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The hydrogen line is a radio frequency emission from cold hydrogen gas in space, detectable due to its wavelength of 21.1 cm. Electrons emit electromagnetic radiation when they descend energy levels, with frequency proportional to energy. Hydrogen atoms emit radiation at 21 cm due to a spin change, which is detectable by radio astronomy and has allowed us to map our galaxy. The hydrogen line is also significant for SETI as it represents a unit of measurement that aliens are believed to understand.
The hydrogen line generally refers to radio frequency emissions of cold hydrogen gas into interstellar space. There are large amounts of hydrogen floating around in our galaxy and other galaxies. Some of this gas is heated by nearby stars, causing them to emit electromagnetic radiation in the visible spectrum, in other words light. Much of it, however, is away from any heat sources, but is still detectable due to the fact that it emits electromagnetic radiation at a wavelength of 8.3 inches (21.1 centimeters), within the radio portion of the spectrum. This is known as the 21cm line, or hydrogen line, and its existence was predicted by Dutch astronomer Hendrik van de Hulst in 1944.
According to quantum theory, the electrons in an atom can only have certain fixed energy levels, with nothing in between. The lowest energy level is known as the “ground state”. Electrons can absorb energy, causing them to “jump” to a higher energy level, but sooner or later they will fall back to a lower level, and eventually to the ground state, with the time taken inversely proportional to the amount of excess energy. When an electron goes down a level, the extra energy is released as electromagnetic radiation with a frequency that corresponds to the difference in energy between the two levels.
The frequency of electromagnetic radiation is proportional to its energy: the higher the energy, the higher the frequency. This relationship is described by Planck’s equation: E = hf, where E is energy, f is frequency, and h is Planck’s constant, which has a value of approximately 6.626 * 10-34 joule-second. Wavelength can be calculated simply as the speed of light divided by the frequency. Thus, when an electron descends from a higher to a lower energy level, electromagnetic radiation will be emitted with a certain frequency and fixed wavelength, which is related to the difference in energy. This radiation shows up as narrow lines on an emission spectrum.
Each element has a characteristic and unique emission spectrum made up of a series of lines at specific wavelengths. The spectral range of hydrogen contains a number of spectral lines, four of which are within the visible part of the spectrum. One of these, a red line known as H-alpha, is widely used in astronomy to detect ionized hydrogen in nebulae. These emission lines for hydrogen can each be thought of as a hydrogen line, but the term more commonly refers to the radio emission produced by cold hydrogen gas at a wavelength
of 21 centimeters. This is due to a different physical process. However, the same rules regarding energy, frequency and wavelength still apply.
Electrons and protons have a quantum property known as “spin” which can have two possible directions. Since a hydrogen atom consists of a proton and an electron, it can have the two spins in the same direction or in different directions. In the former case, the atom has a little more energy and will eventually drop to a lower energy state from the electron changing its spin. The extra energy is emitted as electromagnetic radiation and because the energy difference is small, the radiation has a long wavelength and a low frequency: 21 centimeters and 1420.4 MHz, respectively. The small energy difference also means that a given hydrogen atom in the same spin state will, on average, take a long time – several million years – to descend to an opposite spin state; however, there is so much cold hydrogen in a galaxy that at any moment enough hydrogen atoms will emit 21-centimeter radio waves to be detectable.
The 21cm line was surveyed in 1951 by Harold Ewen and Edward Purcell. It has proved to be of crucial importance in radio astronomy. Much of our galaxy is hidden from view by large clouds of dust that do not allow starlight to pass through them. The radio waves, however, are not obstructed by dust clouds, and since there is a great abundance of cold hydrogen in the galaxy, it is possible to observe and map the galaxy using radio emissions on the hydrogen line. Radio astronomy, using the hydrogen line, has allowed us to determine the size, shape and structure of our galaxy.
The hydrogen line also has great significance for the Search for Extra Terrestrial Intelligence (SETI). It is believed very possible that a technologically advanced civilization could use this frequency to attempt to communicate with other civilizations. The frequency was used not only to listen to incoming messages, but also to send them. Spacecraft Pioneer 10 and 11, destined to travel indefinitely through interstellar space, contain plaques depicting the hydrogen line, its wavelength, its frequency, and the physics behind it. It represents a unit of measurement that aliens are believed to understand.
