SaturnFive
SaturnFive t1_ixj2q7e wrote
Reply to ELI5 How can you look at Earth from another point in the universe and see a past version of itself? by SPLIFF_BAYLESS
If you could travel some distance from the earth, say one light year (the distance light travels in a vacuum in one earth year), and you had a powerful enough telescope, you could look at the earth and see the light that was just now arriving. The light would be one year old and thus you'd be looking one year in the past. The further you travel, the longer the light took to reach you, and the further you can look into the past.
It already works this way from the opposite perspective. When we look at stars, exoplanets, galaxies, etc. we are seeing them as they were hundreds/thousands/millions or more years ago, depending on how far away they are. They might be totally different now, but we won't know until new light arrives.
Shorter distances work too. At closest approach, Mars is 182 lightseconds from Earth. Standing on Mars and looking through a telescope at the Earth, you'd see Earth as it was about 3 minutes ago.
SaturnFive t1_ixjg85s wrote
Reply to comment by CletusDSpuckler in ELI5 How can you look at Earth from another point in the universe and see a past version of itself? by SPLIFF_BAYLESS
It's accurate in the sense that is how it would work if you could be a light year away, but you're right, there's presently no way to get that far away. You could still go to Mars and look a couple minutes in the past. :)
The Voyagers are about a light day away, so they could see "yesterday" if they had the imaging capabilities onboard.