Astrophysical Bounds on the Photon Charge and Magnetic Moment

TL;DR

This paper uses astrophysical observations to set stringent limits on the photon’s electric charge and magnetic moment, improving previous bounds and constraining new physics beyond the Standard Model.

Contribution

It provides the most stringent astrophysical bounds to date on the photon charge and magnetic moment, utilizing radio interferometry and birefringence measurements.

Findings

Photon charge < 10^(-32) e from radio interferometry

Photon charge < 10^(-46) e considering charge polarity

Magnetic moment < 10^(-24) e cm from birefringence data

Abstract

If the photon possessed an electric charge or a magnetic moment, light waves propagating through magnetic fields would acquire new quantum mechanical phases. For a charged photon, this is an Aharonov-Bohm phase, and the fact that we can resolve distant galaxies using radio interferometry indicates that this phase must be small. This in turn constrains the photon charge to be smaller that 10^(-32) e if all photons have the same charge and smaller than 10^(-46) e if there are both positively and negatively charged photons. The best bound on the magnetic moment comes from the observed absence of wavelength-independent photon birefringence. Birefringence measurements, which compare the relative phases of right- and left-circularly polarized waves, restrict the magnetic moment to be less than 10^(-24) e cm. This is just a few orders of magnetude weaker than the experimental bounds on the electron and neutron electric dipole moments.