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The passage around this formula

A number makes the scale concrete without pretending to describe an experiment. Take a spin-one-half wavepacket of electron mass, S = $\hbar$/2 , so that $\rho_M$ = $\hbar$/(2$m_e$c) $\approx$ 1.931$\times$10^{-13}\ $\mathrm{m}$ , essentially half the electron’s reduced Compton wavelength. Boost it transverse to its own spin at $\beta$ = v/c = 0.6 . The exact centroid shift between the rest-frame observer and the boosted observer is $|\Delta \mathbf x| = \rho_M\,\beta \approx 1.16\times10^{-13}\ \mathrm{m}$. about a hundred and sixteen femtometres — smaller than a proton’s charge radius, far below anything a position-resolving instrument built from ordinary matter could hope to see directly, and yet an exact analytic evaluation of a closed-form identity, not a…

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