Using a black hole to weigh light: can the Event Horizon Telescope yield new information about the photon rest mass?

Abstract

We point out that data collected by the Event Horizon Telescope or a similar project might yield new information about the photon rest mass $m_{\gamma }$, in the form of evidence that $m_{\gamma }\ne 0$ together with a lower bound on $m_{\gamma }$ or a new upper bound on $m_{\gamma }$. Using Sgr A*, there is scope to improve on the best upper bound obtained via laboratory tests of Coulomb's law by up to nineteen orders of magnitude.