The Science Book

304 MURRAY GELL-MANN

U

nderstanding of the

structure of the atom

has changed dramatically

since the end of the 19th century.

In 1897, J. J. Thomson made the

bold suggestion that cathode rays

are streams of particles far smaller

than the atom; he had discovered

the electron. In 1905, building

on the light quanta theory of Max

Planck, Albert Einstein suggested

that light should be thought of as a

stream of tiny massless particles,

which we now call photons. In

1911, Thomson’s protégé Ernest

Rutherford deduced that an atom’s

nucleus is small and dense, with

electrons in orbit around it. The

image of an atom as an indivisible

whole had been destroyed.

In 1920, Rutherford named the

nucleus of the lightest element,

hydrogen, the proton. Twelve years

later, the neutron was discovered,

and a more complex picture of

nuclei made of protons and

neutrons emerged. Then, in the

1930s, a glimpse of further realms

of particles came from studies of

cosmic rays—high-energy particles

that are thought to originate in

supernovae. The studies revealed

new particles associated with high

IN CONTEXT

BRANCH

Physics

BEFORE

1932 A new particle, the

neutron, is discovered by

James Chadwick. There are

now three known subatomic

particles with mass: the

proton, neutron, and electron.

1932 The first antiparticle,

the positron, is discovered.

1940s–50s Increasingly

powerful particle accelerators—

which smash particles

together at high speeds—

produce large numbers of

new subatomic particles.

AFTER

1964 The discovery of the

omega (Ω–) particle confirms

the quark model.

2012 The Higgs boson is

discovered at CERN, adding

weight to the standard model.

How can it be that

writing down a few simple

and elegant formulae can

predict universal regularities

of Nature?

Murray Gell-Mann

energies, and hence with greater

masses according to Einstein’s

principle of mass–energy

equivalence (E = mc^2 ).

Seeking to explain the nature

of interactions inside the atomic

nucleus, scientists in the 1950s

and 1960s produced an enormous

body of work providing the

conceptual framework for all matter

in the universe. Many figures

contributed to this process, but

American physicist Murray

Gell-Mann played a pivotal role in

the construction of a taxonomy of

fundamental particles and force-

carriers called the standard model.

The particle zoo

Gell-Mann jokes that the goals of

the theoretical elementary particle

physicist are “modest”—they

merely aim to explain the

“fundamental laws that govern all

matter in the universe.” Theorists,

he says, “work with pencil, paper,

and wastebasket, and the most

important of those is the last.”

By contrast, the experimentalist’s

principal tool is the particle

accelerator, or collider.

In 1932, the first atomic nuclei—

of the element lithium—were blown

apart by physicists Ernest Walton

Quarks group

together in twos

and threes to

make hadrons.

Quarks are

detected by colliding

protons in a particle

accelerator.

Formulating the standard

model of particle physics leads theorists to

predict that hadrons (protons and neutrons)

are made of smaller particles

called quarks.