ALVAREZ, LUIS WALTER

(b. San Francisco, California, 13 June 1911; d. Berkeley, California, 1 September 1988)

physics, particles and detectors, geophysics.

Alvarez was among the talented physicists who shaped the course of high-energy particle physics in the twentieth century. He was responsible for advances in instrumentation, organization, and application in that field, and invented scores of devices that made radar, nuclear weapons, and ground-controlled aviation feasible, as well as solving many mundane problems. His ingenuity made him a national resource in matters ranging from the Oppenheimer trial to the investigation of the Kennedy assassination. Among his greatest discoveries was a theory that accounted for the extinctions of dinosaurs and thousands of other species in the Cretaceous-Tertiary transition. He was a charismatic leader of large projects and an ingenious experimenter and inventor.

Luie, as his friends called him, was the son of the well-known physician and newspaper columnist Walter C. Alvarez, and the grandson of Luis F. Alvarez, a Spanish immigrant to the United States who was trained in medicine at Cooper Union Medical School and practiced in Hawaii. Luie’s aptitude for invention was stimulated by the mechanical and electrical instruments in his father’s laboratories at the Hooper Foundation in San Francisco and the Mayo Clinic in Rochester, Minnesota. Alvarez chose to study at the University of Chicago, where he received his BSc, MS, and PhD in physics. During his first years at Chicago, he developed a method of teaching the technique of grazing incidence spectroscopy using a broken phonograph record, a method he subsequently published in School Science and Mathematics. His first scientific paper, written with his advisor, Arthur Compton, the leading American researcher in cosmic-ray physics, reported Alvarez’s experimental verification of what came to be called the east-west effect in cosmic radiation. This was the result of the effect of the magnetic field of the earth on charged particles that make up cosmic rays. The effect had been predicted by Bruno Rossi, who replicated the discovery two months later in Eritrea.

Although he graduated in 1936, when academic jobs were difficult to find, he was asked to become a research assistant at the Radiation Laboratory of the University of California in Berkeley, which became a formal research unit in the same year. Berkeley’s Ernest Lawrence knew Luie well from his visit to the Century of Progress Exhibition in Chicago in 1933, when Lawrence spent a day impressing the young graduate student with his enthusiasm and camaraderie. Luie’s sister had been Lawrence’s secretary and his father sat on the board of the Macy Foundation, which funded Lawrence’s research. Two years after Luie’s arrival, he was appointed assistant professor in Berkeley’s physics department, and he retained his association with the university for the rest of his life.

Always a quick study, Alvarez soon mastered the literature of the field and the operation of the cyclotron, a notoriously difficult particle accelerator. He collaborated with Edwin M. McMillan in the design of the sixty-inch cyclotron and in the development of a means for extracting the beam from the cyclotron, making it unnecessary to shut it down to insert targets, thus greatly increasing the productivity of the machine in the manufacture of radioisotopes. The cut-and-try methods employed in the construction and use of the cyclotron transcended the theoretical understanding of its operation and accustomed Alvarez to the kind of bold leaps that marked the career of his mentor Lawrence. In addition, Alvarez carefully and thoroughly designed and executed his experiments with the cyclotron and prized physics before production of isotopes.

For example, in 1939 Alvarez used the newly completed Crocker medical cyclotron to discover helium-3, an important isotope of helium that, instead of the usual two protons and two neutrons, contains two protons and only one neutron. After Ernest Rutherford’s discovery of tritium in 1934 (an isotope of hydrogen containing one proton and two neutrons), Hans Bethe and others believed that tritium was stable and produced by helium-3’s radioactive decay. In setting up his experiment, Alvarez observed the acceleration of particles of mass three by tuning the radio-frequency field of the cyclotron. Because his source material was extracted from natural gas in Oklahoma, he concluded that helium-3, like its isotope helium-4, must be stable, because the deposits of gas were thousands of years old. With his graduate student Robert Cornog, he went on to find that tritium produced by deuteron bombardment of heavy water was actually radioactive.

