# Multiple images in gravitaional lensing

1. Feb 21, 2012

### Agerhell

On the subject of gravitational lensing there seems to be two rather different phenomena. The "arc" phenomena seems rather plausible to me, if light is affected by gravity which is natural then there should be able to get that kind of behaviour.

But the multiple imaging stuff, seems rather strange to me, why does it occur?

http://en.wikipedia.org/wiki/Gravitational_lens

There are three examples of arc-like phenomena, but there is also one exampel of multiple images, the so called "Einsteins Cross".

http://en.wikipedia.org/wiki/Einstein_Cross

Where, for some reason, there is what looks like for different quasars (or stars, I guess you need some kind of spectra to see that they are quasars) which is said to be four different images of the same Quasar.

Besides from explaining it here, it would be helpful if someone who knows more than me could write something on this on Wikipedia. There is basically only one and not very informative sentence on the subject there.

2. Feb 21, 2012

### Ben Niehoff

You can see multiple images by looking through funny-shaped pieces of glass easily. Why should gravitational lensing be any different? The foreground galaxy is not perfectly symmetrical, that's all.

3. Feb 21, 2012

### cynopolis

True that, but what is the significance of there being four, why not five and a half?

4. Feb 21, 2012

### Matterwave

Basically, the lensing is such that different line-of-sight light rays gets bent to the same destination (your telescope).

The number of images is determined by the geometry. Sometimes you see the images get smeared out too.

5. Feb 21, 2012

### cynopolis

Right the geometry. Doesn't the cross suggest a four-fold geometry, like a square or octahedron of sorts?

6. Feb 21, 2012

### Agerhell

Well then, what is so special about "Huchra´s lens", the galaxy that according to wikipedia is the lens in this particular case?

What is it about Huchra´s lens that creates four images?

According to this picture from the European Space Observatory it looks like an ordinary galaxy:
http://www.eso.org/public/images/eso0847a/

7. Feb 21, 2012

### cynopolis

Imagine the gravitational field of this galaxy was octahedral in shape. It would have to be a hyperbolic octahedron because gravity is hyperbolic. Next the octahedron would have to be aligned with us in such a way that the four images pass around to produce this pattern.

http://bulatov.org/math/1101/img/ideal_octahedron.jpg

This is what I meant when I said a four-fold octahedral symmetry.