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Dirac's prediction of the existence of antiparticles remained in obscurity until the 1930's, when an American physicist named Carl Anderson stumbled upon one while studying cosmic rays (energetic particles that arrive at earth from space). Anderson used a cloud chamber to track charged particles traveling around a magnetic field. He found that some of the particles produced the same trails as electrons, but in the opposite direction, indicating that they had the same mass as an electron but with a positive charge: the positron. This discovery of the first antiparticle came shortly after the discovery of the neutron (1932). It was a jolt to the world of physics, to say the least, to introduce the existence of antiparticles and the idea that particles could be created entirely out of energy by the Dirac process so soon after accepting the radical notion of the neutron- but it didn't stop there. The two fundamental particles that physicists had to contend with in the 1920's has become over 200, with new additions to the particle zoo being postulated every year. And it turns out that the counterparts of every particle of matter, collectively called antimatter, are the most volatile of all of the specimens in the particle zoo.
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