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v=0.5 ceffv = 0.5\,c_{\mathrm{eff}}

Why this formula appears here

A fourth fact transfers, but only with a correction the paper computes rather than sets aside. A quasiparticle wavepacket moving at speed v inside the linear window carries an internal clock — a spinor phase, a zitterbewegung oscillation — that runs slow by the familiar factor γ\gamma = (1 - v2v^2/ceff2c_{\mathrm{eff}}^2)^{-1/2} : 1.15 at v = 0.5\,ceffc_{\mathrm{eff}} , 2.29 at 0.9\,ceffc_{\mathrm{eff}} . That dilation is real and, in principle, measurable, but only inside a window whose edge the band’s own curvature sets. Writing the correction as a term proportional to (k/Λcurv\Lambda_{\mathrm{curv}})^2 , the fractional error in γ\gamma stays under one percent as long as the wavepacket’s momentum sits below…

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v=0.5 ceffv = 0.5\,c_{\mathrm{eff}}

Equation 35 · Crossed Fields

Every Crystal Has Its Own Speed of Light

This equation states an equality: the expressions on both sides have the same value under the article’s assumptions.

A fourth fact transfers, but only with a correction the paper computes rather than sets aside. A quasiparticle wavepacket moving at speed v inside the linear window carries an internal clock — a spinor phase, a zitterbewegung oscillation — that runs slow by the familiar factor γ\gamma = (1 - v2v^2/ceff2c_{\mathrm{eff}}^2)^{-1/2} : 1.15 at v = 0.5\,ceffc_{\mathrm{eff}} , 2.29 at 0.9\,ceffc_{\mathrm{eff}} . That dilation is real and, in principle, measurable, but only inside a window whose edge the band’s own curvature sets. Writing the correction as a term proportional to (k/Λcurv\Lambda_{\mathrm{curv}})^2 , the fractional error in γ\gamma stays under one percent as long as the wavepacket’s momentum sits below…

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