Scientists: Time Travel Is Not Dangerous To Your Past

Like us on Facebook

Lloyd applied quantum mechanics to the problem. Quantum particles can become entangled, which means that if you look at one particle's quantum state, the entangled particle will have the same state. (This seems to violate the principle that nothing can travel faster than light, but you can't use this to send a message because you can never know what state you are going to see beforehand).

The team did an experiment in which they took a single photon and created an entangled state by sending it through two different paths. They then separated them.

They couldn't send a photon back into the past, but they could simulate some of the effects using a technique called post-selection. Post-selection is basically setting up the results you want beforehand - rather like when radio signals are filtered to get rid of the noise. If you remove noise and have a signal left over, you know you did it right.

They then did an experiment in which they looked at the quantum state of the entangled photon.

If the teleportation were successful, then the photon would have "killed itself at a prior time" -- giving a distinctive experimental signature. But that signature was never observed. The photon behaved in a way that wasn't paradoxical, in which the history of the photon was self-consistent.

>

This means that if you were to try and change the past, something would happen to make doing so impossible. If you try to shoot your grandfather, some weird random event will prevent it from happening - for instance, the bullet will miss or the gun would jam. From the point of view of the time traveler, the past always happens so that history is consistent with itself. You're not in any danger of erasing yourself as Marty McFly was.  

Not everyone was satisfied with this idea. David Deutsch, a physicist at the University of Oxford, said he isn't sure the simulation of a closed timelike curve is necessarily going to behave the way a real one would. He notes that the problem with time travel in quantum mechanics is that much of the theory depends on time being forward and relatively linear.

Deutsch himself has a theory of self-consistency, though he said he himself isn't completely happy with it. But one aspect of Deutsch's (and others') ideas is called the "relative state" or "many worlds" interpretation. It basically says that when a quantum state is measured, it takes on a single value for the observer, but all the other values are still equally real.

Experiments with interference patterns seem to show this possibility - famously, photons sent through a set of slits show diffraction patterns that seem to indicate that a single photon can interfere with itself. In that case, paradoxes are partially solved because if you change the past you simply move over to a universe in which the past was changed; your old one hums along as it did before.

But he says the work is still useful. "A lot of axioms in quantum mechanics depend on what happening before or after," he said. "There are formulations that are identical, but people disagree as to which makes sense."