Carl Anderson

A New York boy in a Pasadena basement

Carl David Anderson was born on 3 September 1905 in New York City, the son of Swedish immigrants. The family moved west while Carl was still a boy, and he grew up in Los Angeles in a household that did not own many books on physics. What it did own was a knack for tinkering, and Carl spent his school years pulling apart anything with a wire in it. When he enrolled at the California Institute of Technology in 1923, he intended to become an electrical engineer. He finished his undergraduate degree in 1927, in physics and engineering, and stayed on for the doctorate.

Caltech in those years was being assembled, almost from scratch, by Robert Millikan. Millikan had brought the oil-drop apparatus and the photoelectric measurements from Chicago and was now using cosmic rays as his next great hunt. He needed graduate students with patient hands and a willingness to sit in the cold next to a cloud chamber for entire nights. Anderson fit perfectly. He earned his Ph.D. in 1930, with Millikan as adviser, and stayed on as a research fellow.

Carl David Anderson (September 3, 1905 – January 11, 1991) was an American experimental physicist who shared the 1936 Nobel Prize in Physics with Victor Hess for his discovery of the positron, which confirmed the existence of antimatter.

A cloud chamber and a magnet

The instrument Anderson was using was a Wilson cloud chamber dropped inside a powerful electromagnet. A cosmic-ray particle blasting through the supersaturated vapour leaves a thin trail of droplets along its path. Slide the chamber into a magnetic field, and the trail curves. From the radius of the curve you read off the momentum; from the density of droplets along the track you estimate the speed and therefore the mass.

In the summer of 1932, while sorting through plate after plate of chamber photographs, Anderson found tracks that were the wrong shape. The curvature said they were as light as electrons, but they bent the opposite way through the magnet. The obvious explanation was that they were ordinary electrons running backwards through the chamber. To rule that out, Anderson slid a lead plate across the middle of the chamber. A particle passing through the plate loses energy, and a slower particle curves more tightly. The new tracks curved more tightly on the upper side of the plate, which meant they had been moving upward, which meant they were positively charged.

They were as light as electrons and positively charged. Anderson had no name for them, so in his 1932 paper in Science he coined one: the positron. The discovery was published in Physical Review the following spring under the title “The Positive Electron.” Paul Dirac’s relativistic wave equation, written in 1928, had predicted exactly such a particle, but Anderson said later that he had not been chasing the prediction. He had simply followed the photographs where they led.

”Who ordered that?”

In 1936 Anderson, now an associate professor at Caltech, gave a problem to his first graduate student, Seth Neddermeyer. They had been finding more odd tracks in the cosmic-ray chambers, tracks that curved like a charged particle several hundred times heavier than an electron but far lighter than a proton. After careful measurement they announced a new particle of mass about 207 times the electron.

For a moment everyone, Anderson included, thought it was the meson that Hideki Yukawa had predicted as the carrier of the strong nuclear force. It was not. The new particle interacted too feebly with nuclei to glue them together; it sat between the electron and the proton on the mass ladder, and it served no obvious purpose in the existing theory. The physicist I. I. Rabi, hearing the news, delivered the line that has clung to the particle ever since: “Who ordered that?” The particle is now called the muon, and the question Rabi asked in 1936 is still, in a sense, open.

The 1936 Nobel Prize in Physics went to Anderson and to Victor Hess, the Austrian who had first established that the radiation he was measuring really did come from space. Anderson was thirty-one, making him one of the youngest physics laureates ever. Hess delivered his lecture in Stockholm; Anderson, characteristically, sent a modest description of the cloud chamber and the curving tracks, and went back to Pasadena.

In 1936, Anderson and his first graduate student, Seth Neddermeyer, discovered the muon (or 'mu-meson', as it was known for many years), a subatomic particle 207 times more massive than the electron, but with the same negative electric charge and spin 1/2 as the electron, again in cosmic rays. Anderson and Neddermeyer at first believed that they had seen a pion, a particle which Hideki Yukawa had…

A quiet life at one institution

Anderson stayed at Caltech for the rest of his career. He became a full professor in 1939, served as chair of the physics, mathematics and astronomy division, and retired in 1976 after forty-six years on campus. He married Lorraine Bergman in 1946 and they had two sons. During the Second World War he worked on rocketry rather than the Manhattan Project, partly by choice and partly because the bomb project never approached him with the right offer.

He was not a self-promoter. He almost never gave interviews. Years after the positron paper he gave credit, generously, to a Caltech classmate named Chung-Yao Chao whose own cloud-chamber work had shown the same anomalous tracks a year earlier without their being correctly interpreted. Chao had never been listed as a co-discoverer at the time, and Anderson’s late acknowledgement was characteristic of the man.

Anderson died in San Marino, California, on 11 January 1991, aged eighty-five. His ashes were interred at Forest Lawn in Los Angeles.

In the quantum story, Anderson is the experimentalist who, with nothing more than a cloud chamber and a magnet, gave the world its first piece of antimatter and its first surprise particle from the hidden zoo to come.