The White Noise of Black Holes

It is well-known that a black hole emits thermal radiation, with a temperature inverse to its mass. As viewed from an infinite distance away, the resulting thermal power spectrum of Hawking photons and gravitons decays to zero at very low energies, and therefore contains no "soft" radiation. We show that this picture changes dramatically when a black hole is viewed from a finite distance away. Even at very large but finite distances from the horizon, the infrared tail of the vacuum fluctuations of the electromagnetic and graviton fields display a flat power spectrum of ultra-low-frequency white noise. This noise takes the form of low-frequency dipole (photon) and quadrupole (graviton) fluctuations in the vacuum, in stark contrast to the classical description of a black hole at equilibrium. This white noise can be understood as the cause of the universal decoherence that arises outside black holes, via a process of gravitational stimulated emission of entangling modes that fall back through the horizon.