Gravitational lenses and objects in front of them

  • Thread starter kmarinas86
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  • #1
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Main Question or Discussion Point

Will the image of an object inside a gravitational lens, though on the side of the lens that is facing us, be enlarged/reduced in apparent angular width? If so, how is this quantified, and dimensionless GR values may it be related to (e.g. gravitational length contraction factor, etc.)? Will its brightness be reduced/increased? If so, by how much?
 
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  • #2
DaveC426913
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a gravitational lens ... the side of the lens that is facing us
Neither of these phrases make sense. There is no lens. There is merely the bending of light circumferentially around a massive object such as a galaxy.

Consider something more simple than distant galaxies. The sun acts as a very weak gravitational lens. If we were far enough from it, and a star were positioned just so, we would see a ring around the sun.


But you're talking about objects in the sun or in front of it. Are you suggesting that Mercury's image as it crosses in front of the Sun is somehow distorted by lensing from the Sun?
 
  • #3
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Neither of these phrases make sense. There is no lens. There is merely the bending of light circumferentially around a massive object such as a galaxy.

Consider something more simple than distant galaxies. The sun acts as a very weak gravitational lens. If we were far enough from it, and a star were positioned just so, we would see a ring around the sun.


But you're talking about objects in the sun or in front of it. Are you suggesting that Mercury's image as it crosses in front of the Sun is somehow distorted by lensing from the Sun?
Yes I am...

Check this program out:

http://www.kwakkelflap.com/files/gravlens.zip

Starting with the default settings:

If I increase the mass of the lens by 10 times and move the source to 110 Mpc, I can have a smaller image if I increase the mass by another 10 times at the same source distance of 110 Mpc

I think the program is what I needed. It shows that if you have an object inside a really strong gravitational lens, if it is on the front half of the lens, it will shrink! Anyone skeptical about this program?
 
  • #4
DaveC426913
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"...if you have an object inside a really strong gravitational lens, if it is on the front half of the lens..."

1] The only practical gravitational lenses are massive galaxies or black holes.
2] The only practical objects imagable with lensing is other, more distant galaxies (since anything smaller, such as stars, are zero diameter points regardless).
3] To have an object "inside a gravitational lens" means you're looking at one galaxy that's floating around inside another, larger galaxy or a black hole. This is impossible.


There is no practical way to talk about the lens itself. Even the massive galaxy is not the "lens". The lensing phenomenon is merely an effect that is caused by the bending of light rays. The only way the rays get bent is by being on a trajectory that takes them from behind the massive galaxy to in front of the massive galaxy. Light rays that start off anywhere other than behind the massive galaxy do not get bent.


In conclusion, lensing only works to distort objects behind the massive lensing object.

See attached diagram.
 

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  • #5
DaveC426913
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I think I see where you're going awry.

You're thinking that "the lens" is the rhomboid shape enclosing the distant object, the massive object in the middle AND the Earth, i.e. the "front half" of the lens is anything between the massive galaxy and Earth.

This is not so.

If you were to try to define the boundaries of the lens itself, you would enclose an imaginary area where light rays are in the process of being bent by gravity. Anywhere that the light rays are travelling in straight lines is not part of the lens.

See attached diagram.
 

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  • #6
Chris Hillman
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I'm probably not helping, but someone should point out that the above discussion mostly refers to "weak lensing"; observers very near a black hole observe some rather more drastic "strong lensing" effects. It is not clear to me either what kmarinas is asking about, but http://www.arxiv.org/abs/gr-qc/0612124 should be interesting for those interested in "seeing" simulations of some "strong lensing" effects.
 
  • #7
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An object in front of (say) a black hole (but very close to the black hole, far from the observer) would indeed have a distorted image. I haven't looked into this but I guess I'd expect it to appear reduced (the rest of the sky does, and I'm naively thinking there is kindof an excess of space close in there). Also, provided it didn't eclipse the event horizon, you could in principle see an infinite series of its .. "reflections" ("Einstein rings").
 
  • #8
Chris Hillman
Science Advisor
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No, not at all. It's much more interesting. One place to get started would be Chandrasekhar, Mathematical Theory of Black Holes.
 

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