In observations of a double quasar image produced by gravitational lensing, fluctuations on the intensity of one image appear to be delayed compared with equivalent variations on the other image. Explain.

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Quizplus · Accepted Answer

The delay in fluctuations between the two images is due to the difference in path lengths that the light from the quasar takes to reach us. In gravitational lensing, light from a distant object (like a quasar) is bent around a massive object (like a galaxy) between the observer and the distant object. This bending can create multiple images of the distant object, and because the light can take different paths around the lensing object, these paths can have different lengths. The longer path means that light takes more time to reach the observer, leading to the observed delay in fluctuations between the images.

In Observations of a Double Quasar Image Produced by Gravitational