A Search in Gamma-Ray Burst Data for Nonconstancy of the Velocity of Light

We discuss possible tests of the constancy of the velocity of light using distant astrophysical sources such as gamma-ray bursters (GRBs), active galactic nuclei (AGNs), and pulsars. This speculative quest may be motivated by some models of quantum fluctuations in the spacetime background, and we discuss explicitly how an energy-dependent variation in photon velocity δc/c ~ -E/M arises in one particular quantum-gravitational model. We then discuss how data on GRBs may be used to set limits on variations in the velocity of light, which we illustrate using BATSE and OSSE observations of the GRBs that have recently been identified optically and for which precise redshifts are available. We show how a regression analysis can be performed to look for an energy-dependent effect that should correlate with redshift. The present data yield a limit M ≳ 10¹⁵ GeV for the quantum gravity scale. We discuss the prospects for improving this analysis using future data, and how one might hope to distinguish any positive signal from astrophysical effects associated with the sources.