Luis Walter Alvarez

Luis W. Alvarez
Born	Luis Walter Alvarez (1911-06-13)June 13, 1911 San Francisco, California, USA
Died	September 1, 1988(1988-09-01) (aged 77)
Alma mater	University of Chicago
Awards	Nobel Prize in Physics (1968)
Scientific career
Fields	Physics
Institutions	University of California, Berkeley

Luis W. Alvarez (June 13, 1911, San Francisco, California – September 1, 1988) was an American experimental physicist and inventor, who spent nearly all of his long professional career on the faculty of the University of California, Berkeley. The American Journal of Physics commented, "Luis Alvarez (1911–1988) was one of the most brilliant and productive experimental physicists of the twentieth century."

He was the author of 168 published papers in scientific journals, mostly in the field of physics, and was elected to the National Academy of Science in 1947 and the National Academy of Engineering in 1969. He was a member of the American Physical Society, a fellow in 1939, and served as President in 1969. He was awarded the Collier Trophy by the National Aeronautics Association in 1946. The trophy was presented by President Truman; and won the Presidential Medal for Merit in 1947. In 1960 he was named California Scientist of the Year; in 1961 he won the Albert Einstein Award. In 1963 he was presented the National Medal of Science by Lyndon B. Johnson; in 1965 the Michelson Award; in 1978 he received the University of Chicago Alumni Medal and was inducted into the National Inventors Hall of Fame. In 1987, the US Department of Energy granted him its Enrico Fermi award.

He won the Nobel Prize in Physics in 1968, and received over 40 patents, some of which proved commercially viable.

Biography

The Alvarez family was of Spanish American descent. Alvarez was the son of Walter C. Alvarez, a doctor who for a time was a researcher at the Mayo Clinic, and of Harriet Smythe, and a grandson of Luis F. Alvarez, a doctor in Hawaii who found a better method for diagnosing macular leprosy. His aunt, Mabel Alvarez, was a California artist specializing in oil painting. Alvarez married Geraldine Smithwick in 1936 and had two children, Walter and Jean. In 1958, he married Janet L. Landis and had two more children, Donald and Helen.

Early Work

File:Compton with LWA.JPG

Nobel Laureate Arthur Compton with young graduate student Luis Alvarez at the University of Chicago in 1933 (Photo: LBNL)

Alvarez was educated at the University of Chicago, where he received his bachelor's degree in 1932, his master's degree in 1934, and his PhD in 1936. In 1932, while still a graduate student at Chicago, Alvarez 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 the cosmic rays. Observing more incoming radiation from the west Alvarez correctly concluded that primary cosmic rays were positively charged, a fact unknown at the time. Following the receipt of his degree he joined Ernest Lawrence's group at the Radiation Laboratory at UC Berkeley in 1936.

At the Radiation Laboratory he worked with Lawrence's experimental team and group of theoretical physicists working with Robert Oppenheimer. Of the twenty or so papers he published in these years, there are three areas of accomplishment that are particularly important. He 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. His conclusive results were published in the Physical Review in 1938 with him as sole author.

Another achievement of Alvarez concerned nuclear reactions. 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 (²
H
+ ²
H
→ ³
H
+ p or ³
He
+ n). This is one of the most basic fusion reactions, and the foundation of the hydrogen bomb 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 it was felt that ³
H
would be stable and ³
He
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 ³
He
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 ³
He
component had to be stable. Afterwards Alvarez produced ³
H
using the cyclotron and the ²
H
+ ²
H
reaction and measured the lifetime of the radioactive ³
H
.

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 μ_{0 = 1.93±0.02}, published in 1940, was a major advancement over earlier work. Bloch continued with experimental work in an effort to measure the magnetic moment of the proton. His later work in what would come to be called nuclear magnetic resonance (NMR) would win him the 1968 Nobel Prize.

The 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 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, for this new laboratory, called the Radiation Laboratory.

