Rossi, Cambridge University Press, New York, 1990.
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.
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.
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.
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
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.
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,
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.
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.
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.
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
book is also enriched with some memorable photographs and a brief
autobiographical essay from Mrs. Rossi’s pen.
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
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.
“Thence we came forth to re-behold the stars,” Purgatorio, xxxiv, p. 139.