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 Astronomy and Cosmology
 Astronomy and Cosmology


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Astronomy and Cosmology
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     In 1930, Paul Dirac developed the first description of the electron that was consistent with both quantum mechanics and special relativity. One of the remarkable predictions of this theory was that an anti-particle of the electron should exist. This antielectron would be expected to have the same mass as the electron, but opposite electric charge and magnetic moment. In 1932, Carl Anderson, was examining tracks produced by cosmic rays in a cloud chamber. One particle made a track like an electron, but the curvature of its path in the magnetic field showed that it was positively charged. He named this positive electron a positron. We know that the particle Anderson detected was the anti-electron predicted by Dirac. In the 1950s, physicists at the Lawrence Radiation Laboratory used the Bevatron accelerator to produce the anti-proton, that is a particle with the same mass and spin as the proton, but with negative charge and opposite magnetic moment to that of the proton. In order to create the anti-proton, protons were accelerated to very high energy and then smashed into a target containing other protons.

     Occasionally, the energy brought into the collision would produce a proton-antiproton pair in addition to the original two protons. This result gave credibility to the idea that for every particle there is a corresponding antiparticle. A particle and its antimatter particle annihilate when they meet: they disappear and their kinetic plus rest-mass energy is converted into other particles (E = mc^2). For example, when an electron and a positron annihilate at rest, two gamma rays, each with energy 511 keV, are produced. These gamma rays go off in opposite directions because both energy and momentum must be conserved. The annihilation of positrons and electrons is the basis of Positron Emission Tomography (PET). When a proton and an antiproton annihilate at rest, other particles are usually produced, but the total kinetic plus rest mass energies of these products adds up to twice the rest mass energy of the proton (2 x 938 MeV).

     Although from a distance matter and antimatter would look essentially identical, there appears to be very little antimatter in our universe. This conclusion is partly based on the low observed abundance of antimatter in the cosmic rays, which are particles that constantly rain down on us from outer space. All of the antimatter present in the cosmic rays can be accounted for by radioactive decays or by nuclear reactions involving ordinary matter like those described above. We also do not see the signatures of electron-positron annihilation, or proton-proton annihilation coming from the edges of galaxies, or from places where two galaxies are near each other. As a result, we believe that essentially all of the objects we see in the universe are made of matter not antimatter.

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