Carl David Anderson

American Physicist, Discovered the Positron and the Muon

Background

His father, also called Carl David, was a Swedish immigrant and the family moved to Los Angeles when Carl was just a boy. He started to study Electronic Engineering at California Institute of Technology (Caltech), but changed to physics in his first year. Anderson took J. Robert Oppenheimer’s graduate course in quantum mechanics and stayed with the course even after everybody else dropped out. He received his Ph.D. magna cum landa (top of the class) in 1930 and he remained at Caltech for the rest of his life becoming a professor in 1939.

Cosmic Rays

Anderson’s research advisor was Noble Laureate Robert A Millikan, he suggested that Anderson should study cosmic rays and modify a newly invented device called a Cloud Chamber to study these rays. He designed and built a cloud chamber with powerful electromagnet surrounding his chamber on the top floor of the aeronautics building at Caltech, where there was adequate electric power to run the large electromagnet.

The Positron

After two years studying photos for the cloud chamber, in 1932 Carl took a photograph of a positively charged electron passing through the lead plate in the center of the cloud chamber and moving upwards. It was definitely a positively charged particle and a measure of it mass showed that it was close to that of the electron. Carl reported that he found a new particle, called the ‘Positron’ which had the same mass as the electron but a positive charge. This particle was predicted by Paul Dirac in 1928 but several well known scientists, including Niels Bohr, were sceptical of his findings. It was not accepted until March 1933 when Blackett and Giuseppe Occhialini of Cavendish Laboratories confirmed its existence.

The New Particle and the High Mountain

The best place to examine cosmic rays is at high altitude, so Anderson and his graduate student Seth Neddermeyer took a cloud chamber to the top of Pike’s Peak, they were towed most of the way up. Click here for a full account of the story by Wiliam H. Pickering a colleague of Andersons. During their time on the mountain they took 10,000 photographs and found evidence of a new particle with a mass between that of a proton and an electron. They first called it a mesotron, but it was later called a mu meson and then shorten to meson. This particle had also been predicted by Hideki Yukawa in 1935, but a meson found by Blackett and Giuseppe Occhialini which was heavier then Anderson’s meson fitted better with Yukawa theory. Anderson’s called his particle an “oddball partricle” as it did not fit in with the current model, it was then called a muon.

The War

During World War II, Anderson joined the Caltech artillery rocket project for the Navy. The project developed solid propellant rockets that were launched from ship and planes. He travelled to the Normandy beachhead to see the use of Caltech rockets and help the allies to make them more effective. He was asked by Compton to Head the Manhattan Project to develop the atomic bomb but turned it down as he believed that he didn’t have enough administrative experience and his mother was not well.

The Prize

Carl Anderson won the noble Prize in 1936 with Victor Hess, who was the first to discover cosmic rays. At the time Anderson was so broke that he had to borrow $500 from Millikan for the flight to Sweden for the ceremony. Half of the $20,000 which is part of the prize went to pay for his mother medical expenses.

The Dinner

“In 1962 President Kennedy held a White House dinner for Nobel laureates, and Carl attended. He was honoured to be seated at a table with the Swedish ambassador’s wife on his right, Mrs. Ernest Hemmingway on his left, the President next to Mrs. Hemmingway, and Mrs. George Marshall on the President’s right. Carl recalled the President’s comment that this was the greatest gathering of talent at a White House dinner since Thomas Jefferson dined there alone.” William H. Pickering

The Human Scientist

Anderson’s anti-matter was the first step that led to an understanding of the atomic nucleus. He stayed at Caltech until he retired in 1976 and became a member of the board of the college. An article in Scientific American stated that Andersons “very approachable manner brands him as one of the more human scientists”.

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