Talk:Point particle

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Not just an approximation?[edit]

A point particle is not necessarily an approximation for leptons, they could truly be point particles.Rotiro 08:00, 11 April 2006 (UTC)

So an infinite density of mass? —Preceding unsigned comment added by Piotr Niżyński (talkcontribs) 00:33, 1 February 2009 (UTC)
Not ture. Remember a point-like particle is not a object of zero length, width and height. It is an object that is 0-dimensional, and thus speaking about the "density" becomes meaningless. Density only becomes meaningful when you speak about a material or collection of particles. You can't really speak about the "density" of a single particle, due to it having no size. Not that it's size is 0, and thus anything/0 = infinity(Even that isn't true, it's undefined) doesn't even apply.Cizuz (talk) 04:00, 25 October 2012 (UTC)
With an infinite density of mass the particle must be a black hole. Is an electron a black hole? —Preceding unsigned comment added by 91.7.180.6 (talk) 08:14, 9 March 2009 (UTC)
Not ture. Remember a point-like particle is not a object of zero length, width and height. It is an object that is 0-dimensional, and thus speaking about the "density" becomes meaningless. Density only becomes meaningful when you speak about a material or collection of particles. You can't really speak about the "density" of a single particle, due to it having no size. Not that it's size is 0, and thus anything/0 = infinity(Even that isn't true, it's undefined) doesn't even apply.Cizuz (talk) 04:00, 25 October 2012 (UTC)

Is a point mass and point particle the same thing_ If so, these two articles should be merged. —Preceding unsigned comment added by 200.49.213.61 (talkcontribs) 20:50, 9 August 2006

It should be mentioned that the term "particle" in the standard model is misleading - they are a discrete amount (a quanta) of a wave. Thus in some ways much closer to what is meant by "wave" in ordinary language than to what is meant by "particle". E.g. spacial extension.aharel 15:49, 15 January 2007
The three points above are now addressed in the article. Djr32 (talk) 16:39, 13 December 2008 (UTC)

Thomson[edit]

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

I added a blurb about the (accurate) history of the point particle. So far a very short history. Ti-30X (talk) 02:54, 26 June 2009 (UTC)

Template[edit]

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)

"Pointness" ?[edit]

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)

Electron radius[edit]

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: H. Haken; Hans Christoph Wolf (2000). The physics of atoms and quanta: introduction to experiments and theory (6th ed.). Springer. p. 70. ISBN 3540672745. ).

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)

Heisenberg's principle[edit]

The article needs to discuss how a point particle can (indeed must) simultaneously be delocalised over an appreciable volume of space, to conform with Heisenberg's principle, and yet this is not a contradiction with its having a "pointlike" nature – i.e. the notion of "pointness" is a bit more sophisticated than just where the particle is.

Thus, for example, a photon is a point particle. But without contradiction, a photon emitted by an electron transitioning from an excited state to a ground state is typically about 106 or 107 wavelengths long [1] -- ie of the order of a metre or two long. Under special conditions, it may be possible to manufacture with an even more narrowly defined frequency. Spatially, these would be even longer.

The "pointness" of the particle is to do with the Green's functions it can be analysed into, not the physical space it's actually spread over. Jheald (talk) 14:26, 17 November 2009 (UTC)

