Test mass metrology for tests of the equivalence principle

Abstract

The Equivalence Principle is accepted as one of the most fundamental principles in modern Physics. However, theories towards the unification of the four forces typically predict violations of this principle. Testing it at a high sensitivity is expected to make a breakthrough in the current understanding of Physics. A space-based project, STEP (Satellite Test of the Equivalence Principle), aims at testing the principle to the level of 10\(^{−18}\). This corresponds to an improvement of the current limits, established by ground-based experiments, by approximately five orders of magnitudes. To achieve the sensitivity, imperfections in STEP test masses, such as density inhomogeneity and thermal distortion, could be a problem. This thesis presents preliminary work on the verification of STEP test masses. We have measured density inhomogeneities in materials intended to be used as STEP test masses (beryllium and niobium). In addition, we have developed a device to measure differential thermal expansion of samples that cannot be machined, by using a capacitive sensing method. It is shown that the device has a precision of approximately 0.3 % in the differential thermal expansion of beryllium. This device could in principle be applied for the measurements of the real STEP test masses in the final shape. Our analysis based on the results of our measurements and literature survey shows that it is feasible to obtain materials that satisfy STEP requirements.