# Antiparticles and antimatter

Is there another parallel Universe to ours composed of antimatter?

Is this a question of science fiction or a real question of physics?

According to Einstein's theory of special relativity the energy (E) and momentum (p) of a particle are related by the equation:

E2 = p2c2 + mo2c4

Where mo is the mass of the particle when at rest (the rest mass) and c is the speed of light in free space.

The British physicist Paul Dirac realised that there are two solutions of the equation:

(a) E = + [p2c2 + mo2c4 ]1/2

and

(b) E = - [p2c2 + mo2c4]1/2

For example if we think of the particle as being an electron we understand what is meant by the positive energy state but what about (b) – the negative energy. Dirac called the particle associated with this negative energy a positron – the antiparticle of the electron with the same mass but opposite charge.

The positron was discovered by Carl Anderson in 1932 during cosmic ray studies only four years after Dirac's proposal and this was followed in 1955 by the discovery of the anti proton at Berkeley in California.

If we can have an anti-electron (a positron) and an anti proton then why not an anti neutron and in fact a whole set of anti particles that 'mirror' the particles that make up our universe? These anti-particles would combine to form a 'new' type of matter known as antimatter. In fact antimatter does exists in our universe – although the exact amount is still not known.

## Pair production

When a gamma ray passes close to a nucleus it can interact with that nucleus forming a positron and an electron – this is known as pair production. Matter and anti-matter have been produced from energy (the gamma ray). Figure 1 shows a diagram of this interaction with the positron and the electron curving in opposite directions through an applied magnetic field.

The reverse of pair production can occur when a positron meets an electron. In fact something similar when a particle meets its own antiparticle – the two particles annihilate each other converting their mass back into energy. (Figure 2)
In the case of the positron and the electron the energy produced is two gamma rays each with an energy of just over 0.5 MeV.

Two gamma rays are required rather than one of energy 1.02 MeV to conserve momentum in the interaction.

Such interaction can be observed in particle accelerators.

It is interesting to imagine what would happen if an object composed of antimatter met its corresponding object composed of matter.
Would the objects simply annihilate each other?

I suppose in science fiction, or is it science fact, if you see yourself coming towards you – run away as fast as possible to avoid ending up as a flash of gamma radiation!

### Antimatter and black holes

When matter falls into a black hole the mass of the black hole increases. However, if antimatter is captured by a black hole the antimatter will annihilate some of the matter within the black hole and so the mass of the black hole decreases. If this process continues the black hole will eventually disappear in a flash of radiation. This is the white hole predicted by the English physicist Stephen Hawking.

It is believed that black holes the size of Mount Everest that were formed in the early stages of the Universe are now just reaching their 'full up' state and gamma ray bursts observed in deep space may be due to this effect.

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