From matter to antimatter

If you are a Star Trek fan you probably know that the starship Enterprise is powered by anti-matter. Antimatter is not the stuff of science fiction; it does exist.

As early as 1898, Arthur Schuster, a British physicist, suggested the fascinating idea that an exotic type of matter could exist with properties that mirror those of ordinary matter. In a letter to Nature he wondered: 'If there is negative electricity, why not negative gold, as yellow as our own?' He added that this speculation was just 'a dream'. In 1928 the gifted British theoretical physicist Paul Dirac provided the mathematical basis for Schuster's dream. Dirac predicted that the electron, which is negatively charged, should have a positively charged counterpart: 'This would be a new kind of particle, unknown to experimental physics, having the same mass and opposite charge as the electron. We may call such a particle an anti-electron.'

The discovery in 1932 of the anti-electron (now known as the 'positron', short for 'positively charged electron') in the cosmic radiation by the American physicist Carl Anderson vindicated Dirac's bold prediction. Twenty-three years later, scientists at the University of California at Berkeley created the anti-proton in a particle accelerator. We now know that every fundamental particle has an anti-particle - a mirror twin with the same mass but opposite charge. The idea of anti-particles is now also applied to atoms - anti-atoms, which make up the antimatter.

When anti-matter meets ordinary matter, they annihilate each other and disappear in a violent explosion in which mass is converted into energy as dictated by Einstein's famous equation E = mc2, where E is energy, m is mass and c is the speed of light. The energy released in matter-anti-matter annihilation is awesome: in a collision of protons and anti-protons, the energy per particle is close to 200 times that available in a hydrogen bomb.

If matter and anti-matter annihilate each other, there is no likelihood of anti-matter existing on Earth, or even in the solar system. The solar wind, the spray of charged particles emitted by the Sun in all directions, would annihilate anti-matter. However, scientists speculate that anti-matter could exist in other parts of the universe, but so far they have found no evidence. This has not stopped them from creating anti-matter in the laboratory.

A team of scientists at CERN, the European particle physics lab in Geneva, did just that in early 1996. For about 15 hours they fired a jet of xenon atoms across an anti-proton beam. Collisions between anti-protons and xenon nuclei produced electrons and positrons. These positrons then combined with other anti-protons in the beam to make anti-hydrogen, the simplest anti-atom. Scientists could detect nine anti-hydrogen atoms. Hydrogen is the most simple (just one electron orbiting a single nuclear proton) and most abundant (it makes up about 75 per cent of the universe) of 114 chemical elements known to us. An anti-hydrogen atom would have a positron orbiting a single anti-proton. 'It's really the proof that there is an antiworld', exulted the CERN team's leader, Walter Oerlert of the Institute of Nuclear Physics in Germany. Since 1996, CERN scientists have been regularly synthesising anti-hydrogen atoms and have so far collected several hundred thousand of them. This harvest would provide scientists with an insight into the properties of anti-hydrogen.

So now there is the experimental proof that antimatter does exist, what can it be used for? Because the annihilation of matter and anti-matter creates enormous amounts of energy - hundreds of times as much as generated in a nuclear reaction - it is tempting to look at anti-matter as a potential source of energy. This energy might one day provide the fuel for interstellar voyages, the same way matter-anti-matter annihilation powers the fictional spaceship Enterprise. The amount of antimatter required for space flights is unbelievably small. A few hundred micrograms could fuel a spacecraft to Jupiter, and the round trip would take only a year.

If you find all this a bit too far-fetched, then what about the idea of an anti-universe - a universe parallel to ours. Enter it and you will find your anti-matter counterpart: anti-you. Don't shake hands - you'll annihilate each other.

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