Time Machine

Light travels at almost 300,000 km per second. That's a lot of distance in a second. So in 3 seconds, light would travel almost 900,000 kilometers. And light reflects off objects and make them visible to the human eye. If light doesn't reflect, nobody sees anything.

If I look at you from 900,000 kilometers away, I will see what you did 3 seconds ago. That's looking at what you did in the past. Now if I try to capture the light that got reflected off everything years ago from some far off place, or make a device that can replicate that light using some complex mathematical equation, I can see the past. There's my idea for a device to see the past. Simple.