Cedric Villani recently wrote an article on Landau damping, where at least one topic discussed confused me. Besides discussing the issue of how a process can be microscopically reversible and macroscopically irreversible, Cédric mentions something regarding a "entropy-preserving relaxation mechanism" that is irreversible. Am I missing something? I think the article suggests that something can preserve entropy but still be irreversible, which begs the question of whether or how these two quantities are tied?

Cedric Villani recently wrote an article on Landau damping in collisionless plasmas, where at least one topic discussed confused me. Besides discussing the issue of how a process can be microscopically reversible and macroscopically irreversible, he mentions something regarding a "entropy-preserving relaxation mechanism" that is irreversible.

Am I missing something? I think the article is suggesting that something can preserve entropy but still be irreversible.

This begs the following questions:

• Are entropy and irreversibility synonymous?
• Or are they distinct entities each with their own fundamental properties?