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@jawheele I didn't get it. Do you mean If there is a black hole somewhere in the universe, escape velocity from that black hole at the current position of the earth is also the speed of light, so we cannot escape from it?
So the efficiency of the rocket increases with speed? Also, speed relative to what? If we take the speed relative to a different frame which is moving in the opposite direction at a very high velocity, the energy gain may exceed the total energy contained in the burnt fuel itself, which shouldn't be possible, right?
