Re: Black hole question

From: Anders Sandberg (
Date: Thu Feb 17 2000 - 09:08:43 MST

Rik van Riel <> writes:

> Actually, I think this might be an excellent way to describe
> gravity.
> "Gravity is the projection of mass onto spacetime"
> (like the point on the wall is the projection of the
> light coming out of your laser pen)
> The same analogy can be used to describe how gravity
> interacts with itself. Projections don't appear to
> interact with each other, until you examine it more
> closely.

This is an analogy, and not entirely bad, but it misses out a couple
of things. The biggest problem is that it doesn't really tells us much
about what gravity actually does.

In general relativity matter and energy are described by the
energy-momentum tensor, a kind of field existing at every point in
spacetime describing how much energy and mass there is at that
point. The spacetime is described by the metric tensor, an entity
describing how spacetime curves in different directions at each
point. The Einstein equations tell how these two things interact.

Gravity is simply curvature of spacetime, usually caused by mass being
nearby. Sometimes it can occur that it gets bent because of
neighbouring spacetime, such as when gravity waves propagate through
space, or around a black hole. It can interact with itself (although
it takes very strong fields for anything to happen); sufficiently
strong colliding gravity waves can form black holes for example.

Anders Sandberg Towards Ascension!
GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y

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