Friday, June 30, 2017

Waterhole Radio

The word waterhole evokes a National Geographic image of ungulates gathering around a muddy depression in the savanna to drink and to drink quickly to avoid irritable hippos and hungry crocodiles. Or perhaps a deep depression in the woos, an oxbow off a larger stream with a Norman Rockwellesque  tire swing hung above it. As it turns out this is actually the idiom from which waterhole radio takes it's name.

The more technical term "waterhole radio" describes the electromagnetic spectrum between 1,420 and 1,666 megahertz. This corresponding to wavelengths of 21 and 18 centimeters respectively. This "hole" is a notch in the radio band that is low in noise. With few exceptions, from Earth, the whole of the electromagnetic spectrum is very noisy. So when we point out dishes skyward in attempts to listen to the cosmos, we often focus on those low-noise segments. It was Bernard M. Oliver in 1971, who dubbed the spectral region a Cosmic Water-Hole. "Where shall we meet our neighbors?" he asked. "At the water-hole, where species have always gathered." More here.

Oliver began his career at Bell Telephone, where he stayed through WWII working on radar development, and later pulse-code modulation with Claude Shannon. In 1952 he joined Hewlett-Packard as director of research.  He was elected President of the IEEE in 1965. Five years later  he was supervising  the production of the first hand-held HP calculators in the 1970s. was IEEE President in 1965. He held over 60 patents and was awarded the National Medal of Science in 1986. So how does he connect to Oliver? In 1971 he and John Billingham identified the band in the Project Cyclops report for SETI in a NASA funded study. Below is the opening quote from that study:
Generally speaking the microwave region, between about 1 and 10 GHz is low-noise. But rather than search blindly within that band, Cyclops recommended targeted searches of specific stars, but also in a subset of that Microwave band between 1 and 3 GHz. There were technical reasons, the low end of the band has smaller Doppler shifts, and less stringent frequency stability requirements... but it was also largely free from Oxygen and Water absorption.  But the target, was between the strongest hydroxyl (OH) radical spectral line radiates at 18 centimeters, and hydrogen (H) at 21 centimeters. These two molecules, which combined form water, are widespread in interstellar gas, and their presence radiates radio noise at these frequencies. So here we have two metaphoric signposts, representing the foundation of life as we know it, within the quietest part of the band. The scientists and the poets all agreed this was a fine place to look for alien life.

But perhaps it's not so clear. Critics point out that there are actually four hydroxyl radical spectral lines: 1612.231, 1665.402, 1667.359, and 1720.53. [SOURCE IAU] so the bulls-eye is somewhat less clear, and less poetic. Game theorists have dismissed the waterhole as a Schelling point. It's a focal point based on our own biased expectations. (Named for American economist Thomas Schelling.]) More here. The Fermi paradox aside, after half a century of ardent searching SETI has found nothing, but as Stephen Hawking pointed out... that may not be a bad thing. More here.

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