Talk:Stern–Gerlach experiment

Potential future addition to the article

@D bar x recently added this interesting section. However the primary reference appeared this month, is uncited, and the work proposes rather than reports an experimental result. Based on other discussions on WikiProject Physics I have reverted the addition.

• Very recently, it has become feasible to experimentally test whether a quantum superposition of two spin-direction wavepackets continue through the Stern-Gerlach magnet, as physics textbooks describe , or, rather, if transfer of a momentum quantum from the magnet reduces the wavefunction to a single spin-direction eigenfunction . A very narrow two-slit screen can now be micro-fabricated. Placed at the exit of the magnet, it would display interference if there is no reduction; otherwise, no interference would be visible.

References

Johnjbarton (talk) 22:11, 6 August 2024 (UTC)

TooSoon

I reverted an addition of content related to this new primary publication:

• Devereux, Michael (2024). "A simple, practical experiment to investigate atomic wavefunction reduction within a Stern-Gerlach magnet". J. Phys. B: At. Mol. Opt. Phys.

Generally we use secondary references but there are exceptions for well-cited papers, multiple author papers, or papers from authors with long established publication records. @D bar x you could ask for consensus to include this paper on for example Wikipedia_talk:WikiProject_Physics but I believe the chances are other editors will agree with me that we should wait until this work has more backing. See ef WP:notnews or wp:toosoon. Johnjbarton (talk) 01:57, 28 August 2024 (UTC)

Someone, not me, inserted the last paragraph of the section on Importance. That physicist has also come to recognize that what we read in all our textbooks about the S-G experiment, namely that a coherent superposition of two spin wave packets continuously develops through the magnet, is mistaken. Obviously, the Wikipedia editors accepted that revision. This is not a trivial understanding about quantum measurement and the foundations of Q. M., but instead, is seminal. Just one example: Zurek's decoherence theory (Physics Today, October 1991) is founded on the assumption that there is a continuous wavefunction evolution through the S-G magnet which can be reversed (see his Figure 2). Because absorption of a momentum quantum reduces the superposition to one spin eigenfunction, reversible measurement is not real. So, of course, there is much resistance from those researchers who have uncritically accepted the received explanation. Please ask the editor of the very last paragraph in "Importance" to evaluate the significance of my suggested experiment, which could refute this persistent misunderstanding. As the famous philosopher of science, Karl Popper, explained, it is the refutation of mistaken hypotheses which produces progress in scientific understanding. And a scientific observation is the way that is done (see Stern and Gerlach).

I'll ask others at the Project_physics talk what they think. D bar x (talk) 18:29, 28 August 2024 (UTC)

It doesn't belong. Wikipedia's mission is to follow the mainstream scientific consensus, rather than attempting to lead it by the nose. XOR'easter (talk) 19:22, 31 August 2024 (UTC)

Also, you did add the last paragraph of the "Importance" section. XOR'easter (talk) 00:39, 3 September 2024 (UTC)

I'm about to pull my hair out (what little is left). I've tried, for over an hour to just join the list of participants on Project_Physics talk. Never mind adding a comment there. Why is Wikipedia so absurdly obtuse in this respect?D bar x (talk) 21:06, 28 August 2024 (UTC)

I would think Wikipedia would want to carefully explain whatever subject may be considered, not just tell readers the most popular, consensus opinion. The methods of science have shown, over five hundred years, that clear reasoning and controlled observation provide correct explanations to physical phenomena. (A very restricted subject area.) Proof is in the continued progress made in describing the material world. Consider the contrast between what Galileo discovered by observation of the planets with his telescope, and the demanding restrictions of that time's popular consensus. Was Boltzmann correct that atoms exist, or were his unanimous scientific detractors?

I believe the reason for so very little progress over the last hundred years in understanding the foundation of quantum mechanics is due to reliance on consensus, not objective critical reasoning. Even today, the consensus decrees that the wavefunction spin superposition in a Stern-Gerlach is real. It is, in fact, obvious from just a careful examination of the Schrodinger equation for the S-G magnet that the superposition collapses within the magnet, as detailed in the last "Importance" paragraph of this page. A very simple, practical experiment (the scientific method) very carefully examined by peer review in the Journal of Physics B, not consensus vetting, would show Wikipedia readers that the consensus is mistaken about this. D bar x (talk) 21:24, 1 September 2024 (UTC)

Sorry, but I disagree with your entire line of reasoning. For every Galileo/Boltzmann there are a hundred other theories which looked very promising to adherents but which we no longer consider valid. At the time there was no way to know which was which. Eventually science sorted it out. So it will be for the peer-reviewed paper you mention.

I guess that you mean that 100 years of QM did not turn out the way you wanted, because the idea that there is no progress in QM is absurd.

As for the last paragraph of the Importance section, you can challenge the content as not reflecting the sources or you can challenge the sources as not providing a neutral point of view, but such challenges depend upon a consensus of editors. Johnjbarton (talk) 19:27, 2 September 2024 (UTC)

True. Moreover, many papers make it through peer review and then sink into obscurity. XOR'easter (talk) 00:18, 3 September 2024 (UTC)

Silver atoms or silver ions?

