Re: Black hole question

From: Damien Broderick (
Date: Tue Feb 15 2000 - 19:07:13 MST

At 03:52 PM 15/02/00 -0500, John K Clark wrote:

>Gravity is nothing but Space -Time curvature. If you place a canon ball on
>a rubber sheet the sheet will be distorted but that doesn't prove something
>must come out of the ball to bend the rubber.

Nicely put, but I've always detested that model/image. Unpack it, and you
learn that if you are already standing in a gravitational gradient (on the
surface of the Earth, say), and you put a heavy ball on top of a sheet of
rubber (don't try this with the rubber on top, kids), the ball will deform
the sheet. Well, duh. In the real 4-space this image is trying to model,
there's no additional force to bend the `sheet' by drawing the cannon ball
downward. So it's spurious and circular.

Yes, of course it's the *shape* of spacetime that's being modelled, but I
still feel that the imagery is profoundly misleading at a gut level. So
maybe a better image would be a rubber sheet with a stick poking into it,
deforming the surface? (Practise safe spacetime!)


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