If radio signals attenuate when travelling through space, then what kinds of emissions are we looking for when searching for extraterrestreal life? My understanding is that radio waves travel forever, like ripples in a pond, but attenuate with distance. They get mixed with other signals and become cosmic noise.
I'm looking at these answers, which suggest that the TV and radio signals that originate from earth attenuate and are indistinguishable from background noise after some distance.
This makes me ask:
How do we hope to detect signals from alien civilizations, if those signals attenuate as well?
Are there other types of signals, that can cross vast interstellar distances (> 600 light year) and carry data that is recoverable?
 A: Electromagnetic waves are still the best bet and distances are huge but we’re reasonably good at sending and detecting weak signals, and constantly improving at that job.  We are still communicating with the Voyager probes, even if they
have both reached the interstellar region, and signal strengths have energy 20 billions times smaller than needed to run a digital watch.
For direct “communication”, the basic assumption is that an alien technology is better than ours at sending directional signals.
There is also hope that secondary information might reveal the presence of civilisation: we are still in the infancy of exoplanet exploration and again the hope is that the presence of chemicals in the atmosphere of an exoplanet might reveal signs of civilisation.
A: You are right, an omnidirectional radio broadcast would be very faint by the time it reached earth, and very difficult to distinguish from background noise. Projects such as SETI use large amounts of computing power and sophisticated signal analysis algorithms to try to detect a faint signal with a pattern indicating an artificial origin.
Instead of broadcasting in all directions, an extra-terrestrial intelligence that wanted to say "hello, here I am" could encode information in a powerful narrow laser beam and direct it at the star systems near its home that it thought most likely to harbour life. There are various "optical SETI" projects that are looking for such laser signals.