World War II years

Alvarez would contribute to a number of radar projects at the MIT Radiation Laboratory. From early improvements to Identification Friend or Foe (IFF) radar beacons, now called transponders, to ingenious strategies for preventing enemy submarines from realizing that they had been found by the new airborne microwave radars, Alvarez quickly got up to speed in the business of developing military radar equipment.

One of the first projects was to build equipment to transition from the English long-wave radar to the new microwave centimeter-band radar made possible by the cavity magnetron. In working on the Microwave Early Warning system (MEW), Alvarez invented a linear dipole array antenna that not only suppressed the unwanted side lobes of the radiation field, but also could be electronically scanned without the need of mechanical scanning. This was the first microwave phased-array antenna and Alvarez used it not only in MEW but in two additional important radar systems. The Eagle precision bombing radar was an application of the Alvarez antenna to allow precision bombing in bad weather or through clouds. It was completed rather late in the war, and although a number of B-29s were equipped with Eagle, and it worked well, it came too late to be of much importance.

File:LWA with HST.JPG

Receiving the Collier Trophy from President Truman, White House, 1946 (Photo: LBNL)

The radar system for which Alvarez is best known and which has played a huge role in aviation, most particularly in the post war Berlin airlift, was Ground Controlled Approach (GCA). Using Alvarez's dipole antenna to achieve a very high angular resolution GCA allows ground-based radar operators watching special precision displays to guide a landing airplane to the runway by transmitting verbal commands to the pilot. The system was simple, direct, and it worked well, even with previously untrained pilots. It was so successful that the military continued to use it for many years after the war, and it is in use in some countries even today. Alvarez was awarded aviation's most prestigious award, the Collier Trophy in 1945 "for his conspicuous and outstanding initiative in the concept and development of the Ground Control Approach system for safe landing of aircraft under all weather and traffic conditions". Alvarez spent the summer of 1943 in England testing GCA at the front lines, landing planes returning from battle in bad weather, and also training the English in the use of the system. While there he encountered the young Arthur C. Clarke, who was an RAF radar technician. Clarke subsequently used his experiences at the radar research station as the basis for his novel Glide Path and a thinly-disguised version of Alvarez appears in it.

In the fall of 1943, Alvarez returned to the United States with an offer from Robert Oppenheimer to work at Los Alamos on the Manhattan project. But Oppenheimer suggested that he first spend a few months at the University of Chicago working with Enrico Fermi before coming to Los Alamos. During these months, General Leslie Groves asked Alvarez to think of a way that the US could find out if the Germans were operating any nuclear reactors, and, if so, where they were. Alvarez suggested that an airplane carrying a system to detect the radioactive gases that a reactor produces, particularly xenon 133. The equipment did fly over Germany but detected no radioactive xenon because the Germans did not succeed in building a reactor capable of a chain reaction. This was the first idea of monitoring fission products for intelligence gathering. It would become extremely important after the war.

As a result of his radar work and the few months spent with Fermi, Alvarez arrived at Los Alamos in the spring of 1944, later than many of his contemporaries. The work on Little Boy (uranium bomb) was far along so Alvarez became involved in the design of Fat Man (plutonium bomb). The technique used for Little Boy, that of forcing the two sub-critical masses together using a type of gun, would not work in Fat Man because the high level of background spontaneous neutrons would cause fissions as soon as the two parts approached each other, leading to heat and expansion forcing the system apart before much energy has been released. It was decided to use a nearly critical sphere of plutonium and compress it quickly by explosives into a much smaller and denser core, which was a technical challenge at the time.

