A New Constituent of the Universe

The demands of war lead to technological advances that have consequences for pure research long after. The development of radar during World War II is one such example. The German radar dishes left behind in Holland at the end of the war enabled Oort to make a beginning for Dutch radio astronomy which, as we have seen, led to an understanding of the overall structure and kimenatics of the Milky Way Galaxy. Ewing and Purcell, the discoverers of the 21-cm line of neutral hydrogen, both had strong backgrounds in microwave radar technology, Ewing as a naval radar officer and Purcell as director of the MIT Radiation Laboratory. In the United Kingdom the young radio engineer Martin Ryle made a major contribution to the development of radar and thus to the British defense effort. But after the war, he applied these skills at Cambridge in the development of radio astronomy, in particular, the technique of radio interferometry.

In astronomy whenever a new wavelength window opens, major discoveries follow. This was certainly the case in the early 1950s, when crude radio telescopes began mapping the sky in continuum radiation and, in addition to smooth radiation from the Galaxy, discovered a number discrete sources scattered about the sky. Some were clearly Galactic in origin (they were in the plane of the Galaxy and associated with known objects such as supernova remnants) but others were more uniformly distributed outside of the Galactic plane.