Wed 27 Sep 2006
Nuts About Time? Get Into an Atomic ClockPosted by glenn under Time
DRU SEFTON, Newhouse News Service, reports:
Let us now take 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium-133 to pay tribute to the atomic clock.
That would be one second.
Fifty years ago this October, the atomic clock first became available commercially. It and its descendants use that cesium frequency to keep time, accurately measuring to one-billionth of a second.
“People might say, a billionth of a second? Who cares? What does this have to do with the price of cheese?” said Geoff Chester, spokesman for the U.S. Naval Observatory in Washington, D.C., America’s official timekeeper.
But if the cheese is discussed in an e-mail or cell phone call, that billionth of a second has plenty to do with its price.
E-mail messages and cell phone calls, among other things, use time stamps — precise markers that enable voice and data to be reassembled correctly once they’ve arrived at wherever they’re going. And those time stamps need to be as exact as possible.
So that’s one reason the Atomichron — that first commercial atomic clock — was so important. The hulking machine was unveiled on Oct. 3, 1956, by its developer, MIT physicist Jerrold Zacharias, and his collaborator Richard Daly Jr. at the Overseas Press Club in New York City.
“They probably couldn’t have imagined how small it would get to be,” said Paul Forman, a curator of the modern physics collection at the Smithsonian’s National Museum of American History in Washington. “They worked very hard to bring it down to even that huge size, from room-size laboratory models.”
Nowadays, atomic clocks on Global Positioning System satellites (each has four) are the size of credit cards. The Time and Frequency Division of the National Institutes of Standards and Technology (NIST) in Boulder, Colo., has even developed computer-chip-size atomic clocks.
All of which is quite captivating to the Time Nuts.
A Time Nut, according to Tom Van Baak, is a precision-time hobbyist. They collect these super-accurate timepieces, examine and dissect them, tinker with them to make them run, well, even better than super-accurate.
As Van Baak says on his Web site, www.LeapSecond.com: “The best clocks in my collection … are accurate to better than one microsecond per year. Excluding national government laboratories, my home time lab now has the most accurate clock in the world.”
Time Nuts are not preoccupied with time per se, as in being on time to appointments. They’re into the mechanics of time measurement.
“There are people that race cars, and they’re trying to get every last mile-per-hour out of their engine,” said Van Baak, of Bellevue, Wash. Time Nuts try to hone their timepieces in the same manner, getting ever more accurate.
An atomic clock — of which he has many — has nothing to do with nuclear power, Van Baak explained. “Atomic means that the clock keeps its time not on something large like the sun, or the Earth turning, or the wheel of a watch, but something much, much smaller and much less influenced by external things: an atom. That’s pretty immune to being dropped or bumped, which could upset a mechanical timekeeper.”
Nor are atomic clocks very exciting to watch. “They just sit there,” Van Baak said. “They don’t hum, they don’t tick, there are no moving parts.”
And those “atomic clocks” available for $20 or so aren’t really.
“That’s essentially a radio-controlled, or self-setting, clock,” Van Baak said. A transmitter at the NIST in Boulder broadcasts an atomic-clock signal of time. “The tiny radio receivers in these clocks listen for that signal, then set the clock from it.”
So the Time Nuts obviously look down on these pseudo-atomic clocks, right?
“No, not at all,” Van Baak said. “They get out their magnifying glasses and figure out how these little clocks work. A number of us have taken apart these radio-controlled clocks.”
Pretty soon the Time Nuts will be fiddling around with rubidium fountain clocks. Those are the latest generation of atomic clocks, nearing completion at the Naval Observatory. With them, “in theory,” spokesman Chester said, “we can keep a continuous time scale to a trillionth of a second.”
Rubidium, like cesium, is an element in the alkaline metal family, but it’s more commonly used nowadays for precision timekeeping.
“Cesium atoms were best for the guys to work with in the 1950s, they behaved better under laboratory circumstances,” Chester said. Now, due to “certain quantum mechanical effects way too complicated to explain,” rubidium is superior.
However, rubidium fountain clocks — their atoms are propelled upward and cascade down like a fountain — are also “environmentally cantankerous. They’re sensitive to temperature and humidity changes,” Chester said. So a special building is being constructed on the observatory grounds to house the clocks, which should be online by next year.
Meanwhile, Van Baak continues his quest for ever more exact timekeeping.
“It has happened that I wake up on a Tuesday morning all excited because one of my clocks is only off by seven nanoseconds, but I completely forget it’s garbage day,” he said.
Sept. 27, 2006
(Dru Sefton can be contacted at )