The Earth, with its permanent source of emissions at radio wavelengths, betrays the presence of an advanced technological civilization. For several decades, radioastronomers have studied the possibility of communicating with radio waves with possible extraterrestrial civilizations. To carry their messages they have chosen a specific transition of neutral hydrogen, at a wavelength of 21 cm, which corresponds to a frequency of 1420 MHz. The choice of this frequency is explained by the fact that hydrogen is the most abundant element in the Universe; in addition, the atmosphere of the Earth, like that of all exoplanets that resemble it, is transparent in that spectral region. It is therefore possible to use very large radio telescopes from the surface of the Earth and to monitor the sky permanently (Fig. 9.24).
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