (UTC)

The implicit postulates are already contained in the article. --Michael C. Price ^talk 21:51, 23 February 2008 (UTC)

To MichaelCPrice: At the risk of repeating what has already be said at excruciating length at Talk:Special relativity, the special theory of relativity is NOT based on any particular physical theory such as Maxwell's equations for electromagnetism. The fact that they are used as premise in a few papers does not change the fact that the constancy of light speed is a postulate of special relativity in general. Special relativity is not an ordinary physical theory; it is a CONSTRAINT on physical theories. If you do not understand that, you have no business editing the article. JRSpriggs (talk) 07:50, 24 February 2008 (UTC)

Really JRSpriggs, that is the sort of attitude that does not reflect well on you. I understand the (elementary) point you're making (about constraints) and am disappointed that you have not paid more attention to the debate before (again) pronouncing me an idiot. As I have repeatedly said my concerns are pedagogical and historical: SR developed out of Maxwell's equations. Specifically Einstein derived the constancy of the speed of light from Maxwell. Teaching this makes SR a lot less mysterious to students and deserves a mention in Wikipedia. The issue that you and Denveron keep looping around is correct but totally irrelevant to my concerns. Wikipedia should be big enough to represent both concerns. But it isn't. Why? --Michael C. Price ^talk 17:58, 24 February 2008 (UTC)

Having watched the debate for a little bit now, I think it's safe to say that you (M.C. Price) are confusing inspiration with foundation. Maxwell's equations were certainly the inspiration for Einstein's Special Theory of Relativity. In his first paper on the subject, he refers to an example of an EM thought experiment involving a loop of wire and a magnet. He may also have been aware of the Michelson and Morley experiment, and probably knew that attempts to find or explain the inability to find the "aether" had failed. However, there is nothing in Maxwell's equations which implies that for *all* observers the speed of light is a constant. Certainly there is a constant with the value of the speed of light which had been quickly been used to associate EM waves with light.

This constant, however, did not say with respect to who or what the speed of light should be measured. Perhaps the simplest possible explanation, and not one ruled out in any way, shape, or form by Maxwell's equations was that there was a preferred frame (the aether) with which the speed of light was a constant. Experimental attempts to verify this failed. Einstein's Second Postulate (the invariance of the speed of light) is a second, equally (and as experiments have shown, more) valid attempt to answer the question of with respect to what was the constant in Maxwell's equations measured. Maxwell's equations in and of themselves can not and could not provide evidence for either the aether or for invariance (if you're argument is that they do, then you are free to tell me how...).

However, in a very real sense this is all besides the point. The Special Theory of Relativity, while historically a descendant of Maxwell's equations perhaps is physically independent. The first postulate states that the laws of physics, *whatever they may be* (e.g. Maxwell's equations may or may not be a "correct" law of physics), are the same in all inertial reference frames. If you exclude all knowledge about what the laws of physics are from your derivation of special relativity, you most certainly need the second postulate. Indeed, this is preferable in order to provide the most solid basis for special relativity.

Finally, a few last comments. Much of your argument is based on a footnote to a single paper by Einstein. Granted that Einstein was a rather smart guy (to say the least), relativity did not end with him (and arguably it didn't begin with him either). A footnote in one paper by one person, however brilliant and well respected, is no basis for an argument. I do not disagree that the viewpoint you are espousing is somewhat interesting for historical and physically aesthetic reasons (the idea that there was a kind of hint in Maxwell's equations to special relativity is something I must admit I find interesting), but it is no more than an interesting notion. Maxwell's equations undoubtedly inspired the second postulate, but it is the second postulate, as a postulate independent of all other theory, which is one of the two foundational pillars of special relativity. DAG (talk) 05:10, 25 February 2008 (UTC)

I am not confusing inspiration with foundation, and am at a loss to see why you think this. Since you offer no explanation of why you believe this, I pass over it. Regarding If you exclude all knowledge about what the laws of physics are from your derivation of special relativity, you most certainly need the second postulate. This is true (of course) , but rather misses the point, since the physical and historical origins of a theory are instructive. Penultimately, Einstein probably did not know of the Michelson-Morley experiment when he derived SR (later in his life he said he had heard of it, but earlier on (when his memory was presumably more accurate about recent events) he denied this). Finally, I note that you agree that Maxwell's equations undoubtedly inspired the second postulate -- that is all I'm trying to add to the article. Why the opposition and the multiple claims from multiple editors that I am confused? --Michael C. Price ^talk 09:40, 25 February 2008 (UTC)

