If you put everything in a rest frame, it seems as if it's impossible to tell an isolated system from a closed system (globally in SR, locally in GR). Am I off my rocker to think so? There's at least one catch I've thought of so far: light. I can't say for sure that it satisfies either definition because it can't be dragged into a rest frame, but I also can't say it shouldn't be thought of as a closed or isolated system. It's counterintuitive in a 3space+1time worldview, because EM radiation behaves almost like a dissipative transport process as seen in Poynting's Theorem (flow of charges through an E field gives you a work that is distributed in an irreversible fashion as radiation). But could a 4-volume element full of EM radiation be called an isolated/closed system? If so, is the specific internal energy of that volume element defined? Or am I just completely off my rocker?