December 22, 1998
Article from New York Times

Where Does the Time Go? Forward, Physics Shows

By MALCOLM W. BROWNE

in Lewis Carroll's mirror world of "Through the Looking Glass," it seems perfectly logical that the White Queen, who lives backward, first
bandages her finger, then begins to bleed, then screams, and finally pricks her finger. On paper, if not in real life, the physics governing
many natural phenomena permit time to run either forward, like a swimmer jumping from a diving board, or backward, like a reversed
movie in which the swimmer leaps from the water and lands on the board.

But since a landmark experiment in 1964 by Dr. James W. Cronin and Dr. Val L. Fitch, both at Princeton
University at the time, physicists have known that time reversal is not so neat in the microscopic world of
particles. They found indirect but convincing evidence that sometimes a particle going backward in time fails to
land on the metaphorical diving board; in other words, time, they found, could not be perfectly symmetrical.

Experimenters have now achieved direct confirmation of this unsettling inference.

To no one's surprise, physicists at two big particle accelerators, one in Switzerland and the other in Illinois,
proved that when certain particles go backward in time, their behavior is somewhat different from what it is
when they go forward.

If this sounds baffling to non-scientists, they are not alone; a member of the Nobel committee who sat on the panel that awarded Dr. Cronin
and Dr. Fitch the 1980 prize in physics remarked, "It would take a new Einstein to say what it means."

But one important implication stands out: Time's slippery nature may explain why there was anything left after the Big Bang to build the
universe as we know it. Theorists have concluded that there was a slight imbalance in the amounts of matter and antimatter created at the
birth of the universe. The matter and antimatter believed to have been created by the Big Bang presumably annihilated each other quickly,
leaving only a slight excess of matter -- just enough to create today's matter universe.

Time, which had a role in this, is probably the deepest of all enigmas in physics.

At the everyday level, physicists believe that the "arrow" of time points always in the direction of increasing disorder (or "entropy").

Natural processes run down, order yields to disorder, information disappears, and people grow old, die and decay. These processes mark the
forward passage of time.

But a particle is not like a human being, and when physicists speak of a particle going backward in time, they do not mean that the particle is
a tiny time machine capable of exploring the past.

A leading particle theorist, Dr. Chris Quigg of the Fermi National Accelerator Laboratory in Batavia, Ill., explained: "It's not that antiparticles
in my laboratory are actually moving backward in time. What's really meant by that is that if I think of a particle moving from one place to
another forward in time, the physical process is the same as it would be if we imagine running the film backward and also changing the
particle into an antiparticle."

Three fundamental transformations of particles are involved in all this: the reversal of electrical charge (C), which changes particles into
antiparticles and vice versa; parity reversal (P), the mirror reversal of every dimension in a particle (turning it inside out, so to speak), and
time reversal (T).

Physicists feel comfortable when things can be explained by balance sheets that show everything accounted for. They once believed that the
symmetry of parity -- the original form versus its inside-out version -- was inviolate, meaning that physics in a mirror world would be identical
to our own. But the 1957 Nobel Prize in Physics honored Dr. Tsung-Dao Lee and Dr. Chen N. Ning Yang for discovering that the assumed
symmetry of parity in particles did not exist; that when short-lived particles called K mesons (or kaons) decay, their transformations violate
parity symmetry.

Many theorists expected that this asymmetry would be balanced out by another of the transformations, that of charge. Dr. Fitch and Dr.
Cronin proved, however, that charge symmetry was also violated. That meant that to keep things in balance, the symmetry of time had to be
violated to make up for the symmetry violations of charge and parity. In this way the total package of charge, parity and time, or C.P.T. as
physicists call this combination of interlocking components, would be "conserved," preserving the ideal of a universe that fits neatly together.

"If you believe that charge, parity and time taken together must balance out," Dr. Quigg said, "then if charge and parity are a little funny, and
you divide them into the charge-parity-time package, then time must be a little funny, to compensate. That was predicted by theory. But the
two new experiments by Fermilab and Cern, the European accelerator group, show directly that time-reversed symmetry is violated in just
the direction and amount predicted by theory. Now the C.P.T. ledger book is in balance."

