“I have no idea which model is really right. Probably both of them are wrong,” says Seth Lloyd, a theorist at Massachusetts Institute of Technology. Of course, he adds, the other possibility is that Hawking is correct, “that CTCs simply don’t and cannot exist.”
Seth Loyd and many other theorists, scientists and quantum mechanics are postulating on what time travel would be like if it did exist.
Many hours and many dollars both private funded and tax payer funded are spent in this pursuit of knowledge.
The specific speculation is surrounding CTCs or Closed Timelike Curves
“The source of time travel speculation lies in the fact that our best physical theories seem to contain no prohibitions on traveling backward through time. The feat should be possible based on Einstein’s theory of general relativity, which describes gravity as the warping of spacetime by energy and matter. An extremely powerful gravitational field, such as that produced by a spinning black hole, could in principle profoundly warp the fabric of existence so that spacetime bends back on itself. This would create a “closed timelike curve,” or CTC, a loop that could be traversed to travel back in time.”
“Hawking and many other physicists find CTCs abhorrent, because any macroscopic object traveling through one would inevitably create paradoxes where cause and effect break down. In a model proposed by the theorist David Deutsch in 1991, however, the paradoxes created by CTCs could be avoided at the quantum scale because of the behavior of fundamental particles, which follow only the fuzzy rules of probability rather than strict determinism. “It’s intriguing that you’ve got general relativity predicting these paradoxes, but then you consider them in quantum mechanical terms and the paradoxes go away,” says University of Queensland physicist Tim Ralph. “It makes you wonder whether this is important in terms of formulating a theory that unifies general relativity with quantum mechanics.””
So on the one hand some scientists believe that time travel is not possible because the very essence of gravity bending would tear apart whatever goes through it.
On the other hand other scientists believe that Quantum Particles can get through the curves and establish effective travel.
But in using the quantum theory they are also using the “on / off” nature of quantum particles. Or in the case of humans they are applying a 50 / 50 ration on the probability scale.
“Instead of a human being traversing a CTC to kill her ancestor, imagine that a fundamental particle goes back in time to flip a switch on the particle-generating machine that created it. If the particle flips the switch, the machine emits a particle—the particle—back into the CTC; if the switch isn’t flipped, the machine emits nothing. In this scenario there is no a priori deterministic certainty to the particle’s emission, only a distribution of probabilities. Deutsch’s insight was to postulate self-consistency in the quantum realm, to insist that any particle entering one end of a CTC must emerge at the other end with identical properties. Therefore, a particle emitted by the machine with a probability of one half would enter the CTC and come out the other end to flip the switch with a probability of one half, imbuing itself at birth with a probability of one half of going back to flip the switch. If the particle were a person, she would be born with a one-half probability of killing her grandfather, giving her grandfather a one-half probability of escaping death at her hands—good enough in probabilistic terms to close the causative loop and escape the paradox. Strange though it may be, this solution is in keeping with the known laws of quantum mechanics.”
I say, they are doing nothing more then covering ground that sci/fi writers have been writing about for over a 100 years.
Marty McFly discussed the grand father paradox well before Hawkins did and Stargate discussed the multiple universe theory well before simulations began in the laboratory.
“In 2009 Seth Lloyd, a theorist at Massachusetts Institute of Technology, proposed an alternative, less radical model of CTCs that resolves the grandfather paradox using quantum teleportation and a technique called post-selection, rather than Deutsch’s quantum self-consistency. With Canadian collaborators, Lloyd went on to perform successful laboratory simulations of his model in 2011. “Deutsch’s theory has a weird effect of destroying correlations,” Lloyd says. “That is, a time traveler who emerges from a Deutschian CTC enters a universe that has nothing to do with the one she exited in the future. By contrast, post-selected CTCs preserve correlations, so that the time traveler returns to the same universe that she remembers in the past.”
What is your thought on time travel?[SOURCE](http://www.scientificamerican.com/article/time-travel-simulation-resolves-grandfather-paradox/)