Quantum vibe

At Potsdam's string theory conference, Einstein's heirs try to tie up an explanation for gravity.


Douglas Merrill
August 11, 1999 8:00PM (UTC)

They took the press attention in stride, the 384 theoretical physicists who gathered in Potsdam, Germany, July 19 through 24 to share the latest results of their quest for a mathematical description of the universe's most fundamental laws. Since Stephen Hawking's bestseller, "A Brief History of Time," camera crews have attended their conferences, floating their branch of physics, string theory, esoterically at the edge of public consciousness. Cartoons in the New Yorker use string theory as a punch line, while Hannibal Lecter tries his hand at it in Thomas Harris' latest novel. As occasionally impenetrable physics goes, string theory is hot.

After a key breakthrough four years ago, string theorists are crunching their way through workable problems, and closing in on a theory of quantum gravity -- the problem that Einstein spent the last decades of his life failing to solve.

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The hunt is a slow one. In a way, it's been on for the better part of 300 years, since Sir Isaac Newton described how fast an apple drops toward a dozing physicist's head. Newton was more interested in the motions of the planets, but his mathematics applied as much to heavenly bodies as to falling fruit. (Incidentally, the calculus that he used was as far ahead of the general understanding of mathematics in his day as the math behind string theory is today.)

The public is on to the hunt as well. More than a thousand people turned out on a chilly and rainy German afternoon for a triple bill of string-theory presentations, overflowing the University of Potsdam's auditorium and filling five additional rooms with live video connections. The first people in line arrived three hours before the start of the lecture; there was security and press enough for a rock band. There were even some 300 people who didn't fit into any of the rooms and stood out on the lawn listening to the lecture on the loudspeakers, the world's first stringheads enjoying the quantum vibe.

The concepts these physicists work with daily -- from 11-dimensional descriptions of reality to quantum interactions in black holes -- stretch the imagination. The mathematics involved daunt even other physicists. The most important developments at Strings99 came in noncommunicative geometry, conformal field theories and nonsupersymmetric scenarios -- not exactly household phrases.

But as David Gross of the University of California at Santa Barbara said, "these are good times for string theory." Jobs are available, both for post-docs and on the tenure track. During the coffee breaks, some participants eye papers while others swap gossip on who's doing exciting work. The German Academic Exchange Service provided extra funding so that young and unestablished researchers could meet leading practitioners in person.

The field draws talent from around the world, and supports numerous research centers as well. The Max Planck Institute for Gravitational Physics and the Albert Einstein Institute are the second European hosts in the 11-year series of strings conferences, and in 2001 the meeting will travel to Bombay, India. As the wife of one presenter noted, the physicists are basically regular scholars -- they just happen to be trying to show how the whole universe works.

String theory seems to be the best way to solve a number of highly non-trivial questions, but there is no guarantee that these hundreds of brilliant minds are on the right path. String theory may go the way of theories of the luminiferous ether, a 19th century explanation of light that Einstein and others washed away in the early decades of the 20th. String theorists' life work hangs on a series of suppositions, and while their daily work is to strengthen those suppositions by both fleshing them out and looking relentlessly for holes, there is a chance that the whole enterprise will come crashing down.

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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?

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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.

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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.

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Douglas Merrill

Douglas Merrill is a Senior Research Fellow in the Research Group on the Global Future at the University of Munich's Center for Policy Research.

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