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The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space. This wavelength or frequency falls within the microwave radio region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light.
The microwaves of the hydrogen line come from the atomic transition between the two hyperfine levels of the hydrogen 1s ground state with an energy difference of 5.87433 µeV. The frequency of the quanta that are emitted by this transition between two different energy levels is given by Planck's equation.
Neutral hydrogen consists of a single proton orbited by a single electron. As well as their orbital motion, the proton and electron also have spin. Classically, this is analogous to rotational motion (like the Earth rotating on its axis as it orbits the Sun), but as they are quantum particles the concept has a slightly different meaning.
The spin of the electron and proton can be in either direction - in the classical analogy they are rotating clockwise or anticlockwise around a given axis. They may have their spin oriented in the same direction or in opposite directions. Because of magnetic interactions between the particles, a hydrogen atom that has the spins of the electron and proton aligned in the same direction (parallel) has slightly more energy than one where the spins of the electron and proton are in opposite directions (anti-parallel). The fact that the lowest-energy configuration arises in the anti-parallel spin configuration is an inherently quantum-mechanical result. A proton and electron with anti-parallel spins have parallel magnetic moments owing to their opposite charge. Classical mechanics would predict that this configuration should have higher energy, but a more detailed quantum mechanical analysis shows that the opposite is true.
The lowest orbital energy state of atomic hydrogen has hyperfine splitting arising from the spins of the proton and electron changing from a parallel to antiparallel configuration. This transition is highly forbidden with an extremely small probability of 2.9×10−15 s−1.
This means that the time for a single isolated atom of neutral hydrogen to undergo this transition is around 10 million (107) years and so is unlikely to be seen in a laboratory on Earth. However, as the total number of atoms of neutral hydrogen in the interstellar medium is very large, this emission line is easily observed by radio telescopes. Also, the lifetime can be considerably shortened by collisions with other hydrogen atoms and interaction with the cosmic microwave background.
The line has an extremely small natural width because of its long lifetime, so most broadening is due to doppler shifts caused by the motion of the emitting regions relative to the observer.
During the 1930s, it was noticed that there was a radio 'hiss' that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to be coming from the centre of the Galaxy. These discoveries were published in 1940 and were seen by Professor J.H. Oort who knew that significant advances could be made in astronomy if there were emission lines in the radio part of the spectrum. He referred this to Dr Hendrik van de Hulst who, in 1944, predicted that neutral hydrogen could produce radiation at a frequency of 1420.4058 MHz due to two closely spaced energy levels in the ground state of the hydrogen atom.
The 21 cm line (1420.4 MHz) was first detected in 1951 by Ewen and Purcell at Harvard University, and published after their data was corroborated by Dutch astronomers Muller and Oort, and by Christiansen and Hindman in Australia. After 1952 the first maps of the neutral hydrogen in the Galaxy were made and revealed, for the first time, the spiral structure of the Milky Way.
In radio astronomy 
The spectral line appears within the radio spectrum (in the microwave window to be exact). Electromagnetic energy in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference.
Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the doppler shift that each of these lines has. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has also been calculated using the 21-cm hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy.
Hydrogen line observations have also been used indirectly to calculate the mass of galaxies, to put limits on any changes over time of the universal gravitational constant and to study dynamics of individual galaxies.
In cosmology 
The line is of great interest in big bang cosmology because it is the only known way to probe the "dark ages" from recombination to reionization. Including the redshift, this line will be observed at frequencies from 200 MHz to about 9 MHz on Earth. It potentially has two applications. First, by mapping redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the matter power spectrum in the period after recombination. Second, it can provide a picture of how the universe was reionized, as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 centimeter background.
However, 21 centimeter experiments are very difficult. Ground based experiments to observe the faint signal are plagued by interference from television transmitters and the ionosphere, so they must be very secluded and careful about eliminating interference if they are to succeed. Space based experiments, even on the far side of the moon (which should not receive interference from terrestrial radio signals), have been proposed to compensate for this. Little is known about other effects, such as synchrotron emission and free-free emission on the galaxy. Despite these problems, 21 centimeter observations, along with space-based gravity wave observations, are generally viewed as the next great frontier in observational cosmology, after the cosmic microwave background polarization.