The article keeps referring to the silver as atoms, even through they are being deflected by a magnetic field and must, therefore, be charged.

The reader is left to infer that the atoms must be ionized if they react to a magnetic field.

I think just a shot mention of the fact that they're ionized would be good. Rather than just outright replacing every instance of the word atom in the context of this description.

What do you guys think? VoidHalo (talk) 00:00, 1 March 2025 (UTC)

I suggest you read some of the references. The Friedrich/Herschbach, D. magazine article is a good start. There you will learn about atomic beams and atomic magnetic moments. Or you can just read the Wikipedia article where is says "The experiment is normally conducted using electrically neutral particles such as silver atoms. This avoids the large deflection in the path of a charged particle moving through a magnetic field and allows spin-dependent effects to dominate." Johnjbarton (talk) 02:07, 1 March 2025 (UTC)

S&G experiment impossible with electrons (N. F. Mott 1929)

I specified that S&G experiment is not possible with electrons, as showed by N. F. Mott in the reference I added , but I also hesitate to delete all mentions of electrons in this chapter, so as not to perpetuate this common error suggesting that S&G would be possible with electrons. I am not expert enough to check whether Mott's demonstration would apply, or not, to other particles or ions of spin 1/2 but of different mass, in which case we could just replace the injudicious example of the electron, by an example not concerned by Mott's demonstration, therefore possible at least in principle. Samuel.Damoy (talk) 00:24, 20 October 2025 (UTC)

I changed the content and I think we need to dig into this more. Mott/Bohr has been the subject of considerably new work. We need a review reference and more content around the topic. Johnjbarton (talk) 02:44, 20 October 2025 (UTC)

Hello @Johnjbarton and thank you for this discussion. You do not cite any source for your statement «scientists like Rudolf Peierls asserted that Bohr's claim was incorrect» so I will add the , if you do not. The Dehmelt 1958 reference you cite does not contradict Mott's conclusions on the impossibility of measuring the spin of e- by interaction with a magnetic field gradient as in S&G for Ag atoms, moreover Dehmelt 1958 does not mention the S&G experiment at all, and Dehmelt 1958 is a completely different configuration from S&G: in this type of polarization detector, it is not a magnetic field gradient that is used as in S&G, but the asymmetry of collisions between e- and atoms, which depends on the spin-orbit coupling as explained in Delmett 1958 (and Mott 1929 that I added!). This is also the type of coupling that is used to measure the spin polarization of e- in a...Mott detector! (see Mott scattering, but Mott detector use spin-orbit coupling during collision with a gold surface, rather than with sodium atoms in the gas phase at low pressure as in Dehmelt 1958), Samuel.Damoy (talk) 07:53, 20 October 2025 (UTC)

Thanks, I did have the ref but I think the tool I use to add refers sometimes replaces the previous addition. So when I added Dehmelt, the Garraway ref was removed and I did not notice. Johnjbarton (talk) 16:29, 20 October 2025 (UTC)

Thanks for this ref. Garraway 99, which I will be quite unable to analyze in detail, but which does indeed seem to contradict Mott (and Bohr and Pauli), with serious arguments, by more precise calculations with numerical evaluations. But it remains a proof of concept, without being able to conclude whether an S&G type experiment with e- is actually technically feasible (Garraway explicitly points this out). In this sense, a colleague who is an expert in spintronics pointed me to this experimental article by Kohda et al. https://www.nature.com/articles/ncomms2080, which manages to spin polarize an e- current, in a semiconductor device "inspired by S&G". A good compromise would perhaps be to not suggest that the S&G experiment was carried out as is with e-, as the current version of the article suggests (the illustration video indicates e- to illustrate the experiment when they were atoms!), but that experiments were inspired by S&G, to separate e- according to their spin in semiconductors? Samuel.Damoy (talk) 17:52, 20 October 2025 (UTC)

I think the Kohda paper seems like a reliable source and could be added to the article.

I don't understand what kind of compromise you are looking for. We can't say the experiment has not been done without an source which says that. If we say it has been done, we need a source. Do we? Johnjbarton (talk) 18:56, 20 October 2025 (UTC)

What I mean by compromise is that we can remove everything in the current version of the article that suggests that the S&G experiment was carried out with e-, because we don't have a source to confirm it, but on the contrary a source that indicates that it is only a theoretical possibility without guarantee of experimental feasibility (for the illustration video it's a shame because the video is well done, but the mention of e- instead of atoms is clearly misleading and unacceptable). But we can still add Kohda's article by specifying that even if it is inspired by S&G, it is a different experiment from S&G, since it is carried out with an e- current in a semiconductor device Samuel.Damoy (talk) 19:18, 20 October 2025 (UTC)

I think your latest edits (clarification of the error on the e- in the video description, separate treatment of the case of charged particles) answer my remarks, thank you. Do you also want to take care of adding Kohda et al.? (otherwise I can do it, but later, and I specify that I do not speak English fluently so I use an auto-translator) Samuel.Damoy (talk) 19:48, 20 October 2025 (UTC)

Give it a spin and I'll check in. Johnjbarton (talk) 23:00, 20 October 2025 (UTC)