The Universal Arrow of Time I: Classical mechanics

Understand

Statistical physics cannot explain why a thermodynamic arrow of time exists, unless one postulates very special and unnatural initial conditions.

• Yet, we argue that statistical physics can explain why the thermodynamic arrow of time is universal, i.e., why the arrow points in the same direction everywhere.
• Namely, if two subsystems have opposite arrow-directions at a particular time, the interaction between them makes the configuration statistically unstable and causes a decay towards a system with a universal direction of the arrow of time.
• We present general qualitative arguments for that claim and support them by a detailed analysis of a toy model based on the baker's map.