What does this equation mean?

Trapped-ion chains make that material-dependence concrete, because they let experimenters dial the range of the interaction and watch the cone respond. Philip Jurcevic and collaborators encoded a tunable-range Ising model on chains of trapped calcium-40 ions, with couplings falling off as 1/|i-j|^{$\alpha$} for an adjustable exponent $\alpha$ , and extracted the fastest magnon velocity the resulting dispersion supports. At short range the measured velocity agrees with the ordinary nearest-neighbour prediction; as $\alpha$ is pushed toward zero, toward an interaction that barely decays with distance at all, the velocity is consistent with growing without bound, and the light-cone picture stops holding [ 3 ] . The cone, in other words, is not a universal feature of matter. It is a feature of local matter, and a long-range enough crystal need not have one at all.