March 14, 2018, was a Wednesday, Pi Day, 139 years after the birth of Albert Einstein. One-hundred-thirty-nine is a happy prime number. On that day, legendary Cambridge physicist Stephen Hawking passed away.
June 29, 2009, was a Sunday, one day before pyramid schemer Bernie Madoff would get sentenced to 150 years in prison. Hawking sat in his wheelchair at a cocktail party at Gonville & Caius College, Cambridge, beneath a banner reading, “Welcome Time Travellers.” Invitations were only revealed after the party, so only the host showed up, though he did not provide any specific evidence that he was not himself a time traveler. It was at least partly a stunt for a television show, the sort of combination of serious and silly that Hawking was known for.
July 15, 1992, was another Wednesday, the day that Bill Clinton became a presidential nominee. Also, a new issue of the journal Physical Review D was released, with a paper called “Chronology Protection Conjecture,” by Hawking. In it, he posits that the laws of physics may actually prevent the possibility of time travel, something he would believe—with the occasional caveat or soft spot for M-theory—throughout his life. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” he wrote. He builds his argument carefully across nine pages, with lines like, “As one approaches a closed null geodesic γ in the Cauchy horizon, the propagator will acquire extra singularities from null geodesics close to γ that almost return to the original point.” Clearly. The paper closes with his clearest evidence that time travel is not possible: “the fact that we have not been invaded by hordes of tourists from the future.”
December 24, 2018, was a Monday, Christmas Eve, and we still don’t have any probable routes back in time, at least in this universe. It’s still not possible to travel faster than the speed of light (we haven’t even gotten close), wormholes remain theoretical, and and we don’t know of any exotic matter with a negative energy density that would keep one open long enough for a person to pass through. So for now (and then and a thousand years from now), the universe remains safe for historians, and we will not have been able to attend Hawking’s party. But we still have all the history he made.