ObNitpick: The idea that a photon is "1e+6 to 1e+7 wavelengths long" is a bit of a non-sequitur. By definition, most of its energy is contained with a region on the order of one wavelength in size, if I understand correctly. The tails of its wavefunction are arbitrarily large, but to describe that as the photon's size is grossly misleading.
I think what you're seeing here is uncertainty in the position of a photon with well-defined momentum, which is a different concept. --Christopher Thomas (talk) 18:15, 17 November 2009 (UTC)
Not really. The typical visible-light photon in the wild is indeed a wavepacket about 1e+6 to 1e+7 wavelengths long; and it takes about 1e+6 to 1e+7 frequency cycles to interact with an electron. (See the link above, which includes an animation). It creates an electromagnetic disturbance of about that length; so it's not an inaccurate description to call that the length its energy is spread out over. Jheald (talk) 18:34, 17 November 2009 (UTC)
I can trivially easily generate a photon wavepacket on the order of one wavelength long, by driving a radio antenna for one cycle. The end result is a photon that's very well-localized in space, but has poorly-defined momentum (if you take the Fourier transform, you get frequency components spread quite far from the nominal driving frequency). You have the same sort of thing happening with fibre optic communication systems (they modulate light at sub-nanosecond time intervals, which only works with spatial localization of tens of centimetres or less). Per my original statement, your "several metres" figure seems to be the position uncertainty resulting from the fact that the transition has a well-defined energy drop, rather than being intrinsic to all (or even most) photons. --Christopher Thomas (talk) 03:16, 18 November 2009 (UTC)
Yes but a wavelength is still a lot bigger than a point -- especially a radio wavelength (~ 1500m)! Also, to be appreciable emissions at radio wave frequencies have to contain lots of photons (because their individual energy is so small), so I am dubious as to whether you would be getting a single photon only one wavelength long, as opposed to a superposition of different photons each a number of wavelengths long. I take your point that of course there is the trade-off between good definition in frequency and good definition in position, and it would be interesting to know some of the maths behind how the photon-generating transitions are made to happen more quickly in the solid state devices than say in an excited hydrogen atom, as this may permit. But nevertheless, "tens of centimetres" is still only an order of magnitude less than the 3 metres I was giving as a ball-park figure; it still represents a million wavelengths. And the main thing I'm trying to flag remains: it is still an appreciable size, not a point. Jheald (talk) 10:22, 18 November 2009 (UTC)
Any given particle's wavelength will be nonzero. What "point particle" means is that you can make the particle's de Broglie wavelength arbitrarily small without any sign of more detailed structure showing up. Contrast that with, say, a proton, which will have an effective radius of about 0.8 fm no matter how much energy you give it.
Regarding single-photon detection of radio waves or microwaves, radio telescopes do this all the time. You just have to cool the detector enough that thermal noise doesn't swamp the signal you're trying to measure. For the math behind excitation of radio waves, a good classical mechanics textbook with a section on Maxwell's equations would be the place to look. For the shapes of photon wavepackets, an introductory book on quantum mechanics would be needed (classical mechanics books mostly treat photons as non-quantized plane waves of arbitrary extent).
Regarding generating short photon pulses, doing it requires a certain amount of uncertainty in the energy level of the transition, in order to sufficiently localize the time of emission. The type of system to look at would be diode lasers for the driving modulators, and erbium doped fiber lasers for the power amplifiers. It's hard to modulate faster than sub-nanosecond, because of the way most of these systems work, but you get around that by using frequency division multiplexing (called wavelength division multiplexing at optical frequencies). Er-doped fiber lasers have a gain-bandwidth product of about 1e+11, meaning the fastest signal they could modulate without attenuation would be about 1e+11 Hz (3 mm wavelength). Nothing prevents lasers with faster modulation from existing; that just reflects the limits of Er-doped fiber lasers specifically. --Christopher Thomas (talk) 18:48, 18 November 2009 (UTC)
The first three paragraphs give accurate descriptions of point particle. In particular the third paragraph of the introduction addresses your concern. It defines a point particle as it is from the point of view of quantum mechanics. There are other point particles not mentioned here. Particularly the particles of the lepton family (muon etc.,etc.) And apparently the photon.
I am satisfied that the point particle is generally defined in the introduction, including related to QM. And upon reading the introduction again, there are nuances of difference between the three paragraphs - but, apparently all three are describing "point particle" in different contexts. As with many things in the Sciences and on Wikipedia, different contexts attribute different meanings or definitions.
If you want to write up a section on the photon, please feel free to do so. I just had an idea. We can do the photon by consensus. I am going to check the Wikipedia photon article first and see what it says. Sorry, about your distress concerning this article. Why not a write a blurb expressing what you mean about point particle in QM and make it a first section, after the introduction? Also, I guess I should have put this article on my watch page sooner, because I wrote the content after the introduction. It is just it hasn't had many page views. Steve Quinn (formerly Ti-30X) (talk) 22:06, 17 November 2009 (UTC)
Also, I have to be honest. Adding that a "point particle can (indeed must) simultaneously be delocalised over an appreciable volume of space" is probably too technical for this article. This article is geared toward a general readership. The more technical characteristics are dealt with in other articles, I am sure. If you read through the other physics articles related to wave function, Heisenberg principle, specific particles, etc., etc., I am sure that you will see the techhical aspects have been dealt with. If your issue is with the description of the photon, then I think it would be more appropriate in the photon article. I will put a general blurb about the photon in this article and then point to the main article. Steve Quinn (formerly Ti-30X) (talk) 22:50, 17 November 2009 (UTC)
I'm sorry, but the third paragraph doesn't address the point at all -- namely that a "point particle" in quantum mechanics is not localised to a point. It's not just about photons: think of an electron delocalised all over a molecular orbital: it's an issue with any so-called point particle in quantum mechanics; and it is fundamental. We need to be much more careful to explain what we mean by "point particle" in this context.
For the record, I've also raised this issue at WT:PHYSICS, in the hope that somebody with some experience in explaining this and better references will come forward. Jheald (talk) 22:58, 17 November 2009 (UTC)
There is nothing in any of the paragraphs that explicitly, or implicitly states that a point particle is localized. Also, I have to agree with Christopher Thomas - the definition that you are providing is not really related to point particle. The definition sounds more like position as related to momentum, or vice versa, in the Heinsenberg principle. The defining principle of a point particle is that it lacks spatial dimension. It has nothing to with being localized or de-localized. It is called a point particle because like in mathematics (geometry) a point has no dimensions. Point - no dimensions. Line- one dimension. a rectangle - two dimensions. This is why it is called a point particle. Particular "types" of point particles include point masses and point charges, point particles whose only attributes are their mass and charge respectively.
Also the second paragraph is a defintion of point particle different from the definition in QM. I just wanted to point that out. Steve Quinn (formerly Ti-30X) (talk) 00:08, 18 November 2009 (UTC)
But this begs the fundamental question: if a point particle is not localised to a point, then in what sense is it a point particle? If it is spread over a volume of three dimensional space, then in what sense does it lack spatial dimension?
This the article doesn't even start to attempt to explain (and needs to). Jheald (talk) 00:18, 18 November 2009 (UTC)
Jheald, I believe I have found a reasonable explanation to your question. I have a reply for you over at Electron talk page. I don't if I should have wrote it here or there. But there it is. Maybe someone can help me research this, because it appears I have a source that can explain this better - I hope. Steve Quinn (formerly Ti-30X) (talk) 02:04, 18 November 2009 (UTC)

