Wikipedia:Reference desk/Archives/Science/2024 January 21

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January 21

Photon gas

Richard A. Mould, claims in his book "Basic Relativity" (New York, Springer-Verlag, 1994), p.119, that although a photon gas has no rest mass, it has [been empirically proven to have] a positive relativistic mass. Can anyone supply an evidence for that claim? By the way, here is what he exactly states about the photon gas:

"Its passive gravitational mass is equal to its relativistic mass (which equals its total energy divided by c^2), so that when it is placed on a scale in a gravitational field g, its weight is equal to its total energy divided by c^2 times g. Furthermore, if the gas is accelerated horizontally [that is, without moving away from/to the gravitational field] it will display inertial properties also equal to the total energy divided by c^2, even at non-relativistic accelerations."

HOTmag (talk) 17:36, 21 January 2024 (UTC)

Consider two photons of equal energy E flying in opposite directions along the x-axis. The 4-momentum of the first photon is (E, p, 0, 0) = (E, E, 0, 0), its mass is $m^{2}=E^{2}-p^{2}=0$ (I'm using units where c=1). The 4-momentum of the second photon is (E, −p, 0, 0) = (E, −E, 0, 0), its mass is $m^{2}=E^{2}-(-p)^{2}=0$. The 4-momentum of the two photons combined is (E+E, p+(−p) , 0, 0) = (2E, 0, 0, 0), its mass is $m^{2}=4E^{2}$ or $m={\frac {2E}{c^{2}}}$ with c put back in. Therefore, a system consisting of two photons has finite mass even though the photons individually have zero mass. This is true for any two photons as long as they don't travel in the same direction. Now generalise to a photon gas and you have. I've said it many times before: Relativistic mass applies to systems that have internal degrees of freedom. It does not apply to elementary particles without substructure. And don't give me any of that "semantics" stuff, I'm tired of hearing that. --Wrongfilter (talk) 18:04, 21 January 2024 (UTC)

This discussion seems to be confusing invariant mass with relativistic mass. The photons individually have zero invariant mass, but the invariant mass of the system of the two photons is nonzero.

Relativistic mass, on the other hand, seems to be out of fashion as a category. There's nothing really wrong with it; it's just not generally found to be the best bookkeeping method for describing things.

But why in the world you would use it, but then exclude elementary particles, makes no sense to me whatsoever. Wrongfilter, I really think you owe more explanation than "I've said it many times before", because I think you're simply incorrect here. --Trovatore (talk) 19:21, 21 January 2024 (UTC)

Let's throw out the term "relativistic mass" entirely then, and call it "invariant mass", fine with me. What I meant is mass that can arises from energy in internal degrees of freedom (which is a relativistic effect), but maybe the wrong term is stuck in my aging brain. I'd be happy with simply calling it "mass", anyway, since mass is invariant (to changes of reference system or, equivalently, centre-of-mass motion). Apart from that, the explanation is in the maths above. --Wrongfilter (talk) 19:28, 21 January 2024 (UTC)

Thank you for your detailed response.

As for your answer (except your two last sentences which I didn't understand): Please notice the equations you've used refer to the invariant mass only. So yes, it's a well known fact that a given system consisting of two photons that move in opposite directions has a positive invariant mass, even though each photon in that system has no invariant mass.

But I didn't ask about the invariant mass, but rather about what Mould (the author whose book I've quoted from) had meant: It seems he had made a distinction between an invariant mass and a relativistic mass, claiming an empirical claim, that even though a gas photon has no invariant mass it does have a relativistic mass. For us to get better what Mould had meant, let me quote from Max Jammer's book: "Concepts of Mass in Contemporary Physics and Philosophy" p. 56, about what Mould had meant: "he [Mould] illustrates it in terms of a photon gas, which has a rest mass equal to zero but, contrary to what is commonly thought, is not weightless", and then Jammer quotes exactly what I've quoted from Mould's book in my first post. So it seems Mould hadn't used the equations you've used, but rather had relied on an empirical evidence, and that's why I asked if anybody could explain what Mould had meant.

