Why do we observe in the “waterhole” frequency range?

The Oliver (1979) paper addresses the question which frequency range is optimal for SETI search.

The author first argues that advanced civilizations will choose the least expensive means of communication to establish contact with other civilizations. And clearly detecting electromagnetic waves is much more cost effective than sending space probes and spaceships. Then the author begins arguing why the electromagnetic waves, especially low-energy photons suits best for SETI search because they are massless, chargeless.

Further, the author makes the case for the optimal spectral region for SETI by analyzing where the galactic and cosmic radiation background are the smallest. This constrains the frequency range to be 1 to 60 GHz. Further complications such as the water absorption lines in the atmosphere further constrains the terrestrial microwave window to be between 1 to 10 GHz. The author also argues that the noise level will be much higher if we go to optical frequency range. There are additional merits using the 1 to 10 GHz frequency range such as great collecting area, higher beacon powers and narrower bandwidths.

Next, the author considers the doppler shift caused by the rotation of the Earth on the microwave signal. Then the minimum of total receiver noise could be achieved around 1.5GHz. Finally, the author makes his comment on the frequency range chosen for the Cyclops report (1420 MHz to 1662 MHz). He is pretty optimistic because we are now at least know where to look for SETI signal and waterhole is best for searching for life since all life we know so far depends on water.

The reason the paper is assigned is because unlike the Cocconi and Morrison (1959) paper, the paper theoretically derives the best frequency range to observe. One possibility the paper does not consider as important as radio SETI is the possibility of optical SETI, which is a more recent SETI research direction.

One limitation of this paper is that it assumes alien civilizations possess the same technology as we do so they will also look in the waterhole frequency range which is not always the case.