Time travel is the concept of moving backwards and/or forwards to different points in time, in a manner analogous to moving through space. Additionally, some interpretations of time travel suggest the possibility of travel between parallel realities or universes. Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation in the theory of general relativity, it is currently unknown whether the laws of physics would allow backwards time travel. Any technological device, whether fictional or hypothetical, that is used to achieve two-way time travel is known as a time machine.
The dream of time traveling, to the past or future, is probably as old as the human imagination. When H.G. Wells published The Time Machine in 1895, he called it a "scientific romance" because no one knew whether time travel was possible.
A mere 10 years later, Albert Einstein would put forth his theory of special relativity, and part of the question would be answered--to the astonishment of many--in the affirmative.
One of Einstein's predictions, now verified by countless experiments, is best illustrated by the parable of the twins. One twin stays home while the other makes a round-trip voyage into outer space, traveling at nearly the speed of light for 10 years, as measured by the stay at home twin. When the traveled twin returns, she finds her sister has aged 10 years, while she has hardly aged at all. The traveled twin has jumped 10 years into the future.
This is the "time-dilation" effect of special relativity, and although it is most noticeable when extreme velocities are involved, it is happening around us all the time. As we move relative to each other we are--all of us--traveling into the future at different rates. The differences in these rates are very small, sure, but they are real. Time travel into the future is inescapable, a consequence of the structure of the universe.
Time traveling to the past, or returning back from a trip to the future, is a somewhat more challenging proposition. Until a few decades ago, the subject was consigned to science fiction. In fact, a query from a first-time science-fiction author provoked the beginnings of the first serious and sustained study.
In 1985, astronomer Carl Sagan was working on the manuscript for his novel Contact. The book's heroine required some means of rapid interstellar transit, and since Sagan wanted to get the physics right, he solicited advice from his friend Kip Thorne, a Caltech theoretical physicist. Thorne recommended the use of a "wormhole," a tunnel-like shortcut through space and time predicted by Einstein and well known among science fiction aficionados. Sagan dutifully incorporated the suggestion.
That same year, Thorne realized that if you treated the two mouths of a wormhole as you treated the twins--keeping one mouth fixed, moving the other at a velocity near the speed of light and then returning it to the vicinity of the fixed mouth--you could create a time machine. If the traveling mouth had been moving for 10 years as measured by the fixed mouth, then Thorne could jump into the traveling mouth and emerge from the fixed mouth 10 years into the past.
Physicists had been skittish on the subject of time travel, considering it science fiction. But Thorne's work was license to take it seriously, and suddenly there appeared a torrent of papers, many of which were published in the most prestigious journals. By the mid-1990s there were at least half a dozen ideas for other ways to twist and fold space-time like origami.
All this thinking was decidedly theoretical--no one was building a time machine in his basement. One reason was that in most cases, the plans required a kind of anti-gravity called negative energy to sustain the warping of space and time. Negative energy is difficult, if not impossible, to produce in the quantities necessary. Still, the idea of time travel was getting serious attention.
Naturally, not all that attention was enthusiastic. Stephen Hawking, for one, suspected that by some as-yet-undiscovered mechanism, nature prohibited traveling back in time. One sticking point was the "grandfather paradox": If I traveled back in time and killed my grandfather, I could not have been born. But if I have not been born, I cannot live to travel back and kill my grandfather.
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The Russian-born physicist Igor Novikov, an enthusiastic investigator into the subject of time travel, has suggested that the paradox doesn't apply because space-time is probably self-consistent. That is, I may be able to travel back in time and somehow become interwoven into a past of which I was already a part, but I will not be able to kill my grandfather, quite simply because I have not killed him already. |
Novikov has also thought a good deal about the other time travel conundrum--the "bootstrap paradox." Suppose I travel to 2009, find a design for a zero-emission automobile engine and return with it to 2008 and patent it. Suppose further that the patent is developed into the design that I find in 2009.
The obvious question: Who would have invented the zero-emission engine? The answer is, no one would have invented it. The design would have been generated quite literally from nothing, courtesy of a time machine and (perhaps) a skirting of some yet-to-be-written intellectual property laws.
British physicist David Deutsch, invoking the "many-universe" interpretation of quantum mechanics, believes that "pastward" time travel would require travel to another, parallel universe--one in which I could kill my grandfather and in which I (therefore) would never be born. Via a time machine, I would have removed myself from this universe to take up residence in that one.
The idea has some interesting implications. Deutsch has suggested that one reason we have detected no extraterrestrial civilizations may be that, using time machines, they have left this universe, preferring to live in another.
Metaphysical and philosophical questions aside, exactly how realistic is the physics of pastward time travel? Each of the several schemes for making a time machine creates a region in which pastward time travel is possible and separates it from a region in which time travel is impossible. The boundary between these regions, the "chronology horizon," has remained a mystery, in part because its nature depends upon the characteristics of space-time on the smallest possible scales.
We have at best a dim understanding of these scales, and we will not have a real understanding until we have developed a full theory of quantum gravity. This is the holy grail of theoretical physics: the so-called "theory of everything" that would eliminate disparities between relativity (which explains nature on very large scales, where gravity becomes important) and quantum mechanics (which explains nature on very small scales, where quantum effects become important).
Some physicists think the theory of everything is 10 years away; others suspect it is a good deal further off. For the moment, then, the question of whether time travel is possible has been put on hold.
The recent (and, no doubt, temporary) decline of interest in traveling to the past is welcomed by physicists who argue that work in less fanciful areas might yield a greater intellectual profit. New Zealand physicist Matt Visser, himself the architect of a number of theoretical time machines, calls that attitude overly cautious and "boring."
More than two decades after Thorne's seminal work, we still don't know whether time travel is possible. But one thing is certain: Even as an idea, it's anything but boring.
David Toomey is an assistant professor of English at the University of Massachusetts Amherst. His most recent book is The New Time Travelers: A Journey to the Frontiers of Physics .
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