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Gravitational lensing as a way of observing Milky Way?

  1. Sep 29, 2015 #1
    Hi Everyone,

    first post here, another enthusiastic amateur I'm afraid so please excuse my general ignorance!

    I struck me the other day that it should be possible to find an arrangement of large galaxies that would allow light emitting from our galaxy to be bent back towards us. This would give us an outside view of our own galaxy once we un-distort the returning light. I've attached a sketch to clarify what I'm talking about.


    Would this be theoretically possible? I've been trying to find out, without luck, how much galaxies bend light to estimate how many galaxies you'd need to bend the light through 180°. The fewer and closer the galaxies the better to give us the maximum image size.

    While searching for information on this idea, I found a similar idea on another site where someone proposes using a nearby black hole to bend light from earth through 180° as a way of seeing back in time due to the distance the light has to travel. This is a great idea but seems much less likely considering how close the black how would have to be in order for us to have any chance at resolving an image of earth through it's gravitational lens. Plus the closer the black hole the closer in time to the present the image would be.

    Observing our whole galaxy, even just the shape of it would surely tell us a great deal about where we're from.

    I'd love to hear thoughts on this idea from someone who actually knows what they're talking about, rather than ramblings of a hyped up scifi fan like me!

  2. jcsd
  3. Sep 29, 2015 #2


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    Galaxies are not nearly heavy enough to give you this magnitude of gravitational lensing. A quick estimation based on the Buckingham pi theorem would tell you that a light signal passing the Milky Way at the galactic radius would be bent by of the order of ##10^{-5}## radians.
  4. Sep 29, 2015 #3
    ah ok, so you'd need over 15,000 perfectly aligned milky way sized galaxies to have any hope of this working...

    What about the other idea, would a rouge black hole be able to bend light around in 180°?
  5. Sep 29, 2015 #4


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    Even with a supermassive black hole only a tiny window would exist where photons would be bent so severely. The image would be unimaginably faint.
  6. Sep 29, 2015 #5
    Although I guess in principle it could happen if there were such a thing as an ultra-hyper-super massive hole having the mass of many galaxies.
  7. Sep 30, 2015 #6


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    Yes, in principle such a monster could perhaps amplify the image. I think an 'Einstein ring' representing the observers own galaxy is a theoretical possibility. It may require an isolated "ultra-hyper-super massive" BH, i.e. with no accretion disk or other interfering material. Anyway, this is speculative.
  8. Oct 20, 2015 #7
    You would need over 94,000 perfectly aligned galaxies. Don't forget 2*pi*rad=360°. And we'd be observing light emitted billions of years ago. Statistical probability of that many galaxies lining up for us and the time to catch that glimpse in a universe of moving galaxies means no. Never. Won't happen. Good thought experiment though.
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