R. Michael Barnett of the Lawrence Berkeley National Laboratory and Helen Quinn of the Stanford Linear Accelerator Center offer this answer, portions of which are paraphrased from their book The Charm of Strange Quarks:

In 1930 Paul Dirac formulated a quantum theory for the motion of electrons in electric and magnetic fields, the first theory that correctly included Einstein's theory of special relativity in this context. This theory led to a surprising predictionthe equations that described the electron also described, and in fact required, the existence of another type of particle with exactly the same mass as the electron but with positive instead of negative electric charge. This particle, which is called the positron, is the antiparticle of the electron, and it was the first example of antimatter.

Its discovery in experiments soon confirmed the remarkable prediction of antimatter in Dirac's theory. A cloud chamber picture taken by Carl D. Anderson in 1931 showed a particle entering from below and passing through a lead plate. The direction of the curvature of the path, caused by a magnetic field, indicated that the particle was a positively charged one but with the same mass and other characteristics as an electron. Experiments today routinely produce large numbers of positrons.

Dirac's prediction applies not only to the electron but to all the fundamental constituents of matter (particles). Each type of particle must have a corresponding antiparticle type. The mass of any antiparticle is identical to that of the particle. All the rest of its properties are also closely related but with the signs of all charges reversed. For example, a proton has a positive electric charge, but an antiproton has a negative electric charge. The existence of antimatter partners for all matter particles is now a well-verified phenomenon, with both partners for hundreds of such pairings observed.

New discoveries lead to new language. In coining the term "antimatter," physicists in fact redefined the meaning of the word "matter." Until that time, "matter" meant anything with substance; even today school textbooks give this definition: "matter takes up space and has mass." By adding the concept of antimatter as distinct from matter, physicists narrowed the definition of matter to apply to only certain kinds of particles, including, however, all those found in everyday experience.

Any pair of matching particle and antiparticle can be produced anytime there is sufficient energy available to provide the necessary mass-energy. Similarly, anytime a particle meets its matching antiparticle, the two can annihilate each anotherthat is, they both disappear, leaving their energy transformed into some other form.

There is no intrinsic difference between particles and antiparticles; they appear on essentially the same footing in all particle theories. This means that the laws of physics for antiparticles are almost identical to those for particles; any difference is a tiny effect. But there certainly is a dramatic difference in the numbers of these objects we find in the world around us; all the world is made of matter. Any antimatter we produce in the laboratory soon disappears because it meets up with matching matter particles and annihilates.

Modern theories of particle physics and of the evolution of the universe suggest, or even require, that antimatter and matter were equally common in the earliest stagesso why is antimatter so uncommon today? The observed imbalance between matter and antimatter is a puzzle yet to be explained. Without it, the universe today would certainly be a much less interesting place, because there would be essentially no matter left around; annihilations would have converted everything into electromagnetic radiation by now. So clearly this imbalance is a key property of the world we know. Attempts to explain it are an active area of research today.

In order to answer this question, we need to better understand that tiny part of the laws of physics that differ for matter and antimatter; without such a difference, there would be no way for an imbalance to occur. This distinction is the subject of study in a number of experiments around the world that focus on differences in the decays of particles called B-mesons and their antiparticle partners. These experiments will be done both at electron-positron collider facilities called B factories and at high-energy hadron colliders, because each type of facility offers different capabilities to contribute to the study of this detail of the laws of physics--a detail that is responsible for such an important property of the universe as the fact that there is anything there at all!

Maria Spiropulu is a physics doctoral candidate at Harvard. Her response follows:

Let's start by defining matter. People have asked "what is matter?" for quite a long time. Democritus, the ancient Greek philosopher and mathematician, envisioned structure in the building blocks of everything and he called the basis for this structure an atom; he wrote, "nothing exists except atoms and empty space: everything else is opinion." At the atomic level, the world can be described in terms of the elements, including hydrogen, oxygen, carbon and the like.

As it turns out, though, atoms are not the fundamental constituents of matter. When we zoom closer into matter, by probing at smaller distances, the subatomic world unfolds. The closer we look, the stranger this world, the quantum world, actually behaves. We can not make a direct connection with it: at a small scale, objects do not behave like rods or balls or waves or clouds or anything we have ever directly experienced. But the quantum mechanics of this world does let us describe how atoms form molecules.

