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Conference Philos Physicists*
June 19, 2001

Physics' Big Puzzle Has Big Question: What Is Time?


STILLWATER, Minn. — When philosophers debate the nature of time and space, a listener is liable to walk away muttering something like "Whoa. . . ." On the same question, a technical exchange among scientists is more likely to elicit a "Huh?" A conference here this month brought together both camps to explore that question — which happens to lie at the heart of the most important unsolved problem in physics — in the hope of bringing forth a satisfying "Aha!" of discovery.

That problem is the search for a theory that encompasses both the effects of gravity, described by Einstein's theory of general relativity, and the fuzziness that occurs in the realm of tiny particles according to quantum mechanics. For a century, technical difficulties have frustrated all attempts to develop a theory that holds where both gravity and quantum effects are crucial, like at the centers of black holes or during the first moments of the Big Bang explosion in which the universe is thought to have originated.

The conference, called the Seven Pines Symposium, drew together some two dozen physicists, historians and philosophers. It was in part an effort to step back from the technical morass and identify the roots of the problem. As the four-day meeting developed, it became clear that those roots run so deep that time — and to a lesser extent, space — may not even be the same actors in unified theories based primarily on relativity as in those based on quantum mechanics. In short: "Arrrgh."

"Many approaches have run into major stumbling blocks that seem to require some new understanding of space and time," said Prof. Robert Wald, a physicist at the University of Chicago. Calling on the image of blind men feeling their way around an object, Professor Wald said, "I don't see any evidence that they're talking about different parts of the same elephant."

Professor Wald was quick to add that the conference should not be seen as a desperate move by scientists to seek philosophical enlightenment on questions that have stymied the physicists. But another physicist, Prof. Abhay Ashtekar of Penn State, where he is director of the Center for Gravitational Physics and Geometry, conceded that "there is a little bit of shaking of confidence" among scientists thirsty for a breakthrough.

"That's the whole point in stepping back," Professor Ashtekar said. "I think somehow the mind is becoming a little more open to ideas coming from everywhere else."

The historians and philosophers occasionally led the scientists on a merry chase through foreign terrain. Prof. John Earman, who is in the history and philosophy of science department at the University of Pittsburgh, said the structure of relativity theory suggested that time could merely be a "psychological illusion" that was important to humans but not a fundamental element of any unified theory.

At this, Prof. Serge Rudaz, a physicist at the University of Minnesota, started looking around the room in surprise. "That sounds pretty radical to me," he said. "Am I the only one?" He was not.

But another philosopher, Prof. Nick Huggett of the University of Illinois at Chicago, suggested that success could be near. It was precisely by struggling, and occasionally blundering, with basic definitions of space and time that great scientists like Newton and Descartes made crucial progress in framing less ambitious theories, he said.

"These thinkers faced similar problems to those encountered today in the development of quantum theories of gravity," Professor Huggett wrote in an introduction to his talk.

Although no immediate resolution appeared, the symposium did call into sharp relief the problem of exactly what time is, a question whose solution, said Prof. Karel Kuchar, a physicist at the University of Utah, "is simply the wind that precedes the storm of any future theory."

The crux of their problem is that time itself looks very different depending on whether scientists try to construct a final theory by starting with quantum mechanics and adding gravity, or vice versa.

For all their strangeness and sophistication, including predictions that a particle can be in many places at once or have irreducibly uncertain speeds and positions, theories based on quantum mechanics and particle physics assume that somewhere, the regular tick-tock of ordinary time is being measured by something like a Swiss watch or a planet whirling around a star.

That crutch is a legacy of the classic formulation of quantum mechanics, which divides the universe into "observers" who make measurements and particles that are measured. Relativity theory could not be more different, focusing on how the gravity of massive bodies bends the structure of time and space. Like marbles rolling on a warped rubber surface, the bodies then move about in ways determined by the bending of space-time, and so on: everything is dealt with together, including any observers.

That is why if scientists go in the other direction and "quantize" relativity theory, they end up with a theoretical universe in which not only particles, but also time and space themselves are shifting and indeterminate, as elusive as the ripples on the bottom of a pool.

Although many physicists expect that the universe really does shift and shimmer on tiny scales, where quantum effects should bend space- time just as gravity does on large scales, the absence of a reliable "background" means that there is no Swiss watch, even in theory, for the particle physicists.

Nevertheless, Prof. Jeffrey Harvey of the University of Chicago said he believed that string theory — a particle theory whose ambitious goal is to explain all the known forces in nature as different facets of the same diamond, so to speak — is by far the best bet for unifying physics.

Roughly speaking, the theory assumes that the vibrations of unimaginably tiny objects called strings and branes correspond to all particles that have so far been discovered, along with a slew of others that have not. The vibrating thingies supposedly exist in many more dimensions than the four that humans are familiar with, but the extra ones are considered to be somehow curled up like arthritic fingers and so small that they are not apparent.

Because all force-carrying particles are included, including gravitons, which theoretically transmit gravity, string theory has the potential to unify all of physics. But because it exists so far only in fragmentary form, Professor Harvey said, string theory must assume that a particular space-time background exists, rather than letting one emerge naturally from the interactions of the particles.

"If you ask a string theorist, `Tell me how to formulate your theory in a way that doesn't involve any choice at all of a background space-time,' they throw up their hands and say, `We don't know how to do that,' " Professor Harvey said.

But the relativity theorists don't have it any better. For them, time and space begin to mix together in incomprehensible ways when quantum effects are added to Einstein's equations. In essence, they often cannot even find time as an entity distinguishable from space in the mathematical mishmash that results.

"Relativity glued almost everything to everything else," said Professor Kuchar, of the University of Utah, and the consequence is head- spinning confusion when quantum mechanics is added to the theory.

The problem could mean that quantum relativity is simply wrong, that time is not so important after all or that a new definition of time in the quantum realm must emerge. It could be, in Professor Earman's somewhat chilling conjecture, that time is an illusion important only to humans, not to physics.

One possibility, said Professor Wald, of the University of Chicago, is that time will ultimately have meaning only as correlations between events. For example, cosmic events like a stellar explosion could be referred to the size of the ever-expanding universe at the moment they happened, rather than to some abstract notion of pure time. But even then, "one runs into all sorts of obstacles," Professor Wald said.

In a sense entirely appropriate to a philosophical gathering, the participants seemed to agree only on what time could not be. But the symposium had a surprising ending when Professor Ashtekar, rather than one of the philosophers, turned to poetry for a note of hope. The Chinese sage Lao Tsu, he said, looked at time and space in a way that might apply to string theory and relativity:

These two spring from the same source but differ in name;

this appears as darkness.

Darkness within darkness.

The gate to all mystery.

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