Free-fall of photons in a planar optical cavity

TL;DR

This paper demonstrates that photons trapped in a planar optical cavity experience gravitational acceleration, revealing a relativistic effect of gravitational redshift on stationary light within the cavity.

Contribution

It introduces a relativistic analysis of photon free-fall in a planar cavity, linking gravitational redshift to photon acceleration in a novel experimental setup.

Findings

Photons in a cavity accelerate under gravity consistent with free-fall.

The effect is explained as a gravitational redshift applied to stationary photons.

The phenomenon aligns with non-relativistic predictions at early times.

Abstract

We consider a planar optical cavity with its main axis oriented perpendicular to the earth's local gravity field. We examine the motion of a wavepacket of light trapped within the cavity spacer under the action of gravity. Within a relativistic framework, we find that the wavepacket initially at rest in terms of in-plane group velocity is accelerated by $g$ at early time in agreement the non-relativistic limit of free-fall. We show that this phenomenon is a manifestation of gravitational redshift, applied on photons at rest within the cavity.