From Shockwaves to Gravitational Memory: New Approaches to Flat Space Holography

We discuss shockwave geometries and the connections to aspects of flat space holography. We first show how the ’t Hooft commutation relations of shockwave operators are equivalent to the commutation relation between soft and Goldstone modes familiar from celestial holography. We demonstrate this equivalence via a diffeomorphism that takes the shockwave metric to a memory metric.  Next we show that shockwaves from vacuum fluctuations, fixed by the ’t Hooft commutation relations, give rise to modular fluctuations with an area law ⟨ Δ K2 ⟩ = ⟨ K ⟩ ~ A/4 GN.  Finally, we discuss metric fluctuations from shockwaves produced by vacuum energy fluctuations, and consider observational signatures in interferometers.