Wearing a helmet and flak jacket and standing in front of The Great Artiste, Tinian 1945 (Photo: LBNL)

To create the symmetrical implosion required to compress the plutonium core to the required density, thirty two explosives were to be simultaneously detonated around the spherical core. Using conventional explosive techniques with blasting caps, efforts to accomplish simultaneity to within a small fraction of a microsecond were discouraging. Alvarez directed his graduate student, Lawrence Johnston, to develop a method of using a large capacitor to deliver a high voltage charge directly to each explosive lens, replacing blasting caps with exploding-bridgewire detonators. The exploding wire detonated the thirty two charges to within a few tenths of a microsecond. The invention was absolutely critical to the success of the Trinity and Nagasaki plutonium bomb explosions. Alvarez himself comments on this in his autobiography "Alvarez, Adventures of a Physicist" by saying:

"With modern weapons-grade uranium, the background neutron rate is so low that terrorists, if they had such material, would have a good chance of setting off a high-yield explosion simply by dropping one half of the material onto the other half. Most people seem unaware that if separated U-235 is at hand, it's a trivial job to set off a nuclear explosion, whereas if only plutonium is available, making it explode is the most difficult technical job I know."

Working with his student, Lawrence Johnston, Alvarez's last task for the Manhattan Project was to develop a set of calibrated microphone/transmitters to be parachuted from an aircraft in order to measure the strength of the blast wave from the atomic explosion. This would allow the scientists to be able to calculate the energy of the bomb. Flying in the B-29 The Great Artiste in formation with the Enola Gay, Alvarez measured the blast effect of the first bomb which fell on Hiroshima. A few days later again flying in the Great Artiste, Johnston used the same equipment to measure the strength of the Nagasaki explosion.

Later life and career

Celebrating winning the Nobel Prize, October 30, 1968 (Photo: LBNL)

Returning to the University of California as a full professor, Alvarez had many ideas about how to use his wartime radar knowledge to improve particle accelerators. Though some of these were to bear fruit, the "big idea" of this time would come from Edwin McMillan with his concept of phase stability which led to the synchrocyclotron. Refining and extending this concept, the Lawrence team would build the world's then-largest proton accelerator, the Bevatron, which began operating in 1954. Though the Bevatron could produce copious amounts of interesting particles, particularly in secondary collisions, there were very few techniques up to the task of detecting these complex interactions.

Seizing upon a new development to visualize particle tracks, created by Donald Glaser and known as a bubble chamber, Alvarez immediately realized the potential of the device if only it could be made to function with liquid hydrogen. Hydrogen, comprising only protons, made the simplest and most desirable target for interactions with the particles produced by the Bevatron. He immediately began an intensive development program to build a series of small chambers, and championed the device to Ernest Lawrence.

The Glaser device was a small glass cylinder (1 cm × 2 cm) filled with ether. By suddenly reducing the pressure the liquid could be placed into a temporary superheated state which would only boil along the disturbed track of a particle passing through. Glaser was able to maintain the superheated state for a few seconds before spontaneous boiling took place. The Alvarez team quickly built chambers of 1.5 in, 2.5 in, 4 in, 10 in, and 15 in using liquid hydrogen and constructed of metal with glass windows so that the tracks could be photographed. Another breakthrough was to cycle the chamber quickly in synchronization with the accelerator beam, take the picture, and then recompress the chamber in time for the next beam cycle.

Ultimately, this program would build a liquid hydrogen bubble chamber almost 7 feet (2 meters) long, employ dozens of physicists and graduate students together with hundreds of engineers and technicians, 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. All of this work would result in the Nobel Prize in Physics for Alvarez in 1968

"For his decisive contributions to elementary particle physics, in particular the discovery of a large number of resonant states, made possible through his development of the technique of using hydrogen bubble chambers and data analysis."

In 1964, Alvarez proposed what became known as the High Altitude Particle Physics Experiment (HAPPE), originally conceived as a large superconducting magnet carried to high altitude by a balloon in order to study extremely high-energy particle interactions. In time the focus of the experiment changed more toward the study of cosmology and the role of both particles and radiation in the early universe. This work was a large effort, carrying detectors aloft with high-altitude balloon flights and high-flying U2 aircraft, and was an early precursor of the COBE satellite-born experiments on the cosmic background radiation (which resulted in the award of the 2006 Nobel Prize, shared by George Smoot and John Mather.)

