How the Einstein Ring Could Help Us Build the Greatest Telescope of All Time
On Einstein Rings and the ultimate telescope
Like almost all good discoveries, this one was made by accident. In September of 2023, two months after Euclid set off on its six-year-long mission to explore the cosmos, the telescope sent back a set of images. They were, quite bluntly, nothing special to look at. Indeed, Euclid’s engineers were not yet interested in pretty pictures of the stars and galaxies, but were instead taking calibrations and checking for ice on the observatory’s instruments.
As a result, the images sent back were deliberately blurred and out of focus. Neither were they supposed to be of anything particularly interesting: the galaxy they showed, NGC 6505, was thought to be as unremarkable as any of the countless others strewn across the heavens. Yet when Bruno Altieri, an engineer in Spain, picked up the images and started to look through them, he noticed something astonishing.
Around NGC 6505, he found, was an almost perfect ring of light. Later photographs, taken with the full power of Euclid’s cameras, confirmed its presence: it was not, they showed, any artifact of ice or of the camera lens. Instead it was an Einstein Ring: a strange and beautiful consequence of Einstein’s theory of relativity.
NGC 6505, it turns out, lies almost directly in front of another galaxy, one placed about four billion light years further away and, so far, unnamed. As light rays from that distant galaxy pass NGC 6505 they are curved and focused on a line running through the solar system. From our vantage point, then, that distant galaxy appears not as a faint smudge, but as a ring of light encircling the closer galaxy.
Such rings are, in general, rare. Einstein himself thought it unlikely we’d ever find one. In the paper he wrote describing the mathematics of such objects he ruled out the possibility, arguing that they’d be too small and too rare to ever be found. But time has proved him wrong: improved telescopes and the immensity of the universe mean we have by now found a few hundred scattered across the heavens.
Yet most are extremely far away and hard to make out with any clarity. Altieri’s Ring, as astronomer Connor O’Riordan pointed out, differs. It is one of the closest known to Earth, and one of the brightest and clearest ever found. Even more remarkably, it was spotted in a galaxy we thought we knew – NGC 6505 was first discovered over a century ago. No one, until Altieri, had ever suspected the presence of the ring.
Project scientists reckon Euclid will find thousands more Einstein Rings over its scientific career. Another twenty of them, they say, could be waiting to be found in nearby galaxies. But none are likely to surpass Altieri’s ring in beauty or in brightness. It was, surely, the discovery of a lifetime.