Alvarez, Luis Walter (13 June 1911–01 September 1988), physicist and inventor, was born in San Francisco, California, the son of Walter Clement Alvarez, a physician, and Harriet Smyth, a teacher. Alvarez attended the University of Chicago from 1928 to 1936, earning his bachelor of science degree in physics in 1932, his master of science in 1934, and his Ph.D. in 1936. Alvarez married Geraldine Smithwick on 15 April 1936. He had two children with her, including the geologist Walter Alvarez. The marriage ended in divorce. Alvarez married Janet Lucile Landis in 1958 and had two children with her. Alvarez held forty patents for inventions in radar, optics, and electronics. For his work with liquid hydrogen bubble chambers he was awarded the Nobel Prize in physics in 1968.

Alvarez made his first important discovery, the “east-west effect” in cosmic rays, as a graduate student under the direction of Arthur Compton. Using a pair of Geiger counters, Alvarez detected more cosmic rays in the western sky than in the east, demonstrating that most cosmic rays are positively charged. This work was more notable than his dissertation, “A Study of the Diffraction Grating at Grazing Incidence.”

Upon receiving his doctorate from Chicago, Alvarez took a position under Ernest Lawrence at the Lawrence Radiation Laboratory at the University of California at Berkeley. He started as a research associate in 1936, advancing to the rank of instructor in 1938, assistant professor in 1939, associate professor in 1941, and professor in 1946. His first four years at the lab were extremely productive, resulting in a series of fundamental discoveries. During this time he made the first experimental demonstration of K-electron capture by nuclei, developed a technique for producing a beam of slow neutrons, discovered the isotopes hydrogen 3 and helium 3, and made the first measurement of the magnetic moment of the neutron. During these years, Alvarez honed his expertise not just in nuclear physics but also in the collaborative approach to science and the utilization of industrial-scale equipment and commensurate monetary resources in science, as pioneered by Lawrence in Berkeley.

Alvarez took leave from Lawrence’s lab to work on military projects in the Radar Research and Development section of the Radiation Laboratory at the Massachusetts Institute of Technology (MIT) from 1940 to 1943. There he was responsible for the development of several important radar systems. The Ground Controlled Approach (GCA) system is the best known of these: a “blind” landing system designed to allow military aircraft to land in conditions of low visibility, GCA made Allied aerial bombing of Germany feasible in weather conditions previously unsuitable for flight. The system was adapted after the war to make civilian air travel safer as well. Alvarez also invented VIXEN, an airborne radar system that allowed air patrols to detect German submarines on the surface without alerting the submarine radar operator that they were under surveillance. Submarines in World War II spent most of their time on the surface, submerging only to hide from air patrols or to approach enemy ships. The Germans had learned to submerge and hide from an approaching radar signal; VIXEN was designed so that the signal received by the target became weaker as VIXEN got closer. In addition to developing these systems, Alvarez also promoted their use and demonstrated the prototypes to military authorities in the United States and England.

While Alvarez was working on radar systems at MIT, many of America’s physicists were developing the first atomic bomb. Alvarez’s mentor Lawrence adapted his cyclotrons to the problem of separating isotopes of uranium and provided a constant link between Alvarez and the bomb project. In 1943 Alvarez left the Radiation Laboratory at MIT for the University of Chicago Metallurgical Laboratory, where he worked under Enrico Fermi on instrumentation for the first nuclear reactor. Six months later, Alvarez was transferred to the explosives division at Los Alamos, New Mexico, to work on the timing mechanism that synchronized the detonation of the explosives that start the nuclear reaction in the plutonium bomb.

Alvarez also worked on the problem of yield assessment. He designed, tested, and operated the systems that measured the power of the atomic bombs and flew as a scientific observer on the Hiroshima mission. At the end of World War II, Alvarez returned to the University of California and turned his attention back to the construction of progressively larger particle accelerators. By building larger accelerators, more energetic nuclear collisions could be arranged, facilitating a better understanding of the physics of subatomic particles. He was responsible for the design and construction of the forty-foot proton accelerator built in Berkeley in 1947.

