Orchid XP v8 wrote:
> I gather space actually has a slight negative curvature, so it strictly 
> isn't Euclidian...

Well, it isn't uniformly curved, for sure.

>>> I don't know... Normally if you remove external force, pressure
>>> decreases.
>>
>> It's not pressure. It's gravity.
> 
> I know that the way black holes normally form is because matter is 
> crushed by gravity. But you said that to make a small black hole, you'd 
> need an external force to crush it instead.

Yes. But once you have a black hole, you can take away the pressure and the 
gravity will keep it "crushed". Indeed, "black hole" is the extent to which 
you have to "crush" some amount of matter before the gravity alone is enough 
to keep it "crushed". That's kind of the definition of a black hole.

> Also, I'm still not getting how you can have [gravitational] black holes 
> of different sizes. Presumably the force required to crush matter is a 
> constant, so...?

OK, make a black hole that weighs 10x as much as the sun. Now drop 50 more 
of our sun into it. Do you think the gravity will remain the same?

>>> I don't understand.
>>
>> The speed of light in a diamond is less than half the speed of light in
>> free space/vacuum.
> 
> Really? I didn't think it was such a big difference! o_O

The index of refraction (IOR) is the ratio of the speed of light in vacuum 
to the speed of light in that material.

-- 
Darren New, San Diego CA, USA (PST)
  "How did he die?"   "He got shot in the hand."
     "That was fatal?"
          "He was holding a live grenade at the time."

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