As the gravitational wave propagates perpendicular to the plane of inertial masses (in free fall), it is displaced by an amount proportional to the gravitational wave strain. After the gravitational wave has passed, the masses are permanently displaced, due to the gravitational memory effect.[1]
Gravitational memory effects, also known as gravitational-wave memory effects are predicted persistent changes in the relative position of pairs of masses in space due to the passing of a gravitational wave.[2] Detection of gravitational memory effects has been suggested as a way of validating general relativity.[3][4]