Redshift drift and strong gravitational lensing

In general, the cosmological redshift of an object changes with time, a phenomenon known as redshift drift. Although long known theoretically, recently interest has been renewed because of the possibility of measuring it on a reasonable time scale. Strong gravitational lensing offers a possibility to measure it on a much shorter time scale, by making use of a time delay of n years rather than making observations separated by n years, but, perhaps at least in part because of the expectation that the signal would be swamped by a larger change in redshift due to transverse motion of the lens, that has not attracted much interest. I present a method to extract the small signal, making use of the fact that the light-travel time through different parts of an Einstein ring is the same (and hence the difference in redshift due to redshift drift vanishes), thus enabling the measurement of redshift drift on a much shorter time scale, and show how that can help in the measurement of the distribution of (dark) matter.