As far as I can make out, the hallowed principle of conservation of mass-energy (modulo quantum fluctuations) lies on four principles: (1) from the assumption that every effect has a cause (again, modulo the leeway given by Heisenberg), so if there is no mechanism for creation or destruction of non-quantum amounts of mass-energy, then it shouldn't happen (2) creation or destruction of mass-energy has never been measured, (3) the assumption has allowed us to make correct predictions for particle disintegration. (4) it fits in nicely with other conservation principles, such as conservation of baryon number. My two questions (which overlap, I admit): (A) Is there any other basis? (B) Would its violation (outside of quantum physics) lead to any logically contradictory consequences if you subtracted the conservation principles from physical theory? Yes, I know that's a lot to ask for in a thought experiment, but I am just trying to find out whether the justification for the conservation laws is not circular. For example, suppose that somewhere, an extra photon appeared in the vacuum and stayed there, but no one observed this fact at the time. Although conservation laws forbid this, and there would be no mechanism to explain it, nonetheless is there anything to say that the universe would not keep on ticking after this hiccup? Otherwise put, if we posit the existence of other bubble universes with different physical laws, could you have one in which the conservation of mass-energy is violated (with the usual quantum-mechanical caveat)? In philosophical terms, is this conservation principle a necessary or just a contingent principle?