Advances in the Development of the Experiment to Test the Weak Equivalence Principle in Free Fall

Abstract. We describe specific advances in the analysis and development of an experiment to test the Weak Equivalence Principle in free fall inside a capsule (Einstein elevator) released from a stratospheric balloon. The accuracy goal of the experiment is a few parts in 10^15. The attainment of this accuracy would improve the present results in testing the Equivalence Principle by two orders of magnitude. The measurement technique calls for a detector with two sensing masses of different materials, spinning about the detector horizontal axis, to free fall inside the capsule/cryostat for about 25 s. We focus here on the effects on the measurement accuracy of gravity gradients produced by nearby masses, which are non-corotating with the detector. The results of our study on gravity gradients generated by the distributed mass of the capsule/cryostat yield requirements on: (a) the size of the cryostat; (b) the tolerable verticality error of the capsule during the fall; and (c) the centering of the center of masses of the detector’s sensing masses. These analyses on the mass distribution of the experiment carrier have moved the project towards viable design options for the capsule/cryostat.