APOD: A December Summer Night (2023 Dec 23)

[johnnydeep]

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.

Thanks. Rest mass of a photon? I thought it was 0, but apparently, that hasn't been definitively confirmed!

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 m²A_μA^μ 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.

Maybe 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.

[VictorBorun]

I didn't say that photons can't be affected, only that they only can travel in a straight line (geodesic) which they cannot be deviated from. Which is why scattering involves the replacement of one photon with another.

So is a photon that drops in energy while traveling along a space-time geodesic still the "same photon"?

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.

but a travelling photon can keep its polarisation plane too, can't it? I thought this plane can be rotated somewhat by a strong magnetic field too. Or do you tell us to consider the magnetic field as creating virtual electron-positron pairs and scattering the poor photon by elastically absorbing-emitting it?

[johnnydeep]

So is a photon that drops in energy while traveling along a space-time geodesic still the "same photon"?

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.

but a travelling photon can keep its polarisation plane too, can't it? I thought this plane can be rotated somewhat by a strong magnetic field too. Or do you tell us to consider the magnetic field as creating virtual electron-positron pairs and scattering the poor photon by elastically absorbing-emitting it?

It was news to me that photons could be affected by magnetic fields, and apparently, at least in a vacuum, they can't. But it's different for photons travelling through a medium. Not sure how authoritative this person is, but it sounds convincing. There's another very interesting reply there about photons splitting into virtual particle pairs which I won't include, but that's pretty authoritative sounding too!

Actually, fixed magnetic fields have no effect on light propagating through a vacuum and (even for rather large field strengths) negligible effect on light propagating through most materials. Quark-antiquark pairs form another category of particle (meson) altogether, not light. Light is not composed of charged particles.

The interesting cases where magnetic fields do affect light propagation are in materials exhibiting the Faraday effect. In these materials, a magnetic field can change the way the charged particles (mainly electrons) respond to the light electromagnetic field. As a result, the polarization of the light (the plane in which the electric field points) rotates as the light propagates through the material. The direction of rotation depends on which way the field points.