# Wikipedia talk:WikiProject Physics/Archive January 2010

## Stimulated emission

Can someone check that the statement in the lead at Stimulated emission, that the second photon is created with the same phase, frequency, polarization, and direction of travel as the original. is true? I suspect that it might only be true macroscopically, in that these contributions are reinforced by constructive interference and the others destructively cancel, but am not sure. --Michael C. Price talk 10:09, 27 December 2009 (UTC)

AFAICT they have the same wave vector and polarization; essentially, the occupation number of photons in that state increases by 1. Not 100% sure about that, though. ― A._di_M.2nd Dramaout (formerly Army1987) 12:32, 27 December 2009 (UTC)

A simple dumbed down explanation goes as follows. The amplitude A of the process is obtained from some matrix element A = <out|M|in>, where |in> represents the atom in the excited state and one incident photon and |out> is the atom in the ground state and two photons. The stimulated emission effect is the fact that A peaks for the additional photon in |out> in the same state as the original photon. To see without doing any calculations why this is the case, consider the reverse process where the |out> state is the initial state and the |in> state is the final state. So, we have two incident photons and an atom in the ground state, the final state is the atom in the excited state and one photon. The amplitude of this process is, of course, the complex conjugate of the amplitude of the original process, so the absolute value squared is the same.

Now, if you have two incident photons, then either one can interact with the atom, bringing it in an excited state and the other just moves along. So, you have two contributions to the final state, but the photon in the final state can be in a different state. The amplitude for the final state photon to be in some given state is then obtained by taking the inner product of both contributions with that given photon state. If we keep the state of one of the incident photons the same and we look at amplitude for the final state photon to be in the same state then this is clearly maximized if we choose the state of the other incident photon the same as the other photon. Count Iblis (talk) 15:55, 27 December 2009 (UTC)

Stimulated emission is not so much a physical process as a result of using probability amplitudes rather than probabilities when considering particles with Bose–Einstein statistics. The "amplification" is due to the photons (the original photon or photons and the stimulated photon) being indistinguishable, so that you get cross-terms when multiplying the amplitude by its complex conjugate to get the probability. Thus they must have the same properties. (But make sure that the set of properties being considered is a commuting set of observables.) JRSpriggs (talk) 15:59, 27 December 2009 (UTC)
Well, neither the original photon nor the emitted photon can have an exact frequency or direction or phase (by the uncertainty principle). Even a "single-mode" laser has finite linewidth. Both photons have some distribution of frequency and direction and phase. And I think the more distributions overlap, the more stimulated the emission is. The standard formula for stimulated emission rate assumes the distributions coincide. But if the photon quantum states have 99% overlap instead of 100% overlap, the stimulated emission rate will only be 1% lower.
I think people don't usually think of stimulated emission as being related to interference. The main (microscopic) explanations I've heard are
Maybe there are other explanations involving interference, but I haven't heard them.
I think the statement in the lead that you quote is fine. --Steve (talk) 16:05, 27 December 2009 (UTC)
The lead implies that, if an isolated excited atom is struck by a photon, the emitted photon always departs in the same direction as the original photon. Is this correct? --Michael C. Price talk 06:06, 29 December 2009 (UTC)
I say yes. It sounds like you're worried that stimulated emission is like second-harmonic generation where the light goes a certain direction because the different locations add in phase for one light direction and out of phase for other directions. For example, if you use a tiny second-harmonic-generating-crystal much smaller than the wavelength of light, the second-harmonic light would radiate in all directions, because it's so small that the whole crystal is in phase for any direction. Stimulated emission isn't like that. You can make a laser where all the stimulated emission occurs in a tiny area, much much smaller than the wavelength of light, and it doesn't affect how sharp the beam is.
(I haven't heard of that exact experiment with one atom and one free-space photon. Experiments like that usually have a cavity, so there's a priori only one photon direction that the atom can easily radiate into.) --Steve (talk) 07:02, 29 December 2009 (UTC)
Your parenthetical comment suggests that it remains possible that it is more the cavity that enforces the direction of emission, rather than the absorption/stimulated emission per se.--Michael C. Price talk 16:47, 31 December 2009 (UTC)
The role interference plays in this is that the total amplitude is so large because all the photons interfere constructively (which is what JRSpriggs said in a more precise way). (Another thing I've heard—but I don't thing it was intended to be any more than hand-waving—is that if the light shakes the electron in phase with its current motion you get absorption, otherwise you get emission. The QM analogue of this, of course, only applies if you have three eigenstates of H such that <3|H|3> − <2|H|2> = <2|H|2> − <1|H|1> and now you're in the state |2>, otherwise there's no way you have both a significant chance of emitting and of absorbing depending only on relative phases (I guess).) ― A._di_M.2nd Dramaout (formerly Army1987) 17:07, 27 December 2009 (UTC)

The lead implies that, if an isolated excited atom is struck by a photon, the emitted photon always departs in the same direction as the original photon. Is this correct?

