Talk:Point particle

message: I wrote a blurb about J.J.Thomson. Ti-30X (talk) 01:21, 26 June 2009 (UTC)
thread_title: Thomson

message: I am removing the re-improve template stating "This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (May 2009)". 
Everything now has the needed in-line citations. Quinn 16:17, 13 July 2009 (UTC)
thread_title:  Template 

message: It seems like the entire section Point particle properties has little to do with point particles.  It's just a couple of stubs for electron and quark articles.  It doesn't really discuss the "pointness" of electrons or quarks very well.Jpkotta (talk) 18:35, 28 August 2009 (UTC)
thread_title:  "Pointness" ? 

message: I deleted some nonsense stating that the electron has a radius equal to some small number, with a reference to Lederman's book. Lederman's book does not say that the electron radius is equal to that number; it says that its radius is measured to be smaller than that number. As of 2000, all X-ray scattering experiments have shown that electrons behave as point-like particles with no spatial extent (ref: ).
There is a classical radius associated with the electron, which is derived by making the assumption that it is a sphere with all its electrical charge distributed on the surface, and that its rest mass is equal to the electrostatic energy of its charge. This is a theoretical derivation, and is based on classical Newtonian mechanics, which does not adequately describe all the properties of the electron; it has nothing to do with the actual spatial extent of the electron. What this "classical radius" means is that if the electron were a spherical charge, then classical electromagnetism predicts that it would have a radius of this value. In fact, based on which theoretical assumptions are made, one can derive a number of different values, all of which may be said to be the "radius" of the electron in some sense. Any attempt to ascribe such hypothetical "radius" values to an actual, physical radius is misleading at best, and totally nonsensical at worst.—Tetracube (talk) 21:57, 22 September 2009 (UTC)
replies: text: I see this interesting discussion. What theoretical assumptions can be made? Are the obtained values hypothetical? Why is that any attempt mentioned above is negatively labeled?--5.2.200.163 (talk) 16:07, 30 December 2015 (UTC)

text: The issue of electron ionic radius in electrides compounds is also mentioned at talk:electride.--5.2.200.163 (talk) 16:09, 30 December 2015 (UTC)
thread_title:  Electron radius 

message: Here's my understanding of some of the areas I think the article should discuss, to treat what is meant by point particle in quantum mechanics.  As I have said elsewhere, I'd be happier if this could be taken up by somebody more practiced at thinking about and explaining this stuff than I am (and who knows some good citeable references).  But these are (I think) some aspects that the article should examine, not as text to go into the article, but summarising informally:
(Could use more work)
#  "Point particle" does not mean the wavefunction is localised to a point.  Even if this were ever possible, according to Heisenberg's uncertainty principle such an extreme localisation of position would require an extreme (infinite, in fact) spread of momenta, and the wavepacket would immediately start to spread apart.  Instead the wavefunction of a point particle does have finite extent: for example, an electron may typically be spread out all over a molecular orbital; and everyday visible light photons are typically of the order of a couple of metres long (corresponding to a finite natural frequency linewidth).  In quantum mechanics the phrase "point particle" therefore implies something else...
#  One way to describe quantum mechanics is through the path integral formulation. The wavefunction is analysed as the result of summing up a contribution from each imaginable history of the system, weighted by an appropriate complex number amplitude.  (See eg Feynman's QED: Strange theory of light and matter for a popular-level description).  For a quantum mechanical particle to be a "point particle", it means that in each of these histories (which are then added together) the particle at each moment in time appears as a single point-like object.  The "pointlike"ness therefore really refers to the Green's function which is summed up, rather than the properties of the whole wavefunction.
# (Thus a γ-ray can be emitted from a nucleus which is smaller than the wavelength of the γ-ray, as raised in our article photon: so beta-decay of a 60Co nucleus with a radius of about 5 × 10−15m includes emission of a γ-particle of wavelength about 1 × 10−12m. The Green's functions, being pointlike, can start in as small a region as required; the evolution of its wavefunction can be traced by adding up the suitably weighted contribution starting from this quite well-defined place).
#:((Actually, I'm not sure this proves much.  The typical atomic radius is about 1000 times smaller than the wavelength of visible light; and the typical radio antenna is a lot smaller than the wavelength of a radio wave. Need to think some more about this.))
#  This can be observed in scattering experiments – a major topic we ought to have a better article on (cf  as an outline); also in high-precision tests of the electron's dimensionless magnetic moment (g-factor), which are in close agreement with the predictions of QED for a point particle; if there was a cut-off in the length-scale over which the path integrals were done, to give the electron propagator a finite radius, this would produce observable effects at a scale of any more than 1 × 10−18 m. 
#  This contrasts with the results of scattering experiments done on composite objects - for example, nuclei or hadrons. (See eg  and Deep inelastic scattering and the Parton model).  Such experiments reveal that in these targets there is structure, and that for them point-like worldlines can not be used in the path integral.
That's what's in my head; but I wonder if I am making a meal of it, or if there is a more obvious way into it.  Jheald (talk) 12:32, 18 November 2009 (UTC)
thread_title:  Arbitrary section break 

