Wikipedia talk:WikiProject Physics/Archive July 2015

Velocity-addition formula and Thomas precession suggestion

Velocity addition in relativity is, in the usual presentation in a textbook something quite easy. It is captured in essence by the formulae ("standard configuration")

$v_{x}={\frac {v_{x}'+V}{1+{\frac {V}{c^{2}}}v_{x}'}},\quad v_{y}={\frac {{\sqrt {1-{\frac {V^{2}}{c^{2}}}}}v_{y}'}{1+{\frac {V}{c^{2}}}v_{x}'}},\quad v_{z}={\frac {{\sqrt {1-{\frac {V^{2}}{c^{2}}}}}v_{z}'}{1+{\frac {V}{c^{2}}}v_{x}'}}$

and, when confined to a plane,

$v={\frac {\sqrt {v'^{2}+V^{2}+2Vv'\cos \theta '-({\frac {Vv'\sin \theta '}{c}})^{2}}}{1+{\frac {V}{c^{2}}}v'\cos \theta '}},\quad \tan \theta ={\frac {{\sqrt {1-{\frac {V^{2}}{c^{2}}}}}v'\sin \theta '}{v'\cos \theta '+V}}.$

Derivations are two- or three-liners.

Velocity addition in relativity is, when taken to the full extent, as mathematically involved as you wish. Goldstein:

• "The decomposition process [describing successive pure Lorentz transformations as a pure Lorentz transformation preceded, or followed, by a space rotation] can be carried through on the product of two pure Lorentz transformations to obtain explicitly the rotation of the coordinate axes resulting from the two successive boosts [that is, the Thomas rotation]. In general, the algebra involved is quite forbidding, more than enough, usually, to discourage any actual demonstration of the rotation matrix"

Physically, it has implications that made people (non-cranks) see paradoxes (Macanu paradox). I believe the situation was not fully resolved until 1990.

Now we have these two articles where Velocity-addition formula actually treats a little advanced stuff truly belonging elsewhere, while (imo) failing to treat the simple stuff (not all formulae there, complicated proof) adequately, which should include the formulae (all of them), full easy proofs and applications, e.g. aberration of light. Thomas precession is entirely nontechnical today.

I suggest we collect the advanced stuff somewhere, perhaps Thomas precession, perhaps a new article Thomas rotation, which would make sense because Thomas precession really is a physical phenomenon with mathematical root Thomas rotation. Lorentz transformation would make sense too, but I don't know whether people want to allow for that article to swell much more. Meanwhile, Velocity-addition formula should be reduced to the basics as given in textbooks. YohanN7 (talk) 14:50, 5 July 2015 (UTC)

Agreed. For supporting the "advanced stuff" in the Thomas precession article, along with Goldstein's classical mechanics and Jackson's classical electrodynamics (both in their second editions), Barut's book Electrodynamics and Classical Theory of Particles and Fields (uses classical methods) and Ryder's QFT (uses Lorentz group theory) could be good sources. Landau and Lifshitz volumes 2, 3, or 4 possibly may have something on the Thomas precession. Need to check these later. M∧Ŝc²ħεИ_τlk 10:18, 6 July 2015 (UTC)

Nothing in L&L. References will become a problem. The full results on Thomas rotation are fairly new and aren't present in any of the classics. The best references I could find are original research papers, the A. A. Ungar papers from 1988-89. Some of these are referenced in Velocity-addition formula. They are good, but there may still be problems because Ungar took these results and developed a completely new theory by abstraction, see Gyrovector space, an article I just discovered we have, see also my comment at Talk:Gyrovector space. So far nothing is wrong, it is perfectly natural. What seems strange is that it appears as a one-man show. Moreover, Ungar wrote a book on his new theory and the first few chapters can be found online (search his name and go to his university homepage):

• ...
• Chapter 3: The Einstein Gyrovector Space
• ...
• Chapter 5: The Ungar Gyrovector space
• ...

See the problem?

I'd like to see a reference to Ungar's 1988-89 results (we don't need the gyrovector stuff) from a third party. I personally think the results are legitimate, they look credible and they should easily be verifiable numerically, but full proofs aren't published (due to their complicated nature, I believe that actually, referring to Goldstein). YohanN7 (talk) 11:01, 6 July 2015 (UTC)

Of course there's something in L&L. The hyperbolic velocity space of Ungar figures in a problem (with solution) on p. 38. A bit brief for referencing though. This is further referenced in a modern paper as "perhaps the most intriguing approach", search for Relativistic velocity space, Wigner rotation, and Thomas precession (link was blocked). Ungar's book is mentioned in passing. YohanN7 (talk) 11:51, 6 July 2015 (UTC)

I'll copy this discussion to Talk:Thomas precession, and we can continue there. YohanN7 (talk) 12:13, 6 July 2015 (UTC)

---

Opinion needed

Draft:Lifshitz Theory of Van der Waals Force is up for review. I believe Lifshitz's contributions are duly stated in van der Waals force. Would there be any need to have a separate article? I wouldn't think so, as they can always expand the latter one. Any comments are appreciated. Cheers, FoCuSandLeArN (talk) 20:23, 12 July 2015 (UTC)

I would like to see it kept. It is more focused and goes into more detail than the other article. Xxanthippe (talk) 22:26, 12 July 2015 (UTC).

Edits at Nicolas Léonard Sadi Carnot

Someone contacted me about reviewing ongoing dubious edits at Nicolas Léonard Sadi Carnot, (diff of the relevant edits). Looks suspicious to me, but the changes are fairly technical, and other than going through and tagging with a bunch of CNs, I don't think I'll be much help at the moment. Anyone with a bit of time and some understanding of Thermodynamics want to take a crack at this? Previous discussion on Vsmith's talk page and my talk page. 0x0077BE ^{(talk · contrib)} 12:41, 13 July 2015 (UTC)

Requested move for hexaquark → dibaryon.

For some reason this isn't picked up by AAlerts, so I figured I'd give a notice here. You can join the discussion at Talk:Hexaquark. Headbomb {talk / contribs / physics / books} 14:20, 17 July 2015 (UTC)

Lagrangian

Anyone interested in Lagrangian mechanics and field theory may want to see this, since it will affect a number of articles. M∧Ŝc²ħεИ_τlk 21:45, 31 July 2015 (UTC)

AfC submission 28/07

See Draft:Weyl semimetal. Thank you, FoCuSandLeArN (talk) 18:56, 28 July 2015 (UTC)

It seems like a nice article, certainly well-referenced and has nice diagrams (the first diagram could have clearer labeling for the "Fermi arc", but its not a big deal). No expert on the topic, but it could be launched into mainspace for physicists to edit. M∧Ŝc²ħεИ_τlk 21:40, 31 July 2015 (UTC)

This draft has been accepted. Primefac (talk) 21:50, 31 July 2015 (UTC)