> if you can manipulate the metric, you should be able to create (or vanish) energy. No, you can't. That would violate local energy conservation. The fact tha

> if you can manipulate the metric, you should be able to create (or vanish) energy. No, you can't. That would violate local energy conservation. The fact that energy conservation is a local property just means that there is no well-defined global energy in a general spacetime (though there is one in some special classes of spacetimes). It does not mean that stress-energy can be created or destroyed. > with a subluminal warp drive, you could hoist matter out of a gravity well and let it fall down afterwards. But the hoisting process would require you to expend at least as much energy as you would be able to collect from the falling down afterwards.