If all you are trying to say is the historical statement that Maxwell's equations inspired (or at least helped to inspire) Einstein's second postulate. The point of confusion (at least) seems to be that while the second postulate may have been inspired by Maxwell's equations, it does not necessarily follow from Maxwell's equations alone, and this latter statement is what you have seemed to be saying. That is Maxwell's equations provides a hint towards, but no proof of, the second postulate. If you like, the key part is not that the second postulate states that the speed of light is a universal constant, but that it's always observed to have the same value for all inertial observers. This latter statement does not necessarily follow from Maxwell's equations.

As to my statement about excluding all knowledge of what the actual laws of physics are, you are correct that the historical origins of a theory are instructive. But they are not the whole story. Deriving the Special Theory of Relativity from both postulates, without any reference to (or even knowledge of) what the (other) physical laws of the universe are is a necessarily stronger constraint, and a stronger theory, than any possible derivation relying on other physical laws. That is that the second postulate is a much stronger statement than any combination of the first postulate and some set of physical laws (e.g. Maxwell's equations). Basically, special relativity with two postulates is a much stronger (and unavoidable) theory than with the first postulate alone and any other physical theory. DAG (talk) 14:02, 25 February 2008 (UTC)

Re the first paragraph: the second postulate may have been inspired by Maxwell's equations, it does not necessarily follow from Maxwell's equations alone. Of course, it requires the first postulate (of the principle of relativity) and the implicit postulates aforementioned -- but I don't think that is what you meant(?) Or is it? In which case you, me and Einstein are in agreement :-). Re the second paragraph, I completely agree, But [Maxwell's equations] are not the whole story. Have never said they were, but (and please don't take offense as some editors have) this is entirely irrelevant to the pedagogical/historical issue. They are two separate and compatible issues and both should be reflected clearly in the article. One truth shouldn't drive out another. --Michael C. Price ^talk 15:19, 25 February 2008 (UTC)

To MichaelCPrice: You keep saying that we are not listening to you. But it is you who are not listening to us. You dismiss our most important points as incomprehensible. They could only be incomprehensible, if you are either less intelligent than you clearly are or if you are deliberately misunderstanding us. So stop trolling and go away! JRSpriggs (talk) 10:21, 25 February 2008 (UTC)

No, JRSpriggs, I said one of your points (about SR being logically prior to Maxwell and hence C could be imaginary) was incomprehensible -- perhaps unphysical and ahistorical would be have been more pedantically accurate, although certainly incomprehensible to the target audience of relativity newbies. And I have just explicitly said that I agree with the (elementary) point that you and others repeatedly make about the Lorentz transforms since being raised to foundational status etc -- it's just that this is irrelevant to the pedagogical issues and the origin of SR. Please stop assuming that anytime I disagree with you that I'm a troll (or a crank as Denveron does)! Why can you not believe that I think it important to mention the origins of SR from Maxwell, without having a hidden agenda? --Michael C. Price ^talk 10:46, 25 February 2008 (UTC)

At that time there was a significant problem in reconciling Maxwell's equations with other accepted aspects of physics, particularly invariance under Galilean transformation, and also (whether or not Einstein himself considered it) the negative result of the Michelson–Morley experiment. This cast doubt on the correctness, or at least the universality, of Maxwell's theory. Only the postulated background "ether" seemed to offer a reasonable way to save the theory, but that notion was definitively shot down by Michelson-Morley. Einstein's observation was that Maxwell's equations could be saved by replacing the postulate of Galilean relativity with the postulate of invariance of the speed of light (which is compatible with Maxwell's equations), and following that to its logical conclusion he obtained the Lorentz transformations, etc. As Ssiruuk25 (labeled "DAG", no relation) said previously, that was Einstein's "inspiration" (or motivation). The theory of special relativity as originally presented did postulate the constancy of the speed of light, but Einstein soon found that he could formulate the theory without putting so much emphasis on that. I see nothing wrong with JRSpriggs' proposal to reword the disputed text to say "In his second 1905 relativity paper, "Does the Inertia of a Body depend on its Energy-Content?" Einstein relegated the second postulate to a footnote, saying 'The principle of the constancy of the velocity of light is of course contained in Maxwell's equations.'" That avoids getting into the detailed historical considerations, while remaining factually accurate. — DAGwyn (talk) 23:36, 26 February 2008 (UTC)

