The Science Book

272

THE STRANGE

THEORY OF LIGHT

AND MATTER

RICHARD FEYNMAN (1918–1988)

O

ne of the questions to

arise from the quantum

mechanics of the 1920s

was how particles of matter

interacted by means of forces.

Electromagnetism also needed a

theory that worked on the quantum

scale. The theory that emerged,

quantum electrodynamics (QED),

explained the interaction of

particles through the exchange of

electromagnetism. It has proved

very successful, although one of its

pioneers, Richard Feynman, called

it a “strange” theory because

the picture of the universe that

it describes is hard to visualize.

Messenger particles

Paul Dirac made the first step

toward a theory of QED based on

the idea that electrically charged

IN CONTEXT

BRANCH

Physics

BEFORE

1925 Louis de Broglie

suggests that any particle with

mass can behave like a wave.

1927 Werner Heisenberg

shows there is an inherent

uncertainty in certain pairs

of values at the quantum level,

such as the position and

momentum of a particle.

1927 Paul Dirac applies

quantum mechanics to fields

rather than single particles.

AFTER

Late 1950s Julian Schwinger

and Sheldon Glashow develop

the electroweak theory, which

unites the weak nuclear force

with electromagnetism.

1965 Moo-Young Han, Yoichiro

Nambu, and Oscar Greenberg

explain the interaction of

particles under the strong force

in terms of a property now

known as “color charge.”

particles interacted through the

exchange of quanta, or “photons,”

of electromagnetic energy—the

same electromagnetic quanta that

comprise light. Photons can be

created out of nothing for very brief

periods of time in accordance with

Heisenberg’s uncertainty principle,

and this allows fluctuations in

the amount of energy available

in “empty” space. Such photons

are sometimes called “virtual”

particles, and physicists have

subsequently confirmed their

involvement in electromagnetism.

More generally, the messenger

particles in quantum field theories

are known as “gauge bosons.”

However, there were problems

with QED. Most significantly,

its equations often generated

nonsensical infinite values.

Feynman diagrams

show the ways in

which particles can

interact. Here, two

electrons repel each

other by exchanging

a virtual photon.

e1 e2

Virtual photon

Time

Space

Path of first electron

Path of second electron

e1 e2