Attracted by the work of the Fermi group, Alvarez succeeded in extracting a slow neutron beam from the cyclotron that facilitated his measurement of neutron scattering in hydrogen. With Felix Bloch, he determined the magnetic moment of the neutron, a key step in the development of nuclear magnetic resonance imaging (MRI).

Enrico Fermi’s postulation of nuclear decay through the emission of an electron afforded another avenue to knowledge of particles interacting inside the atomic nucleus, including positron emission and orbital electron capture. Gian-Carlo Wick provided theoretical insight into these processes, but it was Alvarez who first observed the capture of electrons from the innermost energy level, the K level. He accomplished this by interpreting the x-rays given off by the electrons as they were captured. Alvarez suggested to his student Phillip Abelson that Abelson might use the same techniques to determine the atomic numbers of the so-called “transuranium elements,” which Enrico Fermi had postulated would result from slow-neutron bombardment of uranium and other heavy elements, but which had defied traditional physical and chemical methods of identification.

When fission was discovered by Otto Hahn, Fritz Strassman, and Lise Meitner in late 1938, Abelson was planning to examine the x-ray spectra of elements below uranium, having failed to find any of atomic numbers exceeding the known ninety-two elements. Alvarez read of the discovery while having his hair cut in the Berkeley campus barber shop, and immediately informed Abelson, who was quickly able to duplicate the German results on neutron bombardment of uranium. Abelson and McMillan soon found evidence of elements heavier than uranium in cyclotron-bombarded uranium foils, resulting in the discovery of neptunium and of plutonium (by Glenn Seaborg working with Emilio Segrè), for which McMillan and Seaborg received the Nobel Prize in Chemistry in 1951.

Radar. A number of military agencies had sought to develop means of detecting enemy aircraft after World War I demonstrated the menace of aerial bombardment, using radio detection and ranging (radar). Because the size of the target that could be located by radio waves was proportional to their wavelengths, development pushed from shortwave radio to smaller and smaller wavelengths. The limits on this development were the magnitude of the electron tubes that generated the waves. A solution to this problem was the substitution of a solid-state device, the cavity magnetron, for the electron tube. This discovery, made by John T. Boot and Herman Randall at Marcus Oliphant’s cyclotron in Birmingham, England, made possible the development of microwave radar.

In the year after World War II began in 1939, President Franklin Delano Roosevelt approved the formation of the National Defense Research Committee (NDRC) under the leadership of Vannevar Bush, president of the Carnegie Institution, to augment military scientific

research. Alfred Lee Loomis, an amateur physicist and philanthropist who had supported Lawrence’s development of a giant cyclotron at the Radiation Laboratory in Berkeley, chaired the Microwave Committee of the NRDC. When Loomis learned of the cavity magnetron from the British, he asked committee member Lawrence to recruit a staff for a new laboratory at MIT to develop it. To cloak the effort, the name “Radiation Laboratory” was adopted. Lawrence recruited Alvarez and sent both Alvarez and McMillan to the MIT laboratory, which was headed by another cyclotron builder, Lee A. DuBridge of the University of Rochester. Alvarez took charge of expediting the development of the first successful radar sets that detected surfaced submarines as well as aircraft. He also devised a means to land aircraft under difficult conditions through Ground-Controlled Approach radar (GCA).

Nuclear Weapons. Alvarez was subsequently summoned to the Metallurgical Laboratory at the University of Chicago, where his former advisor, Arthur Compton, had centralized war work on plutonium, including Fermi’s experimental nuclear reactors (piles) from Columbia University and Seaborg’s plutonium chemistry from the University of California, over Lawrence’s vehement objections that his laboratory could do the job and Compton’s could not in a meeting Alvarez attended. Lawrence, however, continued to develop a means of separating fissile uranium-235 isotopes from uranium-238 using a modified cyclotron as a giant mass spectrometer. Although Alvarez intended to join this effort, illness delayed his transfer, and Robert Oppenheimer recruited him to work at the new Los Alamos laboratory building the atomic bomb.

