On 6 December 2013 03:00, John Clark <johnkcl...@gmail.com> wrote:

> On Wed, Dec 4, 2013 at 8:12 PM, LizR <lizj...@gmail.com> wrote:
> >> The earliest reference I can find is 1783 by John Michell, he  called
>>> them "dark stars", however it had very different properties from  a modern
>>> Black Hole. If I was far from one of Michell's Newtonian dark stars I could
>>> not see it, but unlike a real Black Hole, I could obtain a picture of it
>>> and print it in the newspaper, I'd just have to get closer in a powerful
>>> spaceship. I could even land on the classical dark star, get a sample of it
>>> and then return it to Earth, that sort of thing would be impossible with a
>>> real Einsteinian Black Hole.
>> > That's the one. It was used in a story by Brian Aldiss, I guess before
>> black holes became widely known about in SF circles (which was probably
>> thanks to Larry Niven). Of course one could only "land" on it if one could
>> withstand the gravity,
> If it was large enough the surface gravity on one of Michell's "dark
> stars" could be a earth like 1g or even less. The escape velocity from the
> surface of a object depends on BOTH its surface gravity and how big the
> object is because that determines how fast the gravity weakens with
> distance from the surface, with big objects even a long way away the
> gravity is almost as strong as it is on the surface.  Actually if it were
> big enough even with Einstein's Black Hole the gravity at the event horizon
> could be 1g, although after passing that point of no return you would find
> the gravity increasing continually until it reached infinity at the
> singularity at the center of the hole.

Ah yes I've heard that the gravity at the event horizon can be as weak as
you like with a suitably large hole - that you might not even realise you'd
crossed it (though surely you'd get some optical effects?) So the Michell
star is effectively like a solid version of a black hole's event horizon.
If that makes sense.

> > and only take off if one could travel faster than light
> No, a continuously thrusting rocket could escape from one of Michell's
> "dark stars" as slowly as you'd like just like you can from the Earth, but
> not so with Einstein's Black Hole, from that there is no way out.

I suppose one could have a space elevator or whatever and just climb gently
out of the gravity well.That's very interesting. It makes me rather sad
that such objects can't exist, because they'd make for some interesting
fiction, and maybe fact.

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