What about those mini-black holes that may be produced by the LHC. How do they relate to the black holes that you study?
The black holes that we study as astrophysicists are huge, and they have enormous gravitational pulls. The mini-black holes that might be formed at the Large Hadron Collider are radically different from those huge astrophysical holes. It’s unfortunate that they have the same name because it’s like saying that a human being and an amoeba are the same entity, and giving them the same name because they are living things made out of matter. The black holes that one might see at the LHC are in some sense very distant cousins of huge astrophysical holes, with radically different properties. We will see mini-black holes only if our universe has higher dimensions, and then only if they form and evaporate through what’s called Hawking radiation [a kind of radiation that is hypothesized to escape right along the horizon of a black hole]. A large fraction of the evaporation products go off into the higher dimensions, and so here we are exploring a very different aspect of the warped side of the universe, the influence of higher dimensions. If evidence is seen in the LHC for these mini-black holes, it’ll be convincing evidence of the existence of higher dimensions.
If any civilization attempts to make a time machine, quantum effects will cause the machine to begin to self-destruct.
What are some of the big surprises from your lifetime of studying theoretical astrophysics?
Oh, I think the biggest surprise to me was the discovery of dark energy; that most of the mass in the universe is in the form of this dark energy that extends throughout the universe, and has an enormous tension like an exceedingly stiff rubber band. That I found incredible. I and most of my colleagues didn’t believe it until we saw several completely independent pieces of observational data saying that that’s the case.
From the 1960s through the 1980s, you collaborated quite a bit with Russian physicists. What was it like to work with them during the cold war?
I managed to do this in large part because a Russian astrophysicist named Yakov Zel’dovich took me under his wing. He and Andrei Sakharov had been the principal designers of the hydrogen bomb in Russia. John Wheeler, one of the designers of the American hydrogen bomb, was my Ph.D. thesis adviser, so I was personally close to the designers of both the Russian and U.S. hydrogen bombs. I moved freely back and forth between Russia and the United States as an intellectual gadfly, carrying astrophysics and relativity ideas back and forth and helping the two sets of scientists communicate with each other.
Were you watched and questioned by government agents?
I was pretty sure that the CIA or FBI was bugging my telephone occasionally here in the United States, but they never came to me directly. After I would leave the U.S.S.R., my Russian colleagues were typically debriefed by the KGB about what had happened during my visit. The monitoring was much more intense on the Soviet side. The KGB often tried to use Russian scientists as spies, and this was a painful issue that some of my Russian colleagues had to struggle with. The CIA never, ever tried to use me as a spy.
Science fiction fans love you because in the 1980s you suggested that time travel might be possible by passing through a thing called a wormhole. How would that work?
A wormhole is a hypothetical warp of space that can serve as a shortcut between two different regions of the universe. It’s sort of like if a worm drilled a hole through an apple from one side to the other. If you were an ant and you lived on the surface of the apple, there could be two routes to get from one side of the apple to the other. One is around the outside, on the surface, which we can think of as being like our universe’s gently warped space; the other is down the wormhole. In the case of our universe, the wormhole might be quite short and still reach from, say, our solar system to the center of our galaxy. General relativity says wormholes could exist. When we combine general relativity with quantum theory, we find moderately strong evidence that wormholes cannot exist after all—but we just don’t know for sure yet.
How did wormholes lead to your interest in time travel?
In Carl Sagan’s original version of his novel Contact, he had his heroine traveling through a black hole to a distant part of the universe, and he asked me for advice. I immediately told him, “You can’t do that. Black holes can’t be used in that way,” and I suggested he use a wormhole instead. That got me interested in the issue of whether or not there really could be wormholes that you could travel through, and quite quickly I came to realize that if they did exist, it would not be hard for a very advanced civilization to use a traversable wormhole to make a time machine. That forced me to face the issue of self-inconsistent histories: Could you go back and kill your father before you were conceived? And that question led me to realize that these kinds of thought experiments can be a very powerful way to probe the laws of physics. I had friends who worried about whether I’d gone off the deep end when they first heard about this, but most became enthusiastic after they learned the details.
