In 1932, American physicist Carl Anderson was using a simple cloud chamber to study cosmic
rays—extremely fast-moving particles (usually protons) that come from outer space. When a cosmic ray strikes the relatively
slow-moving atoms that comprise the earth and its atmosphere, it produces a spray of “secondary particles” called a cosmic ray shower. Some of these secondary particles come from atoms that have been shattered—that is, they come from nuclear fission caused by the cosmic ray. Other secondary particles are created from the cosmic ray’s kinetic energy, in the same way that new particles are created by colliding protons in the Large Hadron Collider (as discussed on the previous page).
Anderson noticed that some of the secondary particles produced by cosmic rays behaved strangely when moving through a magnetic field. As you may recall from chapter 2, charged particles experience a sideways force when moving through a magnetic field. (This fact enabled J.J. Thomson to measure the mass-to-charge ratio of electrons, as discussed in chapter 4.) The direction of the sideways force depends on whether the particle is positively or negatively charged. For this reason, positively-charged particles curve in one direction while moving through a magnetic field, while negatively-charged particles curve in the opposite direction. The mysterious particles in Anderson’s cloud chamber curved the same direction as protons, indicating that they had a positive charge. But they curved much more sharply than protons, indicating that they had far less mass and hence could be accelerated more easily. In fact, their condensation trails in the cloud chamber looked just like the trails made by electrons, except that they curved the wrong direction! Anderson concluded that these unknown particles had the same mass and amount of charge as electrons, but their charge was positive rather than negative. He called them “positrons.”
Similar observations had been made previously by other physicists, but no one else had bothered to investigate these particles further. Anderson continued to study them, and found that the same types of particles are sometimes produced when atoms are struck by high-energy gamma rays from radioactive decay. In 1936, he won the Nobel Prize in Physics for these discoveries. The term positron (short for “positive electron”) was first introduced in Anderson’s 1933 paper, “The Positive Electron.” The paper, which includes Anderson’s original photographs of positron tracks in a cloud chamber, can be downloaded here.
As it turns out, the positron is just one of many elementary particles of antimatter. Antimatter is just like matter, but with opposite electromagnetic properties. For each type of matter particle, there is a corresponding type of antimatter particle—or antiparticle—with the same mass but opposite charge. Other properties related to electromagnetism are also reversed. For example, a quantum-mechanical property called “spin” can effect a particle’s behavior in a magnetic field. (We’ll learn more about spin later in this chapter, and also in chapter 7.) Even uncharged particles have spin, so uncharged elementary particles of matter—uncharged leptons called neutrinos—also have antiparticles. Specifically, neutrinos and antineutrinos have opposite helicity, or “handedness,” which means that they spin in opposite directions relative to their directions of motion. Neutrinos are “left-handed,” while antineutrinos are “right-handed.” If you don’t understand what that means, it’s ok—physicists don’t really understand it either. It’s just another fundamental (unexplained) property that some particles have. Spin is analogous in some ways to rotation; that’s why it’s called “spin.” But as we’ll discuss later, elementary particles don’t really spin or rotate in the ordinary sense, and there are numerous ways in which quantum-mechanical spin is quite different from the spinning of wheels and other macroscopic objects. In all, there are 12 elementary particles of antimatter, corresponding to the twelve elementary particles of matter: antiquarks corresponding to each of the six types of quarks, and antileptons corresponding to each of the six types of leptons.
Composite particles of matter—i.e., non-elementary particles like protons, neutrons, atoms, and molecules—also have antiparticles. For example, an antiproton has the same mass as a proton but is negatively charged. It consists of 2 up antiquarks (with charge of -⅔ e) and 1 down antiquark (with charge of +⅓ e), for a total charge of -1 e. An antineutron consists of 1 up antiquark and 2 down antiquarks, for a total charge of zero. (So, an antineutron and a neutron have the same mass and the same charge, but the antineutron is made of antiquarks instead of quarks.) Antiprotons, antineutrons, and positrons can come together to form an entire antiatom, in which positrons orbit a nucleus made of antiprotons and antineutrons. Antiatoms could come together to form antimolecules, and so on. Theoretically, an entire universe—full of stars, planets, and even living creatures—could be made of antimatter instead of matter!
There doesn’t seem to be much antimatter in our actual universe, however. That’s a good thing, because matter and antimatter don’t get along very well. When matter and antimatter bump into each other, the typical result is a process called annihilation, in which most or all of the particles are converted into light, in the form of high-energy gamma radiation!
Positron Emission Tomography (PET) scans, which are used to “see” the flow of blood inside a patient’s body, rely on the gamma rays emitted when positrons and electrons annihilate each other. A small quantity of positron-emitting radioactive material (e.g. carbon-11, which has a half-life around 20 minutes) is put into the patient’s blood stream. The radioactive isotope undergoes beta plus decay, and the emitted positrons quickly encounter nearby electrons. The positrons and electrons annihilate each other, turning into gamma radiation that is detected by the PET scanner.
Conversely, when light and other forms of energy are converted into new particles, both matter and antimatter are usually produced. (This fact gives rise to a rather puzzling mystery about the inception of the universe, as we’ll see in chapter 8.) For example, positrons can be created from the energy of cosmic rays, as Carl Anderson discovered, or from energy released in certain kinds of radioactive decay (as we saw earlier in this chapter). Larger particles of antimatter are created and studied in particle accelerators like the Large Hadron Collider. When high-energy protons collide with each other in the LHC, their kinetic energy is converted into roughly equal numbers of new particles and antiparticles. Some relatively large antimatter particles, including anti-hydrogen and anti-helium atoms, have been produced in this way.