Scientists have conducted experiments that have confirmed that individual photons cannot travel faster than the speed of light in a vacuum.
For a time, it was thought that photons might be able to travel faster than the speed of light. That prospect left open some tantalizing possibilities, because under Albert Einstein’s theory of relativity, if an object could travel faster than the speed of light it could evade principles of causality. That is, an event’s effect, by traveling faster than the speed of light, could theoretically precede its cause, and time travel conceivably could occur. The most recent experiments have ruled out that possibility, as least as it relates to photons.
Fortunately for fans of time travel everywhere (and everywhen), Einstein’s theories still permit random intersections of curved space-time continuums — i.e., wormholes — through which time travel could occur. Thus, it remains possible that Star Trek‘s Dr. Leonard McCoy could inadvertently cause the Nazis to win World War II and that H.G. Wells’ Time Traveller could save Weena from the Morlocks, and you should still take care not to accidentally kill an ancestor and thereby prevent your own birth.