Albert Einstein’s general theory of relativity, completed in 1915, provided the tools needed to describe the universe’s structure and to determine the propagation of light through it. It showed that if one pretends that spacetime has no curvature, then light would appear to be deflected as it passes by a gravitational potential well. Such bending—now called gravitational lensing—was seen for the first time during the solar eclipse of 29 May 1919 by British expeditions in Sobral, Brazil, and on Príncipe Island, off the west coast of Africa. The expeditions observed stars near the edge of the Sun and measured their displacement to be closer to Einstein’s prediction of 1.7 arc-seconds than to the Newtonian prediction of half that value. The 1919 measurements represented a major triumph for Einstein’s theory.
For 60 years, the deflection of light by the Sun was the only known example of gravitational lensing. However, that changed...
