It has been predicted by general relativity theory that gravitational radiation changes the spacetime permanently which would show in a permanent displacement of the test masses in a detector. This phenomenon is called the memory effect of gravitational waves. There are two types of displacement memory, one going back to Ya. B. Zel'dovich and A. G. Polnarev and one to D. Christodoulou. The former is sourced by the change of a particular component of the curvature tensor (and called ‘ordinary memory’) the latter by fields that reach null infinity (and called ‘null memory’). Gravitational waves were measured for the first time in 2015 by LIGO and several times since then. It is believed that the memory effect will be detected in the near future. In the meantime, more types of memory effects at lower order have been derived.
In recent years, I found new types of gravitational memory for various classes of spacetimes, which show at highest order. Earlier on, with D. Garfinkle we showed that stress-energy that reaches null infinity also contributes to the null memory. Moreover, we found two electromagnetic analogues of memory for the Maxwell equations. In this talk, I will highlight the concept of memory and present some of the recent new gravitational memories as well as
joint work with D. Garfinkle on electromagnetic analogues. In particular, we find analogues in the electromagnetic theory of ordinary and null memory.
