| June 19, 2001
Physics' Big Puzzle Has Big Question: What Is Time?
By JAMES GLANZ
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.
(Original Len: 9285 Condensed Len: 9560)
Created by Eintime:CondenseHtmlFile on 060514
@ 16:53:28 CMD=RAGSALL -LP83
(Len=9733)
|