Talk:Quantum theory

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Articles for deletion This article was nominated for deletion on October 19, 2006. The result of the discussion was keep.

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[edit] expert needed

This article arised from a discussion in talk:quantum mechanics where it appeared that wikipedia is not consistent with what is quantum mechanics as opposed to other quantum theories.

An expert is needed to clarify what are all the quantum theories, and define what they are (scope, limitations), and their relations with each other (from a history and scope point of view. (I'm no such expert). In particular, he should clarify whether quantum mechanics is relativistic or non-relativistic. Hopefully, it will then be easier to bring consistency in all the specific articles, and convey a clear picture to an engineer like me.Pcarbonn 05:47, 30 August 2006 (UTC)

How about making this a disambiguation page? --Michael C. Price talk 12:05, 30 August 2006 (UTC)
I now see that you have made several edits to this article: they look great ! Thanks for the clarification. Pcarbonn 17:30, 30 August 2006 (UTC)

Glad you like them. On second thoughts I left the QM entry mute on the question of whether it is relativistic or non-relativistic. The Schrodinger equation is non-relativistic, but the Dirac equation is relativistic. Things aren't clear though, since the Dirac equation sort of straddles both QM and QED. Best to keep quiet on the subject and let the respective articles cover the subject.

Gravity should really be gravitational force or interaction since, according to some purists, gravity refers to the gravitational force on Earth alone. (I'm not entirely convinced by that argument so I'm going to leave it as it is, but some people feel strongly about this.) --Michael C. Price talk 20:39, 30 August 2006 (UTC)

I don't see the need for this article. If it stays, I suggest changing "Quantum mechanics" to "Non-relativistic quantum mechanics". Plank's constant was discovered by finding that photons need to be quantized to explain black body radiation. (Maxwell's equations are relativistic.) Schrödinger, started with the Klein-Gordon equation and took the non-relativistic limit. So relativity has been in QM from the start; it is just harder to calculate. Non-relativistic QM has always been considered an approximation to relativistic QM, even before it was known that that would take the form of QFT. QM is more of a description of nature than a dynamical theory, and certainly not a way of calculating as is QFT. QFT uses the QM description.

I agree that it is possible that there might be changes to QM when it is merged with GR, but that is not clear yet. Field theory might change but in a way that does not take it out of quantum mechanics.

I have always found the term "second quantization" confusing and Roger Penrose supports this view in The Road to Reality.

As I pointed out on the QM talk page, relativity is not needed to have particle number not conserved. Phonons appear and disappear in non-relativistic QM. David R. Ingham 23:05, 1 October 2006 (UTC)

Phonons are a product of a QFT (which may be non-relativistic), not QM -- solid state physics, as mentioned in the article. --Michael C. Price talk 06:33, 2 October 2006 (UTC)
That makes QFT a sub-field of QM, as I thought, because phonons (and their nuclear equivalent) are predicted by the many body Schrödinger equation. Since I see no other objection, I am nominating the article for deletion. There is at least one type of quantum physics: the Old Quantum Theory, that is not QM, but that is now considered an approximation to QM. So the central idea of this page is an error. David R. Ingham 23:34, 19 October 2006 (UTC)

[edit] History section

Perhaps we should pull the early history of quantum theory into this article? --Pjacobi 19:59, 24 September 2006 (UTC)

Only if we deleted the Quantum_mechanics#History section to remove duplication. --Michael C. Price talk 20:28, 24 September 2006 (UTC)
Split at 1925, expand the old quantum theory part. --Pjacobi 19:43, 25 September 2006 (UTC)

[edit] Merge?

  • con --Pjacobi 19:42, 25 September 2006 (UTC)
  • con: Pcarbonn 16:13, 26 September 2006 (UTC)
  • con: Kbrewer 03:34, 29 September 2006 (UTC)
  • strong pro: QM and QT differ only at the Quantum Hamiltonian. --Andrej Westermann talk 06:20, 29 September 2006 (CEST)
Comment: That is a big difference, even if true. --Michael C. Price talk 07:24, 30 September 2006 (UTC)
I've removed the merge tag: there was only one vote in favour. --Michael C. Price talk 15:31, 6 October 2006 (UTC)

[edit] Sources requested

The AfD is clearly a bust, but this article definitely needs work. Rather than arguing about what definitions we should use (a dead end, since it's borderline OR), we should actually find some sources.

The terms in this article are used as though they're universal, when in fact I'm not so sure that the distinction between "quantum theory" and "quantum mechanics" has a universally-agreed-upon definition. We need sources for these terms, and we need to state clearly whether the terms are universal or only used by some people. -- SCZenz 22:38, 20 October 2006 (UTC)

[edit] Some specific concerns

As marked in the article, here are some of my specific concerns:

  1. "Quantum mechanics" is not always used to refer only to non-relativistic quantum mechanics. We need to be clear on this
  2. "Second-quantization" is an old term, reflecting an outdated understanding of Quantum Field Theory and its relation to non-relativistic quantum mechanics.
  3. "Third-quantization" is not a commonly used term as far as I am aware. If my impression is correct, we should not be presenting the term as though it is.

That's it for now, I think. -- SCZenz 23:22, 20 October 2006 (UTC)

I agree. AFAIK third quantization was used for a short while in the early days of quantum gravity. The term Second Quantization is still used a lot in condensed matter physics. In that context it is perhaps a more natural term than in QFT. Of course, there is only one quantum theory so, if you quantize something "for the second time", then you must have quantized minus one times, i.e. gone from a quantum description to a classical description, in some intermediary step. This is indeed what happens when one considers the Schrödinger equation as a classical field equation that describes a many particle system.

