SETI is a large-scale effort that requires extensive collaboration. One Planetary Society-funded project, for example, involves searching a wide range of radio frequencies using the world’s largest steerable radio telescope, the 100 meter Green Bank Telescope in West Virginia. Their observations focus on 100 stars known to have planets around them, and due to the width of the radio telescope field of view, they pick up tens of thousands of additional stars and planetary systems. Citizen scientists from around the world then step in to help classify signals in the data to sort out the many sources of radio signals produced by intelligent life here on Earth and highlight the most promising signals in the data. Finally, professional SETI researchers revisit those promising signals to assess their potential as signs of intelligent life.
All of this takes hundreds of hours, and yet only represents a search of a tiny portion of space. The scale of the Universe makes the scope of SETI truly immense, which may be one reason why, so far, we haven’t found what we’re looking for.
Sending out signs of our existence
If intelligent aliens are out there, it’s fairly likely that they’re also conducting their own version of SETI. We are as likely to be detected by a civilization as advanced as our own as we are to detect them.
The signals that are most likely to be picked up by other life forms are those we send out deliberately for this purpose. All electromagnetic communication travels at the speed of light, but some signals make it farther than others, decreasing in intensity as they get farther from their source. The distance a transmission can reach depends on several factors, including the power of the transmitter and the frequency of the signal.
It’s a common science fiction idea that far-off aliens might pick up our stray television transmissions, but this isn’t likely to happen. Television signals are broadcast using fairly weak transmitters since they aren’t intended to go very far. The earliest television broadcasts have likely traveled beyond our Solar System by now, but they’re exceedingly weak by the time they reach interstellar space, and would be practically indistinguishable from the background noise of the Universe.
Some of our radar signals are likely to travel farther. Missile detection radar uses very strong transmitters, as do planetary radar systems like the now-defunct Arecibo Observatory, which used to use radar to detect near-Earth asteroids and other distant objects. It’s possible that intelligent aliens could pick up those radar signals and identify them as being of technological origin, provided those aliens weren’t much farther than the closest neighboring star system.
Humans have intentionally sent several strong signals into space with the intention of having them detected by extraterrestrial civilizations. These efforts are called METI (Messaging to Extraterrestrial Intelligence), CETI (Communication with Extraterrestrial Intelligence) messages, or forward/active SETI.