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Clock to Be Tested in Microgravity

Photonics Spectra
Aug 2002
Brent D. Johnson

When the "primary atomic reference clock in space" project is launched in 2005, it will measure the relativistic effects of gravity on time. The project is the first flight to the international space station of NASA's Laser Cooling and Atomic Physics program, part of the Fundamental Physics in Microgravity program.


The primary atomic reference clock will be launched into space to study the effects of gravity on time. Its ticking rate will be compared with that of the National Institute of Standards and Technology clock on the ground.

The last time an experiment of this type was attempted was in 1976 when a hydrogen maser clock was launched on a Scout D rocket by the Smithsonian Astrophysical Observatory in Cambridge, Mass. Gravity Probe A monitored the continuous rate of the clock as it traveled farther away from Earth's gravity. However, the primary atomic reference clock experiment is different in that it will compare two nearly perfect clocks -- one on the ground and one in orbit -- each designed to lose less than one second in 60 million years.

To test predictions

"Relativity predicts that time is fundamentally altered by gravity," said Bill Klipstein, the project scientist. Because he and Don Sullivan, co-principal investigator, believe that relativity breaks down at some level, they plan to use global positioning system satellites to compare the rate at which the cesium-based clock ticks on the international space station with the rate of the clock at the National Institute of Standards and Technology, which is used to define the base unit of time. There are different theories that try to unite the forces of nature, Klipstein said. The project will test predictions about the nature of time and space to help uncover a final theory of gravity.

This kind of measurement requires stable instrumentation. In a ground test of its latest atomic clock intended for the international space station, the Jet Propulsion Laboratory is using a Vortex external-cavity diode laser from New Focus to cool the cesium atoms. Klipstein said that the researchers are using this laser because they need high reliability, tunability and a narrow frequency that hits the cesium wavelength.

One advantage of the laser is that it runs a single frequency without mode hopping. The single-transverse mode is required for stable delivery of laser light via single-mode optical fibers to the clock. The lasers are employed to collect, cool and "toss" atoms through the clock, and for the primary detection of the clock signal.

However, operation of a laser in space presents problems. The length of the resonator cavity changes in a vacuum, which must be accounted for. In addition, the device must perform for two years without intervention.

The scientists are testing the components for temperature cycling -- so far, with positive results. They also put the laser on a shake table to simulate the effects of the space shuttle launch, and it survived without modification.


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