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Comment by rabito | Hacker News Reader

I wonder if the space region in the interior of a black hole expands the same way as regular space around it.

If so, it would seem to be possible for a super massive black hole to be so big that it would be impossible to actually reach the singularity. If there is enough space the singularity would "move" faster than light. Essentially another event horizon within the black hole protecting travelers from the singularity.

This would lead to a stable universe like ours. Though about this a few days ago and it seemed like an interesting idea.

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I remember from a youtube video that the spacetime inside the black hole flipped around, where space becomes temporal dimension and time becomes spatial dimension (i.e. time is traversable in either direction while the singularity becomes an inevitable place in the "future").

That is true, the time and space axis gets flipped at the event horizon. This also happens to a fast moving objects due to relativistic effects, although the roatation is usually not very high unless you go really fast. Seehttps://en.m.wikipedia.org/wiki/Minkowski_diagram

However that doesn't mean you can't move when you cross the event horizon. You can do that like usual (except for gravity pulling you inside. You can. It only means outside observer won't be able to see what you're doing, because of the time dilation (and the event horizon of course).

This kinda implies time and space are the same thing, it just depends on your point of view. Still have to see about that :D

To be pedantic, it's not that they are the same thing, but that they are intrinsically tied together: Time begins at the moment of first inflation.

> the time and space axis gets flipped at the event horizon. This also happens to a fast moving objects due to relativistic effects

Neither of these things are true, and I have no idea how you are getting either of them from the Wikipedia article you linked to. Penrose-Terrell rotation, which affects how objects appear to observers when they are moving at relativistic speeds relative to those observers, does not "flip" the time or space axes; it's an optical effect due to the finite speed of light.

As I noted in another post upthread, the "flip" of time and space axes inside a black hole's horizon is a property of a particular choice of coordinates, not of spacetime itself.

> This kinda implies time and space are the same thing

No, they aren't, because there is still a fundamental difference between timelike and spacelike directions in spacetime. That is true regardless of how you choose coordinate axes.

People often mistakenly try to map space time onto an ether-like flat background, treating spacetime like a drawing where the paper is the background. It's not an uncommon kind of fumbling as someone tries to grasp these things.

I wonder if inside the black hole the singularity behaves more like past than it behaves like future.

No, it doesn't. The singularity inside a black hole is a future singularity, not a past singularity.

Just to clarify, you mean it's a future singularity in the entropy sense, or in some other sense?

Iff I understand correctly, what pdonis is saying is that it is in your future. It’s where (when?) you run out of future to go into, in a way which may or may not be analogous to the North Pole being the coordinate singularity where you run out of north to move in.

So it's the end of time in a sense?

That’s my understanding, but I’m just an enthusiastic amateur and that doesn’t take you very far with relativity.

This is from PBS Spacetime series [1]. A fantastic channel, and they did a number of videos on black holes.

[1] https://youtu.be/KePNhUJ2reI

> spacetime inside the black hole flipped around, where space becomes temporal dimension and time becomes spatial dimension

No, this is not correct. The "flip" is in a particular choice of coordinates, not in spacetime itself.

> time is traversable in either direction while the singularity becomes an inevitable place in the "future"

It is true that the singularity is inevitably in the future--that's because inside the horizon, the future "time" direction points towards the singularity. But time is still only traversable into the future; you can't "reverse" your travel in time inside the horizon.

If black holes are universes, then what happens when 2 black holes merge? What happens when universes merge? The sudden appearance of more stuff? More space? Existential cataclysm?

And what happens when they finally evaporate due to Hawking Radiation?

With our current theories such questions are nigh intractable.

Given that "uni" means one, what could "universes" even mean?

It's simply the plural of "universe". If there actually is more than one universe, it would imply that "universe" was a misnomer, much like "atom" turned out to be.

People might keep using the word with a new definition. The new definition could be something like, "a region of the multiverse that is causally disconnected from other regions of the multiverse under such-and-such conditions." If those conditions change, two universes could become causally connected and merge into one universe. I imagine inhabitants of those universes would (if they look in the right place) witness events with no recognizable cause until the merger was complete.

Or what we currently call the universe might be renamed a sub-universe within the actual "one" universe, which was much bigger than we thought. Then the question is what would happen when sub-universes merge. Same question, different glyphs.

The same implication as unicycles.

Check out Lee Smolin's Fecund Universes Theory. http://evodevouniverse.com/wiki/Cosmological_natural_selecti...

Whoa that sure sounds plausible.

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