A Rip in Time

By Stephen Reucroft and John Swain,
Globe Correspondents, 1/3/2000

Before Albert Einstein, time was simple and obvious. It was nature's way of not having everything happen all at once. Relentless and unchangeable, it flowed uniformly, dragging us along with it as helpless specks of flotsam and jetsam.

But in the early part of the 20th century, Einstein's two theories of relativity changed forever what we think we know about time and gave birth to a field of research that continues heatedly today. Perhaps at no other point in history have so many been fixated on the nature of time, especially as we step lightly into the new century.

More scientists than ever now believe that traveling back or forward in time would not necessarily violate the laws of physics and that even Einstein's theories support the notion. But how would it happen? Could scientists really build a time machine?

One clue currently comes not from science but from science fiction. The bestselling novel, ''Timeline,'' by Michael Crichton, has people zapping through time and parallel universes in an adventure that connects Medieval France to present day. Crichton himself writes that his book is ''firmly in the realm of fantasy.'' But as with his other prescient novels ''The Andromeda Strain,'' ''Airframe,'' and most notably ''Jurassic Park,'' readers are left with the impression that there's a solid scientific foundation supporting the fiction.

And there is. Though there would have to be major technological breakthroughs in this new century to make it work, it could work.

Space-time
Actually traveling back in time would seem to defy both logic and scientific rules. But with his theories of special relativity, Einstein showed us that time is neither constant nor inflexible. It becomes relative, so that the time between two events can be different for observers at rest and ones in motion and it becomes mixed up with space and fused into one continuous object we call ''space-time.'' Even weirder, in general relativity, the presence of matter can cause space-time to bend and twist, leading to the phenomenon we all know as gravity.

The Earth itself distorts space-time, and in fact, time passes a little more slowly at ground level than it does farther away from the gravitational pull of the earth. By choosing to live close to sea level you are, in effect, choosing to slow your motion through time. Not much, but it is a definite effect. Classical physics, therefore, lets us drag our heels a bit, but it doesn't seem to let us go backward.

But quantum mechanics, the rules that determine behavior in the microworld, may. When objects get really small, and the energy gets large, strange things happen.

Amazingly, quantum particles routinely travel backward in time. Antimatter, the exact counterpoint of matter which is known to exist in very small quantities and is one of our main research tools, is best described as ordinary matter going back in time. So far there hasn't been any way to use this to send messages to the past, but already quantum mechanics lets some weird things happen to our notions of time and space. Theoretically anyway, particles can exist in two places at the same time, or simultaneously in different universes, or can travel back and forth between dimensions.

The multiverse
In ''Timeline,'' Crichton pushes the rules. Instead of using quantum mechanics as a calculational tool, he interprets them literally, and assumes that parallel universes actually exist and in fact are being created every second.

Put another way, everything in the world that can happen, does happen, only it occurs in an infinite number of separate universes which together form a multiverse containing every possible course of action. Under this notion, there is a universe in which Abraham Lincoln was not assassinated, where the Allies lost World War II, and the alternative to your decisions about whom to marry, what job to take, are played out along with all the possible repercussions that result.

This is actually a valid interpretation of what we really know about time, but it falls in the realm of philosophy rather than physics because it doesn't predict anything. It's an interpretation of what might be going on, but like the existence of God, it's not testable. An advance in science or technology might someday point the way to a test, and that's where Crichton's fictional high-tech company, ITC, comes in. ITC has figured out how to take the philosophical question - ''Are there parallel universes? - and turn it into science - ''Let's go see.''

Quantum foam
To reach these other universes, Crichton uses something called ''quantum foam,'' another concept that's rooted in real physics.

If matter is governed by the weird fuzzy laws of quantum mechanics at a small scale, you might expect that at very small scales this will lead to fuzzy behavior of space and time itself. A description of what goes on when you mix general relativity and quantum mechanics does not yet exist, but we can hazard a good guess that at very small scales space-time will cease to be smooth, and take on a foamy look where concepts like ''here'' and '''there'' themselves get mixed up.

