Gravitational Lens

It may be that the intermediate object is asymmetrical, or it may be that there are two objects creating a diffraction pattern in two axes, or causing a quadrupole lensing effect.

Hi George

AFAIK, multiple images happen when the lensing mass concentration is not homogeneous and symmetrical like a black hole or star. It is normally clusters that act as lenses for those images and they have many 'clumps of matter' inside - i.e. multiple lenses in a complex configuration.

-J

Hi Jorrie. Sorry for my late response. I, also, supposed that the multiple images are coincidental due to the non-homogeneous concentration of the mass (and I think that this is rather usual). However, it's difficult to consider that. As an example, let's consider the case of two (equal and symmetrical arranged) masses bending the light of a faraway object. I made a rough drawing depicting this case.

Blue lines show the light beams arising from the distant object (A) which are bent by the intermediate masses (M & M) and eventually end up on the observer's eye. (I, also, show some other lines). So, the observer sees multiple images (as in the following figures).

In Figure1 we get 3 images (A₀, A₁, A₂) considering that the gravitational fields are rather weak (or the distance between the two masses (M & M) are rather large). In Figure2 we get 5 images (A₀, A₁, A₂, A₃, A₄) considering that the gravitational fields are rather strong (or the distance between the two masses (M & M) are rather small). The stronger the gravitational fields are, the larger the distance is between the images A₃ and A₄. However, I'm not sure if these are the only images that we get. I think that we should use a simulation software in order to see the full results, even in this rather simple case. (BTW, is there any free simulation software for this kind of stuff???...)

Also consider not only two as depicted in your designs, but multiple masses being piled the one over the other (modeling maybe a long galaxy). In this case, we would have had two distinct mirror images on either side of this mass formation. The longer this "pile" is the more clear the mirror images. The shorter, the more they would spread to look like circle arcs.

Hi George, here is the best (quite technical) description of how the Einstein Cross might work that I found so far: http://physics.stackexchange.com/questions/14056/how-does-gravitational-lensing-account-for-einsteins-cross. The nearby galaxy in Einstein's Cross has an elliptical mass distribution (as we see it) that is wider in the direction of the short leg of the cross. "This type of lensing is achievable in such a configuration, when the lensing object is relatively close to us, so that the rays pass the central region, where the quadrupole moment asymmetry of the gravitational field is apparent".

Note that contrary to what you have hinted in your opening post: "Picture1 shows the '4 images' of a distant galaxy and it is known as the "Einstein Cross"", the central dot is die active galactic nucleus (agn) of the 'nearby' (500 Mly) galaxy, imaging a distant (10,000 Mly) quasar (http://as.ua.edu/ay/keel/agn/qso2237.html). The lensed images are a combination of the curvature caused by the stars inside the agn, which might have a central black hole, and the spiral arms, which might have a lot of dark matter as well.

The stackexchange ref gives a few links to simulators, but I did not spot any one usable for such a configuration. It seems to be a tough task to simulate such a complex case, but I guess it has been done by academics.

-J

Thanks for the reply, Jorrie. I'll take a look at those links.

(Actually, in my previous post, I didn't try to explain the 'Einstein Cross' but just to think about a simple case of two lensing masses as shown. By chance, it seems that we get those 5 images (sth like the 'Einstein Cross') but -as I said- I'm not sure if these are the only images that we get. Even this rather simple case needs a lot of study and, probably, a simulator in order to get the full picture.)

Hello George, yes, I suppose 4 masses in a chance 'box' configuration may give you 5 images. In reality, I think only even numbers of separate point-like images have ever been observed. One must keep in mind that what is actually observed is point-like, unless it happens to be arcs/rings. AFAIK, the observed objects are simply too far away to show any other dimension. The photos are long exposure CCD-type, with the inevitable leaking of charge to neighboring pixels.