X-Raying the Pyramids with Egyptologist Ahmed Fakhry and Team Leader Jerry Anderson, Berkeley, 1967 (Photo: LBNL)

In yet another stunning display of his imagination together with his ability to harness this imagination to realizable experiments, Alvarez proposed in 1965 to "X-Ray" the Egyptian pyramids to search for unknown chambers. Using the naturally occurring cosmic rays his ingenious scheme was to place spark chambers, standard equipment in the high-energy particle physics of this time, beneath the second pyramid of Chephren in a known chamber. By measuring the counting rate of the cosmic rays in different directions the detector would reveal the existence of any void in the overlaying rock structure.

Alvarez assembled an international team of physicists and archeologists from both the United States and Egypt, the recording equipment was constructed and the experiment carried out, though it was interrupted by the 1967 Six-Day War. Restarted after the war the effort continued recording and analyzing the penetrating cosmic rays until 1969 when Alvarez reported to the American Physical Society that no chambers had been found in the 19% of the pyramid surveyed.

In November 1966, Life Magazine published a series of photographs from the famous film that Abraham Zapruder took of the Kennedy assassination. Alvarez, an expert in optics and photoanalysis, became intrigued by the pictures and began to delve deeply into what could be learned from the film. The result of this was that Alvarez proved conclusively both in theory and experiment that the backward snap of the President’s head was completely consistent with his being shot from behind, which would have been the case if Lee Harvey Oswald were the assassin. He also investigated the timing of the gun shots and the shockwave which disturbed the camera, the speed of the camera, and pointed out a number of things which the FBI photoanalysts either overlooked or got wrong. While the results were not of enormous importance (the paper was intended as a tutorial), the pedagogical aspect of the paper, as well as its informal advice for the physicist intent on arriving at the truth is compelling.

Dinosaur Extinction

Luis and Walter Alvarez at the K-T Boundary in Gubbio, Italy 1981 (Photo: LBNL)

In 1980, Alvarez unveiled a scientific finale which would bring him greater worldwide recognition than all of his previous work. Working with his son Walter Alvarez, a geologist, the father–son team "uncovered a calamity that literally shook the Earth and is one of the great discoveries about Earth’s history"

Walter Alvarez was doing geological research in central Italy during the 1970s on the walls of a gorge whose limestone layers included strata both above and below the so-called K–T boundary (sometimes called the K-Pg), the boundary between the Cretaceous and Tertiary periods corresponding to a time of 65.95 million years ago (Kuiper et al., 2008). Exactly at the boundary is a layer of clay about 1 cm thick. Walter removed a small piece of the rock containing both sections of limestone and the clay layer and later showed it to his father. Walter said "This layer marks where the dinosaurs and much else went extinct. Nobody knows why. Or what the clay is about. A big mystery!", and intended to discover why from then on.

One of the first things Alvarez did was to try to figure out how long it had taken to lay down the centimeter of clay. It was not an easy question to solve considering the events happened 65 million years ago, but he thought he might be able to use the very slow deposition of certain elements, namely iridium, gradually being deposited on earth from the cosmos as a kind of clock. Elements of the platinum group, including iridium, are rare in interplanetary space, but much more abundant than on Earth. There is very little iridium in the Earth's crust, but the planet is constantly bombarded with micrometeorites which, as they burn up in the atmosphere, lightly dust the surface with iridium at a constant and known rate. Alvarez reasoned that if there were a lot of iridium in the clay layer, then the clay layer had formed over a long time, and if there was very little, then the clay layer had formed in a short time. So he went looking for iridium, but he was about to stumble upon one of great discoveries about the history of the earth.

