To be extremely pedantic, operations on physical systems can be performed and perfectly reversed without loss of energy, but you couldn’t ever extract anything anywhere along the way - not even direct evidence that it happened. Our models predict that this happens literally all the time in quantum mechanics.
Edit: fun fact: this prediction is actually central to what makes quantum computers work.
That’s a good point. I hadn’t considered that, as I’m a chemist approaching thermo from the stat mech/free energy POV. I was mostly just thinking that a process where ΔG<0 is spontaneous but if ΔG=0, the system is at equilibrium, so nothing happens.
Now that I type it out, even I know that’s not exactly accurate as equilibrium is a dynamic state with lots of things happening at the molecular/electronic scale, so I guess I should have added a qualifier of “at the macroscopic scale” to my original post.
