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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 ]
and
(b) E = - √[p2c2 + mo2c4]
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)
positron
nucleus
gamma ray
electron
Figure 1
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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. Notice that these tow gamma
rays are emitted in opposite directions to each
other (momnetum is a vector quantity).
electron
positron
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?
Figure 2
gamma ray
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!
Matter – antimatter collision
Figure 3
The existence of antimatter is required because if we think about the creation of an electron
‘on its own’ then charge would not be conserved. The positron is required to conserve charge
in the creation of the particles.
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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