Strict limit on CPT violation from polarization of γ-ray bursts

We report the strictest observational verification of CPT invariance in the photon sector, as a result of γ-ray polarization measurement of distant gamma-ray bursts (GRBs), which are the brightest stellar-sized explosions in the Universe. We detected γ-ray polarization of three GRBs with high significance levels, and the source distances may be constrained by a well-known luminosity indicator for GRBs. For the Lorentz- and CPT-violating dispersion relation E±2=p2±2ξp3/M_Pl, where ± denotes different circular polarization states of the photon, the parameter ξ is constrained as |ξ|<O(10^-15). Barring precise cancellation between quantum gravity effects and dark energy effects, the stringent limit on the CPT-violating effect leads to the expectation that quantum gravity presumably respects the CPT invariance.