The concept of gravitational-wave detection is based on monitoring two freely falling bodies. As long as all other disturbances can be sufficiently reduced and the two bodies are truly moving under the effect of gravity only, a gravitational wave passing between them would change their separation. LISA Pathfinder will test the underlying and most challenging condition for such experiments: whether it is possible to put two test masses into a near-perfect gravitational free-fall.
Even in space, realising a freely falling system is very complex. There are many non-gravitational forces at play, including radiation pressure from sunlight, charged particles from the solar wind and impacting micrometeoroids, as well as internal effects caused by the spacecraft and its instruments.
For this reason, LISA Pathfinder is a high-tech box that surrounds two freely falling test masses without touching them, shielding them from outside influence by constantly applying tiny adjustments to its position.
LISA Pathfinder is not aimed at the detection of gravitational waves themselves. Rather, its goal is to prove the innovative technologies needed to reduce external influences on two test masses and to measure their relative motion with unprecedented accuracy, tracking their free-fall by more than two orders of magnitude better than any past, present or planned mission.
LISA Pathfinder will create the most ‘silent’ place in the Solar System and measure how quiet it actually is.
Source: ESA
