The Real Rules for Time Travelers

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.

[via Digg]


7 Responses to The Real Rules for Time Travelers

  1. All Carroll is doing is assuming things cannot be changed by people from the future (the "it just is" argument) and then makes examples based on that, rather than actually giving a reason for WHY things cannot be changed in the first place.

    Anyone can do that:

    You can mess things up with time travel. You go back in time and kill your younger self.

    There we go – I just did the same as him. Said a statement based on something no-one can prove nor disprove then gave an example. Someone write an article about it!

  2. So many people have tried to tell us such rules. The only ones that we can be sure of within our little piece of Space-Time are:

    One can travel into the future, but not into the past. No limitations are placed on the speed of the travel.

    One can see into the past, but not into the future.

    In fact, these two rules can be used to define time.

  3. My personal favorite is the compression theory.

    Since the universe is expanding, in order to go back in time you have to deal with an extreme amount of compressive forces or be turned into a tiny, very dense remnant of your former self.

  4. How about admitting that we don't know what is or isn't possible? I'm a physicist (well, I studies physics) and I admit it. SciFi authors tried a lot of ideas in this space, but at least they don't pretend it's anything but fiction!

    Saqm Hughes has a nice review of all the SciFi time travel models, at

  5. I agree with Nathan Zeldes here. It´s compleatly ridiculus to make arguments about how time travel "would" work.

    This is nothing new; philosophers have thought about weather we live in a deterministic universe way before the modern idea of time travel even existed.

    What Carroll is suggesting is simply that we live in a deterministic universe and that if someone does travel back in time they will become what Donnie Darko fans would call 'the Manipulated Living' ;)

  6. You may not remember a classic 1941 Heinlein story "By His Bootstraps", which elaborates on this theme. Quite a bit. The protagonist is, at various stages in his lifeline: his father, his mother, his midwife, his adopted parent, his trainer. In the end, he observes: "I know where I came from, but where did all you lot come from?"

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