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ScienceWeek
PARTICLE PHYSICS
ON ELEMENTARY PARTICLES IN PHYSICS
Editor's note: The following views of R.E. Marshak, written more than 40 years ago, were published shortly before the introduction of the idea of "quarks" by Murray Gell-Mann revolutionized particle physics. The words are still relevant: the term "elementary particle" is unstable.
The following points are made by R.E. Marshak (Science 29 Jul 1960 132:269):
In popular usage, the term "elementary particle" signifies an ultimate constituent out of which all matter is compounded. In physics, the usefulness of the concept depends very much on our state of knowledge, the hierarchy of forces with which one is dealing, and the order which is introduced into the description of the empirical facts.
From one point of view, any particle with a well-defined mass, charge, and intrinsic angular momentum (or spin) is an elementary particle and, in this context, even a molecule could be regarded as elementary. However, when the electromagnetic law of force was established between the atoms in a molecule and also between the electrons and the positively charged nucleus of the atom, it was much more convenient at that stage to think of the electron and the various types of atomic nuclei as the elementary particles.
When quantum mechanics was developed and the wave-particle dualism became an essential ingredient of our understanding of all atomic phenomena, it was proper to add the photon to the list of elementary particles.
When the neutron was discovered and the existence of a distinct nuclear force was established, it became more advantageous to think of the neutron and proton as the elementary particles out of which atomic nuclei are built up. Within this context, for example, the completely stable deuteron, with a well-defined mass, charge, and spin, is considered a composite structure, whereas the unstable neutron is treated as an elementary particle. The reason for this anachronistic point of view is that within the hierarchy of strong (or nuclear), electromagnetic, and weak forces, the neutron lives for a very long time [10^(3)] seconds on the nuclear time scale of 10^(-23) second."
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THE NEUTRINO
The history of particle physics during the first 30 years of the 20th century is an excellent example of the intimate interplay between theory and experiment. One of the central problems in the physics of matter during this period was to understand the emissions of radioactive substances first discovered in 1896 by Henri Becquerel (1852-1908). Spontaneous radioactive decay is essentially a spontaneous transmutation of an unstable atomic nucleus (nuclide) A into nuclide B, with nuclide A initially in a higher energy state and losing energy to transmute into the "daughter" nuclide B.
During the early years of particle physics, the energy loss was considered to be accomplished by emission of one of three types, depending on the nature of nuclide A: positively charged alpha particles (helium nuclei), negatively charged beta particles (electrons), or neutral gamma rays (high energy electromagnetic radiation). Since the energies of decaying nuclides and daughter nuclides are fixed according to nuclide identity, one would expect the observed energies of the 3 types of particles to also be fixed for each species of decaying nuclide. During the period before 1927, this was known to be true for alpha particles and gamma rays, but there was intense controversy about whether it was true for beta particles. Indeed, some early experiments indicated that it was not true for beta particles, and this posed a problem, since conservation laws require an accounting for all the energy and the numbers for beta decay did not add up.
The controversy continued for nearly 30 years, particularly among experimentalists who disagreed concerning experimental methods and interpretations of experimental results, until finally in the late 1920s it was conclusively demonstrated by experiment that during the beta-decay process high-speed electrons of various energies are emitted with a continuous beta-emission energy distribution spectrum (i.e., a plot of the number of electrons vs. energy of these electrons) over the range of energies.
Given the experimental evidence of a continuous beta-decay spectrum, theoreticians tackled the problem of accounting for beta decay without violating conservation laws. In 1930, Wolfgang Pauli (1900-1958) proposed that when a beta particle was emitted, another particle, without charge, and perhaps without mass, was also emitted, and that this second particle carried off the missing energy. Enrico Fermi (1901-1954) suggested the particle carrying the missing energy be called "neutrino", which is Italian for "little neutral one", and in 1934 Fermi incorporated the neutrino into his theory of beta decay.
Most theoretical and experimental physicists immediately accepted the proposed existence of the neutrino as the best solution to an important puzzle, but it was not until 1956 that Frederick Reines (1918-1998) and Clyde Cowan (1919-1974) managed to finally obtain experimental evidence for the existence of the elusive neutrino by means of experiments involving emission beams from a fission reactor. Enrico Fermi received the Nobel Prize in Physics in 1938; Wolfgang Pauli received the Nobel Prize in Physics in 1945; and Frederick Reines received the Nobel Prize in Physics in 1995. (Clyde Cowan was not eligible for the Nobel Prize at the time it was awarded to Reines, since the Nobel Prize is not awarded posthumously.)
