Testmassen

Testmassen (test masses) are physical bodies used in experiments to probe external forces, such as gravity or electromagnetism, while perturbing the field as little as possible. In many contexts a test mass is treated as a point-like object with negligible own field compared to the field it experiences, so its motion serves as a faithful indicator of the external force or metric.

In gravitational physics and general relativity, a test mass is expected to follow a geodesic of spacetime in free fall, provided its own gravitational influence is negligible and tidal effects are small. This concept underpins the interpretation of measurements of gravitational fields and spacetime curvature, as the mass itself does not significantly distort the field being studied.

On Earth, test masses appear in precision experiments and gravitational-wave detectors. In LIGO and Virgo, for example, the end mirrors acting as test masses are each about 40 to 42 kilograms and are suspended in ultra-high vacuum optical cavities. They are designed to be as nearly freely falling as possible, isolated from seismic and other disturbances so that passing gravitational waves induce tiny changes in the light phase between the mirrors.

In space, drag-free missions employ freely floating test masses to provide an inertial reference. LISA Pathfinder used two approximately 1.96 kilogram gold–platinum cubes as test masses inside a gravitational reference sensor; the spacecraft used micro-thrusters to follow the masses, creating a nearly undisturbed free-fall environment for detecting low-frequency gravitational signals.

Key design goals for test masses include minimizing non-gravitational forces, controlling charge and magnetic interactions, and ensuring symmetric mass distribution to reduce spurious effects.