So we will have to wait until we ourselves become such a civilization?

Probably, although some of the physicists interested in the subject, as well as numerous science fiction writers before them, have suggested the possibility of “natural” wormhole time machines, or wormhole time machines created by extraterrestrial civilizations. It is possible that somewhere in the universe, they exist today.

This sounds like fairly wild science fiction. Why are physicists interested at all?

Imagining exactly how a time machine would operate is a “thought experiment,” and thought experiments--the most famous of which may be Einstein’s imagining what it would be like to ride a wave of light--lead to fascinating places.

Such as?

The inquiry into the viability of time machines has offered a deeper understanding of quantum mechanics, and has raised questions that are helping to discern the outlines of a theory of quantum gravity—the “theory of everything” that promises the unification of all physical laws. It also leads into more philosophical areas—and some interesting paradoxes.

Paradoxes?

In the view of many, pastward time travel would create paradoxes that would invert cause and effect. They come in two classes.

One is the “grandfather paradox.” If I travel back in time before my birth and kill my grandfather, I cannot have been born. If I was never born, then I cannot have traveled back in time to kill my grandfather. But I had to kill my grandfather in order to bring about the circumstances in which I was never born. And so forth.

The other is the “bootstrap paradox.” Suppose I find a copy of Hamlet in a used bookstore, travel via time machine to London in 1601, and give it to Shakespeare. He copies it out in his own handwriting, and presents it to his acting company as his own. It is subsequently recopied, printed and reprinted, and after many generations and many editions, I find the copy that I give to Shakespeare. The question then becomes, who wrote the play? Shakespeare did not. He copied it from the book I gave him. But I did not, either. I found it in a bookstore. Hamlet then, would seem to have been created from nothing.

These are plot points in a lot of science fiction books and films. Why do physicists care about them?

The idea that cause precedes effect is so fundamental to Newtonian mechanics, special relativity and quantum field theory that in those subject areas, any hypothesis that purported that effect might precede cause would be rejected out of hand. That time machines seem to allow such inversions is deeply troubling to many physicists. In fact, Stephen Hawking has proposed a “chronology protection conjecture,” his own informed suspicion that by one or more mechanisms, the laws of physics prohibit the creation of time machines.

So what your book says is that almost in spite of the science fiction, physicists are serious in thinking about pastward time travel?

Yes. But that doesn’t mean it isn’t fun, too. Perhaps it’s best called “serious fun.”

  

How did you decide to write about physicists and time travel?

It came as a surprise to me when, a few years ago, I read that physicists had been thinking seriously about pastward time travel and exactly how it might be accomplished. The scenarios imagined were, to be sure, theoretical. All involved unrealistically large amounts of energy and warping space. Nonetheless, they violated no known laws of physics. So far as anyone knew, they were possible.

Why only “pastward”? Weren’t they interested in time travel to the future?

Futureward time travel happens all around us, all the time. An extreme case is illustrated by the parable of the twins. One twin stays at home, while her sister takes a round-trip voyage far out into space, at nearly the speed of light, for a little over ten years as measured by her sister on Earth. Upon her return, the “traveled” twin will find her sister, and everything on Earth, ten years older, while she has barely aged at all. As measured by her own clock, she has traveled into the future. Although the effect is considerably less for those of us moving at more modest speeds, it is an inevitable consequence of Einstein’s theory of special relativity. It is both real and inescapable.

Could you describe how pastward time travel might work in, say, 200 words?

I’ll try. There are several schemes. The first was described in a 1988 paper by Caltech theoretical physicist Kip Thorne and his (then) graduate student Mike Morris. It was an ingenious mix of general relativity and special relativity. First, as allowed by general relativity, one would bend space so as to create a wormhole--that is, a connection between two places that in three-dimensional space are widely separated. Then, as allowed by special relativity, one would treat the wormhole mouths like the twins of the parable described above, moving one mouth at near light velocity for a little over ten years as measured by a clock in the vicinity of the ‘untraveled’ wormhole. A person would then be able to enter the ‘traveled’ mouth and emerge from the untraveled mouth ten years in the past. The wormhole would operate as a time machine. (That was 145 words, not counting “I’ll try.”)

Your book claims to describe an entire field of inquiry. Are there other types of time machines?

Yes. In 1992 J. Richard Gott at Princeton described a time machine that would use hypothetical entities known as “cosmic strings”--incredibly dense filaments of energy left over from the first moments in the life of the universe. Two such strings, moving past each other, would create a fault line in space, which Gott thinks might be exploited for travel pastward. The others also take advantage of effects of bending or warping space. For instance, although no configuration of matter and energy can move faster than light, a bubble of space could. In theory, you could catch a ride inside such a bubble, and return to your starting point before you left. Or, one might shrink space in a region along a trajectory, making a “tube” of shrunken space that would allow faster travel in one direction between the points it connected than would be possible otherwise. In principle, two such tubes, laid head to toe, might be used as a time machine. In all, there are at least half a dozen ideas for time machines, not counting variations.

Could we build one?

If “we” means humans at present, no. All would require the control of vast amounts of energy – the output of something like two thirds of the stars in the Milky Way Galaxy. When the physicists describing them need to invoke a builder, they appeal to a hypothetical “suitably advanced civilization.”