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August 2, 2005 NEW YORK TIMES

Lacking Hard Data, Theorists Try Democracy


TORONTO - Science is not done by popular vote. But nothing can be taken for granted when string theory is the subject.

So when Stephen Shenker, a Stanford University theorist who was moderating a panel discussion here on the future of the putative theory of everything, asked for a show of hands on the fate of a strange number known as the cosmological constant, some 400 physicists and mathematicians were happy to swallow their doubts and vote.

The panel discussion, titled "The Next Superstring Revolution," took place at Strings05, the latest yearly congress of string theorists, held in mid-July at the University of Toronto. That it was a somewhat unusual occasion was not lost on anyone. No other field of science, said Jacques Distler, from the University of Texas, would be presumptuous enough to have a meeting about its next revolution.

But then no other field of science is like string theory, which says that nature with all its forces and particles is composed of tiny strings wriggling in 10 dimensions. In the last two decades it has taken academia by storm, and it has been rocked by periodic reinventions.

The first happened in 1984 when it was shown that a consistent theory of all the forces of nature could be constructed from strings. The second occurred in 1995 when relationships between the five different kinds of string theory suggested that they were all separate manifestations of a single underlying 11-dimensional theory known as M-theory.

As a result, string theory conferences have often been the scene of drama and, one year, even dancing and singing, when new developments rocketed the field into new dimensions of their mathematical wonderland.

But despite success in formulating a mathematically consistent theory that unifies gravity and the rest of nature, a goal that eluded Einstein, progress has stalled on other fronts, like going beyond consistency to proving that string theory is actually right. One problem is that "stringy" effects manifest themselves only at energies orders of magnitudes beyond the capabilities of any particle accelerator that could be built on Earth, meaning that the theory cannot be tested or guided by experiment.

"We've done very well for the last 20 years without any experimental input," said Michael Douglas of Rutgers.

Some string theorists (there are no string experimentalists) hope they might get some help soon. In 26 months, the most powerful particle accelerator ever built, the Large Hadron Collider, or L.H.C., will begin colliding protons with energies of seven trillion electron volts apiece at CERN in Geneva.

One long shot is that the accelerator will find evidence of particles disappearing into the extra dimensions demanded by string theory, which Dr. Douglas said would be "one of the great triumphs of human history."

Nima Arkani-Hamed, a Harvard physicist, called the Large Hadron Collider "an unprecedented opportunity for discovering new physics."

But, he said in a talk here, there is a problem. What physicists most expect to discover with the Large Hadron Collider is a new phenomenon called supersymmetry (which would manifest itself as a passel of new particles). Supersymmetry is predicted by string theory but alas is not exclusive to it. In fact there are tens of thousands of different models of supersymmetry, he said, so many that physicists will have trouble figuring out which one is right from a given set of data.

To dramatize the problem, Dr. Arkani-Hamed and others have prepared a simulated set of data from the collider and challenged their colleagues, in what they have dubbed the L.H.C. Olympics, to figure out what it means. For those who want to play, more information is available on the Web site

The panel discussion on the next revolution attracted a full, if somewhat dubious crowd. One of the panelists, Eva Silverstein of Stanford, joked when her time came that her friends in high-energy physics were amused "when we sit around and tell about our feelings."

Dr. Shenker, a Stanford theorist who has tousled blond hair going gray, was drafted as moderator by the organizers, so the joke went, because he "was not only respected, but feared." He had recruited his panel with an accent on youth, on the ground that the progenitors of the previous revolutions were unlikely to be the makers of the next.

Dr. Shenker began by reminding those present that the grist for previous revolutions had come from ideas that had been overlooked. "So it's a good bet," he told the crowd to much laughter, that the next revolution would be based on ideas "that are being ignored or dismissed by you."

One of those ideas, he noted, which has hardly been overlooked in debates in recent years, is what string theorists call "the landscape," a sort of meta-universe of gazillions of possible solutions - at least 10500 - of the string theory equations. Since that's enough different universes for string theory to predict just about anything, it raises the question of how such flexibility can be tested.

