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.
Hubble image of galaxy cluster Abell 1689, showing a large number of lensed arcs (click on the image for larger version). These arcs are distorted images of background galaxies, gravitationally lensed by the mass of the cluster.
The above image of galaxy cluster Abell 1689 is a prime example of gravitational lensing. Throughout the image are numerous small arcs, streaks, and strange-looking objects. Most ...