Comment on "Irreversibility and Fluctuation Theorem in Stationary Time Series"
In their recent paper [Phys. Rev. Lett. 98, 094101 (2007)], A. Porporato et al. studied the irreversibility and fluctuation theorem for stationary time series. In this comment, we point out that the fluctuation theorem is in fact the trivial outcome of a symmetry operation, and hence its physical contect is less convincing.