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Antimatter
According
to physics, for every kind of elementary particle there is an
antiparticle, some of whose physical properties are the same,
but others
(such as the electrical charge) are exactly the opposite.
Matter made up of antiparticles is called antimatter. When a particle
collides
with its antiparticle, they annihilate each other and their mass
is converted
into energy according to the mass-energy relation E = mc².
2-7
a
History
In 1928,
Paul Dirac was the first to come up with a mathematical formulation
for the electron in accordance with relativistic quantum mechanics
(a
combination of the special theory of relativity and quantum mechanics).
His
description predicted that the antiparticle of the electron should
also exist.
In 1932, the particle was discovered by Carl Anderson. He saw
traces of a
particle in a nebula that corresponded to an electron, but in
a magnetic field
symmetrically bent in the other direction: it had to be positively
charged.
He called this antiparticle of the electron positron.
2-7
b
Properties
An antiparticle
has the same mass and spin as the corresponding particle.
The other properties (namely the conserved quantities) are exactly
the
opposite. For example, the anti-electron (also called positron)
has mass
0.511 MeV and spin ½, just like the electron, but in contrast
to that lepton
number -1 and electric charge +1.
If a particle
is unstable, so is its antiparticle, with the same average lifetime.
The decay products of the antiparticle are then the antiparticles
of the decay
products of the particle.
The above
examples are fermions, from which matter is built up. Bosons
are different: these are interaction particles that do not belong
to matter or
antimatter. For example, the positive pion p+ and the negative
p- are each
other’s antiparticle, but they occur under the same conditions
and there is no
reason to call one a particle and the other an antiparticle. The
neutral pion
p0, the photon and many other neutral bosons are their own antiparticle;
there too the term antimatter has little or no meaning.
Antimatter occurs on Earth under normal conditions. It is created
in the
laboratory or in a particle accelerator, or it is created in nuclear
reactions.
A PET- scan uses radioactive substances that emit a positron (anti-electron)
as a result of a nuclear reaction. In 2002, CERN succeeded in
combining
antiprotons and positrons to form antihydrogen atoms and to study
their
properties. They turned out to behave just like ordinary hydrogen.
At the
end of 2010, 38 antihydrogen atoms were created and trapped for
one
tenth of a second. On 6 June 2011, CERN set a new record: it succeeded
in holding 300 anti-hydrogen particles for more than 15 minutes.
