Why do black holes have more gravity than anything else?

06 September 2016

Question

Why do black holes have more gravity than anything else? Doesn't a black hole have the same mass as the star that it was created from and doesn't an object's gravitational field depend on its mass? Or is it that if that mass takes up less space it becomes more gravitational? In other words does a pound of gold actually weigh more than a pound of feathers?

Answer

Kat Arney put this to David Rothery...

David - Well, the answer is yes to both those bits at the end. I think the misconception that Byron has is that black holes have more gravity than other objects. It's the strength of the gravity close to the event horizon which is sort of the boundary of the black hole. If you were to take the sun and magically compress it to the density that it would require to become a black hole, the gravity at the event horizon, new surface around that would be extremely strong. But out here at the Earth, we'd notice no difference. So the total gravity around a black hole doesn't depend on whether it's a black hole or not. It just depends on the mass inside it. The sun can't become a black hole naturally. You have to have a mass of - it's the Chandrasekhar limit which I think is 1.4 solar masses or something - before a star has enough strength or gravity to force its matter into the density required to become a black hole. There are some super massive black holes, which are billions of solar masses base of the things, colliding black holes that were detected by the gravity waves detections. Those were really enormous objects and of course, have lots of gravity because they've got enormous amount of mass in. But the short answer is the strength of gravity at a distance of a black hole is independent of whether or not it's a black hole. It just depends on how much mass has gone into it.

Kat - Because it's all crushed into tiny, tiny space.

David - It's all crushed into such a small space that the gravity really close to it is so strongly that light can't get out and you couldn't get that close without being ripped apart by the tides.

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