136 The Poetry of Physics and The Physics of Poetry
The energy released in nuclear fission results from the destruction of
matter in the splitting of Uranium 235 into Barium and Krypton and
3 neutrons. The total mass of all of the products of the fission reaction is
less than the masses of the original uranium nucleus and the neutron,
which triggered the reaction. Another example of a nuclear reaction
in which energy is released is thermo-nuclear fusion, the process
responsible for the energy of the Sun. The mass of the final product of
fusion, the helium nucleus, is less than the sum of the masses of the
hydrogen nuclei, which combined or fused together to form the helium
nucleus. In both nuclear fission and fusion the mass that is destroyed in
these reactions is converted into pure energy.
Another process whereby energy is converted into matter and back
into energy is the process of pair creation and annihilation. In this
process, energy, in the form of light, is converted into the masses of an
electron and an anti-electron. An anti-electron or positron is a particle
with the same mass and spin of an electron, but which has the opposite
charge. An anti-electron, like an electron, is stable by itself. However,
when an electron and anti-electron make contact, they annihilate each
other and change back into light energy. The creation and annihilation of
electron-positron pairs is a process whereby light energy is converted
into matter. When an anti-electron and an electron collide they annihilate
each other and their mass is converted back into pure energy in the form
of photons. We will study this process in greater detail when we come to
our study of elementary particles at which time we shall encounter other
examples, which demonstrate the equivalence of mass and energy.
Before completing our study of the Special Theory of Relativity, we
turn to one of its first applications in which Einstein demonstrated the
equivalence of the electric and magnetic forces. Maxwell had shown,
through his equations, that the electric and magnetic fields were
intimately connected to each other. Einstein was able to demonstrate the
actual equivalence of these two forces, however, by using his principle of
relativity, which states that the laws of physics for two observers moving
at a constant velocity with respect to each other are the same.
Einstein asked us to consider two charged particles moving along
two parallel lines at constant speed with respect to a stationary observer.
The stationary observer will observe two forces between the charged
particle. One force, the electric force, arises solely by virtue of their
charge. The other force, the magnetic force, arises by virtue of both their
charge and their velocity. From the point of view of an observer moving