Alvarez’s achievements at Los Alamos played a large part in making the implosion bomb possible. The designers thought at first to make a plutonium bomb achieve criticality through firing one fraction of a critical mass into another fraction, as was done with their uranium weapon. However, Emilio Segrè discovered that their reactor-made plutonium contained an isotope with a high probability of spontaneous fission. Thus, two separated fractions would not be stable long enough to allow the shotgun design. An alternative suggestion explored by Seth Neddermeyer and Ed McMillan, who had preceded Alvarez at Los Alamos, was implosion of a subcritical mass of plutonium by a sphere of high explosives. Working with Harvard chemist George Kistiakowsky, who developed the high-explosive lenses that focused the inward-moving wave of the high-explosive blast, Alvarez devised a system that simultaneously ignited thirty-six detonators regularly spaced on the outside of the high explosive to successfully compress the nuclear fuel to criticality. In order to determine the effectiveness of this system, Alvarez detonated spheres of highly radioactive radium-lanthanum with high explosives inside modified Army tanks; the resulting gamma-radiation showed the regularity of the implosion.

The final test of this system was made at an Army bombing range north of Alamogordo, New Mexico, in the Jornada del Muerto, which Oppenheimer had designated Trinity site. Alvarez, who had been asked to design a means of measuring the force of the blast, observed the test from a B-29 flying twenty-five miles away. He convinced Oppenheimer that he should be allowed to use the blast detectors he had created in combat. Hence, Alvarez flew with the mission that dropped the first atomic bomb—which was of the original uranium shotgun design—on Hiroshima, releasing his parachute-borne detectors from an accompanying B-29. He was thus present at the explosions of both the first uranium and the first plutonium bombs.

Alvarez returned to accelerator work at the Radiation Laboratory in Berkeley after the war. The Manhattan Engineer District provided Alvarez and his colleagues with generous funding to complete a 184-inch synchrocyclotron and an electron synchrotron based upon McMillan’s newly discovered principle of phase stability, and a proton linear accelerator designed by Alvarez based on his experience with microwave power generators in the radar program. Alvarez’s group included both his former graduate students and a number of talented individuals he had encountered during the war, most notably Wolfgang Panofsky, who subsequently led the effort to create the Stanford Linear Accelerator (linac). Alvarez’s machine made modest contributions to studies of nuclear scattering but was soon abandoned for higher-energy machines. The design, however, was incorporated as an injector of protons and heavier ions in many postwar synchrotrons.

After the first Soviet atomic bomb was detected in September 1949, Lawrence again mobilized Alvarez for nuclear weapons development. With Edward Teller, the two men successfully advocated the development of the “Super,” or hydrogen bomb, over the opposition of the General Advisory Committee (GAC) of the Atomic Energy Commission, chaired by their former University of California colleague, Robert Oppenheimer. Lawrence had proposed to build a heavy-water reactor to provide tritium for the Super, but when the GAC rejected the project, turned to the manufacture of fissile materials by electronu-clear means. The Oak Ridge and Argonne National Laboratories retained their reactor responsibilities.

At a formal Naval Air Station in Livermore, California, Alvarez’s linac was scaled up some four hundred times to produce deuterium ions that could be turned on various targets, depending on the material desired. Although the prototype was made to work with great difficulty, discovery of uranium on the Colorado plateau made such an expensive alternative unnecessary.

The crash program to develop the hydrogen bomb interfered with Alvarez’s scientific career, as had World War II. Although developments in liquid-hydrogen technology associated with the first thermonuclear tests bore fruit in his later development of the bubble chamber, he paid a price in terms of his career as a research scientist. He also lost the esteem of his fellow physicists when he sided against Oppenheimer in the advocacy of the Super. When Oppenheimer was called to account by political enemies in the Eisenhower administration in 1954, Alvarez testified, along with Edward Teller, in support of the interpretation of Oppenheimer’s obstruction to this development as unwise. He did not go so far as Teller in suggesting Oppenheimer’s disloyalty, but helped to give the hearing panel a reason to terminate Oppenheimer’s clearance.

