<< Back<< The “special” frequency of the Hydrogen


By Flavio Falcinelli

Radio astronomy enthusiasts are well aware that the frequencies around the hydrogen line (1420 MHz), with their receiving devices, are also used for S.E.T.I. researches (Search for Extra-Terrestrial Intelligence). These tools, in fact, are perfect to set up a SETI activity at amateur level, if one just optimizes and specializes the management and data analysis software.

The idea of assessing to what extent the radio waves emitted from the Earth are admissible in interstellar space came from two physicists at Cornell University, Giuseppe Cocconi and Philip Morrison: they have demonstrated, in a famous and innovative article of 1959, how the most suitable radio waves to propagate information in outer space are the ones next to the wavelength of 21 cm, the neutral hydrogen line. This frequency would be a "natural" tune easily distinguishable from the background noise, known to every hypothetical extraterrestrial civilization. It represents "a unique and objective sample of frequency, necessarily known to any observer in the universe."

"....It is also reasonable to expect that receivers at this frequency are built from the beginning of the evolution of radio astronomy....".

In SETI’s history, the article by Cocconi and Morrison explains the scientific motivations and represents the beginning of the search for extraterrestrials "smart” radio signals.

Subsequently were also offered other interesting frequencies corresponding to the spectral lines of molecules discovered in interstellar space, all valid for interstellar communications, since they have good chances of being accidentally revealed by hypothetical extraterrestrial civilizations during a scan of the sky, if not only to astrophysical interest.

Among the experts it is customary to identify with the suggestive name of "waterhole" the band from 1420 MHz to 1700 MHz, the water hole around which the galactic civilization gather, like animals in the jungle. The water, in fact, is formed from hydrogen (natural frequency of 1420 MHz) and the OH molecule (1665 MHz frequency): if it has an essential role in life and if we consider the technical advantages offered by interstellar communications in this frequencies band (the cosmic background noise is minimum), it is reasonable to start from these frequencies the search for possible extraterrestrial radio signals.

Much documentation is available on the web about the different philosophies and SETI search strategies, both academic and amateur level. There are also many groups of amateur radio astronomers involved in this type of research, sometimes coordinated by institutional radio-observers.

The challenge is simply (so to say....) this: you need to gear up to receive very weak signals near the neutral hydrogen line. Fascinating activity, although a bit "nebulous"... The reasons that could stimulate a fan to invest resources and time in this work are:

  • Intelligent extraterrestrial civilization distant no more than 100 light years, equipped with radio telescopes similar to that of Arecibo (diameter of about 500 meters), could transmit radio signals strong enough to be detected on Earth with amateur equipment, for example using parabolic antennas with a diameter of the order of 2-3 meters.
  • Some amateur radio astronomer could have dumb luck....
  • Continuing discoveries of new planets are documented (including Earth-like) external to our solar system.
  • In interstellar clouds there have been discovered (thanks to radio astronomy) many complex organic molecules, such as those necessary for the chemistry of (our) life.
  • The current technology provides the tools (radio telescopes), and analytical strategies of signals necessary to organize the first research efforts.