Wormholes Untangle a Black Hole Paradox | Hacker News Reader

Wormholes Untangle a Black Hole Paradox

quantamagazine.org

30 pointsdigital5510 comments

Can someone explain why black holes are often assumed to "lead somewhere"? Aren't they just Very Dense Objects in space--hence, they lead to the surface of the object?

Also, the event horizon can be quite wide, but why the assumption that there's some kind of "singularity" inside? There's a center of mass, but what's to say the central massive object is not quite large in volume and organized at scales/levels near the bottom of the planck scale? Or is that what they mean by singularity (not so much a single point in space as a small but quite massive object residing within the event horizon)?

>Can someone explain why black holes are often assumed to "lead somewhere"?

I guess because the thought that one universes black hole is anothers big bang is intriguing?

(not saying this is correct but it sure sounds like something I could fool an untrained person into believing.)

I'm not a physicist but my understanding of it is that inside a black hole all spatial dimensions inescapably point toward the singularity. If we were inside a black hole then we wouldn't also observe the universe expanding in all directions like ours is (unless time was running backwards?).

There are a lot of questions in there but I can try and answer a few:

1) Leading somewhere...

It's not really assumed they do but some speculate that they could.

2) Surface of the object/singularity/wide event horizon.

This is tricky to explain quickly and simply but here goes. In a very simple sense, a 'singularity' is really a name given to a thing which can't be explained by known laws or perhaps involves an infinity that shouldn't be able to exist in reality or some such. It's a name for a thing that by it's existance causes a 'problem' in that we an't fully explain it.

That's the layman's definition I often hear but in reality to be a singularity it needs to fit a set of criteria regarding geodesics and curvature etc that wikipedia can explain.

In the case of black holes it's used to refer to the infinitely dense mass that 'is' the black hole, or rather the point at which you'd reach if theoretically you fell all the way down to it. This isn't a small but massive object - it's an infinitely dense object.

So...there's the matter of infinitely dense. Once something loses the ability of its outward pressure of its mass etc to resist the inward crush of gravity, you get an object that becomes denser and denser and denser ad infinitum because it continuously collapses under its own gravity. It simply continues to collapse inwards on itself forever becoming 'infinitely dense' as it gets smaller and smaller.

There is no known reason why it wouldn't continue to collapse inwards on itself to an infinitely small point. There is no known reason why it would stop at the planck scale, no mechanism to suddenly overcome the immense gravity, and that's the problem really.

There is an assumption of a singularity because in order to create a 'strong' enough curvature of space-time such that light couldn't escape (i.e. to form an event horizon), you need to have a sufficiently dense object. The only way to achieve this is with an object that has collapsed past the 'point of no return' and continues to collapse in on itself (i.e you need a singularity).

Other fantastically dense objects like neutron stars lack the density, you've got to go 'super dense' and then you just get the run away effect and a singularity.

Very interesting, thanks!

Doesn't any spherically symmetric configuration of mass compressed into an area smaller than the Schwarzschild radius create a gravitational force strong enough such that light can't escape? If there was an unknown force that could prevent the collapse at some level below the Schwarzschild radius it would still be dense enough to capture light. And there certainly could be since our understanding of things at the Planck scale isn't complete.

First question answer: yes

Second question: yes maybe. No one knows.

An interesting aside is that infinite density occurs only (I think) with no rotating singularities which don't exist in reality. Anything rotating will produce a non-infinitely dense singulatiry somehow, though the details of the maths behind that elude me.

Kerr vs non Kerr blackholes is where I usually start replying with "we don't know" to every second question.

But broadly speaking the inside is theorised to be a 1 dimensional torus having only diameter as a measurable quantity other than the location of its center of mass. This is opposed to a zero dimensional point having only the location of its center of mass. This means the mass is distributed through the torus and is "less infinite" in the aleph zero vs aleph one kind of way.

It just gets weirder from there haha

In general relativity a black hole actually has zero volume and thus infinite density. If it has angular momentum it will collapse down to a ring of zero thickness and finite radius less than the Schwarzschild radius.

So basically the assumption for the singularity is because the mathematics of general relativity leads to (coordinate independent) divergence at the center of the black hole. What the physical significance of that is and how it jives with our understanding of quantum mechanics isn't fully understood.

Interesting using gloves for an analogy.

I've gotten so use to using 1's and 0's.

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