Re: Black hole question

From: John Clark (
Date: Tue Feb 15 2000 - 23:34:55 MST

Damien Broderick <> Wrote:

>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.

I agree the analogy gives no insight as to why mass distorts Spacetime,
nobody knows anything about that, but I find it helpful in understanding how
things move given that distortion. If no force is acting on an object and so
is freely falling then it must be moving in a straight line, or rather the analog
of 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
object, and if you distort that object you'll change the way things move.

It can explain why the earth orbits the sun. Call one point in time "now" and
put a dot on the surface of the hyper object at that point. Change only the time
dimension, find the new point on the surface and call it "then". If you draw the
shortest line connecting the two points on the curved Spacetime surface the
sun produces then you'll find out how the Earth moves between now and then.

      John K Clark

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