The Science Book

292

THE UNITY OF

FUNDAMENTAL

FORCES

SHELDON GLASHOW (1932–)

T

he idea of forces of nature,

or fundamental forces, goes

back at least to the ancient

Greeks. Physicists currently

recognize four fundamental forces—

gravity, electromagnetism, and

the two nuclear forces, weak and

strong interactions, which hold

together the subatomic particles

inside the nucleus of an atom.

We now know that the weak force

and the electromagnetic force are

different manifestations of a single

“electroweak” force. Discovering

this was an important step on

the way to finding a “Theory of

Everything” that would explain the

relationship between all four forces.

The weak force

The weak force was first invoked

to explain beta decay, a type of

nuclear radiation in which a

neutron turns into a proton inside

the nucleus, emitting electrons or

positrons in the process. In 1961,

a graduate student at Harvard,

Sheldon Glashow, was given the

ambitious brief to unify the theories

of weak and electromagnetic

interactions. Glashow fell short of

this, but did describe the force-

carrying particles that mediate

interaction via the weak force.

IN CONTEXT

BRANCH

Physics

BEFORE

1820 Hans Christian Ørsted

discovers that magnetism and

electricity are aspects of the

same phenomenon.

1864 James Clerk Maxwell

describes electromagnetic

waves in a set of equations.

1933 Enrico Fermi’s theory

of beta decay describes the

weak force.

1954 The Yang–Mills theory

lays the mathematical

groundwork for unifying the

four fundamental forces.

AFTER

1974 A fourth kind of

quark, the “charm” quark, is

discovered, revealing a new

underlying structure to matter.

1983 The force-carrying W

and Z bosons are discovered

in CERN’s Super Proton

Synchrotron in Switzerland.

Messenger particles

In the quantum mechanical

description of fields, a force is

“felt” by the exchange of a gauge

boson, such as the photon, which

carries electromagnetic interaction.

A boson is emitted by one particle

and absorbed by a second. Normally,

neither particle is fundamentally

changed by this interaction—an

electron is still an electron after

absorbing or emitting a photon. The

weak force breaks this symmetry,

changing quarks (the particles

that protons and neutrons are made

from) from one kind to another.

Decay of particles via the weak force

drives the Sun’s proton–proton fusion

reaction, turning hydrogen into helium.

Without it, the Sun wouldn’t shine.