Pole-dipole model of massless particles. II

The pole-dipole equations derived in a previous paper for massless particles (whose defining relation is that the energy-momentum tensor has zero trace) are examined. If the reference point ${X}^{i}$ describing the motion is not somewhere on the disk perpendicular to the three-velocity through the energy center, then ${X}^{i}$ describes a null geodesic with no assumptions. The property that ${X}^{i}$ is the particle's energy center in some local reference frame (called a $C$-frame) is shown to be a constant of the motion. If the energy of the particle in the $C$-frames is not zero, then the trajectory is again a null geodesic. The conditions on the curvature needed to have the momentum parallel to the four-velocity are determined. Helicity properties are also discussed.

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