system
Given the following post, respond as if you're commenting on Reddit:
human
Can we bounce a signal off something in deep space and see Earth`s past?

I have no idea how science works, I'm in sixth grade, so don't get mad. But couldn't we bounce some kind of energy signal off something in deep space, have it pointed back at earth, decode it, and see what Earth looked like in the past?
gpt
This is a great question, and it's actually exactly how science works. In science you ask questions that you don't know the answer to, and then you try to find out what the answer is.

When you look at yourself in a mirror, you're actually seeing what you looked like in the past. This is because it takes light a finite amount of time to travel the distance from your face to the mirror, and then back to your face. So, if there was a giant mirror one light year away from earth (one light year is the distance that light travels in one year, about 5.8 trillion miles), when we looked at the mirror we would see what the earth looked like two years ago. This is because the light leaves the earth, spends a year traveling through space until it hits the mirror, then bounces back towards earth and spends another year traveling back to earth where it hits your telescope.

If someone went and built a mirror 5.8 trillion miles from earth today, the light from a year ago would be the first light to hit the mirror. This light would turn around and spend a year traveling back to earth, so a year from now we would be able to see the earth as it was a year ago. So, on August 21st, 2015 we would be able to see what the earth looked like on August 21st, 2013.

Also note that this is a very simplified example, where we're assuming a perfectly reflective surface and a perfectly empty universe between the earth and the mirror.