message: Here I just rewrote the article, deleting everything about the taxonomy of the various elementary particles. As Djr said, we already have an article on elementary particles, and that's the right place to discuss elementary particles. I put in a new section on quantum mechanics primarily to address the issue of what people mean when they say that an elementary particle has zero size...when the wavepacket can be very spread out (see discussions above). This question is very on-topic in this article IMO, unlike the stuff I deleted about decay rates and color charges and the photon mass and on and on. [Again, very good material, but not on-topic in this article, only in elementary particle, IMO.]
I think everything Jheald said just above would absolutely be a great way to expand/rewrite that quantum-mechanics section. --Steve (talk) 10:21, 25 February 2011 (UTC)
replies: text: I agree with the jist of it. Although technically some of that was more appropriate for subatomic particle than elementary particle. Headbomb {talk / contribs / physics / books} 10:30, 25 February 2011 (UTC)
thread_title: Rewrote to delete most elementary particle stuff

message: What means "exsitence" please? P0M (talk) 01:57, 20 November 2009 (UTC)
replies: text: Fixed. --Christopher Thomas (talk) 02:51, 20 November 2009 (UTC)
thread_title:  New vocabulary item or a spelling error? 

message: Alright, I gave the article a trim (removed the infobox-like stuff since it was outdated and is best left for the indivual articles on these particles), chopped the see also section which contained stuff completely unrelated to point particles, and prettied up the references (and properly formatted them) so they don't get in the way of editing (they still need to be sorted by either author or by date). Lots still left to be done, but at least that's out of the way. Headbomb {ταλκκοντριβς – WP Physics} 16:00, 22 November 2009 (UTC)
thread_title:  Tidy up 

message: In section Pointmass:physics, the article references something called a Center of mass circumference. And requires non-overlapping center of mass circumferences for some reason or another. What is a center of mass circumference? the link currently points to center of mass, and circumference, but neither of these give any clarity to what is meant by their use together. Please Help. 134.29.231.11 (talk) 17:56, 24 February 2011 (UTC)
replies: text: I changed it.--Patrick (talk) 00:50, 4 March 2012 (UTC)
thread_title:  Confusing term or Non-term? 

message: Shouldn't the other-language versions for this article include the various other-language articles corresponding to http://ru.wikipedia.org/wiki/Материальная точка? It seems bizarre, for instance, that the Italian article "Punto materiale" and the Spanish article "Punto material" (each literally meaning "material point") are connected to this article but not connected with the Russian "Материальная точка", which also means "material point". Furthermore, the Russian term (along with, presumably, the other terms used for the articles connected to it as other-language versions) has the same meaning as "point particle" in physics (cf. Landau and Lifshitz's Course of Theoretical Physics, which begins with the statement "One of the fundamental concepts of mechanics is that of a particle", with the footnote "Sometimes called in Russian a material point.")  — Preceding unsigned comment added by Vorziblix (talk • contribs) 00:22, 4 March 2012 (UTC)
thread_title:  Other languages 

message: At one point in the article the wave packet is delocalized, but as a superposition of quantum states (what else?) it seems to be localized. I'm confused. 2A02:A463:2848:1:C0FF:DBE0:6E0F:DBD6 (talk) 14:09, 4 September 2023 (UTC)
thread_title:  Wave packet