Trace expressions and associated limits for equilibrium Casimir torque

We exploit fluctuational electrodynamics to present trace expressions for the torque experienced by arbitrary objects in a passive, nonabsorbing, rotationally invariant background environment. We present trace expressions for equilibrium Casimir torque which complement recently derived nonequilibrium torque expressions and explicate their relation to the Casimir free energy. We then use the derived trace expressions to calculate, via Lagrange duality, semianalytic structure-agnostic bounds on the Casimir torque between an anisotropic (reciprocal or nonreciprocal) dipolar particle and a macroscopic body composed of a local isotropic electric susceptibility, separated by vacuum.