Re: APOD: A December Summer Night (2023 Dec 23)
Posted: Mon Dec 25, 2023 12:39 am
Chris Peterson wrote: ↑Mon Dec 25, 2023 12:27 amMaybe not. But I'll be surprised if it turns out to be anything else. It is, of course, the rest mass that is zero. Which is why it cannot ever be at rest, or have any speed other than c. Because it is moving, it has what is an effective mass (which is consistent with having momentum). And that makes sense, given that mass and energy are equivalent.johnnydeep wrote: ↑Mon Dec 25, 2023 12:21 amThanks. Rest mass of a photon? I thought it was 0, but apparently, that hasn't been definitively confirmed!Chris Peterson wrote: ↑Mon Dec 25, 2023 12:02 am
Sure. A photon only has a few properties. The invariant ones are its rest mass and speed (those are actually related). The only meaningful quantum property is its momentum. That changes due to its motion through expanding space. In fact, I don't think anything else can change the momentum of a photon without replacing it with a new one.
https://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass:~:text=time.%5B30%5D-,Experimental%20checks%20on%20photon%20mass,-%5Bedit%5D wrote:
Experimental checks on photon mass
Current commonly accepted physical theories imply or assume the photon to be strictly massless. If photons were not purely massless, their speeds would vary with frequency, with lower-energy (redder) photons moving slightly slower than higher-energy photons. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the upper bound on speed that any object could theoretically attain in spacetime.[31] Thus, it would still be the speed of spacetime ripples (gravitational waves and gravitons), but it would not be the speed of photons.
If a photon did have non-zero mass, there would be other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra physical degree of freedom. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would allow the presence of an electric field to exist within a hollow conductor when it is subjected to an external electric field. This provides a means for precision tests of Coulomb's law.[32] A null result of such an experiment has set a limit of m ≲ 10−14 eV/c2.[33]
Sharper upper limits on the mass of light have been obtained in experiments designed to detect effects caused by the galactic vector potential. Although the galactic vector potential is large because the galactic magnetic field exists on great length scales, only the magnetic field would be observable if the photon is massless. In the case that the photon has mass, the mass term ½ m2AμAμ would affect the galactic plasma. The fact that no such effects are seen implies an upper bound on the photon mass of m < 3×10−27 eV/c2.[34] The galactic vector potential can also be probed directly by measuring the torque exerted on a magnetized ring.[35] Such methods were used to obtain the sharper upper limit of 1.07×10−27 eV/c2 (the equivalent of 10−36 daltons) given by the Particle Data Group.[36]
These sharp limits from the non-observation of the effects caused by the galactic vector potential have been shown to be model-dependent.[37] If the photon mass is generated via the Higgs mechanism then the upper limit of m ≲ 10−14 eV/c2 from the test of Coulomb's law is valid.