Methodology and instrumentation for testing the weak equivalence principle in stratospheric free fall

The use of the GiZero free-fall facility for testing the weak equivalence principle is discussed in this article. GiZero consists of a vacuum capsule, released from a balloon at an altitude of 40 km, which shields an experimental apparatus free falling inside the capsule itself. The expected residual acceleration external to the detector is 10(-12) g (with g the Earth's gravitational acceleration) for the 30 s free fall. A common-mode rejection factor of about 10(-4) reduces the residual noise differential output to only 10(-16) g. The gravity detector is a differential accelerometer with two test masses with coincident center of masses (i.e., zero baseline) with capacitive pick ups. Preparatory experiments have been conducted in the laboratory with a precursor detector by measuring controlled gravity signals, at low frequency, and by observing the Luni-Solar tides. The estimated accuracy in testing the weak equivalence principle, with a 95% confidence level, is 5 x 10(-15) in a 30 s free fall. When compared to orbital free-fall experiments, the GiZero experiment can be considered as a valid compromise which is able to satisfy the requirement for improving significantly the experimental accuracy in testing the equivalence principle with a substantial lower cost, the ability to recover the detector and to repeat the experiment at relatively short time intervals. (C) 1998 American Institute of Physics. [S0034-6748(98)00412-2].