It should be clear that this is not correct. Just compute the differential cross section for this process. All you'll find is that the amplitide is maximized when the emitted photon is in the same state as incident photon, which can be easily understood per the reasoning involving the inverse process given by me above, which implicitely uses Bose-Enstein statistics for photons. Count Iblis (talk) 01:10, 1 January 2010 (UTC)

Yes, it seemed to me that this was not correct, and that therefore the lead needs modifying. How this ties in with JRSpriggs answer is not clear to me. The sense I'm getting from all the replies is that bosonic clumping makes the additional photon more likely to be emitted in the same direction, but not a 100% likely (i.e. a certainty). However Spriggs indicates this might be 100%, and his argument seems interesting, so I'm still not clear. --Michael C. Price talk 07:58, 1 January 2010 (UTC)
It is certainly possible for the emitted photon to be in a different state than the given photon, but in that case the presence of the given photon does not stimulate the emission, that is, the probability of the emission in that state is the same as if the given photon were not present. This is the same conclusion as one gets from Count Iblis's argument. JRSpriggs (talk) 20:13, 1 January 2010 (UTC)

## Particle in a box

Previously posted at WikiProject History of Science

I've been doing quite a lot of work on the Particle in a box article, trying to make it more descriptive and less mathematical. However, consensus seems to be that we need a history section. I could come up with some very general waffle about the early days of quantum mechanics, but if anyone can help with specific details about who first developed the particle in a box model, then I'd be very grateful. Thanks, Papa November (talk) 17:19, 2 January 2010 (UTC)

## Experts needed at Talk:Big Bang

Amidst the usual cruft, someone posted a very interesting question (at Talk:Big Bang#Underlying Assumptions). The question is about what the exact domain of applicability was of the proof that an expanding universe must have expanded from a singularity. This is way out of my league to address, but I seem to recall that there were at least a couple of GR specialists among the WP:PHYS crew. --Christopher Thomas (talk) 04:03, 6 January 2010 (UTC)

## Talk:Rydberg matter

There a potentially 'ticking-time bomb' situation developing at Rydberg matter. An editor with a COI wrote a good chunk of the article, and another suggested that the article does not reflect mainstream physics on the issue. Extra pairs of eyes on the page would be nice. It's relatively civil now, but the tone of the discussion leads me to think that it could get ugly soon. Headbomb {ταλκκοντριβς – WP Physics} 02:07, 3 January 2010 (UTC)

Yes, the article looks stuffed full of actual or potential POV, OR and ownership issues. Xxanthippe (talk) 02:30, 3 January 2010 (UTC).
It might be starting to get ugly. I've tried to diffuse the situation before it gets out of control (will I be successful, I don't know). So please take a look at Rydberg matter and lend a hand. Headbomb {ταλκκοντριβς – WP Physics} 23:17, 6 January 2010 (UTC)

## Extensive anon rewrite of Special relativity

has undergone an extensive rewrite over the last few days by an anonymous user (171.66.x.x; so far, 171.66.105.55  and 171.66.91.179 , resolving to stanford.edu). These changes have been controversial. I invite all experts in the subject to review the article history and the talk page thread regarding the changes. --Christopher Thomas (talk) 17:20, 8 January 2010 (UTC)

## Schwinger's variational principle

Schwingers Variantionsansatz in the German Wikipedia seems to be a translation of the stub Schwinger's variational principle. The paper cited in the article (Schweber 2005) does not make any reference to a "variational principle" but to "Schwinger's quantum action principle". Thus, my question is whether the expression actually exists, or whether it is to be deleted as a newly invented expression following Wikipedia:No original research.

Our understanding is that there were several contributions (independent research of Tomonaga, Schwinger, Feynman, and Dyson) that lead to what we call perturbative Quantum Field Theory our days. Peskin/Schroeder refers to Schweber: QED and te men who made it for details. Itzykson/Zuber recover Schwinger's quantum mechanical action principle by extending the stationary phase approach to operators; however, this is meant as a description of the approach rather than an encyclopedic phrase (although admittedly appearing as "Schwinger's action principle" in their table of contents).

