  A sum rule for tidal Love number is derived from quantum field theory
computations, which relates tidal susceptibility of a spinless body to
transition rates of its single graviton emission processes. An analogous sum
rule for electromagnetism is given as an example, which is substantiated by
comparing to the solved problem of the hydrogen atom. Based on the
semiclassical Hawking radiation spectrum, a finite nonvanishing value for
quantum corrections to the Love number of Schwarzschild black holes in general
relativity is computed using the sum rule, which is known to classically
vanish.
