# Does gravitational lensing provide time evolution information?

When we see multiple images of the same object because of the phenomenon of gravitational lensing, do all the images show the lensed object at the same point in time?

Does it take the same amount of time for light from the lensed object to reach us no matter what image we look at or are we watching the lensed object at different points in time and if so, how big can the difference in time typically be?

A rule of thumb appears to be that the delay is of order $$r_s/c$$, where $$r_s = 2GM/c^2$$, where $$M$$ is the mass of the lens interior to the path of the light ray. $$\tau \sim 10^{-5} \left(\frac{M}{M_{\odot}}\right)\ {\rm sec}$$ So for a $$10^{12}M_{\odot}$$ foreground lensing galaxy (a big spiral like the Milky Way), the delay is about 100 days.