BEN BOVA: Dec. 12, 2010 ... Time travel.

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Ever since H.G. Wells published The Time Machine (in 1895), people have been fascinated by the concept that one might be able to travel into the distant past, or the far future.

Sheer science fiction? Not really. Physicists have seriously considered the possibility of time travel.Actually, we are all time travelers, moving relentlessly into the future at the pace of one second per second. But might we be able to jump hundreds of years ahead? Thousands? Or go back into the past?

Most people scoff at the very thought of such time travel. If anyone had built a time machine, we would have time travelers among us today, they claim. It doesn’t matter when in the future (or even the past) the machine was invented; time travelers would be here with us now just as surely as tourists from overseas visit our shores.

The fact that nobody’s seen a visitor from the future (or past) shows that time travel doesn’t exist, say the doubters.

Well, how would you tell a time traveler if you met one? Would he or she be likely to say, “Hi, I’m from the thirty-third century. Nice little era you have here.” Not likely. I suppose that time travelers would try to blend in to the era they’re visiting and not call attention to themselves.

Unless they had a specific agenda.

The fact that we haven’t bumped into any time travelers isn’t concrete evidence that time travel is impossible.

Ahah! Say the skeptics. But time travel creates all sorts of paradoxes. The most famous example is known as the grandfather paradox. You travel back in time and murder your grandfather while he’s still a lad. Which means he didn’t live long enough to sire your father. Which means you were never born. Which means you couldn’t have gone back in time to bump off your grandfather in the first place. QED.

Maybe. Maybe not.

Physicists who have seriously considered the possibilities of time travel point out that there is nothing known in modern physics that says time travel is impossible. Einstein’s relativity theory allows the possibility, in fact.

Einstein’s work shows that time is a dimension, like the three dimensions of space. We have length, breadth, height — and time. Physicists talk of spacetime: the four dimensions together.

Wells used this point in his story, and I wrote a short story, titled “Inspiration,” in which Wells’ tale inspires a teenaged Einstein to start thinking along those lines and eventually come up with relativity.

But — and it’s a big but — to move into the future or the past, you need to warp spacetime, and that takes some doing. You might have to get very close to a black hole, the remains of a massive star that’s collapsed down into a core that’s so dense that its gravity prevents even light from escaping it clutches, and does indeed warp spacetime.

Black holes, then, might be natural time machines.

Back to the grandfather paradox.

For the better part of a century science fiction writers have toyed with the idea of parallel universes, sometimes called alternate universes. These are universes that exist in addition to the one we happen to be in.

More recently, theoretical physicists have looked into this possibility of branching universes. Suppose you have a choice of two shirts, one red, the other blue. The universe splits, or branches, when you decide which one you will wear. In one universe, you wear the red shirt. But there’s an alternate universe in which you wear the blue shirt.

The universe splits at each decision point. Thus there could be a universe in which you do indeed murder your young grandfather, and you disappear from existence in that universe. But there’s a parallel universe in which you don’t bump off your ancestor, and you continue to exist.

The two universes start out very similar, except for that one decision, but branch off in different directions.

Thus time travel could exist, and the universe branches apart every time a person makes a decision. There could be a myriad of parallel universes, and we’d never know it.

Sheer science fiction? Maybe not.

In this universe Ben Bova is the author of nearly 125 books, including “The Return,” his latest futuristic novel. Dr. Bova’s website address is www.benbova.com.

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