Relativity at four solar radii [using ion trap clocks]

In the strongly time dilated space-time curvature at four solar radii, time runs slower than on Earth by about one half microsecond per second. Three atomic clocks based on hyperfine transitions of Hg⁺ (Z=80), Cd⁺ (Z=48), and Yb⁺ (Z=70) are different in their electromagnetic composition (given by the Casimir hyperfine factor) and will be inter-compared during a solar flyby to determine whether clocks of different physical makeup will measure the same time interval in the strong space-time curvature near the Sun. The atomic clock hardware for the Space Time mission is a modification of the linear ion trap frequency standard (LITS) currently being deployed in the Deep Space Network stations worldwide. A laboratory prototype has shown ultra-stable operation in a package far smaller than other clock technologies and represents the state of the art for atomic clocks. The instrument for this mission is composed of three ion trap clocks in a package where much of the hardware is common to all of the clocks. Because some of the clock systematic frequency perturbations will be common to all three clocks and will have a characteristic signature that can be identified and removed from the difference of the clock frequencies, relative stability´s to 10^-16 in the inter-comparison can be reached, making a 10^-10 test of the Equivalence Principle. The local oscillator will simultaneously interrogate each of the three clock transitions thereby removing LO noise in the inter-comparison, greatly improving short term clock noise so that 10^-16 resolution in the difference in clock rates can be obtained within the 15 hour close encounter. Because ion trap based clocks are relatively immune to temperature and magnetic field changes, a simple, robust electronics package is sufficient for ultra-stable operation