I am going to continue this discussion over here. My first understanding of the point particle came from reading the "God Particle:" by Lederman. On page 141 and 142 he discusses electron and refers to it as a point particle. It has the "spooky" QM qualities of lacking radius, but has mass and charge and has spin. If you link to the book on google books and enter "electron" in the search function you will see page 142 as one of the links. Scroll back from 142 to 141 and read forward. Steve Quinn (formerly Ti-30X) (talk) 02:29, 18 November 2009 (UTC)

Steve, you say "There is nothing in any of the paragraphs that explicitly, or implicitly states that a point particle is localized." I disagree, here's the first two sentences of the article: "A point particle...is an idealized object heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space." Now we know that the electron in hydrogen has spatial extension: It is in a 1s subshell, where it is spread throughout a volume of about 10-30 m3. (See Bohr radius.) I conclude that either (A) The electron in hydrogen is not a point particle or (B) The first two sentences are wrong. Do you agree? :-) --Steve (talk) 02:52, 18 November 2009 (UTC)
Steve, I agree - I see what you are saying. I agree that the first sentences appear to be wrong. However, it does say "an idealized object". I didn't write the first three paragraphs, so I want to be very careful not to throw out material without first figuring out what the heck it really means. So, "idealized object" to me means some sort of mathematical or conceptual construct. What is needed here is some clear reliable sources about this subject. My next question is are we nit picking when we say an electron cloud has spatial extent?
However, this helps to validate one thing. Lederman wrote and gave a radius for the electron - again - on page 142. This radius was realized, I think in 1990. If you notice in the talk page above, someone scoffed and took that out of the article.
We could remove the first two sentences of this article by consensus, if this helps avoid confusion for the general reader of the article. And if we discover these are really part of the definiton we could restore them.Steve Quinn (formerly Ti-30X) (talk) 03:30, 18 November 2009 (UTC)
Steve, I found a source that may help here. I just have to get instructions on how to set up the link for google books with a page number so I can share the wealth. Hopefully, I will be able to do this soon. The book, at google books is Analytical Dynamics: A New Approach by Firdaus E. Udwadia, Robert E. Kalaba. I entered "point particle" into the book search function and got back 18 results. Page 2 is really interesting. Steve Quinn (formerly Ti-30X) (talk) 03:47, 18 November 2009 (UTC)
It appears that this book supports the content of the first paragraph, and probably the second pargraph. Anyway, I am going to bow out and give the other editors of electron, who brought that article to Featrued Article, a chance to catch up. I have a feeling they are much more knowledgeable about either of these articles than I am. There is a reason why they wrote point particle in the first introductory paragraph, of electron. Steve Quinn (formerly Ti-30X) (talk) 04:30, 18 November 2009 (UTC)
Lederman's book does not say that the electron has a radius or that it was realized. It just gives the upperbound for the electron radius as it was known in 1990, and concludes that this is inline with the assumption that the radius is zero. The book even puts emphasis on the "less than" preceding the value on page 142. (TimothyRias (talk) 11:11, 18 November 2009 (UTC))
Timothy is correct. I misread this page - Lederman just gives the upper bound for the radius in 1990. Thanks for clearing up the confusion. Steve Quinn (formerly Ti-30X) (talk) 14:32, 18 November 2009 (UTC)

