If you point your radio telescope at the sky there are all kinds of signals. Some come from the galaxy itself. Some come from our atmosphere. If you made a diagram of this unavoidable noise, you would notice that it's really noisy at low frequencies because of the galaxy. It also becomes noisy at higher frequencies due to atmospheric noise. Between the two noisy regions you would find a relatively quiet region -- from about 1 GHz to about 10 GHz. (1 GHZ or 1 GigaHertz is 1 billion Hertz or 1 billion vibrations per second. This part of the radio spectrum is just above the part used for your electronic pagers and many wireless phones.)
Nature provides an even nicer way to further refine our frequency range. The simplest "stuff" of the universe, neutral hydrogen gas in interstellar space, emits radio signals at 1.42 GHz. Another molecule in space, the hydroxyl ion, or OH, ion emits at about 1.64 GHz. Now if you look at these two, H and OH, you would see that together they make up the compound of water HOH (or more commonly H2O). Life as we know it requires water to evolve and exist. The frequency range between these two emissions, from 1.42 to 1.64 GHz, is therefore a quiet region of the spectrum called "the water hole." Where would you expect water-based intelligent civilizations to meet? Around the water hole, of course! This would be a good, and nicely limited, range of frequencies to start our search. We can always broaden our net at a later date.
Water Hole

Interfering noise (in red) is the combination of lower-frequency galactic noise and higher-frequency noise from our atmosphere. The "water hole" (in blue) is between the lines for hydrogen (H) and the hydroxyl (OH) ion. There is very little noise near the "water hole."

An important fact to note here is that the narrower a signal (the more refined the frequency), the more efficient it is for our friends to send. A narrower frequency signal is also easier for us to pick up above the normal noise level. Unfortunately, very narrow frequency signals mean that we have to look at millions of very narrow frequencies to find just the right one. More on this later.

What kind of signals are we likely to receive? What should we be looking for? There are really two possibilities. Either the other civilizations are intentionally sending out a signal that is expressly meant to get our attention, or, like us, they just happen to be doing their own business and some of their signals are "leaking out." The earth broadcasts lots of stuff unintentionally. Our radio and television transmitters are very strong. Our military radars are even stronger. We've been leaking this stuff for at least 50 years, so our signals are about 50 light-years out. Not very far, but we are still technological infants. It seems like we should try to search for the leakage-type signal and assume that we will also pick up anything that is beamed at us.

What Do We Look With?

As a signal spreads out from its source, it gets weaker and weaker. The distances between stars are so vast that any signal that reaches the earth will be very weak (unless it's being beamed at us). To gather up enough of this signal we need a giant "ear." In our case, that means using a huge radio telescope.

"SERENDIP: The Search for Life"


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