Gyroscopic Gravitational Memory

Understand

We study the motion of a gyroscope located far away from an isolated gravitational source in an asymptotically flat spacetime.

• As seen from a local frame tied to distant stars, the gyroscope precesses when gravitational waves cross its path, resulting in a net "orientation memory" that carries information on the wave profile.
• At leading order in the inverse distance to the source, the memory consists of two terms: the first is linear in the metric perturbation and coincides with the spin memory effect, while the second is quadratic and measures the net helicity of the wave burst.
• Both are closely related to symmetries of the gravitational radiative phase space at null infinity: spin memory probes superrotation charges, while helicity is the canonical generator of local electric-magnetic duality on the celestial sphere.