Gravitational Lensing

 "It suddenly struck me that that tiny pea, pretty and blue, was the earth.  I put up my thumb and shut one eye, and my thumb blotted out the planet earth.  I didn't feel like a giant.  I felt very, very small." - Neil Armstrong

Black holes are weird.  It's hard to fathom the notion of a gravitational field so strong that not even light can escape from it.  There are all sorts of crazy physical effects that happen around black holes, but the topic of this post is "gravitational lensing".  

Gravitational lensing is a fascinating visual effect that causes distortions of an object behind a black hole.  That is, when a black hole (or other dense object, such as a massive galaxy) lies between an object and the observer, the black hole bends light from the background object in strange ways - much as an optical refractor lens would do.

Einstein - of course! - predicted the possibility of gravitational lensing.  He calculated that space (actually space-time) would be warped by massively dense objects, and cause light from a distant object to have to travel around the curvature of space-time caused by the dense object.  This would mean that the observer would see the distant object in a different place than where it actually exists.

 Note that the light doesn't actually bend in a curve.  The light travels in a straight line, along space-time that is bent.  Weird.

There's more - the effect is not perfect, like our diagram above.  These distant objects don't usually align perfectly between the black hole and us, and there may be several gravitational lenses between, so the images of the background objects aren't viewed with the clarity of a fine optical telescope.  Instead the background images are warped and twisted.  The background objects form arcs, twists, and sometimes you get multiple images of the same object.  Crazy stuff.

Let's look at a few. 

Below is an Einstein Ring.  In this case, a dense object in the foreground is affecting the light from an object behind it, making a circular pattern - the Einstein Ring.  We have good enough alignment with the other two objects that light from the background object travels around the foreground object in all directions.  The background object appears to us as a circle.  In reality the object behind is a point, but it appears as a giant arc because it has to travel around an object in between.

Next we have an Einstein Cross, in which multiple images of a background object appear (A, B, C & D).  These are all the same object - a distant quasar - but the light from that single distant object has traveled four separate paths around a galaxy's core to reach us.

More recently, the James Webb Space Telescope has provided some really beautiful examples of gravitational lensing.  The wavy lines and arcs at the lower right are distant objects, whose true form has been blurred by gravitational lensing.

Below are several images of Einstein Rings from the Hubble Space Telescope.  Very cool stuff!