What is a Gravitational Lens?

Gravitational lenses are a beautiful quirk of nature (see above) which were predicted by Einstein’s general theory of relativity, which he himself wrote about in an article 1936 which made the phenomena famous; however the first discussion of gravitational lensing is sometimes attributed to Chwolson, O. (1924), or Klin, F. (1936). 

But down to business, what is a gravitational lens? Seen above in the gorgeous image of the Horsehoe Einstein Ring snapped by NASA’s Hubble Space Telescope, a gravitational lense is simply the lensing, or bending, of light of a distant galaxy around a massive gravitational source in the foreground.

We know that for the better part, light travels in straight lines through space; but in this case it would appear to not be so. Upon closer inspection and through the use of Einstein’s general theory of relativity we can understand that it is not the light that is bent, but rather the space is travels through. That is to say, the massive gravitational source in the foreground, be it a black hole or a galaxy, warps the fabric of spacetime so much that the light follows the curved geometry of space around the galaxy, causing this lensing effect.

We are used to thinking of light in two dimensions as per the ray diagrams we draw in physics class at high school or university, but in the case of a galaxy or a black hole, the structure is 3D and thus the lens operates in all three dimensions where the greatest effect is closest to the gravitational source and the least furthest away from it giving rise to a focal line, instead of a focal point. This line is drawn between the galaxy and and the gravitational source and at various points along the line the observer will see either a complete circle, a horseshoe or smaller arcs.