The first known clock humans used was a sundial of some kind, which told us the time with as much precision as we needed: how high is the sun in the sky, and how long will it be till dark?
A sundial tells more exactly than the eye alone where our real clock, the sun, is. It’s not surprising that people use the sun’s position to give shape to the events of their lives, since all life owes its existence to the effects of the sun’s energy, and indeed evolved in rhythm with its presence and absence.
The first use of time, then, is to stay in touch with one’s physical environment and its cycle of light and dark, hot and cold, high tide and low tide, growth and decay: we must synchronize with the environment our own cycles of hunger, thirst, and sex.
For such uses of time we must know where the sun is. But people often want to synchronize themselves not only with the physical world, but with other people: to catch trains, work in offices, eat lunch together, and so on.
In order to coordinate our activities, we must give time a name that is understood by all concerned. Thus, the key difference between dealing with the natural and human worlds is that whereas the earth and sun tell us what time it is, we must agree with the rest of humanity on what time it is for time to mean anything.
For the entire technological world, the agreed keeper of time is the Bureau International de l’Heure (BIH) in Paris, which stays in constant contact with laboratories and observatories in seventy countries, all of which contribute to the official “correct time.”
The clocks at the BIH and at the headquarters of national organizations such as our National Bureau of Standards are impressive in their precision. Coordinated Universal Time (abbreviated UTC, for some reason), the standard they all keep, is exact to within a few billionths of a second.
The clocks used by the BIH and its members owe their regularity and accuracy to the properties of the element cesium, a silver colored alkali metal. A cesium clock is actually a machine that produces a tone of exact pitch and frequency.
An electromagnetic signal’s frequency is a statement of how often the photons that make it up vibrate each second. But since it is the second we want to define and maintain, we say instead that the second is some number of photon vibrations.
How can we fix vibrations within a time frame without consulting the clock we want to set? Cesium atoms perform an electromagnetic “flip”, their electrons, which spin like tops, suddenly point their axes of spin in a different direction, only at a certain frequency. If we agree that a second will be counted out at whatever frequency is required to make the cesium atoms flip, or resonate, we know that our frequency and therefore our second are as stable as the flipping property of cesium, which is very stable indeed.
The member nations of the BIH have agreed that 9,192,631,770 photon pulses, cesium’s flipping frequency, is a good length of time for a second, so that’s what a second is. For convenience’ sake, it’s very close to a second in the solar system 1/31,536,000 of the time it takes Earth to go around the sun, but the cesium atom is a more precise standard.
Having defined seconds, we can add them in computerized counters to correspond to recognizable times of day such as noon, midnight, and 4:00 A. M. But how do you tell people the time without time passing as you tell them?
Countries keep their own standard cesium clocks, which they set by bringing them into phase with a portable one brought from Paris in a plane, tended by a team of technicians to keep it “ticking” correctly.
Each geographical region of the world that uses exact time thus can have its own keeper of UTC and can broadcast its own time signals to users by radio. The users, in maintaining their own clocks, must allow for the time it takes the signal to reach them from its source. In the United States, the National Bureau of Standards broadcasts on station WWV in Fort Collins, Colorado. Radio waves are electromagnetic and travel at 186,000 miles per second (the speed of light), thus taking about .012 seconds to reach New York from Fort Collins.
Who needs to know the time to within billionths of a second? It is technology that has created the need for both the exact time and the hardware with which to keep it. In ship and air navigation systems, a craft’s position is gauged relative to broadcast towers thousands of miles to either side of it.
If a broadcast wave pattern sounds simultaneously from Washington, D.C., and Greenwich, England, and reaches a ship sailing in the North Atlantic, the ship can compute its position by calculating which “beep” traveling at the speed of light reaches it first, and by how many parts of a second. The time measurement must be accurate; an error of a hundred thousandth of a second can put the location estimate off by 2 miles.
The telephone company needs to know the exact time, too. It sends several conversations along a single wire, to save time and space, by multiplexing, breaking each conversation up into impulses much shorter than a word (in fact, lasting only a millisecond) feeding the interspersed “bits” in sequence along the wire, unscrambling the bits at the other end, and reassembling the conversations. The multiplex transmitter sending bits down the wire must be synchronized perfectly with the receiver sorting them out; otherwise, the different conversations will receive each other’s bits and get hopelessly garbled.
Others who depend on exact time measurement include television and radio stations, physicists, astronomers, and the electric power companies that supply customers with 60 cycle per second alternating current to run their electric clocks.
The resonance of the cesium atom is one of the most regular things we know, much more accurate than the turning of Earth itself, from which man originally derived his concept of time.
Earth has been slowing in its revolutions by about a second every year because the moon’s gravity drags at the oceans and causes friction; in some years the effect is greater than in others. We know this by keeping track of the positions of the stars and planets relative to the turning Earth. (Paleontologists who have measured fossils of ancient coral, which made annual growth rings as trees do, say that 600 million years ago, a day on the earth was only twenty one hours long.) A cesium clock, on the other hand, would take 370,000 years to lose a second. Thus, by the end of a year the solar system and a cesium clock can be in serious disagreement.
A compromise has been struck between the sun and our clocks: every year the BIH in Paris adjusts the time to conform to our relation to the sun, so that “high noon” in the sky remains noon on our clocks as nearly as possible. In recent years the BIH has added a “leap second” between December 31 and January 1, making one 61 second minute, to wait for the “sun time” to catch up. If the world speeds up, we may need short minutes every few years. Says Dr. James A. Barnes of the National Bureau of Standards in Boulder, Colorado: “When the error gets too bad, we have to reset the clocks, since it’s more difficult to reset the earth.”
If you need to know the time to a few billionths of a second, call the National Bureau of Standards; but don’t forget to allow for the time the signal takes to reach you at the speed of light.
