by Paul Levinson

Time travel is my favorite kind of science fiction – precisely because it’s almost certainly impossible. And because it’s so likely impossible, seeing how time travel stories can work, can make sense, is a special kind of fun.

Why is time travel impossible?

Well, if you travel to the past to change whatever event, and you succeed, how would you have had knowledge about the event you went back to change in the first place? This is often called the grandfather paradox – if you go back in time and accidentally kill your grandfather, then you wouldn’t have been born, so how could you have gone back in time in the first place – but there’s no need to kill anyone in your family for this paradox to be upon you, the time traveler. Anything you do, when you travel back in time, that invalidates the reasons or necessary ingredients for the trip, would trigger the paradox.

There are lots of ways out of this, such as the multiple words interpretation, which works like this: Time traveler 1 (TT1) from World 1 travels back in time, and accidentally prevents either set of grandparents from meeting. So TT1 is never born. And in that world – call it World 2 – since TT1 doesn’t exist, there may never be any time travel. But that’s ok, because we could day TT1 from W1 went back in time, stopped the grandparents from meeting, which resulted in W2 with no TT1 or TT2. No paradox at all with these multiple worlds.

But if our existence really consisted of an infinite number of multiple worlds or realities, with a new one triggered with every action of the time traveler, that would make for an existence far more insane than just our normal world with time travel, right?

Ok, but what about travel to the future? No grandparent paradoxes there, but we run into other problems. If I travel one day into the future, and I see you wearing a blue hat, what does that mean for you? That you have no choice but to wear a blue hat? Well I don’t know about you, but I think I have a choice about what color hat to wear tomorrow, or not wear any hat at all. We call that free will. Don’t you think you have that ability too, or do you think everything you do from now on is available to the scrutiny of anyone in your vicinity who travels into the future, which would result in your doing just that, and only that, whatever other ideas you might have right now about what to do tomorrow?

Time travel is such an enjoyable exercise for the mind that I get a kick just thinking about it, and writing about those paradoxes and loops. But reading a great novel like Isaac Asimov’s The End of Eternity, or seeing a movie like 12 Monkeys, or a new television series like Travelers – well, that’s always a rare treat indeed. If you’ve read this far, I’d bet that at least some part of your brain agrees….

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