According to Special Relativity, objects moving fast have more mass than objects at rest. So what if there is an object having a mass density near that required to create a black hole. Now if it were moving fast enough as well, it would gain enough mass density to become black hole. But to the observer moving in its frame of reference it would not. How can an object both be a black hole and not be a black hole? To an observer moving fast enough, every object would seem to be a black how, right?