The following points are made by Allan Franklin (Physics Today 2000 February):
1) The author points out there were two major responses to the establishment of the continuous energy spectrum of beta decay. One idea, favored by Niels Bohr (1885-1962), was that energy might not be conserved in beta decay. But work on the *Compton effect provided evidence against this view. The second major response was Pauli's "desperate way out", Pauli suggesting that a very light, neutral particle was also emitted in the beta decay. Pauli originally called this particle the "neutron", but Fermi christened the particle the "neutrino" and quickly incorporated the neutrino into a successful theory of beta decay.
2) During the next few decades, Fermi's theory was strongly supported by experimental observations, and that success provided most physicists with sufficient evidence for the existence of the neutrino. As stated by Frederick Reines, for 26 years before the existence of the neutrino was experimentally demonstrated, "the [Fermi] theory was so attractive in its explanation of beta decay that belief in the neutrino as a 'real' entity was general."
Editor's note: Although modern views of beta decay and the neutrino are more complex than the views held in the early years of the 20th century, a remarkable group of early experimental and theoretical particle physicists (only some of whom are mentioned in this SW brief) provided the foundation that still supports our understanding of the atomic nucleus and radioactive decay.
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Notes:
Compton effect: (Compton scattering) In general, the reduction in the energy of high energy photons when the photons are scattered by free electrons, the electrons thereby gaining energy, with total energy conserved. The effect was discovered in 1923 by A.H. Compton (1892-1962). Compton received the Nobel Prize in Physics in 1927.
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DISCOVERY OF NUCLEAR FISSION
The story of the discovery of nuclear fission can be briefly summarized as follows:
In 1932, James Chadwick (1891-1974) discovered that beryllium, when exposed to bombardment by *alpha particles, released an unknown radiation that in turn ejected protons from the nuclei of various substances. Chadwick interpreted the radiation as consisting of particles of mass approximately equal to that of the proton, but without electrical charge, and these particles were named "neutrons".
In 1934, Enrico Fermi (1901-1954) and his associates, in a systematic project involving the bombardment of various elements with neutrons, discovered that at least four different radioactive species resulted from the bombardment of uranium with neutrons.
In 1939, Otto Hahn (1879-1968) and Fritz Strassman (1902-1980) demonstrated that these radioactive species produced by the bombardment of uranium atoms were barium, lanthanum, and other elements.
Immediately afterward, also in 1939, Lisa Meitner (1878-1968) (a former long-time collaborator with Hahn and Strassman) and Otto Frisch (1904-1979) demonstrated that the liquid-drop model of the nucleus proposed earlier by Niels Bohr ((1885-1962) provided a qualitative theoretical interpretation of the Hahn-Strassman observations, suggested that the uranium atoms subjected to neutron bombardment split approximately in half, named the process "nuclear fission", and showed that a large energy release should accompany the event.
Finally, and also in the year 1939, Frederic Joliot-Curie (1900-1958) and others demonstrated that several neutrons were emitted in the fission of uranium-235, which immediately suggested the possibility of a self-sustaining chain reaction producing enormous energy. In 1942, Enrico Fermi and his group at the University of Chicago demonstrated this chain reaction in a laboratory under the university athletic field, and this led to the construction of the first atomic bomb.
Much has been written about the role of Lisa Meitner in the history of nuclear fission. She was Otto Hahn's close scientific collaborator for 30 years, and it is generally agreed that Meitner's realization (with her nephew Frisch) that neutron bombardment of uranium split the uranium atom into two parts of nearly equal mass was as important as the experimental work of Hahn and Strassman. It was Hahn alone, however, who received the Nobel Prize in 1945 for his work on nuclear fission.
Apart from the recurrent apparent errors made by Nobel award committees, one reason for Meitner's neglect was certainly the fact that in 1938, although a highly respected professor of physics, she had to abandon her close collaboration with Hahn and flee Germany. It was in 1938 that Nazi Germany annexed Austria, and all Austrian citizens such as Lisa Meitner automatically became German citizens and subject to the laws of Germany. Lisa Meitner had been baptized as an infant and raised as a Protestant, but she had one grandparent who was Jewish, and because of that her post at the Kaiser Wilhelm Institute in Berlin was terminated in 1938, and friends fearful for her life quickly smuggled her out of Germany to Sweden. Hahn and Strassman remained in Germany and continued their work on uranium fission; Meitner was abruptly relegated to a long-distance theoretical counselor.
