Gravitational Lensing

Imagine a bright object such as a star, a galaxy, or a quasar, that is very far away from Earth (say...10 billion light years). For our discussion, let us imagine we have a quasar. If there is nothing between it and us, we see one image of the quasar. Yet, if a massive galaxy (or cluster of galaxies) is blocking the direct view to the quasar, the light will be bent by the gravitational field around the around the galaxy and we will see twin images of the quasar [see Figure 1]. This is what is called "gravitational lensing," since the intervening galaxy acts as a lens to focus the image of the distant quasar to a new location. (Of course if the galaxy were perfectly symmetric with respect to the line between the star and the Earth, then we would see a ring of image stars!)

Now, if the massive galaxy is off-center (as might be expected) with respect to the line between the quasar and the Earth, then the two light paths would be different distances around the galaxy. This makes the twin images be formed at different distances away from the actual quasar.

Finally, since the distances between each of the objects is so great, the radius of the galaxy and the mass distribution of the galaxy are well approximated by point masses (the error is small). Thus, one can use simple geometry (knowing the mass of the galaxy, the distance of the galaxy and the two images) to estimate the distance to the actual quasar.

As an example of what gravitationally lensed objects would look like, check out the Hubble Space Telescope image (http://oposite.stsci.edu/pubinfo/gif/A2218.txt)below.