Science Questions

Why do black holes have more gravity than anything else?

Mon, 5th Sep 2016

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Question

Byron Donovan asked:

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...A black hole

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. Itís 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, weíd notice no difference. So the total gravity around a black hole doesnít depend on whether itís 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 Ė itís 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 theyíve 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 itís a black hole. It just depends on how much mass has gone into it.

Kat - Because itís all crushed into tiny, tiny space.

David - Itís all crushed into such a small space that the gravity really close to it is so strongly that light canít get out and you couldnít get that close without being ripped apart by the tides.

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It is a matter of size the strength of gravity depends not only on the mass of the objects but upon the distance between them.
there are a lot of unknowns about black holes with talk of singularity's so let us consider their close relatives neutron stars in these although the mass may be only about twice that of the sun the matter is so compressed that the radius is about 10Km whereas a normal star of the same mass would have a radius of 500,000Km therefore a body on the surface would be subjected to gravitational force 50,000 times as great due to its proximity to the centre of gravity. syhprum, Mon, 22nd Aug 2016


Black holes don't have more gravity than anything else. They're merely so compact that you can get closer to them and since the strength of the field increases as you get closer they appear to have more gravity, but they don't. Recall that the strength of a gravitational field is g = GM/r2. This only holds outside of spherical bodies or so far from an object that the shape of the object is negligible. Thus for a star this only holds outside the star and the strength of the field decreases linearly when inside. But a black hole can be merely inches in diameter so by the time you get that close the tidal forces would have shredded you by the time you got there. PmbPhy, Tue, 23rd Aug 2016

The above responses have talked about the strength of the gravitational force close to a neutron star or black hole.
A related way of looking at this question is the escape velocity, which is a measure of the energy required to launch an object from the surface.

If a neutron star has 1/50,000 the radius of the original star (but the same mass), then the escape velocity will be over 200 times greater than the original star.

A black hole is crushed down to such a small size that the escape velocity exceeds the speed of light. And, according to Einstein, you can't accelerate any massive object to a speed faster than light, so nothing can escape - not even light.

See: https://en.wikipedia.org/wiki/Escape_velocity evan_au, Tue, 23rd Aug 2016

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