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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Could you explain how gravitational lenses work, and what determines the number of images you see?

I can give it a try!

One of the more interesting prediction by Newtonian physics and Einstein's General Theory of Relativity is that in the presence of matter, the path of a light ray can get bent. Only Einstein's theory gets it exactly right, however, because Newtonian physics fails to include the effect of the bending of space-time by a gravitational field. Newtonian gravitation theory assumes that space-time remains flat.

During the 1980's, astronomers have found a number of examples in which the image of a distant quasar is distorted by the gravitational field of some galaxy along our line of sight to the quasar. Such gravitational lensing leads to several peculiar kinds of image distortion depending on the exact geometry and distances between the quasar, the 'lensing galaxy', the shape of the lensing gravitational field, and the distance to the earth. For the Earth, quasar and lensing galaxy located exactly along the same line, and for a quasar with no resolvable structure, you get what is called an Einstein Ring. For any other geometry in which the lensing galaxy is not on the line between the Earth and quasar, you will get a set of multiple images of the quasar surrounding the image of the lensing galaxy. The number, shape and intensity of these images is determined by what opticians called 'caustics'.

There are no simple rules for determining how many images or arcs ought to form. The 'Einstein Ring' object MG1131+0546 is a perfect ring with two spots. The 'Einstein Cross' object G2237+0305 , observed by the Hubble Space Telescope, is 4 images of a quasar and a 5th image in the center formed by the lensing galaxy itself. Giant arcs in the distant galaxy cluster Abell 370 are formed by lensing of galaxy images into portions of rings; and the 'Double Quasar' 0957+5614, is a pair of images of a distant quasar split by a foreground galaxy. The various geometries of quasar, galaxy and Earth control the intensity and locations of lens images.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.