Testing relativity with a laser-cooled cesium clock in space

NASA has recently funded a collaborative project between the National Institute of Standards and Technology and the University of Colorado to develop a laser-cooled cesium clock for scientific and technical applications in space. The clock design studies are de scribed in these proceedings by S. Jefferts et al. The greatly improved performance which is projected for such clocks provides new opportunities for testing predictions of special and general relativity. Significant improvements are possible in the measurement of gravitational frequency shifts, in determining the limits on detectability of motion relative to a preferred frame from a Kennedy-Thorndike interferometer experiment, and in tests of the principle of local position invariance. Experiments like that of Michelson and Morley, and tests of local Lorentz invariance (time dilation), can be performed with limits comparable to the best results obtained on earth. This paper discusses what could be achieved with such experiments using a cc slum clock having a projected long-term stability of less than 10(-16) at one day, and a short-term stability of less than 10(-15) at about 10 minutes, corresponding to a time dispersion of about 0.25 ps characterizing clock Performance during one pass over a ground-based observing station. Requirements on spacecraft tracking, and on time transfer precision between a clock in low earth orbit and a ground-based time reference will be discussed.