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Dreamworks’ remake of The Time Machine is the latest expression of our fascination with time travel.

Haven’t you noticed the irony of asking the question “when?” about time travel in your March feature? Can’t we assume that if it was ever going to happen we would have already heard?

Peter Rosenl
Ithaca, N.Y.

Time travel, as described by most science fiction writers, would allow a person to journey anywhere into the past or the future. According to the fantasy version of time travel, if we created a time machine tomorrow, we would have the power to travel back to the time of the dinosaurs or explore what the distant future holds for humankind. Similarly, some fiction writers claim, if an advanced civilization designed a time machine many millennia from now, they would be able to venture back to our time and share with us the knowledge that took them thousands of years to develop. Yet no members of this future civilization have arrived, leading experts on the subject as notable as the physicist Steven Hawking to wonder “If time travel is possible, then where are all the time travelers?”

The answer lies in the difference between science fact and science fiction. Work begun in the 1980s by physicist Kip Thorne has shown that it is theoretically possible to travel backward in time. While the technology needed to construct an actual time machine is many centuries away (it would require at the very least manipulating the fabric of space-time, interwoven space and time as imagined by Einstein, as if it were Play-Doh), nothing in the known laws of physics prevents it.

But just because the laws of physics may allow us to travel backward in time, that doesn’t mean we will be able to travel as far back as we please. Every physically plausible time machine ever conceived has a limit: No traveler would be able to journey back to a time before the machine was created. If we were to build a time machine tomorrow, travelers in the year 2050 could travel back to tomorrow but no further.

This limitation is an unavoidable byproduct of the nature of physically possible time machines. All possible time machines work by desynchronizing two areas of space-time-that is, slowing one area of space-time down relative to another (for more detail, see “The Physics of Time Travel,” March). If we begin this process of desynchronization tomorrow, and continue it for 100 years, then one area of space-time will age only, say, five years, while everything else will age 100 years. Stepping into the younger region many years from now will carry you back 95 years in time, but no further, and certainly never before you began to build the machine.

Edited by Bob Sillery
Research by Brad Dunn and Michael Moyer