HOLD a tiny rubber band 10 inches from your eye. It appears to be what it is - a closed loop of rubber. Put it 10 yards from your eye. Now it appears to be a dot, a single point whose features are undiscernible. Put it 10 miles from your eye. You see nothing at all. That rubber band, many physicists now agree, is like a fundamental particle of matter. The naked eye can't see such particles at all. They're simply too small.

How can you see them at closer range? A particle accelerator helps. Like a giant microscope, it uses high energies to let you ``see'' tiny particles. Even that, however, doesn't provide the high resolution needed to see the outlines of the rubber band. It sees the rubber band, all right - but as a dot, not a loop.

But what if you had an accelerator that generated the kind of energy available at the Big Bang? It would be like seeing a particle up close through a mammoth microscope.

And would those dots still be dots?

No, says string theory, a new and promising way of looking at matter that in the last five years has seized the imagination of the physics community. Those dots, say string theorists, only look like dots. In fact, they're really like rubber bands - or, more accurately, loops of string. What's more, the 60 or so particles (depending on who's counting) in the ``particle zoo'' that makes up matter turn out to be, in this theory, different manifestations of a single loop-shaped object.

Then why do all these particles - quarks and leptons, anti-quarks and anti-leptons, and all the rest - seem to be so different? According to an extension of string theory called superstring theory - which incorporates a new symmetry called ``supersymmetry'' - the differences arise because these loops vibrate in different ways. Set a loop oscillating in a certain way, and it will appear to have certain properties. Seen from a distance, it might appear to be a ``charmed'' quark. With a different oscillation, the same string might seem to be a muon. If we could only see it up close, however, it would reveal itself to be just another dancing, jiggling, rolling pattern being played on a one-size-fits-all loop.

If that sounds strange, imagine a blind Martian coming to earth and hearing a one-stringed violin. He could be forgiven for thinking that the instrument had dozens of strings - one for each note he hears. In fact, all those notes come from just the one string. The secret? Each note results from a different vibration of the string.

So promising is the superstring theory, physicists say, that it appears capable of embracing a vast array of phenomena and explaining them as parts of a splendid, well-balanced whole. Even the four forces of nature - gravity, electromagnetic force, the strong force that binds protons and neutrons into the nucleus, and the weak force responsible for radioactive decay - may prove to be different manifestations of a single force. Physicists refer to supersymmetry, only half jokingly, as the TOE - the theory of everything.

But the TOE is not without its problems. ``Right now,'' says Rockefeller University physicist Heinz Pagels, ``it's turned out to be a theory of nothing. By that I mean that, although it's extremely elegant both conceptually and mathematically, it has failed to make contact not only with experiment, but with the ordinary theories that we now know describe experiment.''

The problem, in part, is one of scale. These strings or loops, according to the theory, are just 10 to the -33 centimeters long. That means that it would take a million of them - multiplied by a billion, then by another billion, then by still another billion - to add up to a centimeter. Even the mammoth superconducting supercollider - the SSC, currently proposed by the United States Department of Energy - will develop energies that will ``see'' only to the range of 10 to the -15 centimeters.

Moreover, such strings can't exist in our ordinary three-dimensional universe. ``It appears that the superstring theory implies that space-time is 10 dimensional,'' says cosmologist Michael Turner. That means nine spatial dimensions and a 10th in time. In our everyday world, he notes, ``we know of three spatial dimensions. That would say that we missed twice as many.''

Where are they? The best explanation, apparently, is that they're somehow ``folded up'' within the three-dimensional world, like leaves waiting to mature.

And then there's the mathematics, widely described as immensely challenging. So complex is it, in fact, that superstring theory has had to await the discoveries of highly elaborate mathematics for it to progress. Result: There has been a plethora of solutions to these theoretical problems, many of which claim to be good descriptions of the universe.

Despite the obstacles, however, physicist John Schwarz - who, with British physicist Michael Green, is one of the founders of string theory - is convinced that there are now only three possible string theories that could be right.

``Once you've said which of those theories is the right one,'' he explains across the cluttered desk in his office at the California Institute of Technology, ``you've given a completely unambiguous fundamental theory of nature. And now all you have to do to describe all of physics is to solve the equations.''

``That's, of course, the hard part,'' he adds with a chuckle.

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