system
Given the following post, respond as if you're commenting on Reddit:
human
If I we were to place an optical telescope 100 light-years away from Earth, and look back at it, would we see Earth 100 years ago?

This is something that recently occurred to me. 

Assuming perfect conditions, if an optical telescope with unlimited power were to be placed 100 light-years away from Earth, then pointed back at it, would we be able to see life 100 years ago?


~~Also, I'd add the "Astronomy" tag, but at the top of the submission page it says 'Add flair to your post by clicking the "flair" button under your title.', and I see no such button.~~

**edit:** Found the button.
gpt
This is correct - a telescope that is currently 100 light-years away from Earth would see Earth as it was 100 years ago...but there are a few caveats:

- First, you'd need a big telescope - like, *really* big. To even see the Earth near the Sun, you'd require a telescope hundreds of times larger than anything that's ever been built. As it stands, the planned James Webb Space Telescope could not spot an Earth-sized planet 1 AU (the Earth-Sun distance) from our nearest neighbor, Alpha Centauri, and 100 light-years is about 25 times further. 

- Second, seeing Earth as anything more than a single pixel would require another few orders of magnitude in telescope size.

- Third, you couldn't carry a telescope out to that distance from Earth to see into our planet's past, because the trip would take greater than 100 years. (since you are confined to speeds less than the speed of light).