|There was a conference for time travelers at M.I.T. earlier this spring.
Remembrance of Things Future: The Mystery of Time
By Dennis Overbye
I'm still hoping to attend, and although the odds are slim, they are apparently not zero despite the efforts and hopes of deterministically minded physicists who would like to eliminate the possibility of your creating a paradox by going back in time and killing your grandfather.
"No law of physics that we know of prohibits time travel," said Dr. J. Richard Gott, a Princeton astrophysicist.
Dr. Gott, author of the 2001 book "Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time," is one of a small breed of physicists who spend part of their time (and their research grants) thinking about wormholes in space, warp drives and other cosmic constructions, that "absurdly advanced civilizations" might use to travel through time.
It's not that physicists expect to be able to go back and attend Woodstock, drop by the Bern patent office to take Einstein to lunch, see the dinosaurs or investigate John F. Kennedy's assassination.
In fact, they're pretty sure those are absurd dreams and are all bemused by the fact that they can't say why. They hope such extreme theorizing could reveal new features, gaps or perhaps paradoxes or contradictions in the foundations of Physics As We Know It and point the way to new ideas.
"Traversable wormholes are primarily useful as a 'gedanken experiment' to explore the limitations of general relativity," said Dr. Francisco Lobo of the University of Lisbon.
If general relativity, Einstein's theory of gravity and space-time, allows for the ability to go back in time and kill your grandfather, asks Dr. David Z. Albert, a physicist and philosopher at Columbia University, "how can it be a logically consistent theory?"
In his recent book "The Universe in a Nutshell," Dr. Stephen W. Hawking wrote, "Even if it turns out that time travel is impossible, it is important that we understand why it is impossible."
When it comes to the nature of time, physicists are pretty much at as much of a loss as the rest of us who seem hopelessly swept along in its current. The mystery of time is connected with some of the thorniest questions in physics, as well as in philosophy, like why we remember the past but not the future, how causality works, why you can't stir cream out of your coffee or put perfume back in a bottle.
But some theorists think that has to change.
Just as Einstein needed to come up with a new concept of time in order to invent relativity 100 years ago this year, so physicists say that a new insight into time - or beyond it - may be required to crack profound problems like how the universe began, what happens at the center of black hole or how to marry relativity and quantum theory into a unified theory of nature.
Space and time, some quantum gravity theorists say, are most likely a sort of illusion - or less sensationally, an "approximation" - doomed to be replaced by some more fundamental idea. If only they could think of what that idea is.
"By convention there is space, by convention time," Dr. David J. Gross, director of the Kavli Institute for Theoretical Physics and a winner of last year's Nobel Prize, said recently, paraphrasing the Greek philosopher Democritus, "in reality there is. ... ?" his voice trailing off.
The issues raised by time travel are connected to these questions, Dr. Lawrence Krauss, a physicist at Case Western Reserve University in Cleveland and author of the book "The Physics of Star Trek," said. "The minute you have time travel you have paradoxes," Dr. Krauss said, explaining that if you can go backward in time you confront fundamental issues like cause and effect or the meaning of your own identity if there can be two of you at once. A refined theory of time would have to explain "how a sensible world could result from something so nonsensical."
"That's why time travel is philosophically important and has captivated the public, who care about these paradoxes," he said.
At stake, said Dr. Albert, the philosopher and author of his own time book, "Time and Chance," is "what kind of view science presents us of the world."
"Physics gets time wrong, and time is the most familiar thing there is," Dr. Albert said.
We all feel time passing in our bones, but ever since Galileo and Newton in the 17th century began using time as a coordinate to help chart the motion of cannonballs, time - for physicists - has simply been an "addendum in the address of an event," Dr. Albert said.
"There is a feeling in philosophy," he said, "that this picture leaves no room for locutions about flow and the passage of time we experience."
Then there is what physicists call "the arrow of time" problem. The fundamental laws of physics don't care what direction time goes, he pointed out. Run a movie of billiard balls colliding or planets swirling around in their orbits in reverse and nothing will look weird, but if you run a movie of a baseball game in reverse people will laugh.
Einstein once termed the distinction between past, present and future "a stubborn illusion," but as Dr. Albert said, "It's hard to imagine something more basic than the distinction between the future and the past."
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