Per Carlson, in his article “A century of cosmic rays” (Physics Today, February 2012, page 30), states that the investigation of cosmic rays opened up the field of particle physics. However, he neglects the importance of nuclear emulsions in the progress of particle physics, and he fails to mention a key figure, Austrian physicist Marietta Blau (1894–1970), who pioneered the development of photographic emulsions capable of reliably and quantitatively imaging high-energy nuclear particles and events, including reactions induced by cosmic radiation.1
Between 1923 and 1937, Blau contributed to essentially every aspect of emulsion physics. In 1925 she successfully distinguished the tracks of alpha particles, fast protons, and background events in commercial emulsions, and in 1927 she determined proton energies by measuring the distances between the exposed grains in their tracks. To record the long tracks of fast protons more accurately, she enlisted British film manufacturer Ilford to thicken the emulsion on its commercial film, and she experimented with every other emulsion parameter—grain size, latent image retention, development conditions—to improve the visibility of alpha-particle and fast-proton tracks.
Beginning in 1932 Blau and her assistant Hertha Wambacher determined neutron energies by measuring the tracks of recoil protons in the hydrogen-rich emulsions; in 1936 they began using emulsions for quantitative studies of cosmic radiation by exposing stacks of photographic plates for several months at an elevation of 2300 meters. As expected, they recorded the tracks of extraterrestrial protons and neutrons, but to their surprise they also discovered several “stars,” which they realized could only have been formed by cosmic particles explosively disintegrating heavy nuclei in the emulsion. That discovery, in 1937, created a sensation among nuclear and cosmic-ray physicists worldwide, and by demonstrating that nuclear emulsions had come of age for recording rare high-energy nuclear events, it paved the way for further research in particle physics. Cecil Powell, for example, began using photographic film only in 1938 after he, like all others in the field, recognized the significance of Blau and Wambacher’s discovery and the advantages of the emulsion method.2 (See also the article by Peter Galison in Physics Today, November 1997, page 42, and his response to letters in August 1998, page 81.)
In considering 20th-century history, one must be alert to the effects of racial persecution and forced emigration on the attribution and recognition of scientific work. Blau, a Jew, was forced out of Austria in 1938. For 10 years she led a fragile existence as a refugee in Oslo, Norway; Mexico City; and New York, unable to pursue her own research. Meanwhile, in Vienna, her former associates, all ardent Nazis (including Wambacher), expropriated her work and suppressed her name.3
Blau’s marginalization is apparent in the decision process for awarding the 1950 Nobel Prize in Physics to Powell. Blau and Wambacher were also nominated that year, by Erwin Schrödinger. Although the 1936 physics Nobel for Victor Hess and Carl Anderson provided a precedent for dividing the award between a fundamental early discovery and one that came later, it is evident that the women were not objectively considered. Instead, the documentation shows that the Nobel physics committee prepared a blatantly inaccurate assessment that denied the importance and priority of Blau’s and Blau and Wambacher’s work. The women’s names are entirely absent from the published Nobel texts for that year, although other scientists are mentioned who were never nominated and contributed far less. Powell himself did not cite Blau in his Nobel lecture.4
As a result, Blau was almost entirely written out of the history of 20th-century physics. Although recent research has helped to bring her back in, Carlson’s article shows how persistent the established narrative can be. In the interests of basic accuracy and historical sensibility, Marietta Blau and her contributions deserve a prominent and permanent place in any history of cosmic-ray and particle physics. And I believe it is also important, given the much-discussed problem of the underrepresentation of women in physics, that Physics Today take care not to overlook women who, like Blau, made major contributions to physics.