History has its own balance sheet: Until 1997, element 105 was unofficially known as hahnium. In 1997, the International Union of Pure and Applied Chemistry adopted the name dubnium for element 105 and the name meitnerium for element 109. The element hahnium no longer exists.
The following points are made by E. Wiesner and F. Settle Jr. (J. Chem. Educ. July 2001 78:889):
1) The authors point out that while the history of nuclear fission is a testament to the powers of scientific investigation, the history and the subsequent recognition for the discovery provide examples of human fallibility. The political environment in Nazi Germany that surrounded the discovery, personal prejudices among scientists, and the power of the discovery itself brought fame to Otto Hahn while relegating his collaborators to relative obscurity. The neglect of Lisa Meitner, in particular, reveals shortcomings of judgment in the scientific community. Although she was a driving force in nuclear physics and intimately involved in the discovery of nuclear fission, her contributions were almost lost in the aftermath of World War II when the Nobel Prize in Chemistry was awarded in 1945 to Hahn.
2) The authors point out that when Hitler completed the takeover of the German government in 1933, one of the many "reforms" enacted that year was the Reestablishment of the Professional Civil Service Act. This law removed Jews from any government-related jobs and resulted in the decimation of German universities and research institutes (*Note #1). At the Kaiser Wilhelm Institute for Chemistry in Berlin, Lisa Meitner, who was one-quarter Jewish, became subject to this new policy. As an Austrian citizen, she had received a temporary respite, but with Germany's annexation of Austria in March 1938, Meitner became by law a citizen of the German Reich and many of her colleagues feared for her safety. In July 1938, with the help of friends, Meitner escaped from Germany to Stockholm, where she was given a position in the Nobel Institute of Physics, a new institute directed by Karl Manne Siegbahn (1886-1978).
3) The authors point out that Otto Hahn's ambiguous role in Meitner's escape from Germany reflects his later attitudes toward her role in the discovery of nuclear fission. Immediately after the annexation of Austria, Hahn became disturbed by Meitner's presence at the Kaiser Wilhelm Institute of Chemistry and its possible implications for the institute. He apparently discussed the situation with one of the institute's sponsors, thereby bringing Meitner's situation to the attention of the Ministry of Education. On a personal level, however, Hahn apparently provided Meitner with a great deal of support. When Meitner left Germany quickly with little preparation, Hahn took care of her belongings and other personal affairs. The authors state: "Thus while Hahn cared for Meitner, he lacked the integrity to take any risks on her behalf."
4) The authors point out that while Meitner's contributions to the discovery of nuclear fission were real and substantial, her absence from Berlin during the actual discovery was a significant factor in her neglect. Hahn and Strassman could not admit to any collaborative work with an exiled Jewish scientist. Although Hahn included a reference to the interpretations of Meitner and Frisch in his February 1939 paper on the verification of barium and other products from the neutron irradiation of uranium, his words minimized the importance of the Meitner-Frisch contribution. But even after the war, when Hahn was free of the restraints of the Nazi regime, there is evidence that Hahn was unwilling to admit the contributions of Meitner and Frisch to his work on nuclear fission. The authors state: "Although [Hahn] acknowledges the contributions of Meitner and Frisch in his autobiography, there are also indications that he simply did not understand Meitner's later theoretical contributions and therefore felt she had contributed little to the discovery."
Editor's note: In addition to her experience with the Nazi regime in Germany, Lisa Meitner had life-long experience of discrimination against women in science. After she obtained her doctorate, the great chemist and Nobel laureate Emil Fischer (1852-1919) allowed Meitner to work in his institute provided she never stepped into any laboratory in which males were present. Meitner was given space in an old carpentry shop. Also, appended below is a brief report concerning Marietta Blau, an Austrian female physicist and contemporary of Lisa Meitner, who also had to flee the Nazis. Blau was not as fortunate as Meitner in recovering some of her life.
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Notes:
alpha particles: These particles are identical to helium-4 nuclei: each particle contains two protons and two neutrons.
