Star Trek fans are familiar with the idea of antimatter. It powers the mythical future starships. It sounds like a wild science fiction concept, but physicists study real antimatter particles. Antimatter exists in cosmic rays, is produced in thunderstorms, and as alluded to in Star Trek, annihilates with matter to produce energy.
Theoretical Prediction of Antimatter
In the 1920s P.A.M. Dirac, a British theoretical physicist, developed a formulation of quantum mechanics that included the effects of Einstein’s special relativity theory. Dirac’s equation describing the total energy of the electron included a square root. It is often ignored, but mathematically square roots always have two solutions: a positive and a negative solution.
The negative solution led Dirac to predict that there is a particle corresponding to the electron, which physicists call the positron or the antielectron. According to the theoretical equations, the positron has exactly the same mass as the electron, but it has the opposite electric charge. The amount of charge is equal, but the positron has a positive electric charge compared to the electron’s negative charge. Dirac shared the 1933 Nobel Prize in Physics for his prediction.
Based on Dirac’s theory and subsequent experimental discoveries, physicists eventually concluded that each matter particle has a corresponding antimatter particle with the same mass and opposite electric charge.
Elementary particles have other properties besides mass and charge. One of these properties is spin, which measures the intrinsic angular momentum of the particle. Antimatter particles have the same spin as the corresponding matter particles.
If an electric charge distribution is spinning, it will have a magnetic effect which physicists call the magnetic moment. Because antiparticles have the opposite charge yet same spin as their corresponding particles they also have the opposite magnetic moment.
In the 1950s physicists started discovering many more exotic elementary particles. Some of these particles decayed more slowly that physicists would have expected, so they were labeled as strange. Physicist then began assigning strangeness numbers to elementary particles as a measure of the strangeness property. Antiparticles have a strangeness that, like charge, is equal and opposite to the corresponding particles.
Discovery of Antimatter
Carl Anderson shared the 1936 physics Nobel Prize for discovering the positron that Dirac had predicted. This discovery helped convince physicists that Dirac’s theory had physical reality and that positrons were more than a mathematical quirk.
Anderson studied cosmic rays with a Wilson cloud chamber. Cloud chambers produce tracks of elementary particles. Anderson found tracks that had spiraled in the opposite direction as electrons, indicating that they had the same mass but opposite charge as electrons.
In 1955 physicists Emilio Segrè and Owen Chamberlain used a new particle accelerator to discover the first antiprotons and antineutrons. They won the 1959 Nobel Prize for this discovery.
Matter-Antimatter Annihilation and Pair Production
Matter particles occasionally come into contact with their corresponding antimatter particles. When this happens, the two particles mutually annihilate each other in a burst of high energy gamma-rays. The gamma-ray energy depends on the total mass of the particles according to Einstein’s mass energy equivalency equation. Any kinetic energy the particles may have also adds into the total gamma-ray energy.
The reverse reaction can also happen. Under the right conditions a sufficiently energetic gamma-ray can produce a particle-antiparticle pair. Pair production usually occurs when the gamma-ray interacts with an atomic nucleus. The nucleus recoils to allow momentum to be conserved in the reaction.
Despite sounding like science fiction, antimatter is a real concept in physics. Perhaps in the distant future, our descendants will use it to power starships.