Why do black holes have such strong gravity?
In: Physics
Gravity is generated by matter, and black holes are unfathomably dense collections of a lot of matter.
Gravity is stronger the closer you are to the center (§). Quadratically so: twice the distance means four times less gravity. So if you compress Earth by a factor of 100, then standing on it makes you experience 10,000 times what you are used to.
People often say black holes are incredibly dense, but this is not really always the case. The heavier it is, the less dense, and some of the largest known black holes have a density below yours; it can even be less than air! This happens because the mass (~ gravity) of an object made of a single material grows with the third power of the size, while the larger distance from the center only makes the force quadratically weaker as described above.
(§): this only applies as long as you are “outside” the thing, if you enter deep into the Earth, then the entire shell above you won’t actually contribute anymore. You will feel essentially no gravity at all close to the middle.
They don’t have stronger gravity of anything else with equal mass. But they are dense, we’ll give them that.
Black holes have exactly the same gravity as anything else with the same mass. They just happen be extremely massive. The amount of mass makes gravity so great that nothing can escape.
If the sun were replaced by a black hole with the same mass, the solar system would become dark but all the orbiting planets and asteroids would continue in the same orbits.
there’s two things actually. Mass, and distance.
Mass, BH can have an enormous amount of mass, the equivalent of many many suns.
Distance, BHs are very dense and have a small volume (relatively speaking) and since gravity falls off as radius squared, if you are really close to a huge dense mass, the gravity ‘force’ can be very strong.
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