Friday, 5 August 2016

Does antimatter matter?

Cloud chamber image of the first observed positron
(Source: Wikipedia)
When I was a teenager and ill I used to revert to childhood reading - Famous Five and the like. Recently confined to bed with norovirus, I found that all my brain could cope with was guff such as the output of Dan Brown, so I gritted my teeth and read Angels and Demons. As always with Brown, most of the 'fact' content of the book was anything but factual. However, I thought it would be worth a quick trip into the nature of the central McGuffin of the story, antimatter. It may be Brown's super bomb and the power source of the fictional USS Enterprise, but it is real.

Antimatter is like the familiar stuff that makes up our world, but charged particle have the opposite charge (it's a little more complicated with uncharged particles). Instead of negative electrons, antimatter has positive anti-electrons, better known as positrons. Replacing positive protons in the nucleus, an anti-atom would have negatively charged anti-protons. It’s possible in principle to do anything with antimatter that can be done with ordinary matter. You could build an anti-table or an anti-house on an anti-world as long as you can handle the material. Antimatter has mass and behaves much like ordinary matter does (though as mentioned previously on this blog, it's just possible that it doesn't have quite the same attitude to gravity). But don’t expect to go out and buy some. Doing anything practical with antimatter is tricky. When matter and antimatter get together, both are destroyed, converted into pure energy.

The simplest reaction between the two is when an electron and a positron combine. The mass of the particles is converted into energy in the form of two photons of light (gamma rays), the amount predicted by Einstein’s famous equation E=mc2 – the energy produced is equal to the combined mass of the particles multiplied by the square of the speed of light. Because of this tendency to annihilate, very little free antimatter is found in the universe - at least anywhere we can see.

There is still a debate about where all the antimatter has gone. The Big Bang, starting with pure energy, should have produced equal amounts of matter and anti-matter, which then could wipe each other out, reverting to a universe full of energy. That this didn’t happen is usually explained by assuming that subtle differences in the properties of matter and anti-matter meant that there was a little extra matter left over. As few as one particle in a billion may have survived the initial matter/anti-matter wipe-out. But that was enough. However, this story is speculative at best at the moment.

The amount of energy generated from the interaction of matter and antimatter is vast. One kilogram of matter/antimatter coming together would produce around 1017 joules (1 with seventeen 0’s after it). That’s the energy output of a decent sized power station running for six years. However, don't expect, as Dan Brown appears to think, that antimatter is a new and wonderful power source. It's just a way to store energy - it takes significantly more energy to make the antimatter than is released when it annihilates. It's not a source of energy in the way a fuel in the way that sunlight or gas or radioactive materials are - instead it's more like a battery where you can store a vast amount of energy in a very small space. It's a battery with serious attitude.

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