Macroscopic arrow of time from multiscale perspectives

Abstract: Fundamental laws of physics are symmetric under time reversal ($T$) symmetry, but the $T$ symmetry is strongly broken in the macroscopic world. In this Perspective, I review $T$ symmetry breaking frameworks: \textit{second law of thermodynamics, multiscale energy transfer}, and \textit{open systems}. In driven dissipative nonequilibrium systems, including turbulence, the multiscale energy flux from large scales to small scales helps determine the arrow of time. In addition, open systems are often irreversible due to particle and energy exchanges between the system and the environment. Causality is another important factor that breaks the $T$ symmetry.