How gravity bends light

Light travels in straight lines, right? Not quite. According to Einstein's general relativity, mass warps space and time, and light passing a massive object follows that warp and bends. A big enough mass, like a galaxy cluster, acts as a gravitational lens, bending, distorting and magnifying the light of whatever lies behind it.

The first test came during the total solar eclipse of 29 May 1919. With the Sun's glare blocked, expeditions to Príncipe in West Africa and Sobral in Brazil photographed stars near the Sun. On Príncipe the astronomer Arthur Eddington woke to a storm and heavy rain, and was then so busy changing photographic plates that he barely saw the eclipse. But the plates held the answer: the stars' light had been slightly bent, so they appeared slightly out of position. The result made front pages and made Einstein world-famous.

Today lensing is one of astronomy's most useful tools. When a source, a lens and an observer line up perfectly, the source appears as a glowing ring, an Einstein ring. Because most of the mass in lensing galaxy clusters is invisible dark matter, mapping how background light is distorted lets astronomers chart where that dark matter sits.

Lensing can even play with time. In 2014, the Hubble telescope saw a supernova, named Refsdal, split into four images by a galaxy cluster. Its light had also taken another, longer path. Astronomers predicted where it would show up, and in December 2015 it did. The same explosion, seen twice, a year apart.