Harold Ewen

Harold Ewen, an American physicist, is best known for being the first person to detect the 21-centimeter radio line emitted by interstellar hydrogen. Working alongside Edward Purcell at Harvard University in 1951, this discovery proved that neutral hydrogen gas exists throughout the galaxy. It effectively gave birth to the field of modern radio astronomy.

The 21-centimeter emission comes from cold, neutral hydrogen atoms, which is the most abundant element in the universe. Because radio waves can pass through the thick dust clouds that block optical light, Ewen and Purcell's discovery allowed astronomers to map the spiral structure of the Milky Way galaxy for the very first time. It remains a fundamental tool for studying the cosmos.

Yes, Ewen famously constructed the horn antenna used for the detection on the grounds of the Lyman Laboratory of Physics at Harvard University. Working on a tight budget as a graduate student in 1951, he built the receiving equipment from surplus military parts left over from World War II. Despite its modest appearance, the horn antenna successfully picked up the faint interstellar signal.

After building their receiver, they pointed the horn antenna at different parts of the Milky Way and noticed a consistent signal that matched the predicted Doppler shifts of the galaxy. To confirm their findings, they communicated with Dutch astronomers Jan Oort and C.A. Muller, who were conducting similar research. This collaboration helped validate the 1951 discovery across the global scientific community.