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We have encountered galaxies that act as lenses, magnifying the light coming from behind them. This is super fascinating and it makes me wonder what other optic tools we might have at our disposal.

The question of what we know about our own galaxy is obviously fascinating because it's kind of the the one we have the worst view at. There's so much noise around us it's hard to get a macroscopic view of the galaxy. It occurs to me that if we could stick a mirror out and look at our galaxy from the outside, maybe we have a better view.

Of course, we don't have the technology to make a very useful mirror. And it wouldn't be very efficient. If anything, we would simply launch a probe out there and send back pictures and science data.

But then I got to thinking maybe there's a naturally occurring mirror we could leverage. We know there are entities that can bend light, are there any that bounce it? It would be incredibly advantageous if there was one that allowed us to see our own galaxy, but also it seems unlikely. But are there -any- objects out there that reflect light like a mirror allowing us to see around them or behind them or their "backside"?

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    $\begingroup$ I don't know the answer to your question, but I would say that if a mirror existed, it would not be a reflector, but an object which bends light such that it reverses the direction of the light $\endgroup$ – Beta Decay Sep 13 '17 at 17:44
  • $\begingroup$ +1 Great question! Not very many people anticipated just how useful and productive gravitational lensing would be as an observational astronomy tool, so I'd suggest that a question about some kind of mirror deserves some careful thought. I think it's important to note that you left the mechanism wide open and did not imply it had to be gravitational in nature. What tends to distinguish mirrors from lenses is the thickness of the active region, usually a fraction of a wavelength to a few wavelengths, so we might be looking for things that are wide, but have strong longitudinal gradients. $\endgroup$ – uhoh Sep 14 '17 at 8:11
  • $\begingroup$ Plasmas can interact strongly with waves below their plasma frequency, so one place to look would be at very low frequency RF and regions of charged particles with a strongly non-equilibrium spatial distribution. $\endgroup$ – uhoh Sep 14 '17 at 8:13
  • $\begingroup$ Are there any bodies with negative mass in space? $\endgroup$ – Wayfaring Stranger Sep 15 '17 at 15:29
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Very likely not.

If it's a gravitational process like galaxy lensing, then a "mirror" would require a very large deviation. That's extremely unlikely with this process.

With other processes, e.g. classic reflection, it seems even more unlikely. We don't know any natural processes that could craft mirrors at cosmic scale.

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You could argue that a neutron star satisfies your final criterion (letting us see behind them). The intense gravitational field at a neutron star's surface serves as a gravitational lens - bending not just light from other stars, but emissions from the neutron star itself! For normal stars, this effect is small, but it becomes more dramatic for compact objects like neutron stars. Rob Jeffries wrote a nice answer explaining the geometry of the situation here.

This sort of gravitational lensing is probably the only phenomenon that approaches what you're looking for. A reflection nebula sort of acts like a mirror, reflecting the light from one or more nearby stars, but because of the scattering (and the somewhat low dust density, relative to an actual mirror), we can't use nebulae to actually see the stars. We can get a decent idea of what sort of light they're emitting (although the scattered light is still a bit different), but then again, we can already get that by just observing the stars normally.

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I think if you shoot a laser beam tangent to just outside event horizon of black hole then it might reflect light as mirror reflects incident ray hitting its surface at zero degrees.

but again this is bending of light not reflecting it.

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IMHO the most likely way a cosmological mirror might come into existence is if some localized conditions led to a "cloud" of particles which cause phase conjugation . Since this requires, in general, not only a highly ordered collection of particles/material but also an appropriate 'pump' (optical or otherwise), I'd be both thrilled and astonished if we were to detect such a configuration.

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