Keeping Time
Time marches on, the vintage newsreels blare. We can't stop it. We have real trouble defining it. But we have done a remarkable job of figuring out how to measure its passage.
Admittedly, the first step was fairly obvious. The Sun rises and sets. We wake, we sleep. Then we wake again: Day 2. Soon, however, early humans began to notice larger increments. The cycles of the moon produced the month. The return of seasons yielded the year. Recognizing these patterns would quickly prove crucial to the development of agriculture and trade.
© 2004 James Collins
Kurt Gibble with his atomic clock
Breaking the day into sub-units took a little longer. In due time, however, our ancestors were parsing out hours, and minutes, and seconds. These details — and precision in measuring them — became particularly important for the business of navigation. For mariners judging longitude by the stars, knowing the exact time was crucial to getting a bead on where you were.
Galileo gets credit for realizing that a pendulum's period — the time it takes to swing back and forth - is of constant duration. But it was Dutch astronomer Christiaan Huygens, in 1656, who first applied this idea as a useful measure. By harnessing a swinging pendulum to an interlocking series of gears, Huygens showed, you could keep count of all those periods, and translate the "ticks" into moving hands on the face of a clock.
The Atomic Age
"Atomic clocks work on the same principle," says Kurt Gibble, associate professor of physics at Penn State, who gave the third lecture in the 2004 Frontiers of Science series. According to the laws of quantum mechanics, Gibble explained, atoms can only have discrete, sharply defined energies. "They have to be either in one state or the other. They can't be in-between." By shining light on an atom in a low energy state, you can excite it, driving it to a higher energy state. But this transition, for a given atom, occurs only at precisely what is known as that atom's resonant frequency. The incoming light has to match this frequency exactly, or the atom will not be excited.
Every atom of a given type has the same resonant frequency. An atomic clock takes advantage of this universality, deploying the chosen atom (actually a gaseous cloud of atoms of the same type) as an extremely accurate frequency regulator. The transition-triggering frequency of the light, kept stable by the regulator, stands in for a pendulum.
Nobel laureate Isidor Rabi hatched the idea for an atomic clock back in the late 1930s, as an offshoot of his pioneering work on the fundamental properties of atoms and molecules. By 1949, the National Bureau of Standards (now the National Institute of Standards and Technology, or NIST) had produced the first such timepiece, using an ammonia molecule as its core. Since then, better and better atomic clocks have been built using atoms of cesium 133, a heavy element whose high resonant frequency produces a high tick rate. (Counting more ticks over a given interval, Gibble explained, allows you to make a more accurate clock.) In a standard atomic clock, a gaseous beam of cesium atoms is fired through a vacuum chamber, where it is zapped with brief pulses of light — actually, microwave energy. If the pulses are of the right frequency, they goose the atoms into changing states.
By 1967, the precision of such clocks had become so refined that the definition of a second was changed. No longer would time's fundamental unit be based on dividing up the period of Earth's rotation, a duration which can be slightly affected by physical forces like friction. Instead, "a second is defined by measuring the transitions of a cesium atom exposed to microwave radiation at the proper frequency." By international agreement, that's exactly 9,192,631,770 ticks per second.
Just Chill
The best of the beam atomic clocks, called NIST 7, is a thousand times more accurate than the 1967 version. Amazingly, according to Gibble, it's dead-on to within one second over six million years. And recent advances in physics have uncovered several approaches to even greater precision.
"NIST 7 uses room-temperature atoms," Gibble explained, "At room temperature, atoms move at the speed of sound, or a jet airplane." At that velocity, it took only a thousandth of a second for a cesium atom to fly through the ten-foot tunnel that houses NIST's vacuum chamber. "That brief observation period limits the accuracy of the clock."
Now, however, there's a good way to get those atoms to chill out. In 1997, Stephen Chu, Claude Cohen-Tannoudji and William Philips won a Nobel prize in physics for their work on a technique called laser cooling. If you bombard atoms with laser light tuned to just the right frequency, Gibble explained, "the momentum of the atom is taken away by the momentum of the light." It's not unlike the way velocity is transfered when two pool balls collide. And the result of losing speed is rapid cooling.
By this technique, Gibble said, "You can cool a gas of atoms to within one one-millionth of a degree of absolute zero — and you can do it in a thousandth of a second. In that fraction, "the atoms go from the speed of a jet airplane to the speed of an ant."
Clock-makers have learned to lengthen observations even more by configuring their instruments vertically, as atomic "fountains." In a fountain set-up, target atoms are cooled and collected at the bottom of the vacuum chamber by a set of six lasers placed opposite each other and aimed toward the center. Then a small change in frequency sends the huddle of atoms up through a microwave trap, where they undergo the tell-tale transition. All the lasers are then switched off, and the atoms fall back down through the trap.
"It looks like a water fountain," Gibble said, and — by making use of gravity — it doubles the available observation time. "You get about a half second on the way up, and another half second on the way down. With a full second to look at the atoms, you can make a very accurate clock."
But cooling atoms to near absolute zero also creates some distracting quantum effects. At very low temperatures, he noted, atoms look more like waves than they do particles. "They get bigger, and when they're bigger, they're more likely to collide — and every time there's a collision it gives your pendulum a little kick."
To Infinity and Beyond
To get around this problem, Gibble and his graduate student Chad Fertig designed and built a clock that substitutes rubidium atoms for the standard cesium. They showed that "rubidium atoms don't collide as often," Gibble explained, "and their collisions don't have as great an effect." This work has led other groups around the world to build similar clocks based on rubidium.
One of these clocks, built by French researchers, is currently the world's most accurate clock. Rubidium clocks now under construction at the U. S. Naval Observatory will soon be the basis for Earth's Global Positioning System.
In the future, Gibble added, rubidium-based clocks could be capable of an accuracy that is difficult to imagine: within five seconds over the entire life of the universe. "Better clocks," he said, will have potential impacts in many areas, from advanced communication systems, to navigation, and to better tests of the theory of general relativity. For interplanetary navigation -landing on Mars, for example - clocks of unprecedented accuracy will be essential.
That's why Gibble, with colleagues at NASA's Jet Propulsion Laboratory, has been working on a rubidium clock that is deployable in space. By removing the limiting factor on an Earth-bound atomic fountain - gravity -a space-based clock will yield much longer observation times, up to ten seconds, Gibble said.
In addition, he said, he and his clock-making colleagues are working on other approaches: fine-tuning laser cooling techniques to allow time-lapse "juggling" of atoms in atomic fountains; and using ultra-short pulse lasers that allow researchers to count very high tick rates, the ticks of laser light. Fundamentally, he said, there's no end to the possible improvements:
"There's no known limitation to how accurately we can measure time."
