Gravitons and Lightcone Fluctuations

TL;DR

This paper investigates how quantum-induced metric fluctuations from gravitons in a squeezed vacuum state can alter lightcone structures, potentially allowing photons to travel faster or slower than the classical speed of light.

Contribution

It demonstrates that metric fluctuations from gravitons can smooth out lightcone singularities and modify Green's functions, revealing possible superluminal photon propagation.

Findings

Lightcone singularity disappears after averaging over metric fluctuations.

The retarded Green's function becomes a Gaussian, nonzero inside and outside the lightcone.

Photon propagation speeds can be faster or slower than the classical light speed.

Abstract

Gravitons in a squeezed vacuum state, the natural result of quantum creation in the early universe or by black holes, will introduce metric fluctuations. These metric fluctuations will introduce fluctuations of the lightcone. It is shown that when the various two-point functions of a quantized field are averaged over the metric fluctuations, the lightcone singularity disappears for distinct points. The metric averaged functions remain singular in the limit of coincident points. The metric averaged retarded Green's function for a massless field becomes a Gaussian which is nonzero both inside and outside of the classical lightcone. This implies some photons propagate faster than the classical light speed, whereas others propagate slower. The possible effects of metric fluctuations upon one-loop quantum processes are discussed and illustrated by the calculation of the one-loop electron self-energy.