  A short review of special relativistic dynamics describing a particle acted
upon by an arbitrary conservative external force is presented. If the mass of
the particle is zero and the force is central then the equations of motion turn
out to be completely integrable. A well-known result. Hamiltonian flows on the
twistor phase space T are constructed which, for conservative forces and value
of the helicity equal to zero, reproduce equations of motion of the classical
massless particle. For helicities different from zero the same hamiltonian
flows produce equations of motion showing a curious "Zitterbewegung" like
behaviour. A canonical Poincare covariant quantization procedure on T is
suggested. One simple example describing a spinning and massless 3-D quantum
mechanical harmonic oscillator is analysed in some detail.
