So, I am looking a question about adiabatic expansions and the associated entropy changed. Why exactly is the change in entropy for a reversible adiabatic expansion/compression equal to zero? The book says its because there is no heat transfer, but for irreversible adiabatic processes, there is also no heat transfer. So no matter what, shouldn't the entropy of the surroundings for an adiabatic process be zero? In terms of the system, since entropy is a state function, why would it matter whether its reversible or irreversible?