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The current state of the art belief in physics is that it might be possible to create a wormhole and use it later to travel back to the original time of creation of the wormhole allowing time travel. Hawking doesn't think it is possible, because he believes that infinite energy is required to properly place the wormhole. Kip Thorne believes he has a proof that the energy is not infinite, but Hawking found mistake but he has no proof that the energy is infinite. Hawking will not take a bet on whether or not time travel is possible (Hawking and Thorne have engaged in a well known series of bets on questions in physics). This is all according to current theories of physics which are known to be incomplete.