Each image represents light taking a slightly different path around the galaxy. The paths are of unequal lengths. Thus light that left the quasar at a particular instant arrives here at four slightly different times. The astronomers explain that by measuring the delay times in the path lengths and knowing the geometry of the whole arrangement, they can determine the distance to the lensing galaxy. However, to accomplish this, the quasar's components need to be monitored for some time.
Quasars vary in brightness.
In X-rays they can vary significantly in just a few hours. Thus astronomers think they can find out precisely when the same brightness change occurs in each of the components. He and Bautz have identified nine other lensed systems that could also be monitored. One long observation of each source with a superior X-ray telescope could provide enough data to nail down the Hubble constant in the blink of an eye. That, however, may be overly optimistic.