B Is there an inside to a Black Hole?

[PeterDonis]

I think I'd be turning the thread into something else then and derail it.

Yes.

I could discuss it on another thread.

Yes, if you want to discuss these models, please open a separate thread in the Beyond the Standard Model forum.

 

[stevil]

Nothing ever "experiences" time dilation. It's a purely coordinate effect. This is not semantics. What an object experiences is the passing of its own proper time. The object cannot experience how others may measure that time.

Yes, I'm not phrasing my thoughts correctly. I'm not up with the terminology.

 

[stevil]

That's not an assumption, it's a definition of spacetime. There is no such thing as a classical spacetime model where spacetime is not everywhere. The idea doesn't even make sense.

Is SpaceTime infinite in all directions?

I thought in a closed universe it's kind of like a bubble. There are no hard boundaries, no edge to the universe. But light going in a straight line doesn't always go further and further away from its source but eventually starts coming back just due to the curved shape of the universe. So there is no outside of the universe but if we were to model it on a three dimensional euclidean coordinate system (people intuitively think this way) we might point to a coordinate point and say but at this point which is outside the universe. That point doesn't exist, you can't get there, light can't get there, intuitively we think that place should exist but we can't get there.
Kinda like if there were multiple universes and if these were closed, we would have each universe in its own bubble but with no path to get from one universe to another.

Perhaps the above is just nonsense? I don't know enough about physics to know what is nonsense vs what is possible.
But I was wondering if the inside of a black hole were like that too. A space that we intuitively map out in a euclidean model and assume something must be there, but instead it just doesn't exist as there is no path for light to get there, not at any valid speeds.

The alternative is singularities
Singularities seem like nonsense to me (but I don't know enough and I'm not the right person to ask if singularities make sense or not).

 

[PeterDonis]

Is SpaceTime infinite in all directions?

That depends on the particular spacetime. Some are, some aren't.

I thought in a closed universe it's kind of like a bubble.

A closed FRW universe is spatially finite (but without boundary, as you note--topologically it's a 3-sphere), and also finite in time--it has an initial singularity, and (at least in the case of zero cosmological constant) a final singularity as well.

there is no outside of the universe

Yes. You can try to visualize the universe as embedded in some higher dimensional Euclidean space (as you do), but that causes more problems than it solves.

I was wondering if the inside of a black hole were like that too.

Like what? If you mean, like a closed FRW universe, no, it isn't. There are spacelike surfaces inside a black hole that are spatially infinite. As for trying to visualize an embedding of a black hole interior in a higher dimensional space, that's even more problematic than doing it for a closed FRW universe.

The alternative is singularities

Singularities aren't an "alternative" to anything. They are present in some spacetimes (such as both of the examples given above, a closed FRW universe and a black hole interior) but not in others. It just depends on the particular spacetime.

Singularities seem like nonsense to me

Many physicists believe that the presence of singularities in particular classical spacetime models is a sign that those models break down in those regimes, and that some new theory, such as quantum gravity, will be needed to model what actually happens in regimes where our current models have singularities. However, that is still an open area of research.