Originally published 22 June 1992
A lot of very clever people have been thinking about time machines lately.
Make that mega-clever: Steven Hawking, who just may be the cleverest physicist in the world. And MIT’s Alan Guth, who thought up the inflationary universe. And Jerome Friedman, Kip Thorne, Yakir Aharonov, and J. Richard Gott. These guys know more about time warps, black holes, cosmic strings, and parallel universes than anyone in the world. And they are thinking about time machines.
Mind you, they have not yet abandoned their offices and classrooms for the workshop in the basement. They are not cobbling together outlandish contraptions like the jukebox chair with the whirling disk in the film version of H. G. Wells’s Time Machine. They are not souping up a DeLorean with plutonium and a flux-capacitor for a trip “back to the future.”
No, they are scribbling on yellow pads, as usual, trying to decide if the laws of nature allow travel into the past, and if so, how such a journey might be contrived.
They’ve come up with some wild ideas, all of which follow from Einstein’s theory of general relativity, which describes how space and time can be distorted by gravity, or from the strange multiplicity of universes that seem to be allowed by quantum physics. What these clever fellows are looking for is what’s called a “closed timelike curve,” or CTC, a path through the space-time continuum that loops into the past.
Such loops appear to be possible. It is not yet clear whether travel along a CTC is practical.
Time-travel theoreticians have explored the possibility that CTCs might exist near massive, rotating black holes. They have looked at wormholes — short connections in the space-time continuum that link distant regions — and wondered if wormholes might link different times as well. They have suggested that CTCs might be made from lengths of cosmic string, long threads of extraordinarily dense matter that may have been created early in the history of the universe.
Concocting a path into the past might be dangerous. Let’s say some practical-minded engineer of the future figures out a way to snare a couple of very long cosmic strings, and sets them moving in opposite directions at high velocity. According to calculations, this should produce a CTC. He buckles on his crash helmet and goggles and waits to be swept into…
Stand back! Suddenly, black holes bubble up along the cosmic strings, sucking the hapless engineer (and his laboratory, too) into oblivion. Or somewhere.
And what about the self-murder paradox? A time traveler goes along a CTC and arrives in the past. She murders her younger self, making her own future existence impossible. It would seem that time travel is logically impossible.
But what about parallel universes? Let’s suppose a time traveler follows a CTC back in time, but emerges in one of those parallel universes allowed by quantum physics. She murders her younger self in that parallel universe, but her action has no effect in the universe she came from.
Time travel through parallel universes is chancy, since the laws of quantum physics are probabilistic. Even if our time traveler was willing to set out for an unknown, random destination — one of an infinite number of parallel universes — how will she ever get back?
Then, there’s the “why aren’t they here?” argument. If time travel is possible, then at some future time people will certainly figure out how to do it. In which case they should have appeared here now, in our present, their past. As far as we know, no one has announced themselves as a visitor from the future.
Oh, dear. This business of CTCs is rather loopy. But don’t get the idea that the theoretical physicists who consider CTCs are loopy too. They know exactly what they’re doing. They don’t have their eyes on a patent for a time machine (“…the 1999 Turbo-CTCx…”). Rather, they are exploring current theories of physics for internal contradictions.
If it were possible to send even a single electron back into the past, the electron could influence a future that has already occurred. This is the sort of logical contradiction that would force us to rethink our entire notions of space and time. Some physicists would say that if our present theories allow travel into the past, then the theories must be wrong.
But, who knows, maybe what seems to be a logical impossibility is possible. Richard Feynman, the late, great theoretical physicist and wit, asked us to imagine a time traveler who goes into the past to kill her younger self. She aims for the heart, but because her aim is bad, merely wounds her younger self in the shoulder. The reason for her poor aim, of course, is the impaired shoulder she suffers because of that same misdirected shot.
