Hydrogen line
The hydrogen line frequency lies in the L band, which is located in the lower end of the microwave region of the electromagnetic spectrum.
The existence of this line was predicted by Dutch astronomer H. van de Hulst in 1944, then directly observed by E. M. Purcell and his student H. E. Ewen in 1951.
Observations of the hydrogen line have been used to reveal the spiral shape of the Milky Way, to calculate the mass and dynamics of individual galaxies, and to test for changes to the fine-structure constant over time.
The fact that only parallel and antiparallel states are allowed is a result of the quantum mechanical discretization of the total angular momentum of the system.
Because of the uncertainty principle, its long lifetime gives the spectral line an extremely small natural width, so most broadening is due to Doppler shifts caused by bulk motion or nonzero temperature of the emitting regions.
These discoveries were published in 1940 and were noted by Jan Oort who knew that significant advances could be made in astronomy if there were emission lines in the radio part of the spectrum.
[8] The 21 cm line (1420.4 MHz) was first detected in 1951 by Ewen and Purcell at Harvard University,[9] and published after their data was corroborated by Dutch astronomers Muller and Oort,[10] and by Christiansen and Hindman in Australia.
The magnetic field strength of interstellar space can be measured by observing the Zeeman effect on the 21-cm line; a task that was first accomplished by G. L. Verschuur in 1968.
First, by mapping the intensity of redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the matter power spectrum in the period after recombination.
[22] Second, it can provide a picture of how the universe was re‑ionized,[23] as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 cm background.
Space based experiments, even on the far side of the Moon (where they would be sheltered from interference from terrestrial radio signals), have been proposed to compensate for this.
On this map, the position of the Sun is portrayed relative to 14 pulsars whose rotation period circa 1977 is given as a multiple of the frequency of the hydrogen spin-flip transition.
[28][29] The 21 cm hydrogen line is considered a favorable frequency by the SETI program in their search for signals from potential extraterrestrial civilizations.
According to George Basalla, the paper by Cocconi and Morrison "provided a reasonable theoretical basis" for the then-nascent SETI program.
