 Causal cone for the constant-speed wave equation

For $c>0$, the backward causal cone at $(x_{\ast },t_{\ast })$ isIts boundaries are characteristic lines for the one-dimensional wave equation; its base on $t=0$ is the initial-data domain of dependence. The forward cone from $(s,0)$ is $t\ge 0$, $|x-s|\le ct$. Here the cone is set by the model's wave speed, which may be acoustic or a string wave speed; it does not assert propagation at the speed of light.