Send a pulse of white light into an optical fiber, and the red components of its spectrum will reach the far end before the blue components do. Send the same pulse through a vacuum, and the result will be a dead heat between all frequencies. The absence of frequency dispersion is a consequence of a fundamental relativity symmetry known as Lorentz invariance. Some theories of quantum gravity, however, predict that, for very high photon energies, a vacuum behaves like a medium. Researchers at the Large High Altitude Air Shower Observatory (LHAASO) in China have now tested Lorentz invariance by analyzing measurements of a gamma-ray burst (GRB) [1]. The lack of an observable Lorentz-invariance violation allowed the team to raise the energy threshold at which quantum gravity effects might become evident.
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