The waterhole, or water hole, refers to an especially quiet band of the electromagnetic spectrum between 1,420 and 1,666 megahertz, corresponding to wavelengths of 21 and 18 centimeters respectively. The term was coined by Bernard Oliver in 1971. The strongest hydroxyl radical spectral line radiates at 18 centimeters, and hydrogen at 21 centimeters. These two combined form water, and water is currently thought to be essential to extraterrestrial life advanced enough to generate radio signals. Bernard M. Oliver theorized that the waterhole would be a good, obvious band for communication with extraterrestrial intelligence, hence the name, which is a form of pun: in English, a watering hole is a vernacular reference to a common place to meet and talk. Several programs involved in the search for extraterrestrial intelligence, including SETI@home, search in the waterhole.
- SETI: The Radio Search (page 2)
- "What Is the Water Hole" (has a cleaner diagram)
- Planetary.org: A Blueprint for SETI
- How SETI Works Discusses the water hole.
- "waterhole" entry in the The Encyclopedia of Astrobiology, Astronomy, and Spaceflight'
- "The ABCs of SETI: the search for extraterrestrial intelligence"
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