Re: A partial black hole?

On Feb 26, 2:33 pm, Erik Max Francis <m...@xxxxxxxxxxx> wrote:
James Burns wrote:
Let me try one last time to put my question, only now in
the terms which you three have offered me: Is it provably
impossible for a light-like or time-like worldline to
cross from normal spacetime to this "rotated" spacetime
/and back again/ to normal spacetime? It's looking like
the answer is, yes, it's provably impossible.

If by "'rotated' spacetime" you mean the region within an event horizon,
then yes, the answer is that it is impossible. It is almost a
tautology, actually, since the event horizon of a black hole is
_defined_ as the region around the black hole where, if any particle
enters, it cannot get out. All lightlike and timelike trajectories
inside the event horizon point inward; none point outward.

I think the source of my confusion is Frederik Pohl's
Heechee series, part of which takes place inside the
large black hole in the center of our galaxy. In his
universe, conditions inside a black hole aren't too
different from conditions in normal space.

He adds a touch of verisimilitude by saying that the
density of a really large black hole is about that of
somewhat crowded interstellar space. I know that
density goes down as black holes get more massive, but I
never ran the numbers on this point.

Unfortunately, it looks like even a supermassive
black hole is much too small to have a believable
interstellar density.

I'm not familiar with the story, but perhaps he was talking (or
thinking) about a supermassive black hole big enough to get what he
wanted, whether or not it's what we would today call a supermassive
black hole.

He is correct that with a large enough black hole, the tidal forces
could be made arbitrarily low, and the freefall time could be
arbitrarily large, so that you could poke around and live your life and
not really notice anything wrong -- because locally, things are still
fine. Your eventual destruction near the singularity would still be
inevitable, though.

It's essentially a Big Crunch scenario. Or Small Crunch, perhaps,
since the region of spacetime involved is fairly tiny compared the
rest of the universe.
Once you're inside the event horizon, the singularity doesn't quite
exist yet from your point of view, but space appears to be collapsing,
and at some point in the future it will collapse to a singularity, and
there's no way to get around it, 'cuz at that point in time, there's
no extra space left outside the singularity. The only way to avoid the
Big Crunch is to move backwards in time; and since you can't do that,
you can't get out of the black hole.

-l.
.

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