We show how to compute Casimir forces at nonzero temperatures with
time-domain electromagnetic simulations, for example using a finite-difference
time-domain (FDTD) method. Compared to our previous zero-temperature
time-domain method, only a small modification is required, but we explain that
some care is required to properly capture the zero-frequency contribution. We
validate the method against analytical and numerical frequency-domain
calculations, and show a surprising high-temperature disappearance of a
non-monotonic behavior previously demonstrated in a piston-like geometry.Comment: 5 pages, 2 figures, submitted to Physical Review A Rapid
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