"Doc" Ewen and the discovery of Radio emission from HydrogenSome reference material on the web:
The Ewen-Purcell Horn at Green Bank
This horn antenna, now displayed in front of the Jansky Lab at NRAO in Green
Bank, WV, was used by Harold Ewen and Edward Purcell, then at the Lyman
Laboratory of Harvard University, in the first detection of the 21 cm emission
from neutral hydrogen in the Milky Way. The emission was first detected
on March 25, 1951.
Edward Mills Purcell:
Graduated Purdue University 1933, PhD Harvard, 1938
Worked on radar research during World War II
Shared the 1952 Nobel Prize with Felix Bloch for development of the nuclear magnetic resonance method of measuring magnetic fields in the nuclei of atoms.
Harold Irving Ewen:
Graduated Amherst College 1943, MA (1948) and PhD (1951) from Harvard Univ.
Later was President of Ewen Knight Corp (1952-) and Ewen Dae Corp. (1958-)
The Discovery of Hydrogen Radio emission by Ewen and PurcellIn 1950, "Doc" Ewen was working 40 hours a week designing and building apparatus for the new cyclotron at Harvard. In addition, during nights and weekends, he was working on completing a doctorate in physics by building a receiver to detect the 21 cm line of neutral hydrogen, supervised by Purcell. In the photo at right, left to right: Ed Purcell, Taffy Bowen, Doc Ewen.
Ewen proceeded to design the horn antenna and the mixer and receiver, consulting with experts
in these fields, including Sam Silver on antenna design and Bob Pound on mixers.
The receiver used a frequency switching technique to cancel out background noise,
a novel technique for astronomy at the time.
The whole project, from receipt of the $500 to detection of the line, took
one year. Since the work was done on the weekends, the total time spent
working on the project was (2/7) x 12 = 3.4 months. The image at far right
shows Ewen with the completed receiver at the time of detection.
After detecting the 21 cm line, Ewen learned that Prof. Van de Hulst was
spending a sabbatical year at Harvard.
At the suggestion of Purcell, I went to the Harvard Observatory, in mid April, to meet and talk to Van de Hulst for the first time, and to tell him about the discovery. Ed told me that Van de Hulst was teaching a course at Harvard during the spring term. It was the same course that he taught at Leiden, during the prior fall term. When I met with Van de Hulst at the Observatory he called Oort, and I talked with Oort for nearly an hour describing the switch frequency technique. This was the first time that we learned the Dutch had been searching for the line for several years. Had we known, we would not have tried.
The Dutch group adopted Ewen's frequency switching technique and succeeded in detecting the line on May 11.
A further confirmation of the detection was made by Australian radio astronomers. This came about because Frank Kerr was on a fellowship at Harvard that year, on leave from the CSIRO Division of Radiophysics in Australia. On hearing about Ewen and Purcell's discovery, he wrote to J.L. Pawsey describing the experiment. Christiansen and Hindman, members of Pawsey's radiophysics group, built a receiver that used frequency switching, and were able to first detect the line on July 12.
The American and Dutch results were published in the same issue of Nature: Ewen and Purcell, Nature v.168, p.356, 1951; and Muller and Oort, Nature v.168, p.357, 1951. These were followed by a brief note in the same issue from Pawsey mentioning that Christiansen and Hindman had also detected the line. The Australian results were published more fully later: Australian J. Sci. Res., vol. A5, p. 437, 1952.
In the following years, observations of the neutral hydrogen line were
done with increasing accuracy by numerous
astronomers, primarily Dutch, American, and Australian.
The technique proved to be successful in mapping the large-scale
distribution of matter in the galaxy. Observations of the 21 cm line remains a
very important branch of radio astronomy to this day, resulting in ever more
detailed understanding of the interstellar medium in our galaxy, and in
studies of external galaxies.