Time travel
If you don't like the 21st century, you can always go back
By Douglas Bailey, Globe Staff
Space, Captain James T. Kirk used to say, is the final frontier. Clearly the captain - a decidedly 20th-century man - hadn't thought much about the frontier of time. (Curious, since, in several Star Trek episodes, and at least two of the movies, he traveled back in time.)
As we enter the 21st century, space travel has become passe. While we will doubtless reach new heights of space exploration in the next 100 years, will a manned mission to Mars be all that different from the manned mission to the moon? And the space travelers of the future will surely be like the space travelers of today: nameless, faceless, and lacking personality. Can anyone name one current astronaut?
Time is the next, and possibly final, frontier. And its barrier could be broken sooner than you think. At what better point to contemplate fishing upstream or downstream in the river of time than now, as we all prepare our voyage into the next century?
While time travel has been the stuff of science fiction for more than 100 years, it is only recently that physicists have begun seriously researching the topic. Most will tell you that they think time travel is impossible and that their research is intended only to expand our understanding of the nature of time, space, relativity, and such things as wormholes and negative mass, called antimatter.
Don't believe them. It's just that saying that you're researching time travel today could get you disinvited to the department's annual end-of-semester beer bash and buffet. So physicists tend to keep quiet about it. Nonetheless, a posse of well-regarded, highly intelligent, and highly government-grant-dependent scientists are furiously studying the topic, each aspiring to become the inventor of the first time machine.
A trip to the Internet proves this. A search for the words ''time'' and ''machine'' on a Stanford University Web site, where physicists from around the world share literature, turns up dozens of titles such as: ''Time Machines Constructed from Anti De-Sitter Space''; ''Three-Dimensional Billiards and Time Machines''; ''Quantum Field Theory and Time Machines''; and ''Problems in Time Machine Construction Due To Wormhole Evolution.''
Enter ''time'' and ''travel,'' and you get hundreds more.
That time travel is theoretically possible has already been proved by Albert Einstein. Just accelerate at light speed into the cosmos for a period, and when you return, your colleagues will have aged more than you. You will have, in a sense, traveled to their future.
But when we talk about traveling through time, we generally mean visiting the past or the future and then returning home to tell about it. Setting aside the practical problems of how to build a time machine and whether humans can physically withstand the trek across the time barrier, there are more troubling ones, such as the paradoxes that arise. The first one is called the ''free-lunch paradox.'' In this scenario, a person could travel back in time and meet, say, the young John Williams and show the composer all the scores he will write, including the soundtrack to Star Wars. Williams could then simply copy his songs, note by note, earning a free lunch, as well as a boatload of awards, without having to exert any artistic effort.
Another could be called the ''Menendez paradox,'' in which a homicidal time traveler goes back and tracks down his parents before they meet and kills them, thus preventing his own birth. But, of course, if he were never born, he couldn't travel back in time to commit the crime.
A third, similar to the parent paradox, asks: What would happen if you went back in time and shot yourself?
Amazingly, many scientists today feel that these paradoxes can be resolved without violating standard principles of physics and philosophy, though they might get you in trouble with the cops.
One theory simply states that because the laws of classical physics dictate that there can be only one history, something would have to prevent the time traveler from killing his parents. In fact, the act of lining up both his parents in the sights of his gun could cause them to meet, marry, and give birth to him.
Likewise, if you managed to find yourself in the past and fired a shot at yourself, you wouldn't be fatally injured. ''His wound might be of such a nature that he would not be able any more to shoot accurately enough, and that is why later he would only injure his younger self,'' writes S.V. Krasnikov, of the St. Petersburg Observatory, in a September 1998 paper on the paradoxes of time travel.
Though these notions seem to conflict with the concept of free will, an even stranger idea was put forth by Dr. Hugh Everett III, who in 1957 said that different worlds exist for any possibility that can occur. In each of these worlds, everything is exactly the same, except for consequences of the choices that develop. The worlds evolve independently; they cannot communicate with one another, and the people living in them have no idea that the others exist.
In this ''many worlds'' theory, there are no paradoxes. The person who wants to kill his parents would either fail in his attempt or land in the universe in which he succeeded and was never born. The same is true for the free-lunch paradox. In one universe, Williams dutifully wrote all his compositions. In another, his material and fame were handed to him, and he possessed no real talent. Can there be any doubt about which universe we're living in?
Another seemingly logical argument against time travel says that if it were possible, wouldn't we be surrounded by people visiting from our future? Again, scientists have an explanation grounded in physical law that doesn't rule out time travel one day, though it is discouraging for anyone wishing to revisit the 20th century. If this theory is correct, it shuts off travel to the distant past.
Time travel would be dependent, some scientists believe, on the construction of something called a closed timelike curves (CTC) , huge corridors of distorted space and time. Einstein hinted at such things, but the theory was expanded by physicist Kurt Godel in 1949.
Even Einstein was impressed. So was Stephen Hawking. While Hawking discounts the idea that one can travel into the past or future - and believes that even if you managed to build a CTC , you'd never be able to get near the thing without obliterating yourself - he had to agree that Godel's formulas don't rule out the possibility of time travel.
However, the caveat is that you would never be able to travel back beyond the point at which the CTC was built. So, unless we figure out how to build one in the next six months, we will never revisit the 20th century. And there are other problems. Moving just a microsecond into the past ''would require the presence of a space-warping mass equal to one-tenth the mass of the sun,'' write physicists Phillip F. Schewe and Ben Stein.
Other discussions about time travel ignore the likelihood that we will physically traverse the time barrier and instead focus on the possibility that we can build a machine that could send and receive messages across the time threshold.
London physicist Andrew Gray, in a 1998 paper titled ''A Design for a Quantum Time Machine,'' seems confident that such a machine can be built soon. ''Such a device would effectively enable one to see into the future, and thus be a kind of time machine,'' Gray wrote.
But he cautions that unless improvements can be made to the machine as presently designed, there will be no guarantee that the messages we receive from the future will be accurate.
Though some contend that the study of physics has peaked and that the necessary unity between quantum mechanics and general relativity that could produce time travel will never be found - or understood by anyone, if it is - that is a narrow view.
Even physicists who remain skeptical about the prospects of time travel concede that history has taught us that if something can be imagined, it can be created. As we enter the new millennium, a time machine is a less far-fetched idea than a flying machine was at the beginning of the last one.
At the twilight of the 19th century, renowned British scientist Lord Kelvin said he pitied future physicists - because everything was already known, and there was nothing else to learn. He also said that heavier-than-air flying machines were impossible.
Kelvin, of course, was proved colossally wrong, and he should be remembered whenever a modern physicist says that traveling through time will never happen. He or she could be living in the past.
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