Gravitational-lensing time delay
= Gravitational-lensing time delay
{title2=$\Delta t=(D_{\Delta t}/c)\Delta[\tfrac12|\theta-\beta|^2-\psi]$}
For a thin lens, relative arrival times follow the difference of $\tfrac12|\theta-\beta|^2-\psi(\theta)$ between images, multiplied by $(1+z_l)D_lD_s/(cD_{ls})$. Both geometry and <gravitational potential> contribute; an additive delay common to all images is unobservable.