Solution
= Solution
For a radial null ray, $d\theta=0$ and the metric gives $dt=dr$ for the outgoing branch. Starting at $r=t=0$, it reaches $r=R$ at
$$
\boxed{t_1=R}.
$$
= Solution
For a radial null ray, $d\theta=0$ and the metric gives $dt=dr$ for the outgoing branch. Starting at $r=t=0$, it reaches $r=R$ at
$$
\boxed{t_1=R}.
$$