Could it really be possible to travel backward in time?
It’s quite unlikely that one can go backward in time—although it is certainly not ruled out—and it may be that nature has mechanisms to prevent backward time travel. When I was studying this, I came away convinced that the laws of physics can be readily adapted to backward time travel without any serious loss of ability to predict and without self-inconsistencies. I think more interesting was the discovery I made with a postdoc, Sung-Won Kim from Korea, that there is a universal mechanism that always occurs: If any highly advanced civilization attempts to make a time machine for backward time travel, quantum effects will cause the time machine to begin to self-destruct explosively at the moment you activate it. We don’t know whether the explosion is strong enough to always destroy the time machine. We will have to have in our hands the full quantum theory of gravity [a combination of general relativity and quantum mechanics, yet to be understood] to find out the answer.
This kind of research certainly hasn’t hurt your career. Carl Sagan, on the other hand, did have to deal with a backlash because he was writing fiction and thinking about extraterrestrial civilizations. You were friends with him. Did this backlash really damage his career?
He had some backlash to deal with, but I don’t think it hurt his career among his immediate colleagues. He was elected to the National Academy of Sciences and then his election got quashed in the final meeting, when it should have gone through. It was quashed by people who were not close to his field. I think that was typically true, that his immediate colleagues—those who had worked with him, those who knew his research from having read his papers—they had high respect for his scientific work. It was people who had little personal knowledge of his research, who were the cause of whatever problems he had.
There is a rumor that you are working on a sci-fi project with Steven Spielberg. True?
I’m working on a science fiction film with Steven that’s based on a treatment I coauthored with the producer Lynda Obst. I will be an executive producer on the film, basically focused on bringing good science into it. I expect that nothing in the film will violate fundamental physical law, and all the wild speculations in the film will spring from science. The working title is Interstellar, but it’s unlikely that will be the final title. It is a story in which the warped side of the universe plays a major role.
Can you describe some of the bets you’ve had with Stephen Hawking—and who won?
Our first bet was about Cygnus X-1, the first strong candidate for a black hole that anyone had found. Is it really a black hole? Hawking characterized that bet as his insurance policy because he had so much invested in it turning out to be a black hole, so he bet against his hopes. He figured if it turned out not to be a black hole, he at least would get something out of the disappointment. The bet was very nonpolitically correct: He gave me a subscription to Penthouse magazine when I won. We also had another bet: John Preskill and I on one side—Preskill’s a physicist at Caltech—and Hawking on the other. The bet was over whether the laws of nature permit an implosion to produce a naked singularity—a singularity that is not inside a black hole. We bet that it could, and Hawking bet it couldn’t. He had to concede when a naked singularity was actually created in a finely tuned implosion, simulated on a computer. Now we have a new bet over whether a naked singularity could occur naturally in the universe.
What did you win on that second bet?
The loser had to give the winner an item of clothing to hide the winner’s nakedness. Hawking conceded in a public lecture at Caltech, and he had his assistant present to us T-shirts that had a picture of a woman hiding her nakedness with a towel. On the towel was written “Nature Abhors Naked Singularities.”
You also placed a wager on one of the strangest ideas about black holes: Not only do they swallow matter and light, they even obliterate any clues or information about the event. What was the argument in this case?
If you have something that implodes to make a black hole, which then completely evaporates due to Hawking radiation, does all the information that went into the black hole come back out? The fundamental principles of quantum theory say yes, and Preskill took their side. General relativity seems to say no, and that’s the side that Stephen and I took. About three years ago, Stephen found a new way to analyze the evaporation process, a way that convinced him that Preskill was right and that the information could be recovered, in principle. Hawking conceded in a big ceremony at an international meeting in Dublin where I was the chair. But I haven’t conceded yet.
And that was for encyclopedias?
That’s right. The loser was to provide the winner with an encyclopedia of information. So Stephen gave John, who has a fabulous collection of these baseball cards, an encyclopedia of American baseball.
It sounds like Hawking hasn’t done very well in his bets.
He hasn’t won any of these bets yet. I think that characterizes the fact that he’s ready to go out on a limb and challenge people, as a way of trying to foster the forward movement of science.
Are you still in contact professionally with Hawking?
He and I have never written a paper together. His current focus is the birth of the universe. Mine is probing its warped side. I will be going to Cambridge soon and spend a day with him, and we’ll be talking about physics and about life. He’s just finished writing a book for children called George’s Secret Key to the Universe. I’m eager to read it. It should contain gems of wisdom, not just for children but for adults and probably also for physicists like me.
