The waterhole, or water hole, refers to an especially quiet band of the electromagnetic spectrum

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

 between 1,420 and 1,666 megahertz, corresponding to wavelengths of 21 and 18 centimeters respectively. The term was coined by Bernard Oliver. The strongest hydroxyl radical

Hydroxyl radical

The hydroxyl radical, •OH, is the neutral form of the hydroxide ion . Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides or, in...

 spectral line

Spectroscopy

Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 radiates at 18 centimeters, and hydrogen

Hydrogen line

The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum...

 at 21 centimeters. These two combined form water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, and water is currently thought to be essential to extraterrestrial life

Extraterrestrial life

Extraterrestrial life is defined as life that does not originate from Earth...

 advanced enough to generate radio signals. Bernard M. Oliver

Bernard M. Oliver

Bernard M. Oliver , aka Barney Oliver, was a scientist who made contributions in many fields, including radar, television, and computers. He was the founder and director of Hewlett Packard laboratories until his retirement in 1981. He is also a recognized pioneer in the search for...

 theorized that the waterhole would be a good, obvious band for communication with extraterrestrial intelligence

Communication with Extraterrestrial Intelligence

Communication with extraterrestrial intelligence is a branch of the search for extraterrestrial intelligence that focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. The best-known CETI experiment was the 1974 Arecibo message...

, hence the pun, in English a watering hole is a vernacular

Vernacular

A vernacular is the native language or native dialect of a specific population, as opposed to a language of wider communication that is not native to the population, such as a national language or lingua franca.- Etymology :The term is not a recent one...

 reference to a common place to meet and talk. Several programs involved in the search for extraterrestrial intelligence, including SETI@home

SETI@home

SETI@home is an Internet-based public volunteer computing project employing the BOINC software platform, hosted by the Space Sciences Laboratory, at the University of California, Berkeley, in the United States. SETI is an acronym for the Search for Extra-Terrestrial Intelligence...

, search in the waterhole.

External links

• 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"