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Difference between images produced by gravitational lensing

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  1. Feb 16, 2015 #1
    Around massive bodies, light bends so that we can see multiple images of the same object, such as multiple images of the same galaxy behind the massive body.
    I know this seems kind of a dim question, but how can we know that the similar images we pick up are light rays from the same object that have undergone gravitational lensing or whether there is an actual cluster of galaxies that emit nearly identical light? Given an image, how can we know that two "light spots" that seem identical are from the same source or different sources that emit identical light?
     
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  3. Feb 16, 2015 #2

    CWatters

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    I think the spectrum of the light would be the same (because it comes from the same souce object). Perhaps unlikely you would have a cluster of galaxies in the required arc and all emitting the same spectrum?
     
  4. Feb 16, 2015 #3

    CWatters

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    Google found..

    http://csep10.phys.utk.edu/astr162/lect/galaxies/lensing.html

     
  5. Feb 16, 2015 #4

    sophiecentaur

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    This is a problem with all astronomical observations. Resolving power is always limited, with optical lenses and microwave reflectors and an observation made today may suggest a single star but, when a bigger telescope is used, it is resolved into a binary system - or just two stars in nearly the same direction. A massive cosmological object is a lousy lens, in fact - but better than nothing.
     
  6. Feb 16, 2015 #5
    @CWatters Thanks for the link. What do you think should be the maximum separation of two observed images that are identical to one another for them to qualify as product of gravitational lensing? (i.e. at what arc length separation can we be confident that the two images are not from the same object?)
    @sophiecentaur I see, your point is very valid. But I'm talking about two objects that have already been resolved by the telescope. I'm actually more interested in the procedure which can confirm that the two images are from the same/different object(s).
     
    Last edited: Feb 16, 2015
  7. Feb 16, 2015 #6

    sophiecentaur

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    Your "two objects" are, in fact 'two images' (until proved not to be). You could still be looking at an artefact - or not. There are many examples of double images in ordinary optics and it may not be obvious what you are really looking at. There are sometimes strategies for judging whether there are two distinct ones. In your case, I guess you could just look at the spectra of the two images. If they are indistinguishable then you could conclude they are of the same object.
     
  8. Feb 16, 2015 #7

    Vanadium 50

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    You do know that there have been observations of a lensed supernova, one that has appeared in multiple images of the same galaxy. That seems to me to be pretty convincing.

    Otherwise, you need to argue that something like this is going on:

     
  9. Feb 16, 2015 #8

    sophiecentaur

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    Not many examples of two of those going off at the same time!
     
  10. Feb 16, 2015 #9

    Vanadium 50

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    Indeed. There are also predictions on when we will see an image of the same supernova in another part of the lensed image. I think the next such date is a few years from now, but it seems to me that this is awfully predictive.
     
  11. Feb 17, 2015 #10

    sophiecentaur

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    That is really amazing. But shouldn't surprise me, bearing in mind the large number of light years of distance (path lengths) involved. I was thinking of the images arriving at the same time. What sort of accuracy is involved here? Imagine being able to see the very start of the Nova process in detail with a narrow field telescope.
     
  12. Feb 17, 2015 #11
    My bad, I meant two images that have been resolved, not two objects. Thanks for the answer.
     
  13. Feb 17, 2015 #12

    Vanadium 50

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    I don't know (or rather, don't remember) how precise the prediction is. I assume it depends on how well we know the details of the lensing galaxy, and since it's mostly dark matter, that has to mostly come from the other lensed images.
     
  14. Feb 17, 2015 #13

    sophiecentaur

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    Are 'they' actually pointing telescopes in any particular direction in order to catch one?
     
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