March 25: the day hydrogen was heard

On March 25, 1951, a Harvard graduate student picked up for the first time a radio signal that until then had existed only in a calculation made in the middle of a war: the 21-centimeter line of hydrogen, at 1420 MHz.

What happened

The student was Harold "Doc" Ewen, and his adviser was the physicist Edward Purcell. To try to hear the signal, Ewen built a receiver and a horn-shaped antenna and installed it outside a fourth-floor window of the Lyman Laboratory, at Harvard. Purcell secured a grant of 500 dollars for the materials.

On March 25, 1951, the chart showed the signal: radiation from the neutral hydrogen of our galaxy, at a frequency of about 1420 MHz. The result was published in the journal Nature, in September 1951, together with confirmation by teams in the Netherlands and Australia.

The original antenna is now at the National Radio Astronomy Observatory, in Green Bank, in the United States.

Why it matters

Hydrogen is the most abundant element in the universe. Hearing its signal allowed astronomers to map where the gas in galaxies is and, with that, to draw the structure of the Milky Way, including its spiral arms. The hydrogen line became one of the most important tools of radio astronomy.

And there's a beautiful detail: the signal had been predicted on paper in 1944, by the Dutchman Hendrik van de Hulst, when no one had a way to hear it. Seven years between seeing and confirming.

Because of this importance, the band from 1400 to 1427 MHz is protected by international agreement, under the International Telecommunication Union. In it, emissions are prohibited, so that science can keep listening to the sky.

The bridge to Bairo's vision

The project Bairo Leandro Gonzalez Martinez founded is called 1420 because of this signal. For him, 1420 MHz is the frequency of silence: the signal of the simplest and most abundant element, which the world agreed to leave in silence. It's a symbol, and that's how he uses it. No one transmits on this band, not even the project.

The story of the detection is also a story about seeing first. Van de Hulst saw it on paper; Ewen and Purcell confirmed it in the sky. In Bairo's story, the idea is the same as the line attributed to Mike Vance about Walt Disney: first someone sees, then the world confirms. It's in The frequency of silence.

Read also: April 15, the prediction · Frank Drake and the silence of 1420 MHz

Sources

• Harold Irving Ewen: https://en.wikipedia.org/wiki/Harold_Irving_Ewen
• NRAO, "The Ewen-Purcell Horn at Green Bank": https://www.gb.nrao.edu/fgdocs/HI21cm/ephorn.html
• NRAO, "Horn o'Plenty Discoveries": https://public.nrao.edu/gallery/horn-oplenty-discoveries/
• Ewen-Purcell Horn Antenna Historical Marker: https://www.hmdb.org/m.asp?m=58304
• Hydrogen line: https://en.wikipedia.org/wiki/Hydrogen_line
• Protection of the 1400–1427 MHz band (ITU, footnote RR 5.340): https://www.e-callisto.org/generaldocuments/RAS-Allocations-RR2016.pdf