The cosmic bell tolls: when black holes merge, space rings out, and surrounding light begins to tremble in time. Photon polarization mirrors the rhythm of the gravitational ringdown, like a seismograph. The achromatic ~10° rotation is the signature of pure geometry. New polarimeters will allow us to "read" black hole births by the quiver of light.
🎯 Unlike Faraday rotation in plasma, which strongly depends on wavelength, gravity-induced polarization rotation is the same for all frequencies—an achromatic "gravitational Faraday effect," predicted back in the 1950s but now, for the first time, quantitatively estimated for realistic merger scenarios.