Matter is the stuff the world is made of. It takes up space and it has mass. The ancient Greek, Democritus, postulated that all matter is composed of basic building-blocks, atoms. Of course, he put it more memorably than this: "Nothing exists except atoms and empty space, everything else is opinion."
We know now that atoms are made of even more fundamental particles - protons, neutrons and electrons - and that the protons and neutrons are composed in turn of quarks. We also know that for every one of these ordinary matter particles, there is an antimatter particle.
The world is composed almost entirely of ordinary matter, which is just as well, because when a particle encounters its antiparticle counterpart, they annihilate each other in a burst of energy. However, physics tells us that at the birth of the universe, matter and antimatter were formed in just about equal amounts.
In 1930, the English physicist, Paul Dirac, worked out an equation to describe the motion of an electron in electric and magnetic fields. Surprisingly, the equations that described the electron also described, in fact required, the existence of another particle having the same mass as the electron but with a positive instead of a negative charge. This particle, called the positron, is the antiparticle of the electron and is the first known example of antimatter.
Dirac's theoretical prediction was confirmed experimentally in 1931. Carl D. Anderson took a picture in a cloud chamber (a device for visualising particles) of a particle that was positively charged but that otherwise had the same mass and characteristics as an electron. Today, positrons can be routinely produced in laboratory experiments. Any antimatter produced in the laboratory disappears because it soon meets its corresponding matter particle and annihilates.
Each type of fundamental particle must have a corresponding antiparticle. The mass of particle and antiparticle are identical and all other properties are closely related but with the signs of all charges reversed. The proton has a positive electric charge and the antiproton a negative charge.
When a particle meets an antiparticle, they mutually annihilate in a burst of energy, gamma rays and X-rays. Their combined masses are converted into energy, in line with Einstein's famous equation, E=MC2, where E is energy, M is mass and C is the speed of light. The reverse process also works: any matching pair of particle and antiparticle can be produced any time provided there is sufficient energy available to provide the necessary mass-energy.
The universe appears to be composed entirely of ordinary matter. This can be shown in several ways. For example, if there was a significant amount of antimatter in the Milky Way galaxy, then many cosmic rays (sub-atomic particles that strike Earth after travelling long distances through the galaxy) would be made of antimatter. However, we know that at least 999 out of every 1,000 cosmic rays are made of ordinary matter.
Similarly, if other galaxies were made of antimatter we would see X-rays and gamma rays emerging from their interaction with intergalactic clouds - and these are not seen.
Modern theories of particle physics and of the evolution of the universe suggest, even require, that antimatter and matter were equally common during the earliest stages of the universe. Why then is antimatter so uncommon today?
The answer appears to be that the laws of physics are not quite the same for particles and antiparticles. Until 1956 it was believed that the laws of physics obeyed three separate symmetries called C, P and T. Symmetry C means the laws are the same for particles and antiparticles. Symmetry P means the laws are the same for any particle and its mirror image.
Symmetry T means the laws are the same in the forward and backward direction of time.
One of the fundamental forces of nature is the weak nuclear force. In the 1950s and 1960s it was found that if you replaced particles with antiparticles and took the mirror image but did not reverse the direction of time, then the universe did not behave in the same way.
The early universe did not obey the symmetry T. It was expanding; if time ran backwards it would contract. Since there are forces that do not obey the symmetry T, a slightly higher number of matter particles formed than antimatter particles as the universe expanded. For every billion antimatter particles there were a billion plus one matter particles. Most antiparticle/particle pairs self-annihilated, but the small excess of matter particles persisted and constitutes all the matter present in the universe today.
When matter and antimatter collide, 100 per cent of the mass is converted into energy; thus, an engine fuelled by matter/antimatter would be the most efficient propulsion ever devised. The energy released is about 10 billion times the chemical energy used to power the space shuttle.
In October 2000, NASA announced early designs for an antimatter engine. The amount of antimatter needed to power an engine for a one-year trip to Mars could be as little as one millionth of a gram.
Antimatter can be created artificially in the laboratory but, at present, only in very tiny amounts. CERN in Geneva can make only a few billionths of a gram of antiprotons per year, which would light a 100-watt bulb for a mere three seconds.
It will no doubt be possible to increase this production capacity significantly in the years to come. However, it will take tons of antiprotons to allow travel to interstellar destinations.
• William Reville is associate professor of biochemistry and director of microscopy at UCC
