Physical interpretation of the spectrum of black hole quasinormal modes.

When a classical black hole is perturbed, its relaxation is governed by a set of quasinormal modes with complex frequencies omega=omega R +i omega I. We show that this behavior is the same as that of damped harmonic oscillators whose real frequencies are (omega R2+omega I 2)1/2, rather than simply omega R. Since, for highly excited modes, omega I>>omega R, this observation changes drastically the physical understanding of the black hole spectrum and forces a reexamination of various results in the literature. In particular, adapting a derivation by Hod, we find that the area of the horizon of a Schwarzschild black hole is quantized in units Delta A=8pi l Pl2, in contrast with the original result Delta A=4log(3)l Pl2.