Light travels faster than any other means of communication, so a sufficiently advanced civilization may try to directly communicate with other civilizations using light. Beyond direct, purposeful communication, though, our planet is actually broadcasting signals out into space every day in the form of our radio and TV broadcasts. That is, when we broadcast radio signals around the world for you to listen to in your car, those same signals also travel through space, and so any civilization with a sophisticated enough detector can receive, say, the "I Love Lucy" show from decades ago.
By the same logic, if we try, we should be able to detect signals sent directly to us from a distant civilization, or if they also use transmitters to transmit radio or TV type signals, we could detect those signals, too. However, the signal from a radio transmitter dilutes as it moves farther and farther from Earth, so the radio telescopes a distant civilization must have to detect TV or radio signals from Earth would have to dwarf our most powerful radio telescopes on Earth. If you return to the lesson on the electromagnetic spectrum and review, there are a few considerations that we or another civilization might want to take into account when deciding how to communicate from planet to planet:.
The SETI League: Searching for Extra-Terrestrial Intelligence
Since we cannot know ahead of time anything about other civilizations that may be listening for signals from us or who are trying to communicate with us, the best that we can do is take educated guesses at how we might communicate. Scientists who have been pursuing Search for Extraterrestrial Intelligence or SETI research have been, since the s, using radio telescopes to search for signals from other civilizations.