Bubble Chambers and Big Science. Alvarez returned to Berkeley and began a new career in particle physics in 1954 just as the Bevatron, a 6.2-billion-electron-volt proton accelerator, was being completed by the Radiation Laboratory. He recognized that traditional nuclear detectors would not work well with the output of the new machine, which would be the highest-energy machine in existence. Learning of the invention of the bubble chamber by Donald Glaser of the University of Michigan, he undertook to build a series of such chambers at the Radiation Laboratory. As he had done with the MTA, Alvarez scaled up the chambers rapidly with generous subsidies from the Atomic Energy Commission, from 1.5 inches to 72 inches between 1955 and 1959. He recognized the need for automated analysis of the resulting photographs, which were produced at a rate of several million per year, and organized a group of physicist-programmers to provide it. He also encouraged the development of mechanized means of scanning the photographs for interesting events, leading to the development of a series of track-following devices such as the Franckenstein, invented by James Franck. These ancillary devices made the bubble-chamber system an effective and productive means of detecting many new particles and particle resonances. This work resulted in the award of the Nobel Prize in Physics to Alvarez in 1968.

The virtue of the liquid-hydrogen bubble chamber lay in the simplicity of the target—the hydrogen atom. Unlike earlier bubble chambers, which used hydrocarbon compounds, interactions with hydrogen atoms by particles from the Bevatron were simple to interpret. A significant event was more easily identified, permitting semiautomated and automated techniques to be employed. Alvarez’s group introduced techniques comparable to those of the assembly line in automotive manufacture, including division of labor, continuous processing, and quality control. The “industrialization” of particle detectors exemplified in the liquid-hydrogen bubble chamber similarly enhanced productivity.

Alvarez and his group pursued their particles in much the same way Lawrence had sought new radioisotopes in the 1930s. By focusing on one subatomic particle at a time and thoroughly charting its interactions, they were able to resolve important questions in particle physics as well as to suggest new ones. Unencumbered by theoretical constraints, they demolished existing theory and forced new interpretations on the particle physics community. The bubble chamber, like the cyclotron for accelerators, became the detector of choice in high-energy physics laboratories in the 1960s, and, like his mentor Lawrence, Alvarez was generous with help to would-be emulators of his system, supplying bubble-chamber film to those who did not have the means to produce it at their own laboratories.

Public Scientist. After Lawrence died at the age of fifty-seven in 1958, however, Alvarez chose not to follow Lawrence’s example in laboratory administration. Offered the directorship of the Lawrence Radiation Laboratory by University of California President Clark Kerr, Alvarez elected to remain head of his bubble-chamber group. McMillan, who succeeded Lawrence as director, did not succeed in winning Alvarez’s loyalty, and this led to Alvarez’s resignation as associate director and, eventually, his decision to pursue new activities.

Alvarez enjoyed the role of Cold War scientist and turned increasingly to service on panels of scientific advisors to the Atomic Energy Commission and the Department of Defense, as well as other government and private agencies interested in radar, aviation, and nuclear weapons. He was an advisor to the Central Intelligence Agency, National Security Agency, and Federal Aviation Administration, among others. Science policy was largely shaped by physicists of his generation. This branch of science in particular enjoyed substantial federal research funding and, as a part of the elite military-industrial complex, played a crucial role in directing that expenditure. Alvarez was able to acquire funding for his group from the National Aeronautics and Space Administration when he decided to resign from the administration of the Lawrence Radiation Laboratory, for example, to make a search for the magnetic monopoles that physical theory predicted, but which his team failed to find either in meteorites or in lunar rock samples, where long-term cosmic-ray bombardment should have made them.

