1
$\begingroup$

Imagine there is a test mass in the vicinity of spinning black hole, but the test mass is kept in place, i.e. it is not in free fall or orbit. Does it experience frame dragging?

My guess would be 'no', as the Lense–Thirring force should be proportional to the velocity of the test mass, in analogy to the Lorentz force. (?)

$\endgroup$
2
  • 1
    $\begingroup$ What experiment do you imagine would test this? $\endgroup$
    – John Doty
    Commented Dec 18, 2022 at 15:34
  • 1
    $\begingroup$ It is a theory question, but you would need a spinning heavy object and a small test mass in its vicinity. The test mass could be kept in place by rocket thrusters adjusted such the test mass' position does not change (as viewed from far away). An internal accelerometer would give you the all over force of gravity acting on the test mass. You would notice (not) frame dragging by this force (not) pointing straight to the center of the big object. $\endgroup$ Commented Dec 18, 2022 at 15:44

1 Answer 1

0
$\begingroup$

The Kerr metric has an explicit $g_{\phi t}$ component. From this component, the timelike killing vector of the spacetime, which is the "direction" of the timelike geometric symmetry, inherits a $\phi$ component from here.

If you attempt to keep an object at a constant $\phi$ coordinate while it evolves forward in time, it will not respect the time evolution symmetry, and this will show up as a net force on the object that makes it want to "orbit" along with the direction of the "frame".

A general metric tensor for a spinning physical object will be more complicated than the Kerr metric, but you will still have this same general picture emerge.

$\endgroup$
1
  • $\begingroup$ and I should say that this argument is based on the boyer-lindquist coordinates, which are appropriate for this discussion, because they asymptote to "minkowski in spherical coordinates" far from the black hole. $\endgroup$ Commented Dec 20, 2022 at 19:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.