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# Gravitational lens

A gravitational lens is formed when the light from a very distant, bright object (such as a quasar) is "bent" around a massive object (such as a massive galaxy) between the bright object and the viewer. The process is known as gravitational lensing, and was one of the predictions made by Einstein's general relativity.

In a gravitational lens, the gravity from the massive object bends light as a lens might. As a result, the path of the light from a bright object behind the massive one is curved around the massive object. The effect of this is that the light appears to come from different directions than the real object, often creating multiple "lensed images" of the bright object. If the Earth, a massive object, and a bright object are precisely aligned, one would see a ring of the same bright object, that configuration is known as an Einstein's ring. More commonly, the massive galaxy is off-center, creating different numbers of images according to the relative positions of the object-lens-earth, and the shape of the gravitational well of the lensing system.

There are three classes of gravitational lensing:

1. Strong lensing: where there are easily visible distortions such as the formation of Einstein rings[?], arcs, and multiple images
2. Weak lensing: where the distortions of background objects are much smaller and can only be detected by analysing large numbers of objects to find distortions of only a few percent
3. Micro-lensing: where no distortion in shape can be seen but the amount of light received from a background object can be changed.

History According to the general relativity, gravitational fields "warp" the space-time and therefore must also bend the light. His theory was confirmed in 1919 during a solar eclipse, when Arthur Eddington observed the light from stars passing close to the sun was slightly bent, so that stars appeared slighlty out of position.

Einstein realized that it was also possible for astronomical objects, like galaxies, to bend light, and that under the correct conditions, one would observe multiple images of a single source, called a gravitational lens or sometimes a gravitational mirage. But, he put off publishing his ideas for over twenty years, because he felt that these multiple images would be impossibles to see with the technology of the early part of this century.
With the invention of powerful radio telescopes and CCD optical detectors, astronomers can see farther and to resolutions far beyond Einstein's expectations.

The first gravitational lens ever discovered is called the "Twin Quasar" since it's image simply looks like two identical objects; it is officially named 0957+561 A & B. This gravitational lens was discovered accidentally by Dennis Walsh[?], Bob Carswell[?], and Ray Weymann[?] using the Kitt Peak National Observatory 2.1 meter telescope in 1979 -- some 60 years after Einstein predicted this phenomena

The study of gravitational lenses is an important part of the future of astronomy and astrophysics.

Cosmological Applications Another parameter that may come out of the study of gravitational lenses is Hubble's constant which encodes the age and size of the universe. It can be determined, in theory, by measuring two quantities: the angular separation[?] between two images, and the time delay between these images.
There are two contributions to the time delay:

1. the first is the obvious delay due to the difference in optical path length between the two rays.
2. the second is a general relativistic effect, the Irwin Shapiro[?] time-delay[?], that causes a change in the rate that clocks tick as they pass through a gravitational field.
Because the two rays travel through different parts of the potential well created by the deflector, the clocks carrying the source's signal will differ by a small amount.

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