Gravitational waves stretch and squeeze space, but even when the wave has passed there is a residual stretch and squeeze known as gravitational wave memory. In particular, the merger of binary black holes results in gravitational wave memory. I will discuss the prospects for measuring memory from binary black hole mergers.

Though gravitational wave memory is a consequence of the nonlinear Einstein field equations, it can be most easily understood by using linear perturbation theory with the Weyl tensor as the basic variable. Gravitational perturbation theory using the Weyl tensor is analogous to Maxwell's equations for electromagnetism. Correspondingly, there is an electromagnetic analog of gravitational wave memory called electromagnetic memory. I will present a proposal for how to measure electromagnetic memory.