Carl David Anderson discovered the first antiparticle, a particle with the same mass as an electron but with a positive charge. His discovery represented the first evidence that antimatter exists and confirmed a prediction of relativistic quantum mechanical theory.

The first three decades of the twentieth century saw perhaps the most radical conceptual changes in the history of physics, especially at subatomic levels. To this period belong groundbreaking theoretical work on the quantum of energy (Max Planck), the quantum of light and the theory of relativity (Albert Einstein), the theory of atomic structure (Ernest Rutherford and Niels Bohr), the uncertainty principle (Werner Heisenberg), quantum mechanics (Erwin Schrödinger), and relativistic quantum mechanics (Paul Adrien Maurice Dirac). The early twentieth century was also a period of great experimental discoveries, particularly of hitherto unknown constituents of matter. The electronic charge was measured by Robert Andrews Millikan, and the charge-to-mass ratio was measured by Sir Joseph John Thomson. The alpha particle was discovered by Ernest Rutherford, and the neutron was discovered by James Chadwick. It seemed as though the basic structure of matter was being completely unveiled, as though physicists had discovered a way to describe subatomic matter exhaustively in terms of Schrödinger’s and Heisenberg’s quantum mechanical theories. [kw]Anderson Discovers the Positron (Sept., 1932)
[kw]Positron, Anderson Discovers the (Sept., 1932)
Positrons;discovery
Antimatter
[g]United States;Sept., 1932: Anderson Discovers the Positron[08120]
[c]Physics;Sept., 1932: Anderson Discovers the Positron[08120]
[c]Science and technology;Sept., 1932: Anderson Discovers the Positron[08120]
Anderson, Carl David
Dirac, Paul Adrien Maurice
Millikan, Robert Andrews

Thus, by the mid-1920’s, it had been established that matter consists primarily of heavy particles called protons, with a positive electric charge, and very light, negatively charged particles called electrons. The neutron—an electrically neutral particle, now known to be the third primary constituent of matter—had not been discovered yet. The neutron was discovered in 1932 by Chadwick.

Dirac, a physicist at St. John’s College, Cambridge, was convinced that the quantum theory and the theory of relativity needed to be combined. This combination was accomplished in 1928, when Dirac formulated relativistic quantum theory. The mathematically elegant theory led to the conclusion that every particle had to have an antiparticle—a counterpart that was oppositely charged and that had an opposite “spin” (a property of every particle that is difficult to measure). This conclusion meant that there should be positive electrons and negative protons. It even led Dirac to expect there to be antineutrons, which, like neutrons, would have no charge, but would have a spin opposite to that of neutrons. Dirac’s relativistic quantum mechanics thus predicted theoretically the existence of “antimatter.” Antimatter particles, however, had yet to be observed.

In 1927, Carl David Anderson, a physicist at the California Institute of Technology (Caltech) in Pasadena, started to study elementary particles by investigating cosmic radiation. Cosmic radiation Cosmic rays is a continuous stream of radiation, originating from nuclear reactions on the Sun and elsewhere in the cosmos, that flows into Earth’s atmosphere. It was not yet clear if these “cosmic rays” contained particles in addition to high-energy radiation. Early experiments could not detect any particles. Anderson began his work at Caltech in the Norman Bridge Laboratory of Physics under Millikan, the physicist who had first measured the charge on an electron. Their research goal was to find the nature of cosmic radiation. As Millikan’s junior research colleague, Anderson was first given the job of planning and directing the research.

The necessary experimental equipment was ready for operation in the summer of 1931. The technique used was to send up balloons containing instruments containing water vapor called cloud chambers. When a charged particle entered one of these chambers, it left a track. By studying the track, the mass and charge of the particle that caused the track were calculated. In addition, if a magnet was placed in the chamber so that the track was made in the presence of a magnetic field, the way the track curved would indicate whether the particle had a positive or negative charge.

Anderson and Millikan’s equipment took photographs every fifteen seconds. A very strong electromagnet was incorporated into the cloud chamber. The photographs showed several tracks of particles with very high energies. They also seemed to indicate that there were as many positive as negative particles in cosmic radiation, in many instances originating from the same point. The first explanation of the physicists’ empirical observations was that the positive particles whose tracks were observed must be protons, Protons and the negative ones must be electrons. Electrons Nevertheless, the experimental results seemed to show that the positive particles in the cosmic radiation showers had masses close or equal to those of electrons. (A proton has 1,835 times the mass of an electron.)

As Anderson stated in his 1936 Nobel lecture, the assumption that the positive particles had electronic mass “appeared very radical at the time.” Further refined experiments, however, indicated that the positively charged particles observed in cosmic radiation had to be light particles, like electrons, rather than heavier protons. Anderson published a paper in September, 1932, announcing the existence of positive electrons, or positrons.

