Testing the Weak Equivalence Principle with a Free-fallExperiment from a Balloon

Abstract. The paper describes a free-fall facility, called enhanced g-zero, to be dropped from a balloon at an altitude of 40-45 km for testing the Equivalence Principle. The free-fall duration is 30 s for a non-propelled capsule. Present estimates indicate that this facility provides an acceleration noise, away from the walls of the evacuated capsule, not to exceed 10^-l2 g during the fall. The envisaged technique for testing the Equivalence Principle involves the measurement of differential accelerations between two test masses of different materials that are part of a high-sensitivity detector. The detector is housed inside an instrument package cooled at the temperature of liquid helium. This package is first spun about a horizontal axis at a frequency of typically 1 Hz, for providing gyroscopic stabilization and modulating the gravity signal, and then released inside the falling capsule. The estimated accuracy in testing the Equivalence Principle, with 95% confidence level, is 5 parts in lO^l5 in a 30-s free fall.