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Luis Alvarez: Master Inventor

Luis Alvarez
Image: Dutch National Archives
The physicists of yesteryear were a colorful bunch, often dabbling in one discipline for a time before jumping into something entirely new. To many, the bongo-playing, safe-cracking, Nobel Prize-winning Richard Feynman is the quintessential renaissance-man scientist.
There were others. One of Feynman's contemporaries, Luis Alvarez was the man who could seemingly build anything. He was everywhere and played an important role in a surprising number of the big physics discoveries during the middle of the century. 
As a graduate student at the University of Chicago in the mid-1930s, he kluged together a series of Geiger counters onto a wheel barrow, turning it into a state of the art particle detector. He brought it to the top of a hotel in Mexico City, a place with the perfect mix of altitude and latitude to catch the then mysterious solar wind. There he found that the particles flew in mostly from the west, meaning they had to be protons with a positive charge moving laterally across Earth's magnetic field. 
Alvarez then joined Ernest Lawrence's Rad Lab at Berkley and started making major contributions to fundamental nuclear science. He proved a process called K-electron capture can happen, where an atom's innermost electron is absorbed by a proton in the nucleus and emits an X-ray. 
Even more importantly however, he discovered the process that would ultimately lead to the hydrogen bomb. He used his 60-inch cyclotron to bombard the hydrogen isotope deuterium (one proton and one neutron) with more deuterium and measure the products. It turned out, contrary to what people had predicted, that the hydrogen-3 isotope it sometimes produces was unstable, while helium-3 wasn't. Years later, physicists would harness this instability for use in fusion experiments. 
Then World War II started, and Alvarez went to MIT and invented a radar system for planes to land in bad weather. Halfway through the war he transferred to Los Alamos where scientists were building the first atomic bomb. After arriving in New Mexico, he built the firing mechanism for the bomb, and a way to determine its strength by recording changes in air pressure. When the bombs were dropped on Hiroshima and Nagasaki, he flew in a chase plane to measure the explosions.
Alvarez in front of the B-29 that carried him over Japan. Image: LBNL

After the war he went back to Berkeley and built the first practical hydrogen bubble chamber, a device that would become the standard tool for particle physicists for a generation. A high speed particle would fly through the liquid inside, boiling off the hydrogen as it zipped by. The boiling liquid left a small trail of bubbles behind, photos of which could be used to figure out what had passed through. By 1968, he and his team had helped discover so many new particles with the device, he won a Nobel Prize for his work
Photo of particle trails inside a bubble chamber.
Image: Argonne National Lab
He left working on nuclear physics and spent most of the rest of his career developing better detectors and new uses for them. In the late '60s he led a team hunting for secret rooms in the pyramids of Egypt using cosmic rays. When a high energy particle strikes a molecule of the atmosphere, it creates a shower of muons which can whizz through stone. Alvarez figured that by looking for spots that didn't seem to absorb many muons, there might lie an undiscovered room. After years of searching, he came to the conclusion that there weren't any. 
Starting in the 1970s, Luis and his geologist son Walter were the first scientific team to come up with the theory that a meteorite had killed off the dinosaurs 65 million years ago. It was a hugely controversial theory at the time, but evidence kept mounting. Years after Luis's death in 1988, surveyors found evidence of exactly the kind of massive impact crater off the coast of Mexico. Team Alvarez had it right from the start.


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