In terrestrial remote sensing 
The Soil Moisture & Ocean Salinity (SMOS) satellite's main scientific instrument Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) uses the 1400-1427 MHz frequencies (including 1420.406 MHz) to monitor the ocean surface salinity and the soil moisture of the Earth. The choice of HI band results from: 1) much better radiative signature of salinity and moisture in microwave than in higher frequencies, 2) no electromagnetic interference from anthropogenic sources, as HI is reserved for radioastronomy.
Possible uses for SETI 
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, or 168 cm. Similarly the frequency of the hydrogen spin-flip transition was used for a unit of time in a map to Earth included on the Pioneer plaques and also the Voyager 1 and Voyager 2 probes. 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. It is theorized by the plaque's creators that an advanced civilization would then be able to use the locations of these pulsars to locate the Solar System at the time the spacecraft were launched.
The 21 cm hydrogen line is considered a favorable frequency by the SETI program in their search for signals from potential extraterrestrial 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. According to George Basalla, the paper by Cocconi and Morrison "provided a reasonable theoretical basis" for the then nascent SETI program.
Pyotr Makovetsky proposed to use for SETI a frequency which is equal to pi times 1420.4 MHz (pi times 1420.40575177 megahertz = 4.46233627 gigahertz; (2 * pi) times 1420.40575177 megahertz = 8.92467255 gigahertz). Since pi is a transcendental number, such frequency couldn't possibly be produced in a natural way as a harmonic, and would clearly signify its artificial origin. Such signal would not be jammed by the HI line itself, or any of its harmonics.
See also 
- H-alpha, the visible red spectral line with wavelength of 6562.8 Ångstroms
- Hydrogen spectral series
- Hydrogen atom
- Radio astronomy
- Rydberg formula
- Spectral line
- Wow! signal
- "The Hydrogen 21-cm Line". Hyperphysics. Georgia State University. 2004-10-30. Retrieved 2008-09-20.
- Ewan, H.I.; E.M. Purcell (September 1951). "Observation of a line in the galactic radio spectrum". Nature 168 (4270): 356. Bibcode:1951Natur.168..356E. doi:10.1038/168356a0. Retrieved 2008-09-21.
- Muller, C.A.; J.H. Oort (September 1951). "The Interstellar Hydrogen Line at 1,420 Mc./sec., and an Estimate of Galactic Rotation". Nature 168 (4270): 357–358. Bibcode:1951Natur.168..357M. doi:10.1038/168357a0. Retrieved 2008-09-21.
- Basalla, George (2006). Civilized Life in the Universe. Oxford University Press. pp. 133–135. ISBN 0-19-517181-0.
- Makovetsky P. Smotri v koren' (in Russian)
- P. Madau, A. Meiksin and M. J. Rees, "21-cm Tomography of the Intergalactic Medium at High Redshift", Astrophysical Journal 475, 429 (1997) arXiv:astro-ph/9608010.
- B. Ciardi, P. Madau, "Probing Beyond the Epoch of Hydrogen Reionization with 21 Centimeter Radiation", Astrophysical Journal 596, 1 (2003) arXiv:astro-ph/0303249.
- M. Zaldarriaga, S. Furlanetto and L. Hernquist, "21 Centimeter Fluctuations from Cosmic Gas at High Redshifts", Astrophysical Journal 608, (2004) 608 arXiv:astro-ph/0311514.
- X. Chen and J. Miralda-Escudé, "Observing the Reionization Epoch Through 21 Centimeter Radiation", Mon. Not. Roy. Astron. Soc. 347, 187 (2004) arXiv:astro-ph/0303395.
- A. Loeb and M. Zaldarriaga, "Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations Through 21 cm Tomography Prior to the Epoch of Structure Formation", Phys. Rev. Lett. 92, 211301 (2004) arXiv:astro-ph/0312134.
- M. G. Santos, A. Cooray and L. Knox, "Multifrequency analysis of 21 cm fluctuations from the Era of Reionization", Astrophysical Journal 625, 575 (2005) arXiv:astro-ph/0408515.
- R. Barkana and A. Loeb, "Detecting the Earliest Galaxies Through Two New Sources of 21cm Fluctuations", Astrophysical Journal 626, 1 (2005) arXiv:astro-ph/0410129.