Friday, June 17, 2005

No paradox for time travellers

"Some solutions to the equations of Einstein's general theory of relativity lead to situations in which space-time curves back on itself, theoretically allowing travellers to loop back in time and meet younger versions of themselves. Because such time travel sets up paradoxes, many researchers suspect that some physical constraints must make time travel impossible. Now, physicists Daniel Greenberger of the City University of New York and Karl Svozil of the Vienna University of Technology in Austria have shown that the most basic features of quantum theory may ensure that time travellers could never alter the past, even if they are able to go back in time."

Finding this story headlining at The Anomalist came as something of a synchronicity, since I just finished reading a chapter about the quest to avoid causality violation in Jenny Randles' "Breaking the Time Barrier." The article indicates that waves sent into the past will self-destruct, ensuring causality, but what about physical objects?

So far the best "explanation" I've heard is the "principle of least action," which allows for time travel but requires infinite energy in order to change something that would threaten the time traveler's own (future) existence. Maybe.

Then there's the Many Worlds take, in which you can endlessly "violate" causality because you're not even in the same universe you came from and, presumably, anything goes; you're just a blip in the quantum ocean, one of potential trillions of doppelgangers.


razorsmile said...

I'll settle for any of the above as long as I get to be involved.

W.M. Bear said...

I recall an old time travel story (can't recall name or author though) in which a TT goes back in time to try to prevent Lincoln's assassination. It's known historically that the reason Booth was able to enter Lincoln's box at Ford's Theater was that the guard was absent for some unknown reason. Of course, in the story, the twist is that it's the TT himself who distracts the guard by trying to get into Lincoln's box and warn him! In terms of avioiding paradox, this represents one solution to possible TT paradoxes. Events simply incorporate the TT's presence and actions into the original causal chain! Not maybe as sophisticated as quantum waves self-destructing but a great exercise in plot manipulation at any rate!

Ken Younos said...

If we can theoretically go backwards in time, is it also possible for us to go forward in time?

W.M. Bear said...

ken -- Traveling forward in time is "easy" and involves no paradoxes. Relativistic time-dilation slows down time for someone traveling near the speed of light so that, in effect, he or she is traveling into the future. For example, if you could travel at 99.99% the speed of light (practically difficult but theoretically possible), when a year passed for someone at rest, only about 4 days would have passed for you. As you get even closer to the speed of light, time dilation becomes even more pronounced so that you are, in effect, traveling into the future.