This is how Carl Anderson discovered antimatter

Aug 30, 2021 1 comments
positron antimatter carl anderson paul dirac

British physicist Paul Dirac showed in 1928 that every particle in the universe should have an antiparticle with the same mass as its twin, but with the opposite electrical charge. Across the pond, an American physicist would detect the first such particle, four years later.

Carl Anderson, inspired by the work of his Caltech classmate, Chung-Yao Chao, set up an experiment to investigate cosmic rays under the supervision of physicist Robert Millikan. In 1932, he won the Nobel Prize in physics at the age of 31, becoming one of the youngest recipients.

Discovering the positron was no easy feat but the mechanism he employed to do so was fairly simple and ingenious enough to overcome the limited budget. He found the mysterious particle almost by accident with the help of his own improved version of the cloud chamber.

A cloud chamber is a sealed box with water vapor. When a charged particle goes through it, the vapour is ionized and leaves behind a trail. Thus, the trajectory of the particle can be seen virtually. Carl used a mixture of water and alcohol to get clearer photographs.

carl anderson cloud chamber positron antimatter paul dirac

Carl included a Lead plate in the middle to slow down the particles and surrounded the chamber with a large electromagnet, which caused the paths of ionizing particles to curve under the influence of magnetic field.

As can be seen in the picture, the radius of curvature of the track above the plate is smaller than that below. Thus, the particle entered from the bottom, hit the Lead plate and came to a halt above it due to loss of energy. This and the direction in which the path curved helped in identifying that the charge was positive.

That it was antielectron and not proton was determined by the observation that the upper track was much longer in length than predicted for proton. A proton would have come to rest in a much shorter distance. The trajectory observed was that of a particle much much lighter than the proton.

So, that's how the first antimatter was found and Dirac was proven right within a matter of few years. Furthermore, antiproton and antineutron were discovered in 1955 and 1956 respectively. The first antiatom was produced by CERN in 1996.

Why antimatter is important? Because, studies related to antimatter will help in our understanding of the early universe. Also, Positron emission tomography or PET scan is used to detect early signs of cancer. Scientists hope that some day, antimatter may be used for the treatment of cancer. Who knows!?


  1. Physics is always changing. When a new theory came in to exist, existing theory will collapse! now it is true to gravity theory and General relativity. Now a new theory called "Fourth Law of Motion " thrown away both gravity theory and general relativity theory. These two theories available in my web


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