Talk:Quantum mechanics

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)

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) Reply

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) Reply

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) Reply

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) Reply

In the text where it talks about quantum tunneling the "Classic Route" looks a lot like energy states like where it it has the potential energy to tunnel but not to convert potential to kenetic because: 1. not enough potential energy anyway 2. you will always lose energy to friction. It reminds me of a model of polymorph 1 "Ritonavir" (unstable) to polymorph 2 "Ritonavir" (a lot more stable). I did reasearch and found that the dexmethylphenidate ("Focalin") I take is already in a stable form and it is unlikly there is a more stable form. If you want to talk to me, on the pages I will go more into chemistry then physics. MooMooMoo27 (talk) 13:28, 19 May 2026 (UTC) Reply

The first sentence here is pointlessly pretentious:

• Quantum mechanics is the fundamental physical theory that describes the behavior of matter and of light.

This claim is contrary to how we generally describe theories: we don't proclaim any single explanation to be singularly true. QM is "a" theory that describes behavior of matter. The claim is contrary to the source, which does not say "fundamental". The claim is contrary to the rest of the paragraph which describes QM as a foundation of a bunch of other thing that describe the behavior of matter and light. If we delete a couple of words its a couple of words better:

• Quantum mechanics is a physical theory that describes the behavior of matter and of light.

Johnjbarton (talk) 16:51, 5 June 2026 (UTC) Reply

I think fundamental here just distinguishes it from effective or phenomenological theories like electromagnetism or thermal/statistical physics (which also describe light and matter, but as effective theories). Just my two cents. Sławomir Biały (talk) 17:09, 5 June 2026 (UTC) Reply

But these are all just conjectures that predict or agree with observations to varying degrees and circumstances. "Fundamental" expresses a belief rather than a fact; it is unprovable. "Foundational" makes more sense to me as that expresses the breadth of science built on QM; "the foundational" would express the unique character of this theory in modern physics. Johnjbarton (talk) 18:19, 5 June 2026 (UTC) Reply

The difference between "the fundamental" and "the foundational" seems like splitting hairs to me, to be honest. I do not see how one conveys a fact and the other an opinion; to me, either could be read in either way if you really wanted to. Stepwise Continuous Dysfunction (talk) 18:26, 5 June 2026 (UTC) Reply

If "fundamental" means the same as "foundational" then the word is redundant with the end of the paragraph so we can omit it. The end of the paragraph accurate lays out the pervasive role of QM among the sciences. Johnjbarton (talk) 21:13, 5 June 2026 (UTC) Reply

But it doesn't explain that QM has no underlying theory. Stepwise Continuous Dysfunction (talk) 21:17, 5 June 2026 (UTC) Reply

I would guess that almost every physicist would disagree, which is what the whole "interpretation of QM" bit is about. There are aspects which fit that description. Johnjbarton (talk) 21:38, 5 June 2026 (UTC) Reply

Most interpretations of QM do not say that there is an underlying theory of which QM is an approximation or coarse-graining. Everettians accept QM as just true. QBists accept QM as just true. Consistent-historians don't believe in hidden variables. Interpretations that regard QM as a genuinely incomplete theory are all marginal. Stepwise Continuous Dysfunction (talk) 22:16, 5 June 2026 (UTC) Reply

• Comment adding "the fundamental" is not necessary, given the next sentence describes it as the foundation of modern physics. I also mentioned in my edit summary string theory, which may be considered to operate on a more "fundamental" level. I support Johnjbarton's proposed sentence. Polyamorph (talk) 17:45, 5 June 2026 (UTC) Reply

I'm not committed to the specific word fundamental, but I think we need some adjective there, and fundamental is better than any of the alternatives I've thought up so far. As Sławomir Biały pointed out, it's not an effective theory. The fact that it's not known to be an approximation to a deeper truth needs to be said somehow. The remainder of the paragraph doesn't do this; just saying that QM is the foundation of other topics doesn't go sufficiently far. Newtonian mechanics is the foundation of lots of topics, but it's not fundamental in the sense that QM is. I don't think it matters that the word is "contrary to the source", since that line is supposed to summarize the whole shebang and not be based on a single source anyway. Lose the footnote before losing the adjective, IMO. I also don't see the contradiction between saying that QM is fundamental and that it is the foundation of more specialized theories "that describe the behavior of matter and light". The fundamental thing is the fundamental part of the foundations of the more specialized things.

String theory is beside the point. It's not an established theory in the way that QM is, since it remains stubbornly beyond the reach of experiment, and it's not even more "fundamental" than QM, because it incorporates QM by construction. Stepwise Continuous Dysfunction (talk) 18:14, 5 June 2026 (UTC) Reply

As an alternative, we could add a sentence that says, "It is a fundamental theory, in that it is not known to be an approximation to any more basic theory" (or words roughly like that). Stepwise Continuous Dysfunction (talk) 18:27, 5 June 2026 (UTC) Reply

That's too verbose and unsourced. This isn't an uncontroversial thing to say given that it remains an active area of research and many researchers will argue the theory IS an approximation given it is incomplete. Polyamorph (talk) 21:29, 5 June 2026 (UTC) Reply

