Re: That Black-Hole Space-Time curvature thing

Eliezer S. Yudkowsky (
Sun, 14 Sep 1997 21:28:14 -0500

John K Clark wrote:
> If you were falling feet first into a black hole you would be crushed in the
> left right, and front back direction as I've described because spacetime
> would have a positive curvature in those dimensions, like a sphere, so two
> geodesic lines would converge. Things are different in the up down direction,
> spacetime would have a negative curvature in that dimension, like the inside
> of a bowl or a saddle, so two geodesic lines would diverge. You would be
> stretched in the up down direction and crushed in the right left and front
> back direction, an interesting way to die.

Right, crushed into an egg shape, just like the moon.

I disagree with your spatial visualization of divergence, though. It's tides,
which derive from space*time* divergence. When "geodesic lines" diverge in
spacetime, the result is different accelerations and tidal effects. You make
it sound like space is distorted, like someone in Flatland heading over a
'hill' and spreading out all over. But the Flatlandish metaphor is incorrect,
because Flattime is still flat. It's the spacetime divergence ("tide") that
does it, not spatial geometry.

--       Eliezer S. Yudkowsky

Disclaimer:  Unless otherwise specified, I'm not telling you
everything I think I know.