The freest of free falls

The European Space Agency plans to launch a gravitational-wave observatory in 2034, likely based on the Laser Interferometer Space Antenna (LISA) model. Once it is operational, mission scientists will look for tiny, oscillating changes in the million-kilometer lengths separating a triangular arrangement of test masses—the signal that a gravitational wave has passed by. The LISA concept requires that the test masses at each vertex be maintained in nearly perfect free fall. Any other accelerations—due to stray charges or radiation pressure, for example—could mimic the effects of a gravitational wave; the test masses must therefore be exquisitely shielded from all forces other than gravity. Enter LISA Pathfinder, launched on 3 December 2015 and designed to establish that a pair of test masses floating inside the spacecraft could be shielded to the exacting demands of LISA. Today the mission team announced its first result: To within 25%, it has already achieved...