Explore particle physics.
After the discovery of electrons in 1897, protons, neutrons, pions, kaons,
sigmas, rhos, and over 200 other particles have been discovered, some in
cosmic rays and some generated in particle collision experiments.
Matter and energy are interchangable, and thus particles are also energy.
A particle has a rest mass, but waves (photons) have zero rest mass.
Thus, particles are frozen energy states, as are atoms. Many particles
are unstable, and they disintegrate within a short period of time.
The particle advanture home page is a very interesting website for
the general public to explore the zoo of particles.
Electrons and Positrons
The electrons were discovered by Joseph J. Thomson in 1897. He determined
the charge to mass ratio
e/m = 1.759e8 C/g.
Robert A. Millikan determined the charge to be 1.602e-19 C, and thus
the mass of the electron = 0.107e-31 kg.
The concept of antiparticles was proposed by Paul Dirac in 1930, and the
first case of an antiparticle was the discovery of positrons by Carl
D. Anderson in 1932.
Particles and their antiparticles have opposite spin, charge, and
Proton-antiproton and neutron-antineutron pairs
J.J. Thomson saw some positive particles in experiments that led to the
discovery of electrons. This positive particles have about the same mass
as that of a hydrogen atom. Thomson's student E. Rutherford positively
identified them as the nuclei of hydrogen atoms, and called them protons.
A proton has the same amount of charge as an electron, but of opposite kind.
After the discovery of the positron, able scientists looked for the
antiprotons. Some looked for them in the cosmic rays, whereas the group in
California built accelerators to make them. The group led by Segre looked
for the negatively charge heavy particles, and they thaught antiprotons, -p,
will anihilate protons, p, in their collisions, converting all the mass into
energy. What they have found was a starburst of 8 pions
p+ + p-, four of each.
p + -p = 4(p+ + p-)
In the mean time, there was a belief that antiprotons are present in
cosmic rays. The following are some websites regarding antiprotons.
After the discovery of antiprotons, attention was directed to antineutrons, -n.
A year later, Bruce Cork, also working in Berkley, thought that when an
antiproton neutralized its charge with a proton, instead of annihilating
it, the proton and antiproton pair will convert to a neutron and
p + -p -> n + -n.
Cork observed The annihilation of an antineutron and a neutron also produces
a star-burst of charged pions and photons. 114 antineutron annihilations.
Particles and antiparticles have opposite electrical charge in the case of
electrons and protons, but antineutrons, have opposite magnetic moment of
The study of matter and forces holding particles together or forces for
their transformation into each other is called particle physics
If you are interested in this area, a visit of the
CERN-European Laboratory for Particle Physics is worth while.