# Gravitational Lensing

1. Nov 9, 2005

### 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?

Regards,
Sam

2. Nov 10, 2005

### Chronos

Last edited: Nov 10, 2005
3. Nov 10, 2005

### SpaceTiger

Staff Emeritus
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.

4. Nov 10, 2005

### Danger

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

5. Nov 10, 2005

### SpaceTiger

Staff Emeritus
The one in the top left is a drawing. :tongue2:

6. Nov 10, 2005

### Danger

.

7. Nov 11, 2005

### 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.

8. Nov 11, 2005

### SpaceTiger

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

9. Nov 11, 2005

### Chronos

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

10. Nov 11, 2005

### Chronos

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

11. Nov 11, 2005

### SpaceTiger

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

12. Nov 11, 2005

### Danger

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

13. Nov 13, 2005

### Chronos

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

14. Nov 14, 2005

### Danger

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