The importance and variety of the discoveries and contributions of Luis W. Alvarez (1911-1988) are perhaps unmatched by any other 20th-century physicist. He received many awards for his work over the years, including the 1968 Nobel Prize in Physics for his work on a large liquid hydrogen bubble chamber.

Alvarez will probably be best remembered by the public for ingenious experiments that applied physics to other sciences. He x-rayed Chephren's pyramid in Egypt using cosmic radiation, only to find that there were no undiscovered chambers inside. His application of elementary physics to the evidence on the John F. Kennedy assassination verified the Warren Commission finding that only a single assassin was involved. But perhaps his most dramatic discovery was made after his "retirement" by jumping into a totally new field, paleontology and geology. With collaborators that included his son, Walter, he analyzed a 65 million year old clay layer and showed that the great ecological catastrophe that killed the dinosaurs was caused by the impact of an asteroid or comet.

Alvarez was born June 13, 1911, in San Francisco. He began his career at the University of Chicago. His first published paper (as an undergraduate) described a measurement of the wavelength of light using a phonograph record, a parlor lamp, and a yard stick. While reading the original physics literature, he found a paper by Hans Geiger that described a new type of detector for charged particles. He proceeded to construct one of the first Geiger counters in America. Alvarez was the first Chicago undergraduate to present results of his research at the weekly departmental colloquium, sharing the time with a professor who reported on James Chadwick's discovery of the neutron. After hearing the talk, Arthur Compton invited Alvarez to collaborate with him on a study to determine the electric charge of the primary cosmic radiation.

Alvarez's first summer as a graduate student was spent on the roof of the Geneva Hotel in Mexico City, his Geiger telescope resting in a wheelbarrow that allowed him to periodically reverse the east-west orientation of his apparatus. He and Compton determined that the cosmic rays were mostly positively charged, and therefore presumably protons. After receiving his Ph.D. in 1936 Alvarez began work with Ernest O. Lawrence at the University of California, in part through family connections. Alvarez's father, a physician on the staff of the Mayo Clinic, had helped Lawrence get money for one of his cyclotrons, and his sister was Lawrence's part-time secretary. Arriving at the Old Radiation Laboratory, Alvarez made the first of his dramatic career changes as he prepared himself to become a practicing nuclear physicist. First, he became thoroughly familiar with all instruments in the laboratory, their use, and the physics that was being done with them. He did this by helping everyone with their experiments while becoming a skilled machine operator and repairman.

Emerging from the laboratory at each day's end, Alvarez would pick up a couple of volumes of physics journals from the university library; he eventually read every published nuclear physics article held there. Years later he would astonish his colleagues by reproducing a curve or a little known fact gleaned in these early efforts. He could usually cite the authors, journal, year, and often the location of the volume in the library and whether the item was on a right-or a left-hand page. By 1937 Hans Bethe had published his three-part compendium of all that was known about nuclear physics. Alvarez chose first to make a measurement that Bethe said couldn't be done and then to disprove one of Bethe's assertions. In just four years Alvarez discovered the radioactivity of tritium and the stability of helium-3, the magnetic moment of the neutron, and that nuclei cannibalize their own atomic electrons. He also demonstrated the spin dependence of the nuclear force, established a new standard of length using mercury-198, and made the first experimental demonstrations in a field now called heavy-ion physics.

World War II ended Alvarez's nuclear physics career. He soon found himself in Boston, figuring how to apply high-frequency radio waves to achieve military goals. Using optics ideas learned in his thesis work, Luie invented the linear phased array, which formed the basis of EAGLE, the first radar bombing system. He also invented VIXEN, a system to outfox German submarines by diminishing an airborne acquisition radar's power as a surfaced sub was approached, so that the listening skipper would believe the attack plane was going away. Alvarez solved the problem of landing planes in bad weather by inventing the radar-based Ground Control Approach (GCA), for which he won the 1946 Collier Air Trophy.

Upon his return to the Berkeley laboratory after the war, Alvarez made another career change, to that of a particle accelerator physicist. He realized the importance of team research and looked to the methods of Lawrence and Ernest Rutherford. Like them, he displayed an ability to select good people to work with him.

His first postwar machine was the proton linear accelerator, which has become the standard injector for many subsequent higher energy circular machines and is still referred to as an "Alvarez accelerator." While preparing for his nuclear physics class one morning, he invented the Tandem van de Graaff, which was commercialized by High Voltage Engineering. Alvarez was a superb teacher. His course in physical optics was thorough. The students were introduced to the full spectrum of electromagnetic radiation from gamma rays to radio waves with spellbinding tales of how radar was used during the Battle of Britain.

In the mid 1950s Donald Glaser invented a new detector called a bubble chamber. Alvarez immediately saw the potential this had for the study of the newly available high-energy particles, if it could be made to work with liquified hydrogen. He established a group to develop the liquid-hydrogen bubble chamber from the first small steady-state chambers to large pulsed chambers. Characteristically, he grew impatient with the small chambers and proposed a large one 72 inches in length. This was nearly eight times the size of the one then in action at Berkeley, and some people thought this would be too big a step. Alvarez was confident that the chamber could be made to operate and he convinced the money sources to help. The 72-inch chamber aided in the identification of many new particles. It was for this work that he received the Nobel Prize in Physics in 1968.

In 1977 he was presented a piece of rock that had been cut from a hillside in Italy by his geologist son, Walter. The rock had a thin clay layer in it. He was shown how the microscopic fossils ("forams") in the rock became extinct right at the clay layer. These tiny forams had been destroyed at the clay layer. These tiny forams had been destroyed at the same time the dinosaurs had disappeared. Alvarez later described his experience in examining this rock as one of the most exciting moments in his life. The scientific consequences, which include the nuclear winter theory, are still being uncovered by geologists, paleontologists, physicists, chemists, and astronomers.

Alvarez was always solving practical problems that influenced his life. By his early fifties he needed bifocal lenses to correct his eyesight, and this convinced him that there must be a better way to solve this problem. The result was his invention of the variable focus lens and the formation of Humphrey Instruments.

While visiting Kenya, he was frustrated by how the image of the distant animals jumped around in the viewing port of his hand-held camera. He just couldn't hold the camera firmly enough to steady the image. He then invented a series of stabilized optical devices; and eventually he formed Schwem Technologies to develop and market them.

In addition to the 1946 Collier Air Trophy and the 1968 Nobel Prize in Physics, Alvarez also received the Einstein Medal in 1961, the 1964 National Medal of Science, a 1978 membership in the Inventors' Hall of Fame, and the 1981 Wright Prize.

Further Reading

• Alvarez produced an extensive autobiography. A single volume version, Alvarez: Adventures of a Physicist was published in 1987, a paperback edition in 1989. He was honored by his colleagues with Discovering Alvarez; Selected Works of Luis W. Alvarez with Commentary by His Students and Colleagues, edited by W. Peter Trower (1987).