Luis Walter Alvarez (June 13, 1911 – September 1, 1988) was an American experimental physicist, inventor, and professor who was awarded the Nobel Prize in Physics in 1968 for his discovery of resonance states in particle physics using the hydrogen bubble chamber. In 2007 the American Journal of Physics commented, "Luis Alvarez was one of the most brilliant and productive experimental physicists of the twentieth century."
After receiving his PhD from the University of Chicago in 1936, Alvarez went to work for Ernest Lawrence at the Radiation Laboratory at the University of California, Berkeley. Alvarez devised a set of experiments to observe K-electron capture in radioactive nuclei, predicted by the beta decay theory but never before observed. He produced tritium using the cyclotron and measured its lifetime. In collaboration with Felix Bloch, he measured the magnetic moment of the neutron.
In 1940, Alvarez joined the MIT Radiation Laboratory, where he contributed to a number of World War II radar projects, from early improvements to Identification friend or foe (IFF) radar beacons, now called transponders, to a system known as VIXEN for preventing enemy submarines from realizing that they had been found by the new airborne microwave radars. The radar system for which Alvarez is best known and which has played a major role in aviation, most particularly in the post war Berlin airlift, was Ground Controlled Approach (GCA). Alvarez spent a few months at the University of Chicago working on nuclear reactors for Enrico Fermi before coming to Los Alamos to work for Robert Oppenheimer on the Manhattan Project. Alvarez worked on the design of explosive lenses, and the development of exploding-bridgewire detonators. As a member of Project Alberta, he observed the Trinity nuclear test from a B-29 Superfortress, and later the bombing of Hiroshima from the B-29 The Great Artiste.
After the war Alvarez was involved in the design of a liquid hydrogen bubble chamber that allowed his team to take millions of photographs of particle interactions, develop complex computer systems to measure and analyze these interactions, and discover entire families of new particles and resonance states. This work resulted in his being awarded the Nobel Prize in 1968. He was involved in a project to x-ray the Egyptian pyramids to search for unknown chambers. With his son, geologist Walter Alvarez, he developed the Alvarez hypothesis which proposes that the extinction event that wiped out the non-avian dinosaurs was the result of an asteroid impact.
Luis Walter Alvarez was born into a Catholic family in San Francisco on June 13, 1911, the second child and oldest son of Walter C. Alvarez, a physician, and his wife Harriet (née Smyth). His grandfather, Luis F. Álvarez, was a physician born in Asturias, Spain, who lived briefly in Cuba, settled in the United States, and discovered a better method for diagnosing macular leprosy. He had an older sister, Gladys, a younger brother, Bob, and a younger sister, Bernice. His aunt, Mabel Alvarez, was a California artist specializing in oil painting.
He attended Madison School in San Francisco from 1918 to 1924, and then San Francisco Polytechnic High School. In 1926, his father became a researcher at the Mayo Clinic, and the family moved to Rochester, Minnesota, where Alvarez attended Rochester High School. He had always expected to attend the University of California, Berkeley, but at the urging of his teachers at Rochester, he instead went to the University of Chicago, where he received his bachelor's degree in 1932, his master's degree in 1934, and his PhD in 1936. As an undergraduate, he belonged to the Phi Gamma Delta fraternity. As a postgraduate he moved to Gamma Alpha.
In 1932, as a graduate student at Chicago, he discovered the field of physics and had the rare opportunity to use the equipment of legendary physicist Albert A. Michelson. Alvarez also constructed an apparatus of Geiger counter tubes arranged as a cosmic ray telescope, and under the aegis of his faculty advisor Arthur Compton, conducted an experiment in Mexico City to measure the so-called East–West effect of cosmic rays. Observing more incoming radiation from the west, Alvarez concluded that primary cosmic rays were positively charged. Compton submitted the resulting paper to the Physical Review, with Alvarez's name at the top.
Alvarez was an agnostic even though his father had been a deacon in a Congregational church.
Alvarez's sister, Gladys, worked for Berkeley physicist Ernest Lawrence as a part-time secretary, and mentioned Alvarez to Lawrence. Lawrence then invited Alvarez to tour the Century of Progress exhibition in Chicago with him. After he completed his oral exams in 1936, Alvarez, now engaged to be married to Geraldine Smithwick, asked his sister to see if Lawrence had any jobs available at the Radiation Laboratory. A telegram soon arrived from Gladys with a job offer from Lawrence. This started a long association with the University of California, Berkeley. Alvarez and Smithwick were married in one of the chapels at the University of Chicago and then headed for California. They had two children, Walter and Jean. They were divorced in 1957. On December 28, 1958, he married Janet L. Landis, and had two more children, Donald and Helen.
At the Radiation Laboratory he worked with Lawrence's experimental team, which was supported by a group of theoretical physicists headed by Robert Oppenheimer. Alvarez devised a set of experiments to observe K-electron capture in radioactive nuclei, predicted by the beta decay theory but never observed. Using magnets to sweep aside the positrons and electrons emanating from his radioactive sources, he designed a special purpose Geiger counter to detect only the "soft" X-rays coming from K capture. He published his results in the Physical Review in 1937.
When deuterium (hydrogen-2) is bombarded with deuterium, the fusion reaction yields either tritium (hydrogen-3) plus a proton or helium-3 plus a neutron (2H + 2H → 3H + p or 3He + n). This is one of the most basic fusion reactions, and the foundation of both the thermonuclear weapon and the current research on controlled nuclear fusion. At that time the stability of these two reaction products was unknown, but based on existing theories Hans Bethe thought that tritium would be stable and helium-3 unstable. Alvarez proved the reverse by using his knowledge of the details of the 60-inch cyclotron operation. He tuned the machine to accelerate doubly ionized helium-3 nuclei and was able to get a beam of accelerated ions, thus using the cyclotron as a kind of super mass spectrometer. As the accelerated helium came from deep gas wells where it had been for millions of years, the helium-3 component had to be stable. Afterwards Alvarez produced the radioactive tritium using the cyclotron and the 2H + 2H reaction and measured its lifetime.
In 1938, again using his knowledge of the cyclotron and inventing what are now known as time-of-flight techniques, Alvarez created a mono-energetic beam of thermal neutrons. With this he began a long series of experiments, collaborating with Felix Bloch, to measure the magnetic moment of the neutron. Their result of μ0 = 1.93±0.02 μN, published in 1940, was a major advance over earlier work.
The British Tizard Mission to the United States in 1940 demonstrated to leading American scientists the successful application of the cavity magnetron to produce short wavelength pulsed radar. The National Defense Research Committee, established only months earlier by President Franklin Roosevelt, created a central national laboratory at the Massachusetts Institute of Technology (MIT) for the purpose of developing military applications of microwave radar. Lawrence immediately recruited his best "cyclotroneers", among them Alvarez, who joined this new laboratory, known as the Radiation Laboratory, on November 11, 1940. Alvarez contributed to a number of radar projects, from early improvements to Identification Friend or Foe (IFF) radar beacons, now called transponders, to a system known as VIXEN for preventing enemy submarines from realizing that they had been found by the new airborne microwave radars. Enemy submarines would wait until the radar signal was getting strong and then submerge, escaping attack. But VIXEN transmitted a radar signal whose strength was the cube of the distance to the submarine so that as they approached the sub, the signal—as measured by the sub—got progressively weaker, and the sub assumed the plane was getting farther away and did not submerge.