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Could light emitted from Earth theoretically orbit a black hole’s photon sphere and return, allowing us to observe Earth’s past? If so, what fraction of photons would survive this trajectory instead of being captured or scattered? Would the reflected light (e.g., after a ~2,000-year round trip to a black hole 1,000 light-years away) be detectable against astrophysical noise?

Amirhossein Rezaei
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1 Answers1

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Light can get turned almost-180 by a black hole back towards the emitter. The thing is, Earth is dwarfed in luminosity by the Sun, so if we looked towards such a theoretical black hole, we would, at most, see the Sun. That's just a fact - you can draw out the geodesics if you'd like. It would be "easier" if there were two black holes near each other so that light could get sort-of-slingshotted around instead of having to move at exactly the right trajectory to get exactly 180 degrees of turnaround, but whatever, same effect.

Theoretically, that makes this possible. Not practically. You would need a dish on the order of the Earth's radius to be able to resolve dots that tiny from even nearby distances (e.g. Proxima Centauri, etc.). Keep in mind the main way we can currently detect exoplanets is by looking at how their sun dims when they pass in front, even for systems as near as Proxima Centauri, or sometimes direct imaging by combining images for exoplanets that are reflective enough in infrared and far enough from their star. You're talking about looking at planets through a black hole mirror - we don't know of any black holes closer than around 1,500 light years, so planets would be invisible if we can't see them effectively even at under 5.

Also keep in mind that the “distance we’re looking” will be twice the distance to the black hole, and the sliver of space where light gets turned 180 is going to be a lot smaller than a planet - good luck zooming in far enough with a reasonable telescope.

It would take a lot of technological growth to get to the stage where this is possible, and that's all assuming that Earth's reflected light hasn't been somehow dampened by interstellar gas already, but it is not distinctly impossible as far as I am aware.

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