So what is time? If no one asks me, I know; if they ask and I try to explain, I do not know. -- St. Augustine

Like exhaustively trained musicians in a symphony orchestra performing in lockstep with the movements of the conductor's baton, hundreds of millions of us humans carry out our daily activities under the direction of the clock. Few people in industrialized society can escape having their movements directed by its near-constant intrusion. From the President of the United States, facing a full schedule of back-to-back high-level meetings, the duration of each measured in seconds, to the homeless person who has to push his shopping cart out of the city park before the 10 o'clock curfew, we obediently serve the nearest clock. We might briefly escape our duty to it during a night's sleep or a week's vacation. But sooner or later, we inevitably submit to the dictates of the clock.

That's to be expected. Present-day society is so vastly complex that it would destroy itself in a matter of days if the motions of myriad individual parts within government, industry, commerce, and communications were not precisely synchronized. So that each of us may function as a component in this immense machine, our indoctrination into the rule of the clock begins at an early age, with the much despised bedtime, and all too soon proceeds to the loathsome school bell. Later we graduate to the factory whistle or its equivalent.

Humans haven't always lived at the mercy of the clock. Mechanical clocks didn't appear in Europe until around the 13th century. And they didn't come into general use until five hundred years later, when the Industrial Revolution required large-scale coordination of movement. Indeed, our perception of the passage of time may itself be a comparatively recent addition to human consciousness. Some scientists believe that our distant ancestors may have had no concept of past or future. Consciousness rooted in the present may have persisted until as recently as three thousand years ago. According to Princeton University psychologist Julian Jaynes, Homer's epic poem The Iliad, composed in the eighth century B.C., is populated by characters who "did not live in a frame of past happenings...[and] could not reminisce."

In the last few centuries, humans have developed an increasing awareness of, if not obsession with, the passage of time. Along with this preoccupation have come advances in the ability of scientists to measure time. Today's atomic clocks are distant cousins to medieval Europe's crude mechanical clocks, much as the Space Shuttle is related to the hot air balloon.

The precise timekeeping possible with atomic clocks, so called because they keep time by counting the exceedingly regular vibrations in cesium atoms, boggles the mind. Held at a constant temperature and humidity, the cesium atoms in an atomic clock vibrate 9,192,631,770 times per second. The best of these clocks won't gain or lose a single second over the next million years. In fact, scientists have so much confidence in the ability of cesium clocks to measure time that the standard unit of distance, a length of one meter, is now officially defined as the distance light travels in a vacuum in 3.33564095 billionths of a second.

That scientists have so precisely defined distance in terms of time would seem to support the classical view that time is constant and absolute. Time "flows equably without relation to anything external," Isaac Newton proclaimed three centuries ago, and most people today would be inclined to agree. Although time is always on the move, it seems to be the only constant in our experience. A second is a second is always a second. How could it be otherwise?

Albert Einstein showed us how time is inconstant in 1905, with his theory of relativity. Reducing long-held scientific beliefs to ashes, Einstein's theory said that time wasn't absolute, that it can be stretched and compressed and bent. Time passes more slowly the faster we are moving, and it grinds to a complete stop when we travel at the speed of light, 186,232 miles per second. The astonishing possibilities of time's relativity have been popular fodder for science fiction, such as the 1967 film Planet of the Apes, in which space travelers from Earth unwittingly return home several million years in the future, having themselves aged only a few years during the trip.

But we don't have to take a high-speed jaunt among the stars to see time stretch out. A 1971 experiment showed it happens even at speeds familiar to us planet-bound creatures. Four cesium clocks were placed on commercial jetliners flying around the world. On the eastward leg of the trip, the jets traveled faster because the earth's spin in the direction of flight (1,000 miles per hour at the equator) added to their absolute speed. Flying west against the spin of the earth,the jets moved more slowly.

In principle, a jet flying at 500 miles per hour would have an absolute speed of 1,500 miles per hour when flying east at the equator. Flying west at 500 miles per hour, the Earth's spin subtracts from a jet's speed, and the jet's position in space actually moves backward at 500 miles per hour even though the jet is still moving forward relative to the surface of the earth.

In any case, results of the experiment showed that an eastbound cesium clock slowed by 332 billionths of a second relative to a westbound clock. That doesn't seem like much�only one second in about 35 days. But considering that the latest generation of these clocks is accurate to within one second in more than a million years, the relativistic effect is striking.

Doesn't relativity throw a cosmic monkey wrench into the efforts of scientists to define everything precisely in terms of time? Moreover, Einstein revealed that time is affected not only by velocity, but also by gravity. Each point in the universe is subject to a unique combination of gravitational fields. Add to that layer after layer of motion�a universe expanding, galaxies turning, planets orbiting stars and rotating on their own axes�and every point also has a unique velocity. So time must be unique at each point in the universe.

So what is this thing called time, that scientists can measure so precisely but seems never to be quite the same? Time is not a particle or a wave. And though we have identified fundamental fragments of matter and energy, as far as we know there is no fundamental fragment of time. How finely can we slice a single second? The answer appears to depend solely on how deft we are, and not at all on any natural limitation. Perhaps that's because time is not natural at all, but a product of our own imagination.

Yet we pour our lives so completely into the mold of time that its reflection in our lives seems real�at least, so long as it remains in the periphery of our consciousness. But if we stare directly at its reflection, searching for a clue to its essence, it evaporates like a mirage. Time is a riddle that has mystified philosophers and scientists throughout the ages. And a riddle it may remain. After all, who has the time anymore to ponder it?