Astronomers have observed record-breaking photons that strain classical theories of accelerationA little before sunrise on July 4, A.D. 1054, imperial astronomers of the Song Dynasty in China spotted an unknown star lighting up the eastern sky. “It’s as bright as Venus, with pointed rays in all four directions and a reddish-white color,” they wrote in notes delivered to the emperor. The glow, which remained visible to the naked eye during the day for almost a month, was from an explosion caused by the spectacular death of a star located 6,500 light-years away in the constellation of Taurus. Its relics are...