The purpose of the PARCS project is to place an advanced laser-cooled cesium atomic clock in orbit and utilize it to test a variety of predictions of the Theory of Relativity. One of these predictions, made by Albert Einstein in 1915, is that clocks tick slower in strong gravity than they do in weak gravity. An orbiting satellite might place PARCS at an altitude of 220 miles (360 kilometers), where gravity is slightly weaker than that found at the Earth's surface. Thus the PARCS clock aboard the satellite ticks faster than a clock on the surface of the Earth by about 1 second in every 10,000 years.

What We Hope to Find Out:

Such tiny shifts have already been observed in previous experiments-the aim of PARCS is to measure them about a hundred times more accurately than ever before. To observe such a small change in clock rate requires extremely accurate clocks, both in orbit and on the ground. PARCS will be the most accurate clock ever built, keeping time to within 1 second in 300 million years. It will be compared to the master clock of the United States, which is at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. Principal Investigators for the PARCS project are from NIST and the University of Colorado.

An atomic clock consists of two major components: an oscillator, which produces a stable frequency (in other words, something that produces a steady series of "ticks"), and a "frequency checker", which compares that frequency to the natural frequency of an atom. For PARCS, the oscillator will itself be a highly stable atomic clock, a hydrogen maser built by the Smithsonian Astrophysics Observatory. The frequency checker part of the apparatus consists of a beam of very cold cesium atoms, which pass through a pair of microwave cavities, which are used to very accurately measure the natural frequency difference between two internal energy levels of an atom. The hydrogen maser frequency is checked against this frequency and then compared to that of a clock on the ground. Because every cesium atom of the same isotope is identical, we can be sure that any differences in frequency that we see between a clock on the ground and one in orbit are due to relativistic effects.

To achieve the very high accuracy required, PARCS will use atoms that have been cooled to a temperature of just 1 millionth of a degree above absolute zero. This is achieved using a technique called "laser cooling." Photons from several laser beams, each coming from a different direction, bounce off of the atoms, giving the atoms a small push with each bounce. These small pushes serve to slow down the atomic motion, resulting in dramatically cooler temperatures. These lower temperatures allow the natural frequency of the atom to be measured much more accurately. Further improvements can be made by performing the experiment in space. Because objects in orbit are all freely falling (this is what produces the phenomena of "weightlessness"), the atoms can be observed for a longer time before they hit the walls of the container. The longer measurement times yield more precise clocks. As Werner Heisenberg showed in 1927, this longer observation time allows for a more precise measurement of an energy level (this is called "the uncertainty principle for time and energy").

How We'll Conduct Our Experiment: