Paradox Lost: New science says going back in time and changing the past might not change the present

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Sep 8, 2014

Has any serialized sci-fi story ever managed not to dip a toe into the stream of nonlinear time travel? If such a thing exists, I can't think of it. We're all just collectively fascinated by the idea of going back in time and killing Hitler, or going forward in time to see how terrible we'll look when we get old.

But there's never been any real time travel, right? And that is due at least in part to the fact that traveling backward in time would almost certainly create some kind of causality paradox, right?

Well, the bad news is that, no, there probably hasn't been any time travel, at least not happening within our universe. But the good news is that if there was, there's mathematical evidence to suggest you won't have to worry about wacky paradox problems.

Here are the necessary facts you need going in. Warning: serious math and science ahead.

Einstein's theory of general relativity does not prohibit the possibility of traveling back in time. According to that theory, it could be possible to create a gravitational field strong enough to bend spacetime back in on itself. This is referred to as a closed timelike curve, or CTC.

The problem is that most theorists, thinking in relative terms, have said that an object traversing a CTC would inherently create paradoxes. In 1991, though, theorist David Deutch theorized (as theorists do) that paradoxes could be avoided if thought of in terms of fuzzy probability rather than strict determinism. So the theory goes that if you send a fundamental particle back in time to flip a switch that will create itself, it has more or less equal probability that it will succeed or fail. The fact that it could succeed is enough to prevent its existence from becoming a paradox even if it fails.

Which brings us to University of Queensland physicist Tim Ralph and his Ph.D. student Martin Ringbauer, who wanted to devise an experiment to test Deutch's theory. Ralph and Ringbauer decided to study the interactions between pairs of polarized photons within a quantum system. Says Ralph, "Of course, we're not really sending anything back in time but [the simulation] allows us to study weird evolutions normally not allowed in quantum mechanics." The simulation, they suggest, is mathematically equivalent to having a proton travel along a CTC.

According to Ringbauer, "We encode their polarization so that the second one acts as kind of a past incarnation of the first.” Effectively, they have one photon go through a time-loop simulator and then see if it exactly resembles the state of the non-time-traveling photon at the moment prior to the simulation taking place. 

The result? Says Ralph, "The state we got at our output, the second photon at the simulated exit of the CTC, was the same as that of our input, the first encoded photon at the CTC entrance." The experiment was repeated multiple times with the same result.

Conclusion? It's possible that you can go back in time, irrevocably change the past, but still exist in your original state despite the change.

Of course, this doesn't touch on many other time-travel challenges, but that's conjecture for another time. Or in the comments, if you're feeling theoretically physical!

(via The Huffington Post)

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