Symbol v_d

The passage around this formula

The magnitude is the first surprise. In a driven or flowing lattice, the cone speed along the drift is $c_{\mathrm{eff}}$ + $v_d$ in one direction and $c_{\mathrm{eff}}$ - $v_d$ in the other, so the anisotropy between the along-drift and transverse directions is $\delta$ c/c $\simeq$ $v_d$/$c_{\mathrm{eff}}$ — first order in the drift velocity, not the crushing second-order $v^2$/$c^2$ $\sim$ 10^{-8} that made the 1887 experiment so…

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