Re: That Black-Hole Space-time curvature thing

Kennita Watson (
Sat, 13 Sep 1997 08:09:08 -0700


A big hug to John K. Clark (assuming he wants one)!

I spent all these years thinking that people who fell into a black
hole would be torn apart, when actually they would be crushed together!
This explanation is great!

Kennita (still dazzled by the light bulb going on over her head :-) )

(the rest is appended for people who missed it -- delete at will)

John K. Clark wrote:

>On Thu, 11 Sep 1997 Sarah Marr <> Wrote:
> >Ok, so what I've never quite understood is this: why does the
> >extreme curvature of space-time at the event horizon of a small
> >black whole tear the intrepid astronaut to bits?
>If no force is acting on the astronaut except gravity, that is, if he's
>freely falling, then ever particle in the poor man is moving in parallel
>straight lines, or rather the analog of parallel straight lines when you're
>dealing with curved surfaces, a geodesic. 4 dimensional spacetime can be
>thought of as the surface of a 5 dimensional sphere, but that can't really be
>visualized so consider the 2 dimensional surface of a 3 dimensional sphere,
>like the earth.
>If I'm on the equator and want to go to the north pole by the shortest route
>then there is only one path I can follow, a geodesic. Suppose I have company
>on my trip because my neighbor, also on the equator 10 miles west of me,
>is going to the pole by the shortest route too. Both out paths are absolutely
>perpendicular to the equator and at first, when we've only gone a short
>distance, my path and that of my neighbor seem absolutely parallel too, but
>as we continue our journey we start to get closer together until at the pole
>we collide. There was no force pushing us together, it's just what happens
>when 2 people follow a geodesic on a sphere. Even if we were 2 dimensional
>creatures incapable of visualizing the third dimension we could still deduce
>that the ground must be curved in that mysterious direction because we find
>that parallel straight lines converge if they're long enough, if fact that's
>what is mente by a surface being curved.
>At fist our astronaut feels no force as he falls into the central point of
>the black hole, the singularity, because all his parts are moving straight
>and parallel, and if space was flat things would stay that way, but it's
>curved and the curve is increasing. Despite moving in parallel his parts are
>nevertheless getting closes together and will continue to do so until every
>particle in his body occupies exactly the same point in spacetime, he'd be
>crushed to death long before that of course.
> >When space-time distorts, as I understand it, 'new' space-time is
> >not created, existing space-time merely stretches. But it
> >only in the context of an external point of view. Anything within
> >that space-time would not see the stretching
>If our astronaut was the size of a mathematical point then he would feel no
>force and have no problem, but he's so big that his head and feet are in
>VERY different reference frames, and that's not healthy for our poor friend.

Kennita Watson | The bond that links your true family is not one of blood,| but of respect and joy in each other's life. Rarely do
| members of the same family grow up under the same roof.
| -- Richard Bach, _Illusions_