In a previous blog post, "Einstein's Crazy Idea," I discussed how Einstein's theory of general relativity is a reinterpretation of gravity. Newton's original idea of gravity visualized it as a force between massive objects. Einstein instead surmised that the presence of mass warps space, and that curved space-time produces the motions we attribute to gravity. Earth's orbit around the Sun is either a curved path through flat space (Newton) or a straight path through curved space (Einstein).
Both ideas of gravity produce the same observed motions for most cases. But there are a number of situations, generally involving very strong gravitational effects, where general relativity explains phenomena that gravitational forces get slightly wrong. The differences are often subtle and take quite a lot of explanation to appreciate. However, one example is visually obvious: gravitational lensing.
The above image of galaxy cluster Abell 1689 is a prime example of gravitational lensing (click the image to see a larger version). Throughout the image are numerous small arcs, streaks, and strange-looking objects. Most of these are relatively normal galaxies (a few really are just strange-looking objects) whose images have been stretched and twisted by the galaxy cluster and general relativity.
The combined mass of the thousands of galaxies in the cluster (and their associated dark matter) heavily distorts the space-time around the cluster. Light from more distant galaxies passes through that warped space. The images of those distant galaxies become distorted as if they were being seen through an odd-shaped glass lens. In fact, the physics of light redirection using gravity is entirely analogous to that using lenses. It is the optics of complex lenses, but using mass instead of glass.
Newton's gravity can not produce such gravitational lensing. Well, to be complete, a gravitational force could produce half of the lensing effect of general relativity, but only if one assumes that photons (i.e., particles of light) have mass. Modern physics considers photons to be massless particles, and hence gravitational lensing does not exist in Newton's version of gravity, only in Einstein's general relativity.
For that reason, I like to say that pictures of gravitational lensing are visual "proof" of general relativity. You don't have to delve into the astronomy, physics, or complex mathematics -- just examine the image. Such distortions arise from general relativity.
Now, the visual distortions may be easy to spot, but that's not to say that these images are easy to interpret. Just the opposite is true. I'll provide some examples of the complexities of understanding gravitational lensing in my next blog post.
[NOTE: This post is the second in a series of four, and is a slightly modified version of the same post on the Frontier Fields blog.]