Light is redshifted/blueshifted when travels in gravity by an amount proportional to the gravitational field, as if it had effective mass . The photon acquires mass inside a superconductor making the electromagnetic interaction of a short range https://www.nature.com/news/photons-pair-up-like-superconducting-electrons-1.22868. . Note that photons in vacuum are massless. Could we say light also acquires charge when moving under gravity so that its radiation power is given by some Larmor-like power? If that is so then the photon charge will be equal to the Planck charge.
Hi Arbad,
no the photon does not acquire an electrical charge. Intrinsic properties of particles in general are determined by different classes of symmetries (gauge, SU(N), CP, and so on) which are not affected by gravity. So does the electrical charge which corrresponds to an intrinsic property of a particle (Isospin symmetry). Now, in very simple grounds, GR does affect massless particles because they do have relativistic mass. But GR, as a geometric theory cannot affect the quantum intrinsic property of a particle. Just to give you an example: Isospin (I) invariance leads to the principle of charge independence (CI). If H represents the Hamiltonian of a physical system, this principle may be stated in the form [H,I]=0. And the format of this statement does not change if the system is imersed in a gravitational field (gravitational force do not "see" isospin).
Hi Arbad,
The mass of a particle should be thought as an invariant measure of its energy, and for most particles that is its energy at rest, only that has a meaning that is independent of the reference frame of he observer. There is no frame in which the photon is at rest so the only invariant value is 0. That a photon should not be assigned a charge can be infererd from the fact that symmetry arguments indicate that there is no natural way to decide if it should be positive or negative,
Dear Daniel,
Thank you for your comment. I would like you to read this paper
Article Photonic Counterparts of Cooper Pairs
Dear Arbab:
Concerning the mass, it can be shown that the gravitational mass of a vessel containing photons increases in proportion to h \nu/c^2 for each photon, where \nu is the frequency of the associated EM wave. Anyway, as said before, the rest mass of the photons is, obviously, zero. That mass is a dynamical effect, so to speak, because of their momentum (and not an intrinsic property of photons themselves).
In the attached notes I illustrate this point both for classical particles in a gas and for quantized EM radiation.
Best Regards,
Luis
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