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Antiparticles and Antimatter
Every elementary particle in the Universe appears to have a partner particle
called its antiparticle that shares many of the same characteristics, but many
other characteristics are the opposite of those for the particle. For example,
the electron has as its antiparticle the antielectron (also called the positron).
The electron and the
antielectron have exactly the same masses, but they have exactly opposite
electrical charges. A particle and its antiparticle are termed a pair.
(Some particles are their own antiparticles. For example,
the antiparticle of a photon is itself a photon.)
The common stuff around us appears to be "matter" (matter composed of particles),
but we produce
small quantities of antimatter (matter composed of antiparticles) routinely
in high energy accelerator experiments.
Pair Annihilation
When a
matter particle meets its antimatter partner they destroy each other
completely
(the technical term is "pair annihilation"), releasing the equivalent of their rest
masses in the form of pure energy (according to the Einstein energy-mass relation).
For example, when an electron meets an antielectron, the two annihilate and
produce a burst of light having the energy corresponding to the masses of the
two particles.
Pair Creation
But the opposite reaction occurs also: two photons can give up their energy to form
a particle-antiparticle pair. This process
is called pair creation. Pair annihilation and
pair creation are illustrated in the adjacent figure. These types of reactions play very
important roles in the big bang. Because there was so much energy available, the very early
universe was a seething inferno
of pair creation reactions converting photons to particle-antiparticle
pairs and of particle-antiparticle pairs annihilating to create photons.
Matter and Antimatter Worlds
Because the properties of matter and antimatter parallel each other, we believe
that the physics and chemistry of a galaxy made entirely from antimatter would
closely resemble that of a matter galaxy. Thus, it is conceivable that
life built on antimatter could have evolved at other places in the Universe,
just as life based on matter has evolved here. (But if your antimatter twin
should show up some day, I would advise against shaking hands--remember that
matter and antimatter annihilate each other!) However, we have no evidence
thus far for large concentrations of antimatter anywhere in the Universe.
Everything that we see so far on large scales seems to be matter, and we only
see antimatter produced for short times in violent collisions. This is something
of a mystery, because naively there are reasons from fundamental physics to
believe that the Universe should have produced about as much matter as
antimatter. We shall return to this mystery later.
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