  General relativity explains gravitational radiation from binary black hole or
neutron star mergers, from core-collapse supernovae and even from the inflation
period in cosmology. These waves exhibit a unique effect called memory or
Christodoulou effect, which in a detector like LIGO or LISA shows as a
permanent displacement of test masses and in radio telescopes like NANOGrav as
a change in the frequency of pulsars' pulses. It was shown that electromagnetic
fields and neutrino radiation enlarge the memory. Recently it has been
understood that the two types of memory addressed in the literature as `linear'
and `nonlinear' are in fact two different phenomena. The former is due to
fields that do not and the latter is due to fields that do reach null infinity.
