13 February, 2008

thor and antimatterAntimatter is often misunderstood. One common misconception is that if a small piece of antimatter ever connects with normal matter then there will be a chain reaction and huge amounts of death and destruction will follow. Another is that there exist antimatter versions of ourselves somewhere out in the universe who have goatees and are irredeemably evil. Neither of these have been shown to be true, but antimatter does have some nice properties.

In essence, antimatter is like normal matter except that some of its properties are reversed. Only some though. Antimatter has positive mass for instance, and can act much like normal matter. The most obvious change is that antimatter has the opposite charge from normal matter. So a proton has positive charge and an anti-proton has negative charge. Similarly an electron has a negative charge and an anti-election (called a positron) has a positive charge. When matter and anti-matter meet they do indeed annihilate each other completely leaving a lot of energy behind. There is no anti-energy so we can detect these matter/antimatter interactions in the same way as any other energy burst.

In the centre of our galaxy there is a large cloud of antimatter. Large as in 10 000 light years across, this is a big chuck of the milky way. It was discovered in the 1970’s when high flying balloons equipped with gamma ray detectors managed to discover a lot of energy coming from the centre of the milky way. The gamma rays all had the same energy level, 511 keV, which is the energy level emitted when a positron and electron collide. For a long time it wasn’t clear what was creating this cloud of positrons, until recently when the area of the cloud was mapped rather precisely onto an area where there are a special form of binary star system. The systems have a low mass star circling a black hole or neutron star. In these systems the low mass star is slowly being eaten by its more massive companion and in some way, not yet fully understood, the interaction is belching out positrons at an incredible rate. Some of the mystery of the antimatter cloud has been solved, but more remains to be discovered.

And in fact we use positrons here on Earth. Positron emission tomography is a way of seeing processes inside people (and animals) without cutting them open. A radioactive substance is introduced into the body, and when it decays into a more stable atom a positron is emitted. This positron then collides with a nearby electron and gamma radiation shoots off in opposite directions at 511 keV. By detecting these two bursts of radiation it’s possible to map where the radioactive substance ended up. This is used to diagnose and monitor various cancers for instance, and is also used when developing new drugs in animals. By using PET scans you can follow the course of where a treatment drug goes without dissecting the test animal.

Even without going for a PET scan you will still manage to encounter antimatter. Just as millions of normal neutrinos (very small elemental particles) pass through your body ever second, so too do antimatter neutrinos whizz happily through you. And these anti-neutrinos neither cause a massive chain reaction which will destroy the world, nor do they cause people to grow goatees and become evil. Well, not that often anyway.


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