What are black holes and what happens to the things that get swallowed up?
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A black hole is what you get when enough mass is collected together.
Mass produces gravitational force that pulls on other masses, trying to collect them together. Gravity has a very long range but is relatively weak, meaning masses pulled together will just bump up against each other. The atomic forces of the atoms will repel each other so even though gravity is still pulling them together they can’t actually occupy the same location.
This is how things work for collections of mass up to a certain size. Things like fusion where huge amounts of heat and light are released (stars) complicate things but generally it is mass being pulled together until it can’t anymore.
If you increase the amount of mass even more things start to get weird. Eventually you can end up with a neutron star, a ball of mass composed entirely of neutrons. Instead of the normal atomic structure of a nucleus of protons and neutrons with electrons forming a field around it, a neutron star is just neutrons crammed against each other as closely as they would be in the nucleus of an atom. Gravity is so strong at this point there isn’t room for anything else. The only thing preventing the neutrons from getting closer is the Pauli Exclusion Principle, which simply put describes the concept that you can’t have two identical things in exactly the same place. The building blocks of matter have certain fundamental “quanta” that make them up, indivisible values. It seems that identical combinations of these values can’t coexist in the same place, and this limits how closely gravity can pull them together.
But what if you get even more mass and consequently more gravity?
This is where things start to get really weird. Gravity isn’t just a force pulling masses together. Instead that is just the most easily observed aspect of what it really is. In truth humanity doesn’t really understand gravity fully. Gravity causes the warping of spacetime. For example if you have two parallel lines and one of the lines passes close by a mass than the other, those lines will no longer be parallel. It shifts the underlying structure of space itself; not “vacuum outside Earth’s atmosphere” space, I mean volume of the universe.
When there is a lot of mass this warping of spacetime gets fairly dramatic. The path of light is significantly bent, or rather the light is continuing to move in a straight line but to an outside observer that line is curved. Eventually with enough gravity the warping of space is enough that while there are directions which point closer to the mass, there are none that point away from it. This is a black hole, and not even light can escape moving close enough to it.
From the outside the black hole is just a completely black circular spot. From any direction you see the entire expanse of the “event horizon”, a spherical boundary where the gravity is strong enough to warp space such that light can’t escape. Any mass can be modeled as originating from a point when at a distance, and black holes are no exception. This conceptual single point is a “singularity” and various theories exist about if the mass actually is concentrated in a single point or if there is more space inside the black hole than appears from outside. There seems to be literally no way to know though.
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