Radio Searches

Since we have thought of beaming radio signals to distant stars, it is likely that other intelligent beings out there would have had the same thought. Therefore, we should look for evidence with our radio telescopes of intentional or unintentional electronic signals coming from extraterrestrial civilizations (perhaps the equivalent of I Love Lucy in the Alpha Centauri system?). The adjacent right figure shows one radio telescope that has been used to search for such signals.

Which Stars?

There are billions of stars that are possibilities, so we need to refine our method. First, at what stars should we look? As we have already reasoned, the most profitable are probably G, K, and cooler F stars, and we would expect that planets and associated life are more likely in single star systems rather than binaries.

What Wavelengths?

Second, to what wavelength should we tune our radio telescope to detect the alien signal? Radio signals are swamped by the background noise of the galaxy for wavelengths longer than about 30 cm and any Earth-like atmosphere absorbs wavelengths shorter than about 1 cm. Within this "radio window" there is still a very large range of wavelengths to monitor for extraterrestrial signals. However, as noted in the right panel, there is a natural window dubbed the water hole between the 21 cm hydrogen line and the 18 cm OH line that we might expect to be a logical place to send signals.