As for your two last sentences (in your first response to me): I didn't understand what you were talking about, because I don't remember I've ever given "semantics" staff. HOTmag (talk) 19:43, 21 January 2024 (UTC)

You did, a few months ago. No you didn't, see below. I apologise. --Wrongfilter (talk) 10:37, 22 January 2024 (UTC) I don't know Mould's book so I can only rely on the passage quoted above; my response is informed by the courses that I've taken and books that I've read (with Schutz' "First course in general relativity" probably the biggest influence). As I understand it Mould always talks about the mass of the gas, never about the mass of any individual photon. In light of that, Jammer's looks like a misunderstanding, but I can't tell without more context. Direct empirical evidence for the mass of a photon gas would seem hard to come by, that'd be a very difficult experiment; I'd like to see that though, if it has been done. However, the mass of a proton arises from essentially the same principle. --Wrongfilter (talk) 20:01, 21 January 2024 (UTC)

Although in case of proton it is gluons, not photons. Ruslik_Zero 20:12, 21 January 2024 (UTC)

Doesn't matter. It's energy in internal degrees of freedom that contributes to the mass of the composite system. --Wrongfilter (talk) 21:12, 21 January 2024 (UTC)

Re. your estimation that Jammer misunderstood Mould: Actually that's what I had suspected before I posted this thread, but if we stick to the way Mould phrased his idea - using the term "relativistic mass", I'm not quite sure Jammer was wrong (even though I still suspect).

Maybe you confuse with someone else? I do remember, that I asked (some months ago) a question about the neutrino's mass, and that I found your reply very thorough and helpful. I also remember someone else asked a question about the neutrino in the same month, and that you had a long conversation with that user. Anyway, I don't think I've ever used the word "semantics" in my posts here or anywhere. HOTmag (talk) 21:39, 21 January 2024 (UTC)

Mould didn't state, in this quote of him, that a photon gas has no mass or invariant mass! Of course laser light's mass-energy should gravitate (has a gravitational field) any counter-propagating light and all matter. For example, see this recent paper which references this one. Modocc (talk) 23:11, 21 January 2024 (UTC)

Thank you for this link. I appreciate that. HOTmag (talk) 09:00, 22 January 2024 (UTC)

Those very interesting references delve into general relativity, whereas I think Mould talks about special relativity (in his quote the gravitational field is external). In GR the source of the gravitational field is not simply the mass itself, but the energy-momentum tensor, which includes energy density (including particle mass, but also kinetic and other internal energy components), pressure, stress (zero for a radiation field) and energy flow. The latter is interesting because it means that the gravitational field depends on the state of motion of the source (relative motion between source and observer, more precisely). This is due to the fact that the gravitational field itself is frame-dependent and has to be transformed between the rest frames of the source (problematic for a light beam, I know) and the observer. --Wrongfilter (talk) 10:01, 22 January 2024 (UTC)

Thank you for this comment. HOTmag (talk) 15:26, 22 January 2024 (UTC)

I loss 1h00 in finding your pb with 2E/c^2 in your text (may be a masked char). Otherwise, it was impossible for me to put latex formula in my question today. In this answer it is impossible too, a mystery. Malypaet (talk) 18:27, 22 January 2024 (UTC)

Can't you find ${\frac {2E}{c^{2}}}$ in his text? HOTmag (talk) 18:41, 22 January 2024 (UTC)

Looks like I forgot the slashes in the closing tags. I need a vacation (in fact, I am on vacation, of sorts). --Wrongfilter (talk) 18:53, 22 January 2024 (UTC)

banned user
The following discussion has been closed. Please do not modify it.
(edit conflict) I'm sorry to hear about your aunt's passing. Maybe Wrongfilter is alluding to 2A06:C701:7469:5D00:79A0:4F6B:4303:B768's reference to "syntax" on 27 December, or even Modocc's 18 November reference to "synthesis" in relation to a comment by 2A06:C701:7463:9900:11BA:FAE2:6F7E:5413. 2A00:23D0:DB9:C701:71A7:949A:7ABB:10F1 (talk) 10:24, 22 January 2024 (UTC) My sincere apologies to HOTmag — it was indeed that lengthy IP 2A06:C701:7463:9900:11BA:FAE2:6F7E:5413 in a comment on 16 November in this thread. --Wrongfilter (talk) 10:37, 22 January 2024 (UTC) I will say no more than that the IP geolocates to Israel. 2A00:23D0:DB9:C701:71A7:949A:7ABB:10F1 (talk) 10:48, 22 January 2024 (UTC) I missed it (15:06, 16 November). 2A00:23D0:DB9:C701:71A7:949A:7ABB:10F1 (talk) 10:53, 22 January 2024 (UTC)