Note #1: In 1933, when the chemist Carl Bosch (1874-1940) cautioned Hitler against the policy of dismissing non-Aryan scientists, pointing out to Hitler the severe damage which this policy threatened to inflict on the pursuit of chemistry and physics in Germany. Hitler responded: "Then we'll just get along without physics and chemistry for a hundred years!"
Related Background:
REVELATIONS CONCERNING LISA MEITNER AND THE NOBEL PRIZE
Science is a human activity, and the prizes that are awarded to individuals in science are perhaps a necessary element in scientific progress. Nearly everyone wants approval and accolades for one's work, and if there is not that motive, there is always the rationalization that receiving an important prize is an aid to getting more funds for research, more equipment, more time, and so on.
One of the most prestigious prizes is the Nobel Prize in the various sciences, and now an article has appeared in the journal Physics Today that tells a sad story based on recently available documents of the Royal Swedish Academy of Sciences, official records of Nobel Prize deliberations.
The following points are made by Elizabeth Crawford et al (Physics Today 1997 September):
1) The authors review the details of the Nobel Prize awards in physics and chemistry in the years 1945 and 1946, in particular the long-puzzling failure of the physicist Lise Meitner (1878-1968), acknowledged as one of the central discoverers of atomic fission, to win either the prize in physics or the prize in chemistry for those years.
2) Meitner was a long-time collaborator of Otto Hahn (who received the 1945 Nobel Prize in Chemistry); she barely escaped the Nazis in 1938 to work in Sweden. The conclusions of the authors: Meitner did not share the chemistry prize in 1945 because of "a mixture of disciplinary bias, political obtuseness, ignorance and haste".
3) As for the physics prize of 1946, for which Meitner was also nominated, the authors conclude the failure to then award her the prize was "a rare instance in which personal negative opinions apparently led to the exclusion of a deserving scientist". Meitner received the U.S. Atomic Energy Commission Fermi Award in 1966.
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MARIETTA BLAU: THE DESTRUCTION OF A CAREER IN PHYSICS
A nuclear emulsion is a photographic emulsion specifically designed to register individual tracks of ionizing particles. The emulsion technique, so important in the early history of 20th century particle physics, was developed in the 1930s primarily by Marietta Blau (1894-1969), an Austrian physicist. Blau was a Jew, and she was forced to flee her laboratory when the Nazis occupied Austria in 1938. The other people in the laboratory, including Blau's assistant, the physicist Hertha Wambacher, were in fact Nazis themselves, and they immediately assumed control of the laboratory and may have ordered the seizure of Blau's notebooks, which were confiscated as she exited Germany from Hamburg.
Although the chemists and physicists of the nuclear emulsion group who remained in Germany and Austria for the most part continued their work and after the war moved into important professorships in those countries, Marietta Blau, the prime force in the early development of nuclear emulsion technology, wandered from country to country as a lost experimental physicist without a laboratory. She died poor and unemployed 20 years after the war ended.
Peter Galison, a physicist and historian of science, has presented the details of the Blau story in his book /Image and Logic: A Material Culture of Microphysics/ (University of Chicago Press 1997). Galison (Physics Today 1997 November) states: "The fate of Marietta Blau signifies what it meant to be a woman, a Jew, and a solitary physicist fleeing the convulsing world of Nazi Austria."
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ENRICO FERMI (1901-1954)
The following points are made by Richard Mertens (University of Chicago Magazine 2001 December):
1) Nothing excited Fermi so much as a new problem to solve. It hardly mattered what the problem was. A former student and later colleague of Fermi recalls one afternoon when Fermi sat with a group of students at a long table in Hutchinson Commons. Fermi looked up at the dirty windows and asked: "Well, how thick will dirt get before it falls off?" The former student states: "He made us feel we could really calculate anything. He, of course, could."
2) Fermi shone brightest at Thursday afternoon seminars in room 480 of the Institute for Nuclear Studies. That was when he and his colleagues gathered to discuss whatever happened to be on their minds. Faced with a problem that stumped other physicists, Fermi would often turn to the blackboard, pick up a piece of chalk, and solve the problem with ease. The astrophysicist Roger W. Hildebrand recalls the day when a colleague wanted to know if the age of water at the bottom of the ocean could be explained by the churning of surface waves. Fermi said, "Let's see if we can work it out." And he did. He did it using characteristics of monster ocean waves that he had learned on a transatlantic voyage. "The oceanographers at La Jolla had worked on it for three months. Fermi worked it out on the blackboard in 15 minutes."
