The Fluctuation-Dissipation Theorem and Dynamical Love Number in Schwarzschild Black Holes

Abstract

We study the real-time response of a Schwarzschild black hole in the Hartle-Hawking state to an external massless scalar field. Using a complexified radial contour to represent the Schwinger-Keldysh closed time path, we describe dissipation, thermal fluctuations and conservative tidal response within a common framework. Before performing tidal matching, we derive the equilibrium fluctuation-dissipation relation directly from the radial construction for every scalar multipole at arbitrary real frequency. In the low-frequency tidal expansion, we recover the known relation between the logarithmic running of the conservative dynamical Love coefficient at order $ω^2$ and the leading dissipative response at order $ω$. We explain how the same leading absorptive coefficient enters both relations at low frequency and trace the unit coefficient of the running relation to the Schwarzschild horizon slope. In the Hartle-Hawking state, this common coefficient relates the logarithmic running directly to the zero-frequency thermal noise. Kubo-Martin-Schwinger analyticity determines the thermal noise from the dissipative response, whereas the near-zone radial equation and tidal matching determine the conservative logarithmic running.