It’s official: math proves it’s way faster to heat something up than it is to cool it back down.
March 20, 2026
Original Paper
Thermal relaxation asymmetry persists under inertial effects
arXiv · 2603.18721
The Takeaway
We usually think of temperature changes as reversible, but this paper shows a deep asymmetry in nature. Whether a system is simple or complex, the path to a higher temperature is mathematically 'shorter' than the path back to a cold state, explaining why reaching equilibrium isn't a two-way street.
From the abstract
We algebraically prove the asymmetry in thermal relaxation in phase space in the entire range from overdamped dynamics to underdamped dynamics. We show that for the same setup as for overdamped dynamics, even in the more general case of phase-space relaxation, i.e., underdamped dynamics, far-from-equilibrium heating is faster than cooling. Upon isolating the relevant relaxational contribution to the entropy production, we find that the asymmetry persist for underdamped dynamics that are linearly