SaturnFive t1_ixj2q7e wrote on November 23, 2022 at 8:32 PM

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.

CletusDSpuckler t1_ixjcdpx wrote on November 23, 2022 at 9:37 PM

I don't think this is accurate. To see the light from the past, you'd have to travel faster than light to get ahead of the current wave front. No traveler starting on the Earth's surface could ever see it at any time previous to the date they left unless I'm very much mistaken.

SaturnFive t1_ixjg85s wrote on November 23, 2022 at 10:04 PM

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.

CletusDSpuckler t1_ixjlgts wrote on November 23, 2022 at 10:42 PM

Not disagreeing, just clarifying. You could look arbitrarily into the earth's past from our reference frame on the planet by viewing it from any distance as long as you did not travel from the earth to get there. When I saw the word travel, that is what I was referring to. If you left from the surface, you could never see anything that happened before you left, no matter how far nor how fast you traveled. That was my point. Which, in retrospect, was not the point you were making.