Drop tower tests of the Weak Principle of Equivalence — One step to space missions for gravitational physics

Experiments on the Drop Tower Bremen aim at a proof of the Weak Equivalence Principle and the determination of the Eötvös factor with an accuracy of less than 10−12. The Drop Tower Bremen offers the unique possibility for this pseudo-Galilean test by dropping two cylindrical bodies of different composition and observing their relative movement. To measure the extremely small displacements, a SQUID-based position detector with an ultra-high resolution has been developed. The low-temperature SQUIDs are unaffected by repeated thermal cycling and have an extremely low-noise-limited resolution of 2 × 10−12m/√Hz. The experiments serve not only to prove the Weak Equivalence Principle with an accuracy not attained before, but also to test and develop components for future space missions aimed at carrying out precise gravitation experiments and geodetic measurements. This report focuses on recent experimental results concerning the position control and measurement.