h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Could you or someone else tell me what anti-matter is? It sounds very interesting, doesn't it, and frightening too as if it could gulp down matter itself.

Antimatter is a theoretical type of matter which is made out of antiprotons, neutrons and positrons instead of protons, neutrons and electrons. Each of the electrically charged particles is replaced by one with the opposite charge, so there is a positively charged electron (positron) smeared around a nucleus made up of negatively charged protons (antiprotons) and neutrons. This type of matter should be just as stable as normal matter, but if it meets up with normal matter, it and the normal matter will annihilate each other and there will be an enormous amount of pure energy (photons) given off, so there will be a huge explosion.

Scientists don't know why there appears to be no antimatter in our universe, since there should have been just as much of it made in the big bang as there was normal matter.

If I understand you're saying that if there is no antimatter anymore there shouldn't be any more matter either as they both should have anihilated each other in the big bang or am I inventing things?

No wait, never mind!

Because the Big Bang should have been symmetrical, there should be no reason to be matter rather than antimatter. There should have been equal amounts of both. At first they thought that perhaps some galaxies are matter and others are anitmatter, but there would be occasional collisions with enormous bangs! This doesn't happen, so we conclude that all of the visible universe is made of matter. Perhaps the scientists theories are slightly wrong, and there is some reason why matter is favoured. Or perhaps there was originally huge amounts of antimatter and it destroyed an equal amount of matter in a big flash, leaving a small amount of one left over and that is the universe we see now.

Scientists have made tiny amounts of antimatter in labs (usually only one atom).

While the Big Bang may have at first had a rather symmetric distribution of everything (in the first moments of the Universe only photons could exist), following the pair creation period supersymmetries broke down, such as when the electroweak force degenerated into the electromagnetic force and the weak interaction. Although this is still mostly a mystery, a slight asymmetry (such as the decay rates of the K0 particle and its antiparticle, in the early universe could have favoured matter over antimatter; the universe began to cool as it expanded reducing the amount of pair-creation and annihilation processes.

A small asymmetry in space itself in the initial few seconds of the universe could also be responsible. Some cosmologists have proposed theories saying that a very slight imbalance in the initial forces governing the universe may have led to 1 quark produced without an antiquark for each 1 billion quarks produced in pair-creation. They claim the billion quarks which annihilated produced the light now seen as the cosmic microwave background, and the remaining quark from each of the billion which disappeared combined with other surviving quarks to form the first atoms. Of course, no one really knows.

In particle accelerators, positrons and other antimatter particles are stored in magnetic field loops until they are to be collided with matter so as to release the most energy and hopefully lead to the larger, more massive particles scientists theorize. This is about the only viable antimatter "container" so far, but indeed, in radioactive processes antiparticles are very much involved, so using radioactive isotopes of certain elements as antimatter storage bins might also work.

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