Alvarez’s career took a major turn in 1953 when he met Donald Glaser, the inventor of the bubble chamber. The bubble chamber is an instrument used to detect and record the paths of subatomic particles. The chamber contains a liquid very near the state where it will boil and become a gas. When a subatomic particle passes through the chamber, small bubbles of vapor form where the particle passed through, leaving a trail of bubbles behind as evidence of the journey. A magnetic field of known intensity is placed near the chamber so that charged particles will be deflected. The direction of this deflection will indicate the charge, and the amount of deflection will indicate the mass. By photographing these bubble trails and analyzing the photographs, physicists can then infer the speed, charge, and mass of the particles that left the trails.

Alvarez and his group made dramatic improvements to all three components of the bubble-chamber detection system—the chamber itself, the devices for recording and measuring tracks, and the computerized data analysis system—allowing them to study effectively the particle collisions created by the world’s largest accelerators.

Where Glaser’s bubble chambers contained ether and were characteristically one or two inches in diameter, Alvarez envisioned a much larger instrument. He used liquid hydrogen, which simplified the nuclear reactions that occurred within the chamber, and, in the spirit of his mentor Lawrence, started making progressively larger instruments. His first chamber was a cylinder 1.5 inches in diameter. He followed this with chambers of 2.5, 3, and 4 inches in diameter. He then built a rectangular chamber 72 inches in length. The smaller chambers started producing results in 1957; the 72-inch chamber began operation in 1959.

Alvarez also automated the analysis of the photographs, developing equipment to record data from the tracks and store them on punched cards for analysis by computers. The computers were programmed to make use of the laws of conservation of momentum and energy to deduce the missing information in the interactions. For fifteen years Alvarez developed the techniques and equipment used in bubble-chamber photography, analyzing millions of photographs each year in search of rare and short-lived subatomic particles. The Alvarez group discovered a large number of fundamental particles, including both mesons and hyperons. These particles had been unexpected by theorists and led to the theory of quarks. The tools and techniques that Alvarez developed were adopted by physics labs around the world, including the Rutherford Lab in England and CERN in Switzerland.

Alvarez applied his expertise in nuclear physics and particle detection to a variety of other problems, most notably his work from 1964 to 1969 on the Second Pyramid at Giza. Archaeologists had discovered three chambers in the Great Pyramid of Cheops and two chambers in the Pyramid of Sneferu, but no chamber was known to exist in Chephren’s Pyramid at Giza. Alvarez proposed that they “X-ray” the pyramid, measuring the resistance that it presented to cosmic ray muons going through it. By setting muon detectors in a chamber under the pyramid, Alvarez and the Joint UAR-USA Pyramid Project were able to demonstrate that the Second Pyramid is solid.

In 1979, collaborating with his son, the geologist Walter Alvarez, Alvarez produced a theory regarding the extinction of the dinosaurs. Walter had discovered that a thin layer of rocks separating Cretaceous and Tertiary rocks contained a much higher than normal level of one isotope of iridium. Luis hypothesized that the iridium was the scattered remains of a huge asteroid that had struck the earth 65 million years ago. He argued that the dust thrown into the air after the impact blocked so much sunlight that photosynthesis by plants was virtually halted, leaving large animals such as the dinosaurs with an insufficient food supply. The correlation of the iridium layer with the impact of a large comet or asteroid was accepted by paleontologists, but Alvarez’s conclusions about the resulting mass extinction remained controversial long after his death in Berkeley.

Alvarez built a career in physics by embracing the best parts of his colleagues’ instruments, improving them in dramatic fashion, and exploiting the power of large-scale efforts in large laboratories. Though he later expressed disdain for this committee approach to science, he was a master of collaborative research. He was best known among physicists for his work in particle physics and among scientists in general for his impact theory of dinosaur extinction. His career exemplified the culmination of the trend toward big science that was initiated in the United States by the work of Ernest Lawrence and accelerated by the rush to build the atomic bomb.