Precession Caused by Gravitational Waves.

We show that gravitational waves cause freely falling gyroscopes to precess relative to fixed distant stars, extending the stationary Lense-Thirring effect. The precession rate decays as the square of the inverse distance to the source and is proportional to a suitable Noether current for dual asymptotic symmetries at null infinity. Integrating the rate over time yields a net rotation-a "gyroscopic memory"-whose angle reproduces the known spin memory effect but also contains an extra contribution due to the generator of gravitational electric-magnetic duality. The angle's order of magnitude for the first Laser Interferometer Gravitational Wave Observatory signal is estimated to be Φ∼10^{-35}  arc sec near Earth, but the effect may be substantially larger for supermassive black hole mergers.