= Causal cone for the constant-speed wave equation
{title2=$|x-x_*|\leq c(t_*-t)$}
For $c>0$, the backward causal cone at $(x_*,t_*)$ is
$$
\mathcal C^-_c(x_*,t_*)=\{(x,t):0\leq t\leq t_*,\ |x-x_*|\leq c(t_*-t)\}.
$$
Its 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 $t\geq0$, $|x-s|\leq ct$. 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>.
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