While the Microwave Window is a “cosmic quiet zone” the existence of the radio emission lines marking the Water Hole illustrates a problem: distinguishing communication signals from natural astrophysical emission. Most cosmic radio sources as “very broadband.” They “broadcast” over a very wide range of frequencies. For example, the radio emission from a quasar would span most of the diagram above and the amount of power would change very slowly over the frequency range. As mentioned above, hydrogen and hydroxyl emit radio waves at particular frequencies. The diagram below shows the power emitted by a cloud of hydroxyl (OH) molecules near a red giant star. The cloud rotates around the star so some of the molecules are moving toward us and some are moving away. The Doppler Effect shifts the frequency of the radio waves to higher (moving toward us) and lower (moving away) frequencies. The resulting spectrum (power vs. frequency) shows two peaks. These features are very narrow compared to most astrophysical sources.
Signals produced by technology for communication can be much narrower than any known astrophysical source. The bottom half of the diagram shows the spectrum of the signal from the Pioneer 10 spacecraft. The frequency scale has been expanded by nearly a factor of 10. The narrow signal component on the left is further expanded by a factor of 100 to show the very narrow frequency components of the data in the signal. This illustrates a clear way to distinguish extraterrestrial communication signals for astrophysical signals: look for very narrow frequency components. Most SETI programs take this approach, breaking the radio spectrum into many millions of very narrow frequency channels
Searching for extraterrestrial signals can be likened to searching for a gold nugget buried in a field. We have identified the field that we will explore: the Microwave Window in the radio spectrum. We have a way to distinguish the gold nugget for other pieces of metal: look for very narrow bandwidth signals. But, with limited resources (telescope time, electronics, etc.) what is the best strategy to search through the field?
There are two basic strategies. We could examine the entire field but only sift through the top few centimeters of soil. If a gold nugget is near the surface anywhere in the field, we’ll have a good chance of finding it. We could study the chemistry and geology of the surface and select particulars spots where we will dig deep looking for gold. If the gold is buried well below the surface, this strategy has the best chance.
In SETI, there are two basic search strategies. Sky surveys sweep a telescope over large areas of the sky looking for strong signals that may come from any direction. Targeted searches point a telescope at the direction of selected stars. By dwelling on a star for long periods of time, a targeted search is sensitive to weaker signals.