Thehydrogen line,21 centimeter line, orH I line[a] is aspectral line that is created by a change in the energy state ofsolitary,electrically neutralhydrogen atoms. It is produced by aspin-flip transition, which means the direction of the electron's spin is reversed relative to the spin of the proton. This is aquantum state change between the twohyperfine levels of the hydrogen1sground state. Theelectromagnetic radiation producing this line has afrequency of1420.405751768(2) MHz (1.42 GHz),[1] which is equivalent to awavelength of21.106114054160(30) cm in avacuum. According to thePlanck–Einstein relationE =hν, thephoton emitted by this transition has anenergy of5.8743261841116(81) μeV [9.411708152678(13)×10−25 J]. Theconstant of proportionality,h, is known as thePlanck constant.
The hydrogen line frequency lies in theL band, which is located in the lower end of themicrowave region of theelectromagnetic spectrum. It is frequently observed inradio astronomy because thoseradio waves can penetrate the large clouds of interstellarcosmic dust that areopaque tovisible light. The existence of this line was predicted by Dutch astronomerH. van de Hulst in 1944, then directly observed byE. M. Purcell and his studentH. I. Ewen in 1951. Observations of the hydrogen line have been used to reveal the spiral shape of theMilky Way, to calculate the mass and dynamics of individual galaxies, and to test for changes to thefine-structure constant over time. It is of particular importance tocosmology because it can be used to study the early Universe. Due to its fundamental properties, this line is of interest in thesearch for extraterrestrial intelligence. This line is the theoretical basis of thehydrogen maser.
An atom of neutral hydrogen consists of anelectron bound to aproton. The lowest stationary energy state of the bound electron is called itsground state. Both the electron and the proton have intrinsicmagnetic dipole moments ascribed to theirspin, whose interaction results in a slight increase in energy when the spins are parallel, and a decrease when antiparallel. The fact that only parallel and antiparallel states are allowed is a result of thequantum mechanical discretization of the totalangular momentum of the system. When the spins are parallel, the magnetic dipole moments are antiparallel (because the electron and proton have opposite charge), thus one would expect this configuration to actually havelower energy just as two magnets will align so that the north pole of one is closest to the south pole of the other. This logic fails here because the wave functions of the electron and the proton overlap; that is, the electron is not spatially displaced from the proton, but encompasses it. The magnetic dipole moments are therefore best thought of as tiny current loops. As parallel currents attract, the parallel magnetic dipole moments (i.e., antiparallel spins) have lower energy.[2]
In the ground state, thespin-flip transition between these aligned states has an energy difference of5.87433 μeV. When applied to thePlanck relation, this gives:
whereλ is thewavelength of an emitted photon,ν is itsfrequency,E is the photon energy,h is thePlanck constant, andc is thespeed of light in a vacuum. In a laboratory setting, the hydrogen line parameters have been more precisely measured as:
in a vacuum.[3]
This transition is highlyforbidden with an extremely small transition rate of2.9×10−15 s−1,[4] and a mean lifetime of the excited state of around 11 million years.[3] Collisions of neutral hydrogen atoms with electrons or other atoms can help promote the emission of 21 cm photons.[5] A spontaneous occurrence of the transition is unlikely to be seen in a laboratory on Earth, but it can be artificially induced throughstimulated emission using ahydrogen maser.[6] It is commonly observed in astronomical settings such ashydrogen clouds in our galaxy and others. Because of theuncertainty principle, its long lifetime gives thespectral line an extremely smallnatural width, so most broadening is due toDoppler shifts caused by bulk motion or nonzero temperature of the emitting regions.[7]
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 propagate from thecentre of the Galaxy. These discoveries were published in 1940 and were noted byJan Oort who knew that significant advances could be made in astronomy if there wereemission lines in the radio part of the spectrum. He referred this toHendrik van de Hulst who, in 1944, predicted thatneutralhydrogen could produce radiation at afrequency of1420.4058 MHz due to two closely spaced energy levels in theground state of thehydrogen atom.[8]
The 21 cm line (1420.4 MHz) was first detected in 1951 byEwen andPurcell atHarvard University,[9] and published after their data was corroborated by Dutch astronomers Muller and Oort,[10] and byChristiansen 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 theMilky Way.[11][12]
The 21 cm spectral line appears within theradio spectrum (in theL band of theUHF band of themicrowave window). Electromagnetic energy in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference.[13] The hydrogen line can readily penetrate clouds of interstellarcosmic dust that areopaque tovisible light.[14] 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, by assumingcircular motion, one can calculate the relative speed of each arm of our galaxy. Therotation curve of our galaxy has been calculated using the21 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. However, a limitation of this method is that departures from circular motion are observed at various scales.[15]
Hydrogen line observations have been used indirectly to calculate the mass of galaxies,[16] to put limits on any changes over time of thefine-structure constant,[17] and to study the dynamics of individual galaxies. Themagnetic field strength ofinterstellar space can be measured by observing theZeeman effect on the 21-cm line; a task that was first accomplished byG. L. Verschuur in 1968.[18] In theory, it may be possible to search forantihydrogen atoms by measuring thepolarization of the 21-cm line in an external magnetic field.[19]
Deuterium has a similar hyperfine spectral line at 91.6 cm (327 MHz), and the relative strength of the 21 cm line to the 91.6 cm line can be used to measure the deuterium-to-hydrogen (D/H) ratio. One group in 2007 reported D/H ratio in thegalactic anticenter to be 21 ± 7 parts per million.[20]
The line is of great interest inBig Bang cosmology because it is the only known way to probe the cosmological "dark ages" fromrecombination (when stable hydrogen atoms first formed) to thereionization epoch. After including theredshift range for this period, this line will be observed at frequencies from 200 MHz to about 15 MHz on Earth.[21] It potentially has two applications. First, bymapping the intensity of redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of thematter 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.[24][7]
However, 21 cm observations are very difficult to make. Ground-based experiments to observe the faint signal are plagued by interference from television transmitters and theionosphere,[23] so they must be made from very secluded sites with care taken to eliminate interference. 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.[25] Little is known about other foreground effects, such assynchrotron emission andfree–free emission on the galaxy.[26] Despite these problems, 21 cm observations, along with space-based gravitational wave observations, are generally viewed as the next great frontier in observational cosmology, after thecosmic microwave background polarization.[27]
ThePioneer plaque, attached to thePioneer 10 andPioneer 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 theVoyager 1 andVoyager 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 theSolar System at the time the spacecraft were launched.[28][29]
The 21 cm hydrogen line is considered a favorable frequency by theSETI program in their search for signals from potential extraterrestrial civilizations. In 1959, Italian physicistGiuseppe Cocconi and American physicistPhilip 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.[30] Similarly,Pyotr Makovetsky proposed SETI use a frequency which is equal to either
or
Sinceπ is anirrational number, such a frequency could not possibly be produced in a natural way as aharmonic, and would clearly signify its artificial origin. Such a signal would not be overwhelmed by the H I line itself, or by any of its harmonics.[31]
Skywaves are not used in the UHF band because the ionosphere is not sufficiently dense to reflect the waves, which pass through it into space. ... Reception of UHF signals is virtually free from fading and interference from atmospheric noise.
