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March 21, 2022

Antimatter, in the form of an antimatter electron called a positron, was predicted by Paul Dirac (typically referred to as P.A.M. Dirac, 1902-1984) in 1931. The positron, called an anti-electron by Dirac, was predicted to have the same mass and the opposite charge as an electron, and it had the interesting property that it would annihilate an electron on contact to produce energy. One interesting antimatter concept is the Feynman-Stueckelberg interpretation that antiparticles are actually regular particles that are traveling backwards in time. The existence of the positron was confirmed by experiment quite quickly, by Carl D. Anderson (1905-1991) in 1932 who observed it in cloud chamber photographs of cosmic rays, and it received its positron name in the paper announcing the discovery.[1-2]

Being the first to publish, Anderson won the Nobel Prize in Physics in 1936, but there was contemporaneous evidence from other research groups. Frédéric Joliot-Curie (1900-1958) and Irène Joliot-Curie (1897-1956), the elder daughter of Pierre Curie (1859-1906) and Marie Curie (1867-1934), had photographs with evidence of positrons, but they thought they were protons. Patrick Blackett (1897-1974) and Giuseppe Occhialini (1907-1993) of the Cavendish Laboratory discovered many opposing photographic spirals of positron-electron pair production in 1932, but they delayed publication to obtain more data. This may have been a consequence of the research philosophy of Cavendish director, Ernest Rutherford (1871-1937);[3] or, as they say," extraordinary claims require extraordinary evidence."

Nobel Prize diploma for Pierre and Marie Curie, 1903

Nobel Prize diploma for Pierre Curie (1859-1906) and Marie Curie (1867-1934), 1903 (left) and a circa 1902 photograph of them with daughter, Irène Curie (1897-1956). They received the Nobel Prize in Physics "for their joint researches on the radiation phenomena discovered by Professor Henri Becquerel". The prize was shared with Henri Becquerel (1852-1908), the discoverer of spontaneous radioactivity. Their daughter, Irène Joliot-Curie (1897-1956), and her husband, Frédéric Joliot-Curie (1900-1958), were awarded the 1935 Nobel Prize in Chemistry for their discovery of induced radioactivity. (Left, Wikimedia Commons image uploaded and retouched by Jebulon, and right, another Wikimedia Commons image. Click for larger image.)

After the discovery of the positron came the more difficult discovery of the antiproton by Emilio Segrè (1905-1989) and Owen Chamberlain (1920-2006) in 1955, for which they received the 1959 Nobel Prize in Physics. Having positrons, antiprotons, and antineutrons allows the existence of antimatter equivalents of the chemical elements, and both antihydrogen and antihelium have been observed.

Alchemists weren't successful in turning lead into gold, but positron emission is capable of changing one element into another. Emission of a positron and an electron neutrino from a nucleus converts a proton into a neutron, thereby decreasing the atomic number; for example,

11C -> 11B + e+ + νe

in which carbon with an atomic number of 6 is transformed into boron with an atomic number of 5. Natural positron emission occurs rarely on Earth, generally induced by a cosmic ray or from a small percentage of decays of the rare isotope, potassium-40. Some other isotopes that exhibit positron decay are nitrogen-13, oxygen-15, yttrium-86, strontium-83, and iodine-124. Positrons have proven useful in medical scans known as positron emission tomography.

Long before Star Trek fueled its warp engines with an annihilation reaction of antideuterium and deuterium, moderated by Dilithium, antimatter had become a topic in science fiction.[4] Dirac's 1933 Nobel Prize lecture speculated that stars other than our Sun might be composed of antimatter, as would their planets.[4] John W. Campbell, Jr., who was editor of Astounding Science Fiction magazine, decided that this idea would make a good science fiction story, so he recruited a member of his author cohort, Jack Williamson (1908-2006), to write such a story. Williamson's story, "Collision Orbit," was published in the July 1942 issue of Astounding Science Fiction under the pen name Will Stewart.[4] The story envisions harvesting antimatter using magnetic fields, and three sequels were later published.[4]

