Talk:Correspondence principle

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[edit] Llamas

"macroscopic systems (springs, capacitors, llamas, and so forth)"

Surely llamas is a jarring and confusing example to use in this instance, if appropriate at all?

My first reaction was to believe it vandalism but it's been in since revision 1. --Air 13:02, 2 Nov 2004 (UTC)

Llamas are macroscopic systems, no? -- CYD

first, I appreciate the Monty Python element of the example. But the progression mechanical component, electrical component, wool-bearing quadraped makes it unclear what's being illustrated : )

is it better to instead say (springs, capacitors, trees, clouds, llamas, and so forth) to indicate we mean 'just about everything above the atomic level'? --Air 11:38, 3 Nov 2004 (UTC)

---

Rbj, I appreciate your taking the time to add the SR example, but it's unfortunately irrelevant. AFAIK, the correspondence principle is always used to refer to quantum mechanics reducing to classical (non-quantum) mechanics. The reason we care about this is that there's nothing in the postulates of quantum theory that says it should reduce to classical mechanics; you have to toss in the correspondence principle if you want to construct a physically correct quantum theory. While it is true that relativistic mechanics reduces to classical mechanics at low velocities, this happens automatically; you don't have to do any fiddling as you do with quantum theories. And it's certainly not referred to as the "correspondence principle". -- CYD

I believe that you are mistaken when you state " ... the correspondence principle is always used to refer to quantum mechanics ". Despite its origin from Bohr, the definition of the correspondence principle has broader context. This definition easily found when Googling pretty much agrees with my textbooks:

Correspondence Principle: any new theory, whatever its character--or details--should reduce to the well-established theory to which it corresponds when the new theory is applied to the circumstances for which the less general theory is known to hold. This principle was first applied to the theory of atomic structure by Niels Bohr in l923. (Weidner and Sells, l960, p. 29) The principle can be applied to great advantage in relativity theory and in quantum mechanics.

here's another from Encyclopedia Britannica:

Correspondence Principle: philosophical guideline for the selection of new theories in physical science, requiring that they explain all the phenomena for which a preceding theory was valid.

SR is a more modern theory than Newtonian Mechanics (and is GR) that has more accuracy than Newtonian mechanics as speeds get very large. When applying relativisitc theory to situations where speeds are lower (or space-time is flatter) should and does break down to the familiar Newtonian mechanics or gravitational model. That is the application of the correspondence principle and someday it will used to apply string theory (or something else as new) to default to traditional quantum mechanics or that new theory will fall by the wayside. If what you say is true, why have that quote from Einstein at the bottom of the article? Einstein didn't do quantum mechanics, why would his quote be relevant unless it applied more broadly than quantum mechanics? r b-j 19:22, 26 Nov 2004 (UTC)

Actually, for whatever it's worth, the majority of Google results points to the quantum mechanical "version" of the correspondence principle. If the Encyclopaedia Britannica uses the "broader context" definition, then it is wrong. It's mind-bogglingly obvious that newer theories of physics must reduce to older theories in the domains where the older theories agree well with experiment -- this is just saying that theories must agree with experiment! Thus, Newtonian dynamics reduces to statics in the limit of zero speeds, electromagnetism reduces to electricity and magnetism at low frequencies, statistical mechanics reduces to thermodynamics at large particle numbers, SR reduces to classical mechanics at low velocities, GR reduces to SR and Newtonian gravity for small spacetime curvatures, etc.; all this was known long before Bohr enunciated the correspondence principle in 1923.

The true significance of Bohr's remark lies in its application to quantum mechanics. In the early days of quantum mechanics, it seemed entirely possible that quantum theory applies only to the microscopic domain, i.e. individual atoms and molecules. This was especially the case before the invention of modern quantum mechanics by Heisenbery and Schrodinger, when quantum theories were constructed on a phenomenological basis (the most famous example being Bohr's atom). Bohr's real and decidedly non-obvious contribution was to state that these microscopic theories must match classical physics if, somehow, one can reach the limit of large quantum numbers -- e.g. if one can make an atom the size of a room! It is interesting to note that even today, no one has been able to satisfactorily show how quantum mechanics reduces to the classical world in a general context. By contrast, the way SR reduces to classical mechanics is obvious and completely "built-in" to the theory. -- CYD

It may be "mind-bogglingly obvious that newer theories of physics must reduce to older theories in the domains where the older theories agree well with experiment", but there have been new theories that fell by the wayside because they didn't pass that test. Before quantum mechanics could be completely accepted, there had to be some experiments (or thought experiments) that, after computing the expectation values, shown argeement, at least in perception, between the old and the new. That philosphical test should have a name and it is silly that some would weaken the language by giving different names to the same principle depending on where that principle is applied.

