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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 an Astrophysicist team.

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