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.
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.
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