Antimatter? - Page 5
The operation of changing particles
with anti-particles is called Charge conjugation (C). Particles
and anti-particles have the exact same mass and equal, but
opposite charges and magnetic moments; if they are unstable,
they have the same lifetime. This period is called the Charge
Conjugation-Parity-Time (CPT) invariance, which establishes
the fact that if you interchange particles for anti-particles
(C), look in a three dimensional mirror (P) and reverse time
(T), you cannot tell the difference between the them. The
most stringent tests of CPT to date are measurements of the
ratio of the magnetic moments of the electron and positron
to two parts in a trillion (R. Van Dyck, Jr. and P. B. Schwinberg,
University of Washington,1987) and measurements of charge
per mass of the proton and antiproton--found to be 0.999,999,999,91
to 90 parts per trillion (G.
Gabrielse, Harvard, 1998).
Antimatter came about as a solution
to the fact that the equation describing a free particle in
motion (the relativistic relation between energy, momentum
and mass) has not only positive energy solutions, but negative
ones as well! If this were true, nothing would stop a particle
from falling down to infinite negative energy states, emitting
an infinite amount of energy in the process--something which
does not happen. In 1928, Paul Dirac postulated the existence
of positively charged electrons. The result was an equation
describing both matter and antimatter in terms of quantum
fields. This work was a truly historic triumph, because it
was experimentally confirmed and it inaugurated a new way
of thinking about particles
In 1932, Carl Anderson discovered
the positron while measuring cosmic rays in a Wilson chamber
experiment. In 1955 at the Berkeley Bevatron, Emilio Segre,
Owen Chamberlain, Clyde Wiegand and Thomas Ypsilantis discovered
the antiproton. And in 1995 at CERN, scientists synthesized
anti-hydrogen atoms for the first time.
When a particle and its anti-particle collide, they annihilate
into energy, which is carried by "force messenger"
particles that can subsequently decay into other particles.
For example, when a proton and anti-proton annihilate at high
energies, a top-anti-top quark pair can be created!
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