What is time?
Ask six people to explain it today, and you may get six answers. A physicist, for instance, might say time is one of the two basic building blocks of the universe, the other being space. For a banker time is money. For a Buddhist monk time is contemplated in nature's eternally returning cycle.A graffito on a cafe wall in Austin, Texas, attempted an answer: "Time is nature's way of keeping everything from happening all at once."
"We have given more attention to measuring time than to anything in nature," says Gernot Winkler, director of time services at the U.S. Naval Observatory in Washington, D.C. "But time remains an abstraction, a riddle that exists only in our minds."
In reporting on the enigma of time in the current National Geographic, John Boslough writes, "We have come to place a premium on measuring the flow of time - as if by measuring it we could begin to understand it."
Today more than half a billion watches a year pour off assembly lines, with some 300,000 sold daily in the United States alone. The world's largest timepiece manufacturer, Japan's Seiko Group, produces more watches than all Switzerland.
More timepieces have been built, Boslough theorizes, than all other machines combined.
Without the clock, and now the watch, there is no modern world, says David Landes, a historian of timekeeping at Harvard University. "It is the difference between a complex, intricately coordinated society and a primitive one only vaguely aware of time's possibilities."
A vast global timekeeping system keeps all civilization synchronized - to within a billionth of a second a day.
In today's technological world, a navigator at sea or aloft, plotting his location by satellite, relies on a time signal accurate to within a single millionth of a second. Deep space probes are guided by radio signals timed to billionths of a second (nanoseconds).
Physicists tracking motion inside an atomic nucleus reckon in picoseconds (trillionths of a second) or even femtoseconds (thousandths of a picosecond). There are more femtoseconds in one second than there were seconds in the past 31 million years.
At some 50 timekeeping stations around the world, atomic clocks allow this remarkable splitting of seconds by counting the oscillations of atoms to tell time. The most precise U.S. atomic clock, located at the National Institute of Standards and Technology in Boulder, Colo., is accurate to within one second in 300,000 years.
A second is virtually infinite, says David Allen, a time theorist at the institute. It can be split into as many pieces as technology allows.
The notion that time is absolute was shattered by Albert Einstein's special theory of relativity. For Einstein time depends on how fast one is traveling. Events determine how fast time passes instead of the other way around.
Not time, but the speed of light (186,282 miles a second) is the only absolute, explains theoretical physicist Stephen Hawking of England's Cambridge University.
In a sense the speed of light is Einstein's master clock, since there can be no velocity without time and the speed of light never varies. So the speed of light is the best way to measure a distance.
Yet gravity affects both light and time. In the universe of Einstein's relativity, gravity is caused by the intrusion of a large chunk of matter like the sun into space and time.
Gravity bends both space and time. In the presence of a large stellar object, light has to travel farther between two points, and time slows down, relativistically speaking. This presents a problem. On the surface of Jupiter, with 318 times the mass of Earth, an atomic cesium clock would run noticeably slower.
A black hole, a superdense body created when a massive star collapses of its own gravitational pull, has a gravity field so intense that not even light can escape its surface. There time would stand still.
"This shows us," says John Wheeler, the Princeton University physicist who gave black holes their name, "that time is a measuring tool, not an absolute flow or a substance."
Without an event, there is no time. This means, Wheeler believes, that time may be a secondary feature of nature, not a basic one. "But we're only talking about the physics of time here," he notes.
What is time? Boslough concludes: "I believe that Professor Wheeler's concept of time comes the closest to an actual explanation: that it is a dimension and, like any other dimension, only a secondary quality in nature."
But, he adds, humans have internalized it so powerfully that time has taken on a meaning all its own.