Photons always propagate at the speed of light and obey the same rules of nature, regardless of their energy. If certain models of quantum gravity or string theory are correct, photons above a certain energy threshold should decay as they propagate through the Universe. The HAWC collaboration just tested this, and found that no such cutoff exists. (NASA/SONOMA STATE UNIVERSITY/AURORE SIMONNET)

Astrophysics Signal Does What The LHC Cannot: Constrain Quantum Gravity And String Theory

Astrophysics has probed a test of a fundamental law, ‘Lorentz invariance,’ well beyond the LHC’s limits. Einstein is still right.

The greatest scientific legacy that Albert Einstein left us is this: that the speed of light, and the laws of physics, appear to be the same for all observers in the Universe. Regardless of where you’re located, how fast or in which direction you’re moving, or when you’re performing your measurements, everyone experiences the same fundamental rules of nature. The symmetry that underlies this, Lorentz invariance, is .

However, many ideas that go beyond the Standard Model and General Relativity — such as string theory or most manifestations of quantum gravity — could break this symmetry, with consequences for what we’d observe about the Universe. A , just published on March 30, 2020, just placed the tightest constraints on Lorentz invariance violation ever, with fascinating implications for theoretical physics.