Van Stockum Dust - An Apparent Paradox

An Apparent Paradox

Consider the thought experiment depicted in the following figure, in which the inessential coordinate has been suppressed:

This figure depicts a thought experiment in which an observer riding on a dust particle sitting on the axis of symmetry looks out at dust particles with positive radial coordinate. Does he see them to be rotating, or not?

Since the top array of null geodesics is obtained simply by translating upwards the lower array, and since the three world lines are all vertical (invariant under time translation), it might seem that the answer is "no". However, while the frame given above is an inertial frame, computing the covariant derivatives

shows that only the first vanishes identically. In other words, the remaining spatial vectors are spinning about (i.e. about an axis parallel to the axis of cylindrical symmetry of this spacetime).

Thus, to obtain a nonspinning inertial frame we need to spin up our original frame, like this:

where where q is a new undetermined function of r. Plugging in the requirement that the covariant derivatives vanish, we obtain

The new frame appears, in our comoving coordinate chart, to be spinning, but in fact it is gyrostabilized. In particular, since our observer with the green world line in the figure is presumably riding a nonspinning dust particle (otherwise spin-spin forces would be apparent in the dynamics of the dust), he in fact observes nearby radially separated dust particles to be rotating clockwise about his location with angular velocity a. This explains the physical meaning of the parameter which we found in our earlier derivation of the first frame.

(Pedantic note: alert readers will have noticed that we ignored the fact that neither of our frame fields is well defined on the axis. However, we can define a frame for an on-axis observer by an appropriate one-sided limit; this gives a discontinuous frame field, but we only need to define a frame along the world line of our on-axis observer in order to pursue the thought experiment considered in this section.)

It is worth remarking that the null geodesics spiral inwards in the above figure. This means that our on-axis observer sees the other dust particles at time-lagged locations, which is of course just what we would expect. The fact that the null geodesics appear "bent" in this chart is of course an artifact of our choice of comoving coordinates in which the world lines of the dust particles appear as vertical coordinate lines.