Radio Astronomy Episode 4, The Discovery of Interstellar Hydrogen Episode 4, The Discovery of Interstellar Hydrogen.

Streaming video - 2017

Not long after the birth of radio astronomy, a Dutch student used what was then known about the physics of atoms to determine that if hydrogen existed in interstellar space, it would produce a specific spectral line at radio wavelengths. In 1951, the line was detected at 21 cm, exactly as predicted. At that moment, our understanding of the universe forever changed.