By Marc Lachieze-Rey (Astrophysicist and cosmologist, research director at CNRS)
Posted Sep 30, 2022 12:43 PM
Space-time is curved, and its curvature is identified with gravitation. This is arguably the most fundamental message of Einstein’s general relativity. As a direct consequence, a light ray does not describe a straight line but follows the spatio-temporal curvature. It therefore appears to us to be inflected and its direction of arrival does not point towards the real position of its source. Such a deviation was actually observed during the 1919 eclipse: that of light rays coming from a distant star, bent by the mass of the Sun (eclipsed by the Moon) passing close to the line of sight. This brilliant, very direct confirmation of Einstein’s theory gave him access to fame.
“Gravitational Arcs”
He had indeed foreseen the phenomenon. And in 1936, he had imagined an “ideal” situation, where a massive object would be located exactly in the line of sight of a distant point source, like a star. This line would then play the role of an axis of symmetry for the phenomenon, in particular for the configuration formed by the light rays coming from the distant source: a kind of spindle with spherical symmetry, having as vertices the source and the observer . They all converge on the latter, with the same angle of deviation from the axis, each giving an image of the star. The observer sees all these images and the symmetry causes them to be distributed continuously, drawing an “Einstein ring”.
Einstein was well aware that such a perfect alignment was improbable and therefore unrealistic; and that in any case such a configuration should remain unobservable because of the limited resolution of the observations. But he always enjoyed “thought experiments” and this one could appear as a kind of template for the actual images.
And indeed, for several decades now, astronomers have observed very many fragmented images of rings, “gravitational arcs”. And very recently, the MIRI instrument (Mid-Infrared Instrument) of the James-Webb telescope has just unveiled an almost perfect one: the image of the very distant galaxy SPT-S J041839-4751.8 (not at all annular in reality) appears to us as a thin ring of light, surrounding the near-point central image of the massive deflecting galaxy.
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