In QFT these are just heuristic arguments that lead to the correct description. But the "classical schrödinger equation" does have physical significance in many condensed matter problems. E.g. the Gross-Pitaevskii equation that describes a Bose-Einstein condensate is vald in the lage N limit and the order parameter is a classical observable (but it does still contain Planck's constant). To go back to the quantum theory you could quantize this. From the equation you can see that you could derive this from a suitable Lagrangian, and the full quantum mechanical description can be regained e.g. using the path integral formalism. Count Iblis 00:31, 21 October 2006 (UTC)

OK, I did some fact and etymology checking and a complete rewrite. Better? Michael K. Edwards 02:44, 21 October 2006 (UTC)

without wanting to play stupid, didn't you think the word quantum needs be specifically included, for easy viewing? --andrej.westermann 02:59, 21 October 2006 (UTC)
I think second quantisation is still widely used today. Also I note the current article incorrectly credits Planck with the invention of light quanta, whereas this was due to Einstein (Planck quantised only the emission and absorption of radiation, which is something else). Also I note the omission of any mention of quantum gravity. Is that intentional? --Michael C. Price talk 03:04, 21 October 2006 (UTC)
"Second quantisation" may be widely used in describing the original canonical quantization treatment of the Dirac equation, but I have rarely if ever seen it used when referring to later approaches such as functional quantization (a.k.a. path integrals) or BRST quantization. (Not that canonical quantization doesn't still have its technical uses; but the linguistic baggage of that era tends to obscure what's going on mathematically.) It should probably be left in the historical section of quantum field theory but deprecated in the actual exposition.
You are quite correct on the second point (Einstein as the originator of radiation quanta per se); see http://punsterproductions.com/~sciencehistory/cautious.htm. Please reword to your satisfaction.
I did not omit quantum gravity. I spoke of "research programs such as quantum gravity and string theory which seek to explain quantum field theory in terms of deeper mathematical principles". Though attempts to quantize general relativity are fascinating (and a great deal more promising IMHO than some other approaches), I think it would be premature to call them more than a research program or to give them excessive weight in this article.
By the way, I will be on wikibreak for several weeks; release cycle and family duties call. Cheers, Michael K. Edwards 04:30, 21 October 2006 (UTC)

[edit] Rename this page?

I'm not sure to what, but perhaps "semiclassical theories", "semiclassical approximations" or something? Going from a fictitious classical Dirac equation to the Lagrangian or Hamiltionian for the quantized dirac field is just a heuristic excercise to find the correct quantum mechanical description for electrons. The reverse process can sometimes be physically relevant. I gave the example of the Gross-Pitaevskii equation above. Similarly you can derive the Maxwell equations plus the quantum correction due to electron loops: the Euler-Heisenberg Lagrangian.

On the more elementary level there you have the various semiclassical approximations like the WKB method etc. Count Iblis 01:03, 21 October 2006 (UTC)

I still don't think it's clear what purpose this page is supposed to serve, whether it's a review or a history of early quantum mechanics or what. I kind of liked the idea of a brief overview/disambiguation of different quantum theories, I simply objected to the way that non-universal terminology was being presented. Perhaps there should be more discussion on this? -- SCZenz 06:06, 21 October 2006 (UTC)
I agree that the page is currently confused and would be better as a straightforward disambiguation page. --Michael C. Price talk 22:47, 21 October 2006 (UTC)

[edit] Start with dicdef?

I don't think the article should start with an explicit defnition of the word quantum. This is an encyclopedia, after all. Perhaps, however, we could put a link to the wiktionary definition of "quantum" at the right of the page in the standard format for such things. -- SCZenz 06:06, 21 October 2006 (UTC)

[edit] Disambiguation Vote

Disambiguate. I can't think of anything that could be in this article (history of the quantum, definition, interpretations, etc) that isn't covered elsewhere, so I suggest that this should be a pure disambiguation page, with a (nested?) listing of the various quantum theories (QM, QED, QFT, QG etc) and associated articles. --Michael C. Price talk 12:42, 22 October 2006 (UTC)

The history of this article shows that the current QM article derived from or rivaled an early version of this page. Next somebody redirected Quantum theory to Quantum mechanics. After that someone observed that not all quantum theories are called quantum mechanics so some discussion was necessary to avoid oversimplification. It seems important to some people, for instance, that "old quantum theory" not be called "quantum physics" or anything that implies that it is currently accepted, "canonical" theory. It appears to me that not everyone refers to different flavors of quantum physics by the same names. Is there any single place to which a reader new to this subject could go to get straighted out about all the ins and outs of these terminological changes? P0M 23:05, 22 October 2006 (UTC)
It's not just terminology changes. It's the fact that there isn't any uniform terminology at all. We need to either aknowledge the multiple ways that terms are used or avoid the issue entirely, but we definitely should not be making our own editorial changes. -- SCZenz 23:21, 22 October 2006 (UTC)
I agree. P0M 00:44, 24 October 2006 (UTC)

Yeah, disambiguate. As I've said above, that made more sense to me. -- SCZenz 23:21, 22 October 2006 (UTC)

YES –Joke 15:28, 23 October 2006 (UTC)

Y --andrej.westermann 17:58, 23 October 2006 (UTC)

[edit] Plain-vanilla disambiguation

I've turned the page into a plain-vanilla disambiguation page. It removes most details, and hopefully sends all arguments about the precise meanings of terms to articles on those terms. What do you think? -- SCZenz 01:37, 24 October 2006 (UTC)

Looks good. --Michael C. Price talk 08:58, 24 October 2006 (UTC)

Yup, this is a good solution. –Joke 05:13, 25 October 2006 (UTC)