Dear prof. Hestenes,
I liked your essay. It is very important, that the helical world line of the Dirac particle, predicted by your Zitter-Dirac model, was confirmed experimentally. There is an important detail, connected with the helical world line. Besides, the helix follows from the Zitter-Dirac model , it follows also directly from the Dirac equation. One can eliminate gamma-matrices from the action for the Dirac equation by means of change of dynamic variables, and one can present the action in terms of hydrodynamic variables. (Yu.A.Rylov, "Dirac equation in terms of hydrodynamic variables" Advances in Applied Clefford Algebras, 5, pp 1-40, (1995)).
Elimination of gamma-matrices discovers two important circumstances: (1) In the hydrodynamic variables the action can be divided into two parts: classical and quantum ones, (2) description of the Dirac dynamic system appears to be nonrelativistic.
The classical part describe the statistical ensemble of classical dynamic systems S_cl. Each dynamic system S_cl has 10 degrees of freedom. Considering the dynamic system S_cl as a particle, one obtains, that the world line of this particle is a helix. Six external degrees of freedom, describing translations are described relativistically, whereas four remaining degrees of freedom describe the particle spiraling. These four degrees of freedom are described nonrelativistically.
Nonrelativistic character of spiraling is a defect of the Dirac equation. This defect does not appear in the consideration of electronic envelopes in atoms, because in this case the intrinsic structure of the electron is of no importance. But it may be important in theory of elementary particle.
It is interesting, whether this defect appears in the Zitter-Dirac model of Hestenes. Although the nature of the spiraling is not yet clear, I believe, that prediction of this spiraling and confirmation it in experiment is a very important achievement.
Sincerely yours,
Yuri Rylov