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Theory Explaining It All Doesn't Quite

Posted on: Wednesday, 8 December 2004, 12:00 CST

They all laughed 20 years ago.

It was then that a physicist named John Schwarz jumped up on the stage during a cabaret at the physics center here and began babbling about having discovered a theory that could explain everything.

By prearrangement, men in white suits swooped in and carried away Schwarz, then a little-known researcher at the California Institute of Technology.

Only a few of the laughing audience members knew that Schwarz was not entirely joking. He and his collaborator, Michael Green, now at Cambridge University, had just finished a calculation that would change the way physics was understood. They had shown that it was possible to write down a single equation that could explain all the laws of physics, all the forces of nature the proverbial "theory of everything" that could be written on a T-shirt.

And so emerged into the limelight a strange new concept of nature, called string theory, so named because it depicts the basic constituents of the universe as tiny wriggling strings, not as point particles.

"That was our first public announcement," Schwarz said recently.

By uniting all the forces, string theory had the potential for achieving the goal that Einstein sought without success for half his life. If true, it could be used like a searchlight to illuminate some of the deepest mysteries physicists can imagine, like the origin of space and time in the Big Bang and the putative death of space and time at the infinitely dense centers of black holes.

In the last 20 years, string theory has become a major branch of physics. Physicists and mathematicians conversant in strings are courted and recruited like star quarterbacks by universities eager to establish their research credentials. String theory has been celebrated and explained in best-selling books like "The Elegant Universe," by Brian Greene, a physicist at Columbia University, and even on popular television shows.

Last summer in Aspen, Schwarz and Michael Green, of Cambridge, cut a cake decorated with "20th Anniversary of the First Revolution Started in Aspen," as they and other theorists celebrated the anniversary of their big breakthrough. But even as they ate cake and drank wine, the string theorists admitted that, after 20 years, they still did not know how to test string theory, or even what it meant.

As a result, the goal of explaining all the features of the modern world is as far away as ever, they say.

And some physicists outside the string theory camp are growing restive. At another meeting, at the Aspen Institute for Humanities, only a few days before the string commemoration, Lawrence Krauss, a cosmologist at Case Western Reserve University in Cleveland, called string theory "a colossal failure."

String theorists agree that it has been a long, strange trip, but they still have faith that they will complete the journey.

"Twenty years ago, no one would have correctly predicted how string theory has since developed," said Andrew Strominger of Harvard. "There is disappointment that despite all our efforts, experimental verification or disproof still seems far away. On the other hand, the depth and beauty of the subject, and the way it has reached out, influenced and connected other areas of physics and mathematics, is beyond the wildest imaginations of 20 years ago."

In a way, the story of string theory and of the physicists who have followed its siren song for two decades begins with the classic "what if?"

What if the basic constituents of nature and matter were not little points, as had been presumed since the time of the Greeks? What if the seeds of reality were rather teeny tiny wiggly little bits of string? And what appear to be different particles like electrons and quarks merely correspond to different ways for the strings to vibrate.

It sounds simple, but that small change led physicists into a mathematical labyrinth, in which they describe themselves as wandering, "exploring almost like experimentalists," in the words of David Gross of the Kavli Institute for Theoretical Physics in Santa Barbara, California. String theory, the Italian physicist Daniele Amati once said, was a piece of 21st-century physics that had fallen by accident into the 20th century.

The string revolution had its roots in a quixotic effort in the 1970s to understand the so-called "strong" force, which binds quarks into particles like protons and neutrons. Why were individual quarks never seen in nature? Perhaps because they were on the ends of strings, said physicists, following up on work by Gabriele Veneziano of CERN, the European research consortium.

That would explain why you cannot have a single quark you cannot have a string with only one end. Strings seduced many physicists with their mathematical elegance, but they had some problems, like requiring 26 dimensions and a plethora of mysterious particles that did not seem to have anything to do with quarks or the strong force.

When accelerator experiments supported an alternative theory of quark behavior known as quantum chromodynamics, most physicists consigned strings to the dustbin of history.

In 1974, Schwarz and Joel Scherk, from the Ecole Normale Superieure in France, noticed that one of the mysterious particles predicted by string theory had the properties predicted for the graviton, the particle that would be responsible for transmitting gravity in a quantum theory of gravity, if such a theory existed.

Without even trying, they realized, string theory had crossed the biggest gulf in physics. Physicists had been stuck for decades trying to reconcile the quirky rules known as quantum mechanics, which govern atomic behavior, with Einstein's general theory of relativity, which describes how gravity shapes the cosmos.

That meant that if string theory was right, it was not just a theory of the strong force; it was a theory of all forces.

"I was immediately convinced this was worth devoting my life to," Schwarz recalled. "It's been my life work ever since."

It was another 10 years before Schwarz and Green Scherk died in 1980 finally hit pay dirt. They showed that it was possible to write down a string theory of everything that was not only mathematically consistent, but also free of certain absurdities, like the violation of cause and effect, that had plagued earlier quantum gravity calculations.

One of string theory's biggest triumphs has come in the study of black holes. In Einstein's general relativity, these objects are bottomless pits in space-time, voraciously swallowing everything, even light, that gets too close. But in string theory they are a dense tangle of strings and membranes.

In a prodigious calculation in 1995, Strominger and Cumrun Vafa, also of Harvard, were able to calculate the information content of a black hole, matching a famous result obtained by Stephen Hawking of Cambridge University using more indirect means in 1973. Their calculation is viewed by many people as the most important result yet in string theory, said Brian Greene, of Columbia.

Another success, Greene and others said, was the discovery that the shape, or topology, of space is not fixed but can change, according to string theory. Space can even rip and tear.

Critics of string theory, meanwhile, have been keeping their own scorecard. The most glaring omission is the lack of any experimental evidence for strings or even a single experimental prediction that could prove string theory wrong the acid test of the scientific process.

This disparity between theoretical speculation and testable reality has led some critics to suggest that string theory is as much philosophy as science, and that it has diverted the attention and energy of a generation of physicists from other, perhaps more worthy pursuits. Others say the theory itself is too vague and that some promising ideas have not been proved rigorously enough yet.

String theory's biggest triumph is still its first one, unifying Einstein's lordly gravity, which curves the cosmos, and the quantum pinball game of chance, which lives inside it.

"Whatever else it is or is not," Harvey said in Aspen, "string theory is a theory of quantum gravity that gives sensible answers."

Source: International Herald Tribune

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