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Gravitational
Lensing
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One 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
(Ref).
Gravitational Lensing of Quasars
Gravitational lensing was actually discovered for
quasars 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
Einstein 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 years apart.
Some References for Gravitational Lensing
See also the discussion of gravitational lensing in the
search for dark matter.
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