But some theorists have suggested this abundance might be a way of explaining the bafflingly small value of the cosmological constant, a force that seems to be accelerating the expansion of the universe. Theoretical calculations suggest it should be 1060 times larger than what astronomers have measured. With so many possible universes available, theorists say, perhaps there is a random distribution of values of this constant and we simply live in a part of this meta-environment that works for us, like krill thriving in cold water.

"If environment selection proves correct, it would be a real revolution," Dr. Shenker said, "and we will look back and say the third superstring revolution was five years early."

But this kind of thinking - often known as the anthropic principle - is anathema to many scientists and nonscientists alike because it seems to place humanity at the center of the universe. Many physicists say it is a cop-out. Science, they contend, should eventually be able to calculate the fundamental characteristics of the universe from first principles. The controversy is often characterized as and East Coast-West Coast split, or even a Princeton-Stanford one.

In a conversation here, Michael Green of Cambridge University, one of string theory's founders, said that if the cosmological constant could be decided in such willy-nilly fashion it wasn't a fundamental aspect of physics and in the deeper long run was of little interest, never mind that it determines the fate of our own universe.

And so the panelists were happy to share their feelings about the anthropic principle.

Joseph Polchinski, a string theorist at the Kavli Institute for Theoretical Physics in Santa Barbara, Calif., suggested that the next revolution had already happened and it was indeed the anthropic principle.

But Nathan Seiberg, from Princeton's Institute for Advanced Study, said, "We will understand the universe and it will not involve the anthropic principle." This, he admitted was a conservative view, even though, as he noted, "I come from a blue state."

Andrew Strominger of Harvard deplored what he felt was an attitude of pessimism. String theory, he said, was not all of physics, but it was a very interesting and dazzling part of it. There are lots of problems, he said, but there is no obstacle in principle to solving them, which he described as "a big intellectual adventure we all get to go through in the next few decades."

String theory, he said, would eventually become an experimental science. There will be things out of left field. "Sooner or later we will get there," Dr. Strominger said, "and when we do we'll all be heroes."

Dr. Shenker then opened the floor to the audience members, not all of whom agreed that string theorists were yet heroes.

Lee Smolin, of the nearby Perimeter Institute for Theoretical Physics, which helped sponsor the meeting, and who works on an alternative quantum gravity theory known as loop quantum gravity, chided the string theorists for their failure to come up with a version of their theory in which the strings make space and time instead of merely wriggling around in space and time. Such a so-called background independent formulation, he said, was Einstein's goal for the invention of general relativity.

"I'm not sure it will be the next revolution," Dr. Smolin said, "but I am sure it will be the last."

Edward Witten, one of the string theory leaders, cautioned against having too many preconceptions about the future, saying that a discussion like the present one would not have anticipated many of the most recent advances in string theory.

Many physicists indeed compared their plight to the situation in physics in the early decades of the 20th century between 1905, when Albert Einstein concluded that atoms and light waves strangely appear to exchange energy only in quantized bits, and 1925, when Werner Heisenberg's quantum mechanics overthrew common sense and cause and effect, at least as they applied on the atomic level.

"We haven't done as well in the last 20 years," Dr. Shenker said.

Leonard Susskind, a Stanford theorist and one of the founders of string theory, replied, "There's nothing to do but just hope the Bush administration will keep paying us."

Amanda Peet of the University of Toronto suggested making string theory "a faith-based initiative," to much nervous laughter.

Repeating his call to optimism, Dr. Strominger said the work would go on. Not everybody needs to be a string theorist or even to agree with it.

"We have a need to understand the truth about the mathematical structure of string theory and of our universe," he said. "Those who want to join us can, those who don't don't have to."

"I don't see any need for pessimism or cause to sell ourselves," he concluded to applause.

At the end Dr. Shenker invoked his executive privileges. He asked the audience members for a vote on whether, by the year 3000, say, the value of the cosmological constant would be explained by the anthropic principle or by fundamental physics.

The panel split 4 to 4, with abstentions, but the audience voted overwhelmingly for the latter possibility.

"Wow," exhaled one of the panel members amid other exclamations too colorful to print here.

"The anthropic principle is out of office," somebody else said.

Dr. Shenker concluded, "We have made some progress in sharing our feelings."

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