mdeng said:
Your point is that Coulomb's law does not tell how to "cancel" (or shield from) a electric field, and yet it can be done by a Faraday cage? If so, what conclusion did you want to draw?
i'll throw the question back to you. do you understand how a Faraday cage works? how does it "block" EM fields in the inside from getting out (or fields on the outside from getting in)? Coulomb's law applied to some charge on the outside still has effect on charges on the inside. but maybe we can find another charge (of opposite sign) that will arithmetically cancel that effect.
However, GR says that there actually are no force to block in the first place. All we have is space curvature. Can this effect (curvature) be canceled or how would we cancel it?
well, if positive masses curve it one way, maybe negative masses (if they existed) would curve it the other way. but the Newtonian model ain't half bad, if you're not a black hole or a universe. it's what they
still use to describe the motion of the planets and nearly everything else in our solar system (sure, it misses very slightly regarding the precession of the perihelion of Mercury's or other planets' orbits, or similar effects for GP-B, but these are tiny). so just as opposite-signed charges move around in the metallic shell of a Faraday cage to cancel (not block) EM fields (and you can use Gauss's Law to show how that works),
if there were negative masses (and Newton's laws of motion and gravitation worked the same way on them, except for the sign change, just like Coulomb's law), it is conceivable that something could be constructed that would behave like a Faraday cage to
cancel (not block) gravitational fields. as a field, gravity is also inverse-square so Gauss's Law works on gravitational fields, too.
sure it would be cool to somehow build one of these gravitational Faraday cages and use them to make a room where you could step in and be weightless. but, besides the fact that we don't see any of those negative masses hanging around anywhere, there are theoretical problems regarding their existence. just as positive masses attact everything (positive masses will even attract a negative mass, if you count the number of minus signs carefully), it turns out that negative masses
repel everything. even other negative masses. you could also make yourself a perpetual motion machine with a mass of +1 kg and another one of -1 kg. put them both in free space a meter apart and watch what happens. the +1 kg mass will attract the -1 kg mass which will move toward it, but the -1 kg mass repels the +1 kg mass and the latter will move away from it. it's like this loser guy in high school that had a crush on this hot chick: he moves closer, she moves away, he moves closer still, she move away more. doesn't ever stop. put them on some kinda radial arms and hook 'em up to a generator and no more energy problems.
so, mdeng, if you find one of them negative masses, let's get together and make a lot of money with it. i'll bring the positive mass. to hell with the Patent Office who say explicitly that they will not patent perpetual motion machines.