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Until that day, they press on. In the hours before his lecture, Hawking sat in a glassed-in waiting room with his assistant, giving brief interviews. For all the attention the Cambridge physicist receives from press and public, his paper for the conference was not on string theory's cutting edge, and he is on his way to becoming a grand old man of the field, more respected for his past work than expected to push things forward. His most famous paper, showing that at the quantum scale black holes may emit some radiation (the emissions were named after him), was published in 1975; his paper in Potsdam was characterized as more cosmology than string theory. Breakthroughs in physics come more often to young men than to older ones, so it is not surprising that a younger generation is carrying the torch. Still, Hawking has a knack for explaining the further reaches of physics to a broad audience, and he plays a key role in keeping physics in the public spotlight.

Hawking's talk, when it came, was conceptually clear, and he characteristically nudged up the subject's accessibility a notch. But though Hawking headlined the show, the real action came from the warm-up act, a younger American physicist with a high voice and a reserved manner at the podium, professor Edward Witten.

Within the string theory fraternity -- and it is still very much a male-dominated field -- Witten, the current occupant of Einstein's position at Princeton's Institute for Advanced Studies, is the man behind most of the excitement. He provided a key conceptual breakthrough in 1995, starting the second revolution in string theory, which has brought such optimism to Potsdam.

Witten unified six competing versions of string theory, showing that they were all specific cases of a more general approach now dubbed M-theory. Depending on whom you ask, the M stands for matrix, mystery, magic or even the mother of all theories. The unification was important, because each theory purported to explain fundamental features of the universe. They were not contradictory, but as one practitioner put it: If we were living in the universe explained by one version, who was living in the other five?

String theorists ask such questions, of course, so that they may get to larger questions about the universe. If their theory holds, humanity will know some strikingly basic facts about everything around us: What was the universe like at its very beginning? How much matter is in the universe? How does gravity work? Where does matter come from? These aren't necessarily practical questions -- Einstein famously observed that gravity cannot explain why two people fall in love -- but they reach out to the very limits of what humans can know. The 1,300 people who came to the public lecture may not grasp the math, but they want to be part of that reach.

The problem the string theorists are working on stems from the two revolutions in physics in the 20th century: relativity and quantum dynamics. Thousands of experiments have shown that both relativity and quantum dynamics describe the universe that we inhabit. They explain and predict observed phenomena at the scales of atoms and of galaxies. They do what physicists want most from a theory: They work. Unfortunately, though, relativity and quantum dynamics give completely different explanations of what gravity is and how it works.

String theory essentially attempts to bridge the gap. The central idea is that instead of thinking of subatomic particles as points, it may be more helpful to think of them as strings that vibrate in particular ways. Like a violin string that vibrates one way to produce C and another to produce F sharp, these subatomic strings vibrate in one way to produce a photon and another to produce the quarks that form protons and neutrons.

Using this metaphor, it may be possible to reconcile quantum mechanics, which deals with gravity as an exchange of particles, with relativity, which considers gravity a fundamental feature of space and time. The math gets fearsome because the familiar four dimensions (three of space plus time) are not enough to describe the framework in which the strings have to work. With Witten's unification from 1995, it takes 11 dimensions to do the job. (This is actually an improvement -- earlier versions of string theory required as many as 26 dimensions for the explanation to work properly.)

Now that the physicists aren't worried about which version of string theory they should use, they're getting down to business. And in this case, business means proving different aspects of the mathematics, prodding, looking for loopholes and contradictions. It also means looking for elements that might be experimentally accessible, but they regard this as unlikely. The next generation of particle accelerators at CERN in Switzerland may be useful to look for supersymmetry, one of string theory's predictions, but the machine will not be ready until at least 2005. Other predictions of the theory are only testable by observing a supernova's collapse into a black hole, or by noting certain effects of existing black holes. The theorists are not holding their breath for either of these eventualities; the possibility exists that we may never know if string theory is true.

If the string theorists succeed, however, our lives may very well change at the most profound levels. We'll know how gravity works, everywhere from within an atomic nucleus to between galactic clusters. We will know what holds the universe together and what makes it work. A branch of physics will be complete, an explanation of life seamless. For now, the hunt is still on.
salon.com | August 11, 1999

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