-J

"... yes, I suppose 4 masses in a chance 'box' configuration may give you 5 images..." I suppose that you meant "2 masses" (as depicted in my previous post) not "4 masses". I think that 4 lensing masses in a quadrangular configuration will give 9 images due to the symmetrical aspects of the configuration, i.e. probably sth like the following drawing.

Figure1 shows the arrangement (black dots=lensing masses & red dot= distant object). Figure2 shows the images that we get (black dots=lensing masses & red dots= images of the distant object).

"I suppose that you meant "2 masses" (as depicted in my previous post) not "4 masses""

I believe that although theoretically possible, the chances of 5 images from 2 masses are negligible, if possible at all. IMO, for basically the same reasons as to why a single point mass can only produce a ring or 2 arcs (not 4). When arcs are tiny enough, they become indistinguishable from points of light, so we will have to establish how many arcs your 2-mass lens can form.

We will have to get hold of a multi-lens simulator to be sure. A fairly huge exercise in ray-tracing...

-J

At the moment I think that gravitational lensing seems to have a similar effect to optical lensing. The case of multiple images will also occur with an object seen by an unamorphic lens with areas of differing densities, or a facetted special effects lens. As a gravitational field, or a magnetic field may not be evenly distributed in space, the resulting trasmission may produce multiple images. The case with the 4 images may be caused by a strong magnetic field transmitting and bending light through 4 different polar field densities and displaced on the lower RH quadrant by another source, like solar wind or cosmic radiation effects our Earth magnetic field.

Science is Food for thought, not dogma, even when all we know seems to prove what we believe. (me)

Can light be bent by passing through an electrical or magnetic field like water bending when passed through a electrostatic field?.

I don't know of any effect where light curves while passing through electric or magnetic fields (i.e., just due to the E or M fields alone, and not due to a plasma), but there are a number of 'effects' on the polarization of light that passes through electric and magnetic fields.

See:

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

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

http://en.wikipedia.org/wiki/Cotton-Mouton_effect

It would be interesting to photograph the Einstein Cross through a polarizer to see if the intensities of the 4 spots of light change as the polarizer is rotated.

Can light be bent by passing through an electrical or magnetic field like water bending when passed through a electrostatic field?.,

Maybe not alone, but magnetic fields can have other effects which could: like plasma, create a light bending effect. Plasma could be ejecting from a black hole at the galaxy centre, or maybe from an old supernova ejection shell?

I also wonder is there may be a coherent light effect (somewhat simulating laser) when we look at such distant objects and could the light be split/polarized on more than one plane?

Are we observing the simpler effect of light of the farther galaxy focused by the cosmic dust/stars concentration in and around the interfering galaxy? This is a common lensing effect seen during air travel near clouds, or observed when caused by suspended ice particles in peculiar conditions. It this scenario, the multiple images would be projections of the actual source on a cosmic dust/ice disk.

This is not my field; just personal interest.

that's interesting. as light passes though a magnifing lens, would there be a time shift due to the longer path light must travel towards the outer side of the the lens?

"...would there be a time shift due to the longer path light must travel towards the outer side of the the lens?"

Depends on what you mean by 'time shift'. Parallel light rays are slowed down exactly enough by the central part of a convex lens to arrive at the focal point in phase with light going through the outer parts of the lens.

In a way its the same with gravitational lenses, because the closer to the lensing mass, the more Shapiro time delay there is. However, gravitational lenses are generally not like convex lenses, as GK has shown in his OP. It bends more near the center, not less...

-J

Hi George,

This link gives a good explanation.

-S

Thanks for the link, StandardsGuy. However, it gives just a simple and general explanation of the phenomenon rather than an answer to my initial question. (Thanks anyway...)

"The fact that the observer may see arcs doesn't change the truth of this: the thickness of the arc won't exceed the diameter of the object."

The objects that are lensed are usually distant stars or quasars, rarely galaxies. Since the distant stars/quasars are point sources, any arc or multiple image will increase the amount of light observed - hence it may be called magnification. This fact has been used to obtain the spectra/redshift of objects that are too dim for direct measurement.

-J