Carl Anderson was born in 1905 to Swedish immigrant parents. He pursued a degree in engineering at Caltech, graduating in 1927. By 1930, he had received a Ph.D. in Physics under the supervision of Robert A. Millikan.
Millian had received the 1923 Nobel Prize in Physics for his measurement of the electric charges carried by the proton and the electron. This "elementary charge" is considered a fundamental physical constant.
Millikan was also one of the discoverers of the photoelectric effect, for which Albert Einstein received the Nobel Prize in Physics in 1922.
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In 1932, as a postdoc, Anderson began investigating cosmic rays which are high-energy protons and atomic nuclei (protons and neutrons) that travel through space at almost the speed of light.
Cosmic rays originate in our sun, outside of the solar system, in distant galaxies and in supernova explosions. Their existence was first discovered in 1912 through balloon experiments.
99% of cosmic rays are the nuclei of atoms that have been stripped of their electron shells, and 1% are electrons. Of the nuclei, 90% are protons, or just hydrogen atoms, 9% are alpha particles, which are identical to the nuclei of helium, and 1% are the nuclei of heavier elements.
However, a very small fraction of cosmic rays is something that in 1932 had never been seen before - particles of antimatter, such as positrons or antiprotons.
The cloud chamber
Anderson was able to see cosmic rays in what ultimately became known as an Anderson Cloud Chamber. It is a sealed environment that contains a supersaturated vapor of either water or alcohol. When a charged particle from a cosmic ray streams through the cloud chamber, it knocks electrons off of the gas molecules inside, and this creates a trail of ionized gas particles.
A mist-like trail appears along the track of the cosmic ray that persists for several seconds. The tracks of alpha particles are straight and thick, while the track of electrons is wispy and curved.
Anderson began photographing the cosmic ray tracks, and in one such photo, there appeared a curved track. Anderson realized that the track could only have been made by a particle that has the same mass as an electron, but an opposite, or positive, charge. Anderson called this new particle a positron.
A particle "zoo"
The positron was the first identified antiparticle. Antiparticles had first been proposed in 1928 by English theoretical physicist Paul Dirac. He proposed that every atomic particle has an antiparticle that shares the same mass, but has the opposite electric charge and other quantum differences. For his discovery, Dirac was awarded the 1933 Noel Prize in Physics along with Erwin Schrodinger.
Following his discovery of the positron, in 1936, Anderson discovered another charged particle in cosmic rays. This new particle had a mass one-tenth that of a proton and 207 times the mass of an electron. It was negatively charged and had spin 1/2, the same as an electron. Anderson named this new particle a "mesotron", but it quickly became known as a meson.
At first, it was thought that this new particle was a pion, which had been predicted by Hideki Yukawa two years earlier in his theory of the strong interaction.
When it became clear that Anderson's new particle wasn't the pion, the physicist I.I. Rabi famously asked, "Who ordered that?" Eventually, Anderson's meson was deemed a mu meson, also known as a muon, and Yukawa's meson became the pi meson, which is also known as the pion.
Anderson's discovery was the first of a long list of newly discovered subatomic particles that became known as the "particle zoo". This was due to physicists' inability to categorize them into a coherent scheme. It wasn't until the discovery of quarks in the late 1960s that the Standard Model of particle physics began to emerge. Today, we know all matter to be comprised of quarks, bosons, and leptons.
Carl Anderson spent his entire career at Caltech, and during World War II, he conducted rocketry research there. Anderson died in 1991.