Constraining Lorentz Invariance Violation with Fermi

TL;DR

This paper uses Fermi observations of a gamma-ray burst to set stringent limits on Lorentz invariance violation, constraining quantum gravity models predicting energy-dependent photon speeds.

Contribution

It provides the first high-precision constraints on energy-dependent photon speed variations using Fermi data from GRB 090510.

Findings

Limits on linear energy dispersion scale exceed the Planck scale

No evidence of Lorentz invariance violation detected

Quantum gravity models predicting such effects are highly constrained

Abstract

A cornerstone of special relativity is Lorentz Invariance, the postulate that all observers measure exactly the same photon speeds independently on the photon energies. However, a hypothesized structure of spacetime may alter this conclusion at ultra-small length scales, a possibility allowed in many of the Quantum-Gravity (QG) formalisms currently investigated. A generalized uncertainty principle suggests that such effects might occur for photon energies approaching the Planck energy, $E_{Planck}=M_{Planck} c^2 \simeq 1.22\times10^{19} GeV$. Even though all photons yet detected have energies $E_{ph}<<E_{Planck}$, even a tiny variation in the speed of light, when accumulated over cosmological light-travel times, may be revealed by high temporal-resolution measurements of sharp features in Gamma-Ray Burst (GRB) lightcurves. Here we report the results of a study using the emission from GRB 090510 as detected by \textit{Fermi}'s LAT and GBM instruments, that set unprecedented limits on the dependence of the speed of photons on their energy. We find that the mass/energy scale for a linear in energy dispersion must be well above the Planck scale, something that renders any affected QG models highly implausible.