Positron discovery
The discovery of the positron, a particle with the same mass as an electron but with a positive charge, marked a significant milestone in the field of physics during the 1930s. This finding emerged from the theoretical groundwork laid by Paul A. M. Dirac's equations, which suggested the existence of antimatter as a counterpart to ordinary matter. The breakthrough occurred on August 2, 1932, when California Institute of Technology researcher Carl David Anderson, using cloud chambers to analyze cosmic rays, captured a photograph of this newly identified particle. Initially met with skepticism, Anderson’s claim was later validated by British researchers, further establishing the reality of antimatter and confirming Dirac's predictions.
The implications of this discovery extended beyond particle physics; it illustrated the principle of energy-mass equivalence as later described by Albert Einstein’s famous equation E = mc². Anderson's pioneering work earned him a share of the Nobel Prize in Physics in 1936, cementing his role in this transformative moment in science. The positron discovery not only enriched our understanding of the universe at a fundamental level but also opened new avenues for research in high-energy physics and cosmology.
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Positron discovery
The Event Discovery of the first experimental proof of the existence of positrons
Dates August-September, 1932
Place California Institute of Technology, Pasadena, California
The positron was the fourth subatomic particle discovered and proved the existence of antimatter.
Prior to the 1930’s, physics had undergone radical change. Even the ultimate constituent of matter was unknown, as the discoveries of both the electron and the proton showed that the atom itself consisted of small particles. Equations in Paul A. M. Dirac’s relativistic quantum mechanics, derived in 1928, predicted that each particle would have a “twin” with the opposite charge; this twin was eventually called “antimatter.” However, no such particles had been detected at that time.
One place where subatomic particles could be detected was in cosmic rays—high-energy radiation emanating from outer space. Working under cosmic-ray researcher and Nobel laureate Robert Andrews Millikan, California Institute of Technology researcher Carl David Anderson designed an experiment to determine the nature of cosmic radiation. He sent balloons into the atmosphere equipped with cloud chambers. These instruments could photographically track particles that passed within them. The tracks determined the mass of a particle, and as the cloud chambers were equipped with strong magnets, the curvatures of the tracks determined whether a particle was positively or negatively charged.
Initial results seemed to show a few particles with the mass of an electron, but the positive charge of a proton, a particle nearly two thousand times heavier. After making additional instrument refinements, on August 2, 1932, Anderson obtained a photograph showing unambiguously a positively charged particle with the mass of an electron. He quickly announced to the world the discovery of a new particle, the “positive electron,” later known as the positron.
Impact
Anderson’s initial claim was met with skepticism. However, in early 1933, British researchers Patrick M. S. Blackett and Giuseppe P. S. Occhialini confirmed Anderson’s results, thereby also confirming both Dirac’s equations and the existence of antimatter. Later work also showed that a high-energy photon could instantaneously change into an electron/positron pair, energy changing into mass as Albert Einstein’s E = mc2 predicted. For his work, Anderson shared the 1936 Nobel Prize in Physics.
Bibliography
Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth-Century Physics. Rev. ed. New Brunswick, N.J.: Rutgers University Press, 1996.
Kevles, Daniel J. The Physicists: The History of a Scientific Community in Modern America. New York: Alfred A. Knopf, 1977.
Segrè, Gino. Faust in Copenhagen: A Struggle for the Soul of Physics. New York: Penguin, 2007.