Solution
= Solution
Reverse the endpoints of the future-directed <causal curve>. The origin can be reached precisely from its <causal past>,
$$
\boxed{J^-(0)=\{(t,x):ct\le-|x|\}.}
$$
This is the orange region below the two null rays in part <1/c>. The origin itself belongs to both the <causal future> and <causal past> by the usual zero-length-curve convention.