Well, actually... "gravity" isn't just some sort of special propery of a planet. Everything has a field of gravity. Even your pencil and shirt or crap or whatever random object you want to point out.<P>My point is, the greater the mass of an item, the greater it's gravity. By hollowing out the planet (a massively difficult deed, I might add, one that would send out enough debris to make a few moons), the gravity of the planet doesn't reverse... it dissipates.<P>So even if a device were made to reverse gravity (which is weird in itself but I suppose it's not arguable since that's basically what it does and all sci-fis are allowed some stuff like that!), the planet no longer has nearly enough gravity to reverse. It would have a lot less than the moon, and that wouldn't be enough to keep people on their feet.<P>And the very very core of the planet is about as hot as the sun... now there's a LOT LESS of our core than sun (I think the sun's mass is that of all other planets combined times 750), but truly difficult to put some sort of fire retardant metal down there.