Such a foam, if it exists, is expected to contain tiny wormholes, tunnels between different times and places that might otherwise be far apart. The technical problem in using them, or indeed in even checking that they exist, is that they're expected to be about 0.000000000000000000000000000000001 centimeters across. That's 32 zeroes after the decimal point. Finding one of these holes and then shoving something through it is clearly not going to be easy.

Crichton knows this, so his solution is to scan the information content of a person (where all the atoms are, and how they're connected), shrink the data using currently available software compression techniques, and send it as a beam of particles like electrons into a wormhole to be reconstructed on the other side.

It's a tall order. Finding a wormhole, assuming they exist, is well beyond what we know how to do now. Getting one that goes to where or when we'd like would probably be a matter of trial and error, and even then there's no guarantee that a wormhole will stay open long enough - remember that they're supposed to exist in a spacetime that is fluctuating wildly.

Still, let's assume that ITC has somehow got itself a traversible wormhole, and all that's left to be done is to digitize a person and send them through.

Human bits
First, to get the information content of a person, Crichton uses nuclear magnetic resonance imaging, somewhat like those used in hospitals today. While this is complementary to an X-ray image which gives you a picture largely based on density (the bones show well), it won't provide enough data to describe a person in detail.

So Crichton relies on a recent spiced-up version of MRI using something called SQUID pickup devices so sensitive they can detect the tiny magnetic field of your heart at distances of a few feet. It's all real technology, but it still isn't going to get you the information you need.

The raw original data for a person would be a staggering number of bits. Of course to recreate a person you don't need an exact copy. Every time you take a breath you're a little different than you were the moment before, and that doesn't seem to matter.

But even if it could be done, the amount of energy needed to cram that data down a wormhole would be enormous. In fact, the energy need to even look at spacetime at the scale of the wormholes is well beyond current accelerator technology.

Even if we cleared those hurdles, could the information really be ''teleported'' to another time? Well, a quantum mechanical particle can be sort of here and there at the same time. What is meant by quantum teleportation is that there's a way to take a particle in a state of here and there and, though you must destroy it, recreate that state in another particle somewhere else.

But this sort of teleportation does not do anything that you'd want in response to ''Beam me up, Scotty,'' and in fact it doesn't move matter at all. What it does do, is allow you to take the quantum mechanical state of one particle somewhere and transfer it to the quantum mechanical state of another one somewhere else.

You still need a particle to copy the state to, and you still need to send the information. Crichton cleverly ducks the recreation of his travelers by assuming that the problem has been solved in some other universe, coincidentally the one he's sending the travelers to. This is sort of pushing the physics a bit, but it's not completely crazy.

Paradoxes
What about paradoxes? What if you went backward in time and killed your grandfather before he met your grandmother? Then you could never have been born, and thus never gone back in time, and so your grandfather would have met your grandmother after all, and...you see the problem. Interestingly enough, simple models of physics in space-times with closed timelike curves (a term which Crichton himself uses, and which describes the idea that you can travel in such a way as to visit the same moment twice) suggest that when you take into account quantum mechanics and the effects from all sorts of parallel universes, the universes that contain paradoxes occur with probability zero. In other words, just as Crichton suggests in the book, there may be no problem with time travel at all. You may be able to go back in time, but everything will adjust so that there's nothing paradoxical.

Science fiction is only a sort of fiction. It doesn't have to be scientifically accurate, though the closer it is to not being completely impossible or provably wrong, the more scientists tend to enjoy it. A lot of our colleagues in the field got started reading science fiction and wanting to be a part of making the future happen, and we have no doubt that this is more true than ever today. We clearly remember reading Crichton's ''The Andromeda Strain'' in elementary school and thinking ''Boy, scientists are cool.'' We're not literary critics, but perhaps we're allowed three words to describe the book: We like it.

Web reading: If you want to find recent research articles on time travel or wormholes, have a look at http://www-spires.slac.stanford.edu/find/hep and search with commands like ''find title wormhole'' or ''find title time machine.''

Stephen Reucroft and John Swain are experimental particle physicists at Northeastern University.