important consequence of the influence of Einstein's
gravitation on light is that gravitational masses can alter the direction
of light and cause lensing effects.
Lensing of Galaxies
The following Hubble Space Telescope image shows a spectacular example
of such a
gravitational lens. The arc-like
pattern spread across the picture like a spider web is an illusion caused
by the gravitational field of the cluster. The arcs are actually distorted
images of very distant galaxies that are being imaged by the gravitational
lens: The cluster is so massive and compact that light rays passing through
it are deflected by its enormous gravitational field, much as an optical
lens bends light to form an image. The figure also exhibits multiple imaging,
which is less common and happens when the distortion is large enough to
produce more than one image of the same galaxy.
The following figure shows a gallery of images giving evidence for
possible gravitational lensing
Gravitational Lensing of Quasars
Gravitational lensing was actually discovered for
before it was found for galaxies. In 1979 two quasars only 6 seconds of
arc apart were discovered and they were found to have identical redshifts
and spectra. The probability of this happening by accident is exceedingly
small, and it was postulated that this pair actually corresponds to the
same quasar that is lensed so strongly by intervening mass that its image
appears double as we view it from Earth. The adjacent image shows the "double
quasar" QSO 0957+561. The two bright blue objects in the center are actually
gravitationally lensed images of the same quasar.
This interpretation was bolstered by the discovery of a galaxy nearly
in front of one of the images and a surrounding cluster of galaxies that
are candidates for the intervening mass causing the lens effect. For a
more detailed discussion of the double quasar and this image, see this
discussion by Bill Keel.
The Einstein Cross
Here is another example of a gravitational lens called the
Cross. In this image, a single object appears as four objects.
A very distant quasar is thought to be positioned behind a massive galaxy.
The gravitational effect of the galaxy has created multiple images through
gravitational lensing on the light from the quasar. The stars in the foreground
galaxy also seem to be acting as gravitational lenses, causing the images
to change their relative brightnesses in these two photographs taken three
Some References for Gravitational Lensing
See also the discussion of gravitational lensing in the search for dark