Tiny violations of the Lorentz symmetry of relativity and the associated discrete $CPT$ symmetry could emerge in a consistent theory of quantum gravity such as string theory. Recent evidence for linear polarization in gamma-ray bursts improves existing sensitivities to Lorentz and $CPT$ violation involving photons by factors ranging from ten to a million.

• Received 23 January 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.201601