Although
Newton was the first to theorize about gravitational lensing, his theory was
discredited by the prevailing understanding of light as a wave, not a particle.
It was not until Einstein formulated his theory of relativity that the concepts
behind gravitational lensing became more widely understood. Einstein's theory
gave predictions qualitatively similar to but quantitatively different than
those made by Newton's theory. The Theory of General Relativity states, among
other things, that highly massive objects with strong gravitational fields are
able to warp space-time and "curve" the very fabric of space around an object.
Any light passing through this curved space-time area would continue in a
straight line while the space-time through which it travels remains curved.
(See Figure 2.) The light emerging from this curved space-time will appear to
travel in a bent direction, rather than in its original trajectory. It may be
easier to picture this by imagining gravity pulling the light rays in toward
the massive object and deflecting them away from their original path.
Our own sun creates gravitational lensing effects which are noticeable during solar eclipses when the light from stars located directly behind the sun is bent around and they become visible to us at the edge of the sun. Deep space objects may similarly lens luminous objects which would normally be hidden behind them.
For
gravitational lensing to occur from the Earth's observational standpoint, the
Earth must be almost directly in line with the high mass object acting as the
lens and the distant object to be lensed. The variations in lensed images we
observe result from differences of alignment of the actual objects being lensed
or of the object doing the lensing. If a point source such as a star or quasar
is being lensed, we will see multiple images. The number of images is
dependent upon the structure of the lens.
In the example shown in Figure 3, light not only passes through the center of
the galaxy, but through the edges of the galaxy as well, warping the light and
bending it towards the observer, creating images at "a" and "b" yielding a
triple image. The number of images is dependent upon the structure of the
lensing galaxy. (See Figure 3.)
Quasars and stars are not singular in their subjectivity to lensing effects. Galaxies may lens other galaxies. The light we see emitted by galaxies is not a point source, but rather an extended source because a galaxy's light is drawn from stars which are spread across hundreds of thousands of light years. When this large mass of light is lensed by another galaxy in our line of sight, we see an arc of light stretching around the lens. If both galaxies are lined up precisely with the Earth, the image of the more distant galaxy will appear as a halo around the nearer galaxy.
The term "dark matter" comprises any non-luminous mass in the universe incapable of emitting light. Distant objects in the universe remain visible to us because they emit light. We are unable to view distant objects not emitting light. The earth, for instance, along with the other planets in our solar system, fit under the classification of dark matter because they remain unable to emit light. They are limited to reflecting the light emitted by our sun. Therefore, our sun is the only body in our solar system that would be incorrectly labeled as dark matter. Dark matter may range from planets to nebulas (conglomerations of interstellar gas).
Quasars
are a particular type of quasi stellar object (QSO). As quasi stellar objects,
they are light sources in space which appear to be stars, but are much larger
and more luminous than stars.
Our current knowledge of quasars remains limited as we are still unable to view
them at closer range. Gravitational lensing may be employed as a means of
magnifying QSOs, allowing us to study these at seemingly closer proximity.
Gravitational lensing may not be readily apparent from pictures. Often, multiple images are blurry, distorted, or unrecognizable as gravitational lensing. Astronomers measure a star's red shift in order to identify multiple images of possibly the same object. The red shift of an object is the stretching of light waves emitted by that object while that object moves away from an observer. The changes of wavelength in the light due to stretching may be measured. When comparing red shifts of similar images, identical red shift measurements indicate a multiple image. The expansion rate of the universe, Hubble's Constant, may be calculated from red shift measurements .