This expression implies that ⟨ ⟨ r 2 ( t ≪ ≪ τ τ ) ⟩ ⟩ ≃ ≃ v 2 ( 0 ) t 2 , indicating that the motion of Brownian particles at timescales much shorter than the relaxation time τ τ of the system is (approximately) time-reversal invariant. On the other hand, ⟨ ⟨ r 2 ( t ≫ ≫ τ τ ) ⟩ ⟩ ≃ ≃ 6 k B T τ τ t / m = 6 μ μ k B T t = 6 D t , which indicates an irreversible, dissipative process.