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Does an observer in that galaxy see our galaxy magnified as well?

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  • $\begingroup$ @curiousdannii: (a) A photographer themselves does not look through the lens, so your analogy is slightly flawed. (b) If you've ever done the classic childhood act of reversing binoculars, it inverts the zoom effect. I'm unsure whether lensing is an absolute value or whether direction matters. So I would argue that if a photographer were to look down a lens, that the subject could also stare down the lens and see related lensing effect, albeit inverted. I'm ignoring silly things like one-way glass here for obvious reasons. $\endgroup$
    – Flater
    Feb 13 at 22:40
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    $\begingroup$ @Flater The photographer does in a SLR camera! Yes there's a related or inverse imaging effect, it's just not symmetrical. My point was more that if you could resolve the camera you might be able to see the photographer's eye magnified through it's lens, but the photographer may be too far away for you to be able to resolve them through it. Telephoto lenses only help people see through one end. $\endgroup$ Feb 13 at 22:59
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    $\begingroup$ @curiousdannii I think for the purpose of the question we can assume that neither the photographer nor the subject are very close to the lens in question, or at least we can entertain the subset of cases in which they aren't. $\endgroup$
    – Flater
    Feb 13 at 23:04
  • $\begingroup$ I have a large dog with jet black fur. For some reason, one of her favorite spots to sleep for the night is at the foot of my bed. More than once I have stepped on her tail or one of her paws while en route for a middle of the night trip to the bathroom. My reaction has always been to apologize profusely. Her reaction has always been along the lines of I saw you, so why didn't you see me? Being able to see something is not necessarily symmetrical. $\endgroup$ Feb 14 at 0:40

2 Answers 2

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It is highly unlikely that a galaxy that we see as magnified by lensing is, has, or will be positioned to see our galaxy similarly magnified. The three galaxies, including the lensing galaxy or cluster, are moving with respect to each other. Our proper motion with respect to the microwave background is about 630 km/s, so these are typically quite high velocities. Now, consider the plane containing the Milky Way (MW), the lensing galaxy, and our motion with respect to the lensing galaxy. It is moving and tilting, but let’s take it as stationary and the other galaxies are moving through it. The magnified image of the MW is always on this plane and it is moving along arcs that start at the lens and curve outward. There is another arc on this plane for magnified regions of space that we see. It also is curved, starting at the lens and curving outward, but backwards in time. We see a galaxy magnified because it crossed the plane at just the right location and time, billions of years ago, so that light from it is lined up to reach us today. But, the curve of images we see and the curves of images others see, go off in different directions because the first is based on where we are when we see it and the other is based on where we were when the light left us. It is not possible for a galaxy to pass this plane twice because the MW has never reversed its trajectory. The curves do join at the lens but the magnification there is very low.

Only if the three galaxies remained in a line for billions of years could they see each other as gravitationally lensed magnified images.

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It depends on the distance between us, the lens galaxy, the magnified galaxy, and many other factors due to the way light travels. A lens galaxy passes the old light of a far-off galaxy and amplifies it in our direction due to perfect alignment.

It does not mean that an observer in the distant galaxy would see the same thing, however. For example, if the lens galaxy/cluster is closer to us than it is to the galaxy that we are magnifying, which it often is, the light of the Milky Way being amplified from the opposite view would not have reached the distant one, and will almost certainly never align since galaxies are constantly changing position. In other words, by the time the magnified light of the Milky Way reaches the other galaxy's original position, it would have already moved an enormous distance, well out of alignment.

The answer: No. Just because a galaxy is amplified for us, it does not mean we are amplified to the other galaxy due to a significant delay in light travel over such distances.

Postscript: Even if both the Milky Way and some distant galaxy were magnified from both perspectives at the same time due to some rare coincidence, being able to see one another still depends on many factors such as luminosity. For instance, if the distant galaxy hosts a quasar, we could see them - though they may not be able see us, at least not as well, unless Sagittarius A* was also a quasar when the Milky Way's light reached the other galaxy.

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