Einstein spent years studying Maxwell's equations and was aware of the failure to detect the aether. That was the inspiration for SR, not the invariant lightspeed. Once he had formulated SR he swiftly realised (according to an Einstein letter, perhaps in Abraham Pais, I'm not sure) that he could replace the assumption of Maxwell's equations by just the constancy of the speed of light, which was what he published in his first paper. I do not regard saying that in the article as getting bogged down in "detailed historical considerations".--Michael C. Price ^talk 00:01, 27 February 2008 (UTC)

AfD nomination of Harteck Process

An article that you have been involved in editing, Harteck Process, has been listed for deletion. If you are interested in the deletion discussion, please participate by adding your comments at Wikipedia:Articles for deletion/Harteck Process. Thank you. TomStar81 (Talk) 18:44, 26 February 2008 (UTC)

The rubber-sheet model

The image on the right is currently used in four articles, general relativity, spacetime, theory of relativity and theoretical motivation for general relativity. It looks like an illustration of the famous rubber sheet model of gravitation, which is an excellent model of Newtonian gravity; in fact, it's a quantitatively exact model once you make enough idealizing assumptions. I suppose it's also a good model of linearized GR. But as far as I know it's completely wrong, quantitatively and qualitatively, as a model of space(time) curvature. So I don't think this image should be in articles about general relativity. Actually, because people so commonly confuse the rubber sheet with spacetime geometry, maybe it should be in some of the articles with a caption clarifying that it has nothing to do with general relativity, and that this is not the right way to picture spacetime curvature.

It's also possible that this isn't supposed to be a rubber sheet, but an actual picture of spacetime geometry (isometrically embedded). In that case, it's the wrong shape; I suppose you could find a spacelike slice through the Schwarzschild geometry that looked like that (or anything else) when embedded, but the "usual" slice (constant Schwarzschild coordinates) doesn't have that shape. Also, even if it's not intended to be a rubber sheet, it still looks like one, complete with a heavy ball on the top making it curve down, and the articles do nothing to dispel that misconception. So it seems worth explicitly pointing out that this is not a rubber sheet—that there is a rubber sheet model of gravitation, but it's a completely different thing.

If this were one article I'd probably just change it, but it seems like a fairly broad problem; given that "everyone knows" that spacetime is like a rubber sheet, there may be more articles involved than the four that use this image. I'm hoping for a go-ahead or whoa-there from the community before I go and edit everything. -- BenRG (talk) 22:04, 25 February 2008 (UTC)

Yes, the rubber sheet does not account for the time dilation near massive bodies which is the cause of the most important effect of gravity in everyday life. JRSpriggs (talk) 03:46, 26 February 2008 (UTC)

I don't think it accounts for the spatial components of the metric either. There's an accidental similarity of appearance between the Schwarzschild embedding and the rubber sheet, but no real connection. The accidental similarity disappears if you draw the embedding so that it bulges upward instead of downward. You can do that with an embedding (only the intrinsic geometry matters) but you can't do it with the rubber sheet (an upward bulge would be a repulsive force). For some reason unknown to me, virtually every embedding found in textbooks bulges downward. If upward-bulging (or sideways-bulging) embeddings had caught on instead, the confusion might never have arisen. The rubber sheet model really has nothing whatever to do with spacetime geometry, no way, no how, as far as I can tell. -- BenRG (talk) 13:27, 26 February 2008 (UTC)

The "rubber sheet" analogy was, as I recall, first used in books and articles for the layman, as an extreme simplification to indicate how an object might be embedded in a curved manifold and how its motion could be influenced somehow by the curvature. I am sure it was never meant as an exact, or even approximate, model. — DAGwyn (talk) 23:50, 26 February 2008 (UTC)

See Kepler problem in general relativity, a very nice article by WillowW (talk · contribs) on a related subject. JRSpriggs (talk) 10:00, 28 February 2008 (UTC)