Carl Sagan, the astronomer, Pulitzer Prize-winning author, and legendary popularizer of science, gave this interview in the mid-1990s during the making of NOVA's program "Time Travel." True to form, he discusses arcane aspects of the field—from how you define time to what it might look like inside a wormhole—with flair and a refreshing dash of humor.
If we could travel into the past, it's mind-boggling what would be possible. For one thing, history would become an experimental science, which it certainly isn't today. The possible insights into our own past and nature and origins would be dazzling. For another, we would be facing the deep paradoxes of interfering with the scheme of causality that has led to our own time and ourselves. I have no idea whether it's possible, but it's certainly worth exploring.
Among the claimed solutions are that you can't murder your grandfather. You shoot him, but at the critical moment he bends over to tie his shoelace, or the gun jams, or somehow nature contrives to prevent the act that interrupts the causality scheme leading to your own existence.
One of Hawking's arguments in the conjecture is that we are not awash in thousands of time travelers from the future, and therefore time travel is impossible. This argument I find very dubious, and it reminds me very much of the argument that there cannot be intelligences elsewhere in space, because otherwise the Earth would be awash in aliens. I can think half a dozen ways in which we could not be awash in time travelers, and still time travel is possible.
Then there's the possibility that they're here alright, but we don't see them. They have perfect invisibility cloaks or something. If they have such highly developed technology, then why not? Then there's the possibility that they're here and we do see them, but we call them something else—UFOs or ghosts or hobgoblins or fairies or something like that. Finally, there's the possibility that time travel is perfectly possible, but it requires a great advance in our technology, and human civilization will destroy itself before time travelers invent it.
I'm sure there are other possibilities as well, but if you just think of that range of possibilities, I don't think the fact that we're not obviously being visited by time travelers shows that time travel is impossible.
The reason this is connected with time travel is because another consequence of special relativity is that time, as measured by the speeding space traveler, slows down compared to time as measured by a friend left home on Earth. This is sometimes described as the "twin paradox": two identical twins, one of whom goes off on a voyage close to the speed of light, and the other one stays home. When the space-traveling twin returns home, he or she has aged only a little, while the twin who has remained at home has aged at the regular pace. So we have two identical twins who may be decades apart in age. Or maybe the traveling twin returns in the far future, if you go close enough to the speed of light, and everybody he knows, everybody he ever heard of has died, and it's a very different civilization.
It's an intriguing idea, and it underscores the fact that time travel into the indefinite future is consistent with the laws of nature. It's only travel backwards in time that is the source of the debate and the tingling sensations that physicists and science-fiction readers delight in.
It's very different from saying that we ourselves could construct such a wormhole. One of the basic ideas of how to do it is that there are fantastically minute wormholes that are forming and decaying all the time at the quantum level, and the idea is to grab one of those and keep it permanently open. Our high-energy particle accelerators don't have enough energy to even detect the phenomenon at that scale, much less do anything like holding a wormhole open. But it did seem in principle possible, so I reconfigured the book so that Eleanor Arroway successfully makes it through the center of the galaxy via a wormhole.