Is time travel possible? This is a perennial question of science fiction, and real scientists investigate the possibility of these sorts of journeys too. The case for forwards time travel is scientifically solid. In fact, many of us have already time traveled in exactly this way. The faster we travel, the slower time passes. Take a high-speed flight around the world, and when you return home you’ll find you’ve experienced slightly less time than those you left behind. Einstein figured this out and he was a bro. But he wasn’t a philosopher. We folk investigate logic, not the flighty field of theoretical physics.
So what about backwards time travel. The possibility of being able to relive previous glories and rectify past failures is an appealing one, but could we ever travel to the past and make these changes? One of the strongest objections to this sort of journey is the Grandfather Paradox. Suppose Timmy–a very troubled young man–develops a seething hatred for his grandfather. Timmy builds a time machine, travels back to 1950, and sets out to kill his grandfather as a child. How does this story unfold? If Timmy succeeds, then his grandfather will never live to have children and Timmy will never have been born. If Timmy was never born, then he never traveled back in time to kill his grandfather, in which case his grandfather lives. But if his grandfather lives, then Timmy was born, and so can travel back in time to murder his grandfather. And so on in circles until your head is sore.
It seems that we can’t make sense of Timmy’s story. It doesn’t seem just false, but incoherent. How do we resolve this problem? The monumental David Lewis, owner of the most majestic philosopher’s beard of the contemporary age, shows the way. According to Lewis, every time only happens once, and whatever happens at that time always happened at the time. There is only one 1950, and Timmy’s grandfather either did, or did not, die in that year. The fact that Timmy’s grandfather survived to father children (and eventually grandchildren) is very good evidence for the latter. Thus, any murder attempt by Timmy did (and will) fail. Maybe Timmy loses his nerve, or maybe his gun jams. Whatever happens, some normal event will prevent Tim from successfully murdering his Grandfather. The moral of the story? Backwards time travel may be possible. Unfortunately, changing the past isn’t. Guts.