It also enables us to depict the "motion" of certain particles inside atoms. Indeed, atoms are made of electrons that whiz around the fixed protons and neutrons in their nuclei, which are made of quarks. These particles all interact with each other by means of "force messenger" particles, such as photons, gluons, W's and Z's. Based on the attributes of these particles, we assign them identification numbers, or quantum numbers. And by means of symmetries and conservation laws involving the quantum numbers of the particles, we can describe their interactions. Examples of such numbers are charge and intrinsic angular momentum, or spin.

If a is any particle and this particle has no attributes other than linear and angular momentum (which include energy and spin), then a is its own anti-particle--one of the constituents of antimatter. For example, the photon is its own anti-particle. If a particle has other attributes (such as an electric charge Q), then the anti-particle has the opposite attributes (or a charge of -Q). The proton and neutron have such attributes. In the case of the proton, its positive charge distinguishes it from the negatively charged anti-proton. The neutron--although electrically neutral--has a magnetic moment opposite that of the anti-neutron. Protons and neutrons have another quantum number called the baryon number, which also has the opposite sign in the corresponding anti-particles.

The operation of changing particles with anti-particles is called Charge conjugation (C). Particles and anti-particles have the exact same mass and equal, but opposite charges and magnetic moments; if they are unstable, they have the same lifetime. This period is called the Charge Conjugation-Parity-Time (CPT) invariance, which establishes the fact that if you interchange particles for anti-particles (C), look in a three dimensional mirror (P) and reverse time (T), you cannot tell the difference between the them. The most stringent tests of CPT to date are measurements of the ratio of the magnetic moments of the electron and positron to two parts in a trillion (R. Van Dyck, Jr. and P. B. Schwinberg, University of Washington,1987) and measurements of charge per mass of the proton and antiproton--found to be 0.999,999,999,91 to 90 parts per trillion (G. Gabrielse, Harvard, 1998).

Antimatter came about as a solution to the fact that the equation describing a free particle in motion (the relativistic relation between energy, momentum and mass) has not only positive energy solutions, but negative ones as well! If this were true, nothing would stop a particle from falling down to infinite negative energy states, emitting an infinite amount of energy in the process--something which does not happen. In 1928, Paul Dirac postulated the existence of positively charged electrons. The result was an equation describing both matter and antimatter in terms of quantum fields. This work was a truly historic triumph, because it was experimentally confirmed and it inaugurated a new way of thinking about particles and fields.

In 1932, Carl Anderson discovered the positron while measuring cosmic rays in a Wilson chamber experiment. In 1955 at the Berkeley Bevatron, Emilio Segre, Owen Chamberlain, Clyde Wiegand and Thomas Ypsilantis discovered the antiproton. And in 1995 at CERN, scientists synthesized anti-hydrogen atoms for the first time.

When a particle and its anti-particle collide, they annihilate into energy, which is carried by "force messenger" particles that can subsequently decay into other particles. For example, when a proton and anti-proton annihilate at high energies, a top-anti-top quark pair can be created!

An intriguing puzzle arises when we consider that the laws of physics treat matter and antimatter almost symmetrically. Why then don't we have encounters with anti-people made of anti-atoms? Why is it that the stars, dust and everything else we observe is made of matter? If the cosmos began with equal amounts of matter and antimatter, where is the antimatter?

Experimentally, the absence of annihilation radiation from the Virgo cluster shows that little antimatter can be found within ~20 Megaparsecs (Mpc), the typical size of galactic clusters. Even so, a rich program of searches for antimatter in cosmic radiation exists. Among others, results form the High-Energy Antimatter Telescope, a balloon cosmic ray experiment, as well as those from 100 hours worth of data from the Alpha Magnetic Spectrometer aboard NASA's Space Shuttle, support the matter dominance in our Universe. Results from NASA's orbiting Compton Gamma Ray Observatory , however, are uncovering what might be clouds and fountains of antimatter in the Galactic Center.

We stated that there is an approximate symmetry between matter and antimatter. The small asymmetry is thought to be at least partly responsible for the fact that matter outlives antimatter in our universe. Recently both the NA48 experiment at CERN and the KTeV experiment at Fermilab have directly measured this asymmetry with enough precision to establish it. And a number of experiments, including the BaBar experiment at the Stanford Linear Accelerator Center and Belle at KEK in Japan, will confront the same question in different particle systems.

Antimatter at lower energies is used in Positron Emission Tomography (see this PET image of the brain). But antimatter has captured public interest mainly as fuel for the fictional starship Enterprise on Star Trek. In fact, NASA is paying attention to antimatter as a possible fuel for interstellar propulsion. At Penn State University, the Antimatter Space Propulsion group is addressing the challenge of using antimatter annihilation as source of energy for propulsion. See you on Mars?

Answer originally posted October 18, 1999