**Publication date:** Feb 1993

**Abstract:**

The radius of gyration is a familiar concept in Newtonian mechanics and

a suitably defined relativistic generalization of it turns out to be

very useful for analyzing rotational effects in strong gravitational

fields. The present paper contains a discussion of the properties of

this quantity and of its level surfaces (the von Zeipel cylinders) and

also of its connection with the effective potential for photon motion

and with ideas of centrifugal force. The direction of increase of the

radius of gyration gives a preferred determination of the local outward

direction relevant for the dynamical effects of rotation, but this

direction becomes misaligned with the global outward direction in

strong-field situations. This misalignment underlies some apparently

counterintuitive behavior of the centrifugal force in strong fields

which has recently been the subject of considerable interest.

**Authors:**

Abramowicz, M. A.; Miller, J. C.; Stuchlík, Z.;