A few points:
1) Black holes can be any size, the secret is that they have to have mass such that at any point on the surface, light doesn\'t have escape velocity. There are very small black holes, the size of a city, with less mass than our sun. However, they are so dense, that light from the surface cannot escape the event horizon. If you increase the mass, the size may not generally get bigger, but the distance from the center of the hole from which light can\'t escape gets bigger.
2) If you want to explain your example, you need reletivaty (something in which I personally don\'t believe, but here is it\'s explanation): First of all, you can\'t travel at c, or even near c. The equation of relativity says that for matter to travel at c, it must become energy. Anything of substantial enough mass to affect a black hole will never be able to approach and then escape as you said; however, if you had:
a)lets say, a large mass neutron star (10s of km across), that was travelling RELATIVE to a black hole very quickly, say, .5 c, close enough that their gravities would affect each other, far enough that the pass would just change their paths, and a
b)black hole that\'s event horizon was literally just above the surface of the black hole object, then in passing, if the neutron star could allow some of the energy/mass of the black hole to escape the hole (for a short time), then the hole object would become visible until that matter was pulled back in.
c) Because the effect would be short lived from the passing object/star, as soon as it was far away, the black hole, which would still exert a gravitational pull, would probably pull back in it\'s mass. Some light may escape, hence energy, but once it had most of it\'s mass back, it would be a black hole again. Even if the lost energy/bit of mass is enough to turn it back into a non b-h object (one who\'s event horizon would be basically at the surface, for all intents and purposes), the gravity/tidal stresses exerted by passing would generate more energy and heat within it, eventually leading to further compression, and it would soon become a black hole again anyways.
d) It\'s not like a magical equation, visible object, very massive... add 1 kg, and it becomes a black hole. Things get very funky when we look at gravity from objects like this. Relativity can\'t explain it well, but gravity becomes MORE than just a force at these powers and distances. Gravity more than just visible space, but time as well. Objects very close to the threshold of a black hole would have some all might wierd properties.
Dislaimer:
I\'m getting most of this out of an astronomy textbook, from a second year physics/astronomy course at university. I could be mis-reading or mis-understanding parts of it.
I personally don\'t believe that relativity is correct, parts of it have been disproven, and it basically is self-fulfilling (it\'s proof).. but right now, it\'s the most adequate explanation for the universe in terms of examining characteristics on or around the speed of light.
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