In 1932, Carl Anderson Saw a Particle Curve the Wrong Way, And Discovered the Positron

Anderson performed experiments with cosmic rays at the California Institute of Technology in 1932, when he employed the cloud chamber to make the particles' tracks visible. Usually, the behavior of the particles conformed to what was expected from Anderson; however, there were some tracks that did not conform to the usual behavior of the particles. One particular particle moved against its natural behavior in the magnetic field, having the same mass but opposite electric charge compared to the electrons.

It intrigued Anderson because the particle behaved contrary to what Anderson knew. At the time of his discovery, Anderson had not thought of seeking anti-particles, as he was just measuring the properties of the cosmic rays. As mentioned in the archives of Caltech, the key photograph that was taken by Anderson of the particle's path was taken on August 2, 1932. In addition, it was identified as a positive electron. Masses did not have to be precisely measured since they could easily be interpreted by the direction and the curvature of the particle.

Science has shown us various examples where minor facts led to great discoveries. Anderson did not ignore his observation but analyzed it together with other similar observations.

How Did One Image Become Evidence?

In a cloud chamber, the presence of charged particles could create visible trails, which, when subjected to a magnetic field, would show curvature depending on the charge and momentum of the particles. The path of Anderson’s particle had an opposite curvature to the electron trail but followed similar curves. This made it possible for him to deduce that this was a particle different from the electron and positron. According to some subsequent discussions, and even Anderson himself in his Caltech oral history, cosmic ray experiments often led to the formation of numerous ambiguous tracks. In order to determine a particle, one had to infer from the available data.

This finding was subsequently confirmed, and Anderson’s particle was named the positron. A publication at the 90th anniversary review in PubMed Central refers to this as the first instance of the experimentally proven anti-matter particle and a milestone from purely theoretical prediction to experimentally observed fact. This was indeed essential. Prior to Anderson’s experiment in 1932, antimatter remained only theoretically known.

This Discovery Changed Physics

This was much more than just adding one more item to the list of known particles. It transformed the way scientists viewed symmetry in nature. The possibility that particles existed that were completely similar to those known before, yet different in charge, led scientists to consider the idea of antiparticles. This idea later resulted in new developments in particle physics, including finding other antiparticles and developing theories explaining the essence of matter. The significance of the positron goes far beyond science itself. Medical technology based on positron emission tomography would never have appeared without this discovery of 1932.

However, the importance of the method used should also be acknowledged. Without the possibility of capturing transient phenomena and converting them into understandable data, the discovery could never have been made. The cloud chamber could hardly be regarded as the ultimate solution, but it gave enough hints to realize something new.