This Water Hole has Nothing to do with Buffalo (or zebras)

The “water hole,” as used in SETI, has nothing to do with holes and only a little to do with water (but not really). It is the range of frequencies between 1420 and 1700 MHz, bounded by the hydrogen line and the hydroxl line (H + OH -> H2O, so sort of related to water but unrelated still to holes). The idea of this water hole was first introduced in 1971 by the Cyclops study who suggested that this band was a place where “different galactic species might meet” just as animals meet at actual water holes.

In an attempt to resurface love for the “Water Hole,” Oliver wrote his paper “Rationale for the water hole” in 1979. This publication was timely in that many companies were submitting proposals and plans to fill this frequency band with what would be interference to astronomers searching for ETI. Oliver suggests (and I agree) that “it would be a bitter irony if the desire to know exactly where we are … were to prevent us from ever knowing where we are with respect to other life,” as many of these proposed technologies were for GPS. Oliver writes his paper methodically, going over requirements for any signal from ETI and arguing for the water hole.

First, any signal sent must be significant in respect to signal to noise.  Oliver immediately dismisses Bracewell’s probes as excessive and expensive. Therefore, the signal must exceed the noise, avoid scattering and deflection by the ISM and other mediums, and be easy to detect. Massive particles are excluded from this as they require too much energy to send and anything holding a charge would be deflected or absorbed. Although neutrinos are nearly massless, they cannot be generated or detected easily and are not usually radiated by civilizations, so no one would start by looking there. In contrast to all of this, light is massless and easy to produce and detect. Low energy photons are also not readily absorbed or deflected, making them a great signal.

Oliver argues for the microwave region as the optimal region since it gives the minimum detectable received power, and grants the smallest cost per unit of collecting area. It also allows for bandwidths narrower than optical. This region is also decently unobscured by the Earth’s atmosphere. Obviously, we need to be able to detect signals through  our atmosphere, but the signal would need to leave the atmosphere of other civilizations (assuming their life and atmosphere are alike to ours). The H and OH lines are also relatively quiet in respect to the receiver noise per channel.

All of this leaves a fairly wide band (2GHz), which is way too wide to search. And thus enters the water argument. Life on our Earth requires water, and water is decently common in the universe. It isn’t too difficult to take these two facts and postulate that most life in the universe probably requires water. So, searching for signals of water make sense. Why that means that the H and OH lines should be the borders of the band we search in, I’m not sure. I suppose the H and OH lines leave a decent sized band that is an appropriate place to search.

Since no better band has been suggested, Oliver argues that this band should be used for SETI and preserved for such reasons. He closes with a lovely quote “If we are to make progress we must proceed on the basis of what we know, and not forever wait for something now unknown to be discovered” that should be on scientific motivational posters.

I personally really enjoy this paper since it is methodical, scientific, and not just random, unsupported postulation. As of now, I’m not sure if the water hole is bogged down with terrestrial signals, but I hope not. I wonder if a call to action (or I supposed inaction) such as Oliver’s need be published every few years to remind people of why it is important. Something to look into.