But there is no established, well-defined, demonstrably successful theory of which it is known to be an approximation. It's not like Newtonian mechanics, which is an approximation to relativistic mechanics in one direction and to quantum physics in another. It's not like thermodynamics, which is a phenomenological theory where we deliberately gloss over the details and work with coarse-grained pictures of the world. Stepwise Continuous Dysfunction (talk) 22:12, 5 June 2026 (UTC) Reply

The addition is not an improvement for two reasons. 1) it replaces a problematic (for some) word with a more verbose problematic sentence. 2) The lead is supposed to be a summary of the article. So it would be better to get this right in the main text, and then update the lead accordingly. Polyamorph (talk) 09:38, 6 June 2026 (UTC) Reply

It's more verbose, but that's the cost of being more clear, rather than trying to pack too much meaning into a single, rather vague adjective.

And the article does discuss the import of Bell's theorem and the relation of QM to other established physical theories. The line in the intro seems a decent summary of all that. It could be improved, as could many places in the article, no doubt.

(This is SCD, editing from a friend's phone.) ~2026-33597-12 (talk) 17:00, 6 June 2026 (UTC) Reply

I'm worried that our opening paragraph may be propagating a Wikipedia-ism. Is there actually an established division in meaning between the terms quantum mechanics and quantum physics? I have occasionally seen authors try to draw a line between them, but overall, my experience is that they're used pretty synonymously. For example, here's Zeilinger:

The quantum system cannot be assigned properties independent of the context of the complete experimental arrangement. This is just in the spirit of Bohr's interpretation. This so-called contextuality of quantum physics is another central and subtle feature of quantum mechanics.

And here's Jauch:

This is of course a classical experiment in optics and has, in this form, nothing to do with quantum mechanics. It becomes an experiment in quantum physics if one does this experiment with individual photons.

The preface to Hall's Quantum Theory for Mathematicians begins, "Ideas from quantum physics play important roles in many parts of modern mathematics. Many parts of representation theory, for example, are motivated by quantum mechanics...". If there's a distinction there, I can't see it. Here is a book that just says "quantum physics (or mechanics)" straight up. Julian Schwinger's Quantum Mechanics (Springer, 2001) says "quantum physics or quantum mechanics" (p. 1). Likewise, Jaeger uses "fundamental quantum physics" and "fundamental quantum mechanics" synonymously in a paper that our article already cites. Jochen Rau's Quantum Theory (Oxford UP, 2021) includes this passage:

Much of conventional quantum mechanics is dominated (and, arguably, obscured) by rather complicated mathematics, such as functional analysis or the theory of partial differential equations. This makes it sometimes difficult to disentangle the physical concepts from the mathematical methods. Introducing quantum theory under the aspect of information processing, on the other hand, would require only minimal mathematics: essentially, just finite-dimensional complex vector spaces. It would thus offer a much simpler, more direct route to quantum physics, all the way to some of the most fascinating conceptual issues.

Here, "quantum theory", "quantum mechanics" and "quantum physics" all appear effectively interchangeable. Stepwise Continuous Dysfunction (talk) 22:09, 5 June 2026 (UTC) Reply

I do agree that "quantum theory", "quantum mechanics" and "quantum physics" are all synonymous, but I don't see where our leading paragraph is suggesting that there exists a distinction. Tercer (talk) 08:36, 8 June 2026 (UTC) Reply

The second line is "It [quantum mechanics] is the foundation of all quantum physics". This implies that the two are related but different. (No one would say "quantum mechanics is the foundation of all quantum mechanics".) "Quantum mechanics" is treated like the part that all these other theories have in common. If the paragraph actually treated the terms as synonyms, it would be more like, "Quantum mechanics, also known as quantum physics, is ... It is the foundation of many disciplines, including quantum chemistry...". Stepwise Continuous Dysfunction (talk) 18:24, 9 June 2026 (UTC) Reply

Fair enough, that sentence is gibberish. I think your proposal is fine. Tercer (talk) 21:31, 9 June 2026 (UTC) Reply

Sources about QFT will explain why it is not QM, but another quantum thing. For example, the preface and prologue of A. Zee's Quantum Field Theory, As Simple As Possible, distinguishes QFT and QM. Thus equating QM with "quantum physics" is logically inconsistent. Nevertheless Zee says "quantum mechanics (or quantum physics as it is often called)". I think this a side effect of the elephant effect. "QM" means lots of different things depending upon the context/writer/reader. On one end it means Schrodinger equation, on the other of physics involving "quantum".

Our intro seems to have some specific definition in mind, but the entire article is about single particle, non-relativistic Schrodinger equation. So rather than the grandiose "foundational", the article actually sensible. Its definition of QM is the simplest model that include key ideas common to quantum theories.

To address your concern:

1. Put quantum physics as a synonym in the first sentence, per Zee.
2. Change the second sentence, eg: "The concepts of quantum mechanics have been extended and applied across many fields like quantum chemistry, quantum biology, quantum field theory, quantum technology, and quantum information science."

Johnjbarton (talk) 19:20, 9 June 2026 (UTC) Reply

The redirect Quantum systems 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/2026 July 21 §Quantum systems until a consensus is reached. Tercer (talk) 22:43, 21 July 2026 (UTC) Reply