  According to conventional wisdom, a system placed in an environment with a
different temperature tends to relax to the temperature of the latter, mediated
by the flows of heat and/or matter that are set solely by the temperature
difference. It is becoming clear, however, that thermal relaxation is much more
intricate when temperature changes push the system far from thermodynamic
equilibrium. Interestingly, under such conditions heating was predicted to be
faster than cooling, which we experimentally confirm using an optically trapped
colloidal particle. More strikingly, we show with both experiments and theory
that between any pair of temperatures, heating is not only faster than cooling
but the respective processes in fact evolve along fundamentally distinct
pathways, which we explain with a new theoretical framework we coin "thermal
kinematics".
