Kerr Solution

Bill Wallace ( (no email) )
Sat, 3 Oct 1998 18:34:55 -0400

What can you tell us of the Kerr solution?

What happens if a gravitating object rotates is the so-called drag effect.
This is easiest to explain using a spherical body (e.g. a star). When the
star is nonrotating, a test mass (a small body) which is shot into the
direction of the centre of mass of the star will stay on a straight
trajectory (on a `radius'). With a rotating star, this is not the case any
more. The rotation somehow manages to `drag along' spacetime around it so
that the test mass would deviate slightly from the straight line path and
take a course into the direction of the rotation.

As you might know, gravity obeys Einstein's Field equations, and every
solution of those equations might potentially be realized in the `real
world'. The solution of the field equations for a spherical, rotating body
is known as the `Kerr solution', and it predicts the mentioned drag effect.
I'm not sure whether the solution for a torus is known (at least I wasn't
able to find anything in that direction), but I'm quite sure that the drag
effect will take place anyway. After all one can show that any mass
distribution of finite extension will more or less look like a point mass
(or a sphere) the farther away you are, but the information about angular
momentum must not be lost.

http://madsci.wustl.edu/posts/907332964.Ph.r.html