Often appearing in crop circles at the beginning of the year as a hello and establish communication with intelligent life. Depicted as a hydrogen atom or cell this is most likely used as a greeting sign from other intelligent lifeforms in the universe.
SETI scans the hydrogen line radio frequency at 1420mhz which is the same frequency emitted by accelerated hydrogen atoms (the most common element in the Universe) and is therefore thought to be a frequency on which intelligent beings might use as a “Hello” signal. Interestingly the famous “Wow!” signal was hypothesised to be of alien origin because it was at this frequency at high intensity. Not long after this event, the crop circle phenomena began.
The Pioneer plaque, attached to the Pioneer 10 and Pioneer 11 spacecraft, portrays the hyperfine transition of neutral hydrogen and used the wavelength as a standard scale of measurement. For example, the height of the woman in the image is displayed as eight times 21 cm. The 21 cm hydrogen line is a favourable frequency used by the SETI program in their search for signals from potential extra-terrestrial civilizations.
In 1959, Italian physicist Giuseppe Cocconi and American physicist Philip Morrison published “Searching for interstellar communications”, a paper proposing the 21 cm hydrogen line and the potential of microwaves in the search for interstellar communications.
