Free-fall of photons in a planar optical cavity

We consider a planar optical cavity with one of its in-plane axis aligned with 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. Using a general relativity framework, we find that the wavepacket initially at rest in terms of its in-plane group velocity, is subject to a non-relativistic free-fall motion of acceleration g in the Earth-bound reference frame.