Talk:Quantum mechanics

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We need a section shortly after the lead

We need a section shortly after the lead titled something like "Quantum Systems" that defines what a quantum system is and goes into detail about the different types of quantum systems. I'm an IP editor so I can't edit the article.

The impetus for this is: There are many articles in Wikipedia which mention the term "quantum system" or "quantum mechanical system"; Like for example: https://en.wikipedia.org/wiki/Spontaneous_emission . I was reading that article and I wanted to know what a quantum mechanical system is. It is even linked in the article. So I followed the link and I get to the page for quantum mechanics, which explains the field of physics but does not readily define what a quantum system or quantum mechanical system is.

So in short, this article needs a section that goes over what a quantum mechanical system is, including the different types of common systems like molecule, atom, subatomic particle, and potentially many particle systems.

Something like the following:

== Quantum Systems == A quantum system is a physical system that can be analyzed using quantum mechanics. Quantum systems are fundamentally irreducible, in that to analyze the system one needs to know the total state of the system to make any useful observation on it. In contrast, an [[open quantum system]] is one where not all the information about the system need to be known to be able to make a useful analysis. Examples of quantum systems include: * [[atom]]s * molecules * subatomic particles * other many particle systems Mathematically, a quantum system is the [[tensor product]] of its component systems.<ref>https://www.sciencedirect.com/topics/engineering/quantum-system</ref> — Preceding unsigned comment added by 96.227.223.203 (talk • contribs)

"Total energy" listed at Redirects for discussion

The redirect Total energy has been listed at redirects for discussion to determine whether its use and function meets the redirect guidelines. Readers of this page are welcome to comment on this redirect at Wikipedia:Redirects for discussion/Log/2025 October 20 § Total energy until a consensus is reached. —Myceteae‍🍄‍🟫‍(talk) 20:59, 20 October 2025 (UTC)

On the unfair criticism about the hidden variables

I have to point out a serious inaccuracy in this sentence:

<<Another possibility opened by entanglement is testing for "hidden variables", hypothetical properties more fundamental than the quantities addressed in quantum theory itself, knowledge of which would allow more exact predictions than quantum theory provides. A collection of results, most significantly Bell's theorem, have demonstrated that broad classes of such hidden-variable theories are in fact incompatible with quantum physics. According to Bell's theorem, if nature actually operates in accord with any theory of local hidden variables, then the results of a Bell test will be constrained in a particular, quantifiable way. Many Bell tests have been performed and they have shown results incompatible with the constraints imposed by local hidden variables.>>

It unfairly casts theories that admit hidden variables in a bad light. In fact, "Bell's theorem asserts that if certain predictions of quantum theory are correct, then our world is non-local" (Sheldon Goldstein et al. (2011), Scholarpedia, 6(10):8378, http://dx.doi.org/10.4249/scholarpedia.8378). Thus, the conclusion is that "local theories as such (whether deterministic or not, whether positing hidden variables or not, etc.) are incompatible with the predictions of quantum theory" (ibidem).

For too long, Bell's theorem has been cited against the existence of hidden variables; but this is wrong. Of course, it is against the use of local hidden variables, but only for the fact that it is against the use of any local theory.

I therefore urge that this sentence be changed.

Furthermore, hidden variables are not in principle "more fundamental" than others (certainly, more peculiar).

User Argoncano Argoncano (talk) 16:53, 12 November 2025 (UTC)

I agree with you in principle, but my understanding is that phycisists generally prefer local non-real theories over non-local real theories.

You are just fundamentally incorrect when you say "Of course, it is against the use of local hidden variables, but only for the fact that it is against the use of any local theory." I'm not checking the source because Bell's theorem simply disproves the existence of local real theories as valid theories.

Otherwise, I don't particularly understand what's wrong with the sentence. There is no accuracy that it has made; I would maybe simply add a sentence outlining that other forms of non-local mechanics have been pursued like Bohmian mechanics, but it is important to mention that whilst significant, they are not mainstream. Oblibion123 (talk) 05:00, 12 December 2025 (UTC)

Quantum Field Theory vs Quantum Mechanics

I want to open a discussion about whether its reasonable to call Quantum Field Theories like QED (Quantum Electrodynamics) as categories of quantum mechanics.

To my understanding, quantum field theory is a part of quantum physics that developed from quantum mechanics to include special relativity (as well as other properties like creation and annihilation of particles).

The article begins with "Quantum mechanics is the fundamental physical theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms." However, whilst clear and informative for an audience unaware of the subject, I am not sure that this is accurate as quantum mechanics is not the fundamental physical theory that describes the behavior of matter and light -- that would be the standard model (or QED or some future model that humans have not come up with). If we include quantum field theory as a sub-category of quantum mechanics, then it would make sense, but again I am not sure that this is an appropriate categorisation.

An initial part of the article says: > Predictions of quantum mechanics have been verified experimentally to an extremely high degree of accuracy. For example, the refinement of quantum mechanics for the interaction of light and matter, known as quantum electrodynamics (QED), has been shown to agree with experiment to within 1 part in 1012 when predicting the magnetic properties of an electron. I do not think quantum electrodynamics is of quantum mechanics, so another test should be likely used. Oblibion123 (talk) 05:10, 12 December 2025 (UTC)

There was a discussion in WT:PHYSICS some time ago about whether we should try to distinguish quantum physics and quantum mechanics (you might want to search through the archives). The consensus was that this would be too confusing, as many sources treat them as synonyms. That makes QFTs (and QED in particular) part of quantum mechanics, and that's why the article is written the way it is.

You seem to have in mind that quantum mechanics means specifically non-relativistic quantum mechanics. But I think it's better to use the longer expression "non-relativistic quantum mechanics" when we mean that to avoid confusing the readers. Tercer (talk) 11:34, 12 December 2025 (UTC)

If this as been the past consensus, then I will not argue with it. It might still be a good idea to just emphasize that QED is the "extension of quantum mechanics" rather than "the refinement of quantum mechanics". Oblibion123 (talk) 20:46, 12 December 2025 (UTC)

I don't think we can solve this without a very convincing source as a basis for discussions. Here is one of the leading books on QFT:

• Quantum field theory is the application of quantum mechanics to dynamical systems of fields, in the same sense that the basic course in quantum mechanics is concerned mainly with the quantization of dynamical systems of particles.
• Peskin, M. E., Schroeder, D. V. (1995). An Introduction To Quantum Field Theory. United Kingdom: Avalon Publishing.

This makes QFT part of "quantum mechanics". Unfortunately I do not know of any term commonly used for the "quantization of dynamical systems of particles" other than "quantum mechanics".

Peskin/Schroeder indirectly explain the additional complication: the Klein-Gordon equation and Dirac equation are examples of relativistic quantum mechanics from the "quantization of dynamical systems of particles" branch of the topic. So, as they point out, the divide is between single-particle relativistic theory and multiple-particle relativistic theory. A multi-particle relativistic problem treated with single-particle theory will result in causality violations. But that won't stop popular discussions under the banner of "quantum mechanics". Johnjbarton (talk) 18:00, 12 December 2025 (UTC)