Gravitational Lensing

BerryBoy

I was wondering...

Why doesn't Gravitational Lensing ever create a ring when bending light from an obstructed star? It seems to always create 2,3 or 4 clones of the star.

I can't get my head around this, maybe I'm missing something fundamental?

Your posts are appriciated.

Regards,
Sam

Chronos

Yes, it's called an Einstein ring. Here is one of the better examples:
http://www.eso.org/outreach/press-re...hot-20-05.html
There are a number of reasons 'pharmaceutical grade' examples of this phenomenon are virtually unknown.

SpaceTiger

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 Blandford & Narayan 1986), 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:

Einstein Rings

Some of these may be simulations, so be sure to check the links if you're curious about a particular object.

Danger

I'm pretty sure that the top left one on the first page is a simulation.

SpaceTiger

The one in the top left is a drawing.

Danger

.

Chronos

A perfect alignment between an object and its gravitational lensing partner is not the reason a perfect lensing event is virtually impossible. There will always be intervening matter that distorts the image.

SpaceTiger

Chronos, asymmetric lenses and sources, intervening material, and imperfect alignments are all valid reasons for why we don't see perfect Einstein rings.

Chronos

Dang, ST, I thought that was what I just said...

Chronos

Dear diary, ST will flunk me if I take his class...

SpaceTiger

You're a strange fellow, Chronos. Never change.

Danger

He doesn't. That's why his laundry costs are nonexistent.

Chronos

You guys are tough on me. I only have a BS and like to argue.

Danger

That's not quite like me, but close. I have a BS in arguing, in that most of my arguments are BS.