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The Question
(Submitted May 05, 1997)
Recently I've heard about antimatter.
Could you describe to me what it is?
Could you tell me why is it so important for
NASA to join matter and antimatter?
The Answer
The existence of antimatter was predicted by the theory of quantum
mechanics of the electron by Dirac in the 1920's. The first experimental
verification came with the discovery of positrons in cosmic rays by
Anderson in 1932 or thereabouts. However, positrons (anti-electrons)
are found in some kinds of naturally radioactive substances also.
Later anti-protons and other anti particles were produced in
accelerators in the laboratory.
An anti-particle is a particle whose properties are exactly opposite
to its corresponding particle. Thus a positron (the particle thought
to be responsible for the gamma-rays which were in the news last week)
has a charge opposite (positive) to that of the electron, and
'lepton number' -1. When positrons and electrons collide they
annihilate each other, and their energies are converted into gamma-rays.
If the positron and electron are at rest (which is unlikely) and
their spins are oriented opposite to each other they produce 2 gamma-rays, each with energy 511,002.7 electron volts. This is the radiation
which was observed by the Compton gamma-ray observatory, and is
considered to be a unique signature of electron-positron annihilation.
This signature is expected to arise from annihilation even if the
positrons and electrons are not at rest, but have moderate kinetic
energies.
Antimatter is interesting partly because of the spectacular and violent
way in which it interacts with normal matter. It is also an open
question why the Universe appears to be relatively empty of antimatter;
theories for the big bang predict approximately equal amounts of matter
and antimatter should have been produced. The positrons which
produced the gamma-rays seen by the Compton observatory were probably
produced by collisions of high energy particles (predominantly ordinary
protons and electrons) accelerated near a black hole.
I hope that this helps to answer your question.
Tim Kallman
for the Ask a High-Energy Astronomer team.
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