3) Part of Fermi's gift as a scientist, as well as his effectiveness as a teacher and his appeal as a human being, lay in his simplicity. He did not care for fancy solutions. He would explain a difficult problem so clearly that he made the answer seem obvious. That was the way it was with Fermi.
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FERMI ON THERMODYNAMICS
Thermodynamics is mainly concerned with the transformations of heat into mechanical work and the opposite transformations of mechanical work into heat. Only in comparatively recent times have physicists recognized that heat is a form of energy that can be changed into other forms of energy. Formerly, scientists had thought that heat was some sort of fluid whose total amount was invariable, and had simply interpreted the heating of a body and analogous processes as consisting of the transfer of this fluid from one body to another. It is therefore noteworthy that on the basis of the heat-fluid theory [Nicolas Sadi] Carnot [1796-1832] was able, in the year 1824, to arrive at a comparatively clear understanding of the limitations involved in the transformations of heat into work, that is, of essentially what is now called the second law of thermodynamics. In 1842, only eighteen years later, J[ulius] R. Mayer [1814-1878] discovered the equivalence of heat and mechanical work, and made the first announcement of the principle of the conservation of energy (the first law of thermodynamics).
We know today that the actual basis for the equivalence of heat and dynamical energy is to be sought in the kinetic interpretation, which reduces all thermal phenomena to the disordered motions of atoms and molecules. From this point of view, the study of heat must be considered as a special branch of mechanics: the mechanics of an ensemble of such an enormous number of particles (atoms or molecules) that the detailed description of the state and the motion loses importance and only average properties of large numbers of particles are to be considered. This branch of mechanics, called "statistical mechanics", which has been developed mainly through the work of [James Clerk] Maxwell [1831-1879], [Ludwig] Boltzmann [1844-1906], and [Josiah Willard] Gibbs [1839-1903], has led to a very satisfactory understanding of the fundamental thermodynamical laws.
But the approach in pure thermodynamics is different. Here the fundamental laws are assumed as postulates based on experimental evidence, and conclusions are drawn from them without entering into the kinetic mechanism of the phenomena. This procedure has the advantage of being independent, to a great extent, of the simplifying assumptions that are often made in statistical mechanical considerations. Thus, thermodynamical results are generally highly accurate. On the other hand, it is sometimes rather unsatisfactory to obtain results without being able to see in detail how things really work, so that in many respects it is very often convenient to complete a thermodynamical result with at least a rough kinetic interpretation. The first and second laws of thermodynamics have their statistical foundation in classical mechanics. In recent years, [Walther] Nernst (1864-1941] has added a third law which can be interpreted statistically only in terms of quantum mechanical concepts.
Adapted from: Enrico Fermi: Thermodynamics. Lectures at Columbia University [US] 1937. Dover Publ. 1956, p.ix.
Editor's note: Fermi gives Mayer's name as R.J. Mayer, which reverses the initials and is an error. It is interesting that Mayer, the discoverer of the first law of thermodynamics and the law of conservation of energy, was not a physicist but a physician, a medical doctor without professional training in physics. When Mayer submitted a paper on the subject to the /Annalen der Physik/, the submission was not even acknowledged and was thrown away. Finally, in 1842, the noted chemist Justus von Leibig accepted the paper for his journal /Annalen der Chemie und Pharmazie/. But once published, the paper was totally ignored, Mayer became depressed, and in 1849 he attempted suicide by jumping from a third-story window. The suicide attempt failed and Mayer became permanently lame as the result of leg injuries. By 1851, Mayer was in a mental institution, and he suffered greatly under the primitive and cruel methods of treatment of the mentally ill. He was eventually released, but he never fully recovered, and he languished in obscurity: when, in 1858, Leibig lectured on Mayer's views, Leibig referred to Mayer as deceased. Then, in the 1860s, recognition finally arrived for Mayer's work, and he received many honors, including the Copley Medal of the UK Royal Society in 1871. Mayer clearly anticipated James Joule (1818-1889) and Hermann von Helmholtz (1821-1894) in the discovery of the law of conservation of energy.
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