Gravitational lenses are part and parcel of the General Theory of relativity. In it, large masses such as stars and planets warp the fabric of space time like heavy balls sitting on a stretched rubber sheet (but transposed into three dimensions rather than two). The objects create indentations around them, which deepen with increasing object mass.
Eddington took part in a trip to the island of Principe in 1919 to witness an eclipse of the Sun in front of the brightest star cluster in the sky, the Hyades. When the bright solar light was blocked out, he saw in photographs that he took that the apparent positions of stars close to the Sun had been shifted away from its position. The closer they were supposed to be, the greater the shift. The shift was in line with the predictions of Einstein’s new theory (as opposed to Newton’s theory with the assumption of photon mass, which would give half the shift, or Newton’s theory with no photon mass, which would give no shift).
Since Eddington’s time, gravitational lensing has become an active observational and theoretical field. New results from teams in America and Germany have identified ways of using gravitational lensing (this time using galaxies passing in front of other galaxies, as is now much more common than using stars passing behind stars) to determine the age of the universe and Hubble’s constant, which defines the rate of the expansion of the Universe.
The new technique uses the fact that an off-axis (ie not perfectly aligned) galaxy lensed by another will be seen as multiple images around the lensing galaxy. These images involve light travelling different optical lengths (distances, with modifications for how difficult it is for light to travel over parts of them). This allows a single lensing event to provide multiple cases of the lensing effect that can then be used with general lensing theory to test it. A bit like solving multiple equations to get hold of certain constants.
It’s not all hard maths and philosophy though, sometimes the lead authors like Philip Marshall can relax with a glass of wine and a candle. It is after all one way to help describe to people how gravitational lensing produces the images it does…