Experiments by Patrick M. S. Blackett Blackett, Patrick M. S. and Giuseppe Occhialini Occhialini, Giuseppe at the University of Cambridge confirmed Anderson’s findings in 1933. Blackett and his coworkers suggested that the positive particle they had found was the antiparticle of the electron that Dirac’s theory had predicted. This discovery was the first evidence of the existence of antimatter. Dirac received the 1933 Nobel Prize in Physics (with Schrödinger) for his experimental confirmation of the theory. Nobel Prize recipients;Paul Adrien Maurice Dirac[Dirac] Anderson received the Nobel Prize in Physics in 1936 (with Victor Franz Hess) for his work on the positron. Nobel Prize recipients;Carl David Anderson[Anderson] Anderson went on to win many other awards for his efforts, such as the Gold Medal of the American Institute of the City of New York (1935), the Elliot Cresson Medal of the Franklin Institute (1937), and the Presidential Certificate of Merit (1945).

The collections of positive and negative particles originating in the same point noted by Anderson now had an explanation. Some of the energy in the cosmic radiation was changing into particles, forming an electron-positron pair, which then sped off in opposite directions under the influence of the magnetic field. This was the first observation of what is known as “pair creation” Pair creation (or pair production). It was also an example of the conversion of energy into matter, as predicted by Einstein’s famous equation E = mc²
. E = mc²
The opposite of pair creation is annihilation: When equal amounts of matter and antimatter collide, they annihilate each other, with their mass being turned into pure energy.

Some of the theories of the early history of the universe involve the conversion of energy into matter through pair creation. According to the big bang theory of the creation of the universe, fifteen billion years ago a gigantic explosion took place that created substance from energy. As many antiparticles as particles would have been created at that moment. The question arises as to why Earth’s part of the universe is predominantly composed of one type of particles (matter: protons, electrons, and neutrons) to the exclusion of the other type, antimatter. It is not known yet if there are galaxies that are predominantly composed of antimatter. Anderson’s discovery was the first evidence that this is a possibility.

The discovery of the positron—the first antiparticle to be discovered—furnished direct evidence supporting Dirac’s theory of the existence of antimatter. Because antimatter is scarce on Earth, scientists began to produce antimatter under laboratory conditions. These experiments required particles at very high energies. At these energies, it was found that many new kinds of reactions occured and new kinds of particles were produced.

The entire field of “high-energy physics,” High-energy physics[High energy physics] for which large and expensive particle accelerators are required, can be considered to have started with Anderson’s discovery of the positron. His discovery gave physicists a clearer understanding of elementary particles. Further, this branch of physics has led to the unveiling of a very complex set of particles and of new forces that operate in the subnuclear realm. Positrons;discovery
Antimatter

• Charlton, M., and J. W. Humberston. Positron Physics. New York: Cambridge University Press, 2001. A lengthy treatise on the study of positrons and of positronium (an unstable form of matter consisting of an electron and a positron in orbit around each other).
• Foot, Robert. Shadowlands: Quest for Mirror Matter in the Universe. Parkland, Fla.: Universal, 2002. An introduction for lay readers to antimatter, dark matter, mirror matter, and other “non-ordinary” types of matter that are predicted by quantum mechanics and relativity. Explains the evidence that such types of matter exist, as well as the implications of their existence.
• Guillemin, Victor. The Story of Quantum Mechanics. New York: Charles Scribner’s Sons, 1968. A general textbook discussion on the history of the development of quantum mechanics.
• Heathcote, Niels H. de V. Nobel Prize Winners in Physics, 1901-1950. New York: Henry Schuman, 1953. Entries contain the details of the discoveries for which the Nobel Prizes were awarded and give lengthy extracts from the Nobel lectures.
• Inman, Fred W., and Carl E. Miller. Contemporary Physics. New York: Macmillan, 1975. A modern survey of physics, this book divides physics into the classical and modern eras. The discoveries of the modern era, including that of the positron, are described.
• Kim, S. K. Physics: The Fabric of Reality. New York: Macmillan, 1975. Starting with the idea of absolute and relative motion, this book attempts to give a simple description of the central concepts of modern physics, including Dirac’s relativistic quantum theory, for the nonspecialist.
• Trefil, James S. The Moment of Creation: Big Bang Physics from Before the First Millisecond to the Present Universe. New York: Charles Scribner’s Sons, 1983. This book is a description of the theories of the starting point and development of the universe and includes a lengthy discussion of the role of matter and antimatter in this development.

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