It’s interesting to note that solutions to the equations of general relativity allow for the existence of “closed time-like curves” in which if you have two contemporaneous observers, the future of one may lie in the past of the other. In other words, Einstein allows time travel. Also noteworthy is that time travel is not disallowed by the laws of quantum mechanics. This guy believes a dedicated effort of scientific research could create a “time telegraph” or “time telephone” inside of a century.