The memory effect is a prediction of general relativity on the same footing
as the existence of gravitational waves. The memory effect is understood at
future null infinity as a transition induced by null radiation from a
Poincar\'e vacuum to another vacuum. Those are related by a supertranslation,
which is a fundamental symmetry of asymptotically flat spacetimes. In this
essay, I argue that finite supertranslation diffeomorphisms should be extended
into the bulk spacetime consistently with canonical charge conservation. It
then leads to fascinating geometrical features of gravitational Poincar\'e
vacua. I then argue that in the process of black hole merger or gravitational
collapse, dramatic but computable memory effects occur. They lead to a final
stationary metric which qualitatively deviates from the Schwarzschild metric.Comment: 5 pages + bibliography. Honorable mention at the Gravity Research
Foundation 2016 Essay Contes
