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The passage around this formula

set purely by the rest mass. For an electron this is $\omega_Z$ $\approx$ 1.55$\times$10^{21}\ $\mathrm{rad\,s^{-1}}$ , an oscillation period of roughly four zeptoseconds in the particle’s own rest frame — a boosted lab observer would clock a longer, time-dilated period by the ordinary factor $\gamma$ , coordinate time and proper time parting ways in exactly the usual manner — with an amplitude of order the reduced Compton wavelength $\hbar$/($m_e$c) , the same length scale, twice $\rho_M$ for a spin-half particle, that bounded the Møller disk above. Schrödinger first noticed this trembling motion, zitterbewegung, in the Dirac coordinate’s equations of motion; Foldy and Wouthuysen’s 1950 canonical…

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