Why can light not escape a black hole?

06 September 2009

Question

If a difference between a star and a black hole is density of matter (black hole being a collapsed star) then why cannot light escape the gravity of a black hole but it can escape a gravity of a star? To my understanding:

1)When a star collapses it does not gain mass, it only becomes denser,
2)Gravity depends on mass, not density.

Answer

Chris - The point he's making is that a black hole is a collapsed star. So, all the mass of the star ends up in the black hole. So, if light can come out of the star in the first place, given that there's no more mass now in the black hole when it's collapsed, what's changed that now light can't get out?Dave - That's right. When you take a star and convert it into black hole, you actually normally lose an awful of mass. It involves all sorts of explosions and lots of energy given off so that black hole normally weighs an awful less than the original star did, but that mass is much, much closer together - it's much more dense. The force of gravity even the Newtonian force of gravity is essentially proportional to the inverse square. So, if you're twice as far away from it, the force gets four times weaker. So, if you take a star and squash all that mass very close together and then you stand on the surface of it, apart from being burned up and everything, you stand on the surface of it then you're going to be a lot closer, a lot more mass. So the gravity is going to be much, much stronger. And once you go into relativity and general relativity then that mass can bend space enough that light always gets bent around and it can never escape at all, ever.Chris - So, if the black hole blew up again and you took the same mass and put it back to something that was the original size of the star - so in other words, the density was low again then it would start to emit light again.Dave - Yeah then light could escape no problem.

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