Return to Cosmic Rays. Alvarez simultaneously searched cosmic rays for evidence of the antimatter that Hannes Alfven had predicted should enter the earth’s galaxy from galaxies made of it and separated from this galaxy by an electron plasma. He used a five-ton superconducting magnet borne aloft by balloons to detect them and, when that failed, undertook a survey with smaller superconducting magnets. As with the magneton, this search served to establish that these visitors were undetectable.

Alvarez also found an archaeological use for cosmic rays. The pyramid of Chephren at Giza had only one subterranean chamber, unlike the two constructed by his predecessors, and Alvarez believed that there might be other chambers that could be revealed by “x-raying” the pyramid with cosmic-ray muons, whose passage through the stone would be less attenuated by such voids in the pyramid’s structure. Despite the Arab-Israeli War of 1967, which forced his team of American and Egyptian physicists to postpone their work, the spark-chamber detectors he placed in the subterranean chamber made an extensive survey of incoming muons and found no hidden chambers. The method is still used in the early twenty-first century to investigate massive structures.

Nobel Prize. In 1968, Alvarez was awarded the Nobel Prize in Physics for his bubble-chamber work and the resulting discoveries. Although he had been disappointed when he did not receive the prize jointly with Donald Glaser in 1961 and trailed his other Radiation Laboratory colleagues Lawrence, McMillan, Seaborg, Segrè, and Owen Chamberlain to Stockholm, he made the most of his undivided funds by inviting the senior members of his group to join him there for the award ceremonies and took some condolence that while not the earliest, his was the longest prize citation for the award. Unlike many Nobel laureates, he resolved not to accept the social liabilities that usually come with the prize, and refused to sign petitions and attend other celebrations, including the Lawrence Radiation Laboratory party held in his honor by McMillan.

Alvarez did enjoy the liberation from his quotidian duties at the laboratory to investigate unusual phenomena. His attention was drawn to the ongoing controversy surrounding the assassination of John F. Kennedy when he was given access to the only visual recording of that incident, the Zapruder film made by an amateur photographer in Dallas at the time. Many critics of the Warren Commission, which conducted an investigation of the assassination and concluded that a lone gunman, Lee Harvey Oswald, perpetrated it, believed the film belied that conclusion. Alvarez systematically tackled the analysis of the film and found that it supported the commission’s conclusion, particularly with respect to Kennedy’s physical response to the fatal shot and with respect to the timing of the three shots. As an example of forensic science, his study was a tour de force that undermined alternative scenarios that had become a leitmotif of the 1970s.

His last experiment in forensic science dealt with the mass extinction of dinosaurs and other species. His son, Walter, who had become a geologist, detected a thin layer of clay laid down at approximately the same period as the dinosaurs’ disappearance, and brought a sample of it to Luis Alvarez’s attention. Intrigued by the association, Luis suggested that they measure the length of time that had been required to make the deposit. In order to determine the age of the clay, Luis thought first to use as tracers radioactive elements produced in the Earth’s atmosphere by cosmic rays (e.g., carbon-14). However, their lifetimes proved an order of magnitude too small, so Luis elected to search for elements that were more abundant in meteorite debris than in normal soil. Iridium was detected by Radiation Laboratory nuclear chemists using neutron activation analysis and proved three hundred times more abundant in the clay layer than in the surrounding limestone layers.

Subsequent investigation revealed that the layer also did not have elements associated with supernovae, thought by some to have caused the mass extinction, and through a process of elimination Alvarez concluded that a ten-kilometer meteor had struck the earth and created a dust cloud that orbited it long enough to cause the extinctions by cooling and interfering with plant growth.

Alvarez’s explanation of the Cretaceous-Tertiary mass extinction has won increasing acceptance among paleontologists, especially since a candidate impact site was discovered in the Yucatan peninsula of Mexico. Although there are competing theories that seek to account for the extinction in terms of terrestrial causes, the Alvarez hypothesis has not been refuted, and he believed it would be the scientific accomplishment for which he would be longest remembered.