Imho, "Schwinger's variational principle" is a non-existing lemma. It should best be deleted. If deletion is not an option, the article should be renamed to "Schwinger's action principle", and should be placed into perspective by mentioning that it is one of several approaches to QFT. Alternatively, the text could be added to a new section "Approaches to QFT" in Quantum field theory.

A deletion/renaming of the English stub would be a good indication on how to proceed with the German page. --Dogbert66 (talk) 22:44, 8 January 2010 (UTC)

article has been moved to Schwinger's quantum action principle. --Dogbert66 (talk) 23:59, 3 November 2010 (UTC)

## Speed of light article up for FAC

See Wikipedia:Featured article candidates/Speed of light/archive2. ― A._di_M.2nd Dramaout (formerly Army1987) 17:11, 10 January 2010 (UTC)

## Sourced content being removed at Tests of general relativity

An anon is removing a sourced section. See [1] and [2]. After my revert he left a message on the talk page. Any experts around? DVdm (talk) 17:55, 10 January 2010 (UTC)

The anon's revert looks reasonable to me, but I'm not an expert either. Xxanthippe (talk) 00:39, 11 January 2010 (UTC).
Seconded, with the caveat that the whole article could probably use vetting by an expert in the field. --Christopher Thomas (talk) 01:10, 11 January 2010 (UTC)

## Quasicrystal

Could perhaps someone knowledgeable have a look at this article? I don't know anything of it but came there from Peer review where an editor User:Bourdillona was inserting self-published material. He seems to have an agenda in crystallography and has seeding several articles with links to his web sites. To be frank, he sounds like a fringe scientist to me, but that doesn't necessarily mean that he is one and his additions are perhaps legitimate. I have no way of knowing and therefore it would be helpful if someone who knows more about this stuff would have a look. Thanks. --Crusio (talk) 15:00, 9 January 2010 (UTC)

Via the user's contributions I came across another article, Modified Bragg diffraction, which I've nominated for deletion for all sorts of reasons, and am now contending with the user removing and falsely disputing the AfD template on the page, if anyone else wants to take a look. --JohnBlackburnewordsdeeds 23:50, 10 January 2010 (UTC)

Sorry. The deletion was inadvertent when I wrote to an old version my mistake.

I have removed the added part in quasicrystal because it violates several WP policies, including WP:COI, WP:RS and WP:OR - the linked web pages are not appropriate references as they apparently copy the journal papers in the midst of review process; even if those results are true, secondary sources are required. Materialscientist (talk) 10:42, 11 January 2010 (UTC)

The secondary source is given: "Nearly free electron bands..." The web pages are subsidiary and the references were deleted. My profile is on Wikipedia. The 'added part' has been on wikipedia for over a year with some discussion and agreement. More debate is needed. —Preceding unsigned comment added by Bourdillona (talkcontribs) 14:49, 11 January 2010

Discussed and agreed where, i.e. links? I've not found it after a quick look in obvious places, and you've mentioned previous discussions also in your edit summaries. The discussion on the talk page suggests there was little agreement before. --JohnBlackburnewordsdeeds 14:56, 11 January 2010 (UTC)

## Combustor

A little cross-wiki-project posting here. I have opened a peer review for the Combustor article over a WP:Aviation. It's a reasonably technical topic, and I'm hoping to get comments from several different groups of editors so that I get a decent feel for how easy the article is to read/understand. Please take a look at the article and leave any comments you may have at the peer review page. Thanks! -SidewinderX (talk) 20:43, 12 January 2010 (UTC)

Wikipedia:Help desk#External Links 2 and User talk:Amanda.nelson12#January 2010 discusses external links to a large online resource of interviews with physicists and some other scientists. PrimeHunter (talk) 17:16, 15 January 2010 (UTC)

The links to AIP oral history are of great importance and should be retained. Xxanthippe (talk) 21:54, 15 January 2010 (UTC).

## Science and Technology Facilities Council

Hi all. Could someone please take a look at the "Funding crisis" section of the Science and Technology Facilities Council article? It appears to be very misleading, particularly the subsection about Jodrell Bank. For a more accurate description, see Jodrell Bank Observatory#Threat of closure and E-MERLIN#STFC.27s Programmatic Review 2007-8. The section also needs updating to describe the latest rounds of cuts; see the STFC-provided information and Paul Crowther's summary from an astronomer POV. Some sort of a balance between the two is needed here, I guess

I have a conflict of interest here - I work at Jodrell Bank, and some of these cuts have directly affected me - so I don't feel that I can directly edit the article. I would also really struggle to maintain a neutral point of view here. However, the section of the STFC article seriously needs looking at IMO, hence my appeal here. Thanks. Mike Peel (talk) 15:02, 16 January 2010 (UTC)

