Editing Hydrogen line (section)

{{Short description|Spectral line of hydrogen state transition in UHF radio frequencies}}
{{For|hydrogen lines in general|hydrogen spectral series}}
{{Use American English|date = March 2019}}
[[File:Hydrogen-SpinFlip.svg|thumb|A hydrogen atom with proton and electron spins aligned (top) undergoes a flip of the electron spin, resulting in emission of a photon with a 21&nbsp;cm wavelength (bottom)]]

The '''hydrogen line''', '''21&nbsp;centimeter line''', or '''H&nbsp;I line'''{{efn|The "I" in H&nbsp;I is a [[roman numeral]], so it is pronounced "H&nbsp;one". It is also called the "neutral hydrogen" line, with the listener obliged to infer from context that "cold neutral hydrogen" at 1420.4&nbsp;MHz or 0.211&nbsp;m is meant.}} is a [[spectral line]] that is created by a change in the energy state of [[Monatomic gas|solitary]], [[electrically neutral]] [[hydrogen atom]]s. It is produced by a [[Spin (physics)|spin]]-flip transition, which means the direction of the electron's spin is reversed relative to the spin of the proton. This is a [[quantum state]] change between the two [[hyperfine levels]] of the hydrogen [[principal quantum number|1]] [[azimuthal quantum number|s]] [[ground state]]. The [[electromagnetic radiation]] producing this line has a [[frequency]] of {{val|1420.405751768|(2)|ul=MHz}} (1.42&nbsp;GHz),<ref>{{cite journal
 | title=Measurement of the unperturbed hydrogen hyperfine transition frequency
 | last1=Hellwig | first1=Helmut | last2=Vessot | first2=Robert F. C.
 | last3=Levine | first3=Martin W. | last4=Zitzewitz | first4=Paul W.
 | last5=Allan | first5=David W. | last6=Glaze | first6=David J.
 | display-authors=1 | journal=[[List of IEEE publications|IEEE Transactions on Instrumentation and Measurement]]
 | volume=IM-19 | number=4 | page=200 | year=1970
 | doi=10.1109/TIM.1970.4313902 | bibcode=1970ITIM...19..200H
 | url=https://tf.nist.gov/general/pdf/13.pdf | access-date=2023-04-30
}}</ref> which is equivalent to a [[wavelength]] of {{val|21.106114054160|(30)|u=cm|ul=}} in a [[vacuum]]. According to the [[Planck–Einstein relation]] {{math|''E'' {{=}} ''hν''}}, the [[photon]] emitted by this transition has an [[photon energy|energy]] of {{val|5.8743261841116|(81)|u=μ[[electronvolt|eV]]}} [{{val|9.411708152678|(13)|e=−25|u=J}}]. The [[constant of proportionality]], {{mvar|h}}, is known as the [[Planck constant]].

The hydrogen line frequency lies in the [[L band]], which is located in the lower end of the [[microwave]] region of the [[electromagnetic spectrum]]. It is frequently observed in [[radio astronomy]] because those [[radio wave]]s can penetrate the large clouds of interstellar [[cosmic dust]] that are [[opacity (optics)|opaque]] to [[visible light]]. The existence of this line was predicted by Dutch astronomer [[Hendrik van de Hulst|H. van de Hulst]] in 1944, then directly observed by [[Edward Mills Purcell|E. M. Purcell]] and his student [[Harold Irving Ewen|H. I. 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. It is of particular importance to [[cosmology]] because it can be used to study the early Universe. Due to its fundamental properties, this line is of interest in the [[search for extraterrestrial intelligence]]. This line is the theoretical basis of the [[hydrogen maser]].