Development of Components for Implementation of Free Fall Experiments to Test the Weak Equivalence Principle

Abstract

In this work I describe the development of an electrostatic positioning system (EPS) for improving the starting conditions of the test masses in a free fall experiment to test the weak equivalence principle. The equivalence of intertial and gravitational mass is one of the fundamental principles in physics. One version of this principle is the weak equivalence principle (WEP), also known as Universality of Free Fall (UFF). It says, a test mass with neglectible self gravitation behaves independently of its properties in a gravitational field. In the recent centuries, the WEP has been investigated by using different methods. My literature recherche shows, that surprisingly few free fall experiments using macroscopic test masses have been performed. And their results have not reached the accuracy of other methods jet. At the drop tower Bremen, a free fall experiment using high precision, Supraconducting Quantum Interference Device (SQUID) sensors was developed. In 2001 it tested the WEP by measuring the difference between the accelerations of two test masses falling freely with an accuracy up to $ eta 10 -7 $. By improving the different components of the Experiment and especially the starting conditions of the test masses, an improvement in the accuracy of several orders should be possible. The center of mass of the test masses should be positioned in the same place within 0.3 mircometer und their velocities should be minimized to 0.1 m/s. Therefore, a electrostatic positioning system (EPS) in axial direction was developed und the first results of the characterisation of the protoptype are presented in this work. The experimental set-up and the controller are described in detail. Characteristic parameters of the system are defined and the experiments to determine them are described. By using a high precision weighing balance, the force exerted on the test mass by the EPS was measured. This results help to modell the system. Measurements of different positionings of the test mass were used to determine the position resolution of the EPS and other characteristic parameters. A suprising result of these measurements is the EPS can be used with and without the wire connected to the test mass for electrically grounding it. The parameters of the controller had not been changed between both measurements and only the scaling factor to convert the voltage signals into position signals changes slightly. The results of the characterisation provide new aspects for simplifing and improving the axial EPS and developing an radial EPS in the future. The combination of both EPS will improve the starting conditions of the test masses and therefore the conditions for the SQUID sensors significantly. This will be a main step in improving the accuracy of free fall test of the WEP up to $ eta 10 -13 $.