MadSci Network: Physics

Re: how does a rotating object affect the space-time field?

Date: Fri Oct 2 03:35:30 1998
Posted By: Georg Hager, Grad student, Theoretical Particle Physics
Area of science: Physics
ID: 906307525.Ph

Dear James!

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.

Hope that helps,

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