  Gravitational shockwaves are simple exact solutions of Einstein equations
representing the fields of ultrarelativistic sources and idealized
gravitational waves (shocks). Historically, much work has focused on shockwaves
in the context of possible black hole formation in high energy particle
collisions, yet they remain at the forefront of research even today.
Representing hard modes in the bulk, shocks give rise to the gravitational
memory effect at the classical level and implant supertranslation (BMS) hair
onto a classical spacetime at the quantum level. The aim of this paper is to
further our understanding of the `information content' of such
supertranslations. Namely, we show that, contrary to the several claims in the
literature, a gravitational shockwave does leave a quantum imprint on the
vacuum state of a test quantum field and that this imprint is accessible to
local observers carrying Unruh--DeWitt (UDW) detectors in this spacetime.