My view is that there's (1) "point particle", a concept in classical physics, and there's (2) "point particle", a concept in quantum particle physics, and they're different concepts. The first two paragraphs define (1), the third paragraph defines (2), the second body section elaborate on (2), the third and fourth body paragraphs elaborate on (1), etc. There's no reason for (1) and (2) to be the same article. (2) is already a very nice article, elementary particle. I would propose to merge the quantum half of this article into elementary particle, and keep this article as the classical concept. Of course we would make it clear in the first sentence that "point particle" can also mean "elementary particle", see elementary particle for more information! :-) --Steve (talk) 05:21, 18 November 2009 (UTC)

The Elementary particle article does a good job of introducing how the properties of such particles fall into patterns, and how they combine to build up more complicated things; but their pointlike nature is handed off to here to explain. And as the elementary particle article introduces, there is doubt as to whether the elementary particles are really pointlike, or whether they could be string-like.
Elementary particle is already pretty full, so I don't think it's a good place to load on a discussion of what is actually meant when a quantum mechanical particle is said to be pointlike, or almost pointlike. I think that discussion is best located here. Jheald (talk) 10:31, 18 November 2009 (UTC)
I agree with Steve - some time ago, this article was about point particles of the type that appear in classical mechanics textbooks (c.f. massless strings and ideal springs). It has since been expanded (1) by merging in the old point mass and point charge stubs, and (2) by adding a lot of discussion on elementary particles, duplicating an already-existing article. Most of 2 isn't a discussion of what it means for a particle to be pointlike, it's "quarks for beginners". "Point particle" is a less clear name than "elementary particle", given that the whole "point-like particle" idea is dubious in quantum mechanics. The spatial extent of a photon belongs in the article on photons or elementary particles, not here. I would suggest moving most of this material to more appropriate articles. Djr32 (talk) 23:28, 18 November 2009 (UTC)
As I have come to understand "point particle" in the age of particle accelerators a couple of sentences in the first paragrpah express the concept of point particles well. "Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space. Particular "types" of point particles include point masses and point charges, point particles whose only attributes are their mass and charge respectively." I think this fits the Quantum mechanical definition as well as the third pargraph. According to the discussions on this page, physicists have been unable to measure the actual size of the electron. As far as it is known, the electron has zero radius.
Concerning this is an entry in one of the above sections from User:Tetracube "As of 2000, all X-ray scattering experiments have shown that electrons behave as point-like particles with no spatial extent (ref: H. Haken; Hans Christoph Wolf (2000). The physics of atoms and quanta: introduction to experiments and theory (6th ed.). Springer. p. 70. ISBN 3540672745. ) I think this user summed it best with: "Any attempt to ascribe such hypothetical "radius" values to an actual, physical radius is misleading at best, and totally nonsensical at worst." This is the same for all particles classified as point particles, to date, these have zero radius, or no spatial extent.
Moving on - below the introductory paragraphs are particles. The particles illustrate the third paragraph, and part of the first paragraph. That was the purpose of "listing" and giving short descriptions of those particles. Yes, there are articles on these particles, but the point is to give a brief overview, with a link to the main article. The short descriptions do not give elaborate, in-depth coverage that the articles do. So, I think it is important to have these there, more for the general reader than anything else. There are plenty of articles on Wikipedia that do this sort of thing. As an aside - I notice there are no (non-composite) vector bosons.
I think the article in total gives a well rounded view of what a point particle is. Whoever, wrote those first three paragraphs had a good grasp of the subject. In my mind "point particles" have an important role in physics, but I may be wrong. Anyway, I don't agree that a merger is needed in this situation. I believe this to be a topic which stands on its own merits and has a place in physics (and Wikipedia physics). Steve Quinn (formerly Ti-30X) (talk) 00:48, 21 November 2009 (UTC)
I entirely agree that this topic has it's place within physics, but I think that place is "elementary particle" rather than "point particle". Moving some of this information into the Elementary particle article would improve that article, and would simplify this one by making it an article about the "point"ness of point particles, rather than being dominated by a lot of particle physics. As it stands, there is an undesirable overlap between the two articles. Djr32 (talk) 12:33, 22 November 2009 (UTC)

Arbitrary section break[edit]

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)

