“Is there any point in hosting a party for time travelers? Would you hope anyone would turn up?
Hawking’s answer: In 2009 I held a party for time travelers in my college, Gonville and Caius in Cambridge, for a film about time travel. To ensure that only genuine time travelers came, I didn’t send out the invitation until after the party. On the day of the party, I sat in college, hoping but no one came. I was disappointed, but not surprised, because I had shown that if general relativity is correct and energy density is positive, time travel is not possible. I would have been delighted if one of my assumptions had turned out to be wrong.”
In “Brief Answers to the Big Questions – The Final Book” by Stephen Hawking.
I'm not really asking a question - a lot of what Hawking talks about really isn't even theoretically testable. Theoretical physics does tend in that direction - often it talks about ideas that are not testable yet, and may not be for a long time, or which are mathematical speculation as much as observation.
If you're into stuff like this, you can read the full review.