Dispersionless chiral interface wavepacket
= Dispersionless chiral interface wavepacket
A branch $E=-\hbar ck_x$ gives a packet $f(y)u\,g(x+ct)$: it translates rigidly in the negative longitudinal direction while remaining transversely localized. Curvature of a more complete physical dispersion can restore packet spreading; the rigid result belongs to the exactly linear model.