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Q&A: Black Holes

Q:
What eventually happens to black holes? This energy that they have in terms of gravitation so that even light is bent and cannot escape, is eventually bound to be exhausted, isn't it? Otherwise, the universal law of energy which states that energy cannot be formed out of nowhere, would be breached. I'm just puzzled as to how the black holes retain this enormous capacity for gravitation. It must take tremendous energy to maintain this power. I am not sure I understand where this energy is coming from. It'd be too unrealistic to assume that the black holes will have this immense gravitational pull forever. Can they use the energy of the substance that they suck from the neighboring stars and gas?

A:
What's happening is that the energy is constantly changing forms. A black hole does not use up its energy by pulling matter in anymore than a hill does by making balls roll downhill. What happens is that the energy changes form. As a particle falls toward a black hole, its kinetic energy, or energy of motion increases, and its gravitational potential energy becomes increasingly negative. Some of the kinetic energy may be transformed into heat through collisions with other particles and some of the heat energy may be radiated away as X-rays or other form of light. Eventually the particle will go beyond the event horizon, which means that no energy can be radiated away, and all the kinetic energy is also captured inside of the black hole. Since energy is equivalent to mass in Einstein's theory of relativity, the mass of the black hole increases slightly.

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