Gravitational Lensing

[Glenn]

An article published today at http://spaceflightnow.com/news/n0402/11lens/ stated that "Einstein's general theory of relativity predicts there should be an odd number of images, yet almost all observed lenses have only 2 or 4 known images."


How and Why does Einstein's General Relativity predict an odd number of images?

Thanks,
Glenn

[RMDumse]

A very simple explanation (perhaps too simple) would be, if there is enough gravitational field on one side of a body to bend light your way, then there will be enough gravitational field on the otherside to bend rays your way. So you will see one image on the central ray (unless the central ray is blocked by the gravitating body, but if that body is "transparent", like a galaxy which is mostly empty space rather than matter, then you will see the primary image). Additional images will appear as multiples of two on either side of the central image.

If you want to get into deeper detail, you could refer to: Burk, W.L. 1981, "Multiple gravitational imaging by distributed masses," Ap. J 244, L1.