Sorry, science-fiction fans: Time travel is impossible. That is the sobering conclusion, anyway, of a Hong Kong-based team of physicists. They found that the maximum speed of a single photon, the basic unit of light, “obeys the traffic law of the universe,” Agence France-Presse reports. The photon cannot go faster than the speed of light—186,282 miles per second—and thus provides no way around the law of physics that an effect can’t come before its cause. Previously, scientists had noticed that in certain mediums, light appeared to exceed its set limit. But when the Hong Kong researchers shot photons through atoms chilled to one ten-thousandth of a degree above absolute zero, they could make more-precise measurements of the waves they generate in the material ahead of them. It turns out even those “optical precursors” can’t break the speed-of-light barrier. Lead researcher Du Shengwang says that means the “information carried by a single photon” can only exist in the present.