= Observer event horizon
{title2=$H^+_\gamma=\partial I^-(\gamma),\quad H^-_\gamma=\partial I^+(\gamma)$}
An observer's future event horizon bounds the events that can ever signal to its future-inextendible worldline. Its past event horizon bounds the events reachable by signals from its past-inextendible worldline. A complete inertial observer in <Minkowski spacetime> has neither. The uniformly accelerated worldline $t=a^{-1}\sinh(a\tau)$, $x=a^{-1}\cosh(a\tau)$ has $I^-(\gamma)=\{t-x<0\}$ and $I^+(\gamma)=\{t+x>0\}$. Their boundaries are its two <Rindler horizons>. This observer dependence is distinct from a black-hole horizon defined using <future null infinity>.
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