Water hole is a SETI term used to describe a narrow portion of the electromagnetic spectrum that appears to be especially appropriate for interstellar communications between emerging and advanced civilizations. This band lies in the radio frequency (RF) part of the spectrum between 1,420 megahertz (MHz) frequency (8.3-inch [21.1-cm] wavelength) and 1,660 megahertz (MHz) frequency (7.1-inch [18-cm] wavelength).
Hydrogen (H) is abundant throughout interstellar space. When hydrogen experiences a "spin-flip" transition (due to an atomic collision), it emits a characteristic 1,420-megahertz frequency (or 8.3-inch [21.1-cm] wavelength) radio wave. Any intelligent species within the Milky Way galaxy that has developed radio astronomy will eventually detect these natural emissions. Similarly, there is another grouping of characteristic spectral emissions centered near the 1,660 megahertz frequency (7.1-inch [18-cm] wavelength) that are associated with hydroxyl (OH) radicals.
As scientists know from elementary chemistry, H + OH = H2O. So there are, as suggested by SETI investigators, two radio-wave emission signposts that are associated with the dissociation products of water that "beckon all waterbased life to search for its kind at the age-old meeting place of all species: the water hole."
Is this high regard for the 1,420 to 1,660 megahertz frequency band reasonable, or is it simply a case of terrestrial chauvinism? Many exobiologists currently think that if life exists elsewhere in the universe, it will most likely be carbon based, and water is essential for carbon-based life, as known here on Earth. Furthermore, for purely technical reasons, when scientists scan all the decades of the electromagnetic spectrum in search for suitable frequency at which to send or receive interstellar messages, they ultimately arrive at the narrow microwave region between 1 and 10 gigahertz (GHz) as being the most suitable candidate region for conducting interstellar communication. The two characteristic emissions of dissociated water, namely 1,420 megahertz for H and 1,660 megahertz for OH, are situated right in the middle of this optimum communications band.
On the basis of this type of reasoning, the water hole has been favored by scientists engaged in SETI projects that involve the reception and analysis of radio signals. They generally consider that this portion of the electromagnetic spectrum represents a logical starting place for humans to listen for interstellar signals from other intelligent civilizations.
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