"it seemed entirely possible that quantum theory applies only to the microscopic domain" , you could make the same claim regarding either general relativity or special relativity (that it only applies to high speeds or extremely strong gravitational fields). until someone (maybe the theory's developer) checks to see that the new agrees with the old where the old was known to be valid, then the theory is suspect. For some reason, i suspect that there was a (natural) change in usage and sematic regarding this, extending the concept to all new theories and only some of the physics community have found out about it. You still haven't answered my question regarding Einstein's quote. What was Einstein talking about, when he said it? Quantum mechanics? Relativity? I am still investigating this. r b-j 22:57, 29 Nov 2004 (UTC)

I have no idea where the Einstein quote came from, but notice that he does not use the term "correspondence principle" in it. Feel free to remove the quote from the article if you think that it is irrelevant.

I don't think it's irrelevant but it supports the notion that the correspondence principle (or something with a different name that I am still asking what that name would be) is a broader notion than just one that shows that quantum mechanics is not inconsistent with Newtonian mechanics (for large masses).

The point that you are steadfastly ignoring is that the way quantum mechanics reduces to classical physics is fundamentally different from the way that, e.g., SR reduces. The "philosphical test" that you describe is a trivial matter for SR and other theories, but a complicated (and still unsolved) matter for quantum mechanics! The point is that quantum theory is not so much a theory as a class of theories, and it is entirely possible to construct quantum mechanical models that have no classical limit. The simplest example of a useful quantum theory that does not possess a classical limit -- and to which the correspondence principle does not apply -- is spin. -- CYD

Nearly all of this is true (however there are models in SR and certainly GR that have no counterpart in Newtonian mechanics) but does not change the point. All modern theories, if they are to be accepted as "true" in some scientific sense, have to be able to agree with the older accepted theories in the domain where those theories were shown to be valid. If that is not the case a hypothisized theory will eventually fail to gain acceptance. Frankly, I have always been taught, that at least in physics, this was called the "correspondence principle". To limit that semantically to only one area of science (or physics) is etimologically (sp??) not economical. Applying the term, with the same meaning of the term, to other areas, does not change its meaning for the original area. However restricting it artificially (or unnecessarily), does change its meaning. r b-j 17:47, 30 Nov 2004 (UTC)

It is even more economical to adopt the principle that theories have to agree with experiment. If old theories agree with experiment in certain domains, this would imply that new theories have to agree with the old theories in those domains, obviating the need for a "correspondence principle" in the sense you describe. Frankly, I've never come across the term except in the quantum mechanical context. String theorists, for example, never refer to the limit in which string theory is supposed to reduce to Standard Model physics as the "correspondence limit". The term classical limit is often used to describe the regime in which both quantum mechanics and relativity reduce to classical physics, and the term thermodynamic limit is used to describe the regime of large particle numbers in statistical physics, but that's about it. -- CYD

I've read all the above comments - quite revealing. The fact is that the term 'correspondence principle' means (at least) 2 things: firstly, the exclusively quantum mechanical one that CYD is referring to and the more general one referring to the reduction of scientific theories to less accurate ones (albeit possibly precursors to the more accurate one). One cannot argue that just because the term arose from quantum mechanics, then it must only be used in that context. The point is that this is an encyclopedia and both meanings must be accommodated. If that means creating another article (and a disambiguation page) then so be it, but for the time being I've added a new section: 'Other uses of the term'.

[edit] Ehrenfest's theorem

Shouldn't Ehrenfest's theorem be mentioned somewhere? My understanding is that the correspondence principle relies heavily on Ehrenfest's theorem.--Michael C Price 11:53, 13 June 2006 (UTC)

[edit] Einstein quote misleading

I'm not sure what the purpose of the Einstein quote at the bottom is. The importance of the correspondence principle in respects to Einstein is not that he accepted that correspondence between QM and classical values was necessary, but because he did not think it was sufficient. Einstein thought that it was obvious that any QM result had to accord with experiment at a bare minimum, but unlike Bohr he did not think that should be the only limiting factor on a new theory — he believed that it should correspond with certain axiomatic and generalized principles, i.e. that it should make physical sense (Bohr considered this to be too limiting a constraint).

I think the invocation of Einstein here is a bit misleading both about Einstein's own opinions, to say the least, but also because nowhere in this article is it reflected why the correspondence principle was considered a radical methodological approach in its time. It is not because of what the principle says, but what it does not say. --Fastfission 00:17, 21 September 2006 (UTC)

I agree, I don't see the relevance of the quote. --Michael C. Price ^talk 06:30, 21 September 2006 (UTC)