Alvarez had access to the nuclear chemists at the Lawrence Berkeley Laboratory and was able to work with Frank Asaro and Helen Michel. The chemists used a technique known as neutron activation analysis and were astounded to discover that exactly at the clay boundary the iridium content was enormous, but not in the limestone on either side. Whatever had caused the iridium content in the clay, it was far too high to have come from micrometeorites. Carefully checking their work, the next step was to determine if the clay from other locations contained the same level of iridium (the K-Pg clay is well known and is distributed world wide), which it did. Within a few years of the publication of their paper, more than 100 iridium-containing clay sites were found. The team, knowing of no terrestrial source which could produce and deliver so much iridium, concluded that the source had to be extraterrestrial. And in the years following their publication the clay was also found to contain soot, glassy spherules, shocked quartz crystals, and microscopic diamonds and other rare minerals formed only under conditions of great temperature and pressure.

They considered a number of possible sources for the iridium anomaly; the passage of Earth through giant nebular clouds, a nearby supernova, and other low probability scenarios. With time, effort, and subsequent experimentation, all of these were eliminated. They were at last reduced to considering a direct impact on the earth by a comet or an asteroid; this was the only hypothesis which could satisfy all of the conditions. The impact theory causing the death of the dinosaurs was born.

After the publication of their seminal paper in 1980 an outcry was heard from some members of the geological community as the impact theory was a significant challenge to conventional theory, and an often acrimonious scientific debate ensued. Ten years after this initial proposal, evidence of a huge impact crater called Chicxulub off the coast of Mexico strongly confirmed their theory. Unfortunately for Alvarez, the finding of the crater was subsequent to his death. Other researchers would later find that the end-Cretaceous extinction event that wiped out the dinosaurs may have lasted for thousands of years instead of millions of years as had previously been accepted. While some scientists believe this extinction resulted from a point event such as an extraterrestrial impact, increased volcanism and climate change continue to be valid hypotheses, and all three are subject to continued study and correlation with the fossil record.

Publications

• "Berkeley Proton Linear Accelerator," Radiation Laboratory, University of California, Berkeley (Oct. 13, 1953).
• "High-energy Physics with Hydrogen Bubble Chambers," Radiation Laboratory, University of California, Berkeley (Mar. 7, 1958).
• “LRL 25-inch Bubble Chamber," Lawrence Radiation Laboratory, University of California, Berkeley (July 8, 1964).
• “Early Days of Accelerator Mass Spectrometry," Lawrence Berkeley Laboratory (May 1981).
• “The Hydrogen Bubble Chamber and the Strange Resonances," Lawrence Berkeley National Laboratory (June 1985).
• “History of Proton Linear Accelerators," Lawrence Berkeley Laboratory (Jan. 1987).

Encomium

Richard Feynman, considering whether to do the O-ring in-ice-water demonstration in the Challenger disaster hearings:

"I think, 'I could do this tomorrow while we're all sitting around, listening to this [Richard] Cook crap we heard today. We always get ice water in those meetings; that's something I could do to save time.' Then I think, 'No, that would be gauche.' But then I think of Luis Alvarez, the physicist. He's a guy I admire for his gutsiness and sense of humor, and I think, if Alvarez was on this commission, he would do it, and that's good enough for me."

See also

• Alvarez Physics Memos

External links

• Nobel biography
• About Luis Alvarez
• IEEE interview with Johnston, patentholder of the exploding-bridgewire detonator
• Weisstein, Eric Wolfgang (ed.). "Alvarez, Luis W. (1911-1988)". ScienceWorld.
• Annotated bibliography for Luis Alvarez from the Alsos Digital Library for Nuclear Issues
• Garwin, Richard L., 1992, "Memorial Tribute For Luis W. Alvarez" in Memorial Tributes, National Academy of Engineering, Vol. 5. Washington DC: National Academy Press.
• Biography and Bibliographic Resources, from the Office of Scientific and Technical Information, United States Department of Energy
• Oral History interview transcript with Luiz Alvarez 14, 15 February 1967, American Institute of Physics, Niels Bohr Library and Archives

Research resources

• Online guide to the Luis W. Alvarez Papers, The Bancroft Library