  Active systems evade the rules of equilibrium thermodynamics by constantly
dissipating energy at the level of their microscopic components. This energy
flux stems from the conversion of a fuel, present in the environment, into
sustained individual motion. It can lead to collective effects without any
equilibrium equivalent, such as a phase separation for purely repulsive
particles, or a collective motion (flocking) for aligning particles. Some of
these effects can be rationalized by using equilibrium tools to recapitulate
nonequilibrium transitions. An important challenge is then to delineate
systematically to which extent the character of these active transitions is
genuinely distinct from equilibrium analogs. We review recent works that use
stochastic thermodynamics tools to identify, for active systems, a measure of
irreversibility comprising a coarse-grained or informatic entropy production.
We describe how this relates to the underlying energy dissipation or
thermodynamic entropy production, and how it is influenced by collective
behavior. Then, we review the possibility to construct thermodynamic ensembles
out-of-equilibrium, where trajectories are biased towards atypical values of
nonequilibrium observables. We show that this is a generic route to discovering
unexpected phase transitions in active matter systems, which can also inform
their design.