## Straw poll at Talk:Quasar

I've been doing a bit of cleanup at Talk:Quasar (added auto-archiving of very-old threads, added datestamps, split off new comments on old topics). One item I don't feel comfortable unilaterally addressing is whether or not the article is overly-technical (per the template on the talk page, which might or might not be obsolete). If you have a few idle minutes, please take a moment to glance at , and to comment in the thread in question. --Christopher Thomas (talk) 08:26, 13 January 2010 (UTC)

A note to WP:WikiProject Astronomy would have been nice... 76.66.197.17 (talk) 06:03, 19 January 2010 (UTC)

## Recurring argument at black hole

Every so often, a high-traffic thread starts at Talk:Black hole about whether there's any direct evidence for black holes, and how strong or weak the circumstantial argument for them is. This time around, we have an editor making changes to the article itself, in addition to posting to the talk page thread. More eyes from experts in the field would be helpful; the relevant thread is at Talk:Black hole#Disclaimer in the Lead. The article is back at its previous consensus-state for now, but the discussion is far from over, so contributions from actual astronomers and astrophysicists would help. If nothing else, the fact that it keeps coming up on the talk page suggests that it could be clarified in the article (the last such discussion resulted in the "observational evidence" section being fleshed out). --Christopher Thomas (talk) 08:33, 17 January 2010 (UTC)

## DYK Nebulium

As this nebulium is more a spectroscopy discovery a look by a person with more knowledge in physics might be good. The article will be on the Mainpage in one hour. Thanks from the WPElements--Stone (talk)

The DYK hook (".. that it took until 1927 to disprove the discovery of the new chemical element nebulium in the Cat's Eye Nebula in 1864?") seems factually correct. You could argue that it was already clear before 1927 that Nebulium could not exist, based on Moseley's work, but we are talking about a period in science when very little was generally accepted: as such, it is valid to wait for the experimental confirmation that oxygen ions were responsible for the spectral line before saying that the "discovery" was "disproved". Physchim62 (talk) 16:29, 21 January 2010 (UTC)
Thank you!--Stone (talk) 06:07, 22 January 2010 (UTC)

## WP 1.0 bot announcement

This message is being sent to each WikiProject that participates in the WP 1.0 assessment system. On Saturday, January 23, 2010, the WP 1.0 bot will be upgraded. Your project does not need to take any action, but the appearance of your project's summary table will change. The upgrade will make many new, optional features available to all WikiProjects. Additional information is available at the WP 1.0 project homepage. — Carl (CBM · talk) 03:46, 22 January 2010 (UTC)

## surface gravity

I have a problem with the explanation found in the second paragraph of the first section:

A large object, such as a planet or star, will usually be approximately round, because large mountains will be squashed down, and large valleys filled in, by the object's own gravity.

It just seems flat out wrong. Can someone who actually understands what is trying to be said here correct the passage? --Izno (talk) 22:51, 10 January 2010 (UTC)

I'll take a look at rewording this. It's an overly-dumbed-down description of hydrostatic equilibrium. --Christopher Thomas (talk) 23:36, 10 January 2010 (UTC)
Thanks. --Izno (talk) 00:19, 11 January 2010 (UTC)
If they said "large, non-rotating object", would that be better?--Michael C. Price talk 20:02, 25 January 2010 (UTC)
It also applies to rotating objects, except that the equilibrium shape is an oblate spheroid rather than a sphere. For not-too-large rotation speeds, the "approximately round" still holds.― A._di_M.2nd Dramaout (formerly Army1987) 20:11, 25 January 2010 (UTC)

## Biocentrism in cosmology

Biocentrism in cosmology has been nominated for renaming

76.66.192.206 (talk) 05:58, 24 January 2010 (UTC)

## List of Feynman diagrams

Just created this. Extremely stub-like and in need of expansion (and attention from experts, especially for the descriptions). Headbomb {ταλκκοντριβς – WP Physics} 18:18, 25 January 2010 (UTC)

My immediate reaction is "aren't there an infinite number of Feynman diagrams?" Physchim62 (talk) 18:24, 25 January 2010 (UTC)
There are infinitely many prime numbers too, but this doesn't stop List of prime numbers from existing. It's not titled "List of all Feynman diagrams"... ― A._di_M.2nd Dramaout (formerly Army1987) 20:00, 25 January 2010 (UTC)
Yes there are, but some diagrams such as those of pair creation/annihilation and box diagrams are definately "textbook cases" of Feynman diagrams, which could all have their standalone articles. This page would list the common/famous/prototypical diagrams. Ideally, there should be a list of the basic diagrams (aka all types of possible vertex), then a list of the "common/famous" diagrams (beta decay, box diagrams, penguin diagrams, ...). Headbomb {ταλκκοντριβς – WP Physics} 23:30, 25 January 2010 (UTC)

