Speakers
Description
Part 1: I will start talking about the constraints imposed on the electromagnetic response of media by microcausality (commutators of local fields vanish outside the light cone) and positivity of the imaginary parts (the medium can only absorb energy from the external field). Modeling the medium as composed of small polarizable particles, one finds that similar positivity bounds apply to their electromagnetic polarizability coefficients. This naturally extends to gravitational systems, offering analogous constraints on the tidal response coefficients of weakly gravitating compact objects.
Part 2: We study general properties of retarded Green’s function without Lorentz symmetry. We present constraints on the dissipative part (imaginary part) solely arising from analyticity in forward light cone. Using multi-variable dispersive representation, we show that retarded Green’s functions are Herglotz functions.