Alvarez’s discoveries and inventions made him one of the most remarkable and versatile scientists of the twentieth century, and his technological accomplishments were both abundant and profitable, giving rise to a variety of spin-off firms from which he drew profit as well as pride. In addition to his many radar patents, he also held patents for electronuclear machines, x-ray detectors for explosives in airline baggage, optical systems, and range detectors for golf carts. For these accomplishments he was elected to the Inventors’ Hall of Fame, and won numerous awards, including the Collier Trophy of the National Aeronautical Association, the National Medal of Science, and the Pioneer Medal of the American Institute of Electrical and Electronic Engineers.

On 1 September 1988, a year after he completed his autobiography, Alvarez died of cancer in Berkeley, California. His work in high-energy physics had transformed experimental practice by importing collaborative research into the laboratory, as historian Peter Galison has shown, and contributed to the triumph of “big science” in the twentieth century.

BIBLIOGRAPHY

WORKS BY ALVAREZ

With Arthur H. Compton. “A Positively Charged Component of Cosmic Rays.” Physical Review43 (1933): 835–836. An account of the discovery of the east-west effect in cosmic radiation.

“Capture of Orbital Electrons by Nuclei.” Physical Review54 (1938): 486–497.

With Robert Cornog. “Helium and Hydrogen of Mass 3.” Physical Review56 (1939): 613.

With Felix Bloch. “A Quantitative Determination of the Neutron Moment in Absolute Nuclear Magnetons.” Physical Review57 (1940): 111–122.

With Hugh Bradner, James V. Franck, Hayden Gordon, et al. “Berkeley Proton Linear Accelerator.” Review of Scientific Instruments 26 (1955): 111–133.

With Frank S. Crawford, Myron L. Good, and M. Lynn Stevenson. “Lifetime of K-Mesons.” Physical Review 101 (1956): 503–505.

With Hugh Bradner, Frank S. Crawford Jr., J. A. Crawford, et al. “Catalysis of Nuclear Reactions by ?-Mesons.” Physical Review 105 (1957): 1127–1128.

“High-Energy Physics with Hydrogen Bubble Chambers.” In Proceedings of the Second United Nations International Conference on Peaceful Uses of Atomic Energy, vol. 30, Fundamental Physics. Geneva: United Nations, 1958.

With Phillipe Eberhard, M. L. Good, William Graziano, et al. “Neutral Cascade Hyperon Event.” Physical Review Letters 2 (1959): 215–219.

With Margaret Alston, Phillipe Eberhard, M. L. Good, et al. “Resonance in the ?-? System.” Physical Review Letters 5 (1960): 520–524.

With Margaret Alston, William Graziano, M. L. Good, et al. “Resonance in the K-? System.” Physical Review Letters 6 (1961): 300–302.

With D. O. Huwe, G. R. Kalbfleisch, Margaret Alston, et al. “The 1660 MeV Y*₁ Hyperon.” Physical Review Letters 10 (1963): 184–188.

“A Physicist Examines the Kennedy Assassination Film.” American Journal of Physics44 (1976): 813.

With Walter Alvarez, Frank Asaro, and Helen V. Michel. “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” Science 208 (1980): 1095.

Alvarez: Adventures of a Physicist. New York: Basic Books, 1987.

OTHER SOURCES

Galison, Peter. Image and Logic: A Material Culture of Microphysics. Chicago: University of Chicago Press, 1997.

Garwin, Richard L. “Memorial Tribute for Luis W. Alvarez.” Memorial Tributes, National Academy of Engineering, vol. 5. Washington, DC: National Academy Press, 1992.

Trower, W. Peter, ed. Discovering Alvarez: Selected Works of Luis W. Alvarez with Commentary by His Students and Colleagues. Chicago: University of Chicago Press, 1987.

Heilbron, John, and Robert Seidel. Lawrence and His Laboratory, vol. 1, A History of the Lawrence Berkeley Laboratory. Berkeley: University of California Press, 1989.

Robert W. Seidel