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I had been fascinated to hear that anything can be swallowed by a black hole. But what causes such a huge amount of gravitational force?

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A huge amount of stuff. –  Olin Lathrop Aug 6 '13 at 13:17
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This sounds like a setup to a yo mama joke. –  mart Aug 6 '13 at 13:42
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Huge is a relative term. A black hole with the same mass as the sun has the same gravitational attraction as the sun. –  Michael Brown Aug 6 '13 at 13:44

5 Answers 5

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As mentioned in other answers, the thing that generates the gravitational attraction, is simply mass. However, the earth also has a lot of mass, as has the sun. In fact, the smallest known black hole has a mass of just 3.8 solar masses (source). So how come this object has become a terrifying black hole that can swallow anything, whereas billions of stars many times more massive have not?

This is the equation that is used to calculate the force of gravity between two objects: $$F_G=G\frac{m_1 m_2}{r^2}$$ $F_G$ is the force of gravity, $m_1$ and $m_2$ are the masses of the two objects, and $r$ is the distance between the two objects. $G$ is a constant.

The difference between a big star and a small black hole is size. A black hole isn't special for its great mass, but for its tiny size. As you can see in the above equation, the force of gravity is dependent upon the masses involved, and the distance between these masses. The smaller the distance, the greater the force. On Earth, the gravity is comparatively weak because we can't get very close to the center of the earth (about 6000 km). Yes, you could dig a hole and go down, but that would actually only make the force of gravity smaller, because you're leaving more and more mass behind you (see this wikipedia article). However, because black holes are much smaller compared to their mass, $r$ can get very small without leaving any mass behind you. This means that $F_G$ can become very big. So big in fact, that not even light can escape.

Note that this means that from a larger distance, black holes behave exactly the same as any other body of the same mass. In other words, if the sun would be compressed, turning into a black hole of the same mass, the orbits of the planets in the solar system wouldn't change. Earth would be just fine, be it freezing.

In summary, mass is what generates the gravity in a black hole, but this gravity can get so strong because black holes are very small relative to their mass, but this doesn't change their effect on objects that are far away.

PS: this is all from a Newtonian point of view. A proper discussion of black holes should include General Relativity as well as Quantum Mechanics, a combination we are still trying to figure out. Nevertheless, there are a lot of people who could tell you a lot of things about black holes that I couldn't. Please forgive my ignorance. :)

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Mass. A black hole is a lot of mass in a small space. From a distance the gravitaional attraction is the same whether from a dispersed amount of mass or from a compact volume.

For scale a very large black hole has a mass millions of times that of our sun. A typical galaxy will exceed this value by a factor of a thousand.

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In non-technical, basic, terms,

Mass, and a small radius .

Instead of getting into GR, let' us just stick with Newtonian gravity.

$$F=G\frac{m_1m_2}{r^2}$$

Is Newton's Universal law of gravitation . If $r$ gets small, $F$ gets big . On the surface of the Earth, and a black hole with the same mass of as the earth , the gravity is thus much stronger on the black hole . To be a black hole (escape velocity at event horizon) , the radius should be smaller than all equal to the Schwarzschild radius:

$$r_S=\frac{2Gm}{c_0^2}$$

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What causes a Black Hole to have a huge gravitational force is its density. As gravity loses force as the distance increases, it gets diversified in mundane less-dense objects, the lower the density of the object the bigger its 'local' gravitational force, to the point when the gravitational force becomes so strong it stops light from leaving within the the "Event Horizon", and distort light in the outside, at this point it has become a Black Hole.

If you are wondering how a big-enough dense object is formed, usually its because a mass-stellar non-dense object (like a star) collapsed in its own after its core couldnt reppel the gravitational force.

Any amount of matter could be turned into a Black Hole if compressed enough.

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The gravity produced is dependent on mass and volume. Following a simple equation of gravity:

$$ F = G\frac{m_1m_2}{r^2} $$

You can see that the gravitational force ($F$) is dependent on the mass and radius; $G$ is a constant. The higher the mass of an object, the higher the gravitational force.The opposite is true with the radius - the lower the radius, the higher the gravitational force.

Although the radius is technically the distance between the two bodies that gravity is acting on, one being the black hole, the radius of the black hole itself is a factor of the radius, just as the radius of the Earth is a a factor for the gravitational force that we feel.

As an added note, as Olin said in his comment, there really are a huge number of factors that go into the amount of gravity produced by a black hole, but for simplicity, most of it is due to mass and size.

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