Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Particles of Antimatter

British physicist Paul Dirac proposed an early theory of particle interactions in 1928. His theory predicted the existence of antiparticles, which combine to form antimatter. Antiparticles have the same mass as their normal particle counterparts, but they have several opposite quantities, such as electric charge and color charge. Color charge determines how particles react with one another under the strong force (the force that holds the nuclei of atoms together, just as electric charge determines how particles react to one another under the electromagnetic force). The antiparticles of fermions are also fermions, and the antiparticles of bosons are bosons.

All fermions have antiparticles. The antiparticle of an electron is called the positron. The antiparticle of the proton is the antiproton. The antiproton consists of antiquarks—two up antiquarks and one down antiquark. Antiquarks have the opposite electric and color charges of their counterparts. The antiparticles of neutrinos are called antineutrinos. Both neutrinos and antineutrinos have no electric charge or color charge, but physicists still consider them distinct from one another. Neutrinos and antineutrinos behave differently when they collide with other particles and in radioactive decay. When a particle decays, for example, an antineutrino accompanies the production of a charged lepton, and a neutrino accompanies the production of a charged antilepton. In addition, reactions that absorb neutrinos do not absorb antineutrinos, giving further evidence of the distinction between neutrinos and antineutrinos.

When a particle and its associated antiparticle collide, they annihilate, or destroy, each other, creating a tiny burst of energy. Particle-antiparticle collisions would provide a very efficient source of energy if large numbers of antiparticles could be harnessed cheaply. Physicists already make use of this energy in machines called particle accelerators. Particle accelerators increase the speed (and therefore energy) of elementary particles and make the particles collide with one another. When particles and antiparticles (such as protons and antiprotons) collide, their kinetic energy and the energy released when they annihilate each other converts to matter, creating new and unusual particles for physicists to study.

Particle-antiparticle collisions could someday fuel spacecraft, which need only a slight push to change their speed or direction in the vacuum of space. The antiparticles and particles would have to be kept away from each other until the spacecraft needed the energy of their collisions. Finely tuned magnetic fields could be used to trap the particles and keep them separate, but these magnetic fields are difficult to set up and maintain. At the end of the 20th century, technology was not advanced enough to allow spacecraft to carry the equipment and particles necessary for using particle-antiparticle collisions as fuel.