This operation relates the external protocols in the numerator and denominator of the Crooks relation. In stochastic thermo, the entropy production is proportional to the log of the following: the probability of a trajectory, over the probability of the trajectory under this operation. In quantum mechanics, this operation is an anti-unitary operator that [emphasize] commutes with the Hamiltonian. Spin-1/2 systems that are invariant under this operation satisfy (*) Kramers’ theorem. Magnetic fields break symmetry under this operation, which is unified (10[1])with symmetry under charge conjugation and parity inversion. Loschmidt’s paradox asks how it is possible for Newton’s laws, which are invariant under this operation, to give rise to a quantity like entropy that only increases. For 10 points, name this operation analogized to playing a movie backwards. ■END■