Beyond the God Particle (44 page)

Particles that have
half-integer multiples
of
for their angular momentum, that is,

are called
fermions
, after Enrico Fermi, who helped pioneer these concepts (with Pauli and Dirac). The main fermions we encounter in most of our discussions are the electron, the proton, or the neutron (and quarks, which make up the proton and the neutron, etc.), and each has angular momentum
/2. We refer to all of these as “spin-1/2 fermions.”

Particles, on the other hand, that have angular momenta that are
integer multiples
of
, such as 0,
, 2
, 3,
,…and so on, are called
bosons
, after the famous Indian physicist Satyendra Nath Bose, who was a friend of Einstein and who developed some of these ideas. There is a profound difference between fermions and bosons that we'll encounter momentarily. Typically, the only particles that are bosons and that will concern us presently are particles like the photon, which has “spin 1,” or one unit of
angular momentum; the quantum particle of gravity, the graviton, which has yet to be detected in the lab, and has “spin 2,” or 2
units of angular momentum; and other particles that are made of quarks and antiquarks, called mesons, that have “spin 0,” or 0 units of angular momentum. Orbital motion also has angular momentum. All orbital motion, in quantum theory, has integer units of
for angular momentum, hence, 0,
, 2
, 3,
,…and so on.

EXCHANGE SYMMETRY

Elementary particles are so fundamental that they have no identifying labels. For example, any two electrons cannot in principle be distinguished from each other. There is no difference between any two electrons in the universe. The same is true of photons, muons, neutrinos, quarks, etc. The quantum effect of this identity symmetry depends strongly upon spin.