It appears that zitterbewegung, a frequency associated with the total energy of a particle or system, is widely considered to be an unphysical quantity (e.g., Kobakhidze et.al.), @Lubos Motl, McMillan). However, a few physicists including Hestenes, Recami et.al.,consider it to be a fundamental, physical quantity.
It appears that, for example, the photon emitted in transitions between two states in an atom has precisely the frequency of the "beat frequency" (difference frequency) between the zitterbewegung frequencies of the two states.
Why is zitterbewegung considered unphysical by most physicists, and has there been a solid refutation of Hestenes' position that zitterbewegung is physical?