Prediction of 21 cm radiation.

Prediction of 21cm line radiation.

Although the Netherlands were occupied by Germany during the years of World War II, Dutch astronomers were nevertheless able at times to meet and discuss astronomy. Copies of the Astrophysical Journal managed to reach Leiden Observatory.

Jan Oort (1900-1992)

The great Dutch astronomer Jan Oort learned of Grote Reber's discoveries in radio astronomy and realized that a radio spectral line would be an important tool for discovering the structure of our galaxy. Oort had spent many years studying the rotation and structure of the galaxy using optical means. He was frustrated by the extensive clouds of dust lying in the galactic plane, which block visible light. One can see only a few thousand light years towards the galactic center because the light of distant stars is absorbed. But radio waves will penetrate the dust and show us the galactic center and indeed the opposite side of the galaxy. The importance of a spectral line is that the frequency of the line will be shifted by the Doppler effect which means that the velocity of the gas can be measured. One can then study the differential rotation of the galaxy and estimate distances to gas clouds, and thus map the distribution of matter in the galaxy.

Oort assigned his student, H.C. Van de Hulst, the job of figuring out what radio spectral lines might exist and what their frequencies would be. Since hydrogen is the most abundant element in the universe, he started his studies with hydrogen. He found that a "hyperfine" transition in the ground state of neutral hydrogen would produce radiation in the radio range, at a frequency of 1420 MHz, or about 21 cm wavelength. In the ground state of hydrogen, the electron can have its magnetic moment either parallel to that of the proton, or anti-parallel. The parallel state has a little more energy, so a transition to the anti-parallel state results in emission of 21 cm radiation.
Van de Hulst's prediction was published in Dutch in Ned.Tijd.Natuurkunde, vol.11, p210, 1945. An English translation is published in "Classics in Radio Astronomy", by W.Sullivan, Reidel 1982.

Building a receiver to detect the predicted line proved to be a daunting task. Grote Reber began design of such a receiver in 1947, but set it aside because of changing jobs and moving to the east coast. Dutch engineers worked on the problem, but the first successful detection was by Ewen and Purcell at Harvard.

Credits: photos of Oort and Oort&van de Hulst from "The Letters and Papers of Jan Hendrik Oort", by J.K.Katgert-Merkelijn, Kluwer, 1997.
Photo of van de Hulst reading paper from "The Evolution of Radio Astronomy" by J.S.Hey, Science History Publications, 1973.