@d31,
"Maybe they're not fixed, but until something shows that to be the case, why assume not?"
The question is, why assume so? The believer in physics, after all, is claiming a lot more than the physics sceptic. He's claiming that if he throws a rock in a particular direction, it will follow a very precise, predictable trajectory, for example. Well, there are millions of trajectories it COULD follow, or for that matter, it could just disappear, or hang there in midair, or turn into a flower. If you're the one making claims about what will happen, I would ask you, again, why assume SO?