# 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'd like to suggest adding the word "small" to the lead sentence.   The existing lead sentence is:

Quantum mechanics is a fundamental theory that describes the behavior of nature at and below the scale of atoms.
This may be clear to educated readers, but it is important that the introduction should be clear and unambiguous to nontechnical readers too. To them the phrase "below the scale of atoms" can be confusing.  Does this mean below the weight of atoms?  Below the size of atoms?  Below the energy of atoms?  Below the complexity of atoms?
I suggest a minor change to resolve this, adding the word "small":    

Quantum mechanics is a fundamental theory that describes the behavior of nature at small scales, at and below the scale of atoms.
I didn't think this three word addition would be objectionable to anyone.  But Remsense reverted; he prefers the original.
Which version is better? --ChetvornoTALK 03:33, 16 March 2025 (UTC)

@Remsense: Your edit summary objection was: "below the scale of atoms" does a lot for many readers, and if it does not then they're not going to get much out of their first read of this article in all likelihood
That's kind of my point.  For technical articles like this, a lot of readers aren't going to be able to understand the article, so the introduction is the only thing they will read.  So "It is particularly important for the lead section to be understandable to a broad readership" WP:EXPLAINLEAD.  We want the introduction to be comprehensible to high school dropouts, English majors, and ballet dancers.  So resolving small ambiguities like this is worth it. --ChetvornoTALK 03:52, 16 March 2025 (UTC)
Agreed whole-heartedly, thanks for starting this discussion. I feel deeply foolish even thinking about writing any prose for this article, but hopefully others will look past it.Remsense ‥ 论 03:57, 16 March 2025 (UTC)
We agree on all principles, it's just a matter of when we think we can fulfill them. Maybe at the scale of the atom and smaller?
That said, question for those who may know: I understand the Planck length to be a hard cap for the smallest distance that our theory can meaningfully discuss—but even so, would it be considered wrong to say quantum mechanics describes nature "at the smallest scales"? That may sounds popsci or whatever, but it seems potentially more flexible in diction and equally informative? Remsense ‥ 论 03:56, 16 March 2025 (UTC)
The wording ...at the scale of atoms and smaller is great --ChetvornoTALK 04:11, 16 March 2025 (UTC)
I'm just brainstorming, and if anything changes I'd very much like a subject expert to sign off on it first—I'd hate to be responsible for introducing inaccuracy or decreasing value at the most high traffic junctures for those learning about the subject. Remsense ‥ 论 04:14, 16 March 2025 (UTC)
Eh. The fragment at and below the scale of atoms is more concise. As long as atoms and everything smaller are mentioned, I couldn't care less. ZergTwo (talk) 05:21, 16 March 2025 (UTC)
On the subject of the Planck length, I agree the article should avoid wording that QM applies "at the smallest scales".  The section "Relation to General Relativity" does mention the Planck length as a lower limit to length in Loop Quantum Gravity.  But if as you say the PL is regarded as the lower length limit to the applicability of QM (which I think is the consensus of physicists) the article should probably say that.  My feeling is it doesn't have to be mentioned in the introduction, and should not change our lead wording  ...at the scale of atoms and smaller.  --ChetvornoTALK 10:59, 16 March 2025 (UTC)
I agree with what you decided here, but sadly the current sentence is simply incorrect. The Feynman source says
“Quantum mechanics” is the description of the behavior of matter and light in all its details and, in particular, of the happenings on an atomic scale.
QM works at all scales and it is our only (ultimate) theory for matter and light. I suggest a better summary would be:
Quantum mechanics is the fundamental physical theory that describes the behavior of matter and of light; its unusual characteristics are typically only clear at and below the scale of atoms.
So "the theory", not "a theory" to make its mainstream character clear. "matter and light" to avoid "gravity" which is also in nature". Set the scale on observed behavior rather than applicability.
(I don't think we should get to hung up on scale in any case, nothing in the bare theory is limited in size). Johnjbarton (talk) 17:37, 16 March 2025 (UTC)
@Johnjbarton thank you as always. I'm always happy when my less sound musings can at least cause someone to notice and make changes that really do help. Remsense ‥ 论 15:28, 17 March 2025 (UTC)

The redirect Quantum realm 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/2024 April 25 § Quantum realm until a consensus is reached. Utopes (talk / cont) 00:25, 25 April 2024 (UTC) 2404:7C80:84:2FF1:4476:5670:8673:3317 (talk) 10:29, 30 May 2025 (UTC)

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)

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)