Time Travel Theory Avoids Grandfather Paradox

From Physorg.com:

The possibility of going back in time only to kill your ancestors and prevent your own birth has posed a serious problem for potential time travelers, not even considering the technical details of building a time machine. But a new theory proposed by physicists at MIT suggests that this grandfather paradox could be avoided by using quantum teleportation and “post-selecting” what a time traveler could and could not do. So while murdering one’s relatives is unfortunately possible in the present time, such actions would be strictly forbidden if you were to try them during a trip to the past.

The model of time travel proposed by Seth Lloyd, et al., in a recent paper at arXiv.org arises from their investigation of the quantum mechanics of closed timelike curves (CTCs) and search for a theory of gravity. In simple terms, a CTC is a path of spacetime that returns to its starting point. The existence of CTCs is allowed by Einstein’s general relativity, although it was Gödel who first discovered them. As with other implications of his theories, Einstein was a bit disturbed by CTCs.

In the new paper, the scientists explore a particular version of CTCs based on combining quantum teleportation with post-selection, resulting in a theory of post-selected CTCs (P-CTCs). In quantum teleportation, quantum states are entangled so that one state can be transmitted to the other in a different location. The scientists then applied the concept of post-selection, which is the ability to make a computation automatically accept only certain results and disregard others. In this way, post-selection could ensure that only a certain type of state can be teleported. The states that “qualify” to be teleported are those that have been post-selected to be self-consistent prior to being teleported. Only after it has been identified and approved can the state be teleported, so that, in effect, the state is traveling back in time. Under these conditions, time travel could only occur in a self-consistent, non-paradoxical way…

[continues at Physorg.com]

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  • tonyviner

    Good thing theses scientists aren't working to fix real problems.

    • gil

      Little do you know how you're going to die from an unfortunate and, for the time, unexplained accident which will be, in fact, caused by your great-great-grandson to be, who will be so appalled with the lives of all the people who were supposed to come after you (these will be all thoroughly documented on Facebook) that he'll work his whole life creating a time-machine just to correct that with no regard for his own, precious little life. And little do you know this will cause a reset in the quantum-timeline which will, in effect, create a space-time continuum discrepancy which will place the whole Universe in a time loop, condemning us and all those who will live from the time of your death until the moment when he successfully comes back in time, to live in the same time period for all eternity.
      If you don't think your untimely death is a real problem, that's fine; but have some respect for the rest of us, okay?

      • tonyviner

        Try as I might, I just can't help but to take you seriously.

        • gil

          Seriously?! :P

          Well, everything I say is only half kidding, so… who knows. Maybe that actually happened to your great-grandfather and we've already been in this time loop for the equivalent of millions of years. Or maybe the whole thing is just a time loop anyhow. :P

  • gil

    Seriously?! :P

    Well, everything I say is only half kidding, so… who knows. Maybe that actually happened to your great-grandfather and we’ve already been in this time loop for the equivalent of millions of years. Or maybe the whole thing is just a time loop anyhow. :P