  1. "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...
  2. 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.
  3. (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.))
  4. This can be observed in scattering experiments – a major topic we ought to have a better article on (cf [2] 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. [3]
  5. This contrasts with the results of scattering experiments done on composite objects - for example, nuclei or hadrons. (See eg [4] 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)

Rewrote to delete most elementary particle stuff[edit]

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)

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)

New vocabulary item or a spelling error?[edit]

What means "exsitence" please? P0M (talk) 01:57, 20 November 2009 (UTC)

Fixed. --Christopher Thomas (talk) 02:51, 20 November 2009 (UTC)

Tidy up[edit]

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)

Confusing term or Non-term?[edit]

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)

I changed it.--Patrick (talk) 00:50, 4 March 2012 (UTC)

Other languages[edit]

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 (talkcontribs) 00:22, 4 March 2012 (UTC)

File:Modeling.pdf Nominated for speedy Deletion[edit]

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Will somebody please explain how a PDF file link is regarded as a visual image? What appears to have happened is that someone has uploaded a PDF file to Commons, and that there is no proper copyright provision for that to have been done. Either the "find" function on Commons is totally useless, or this file has already been deleted.P0M (talk) 14:15, 13 May 2012 (UTC)
I found it by following the RFD.
It appears that the article was uploaded by "Ribarič" and that there is no proof that the person who uploaded it under that name is the Ribarič who is one of the two authors of that article. I have e-mailed the real Ribarič about what has happened, just in case Dr. Ribarič really wants the article to be available through Commons. P0M (talk) 14:50, 13 May 2012 (UTC)

Spherical objects act like points[edit]

, spherical objects interacting in 3-dimensional space whose interactions are described by the inverse square law behave in such a way as if all their matter were concentrated in their centers of mass. This statement seems incorrect. For example when two billiard balls collide they stop going toward each other much sooner than if all the mass was at their center points. So the actual distance traveled between the two billiard balls to the point of impact is less than would be traveled by two point particles. GeeBIGS (talk) 01:46, 10 February 2013 (UTC)

The key words are whose interactions are described by the inverse square law.
The force between two billiard balls does not follow the inverse square law; nor is eg the Lennard-Jones potential between two atoms an inverse square law. But when the interactions do follow an inverse square law -- eg the gravitational interaction between two planet-like spherical bodies -- then, for the purposes of calculating that interaction, all the mass can be treated as concentrated at the two bodies' centres. Jheald (talk) 16:48, 10 February 2013 (UTC)
What if those two bodies collide though? The real bodies would stop when they contacted each other at their surfaces yet if those bodies were instead points they would travel a longer distance before contacting namely the sum of te two radii of the bodies. That is a tangible difference. Can you explain using my example how what I added is inaccurate? 68.36.148.100 (talk) 00:30, 11 February 2013 (UTC)
If they can collide, and/or if they have something worth calling a surface, than their mutual force is obviously not described by an inverse square law. Which means they are not what the article is talking about. Your example is accurate, but off-topic. If the article said "pigs can't fly", then finding a bird that can would not prove it wrong, no matter how good a flier that bird is. — HHHIPPO 07:42, 11 February 2013 (UTC)

OK, this: "If the distribution of matter in each body is spherically symmetric, then the objects can be treated as point masses without approximation, as shown in the shell theorem. Otherwise, if we want to calculate the attraction between massive bodies, we need to add all the point-point attraction forces vectorially and the net attraction might not be exact inverse square. However, if the separation between the massive bodies is much larger compared to their sizes, then to a good approximation, it is reasonable to treat the masses as point mass while calculating the gravitational force." Seems to me to say that what you are saying is correct UNLESS the two bodies are close enough that they are likely to collide. Does it not say that?165.212.189.187 (talk) 13:48, 11 February 2013 (UTC)

Not really. As far as I can see from the text you posted, the author talks only about inverse square interactions, namely gravity. They then make a distinction between spherically symmetric matter distributions where the treatment as point masses is exact, and other distributions, where this treatment is only an approximation valid for large distances.
The problem with your billiard balls is not that they are not spherically symmetric, but that they are subject to other interactions than the inverse square gravity, namely the contact force which is related to the Lennard-Jones potential between atoms mentioned above. The article says nothing about bodies that don't follow an inverse square law, therefore is says in particular nothing wrong.
One could consider adding the billiard balls as an additional case, where the point mass treatment is approximately valid as long as the distances are that large that the non-inverse-square interactions are negligible compared to the inverse-square ones. That would be a useful addition, though not a necessary correction. — HHHIPPO 20:40, 11 February 2013 (UTC)