But when societies clash, there's no higher authority to turn to except the laws of physics: irreconcilable differences between different societies lead to war. The winner gets to impose its moral code on the loser. Thus, over time, the laws of physics determine the moral code of society by means of evolution.
On the face of it, one would think this would not lead to a terribly good dominant moral code.
C.S. Lewis (yeah, him again) holds that there is a good being external to the physical universe that is in some way inserting morality into it in opposition to this evolutionary tendency. I find this is an unsatisfyingly complex counter-hypothesis, I would require considerable evidence to accept it as correct.
I guess what we need to know is exactly what sort of morality do the laws of physics impose. Are they what we have now, or is something meddling? Is physics inherently Evil?
Perhaps we could devise some kind of evolutionary artificial-life simulation that reflects certain important properties of physics, and see what morality emerges. If this morality matched our own, we could discard the external meddler hypothesis. OTOH, if we can prove there is no way a simulation could give rise to our morality no matter how detailed the simulation (a somewhat harder task), we would have to accept the external meddler hypothesis.