# Why can't a system reach equilibrium with non-interacting particles?

I am new to Statistical Mechanics, and have just started reading from this book Tony Genault Statistical Physics where he writes the following (paraphrased for convenience)-

Consider a system of $$N$$ weakly interacting particles. If the energy of one particle is $$\epsilon$$, the total energy of the system is- $$U=\sum_{l=1}^{N} \epsilon(l)$$ Any such expression implies that the interaction energies between particles are much smaller than these (self) energies ε. Actually any thermodynamic system must have some interaction between its particles, otherwise it would never reach equilibrium. The requirement rather is for the interaction to be small enough for the above equation to be valid, hence ‘weakly interacting’ rather than ‘non-interacting’ particles.

And hence my question- Why does non interacting not work?

• Ask yourself what would equilibrium (or non-equilibrium) mean between two systems if they do not interact? Can it be any meaning to that question? Mar 15, 2021 at 15:59