quasar light commonly passes through regions of space that contain monstrous sources of gravity, which wreak havoc on the quasar’s image. These are often hard to detect because they may be composed of ordinary matter that is simply too dim and distant, or they may be zones of dark matter, such as what occupies the centers and surrounding regions of galaxy clusters. In either case, where there is mass there is gravity. And where there is gravity there is curved space, according to Einstein’s general theory of relativity. And where space is curved it can mimic the curvature of an ordinary glass lens and alter the pathways of light that pass through. Indeed, distant quasars and whole galaxies have been “lensed” by objects that happen to fall along the line of sight to Earth’s telescopes. Depending on the mass of the lens itself and the geometry of the lineof- sight alignments, the lensing action can magnify, distort, or even split the background source of light into multiple images, just like fun-house mirrors at arcades.

quasar light commonly passes through regions of space that
contain monstrous sources of gravity, which wreak havoc on the quasar’s image.
These are often hard to detect because they may be composed of ordinary matter
that is simply too dim and distant, or they may be zones of dark matter, such as
what occupies the centers and surrounding regions of galaxy clusters. In either
case, where there is mass there is gravity. And where there is gravity there is
curved space, according to Einstein’s general theory of relativity. And where
space is curved it can mimic the curvature of an ordinary glass lens and alter the
pathways of light that pass through. Indeed, distant quasars and whole galaxies
have been “lensed” by objects that happen to fall along the line of sight to Earth’s
telescopes. Depending on the mass of the lens itself and the geometry of the lineof-
sight alignments, the lensing action can magnify, distort, or even split the
background source of light into multiple images, just like fun-house mirrors at
arcades.