BerryBoy said:
I was wondering...
Why doesn't Gravitational Lensing ever create a ring when bending light from an obstructed star?
The pure ring solution only arises when the lens is situated on a line exacty between a spherically symmetric source and the observer. In general, you'll get more complex behavior.
Gravitational lensing can be thought of in terms of Fermat's Principle (see
http://adsabs.harvard.edu/cgi-bin/n...pe=HTML&format=&high=4349a261f103762"), in which the light traverses paths that are extrema in time. If the source, lens, and observer are all perfectly lined up (and all are symmetric about this line), it stands to reason that there will be multiple extremal paths -- in fact, a ring of them. The symmetry demands that any particular path will have identical counterparts at all angles about the central line (each with the same distance from the line). Thus, if one path is an extremal, then all of its counterparts will be as well.
However, if you disturb this symmetry, then the redundancy of the extremal paths drops significantly. This is when you get the multi-image behavior that we often observe in nature. Not only are the lenses and sources we typically observe
not spherically symmetric, but we never see them in such perfect alignment. Nevertheless, with near-alignment and extended sources, you can still get ring-like objects. A google image search brings up some really nice examples:
http://images.google.com/images?q=e...a&rls=org.mozilla:en-US:official&sa=N&tab=wi"
Some of these may be simulations, so be sure to check the links if you're curious about a particular object.
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