April 15: the hydrogen line predicted on paper

On April 15, 1944, at a colloquium of astronomers in Leiden, in the Netherlands occupied by Nazi Germany, a 25-year-old student presented a prediction that would only be confirmed seven years later.

What happened

The student was Hendrik Christoffel van de Hulst, of Utrecht University. The astronomer Jan Oort, of Leiden, had asked him a question: might there be some spectral line, a signal at an exact frequency, that could be picked up by radio coming from space?

Van de Hulst studied hydrogen first, because it's the most abundant element in the universe. He concluded that the hydrogen atom, in its simplest state, undergoes from time to time a small internal change, and that this change emits radiation at a wavelength of about 21 centimeters, corresponding to a frequency of about 1420 MHz. At the colloquium of April 15, 1944, he presented the result. The work was published in 1945.

Many historians of astronomy mark this date as the birth of Dutch radio astronomy.

Why it matters

In 1944, there was no way to hear this signal. The war was destroying Europe, the instruments didn't exist and no one knew whether the prediction was right. Van de Hulst saw on paper what could not yet be heard.

Confirmation came on March 25, 1951, when Harold Ewen and Edward Purcell detected the line at Harvard. Seven years between prediction and listening. The hydrogen line became one of the central tools for mapping the Milky Way and other galaxies. The exact frequency, later measured with enormous precision, is 1 420 405 751.768 hertz.

The bridge to Bairo's vision

The project Bairo Leandro Gonzalez Martinez founded bears the name of this signal: 1420. He calls the hydrogen line the frequency of silence, because the band from 1400 to 1427 MHz is protected by international agreement, under the International Telecommunication Union, and emissions are prohibited in it. There, one only listens. No one transmits on this band, not even Bairo's project; the number is a symbol.

But April 15 tells something else, even closer to his story. Van de Hulst saw before there was proof. In Bairo's life, this repeats: ideas seen before they became trends, a business seen before it had a path. He says that, after an inner search, he already knew where he wanted to get to and didn't know how; he asked for wisdom to remember the way. First someone sees. Then the world confirms.

Read: The frequency of silence · The God of the impossible

Read also: March 25, the detection · Frank Drake and the silence of 1420 MHz

Sources

• Encyclopedia.com, "Van De Hulst, Hendrik Christoffel" (colloquium of 04/15/1944): https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/van-de-hulst-hendrik-christoffel
• Leiden University, "Leiden astronomer Henk van de Hulst: humble man with great authority": https://www.universiteitleiden.nl/en/news/2021/04/leiden-astronomer-henk-van-de-hulst-humble-man-with-great-authority
• Hendrik C. van de Hulst: https://en.wikipedia.org/wiki/Hendrik_C._van_de_Hulst
• CAMRAS, "With the 21 cm hydrogen line from Kootwijk to Dwingeloo": https://www.camras.nl/en/blog/2021/with-the-21-cm-hydrogen-line-from-kootwijk-to-dwingeloo/
• NIST, hydrogen line frequency: https://tf.nist.gov/general/pdf/13.pdf