Both the Cern and Fermilab experiments measured decay processes of rare particles called neutral kaons and neutral antikaons, which
consist of two quarks. (Protons and neutrons, the particles that make up the nuclei of ordinary atoms, contain three quarks.)

In the Cern experiment, detectors measured the oscillations of kaons into antikaons, and vice versa, as these fleeting particles sped away
from their point of origin. If time were perfectly symmetrical, the rates at which kaons and antikaons are transformed into each other should
be precisely equal. The experiment showed, however, that the rate at which antikaons (which are a form of antimatter) turn into kaons
(which are normal matter) is higher than the time-reversed process in which kaons become antikaons.

Some similar asymmetry could help to explain the presumed excess of matter over antimatter when the universe was created.

But what, if anything, does a particle moving backward in time have to do with conventional time at larger scales?

Mathematical equations governing the laws of motion, electromagnetism and many other phenomena present no difficulty with time reversal.
Nor is time reversal inconsistent with particle physics, which is governed by ordinary quantum mechanics, which for all its celebrated
weirdness operates within a mathematical framework of classical three-dimensional space and time. Quantum mechanics requires no special
direction of time, either forward or backward.

But cosmic relationships are governed by the laws of general relativity, Einstein's theory of gravity, and these have yet to be brought into
consonance with quantum mechanics -- the rules for the behavior of atoms and subatomic particles. In each of these domains, time has a
somewhat different meaning.

Relativity decrees that time is not an absolute quantity. Among the surprising effects of relativity are that a moving clock runs slower than a
clock at rest, and that time on a mountain top runs faster than time at sea level, because gravity is stronger at sea level and gravity slows time
down.

In theory, some scientists have suggested, it might be possible to travel in time using black holes, worm holes, cosmic strings or other
distortions of space-time, but one of the problems with time machines is the "grandmother paradox"; if someone could go back to the past and
kill his own grandmother, a paradoxical violation of the principle of causality would result. The possibility of this happening is one of the main
objections to the idea of time travel, although some physicists have devised ingenious schemes for getting around the paradox.

But does a particle going backward in time pose the same kind of paradox?

Physicists think not.

Noting that the physics of ordinary experience prevents time reversal and violations of causality, Dr. Fitch said in an interview that "things in
the everyday world are statistical in nature, and disorder always increases, fixing the direction of the arrow of time. But time asymmetry for
particles applies to just a handful of individual particles, not to statistical aggregates."

Dr. J. Richard Gott 3d, a Princeton University cosmologist, envisions a possible universe that would be the opposite of ours in every sense.

"I can imagine living in a universe like ours, except that charge, parity and time are all reversed," Dr. Gott said in an interview. "Instead of
expanding from the Big Bang, such a universe would be contracting toward a big crunch, with everything growing hotter."

In a recent paper in the journal Physical Review D, Dr. Gott and his colleague, Li-Xin Li, suggested that the laws of physics "may allow the
universe to be its own mother."

In an antimatter, time-reversed universe, Dr. Gott said, people would remember what we call the future (but not what we think of as the
past), and for them, the backward flow of time would seem as natural as does our sense of forward-flowing time.

But to come to terms with such things physicists need to deal with quanta -- the discrete packets of energy that define the microscopic world:
electrons, photons, quarks and so forth. Even empty space is believed to be quantized -- subdivided into infinitesimal cells.

But so far, despite the best efforts of Albert Einstein and many other theorists, no one has been able to dissect gravity or time into their
component quantum packets, if such exist.

"We're still children as far as quantum gravity is concerned," said Dr. Daniel E. Holz, a relativity theorist at the Max Planck Institute,
Potsdam, Germany.

"We don't know how to quantize time," Dr. Holz said. "You can't make heads or tails of it. When you try to quantize gravity, time is what
sinks you. When we understand what to do with time in quantum gravity we'll have it done. Or turn it around: When we get quantum gravity,
the big revelation will be, aha! So that's the way time works!"

Dr. John A. Wheeler of Princeton University, the cosmologist and astrophysicist who coined the term "black hole" to describe ultradense
objects from which light cannot escape, believes that despite the puzzles and paradoxes posed by time, a fundamental simplicity underlies it.

"It's not so much that there's something strange about time," Dr. Wheeler said in an interview. "The thing that's strange is what's going on
inside time.

"We will first understand how simple the universe is when we recognize how strange it is."