Radio waves from hydrogen

Van de Hulst theorized that cold hydrogen atoms should emit detectable 21-centimeter radio waves in space. Because of the German occupation, instrumentation was not available and so his calculations could not yet be corroborated.
 Hendrik Christoffel van de Hulst Netherlands 1944 Radio waves from hydrogen

Van de Hulst theorized that cold hydrogen atoms should emit detectable 21-centimeter radio waves in space. Because of the German occupation, instrumentation was not available and so his calculations could not yet be corroborated.