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Two galaxies aligned in a way where their gravity acts as a compound lens

(phys.org)
268 points wglb | 1 comments | 16 Nov 24 16:48 UTC | HN request time: 0.211s | source
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bparsons ◴[16 Nov 24 20:12 UTC] No.42158887[source]▶
If the lens curved light back toward us, could we see earth several million years ago?
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Danieru ◴[16 Nov 24 23:09 UTC] No.42160335[source]▶
No, because the light requires twice the time to travel there then back. If Earth did not move relative to the lens, it would work. Sadly we move, a lot, so what was here 2x ago was something not-earth.

To see earth, the lensing would been to be focused on where Earth was 2x ago. Still possible in theory, and you might even argue just as likely as a fully reflecting curve. But you'd not call it "back towards us". It would need to be "curved to where earth was".

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1. deskamess ◴[17 Nov 24 14:48 UTC] No.42164451[source]▶
Would this be the case even if you were moving toward or along-side the 'reflector' (black hole/other body)? For the sake of discussion assume we are at or beyond the focal point.