Just like ancient observers, NASA scientists will soon look to the sky to tell the time.
By 2006, the world's most accurate timekeeping device should be in orbit on the International Space Station. Scientists hope the Primary Atomic Reference Clock in Space, or PARCS, will test the limits of Einstein's theory of general relativity and improve future international timekeeping by being the most precise clock to be taken out of the time-obscuring effects of gravity and magnetism.
Today, the most accurate clocks around are two Earth-bound cesium fountain clocks, used to define the length of a second. They're used to correct deviations that arise in the international timekeeping effort, work undertaken by the Bureau of Weights and Measures in France, which takes time estimates from 200 less exact atomic clocks in 50 countries.
The bureau distributes its measurements around the world through satellite and Internet transmissions.
Now on Earth, and soon on the ISS, cesium clocks operate by tossing a ball of cesium atoms up a meter-long copper tube into a microwave-emitting chamber. The microwaves excite the cesium atoms so that they vibrate.
"Think of it like a pendulum clock, where a pendulum swings back and forth at a certain resonance," said Donald Sullivan, the PARCS experiment's lead scientist. "To tell time with the clock, you must find the center of each swing."
The challenge
NASA scientists say that putting an atomic clock in space will eliminate some of the perturbations the scientists face in using a fountain clock on earth.
"There are a number of non-ideal effects on Earth, but we have a definition of the second that is based on a perfect environment. In space, we get closer to that environment," Sullivan said.
That environment for NASA will be the External Facility of the Japanese Experimental Module section of the International Space Station, where PARCS will be installed.
On Earth, gravity causes the atomic clock's cesium atoms to fall away from the detector almost immediately. This makes it more difficult to find to the center of the atom's swing. In microgravity, Sullivan said, the cesium atoms will move five to ten times slower. The longer interrogation time will allow scientists to find the center of the atom's swing more precisely and more often.
"The longer the atoms are in the interrogating field, the sharper the calculation of the second will be," Sullivan said.
Also, the slower pace will lessen what's called the Doppler effect, in which the faster changes in distance between an object and an observer distort frequencies more than slower changes. The slower cesium atoms will be much like the difference between the sound of horns coming from two cars, when one car is moving at 50 mph and the other at 10 mph.
Earth's magnetic field can also affect readings by causing the atoms to bump into each other, an effect called collision shift, Sullivan said. This will also be decreased on the space station.
Next Page: The real goal is to test time vs. gravity
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