rbj said:do you know what would be the nature of the gravitational field created by a hypothetical infinite plane of mass?
You'd better tell him, I doubt he realizes that the gravitational field at the axis of a large disk does not vary with height until the height is >> radius.
This makes the gravitational field of such a disk essentially constant. It also means that "real" gravitational fields from such a disk do mimic the gravitational fields of an accelerating rocket, including the fact that losely speaking, gravity doesn't change with height. (I'm cutting a few corners here, deliberately, considering my best guess of Lighstorm's level of physics understanding, but I'm not cutting any corners when I say that the gravitational field / metric of a large plane is identical to that of an accelerating spaceship).
It is simply physically difficult (though not impossible in principle) to build a large enough disk to mimic the field configuration (including invariance with height) of a rocket - not impossible as Lighstorm seems to think.
But the whole objection was really poorly motivated in the first place, it was basically missing the point.
In Newtonian physics, we have two sorts of mass - inertial and gravitational. There is no explanation for why they are the same.
The whole point of the equivalence principle is to assume that inertial mass and gravitational mass are fundamentally related, and that the equivalence between gravitational mass and inertial mass is not an accident.
That's the broad picture, the quibbling about the details is missing the point, rather like being unable to see the forest for the trees.
I've personally become somewhat jaundiced about arguing with people with an axe to grind against relativity. I've found it much more useful to talk to people who are actually interested in learning the theory.