When massive bodies accelerate (say, two stars orbiting each other) the space around them doesn't stay calm. It trembles like the surface of a pond after a stone is thrown. These tremors are called gravitational waves. They carry energy away, and the source gradually loses mass.
A team of physicists led by Geoffrey Compère asked: how exactly do you calculate this mass loss? In curved spacetime, simple formulas don't work. They considered weak waves far from the source — where spacetime curvature is already small. It turned out the key lies in multipole moments — something like "fingerprints" of mass distribution and motion: how fast the object rotates, how squashed it is, how its center shifts.
By linking these fingerprints to oscillations of the metric (as physicists call the description of spacetime geometry), the authors derived charge fluxes — quantities showing how much "gravitational substance" leaks into the waves. The calculation relies on the adiabatic approximation: changes in the source happen slowly compared to the wave's passage, like slowly lowering a stone into water while ripples spread quickly.
The result is not just an abstract formula. It connects what can be measured (how the distance between detectors changes) with what happens to the wave source itself. Thus we learn to read the history of black hole mergers from the ripples reaching Earth.
The memory effect in gravitational waves is so weak that after a typical wave from a black hole merger passes, detectors should notice a shift smaller than the size of an atomic nucleus.
In Carl Sagan's novel "Contact," aliens encode a message in gravitational waves — the memory effect could be a natural carrier for such information.