We rigorously show that the probability to have a specific trajectory of an externally perturbed classical open system satisfies a universal symmetry for Hamiltonian dynamics. It connects the ratio between the probabilities of time forward and reversed trajectories to a degree of the time reversal asymmetry of the final phase space distribution in a model-independent framework. Especially, it amounts to a nonequilibrium generalization of the detailed balance between the probabilities of the forward and reversed trajectories under the condition that the initial phase space distribution is described by an equilibrium ensemble. An expression of the microscopic reversibility for the subsystem is also derived based on this relation.
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