In 1924 Mars and the earth were going to to be closer, for a few days, than they had been since 1804. The US Navy was asked to maintain a period of radio silence in order to listen for Martian broadcasts. The Navy managed to remain silent, but so, apparently, did Barsoom.
In 1960 astronomer Frank Drake created Project Ozma, which was earth's first serious attempt to use radiotelescopes to listen for signals from intelligent species elsewhere in the galaxy. Ozma looked specifically at the stars Tau Ceti and Epsilon Eridani, both considered at the time to be likely candidates. They, too, failed to produce an artificially generated radio signal (but Drake did, in the process, manage to discover boring old pulsars). Not especially daunted by not finding obvious signals after searching only two of billions of stars, Drake proceeded to expand his search of the heavens. In 1961 Drake chaired a meeting at the National Academy of Sciences National Radio Astronomy Observatory in Green Bank, West Virginia. Now known simply as the "Green Bank Meeting", this gathering of 12 astronomers, physicists, biologists and others established the first serious scientific protocols for the Search for Extra-Terrestrial Intelligence, or SETI.
In preparing for the Green Bank Meeting, Drake created the now famous equation which bears his name:
It is important to understand that SETI itself is a gross misnomer. It is not possible to search for intelligence. It is possible to search for life on other worlds, and it is possible to search for technology on other worlds, but intelligence itself is almost impossible to identify, even here on earth. We observe behaviors in cetaceans, cephalopods, corvids, canines, felines, pachyderms and even some primates which we interpret as intelligence. On the other hand, we occasionally observe behavior in simple eukaryotes, prokaryotes, viruses and even sub-atomic particles which might be considered intelligent if we observed it in a vertebrate. Even among our own species, we have very little understanding of what "intelligence" means, and how or if that can be qualified or quantified in a meaningful way. But, to the very best of our knowledge, no species on earth other than ourselves has ever developed technology more sophisticated than a broken rock or a digging stick.
Even very advanced human cultures did not invent the technology to transmit and receive radio waves until Marconi. And as human technology progresses, we are now broadcasting fewer and fewer radio signals into space simply by virtue of using more microwavelength communications and fewer medium and high frequency communications. Even if another civilization followed exactly our trajectory of technological development, there may be only a window of less than a century in which they are broadcasting distinct signals in the EM spectrum. We simply don't know.
Searching for extraterrestrial technology by listening in the radio spectrum, because it presupposes that the entities sending the signal have a transmitter which we can receive, is only a baby step above tying a string to two soup cans and holding one can to our ear and the other to the night sky. In other words, the sky may be teeming with technologically advanced civilizations and we'd never know it, because even though they're technologically advanced, we aren't. Whatever the reason (and we'll be exploring some of them in the coming weeks here), we have not yet found a conclusive radio signal from space aliens.
We have, however, found some not-quite-conclusive signals of interest. The most interesting was found by Jerry Ehman in 1977, and is known as the "WOW! signal" because of a hand-scrawled note on the digital printout. It was strong, narrowband, clearly not from our solar system, met all of the parameters SETI researchers have expected, lasted 72 seconds...and was never heard again.
SETI, more properly SETT, continues. Radiotelescopes great and small comb the skies searching for a distinct and unambiguous signal from a technological race beyond our solar system. Whether we will find an unambiguous signal in our lifetimes, or ever, only time will tell.