SETI

SETI stands for Search for Extra Terrestrial Intelligence, it is a non profit organisation that points radio telescopes up on the off chance they pick up a signal transmitted by an alien race. SETI attempts to detect technologically advanced civilisations by picking up stray electromagnetic transmissions. However the universe is a noisy place and many natural processes produce electromagnetic radiation, the key difference being that natural sources tend to cover a large range of wavelengths(a high bandwidth) while intelligently developed sources tend to have much narrower bandwidths, this has two benefits. First it needs less energy and second it keeps frequencies clear for others to use without interference. Another important consideration is the fact that some wavelengths are naturally much quieter than others, with the majority of significant natural signals being in the infra red or higher frequencies (shorter wavelengths). This means we can take an educated guess at the type of signal we want to detect and the range of wavelengths to focus on. Specifically a signal with a narrow bandwidth in the radio or microwave region is very likely to have a non natural source, so this is where SETI looks.

The procedure is something like this, get hold of the biggest radio telescope available, point it at a star then scan millions and millions of really tiny bandwidths and compare them, if a narrow signal is detected then another telescope hundreds or thousands of Km away also focuses on the star to rule out local interference.

To date, and the best of my knowledge only one such signal, now known as the 'wow! signal' has ever been detected, however it lasted for 72seconds and the region in question has never shown anything interesting since so it is still inconclusive. Since the signal was detected back in 1977 the technology was rather primitive so confirming with a second telescope was not possible but interference was ruled out as much as possible given the circumstances.

What the wow! signal was and what it means, in the 2nd link the most interesting part is the last section about hypotheses and speculation.

Related to all this is something called the Drake equation, developed by Frank Drake in 1961 this is an attempt to estimate the number of technologically advanced civilisations currently active in our galaxy, unfortunately most of the values we can only make very broad guesses so it is of little scientific value but still somewhat interesting. Despite being an equation it is very simple so don't worry if you're not a maths person, that link also has a small interactive bit at the end.