So with the shell theorem causing a net zero acceleration at the center of a spherical mass, how does spacetime curvature work on the "inside" of a large mass? Does it cancel out the closer it gets to the center in the same way, or does the curvature work some other way?

So with the shell theorem causing a net zero acceleration at the center of a spherical mass, how does spacetime curvature work on the "inside" of a large mass? Does it cancel out the closer it gets to the center in the same way, or does the curvature work some other way?

I am no theoretical physicist, but as I understand it gravity and mass aren't the only factors that come into play. If your mass is rotating that creates strange effects that could lead to an observer being able to distinguish "empty space" from "being at the center of a large mass with zero net acceleration" (for instance https://en.wikipedia.org/wiki/Frame-dragging). Having an electrical charge might also matter.

I don't know. Physics at this level is pretty much just cool-sounding gobbledygook and doesn't mean a lot to me, since I can't easily tell the difference between legit theoretical physics, legit theoretical physics incorrectly translated by someone else, pseudoscience, or just plain garbage. If this was for a game of some sort, I would definitely prefer to interact with a model of the world that I can relate to. Simple time dilation models can be fun; but mind-bending, unpredictable realistic high energy physics would probably get frustrating if I can't grasp the foundational mechanisms that underlie said physics and manipulate them to my advantage.

I think there is really something paranormal going on

What is paranormal? Isn't this just defined as anything beyond our understanding of science? To a caveman, a zippo lighter or a steam engine would be considered supernatural... Perhaps "supernatural" is the correct definition for things that might always be beyond explanation by current science?

As others have said, even if we are inside a black hole, the fact that we have mass and are constrained by the laws of physics within this universe means we can never prove or disprove the theory... That implies belief and belief alone is the antithesis of science, e.g a religion or supernatural belief.

Do blackholes have an internal space?

I guess the simple answer is that space exists all over the joint, including at the heart of a black hole.
I'm not too sure what your diagram is showing to be honest, so it's hard to comment on more than that.

What actualy made scientific conclusion that inside a black hole is a singularity?

Mainly that the most rigorously tested mathematical models predict such a thing. Not all of them mind you!
Singularities are usually indicative of some piece of missing theory or you just need to find a perspective from which the 'singularity' is not really a singularity at all.

Since the strength of gravity force is not influenced by density, just by the mass

True. But by compressing a mass (and thereby increasing its density) allows particles to move that much closer to the compressed mass without colliding or penetrating it. Being so much closer means they experience a quadratically proportional increase in gravitational attraction.

Consider earth to be as big as a tenis ball, but still having mass of earth. Would objects falling towards it cross speed of light at same particular distance ?

Um, well, I probably know less about tennis balls than I do about black holes.
I maintain a strong suspicion that the size of tennis balls is not related to any cosmological properties related to black holes or the speed of light. I imagine there's some sports standard out there that dictates the size and manufacturers just comply with it.

The radius of a tennis ball is... about 5cm? So, no, the speed of objects falling towards it wouldn't even reach the speed of light in that case.

You would have to squish the Earth to below its Schwarzschild radius to create a black hole; that's about 9mm for the Earth. Much smaller than your tennis ball.

Suppose you squashed the Earth below its Schwarzschild radius, in that case objects in free-fall would exceed the speed of light from compared to a static reference frame. The distance at which it reaches the speed of light is equal to the Schwarzschild radius and is known as the event horizon. Any closer than that and it will be moving faster than light with respect to that static reference frame.

What an outside observer in this reference frame would actually 'see' is rather different though and largely an optical effect caused by the time dilation. They would see the falling object slow down and fade away just outside the Schwarzschild radius.

Of course, in practice we don't observe the real event horizon, just an apparent horizon from our observation point.

Would the ball still be able to keep that much mass in such dense state just by having gravity of that mass?

For a tennis ball sized Earth?
Hard to say for sure. I would think not, personally.
I imagine it would be so hot as to probably evaporate itself as plasma, but that's just a guess.

Maybe you just turned the Earth into a white dwarf?

We have never observed a black hole explosion, nor we do know nothing about black hole life time.

