Bruno Rossi, Cambridge University Press, New York, 1990.

Soon after radioactivity was discovered, its ionizing effect on air came to be known. Then the ionization caused by natural radioactivity was carefully measured by investigators, using a sensitive, but inexpensive electroscope invented by the Jesuit Father T. Wulf. It is not always recognized that the physics of elementary particles, which has occupied center stage in the world of physics during much of our century, owes its origins to the curiosity of individuals who wanted to know at what rate this ionization decreases with altitude. They found, much to their surprise, that at higher altitudes the ionization is actually increasing. The first of these researchers was Father Wulf, who climbed to the top of the Eiffel Tower in Paris to make his measurements. Soon after that, A. Gockel and V. Hess carried their instruments to still greater heights in a balloon. Adter a series of very careful measurements, it appeared that there were some very penetrating rays inundating our atmosphere from high above. This was how cosmic rays were discovered in the first decades of our century.

Rational and systematic as science is, the role of chance factors in its progress is remarkable indeed. For example, when efforts to confirm the reports on the increase in atmospheric ionization with altitude were undertaken by Robert Millikan and his co-workers in 1922, flying high above the Texas landscape, they found that there was no such increase as had been reported by the German balloon-flying physicists. Millikan, who was to coin the term “cosmic rays” later, declared categorically that he had found “definite proof that there exists no radiation of cosmic origin having such characteristics as we had assumed.” There was nothing wrong with Millikan’s observation or data because, because of the Earth’s magnetic field, cosmic ray particles were very sparse in the latitudes where he was working. Now imagine for a moment that the Eiffel Tower had been in San Antonio and that Father Wulf had conducted his experiments there. He would simply have confirmed that the ionization did decrease with altitude (as one expected), and that would have been the end of that. Nobody would have talked of Höhenstrahlen, as Hess called these rays or of cosmic rays. Dirac’s positron idea would have been interesting, but without experimental confirmation. The Yukawa particle would have remained hypothetical, and so on.

Even as the invention of the microscope revealed to human understanding the existence  of a whole new world of living entities that had been beyond our suspicion or imagination until then, so too the discovery of cosmic rays brought within the scope of human knowledge the existence and properties of a plethora of particles that not only populate and interact in the microcosm, but are at the very substratum of the physical universe. Thanks to their study, we are gaining deeper insights into the mysteries and origin of the spatio-temporal arena we call the cosmos.

Among the many patient and dedicated workers whose insights and talents have contributed to our understanding of cosmic rays stands Bruno Rossi. His use of what used to be called electronic valves to construct an instrument with two or three Geiger counters in a row, which would register coincident pulses in the counters, was a gaint step forward in the experimental study of cosmic rays. Using lead shields with his instruments, he not only revealed the great penetrating powers of cosmic rays, but also detected showers of secondary particles. The so-called Rossi curve,2 which is obtained by plotting the number of showers recorded for various thicknesses of lead, furnished not only qualitative understanding of what was going on, but also quantitative confirmation for the Heitler-Bhabha theory of shower production. 3

It was Rossi who first suggested in 1930 that cosmic ray particles, being electrically charged, must be defected by the Earth’s magnetic field towards the east or the west, depending on the nature of the charge.

Rossi was the first to measure the lifetime of the muon (mesotron as it used to be called). He measured the penetrating component of cosmic rays at various altitudes. His first estimate for the lifetime of the muon yielded 2 microns; his second experiment,
2.15 microns.4

The advances that scientists make, the results of their experiments and calculations, all find expression in the countless journal articles that appears year after year. But behind each publication there lies a story that is often interesting, sometimes fascinating or inspiring. Consider, for example, the circumstances that led to Rossi’s work in this context. At the conclusion of an international symposium on cosmic rays at the University of Chicago in 1939, Arthur and Betty Compton invited Rossi and his wife to spend some days at their summer cottage. During a conversation there, Rossi suggested to Compton that the decay of mesotrons (a topic that had been discussed at the conference) could be best explored in experiments on a mountain top. Compton was very enthusiastic about the idea and replied that the Rocky Mountain would be ideal for this. Mount Evans was the highest point in the United States that could be reached by road. An expedition was promptly organized for the purpose. An old bus was requisitioned from the zoology department of the university. It took almost three days for the team to reach Denver, then on to Echo Lake some 3200 meters above sea level, and finally to Mount Evans, a thousand meters higher still. The bus carrying the scientific expedition irritated the impatient drivers of the many cars behind it who were obliged to follow its slow course along the narrow and winding path. This was part of the background to the crucial observations that revealed the decay mode of (what we now know to be) the muon, which was the very first instance of the experimental recording of the decay of an elementary particle per se.