## Course of Theoretical Physics

This is a well-known series of books by Russian physicists Lev Landau and Evgeny Lifshitz. I've detailed the English publication history, but it would be nice to have the Russian side of the story. Thanks. Headbomb {ταλκκοντριβς – WP Physics} 04:10, 28 January 2010 (UTC)

## Definition of second: classical or quantum vacuum?

The article Second seems to imply that the definition is supposed to apply to classical free space, not quantum vacuum, so that one has to correct the measurements for the Lamb shift. This sounds weird to me; I would have assumed that it applied to quantum vacuum, i.e. space without particles or photons. Can anyone who knows better shed light on Talk:Second#Classical or quantum vacuum?? ― A._di_M.2nd Dramaout (formerly Army1987) 11:44, 29 January 2010 (UTC)

First, I should say that I don't think there is any quantum correction to how fast low-intensity light travels, but I'm not an expert. Let's suppose there is, for the sake of argument. If there is, I feel strongly that the "second" would be defined in terms of the quantum vacuum, not the "classical free space". Because the quantum vacuum is the one that actually exists in the universe we live in. The quantum vacuum is the one that's approached closer and closer by better experimental methodologies. NIST and other such organizations are not, to my knowledge, sponsoring theory research into whether there are quantum corrections to how fast low-intensity light travels, even though it quite conceivably could be a significant correction (given how accurate and reproduceable atomic clocks are). They don't mention anything about theoretical quantum corrections in SI literature. It's conceivable that without quantum corrections the speed that low-intensity light travels at could be twice as high or something. (If this sounds crazy, remember that without quantum corrections, the electron has an infinite mass! And lots of other silly things happen too.) If some theorist argued convincingly that there was a factor-of-two alteration of the speed of light accountable to quantum vacuum fluctuations, I can't imagine that SI would say we have to reset our clocks by a factor of two!
In general, I think it's a very weird hypothesis to say that some definition of a real-world unit that measures real-world quantities should refer to anything but the real world and the real laws of physics. The real world is quantum, not classical.
I have argued this at length with Brews, and I believe he would disagree with what I wrote above, but I'm not sure. What do other people think? --Steve (talk) 17:07, 29 January 2010 (UTC)
The discussion is linked above (and my intent was that answers would go there, too). ― A._di_M.2nd Dramaout (formerly Army1987) 17:37, 29 January 2010 (UTC)
Oops sorry. :-) --Steve (talk) 01:19, 30 January 2010 (UTC)

## wiki/Pea_galaxy

I have all but finished this article on the new discovery by the Galaxy Zoo citizen scientists of Pea galaxies. These are some of the most star-forming galaxies in the local Universe, having rates over 1000 times faster than the Milky Way. There has been a link to the this Wiki Physics section on my discussion page, but I have only just plucked up the courage to put it here, as I am not a physicist.

If someone could rate the article- it has all the citations and has been edited thoroughly, so is waiting for a larger audience! http://en.wikipedia.org/wiki/Pea_galaxy

Thanks, Richard Richard Nowell (talk) 10:50, 30 January 2010 (UTC)

## Unreferenced BLP cleanup

There is currently a lot of concern across Wikipedia about how biographies of living people (BLPs) are handled (see discussion). One solution which is being considered is mass deletion of all unreferenced BLPs. While I doubt there are many significant problems in BLPs of interest to the Physics project, to avoid having our articles caught up in the process it would be worthwhile to go through them and try to get them up to the required standard. (I also suspect that getting rid of unreferenced biographies isn't going to help much in solving the real problem of people slipping contentious content about living people into various Wikipedia articles, but that viewpoint seems to be on the loosing side in the discussion!)

Based on the latest bot-generated cleanup listings, I've made a list of the articles which are tagged with WikiProject Physics BPH and also tagged as unreferenced BLPs. I'm going to try to work through the list to check whether articles are really unsourced, whether sources can be found, etc. Any help would be appreciated! Djr32 (talk) 12:38, 31 January 2010 (UTC)

Does a paper written by the person qualify as a reference?Chhe (talk) 20:38, 31 January 2010 (UTC)
If the paper gives his biographical information and is subject to editorial checking to make sure that it is a reliable source WP:RS. JRSpriggs (talk) 21:02, 31 January 2010 (UTC)