Do photons travel faster than gravitons?

TL;DR

This paper investigates how quantum electrodynamics effects in curved spacetime can cause photons to travel faster than gravitons, with implications for gravitational wave and electromagnetic signal timing.

Contribution

It demonstrates that QED effects can lead to superluminal photon propagation exceeding graviton speed in an expanding universe.

Findings

Photons can propagate superluminally due to QED effects in curved spacetime.

Superluminal photon speed can cause electromagnetic signals to arrive before or after gravitational waves.

Results applied to GW170817 suggest potential observational differences in signal timing.

Abstract

The vacuum polarization in an external gravitational field due to one loop electron-positron pair and one loop millicharged fermion-antifermion pair is studied. Considering the propagation of electromagnetic (EM) radiation and gravitational waves (GWs) in an expanding universe, it is shown that by taking into account QED effects in curved spacetime, the propagation velocity of photons is superluminal and can exceed that of gravitons. We apply these results to the case of the GW170817 event detected by LIGO. If the EM radiation and GWs are emitted either simultaneously or with a time difference from the same source, it is shown that the EM radiation while propagating with superluminal velocity, would be detected either in advance or in delay with respect to GW depending on the ratio of millicharged fermion relative charge to mass $\epsilon/m_\epsilon$.