Rossi’s measurements not only gave the mean life of the muon, but also afforded in the process a verification of the time-dilation formula that follows from Einstein’s special theory of relativity.5 Rossi himself gave an elegant exposition of this in his classic treatise on High-Energy Particles. 6

All this and more is narrated by the widely respected physicist Bruno Rossi in his slim volume of autobiographical reminiscences, Moments in the Life of a Scientist. The book presents primarily some scientific highlights from the author’s life. But these are spiced with impressions of his encounters with a number of other outstanding physicists (Hans Bethe, Robert Oppenheimer, Otto Frisch, and others), brief descriptions of places (Ithaca, Summit Lake, etc. and details of some his own work (Rossi curve, cloud chamber pictures, launching of X-ray astronomy, search for X-rays from the Moon, and more). As one of the scientists who worked in Los Alamos during the fateful years of the Manhattan Project, Rossi also experienced a pang of conscience when the first atom bomb exploded over Hiroshima. Considering the potential for the destruction of the human species that has emerged from our knowledge of the physical world and our exploitation of it, Rossi’s words in this context are applicable to the scientific enterprise at large.

Now the terrifying significance of what we had done hit me like a blast. I must admit that at times I felt a certain pride at having played a role in an undertaking of such great difficulty, of such historical importance. But soon this feeling was overwhelmed by a feeling of guilt and by a terrible anxiety for the possible consequence of our work….7


The book is also enriched with some memorable photographs and a brief autobiographical essay from Mrs. Rossi’s pen.

The irony of human history is that even its most vicious characters sometimes unwittingly do some good. Thus Hitler and Mussolini, by their persecution of Jews, provided the United States with considerable scientific brain power during the 1930s. Bruno Benedetto Rossi was one such European scientist who migrated to the United States. One effect of the cultural transplantation on people who move away from their native soil is reflected in an aspect of this book: Rossi, who was born in Venice, Italy has written his autobiography in English; and to pay homage to his linguistic heritage, his dedication includes a famous quote in his mother tongue from Dante’s Divine Comedy. The quote reminds us that we should consider our seeds (descendents) to be made not for living as brutes but for following virtue and knowledge.8 Rossi’s own life, like that of other expatriate scientists who continue to explore the universe in their adopted country, reminds us of yet another of Dante’s lines: “Equindi unscimmo a riveder le stelle.” 9

Rossi’s book should be interesting reading for those who have an interest in more than the principles and formulas of physics; this, one hopes, includes the vast majority of the practitioners and teachers of physics.


    R. A. Millikan and I. S. Bowen, Phys. Rev. 22 198 (1923)
    B. Rossi, Ziet. f. Phys. 82, 151 (1933)
    H. J. Bhabha and W. Heitler, Proc. R. Soc. A 159, 432 (1937)
    B. Rossi, Phys. Rev. 57, 461 (1940); 59, 223, (1941); 61, 675 (1942).
    The idea for this had been suggested by H. Bhabha in a letter in Nature the previous year.
    B. Rossi, High-Energy Particles (Prentice-Hall, Englewood Cliffs, NJ, 1952), p. 157. Rossi also published a very readable popular account of his specialty entitled Cosmic Rays (McGraw-Hill, New York, 1964).
    Book under review: Moments in the Life of a Scientist, p. 98.
  8.         Considerate la vostra semenza fatti non foste a viver come bruti ma per             seguir virtute e conoscenza,” Dante, Inferno, xxvi, pp. 118-120.

                 “Thence we came forth to re-behold the stars,” Purgatorio, xxxiv, p. 139.