No. 56: Mar-Apr 1988
Those mysterious giant blue arcs mentioned above have at least two explanations. The less-favored states that the arcs are formed when a spherical light pulse from a briefly flaring quasar encounters a plane of gas and dust. This matter scatters the light, making it visible to us as a huge arc. The preferred model employs gravitational lensing. Here, the arcs are simply distorted images -- mirages, if you want -- of distant galaxies.
(Two of the many reports on this subject are: Waldrop, M. Mitchell; "The Giant Arcs Are Gravitational Mirages," Science, 238:1351, 1987; and Anonymous; "Giant Arcs: Light Echoes or Lensed Galaxies?" Sky and Telescope, 75:7, 1988.)
Comment. A bit of background: according to Einstein, the presence of matter can bend light rays, just as our atmosphere does when mirages are created by refraction. Gravitational mirages of celestial objects are thus predicted by Relativity.