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

## TL;DR

This paper proposes that observations from the Event Horizon Telescope could provide significantly improved bounds on the photon rest mass, potentially revealing whether it is zero or non-zero.

## Contribution

It introduces a novel method to use black hole imaging data to constrain the photon rest mass, surpassing laboratory bounds by up to nineteen orders of magnitude.

## Key findings

- Potential to set new upper bounds on photon mass
- Possibility of detecting non-zero photon mass evidence
- Improvement over laboratory Coulomb's law tests

## 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.