- TILLWATER, 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.
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- 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.
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- 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."
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- "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."
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- 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.
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- "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."
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- 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.
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- 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.
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- 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.
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- "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.
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- 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."
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- "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.
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- 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.
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- "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.
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- 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.
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- 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.
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- 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:
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- These two spring from the same source but differ in name;
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- this appears as darkness.
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- Darkness within darkness.
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- The gate to all mystery.
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