We’ve read the books, we’ve seen the movies (lots of them), but what does a real physicist say about time travel? Sean Carroll of the blog Cosmic Variance is a Senior Research Associate in the Department of Physics at the California Institute of Technology. He wrote a book called From Eternity to Here: The Quest for the Ultimate Theory of Time. In an excerpt at Discover Magazine, Carroll says if time travel were possible (and it might be), there would be no paradox, because we cannot change what has already happened. Ever. Then it gets weird.

Imagine that we have been appointed Guardian of the Gate, and our job is to keep vigilant watch over who passes through. One day, as we are standing off to the side, we see a person walk out of the rear side of the gate, emerging from one day in the future. That’s no surprise; it just means that you will see that person enter the front side of the gate tomorrow. But as you keep watch, you notice that he simply loiters around for one day, and when precisely 24 hours have passed, the traveler walks calmly through the front of the gate. Nobody ever approached from elsewhere. That 24-hour period constitutes the entire life span of this time traveler. He experiences the same thing over and over again, although he doesn’t realize it himself, since he does not accumulate new memories along the way. Every trip through the gate is precisely the same to him. That may strike you as weird or unlikely, but there is nothing paradoxical or logically inconsistent about it.

OK, this is why I’m not a physicist. I know people have a beginning and end. Even Bill Murray got to break out of the loop once he learned how to treat a woman.

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Tags: time, time travel