Talk:Wave packet

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I really like the Shrödinger part where it is used as a way to show what a dispersive medium can do, but I would like to add that I don't know many particles obbeying the Shrodinger's equation. If you describe photons, you will use Maxwell's equation and quantize them as much as you can, if you look at electrons, you will deal with the Dirac equation. None of them exhibit that dispersive behavior (at least in vacuum). That dispersive behavior is a pathology of the model, it creates faster than light signals. Klinfran (talk) 10:38, 15 June 2024 (UTC)Reply

@Cosmia Nebula added content about quadratic phase but without references. This content cannot be verified. I would normally just revert such edits but later ones on Airy waves did have references. Johnjbarton (talk) 04:18, 29 August 2024 (UTC)Reply

Unfortunately it is not the first time this has happened. I had the exact same issue with recent edits in the wave equation article, which I addressed on her talk page. (I actually suggested the citer tool I found on your userpage, which is amazing)

Then (to a lesser degree) the same happend in the weyl algebra article, which I almost had to WP:TAGBOMB

To her credit, she somewhat improved the referencing afterwards and I do believe it is clearly a case of WP:GF as the mathematics is usually very solid. However, when she goes into "storytelling mode" the situation becomes more problematic; violating WP:NOR.

It feels like referencing is being treated as an afterthought rather than the basis of the edits. If WP:V and WP:NOR is not taken into consideration in future, then she shouldn't be surprised her edits will be reverted. Otherwise we'll keep having this discussion over and over again.

Kind regards, Roffaduft (talk) 06:18, 29 August 2024 (UTC)Reply

I'm trying to read this page but it has animations all over it. So while I try to focus there is always some distracting motion that draws my eye. It sucks. Jayarava (talk) 07:54, 7 April 2025 (UTC)Reply

Maybe there is a way to have an on-demand animation so users would click on the image to see the animation. Perhaps you can ask on WP:Village pump for such an option. Johnjbarton (talk) 17:56, 7 April 2025 (UTC)Reply

At the end of the Historical background section, there is a formula for the time for a wavefunction to double in size. The formula cannot be correct because its units are square root of seconds, rather than seconds. I believe the right hand side of this equation needs to be squared: T approx DeltaX^2 times m over hbar.

I don't know how to do equations in Wikipedia and cannot do the correction. Stuartsamuel123 (talk) 23:11, 14 May 2025 (UTC)Reply

While I agree with your dimensional analysis, reliable sources do not. The given equation does verify with the given source. Other sources discussing the "Ehrenfest time" say is it proportional to one over square root of hbar.

• Schubert, R., Vallejos, R. O., & Toscano, F. (2012). How do wave packets spread? Time evolution on Ehrenfest time scales. Journal of Physics A: Mathematical and Theoretical, 45(21), 215307.
• Shepelyansky, D. (2020). Ehrenfest time and chaos. Scholarpedia, 15(9), 55031.

Later I can check if any of my QM books cover this. Johnjbarton (talk) 01:47, 15 May 2025 (UTC)Reply

I derived the formula from scratch. Then I did Google search with "wave packet doubling time formula" and the result appeared in Search Labs AI Overview. One reference that appeared was Equation (2.11.13) of https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Introductory_Quantum_Mechanics_(Fitzpatrick)/02%3A_Wave-Particle_Duality/2.11%3A_Evolution_of_Wave-Packets

The result is also consistent with the analysis in "Gaussian wave packets in quantum mechanics" of this Wikipedia webpage.

By the way, Ehrenfest time is not the wavefunction spreading doubling time.

I figured out how to implement the math formatting and did it. Stuartsamuel123 (talk) 04:22, 15 May 2025 (UTC)Reply

I reverted your change as it conflicts with the given source. Dimensional analysis also fails for Ehrenfest time, which the reason I pointed to it.

The phys.libretexts.org source is pretty good, but the AI and Wikipedia sources are unacceptable, incorrect for many physics cases. That gives us two different sources with two different results.

I won't agree to a change without a solid textbook source replacing the one in the article now. If the topic is notable this should not be difficult. You can seek other opinions of course.

I think in the meantime we should just remove the content. Johnjbarton (talk) 16:22, 15 May 2025 (UTC)Reply

I removed this disputed content:

• Later in 1927 Paul Ehrenfest showed that the time, ${\displaystyle T}$ for a matter wave packet of width ${\displaystyle \Delta x}$ and mass ${\displaystyle m}$ to spread by a factor of 2 was ${\textstyle T\approx \Delta x{\sqrt {m/\hbar }}}$. Since ${\displaystyle \hbar }$ is so small, wave packets on the scale of macroscopic objects, with large width and mass, double only at cosmic time scales.<ref name="Kragh"/>{{rp|830}}

Note that this is a secondary ref reporting on a historical publication. Ehrenfest's 1927 article is in German but it is short and as far as I can tell the last paragraph contains the formula and it does not have the square root shown here. Johnjbarton (talk) 17:34, 15 May 2025 (UTC)Reply

Stuartsamuel123 is right. Not only the correct formula is easy to derive, he also gave a good source for it. Your sources about the Ehrenfest time are talking about something else. Since Ehrenfest's paper also agrees I think there's no reason to remove the content. Tercer (talk) 18:24, 15 May 2025 (UTC)Reply

I found a different secondary ref that agrees with your correction. This ref has the same formula as Ehrenfest but in English. Thanks for fixing this! Johnjbarton (talk) 18:49, 15 May 2025 (UTC)Reply