Strict Limit on CPT Violation from Polarization of Gamma-Ray Bursts

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We report the strictest observational verification of CPT invariance in the photon sector, as a result of gamma-ray polarization measurement of distant gamma-ray bursts (GRBs), which are brightest stellar-size explosions in the universe. We detected the gamma-ray polarization of three GRBs with high significance, 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/MPl, where ± denotes different circular polarization states of the photon, the parameter ξis 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.