Don't be so sure.
We actually know a number of things about the life cycle of a black hole. Ranging from how they may be formed, to how they grow, how they move, to Hawking radiation and even to several ideas about how they die and what that dying process might leave behind.

frankly, a zero radius doesn't even compute (to me). after a certain point, you can't smoosh the protons, neutrons , and electrons any closer together. unless some neato thing happens in extreme gravity that lets sub-atomic particles co-exist in the same physical space at the same time.

One way I try to make sense of it is to think that if the curvature of space is asymptotic then you aren't really smooshing stuff together at all. It is simply that there is always more space for things to fall a little further 'into' or 'toward' the singularity. Locally, particles always occupy the same amount of space they always did - at least until they are torn apart by tidal forces.

There is a problem with asking if space exists inside a black hole.

How do you define "inside"?

Depending on where you are, the distance of the event horizon changes relative to other objects around you.

Namely, as you fall closer to the event horizon, the event horizon keeps moving away from you in space-time...

Assuming you are a point in space, you will never observe yourself reaching the event horizon.

When I say that the event horzon moves away from you in space time, it's not just a farther distance. Time also stretches... An outside observer will see you slow down and freeze when you approach the horizon. Essentially your distance in "time" becomes "further".

Having said all this, you understand that that are many things one might mean when one says "inside a black hole". For different observers, inside means different things. That's the whole point of Einstein's relativity.

If you want to visualize this in a simple graph:

Take an asymptote of 1/(x^2)

If you get closer to x=0 it looks steeper and steeper.

But if you zoom as you move, as space time is streched, The steepness stays pretty much the same (like zooming into a fractal).

I think it is pretty safe to assume that he meant inside the event horizon.

There is a problem with asking if space exists inside a black hole.

How do you define "inside"?

Depending on where you are, the distance of the event horizon changes relative to other objects around you.

Namely, as you fall closer to the event horizon, the event horizon keeps moving away from you in space-time...

Assuming you are a point in space, you will never observe yourself reaching the event horizon.

When I say that the event horzon moves away from you in space time, it's not just a farther distance. Time also stretches... An outside observer will see you slow down and freeze when you approach the horizon. Essentially your distance in "time" becomes "further".

Maybe I'm wrong (I'm just telling from memory) but I remember that in theory you can pass the horizon (and may even survive if the black hole is large enough), only the outside observers will see you slowing down and "freezing" at the horizon.

I think it is pretty safe to assume that he meant inside the event horizon.

Not "safe" at all.

Inside the event horizon relative to what observer?

An event horizon is always relative to an observer...

Which again leaves the question of "inside" to be subjective.

For example: Alice looks at Bob and Camila, and says: Wow Bob & Camila are almost inside the event horizon!

But Bob looks at Camila and says: Camila is almost inside. But I still have a way to go!

This happens because the gravity of the black hole distorts times, and distances as you approach it.

The very concept of "distance" starts to become meaningless... (how would you define distance)

Another question that hides in the details is: Can Alice and Bob ask something at the same time? The "strange" answer is that they can't. Thing's that happen "at the same time" are also subjective.

So if "time" is subjective, and "distance" is subjective, and "size" is subjective, what do you mean when you say "inside"?

If we were to use the metaphor of a ball placed on a rubber sheet:

The ball, the rubber sheet, and the size of the hole, would look different as you approach them...

Each observer has their own unique view of 4D space. That again is Einstein's relativity.

It sounds like you are saying it is impossible to cross the event horizon, and yet black holes increase in mass as they pull in matter. How can this be?

An event horizon is always relative to an observer...

Specifically, an observer located at infinity.
Observers more local than that can't be sure where the event horizon is or whether they are inside or outside of it. They only see an "apparent horizon" from their frame of reference.

A lot of literature will refer to this apparent horizon as the event horizon though. For a Schwarzschild blackhole that won't grow in its future then the two horizons coincide.

It sounds like you are saying it is impossible to cross the event horizon, and yet black holes increase in mass as they pull in matter. How can this be?

It's definitely possible to cross an event horizon.
What SillyCow is saying is that a local observer free-falling into a blackhole won't be able to notice themselves crossing any particular horizon at all. An apparent horizon (indicated as a region of blackness) always remains ahead of them.