As I understand it, a simple quantum system undergoes transitions via unitary transformations. Unitary transformations preserve inner products, so they are "rigid" rotations of a "configuration" in N-space (assuming that a finite dimensional quantum system can exist). Nevertheless, from a given "state" (whatever that means), not all unitary transformations seem possible. For example, if a system is isolated from its environment, wouldn't it "evolve" in an "inertial-like frame" manner? If the rotation analogy is correct, wouldn't this be a rigid constant-speed spin about some axis/plane? Wouldn't this spin have some "angular momentum" (resistance to slowing down) ? Aren't there other unitary evolutions which will require some external "effort" to change its course -- i.e., axis ? And in this case, wouldn't reversibility require the re-appearance of the conjugate of this "effort" to change the course back?