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Time Travel: Possible or Not?

By Tom Nugent

There was a young lady named Bright,
Whose speed was faster than light;
She set out one day in a relative way,
And returned home the previous night.

—British scientist Arthur Buller, writing in Punch, 1923

It’s one of the most fascinating and controversial questions now being asked in the world of physics.

Will it someday be possible for human beings to travel back through time, or do the laws of physics actually operate to make such a journey impossible?

MIT physicist Eddie Farhi, an internationally renowned expert on general relativity, minces no words when faced with this question, which once existed only in the minds of humorists and science fiction writers like Kurt Vonnegut and Philip K. Dick.

“No way,” he replies.

“I’ve given a lot of thought to the problem of time travel in recent years,” adds Farhi, director of MIT’s Center for Theoretical Physics, “and everything I’ve ever looked at suggests that the laws of physics conspire to prevent you from going backward in time.

“If you think about it for awhile, certain logical paradoxes arise. For example, time travel would allow you to go back into the past and kill your parents before your own birth—which means you would never have been born.”

While many physicists share Farhi’s skepticism about time travel, several well-known investigators insist that future technological breakthroughs may indeed permit human beings to move back and forth through history. For these cosmic analysts, the idea of time travel—as described in H. G. Wells’ classic science-fiction novel of 1895, The Time Machine—seems at least theoretically possible, given the recent discovery that particles appear to move backward through time in the microscopic world of quantum physics.

Princeton physicist J. Richard Gott caused a stir, for example, by suggesting in the March 4, 1991, issue of the journal Physical Review Letters that time-travelers might be able to take advantage of the “warped spacetime” created by “two infinite parallel cosmic strings” in order to go backward through the dimension of time. According to Gott, travelers might be able to enter a different kind of spacetime by encircling the fast-moving strings and return to their own pasts.

Responding to Gott’s challenge in the same journal about a year later, Farhi and his MIT colleagues Sean Carroll and Alan Guth argued that building such a time machine was clearly impossible, given the apparent physical limitations of our universe.

In a response to the Princeton scientist, the MIT naysayers wrote, “We find that there is never enough mass in an open universe to build the time machine. . . . The Gott time machine cannot exist in any open . . . universe for which the total momentum is timelike.”    

Can Farhi translate that for us?

“As we worked on the equations,” he explains, “what we discovered was that you really could not construct such an object because the construction would require more than half the energy in the entire universe.

“Putting together that much energy to build your time machine would be rather daunting, to say the least,” Farhi adds.

According to the MIT expert, both the logical and physical obstacles to time travel are simply overwhelming. “I think the idea of going back through time to explore past worlds is an intriguing fantasy,” he says with a whimsical smile, “but the laws of physics clearly indicate that it will remain a fantasy, at least in the universe we now seem to inhabit.