II.

Fermions Versus Bosons

Fermions differ from bosons, the other main group of fundamental particles, in that they follow different rules of behavior. The basic difference in their behavior is defined by a rule of physics called the Pauli exclusion principle. The Pauli exclusion principle, developed by Austrian-born Swiss physicist Wolfgang Pauli, states that two identical fermions cannot occupy the same space. Bosons do not behave according to the Pauli exclusion principle.

The fact that every fermion must be distinct from every other fermion has important consequences for the way the universe works. For example, electrons are tiny negatively charged particles that surround the nucleus of an atom. Electrons are classified as fermions. No two electrons can occupy the same space, therefore they must arrange themselves in a complex way around atoms so that each electron avoids occupying the same place that another electron occupies. The way electrons are arranged in the atoms of an element determines the chemical and physical properties of the element. If fermions were not distinct from bosons, all elements might share the same chemical and physical properties.

Fermions and bosons also have different values for a property of particles called spin. Spin is a measurement of a particle’s rotation, or angular momentum. Physicists usually express a particle’s angular momentum as a multiple of the constant number h/2p, where h is a number called Planck’s constant and p is a constant number approximately equal to 3.14. Planck’s constant is equal to 6.626 × 10^-34 joules-sec. (This is a very small number; written out, it would be a decimal point followed by 33 zeroes, then the digits 6626.) The number that, when multiplied by h/2p, expresses a particle's angular momentum is called the particle’s spin. Fermions all have spins that are odd multiples of y (y, 1y, 2y, and so on). Bosons have spins equal to whole numbers (0, 1, 2, and so on).