How do you determine the velocity of a person in a distant o

How do you determine the velocity of a person in a distant observer\'s rest frame in general relativity?

The Kerr geometry is described by the Boyer-Lindquist metric where R and p are defined by and delta is the horizon function defined by Argue that a person who is moving along a worldline with dr - dtheta = 0 in Boyer-Lindquist coordinates will appear to be at rest as perceived by a distant observer at rest at infinity. ( The Boyer-Lindquist metric is independent of time t, and goes over to flat space at infinity. A distant observer at rest is themself moving along dr - dtheta = dphi = 0.]

Solution

Apparently in the Boyer-Lindquist coordinates frame, near the Kerr metric, the metric does not depend on the parameters psi and t. In other words, their time derivatives are Killing vector fields. The solutions of the Kerr metric contains two surfaces of infinite redshifts. At infinity for an observer, the motion of a particle at this infinite redshifts would be a stationary.