LISA Pathfinder proof-mass under test

This photograph shows the LISA Pathfinder gold-platinum proof-mass being placed over the high sensitivity balance used at the International Bureau of Weights and Measures (BIPM, France) to measure the proof-mass's magnetic susceptibility .

Once in space, LISA Pathfinder's proof-masses will be used to detect forces as small as the weight of a typical bacterium, and test the theory of General Relativity.

The perfect proof-mass is one that responds only to the force of gravity. Therefore, scientists needed to precisely measure how the LISA Pathfinder proof-masses respond to magnetic forces so that they can compensate for this in their calculations.