Re: A partial black hole?

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.

For any reasonably realistic black hole we can imagine today, though, it 
wouldn't work, as you say.  Note that when we're talking about tides 
killing you near the singularity of a mundane stellar-massed 
Schwarzschild black hole, we're really being kind:  The tides near the 
_event horizon_, before you've even passed the point of no return, would 
be plenty fatal as it is.  Not to mention the radiation from any 
accretion disk that might be present (though usually in these thought 
experiments we assume that it's an isolated black hole to make things 
simpler).

-- 
Erik Max Francis && max@[EMAIL PROTECTED]
 && http://www.alcyone.com/max/
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   Chastity the most unnatural of the ***ual perversions.
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