Wondering if a naturally occurring gravitational lens (such as one created by a star or black hole) could be used in the future by someone searching for "ancient" (ie, our recent) radio and TV emissions. Might that be a way of reducing the inverse-square law's effect on those signals? I know there have been proposals to use the Sun as a gravitational lens to detect alien microwave transmissions. I'm wondering if, by analogy, one could use the same technique to find human broadcasts, many years in the future.

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    $\begingroup$ Do you mean some distant aliens listening to our broadcasts, or somehow bending the waves to go back to Earth? $\endgroup$
    – Alexander
    Commented Feb 27, 2018 at 20:56

1 Answer 1


This scheme was mooted on the Halfbakery some time back.


Gravity Time Mirror: Seeing our past using the sling-shot effect of a black hole's gravitational pull on light. Using a very powerful telescope, we could look right at the very edge of a black hole where a light path made it 180degrees around the black hole due to the slingshot effect of gravitation. If the black hole were, say, 1000 light years distance from us, we would be looking back at ourselves 2000 years ago, a gravitational mirror to the past.

— BruceRH90, Dec 03 2004

Pretty slick idea. I think it would work better with radio (as you propose) than with visible light.


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