This article by Charles Townes from 1983 discusses the optimum frequency we should use to listen at for ETI. It discusses how all the previous SETI searches have focussed in the microwave due to the obsession with the water – hole and the potential ‘fallacy’ of this obsession. It must be noted that this article was written in 1983, before many of the technological advancements of the 21st century.
The author talks about how the diffraction limit affects observations, and the advantage of directed observations over a beamed observation. It also suggests that the transmitter might want to correct for barycentric movement to ensure that the signal is in the ‘rest frame’, and then the receiver could correct for their respective motion. I believe this drift is an important factor that needs to be taken into account when we decide the bandwidth of future SETI searches (cue: Sofia’s project).
To differentiate between the microwave and the infrared, the author compares the detectors available and the background contributions in the two domains. Since the diffraction limited solid angle is different in the two regimes, the background plays a higher role in the microwave and radio.
The suggestions of the paper have aged though since photon counters are no longer the most sensitive means of detecting optical and IR light. It does not extrapolate into the future for potential telescope primary sizes, however the detector assumptions made have fallen way by the side due to advancements in the field.
All that being said, I believe this paper is important as it is the first one which goads us to not be myopic and consider looking outside the water-hole (21cm) for communication with ET.