Inside planets, touching black holes, and breaking gravity.

[1011686]

I just feel like talking about some interesting physics. So, the law of gravity state that force between two masses is inversely proportional to distance between them. Distance between them actually refers to distance between their centres of mass though.
However, if you travel to the center of, say, a planet (somehow), it would actually exert a force of roughly 0 on you. This is like a riddle i once heard; if you were somewhere inside a hollowed out asteroid, would be drawn to the edge, the center, or stay where you were?
This riddle works on what i said before, that gravity goes different inside of planets, and in fact any other object. The reason is that inside such an object, some of mass is actually pulling you away from the center. Once you get to the center, all the object's mass is pulling away from the center, and you'd feel negative pressure.
It would help if i had a diagram

But, on to the more theoretical part. Consider black holes. So, a black hole is an amount of mass compacted to a certain size. Once it reaches that density, the object's own gravity exerts more force on itself than it's atomic resistance to compression. At this size, the smaller the object gets, more effect it's gravity has on itself, so it just gets smaller and smaller until size is 0.
Think about this. The observable black hole is it's event horizon, which changes in size depending on the black hole's mass, but the actual blackhole, called a singularity, always has 0 volume. If you travel to the center of a black hole, you wouldn't experience the weakening effect i described earlier. You would continually experience the whole gravity, which would increase vastly as you got closer and closer.
And then, you touch it.
Now the law of gravity involves deviding by the distance between the two masses. If you touched a black hole, the distance between your hand and the singularity would be 0.
The equation for the force you would feel would involve deviding by 0!
So yeah, just another reason why there so many weird theories hypothesised about what goes on inside black holes.


(P.S. the force isn't simply infinity. I could explain why, but it would take a lot pf writing and be hard to explain without diagrams.)

[Zawadx]

Actually, the event horizon of a black hole has spacetime curved to such an extent that time appears to stand still there (so if you were to observe someone entering a black hole, he would appear to stay forever frozen at the event horizon. So at the point Einstein's General Theory of Relativity breaks down, and what happens inside the black hole can't be determined by it. For that we need the quantum theory of Gravity... whatever that is.

Newton's law is simply an approximation of General relativity and it would also break down at the event horizon, especially since time stops at that point. Newtonian physics has the concept of absolute time which runs at the same speed independent of observer, so it can't deal with the curvature of spacetime.

[godofdeath500]

A black hole's gravity is so strong at the center that it breaks down everything that enters it down to minute atoms, and further breaks them down into nothing but dark energy. Its so strong, that light cannot escape it, and the inside is actually absolute black.

Zawadx is correct about the event horizon, as well. The Space/Time continuum breaks down completely and utterly at the point of maximum gravity. (I don't know if I said that properly.)

[1011686]

Time doesn't stop at the event horizon, it just appears that way to an observer from the outside due to light not coming from inside the event horizon. If you actually traveled into a black hole, you wouldn't experience any sudden change at the event horizon.
Also, it seems reasonable to assume that gravity would increase uniformly through the black hole. The event horizon is where light can't escape, nothing more.

One last thing, where did you hear black holes turn particles into dark energy? i've never heard that before

[andretimpa]

Event horizons depend on the observer (as everything in relativity). Someone on free fall towards the black hole doesn't see the horizon. Someone keeping themselves from falling in the black hole (far away, orbiting) sees an horizon and they see time stopping for anything falling inside. Because not only light but any physical information takes a diverging amount of time to reach observers. This creates a funny situation where you can't see something finish falling in the event horizon unless there is no way for you to escape the horizon too.

About the predictions (just relativity) about what happens inside of the event horizon; it depends on the mass, electrical charge and angular mommentum of the black hole (black holes spin too):

Roughly speaking mass increases the event horizon, but charge and angular mommentum reduce it, making it theoretically possible for black holes with no horizon to exist  (https://en.m.wikipedia.org/wiki/Naked_singularity). Rotation also causes all sort of weird effects, with singularities no longer being points (it becomes a ring) and an inner event horizon (http://www.technology.org/texorgwp/wp-content/uploads/2014/03/blackhole2.jpg). There is a lot of controversy if these things really exist or if they are signs of limitations of relativity due to neglecting quantum effects.