You've probably heard the trope about how aliens have been watching old episodes of "I Love Lucy" and might think these are our "historical documents". How far have our signals reached?
Television transmissions expand outward from the Earth at the speed of light, and there's a trope in science fiction that aliens have learned everything about humans by watching our television shows. If you're 4 light-years away, you're see the light from the Earth as it looked 4 years ago, and some of that light includes television transmissions, as radio waves are just another form of electromagnetism – it's all just light.
Humans began serious television service in the 1930s, and by the modern era, there were thousands of powerful transmitters pumping out electromagnetic radiation for all to see. So are aliens watching "I Love Lucy" or footage from World War II and believing it all to be part of our "Historical Documents"?
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