Kip S Thorne - Is Backward Time Travel Compatible with Quantum Physics?

The laws of classical physics, particularly Einstein’s general relativity, permit backward time travel. However, there is evidence but no firm proof, that quantum physics effects will cause any machine for backward time travel to self destruct at the moment it is first activated. The laws of quantum mechanics as usually formulated seem unable to deal with backward time travel. However, an alternative formulation of the quantum laws, due to Richard Feynman and extended by James Hartle and Murray Gell-Mann, appears to be fully compatible with time travel, but gives some strange predictions that I shall describe. What does all this mean for the nature of time in our universe?