Antimatter particles have charge and magnetic moment of equal magnitude but opposite sign, and their mass appears to be the same. There has been a continued effort to verify this equality of mass between protons and antiprotons. A huge international team has just determined that the antiproton-to-proton charge/mass ratio is the same at a 16 parts per trillion level, the implication being that the masses of the proton and antiproton are equal.[5-7] The team has members from RIKEN (Saitama, Japan), the Leibniz Universität Hannover (Hannover, Germany), Physikalisch-Technische Bundesanstalt (Braunschweig, Germany), CERN (Meyrin, Switzerland), the Max-Planck-Institut für Kernphysik (Heidelberg, Germany), the University of Tokyo (Tokyo, Japan), GSI-Helmholtzzentrum für Schwerionenforschung (Darmstadt, Germany), the Johannes Gutenberg-Universität (Mainz, Germany), and the Johannes Gutenberg-Universität (Mainz, Germany). Their results were published in a recent issue of Nature.[5]

This is the latest result from the Baryon Antibaryon Symmetry Experiment (BASE) at CERN, and the measurement is an improvement of a previous measurement that had a 1.5 parts per billion precision.[5-6] BASE uses antiprotons from CERN's antimatter factory.[6-7] The result derives from the combination of four independent long-term studies done over a span of 1.5 years, from December 2017 to May 2019.[5,7] These studies used different measurement methods and experimental set-ups, so they had different systematic errors.[5]

In order to compare protons and antiprotons directly, the experiment uses negatively charged hydrogen ions as the proton reference so the negatively charged particles can both be measured in the same device called a Penning trap.[6-7] The Penning trap uses magnetic fields and electric fields to trap the particles in a cyclotron resonance, and the frequency of the resonance gives the charge-to-mass ratio.[6-7] The mass difference between the hydrogen ion and a bare proton is known with an uncertainty of 0.03 parts per trillion.[6-7] The experiment obtained more than 24000 cyclotron-frequency comparisons, each lasting 260 seconds.[7]

The BASE antiproton Penning trap system

The Baryon Antibaryon Symmetry Experiment (BASE) Penning trap.

CERN image.

According to the standard model of particle physics, matter and antimatter should be mirror images of each other with regard to CPT symmetry, and they should have equal mass, equal but opposite charge, and respond equally in magnitude to the fundamental forces.[6] Any slight difference between the masses of protons and antiprotons, or between the ratios of their electric charge and mass, would violate CPT symmetry and indicate new physics beyond the standard model.[7] Such a difference might explain why the universe is nearly devoid of antimatter, although equal amounts of antimatter and matter should have been created in the Big Bang.[7]

Since the experiment was conducted over multiple years, it was possible to test the so-called weak equivalence principle, which requires matter and antimatter to fall at the same rate in the same gravitational field.[6-7] Since the Earth has an elliptical orbit around the Sun, the Earth-Sun distance changes and the Sun's gravitational potential is about 3% stronger in January than in July.[6] The research team could find no variation in the cyclotron frequency ratio between winter and summer.[6]

Views of the BASE experimental zone, located in the CERN antiproton decelerator.

As these views of the Baryon Antibaryon Symmetry Experiment (BASE) show, experimenters need to be electronics experts and plumbers. The experiment is located in the CERN antiproton decelerator. (CERN photographs by Stefan Ulmer, who was an author of the study.)


  1. Carl D. Anderson, "The Positive Electron," Physical Review, vol.43, no. 6 (March 15, 1933), pp. 491ff., DOI:https://doi.org/10.1103/PhysRev.43.491.
  2. This Month in Physics History - Discovery of the Positron, August 1932, APS News.
  3. Richard Reeves, "A Force of Nature: The Frontier Genius of Ernest Rutherford," W. W. Norton & Company, December 17, 2007, 208 pp., ISBN 978-0393057508 (via Amazon).
  4. William S. Higgins, "Antimatter's science fiction debut, Fermilab Symmetry Magazine, September 1, 2008.
  5. M. J. Borchert, J. A. Devlin, S. R. Erlewein, M. Fleck, J. A. Harrington, T. Higuchi, B. M. Latacz, F. Voelksen, E. J. Wursten, F. Abbass, M. A. Bohman, A. H. Mooser, D. Popper, M. Wiesinger, C. Will, K. Blaum, Y. Matsuda, C. Ospelkaus, W. Quint, J. Walz, Y. Yamazaki, C. Smorra, and S. Ulmer, "A 16-parts-per-trillion measurement of the antiproton-to-proton charge–mass ratio," Nature, vol. 601, no. 7691 (January 5, 2022), pp. 53-57, https://doi.org/10.1038/s41586-021-04203-w.
  6. Michael Schirber, "Antiproton Mirrors Proton," Physics, vol. 15, no. 8 (January 19, 2022).
  7. BASE breaks new ground in matter–antimatter comparisons, CERN News, January 5, 2022

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