QED: The Strange Theory of Light and Matter

163 Pages·1988·7.09 MB·English

by  Richard P. Feynman

PRINCETON SCIENCE LIBRARY

The Strange Theory of
Light and Matter

Richard P. Feynman

4 Chapter 1

ove us area of physies and 1 dink its wonderful i is
valled quantum elecuodynamies, or QED for short.

My main purpose in these lectures is to describe as ac~
curately ag can the strange theory of Tigh and guater—
or more specifically, the interaction of light and electrons,
Avs going ro take along time to explain all the things I want
to, Flowever, there ace tour lectures, so Pin going to ake

‘my fime, and we wil yet everything all vight

Physics has a history of synthesizing many phenomena

were pheaumena of motion and phenomena of heat; there
were phenomena of sound, of Hight, and of gravity. Bue it
‘was soon discovered, after Sir Issac Newton explained the
laws of motivn, that some of these apparently different
things were aspects of the same thing. For example. the
phenoinena of sonul could be completely understoad as
the morion of acoms in the air. So sound was ne longer
considered something in aldigon ta motion Tt was also
covered Ural heal phenetbenu ave easily underswundabte
from the laws of motion. In this way, great globs of physics
theory were synthesized into-a simplihed theory. The the-
cory of gravization, an the other hand, was not understand-
able from the laws of mocion, and even today it stands
isolated fram che other thearies, Gravitation is, so Lae, ot
understandable in cerms of other phenomena,

After the synthesis of Ue phenomen: af motion, sound,
and heat, theie was dhe discovery of « number af phenom

‘oni Utat we call eleetrieat anal maguetic. In 1874 these plc
nomena were synthesized with the phestomena of light and,

? i James Cleric
proposed! that light is an electromagnetic wave, So ac that
stage, there were the laws of motion, the laws of electricity
and magnetism, ane the Laws of gravity
Around 1900 a dicory was developed to explain what
matter was. It was called the electron theory of matter, amd:
Introduction 5

it suid that there were litle charged particles inside of
aroms. ‘This theory evolved gradually to include a heavy
nucleus with electrons going around! it
‘Attempts c understand the nietion of the electrons
going around the anelens by nsing mechanical lney—anal-
fate way Ne Lhe Lanes af i

‘out how the earth went around the sun-—were a real failure:
ull kinds of predictions came out wrong. (Incidentally, she
theory of relativity, which you all understand to be a geet
revolution in physics, wus also developed at xbout that time.
But compared to this discovery that Newton's laws of mo-
tion were quite wrong in atoms, the theory of relativity was
only a winor modification) Working out another system,
to replace Newton's laws took a long time because phe-
nomena al the alomnic tevel were quite strange. One act,
use one's common seuse in order to perceive what was
happening at the atomic level. Finally. in 1926, an "uncom
esernsy"Lh Jievelopesl io explain Lhe"

of behavior” of electrons in matter, Ielooked cockeyed, but,
in reality it was nt: it eas called the theory of quantara
mechanics. ‘The word “quantum” eefiers lo dais peculiaa
aspect of nature that gocs against common sense, It is chis,
aspect that Lam going to tell you about

Lhe theory of quantum mechanics also explained all
kinds of devas, such as why an oxygen atom combines with
wu byelevgen atoms w make waler, aul so on, Quantaca
mechanics thus supplied the theory behind chemistry. So.
Fundamental thenretical chemistry is weally physics

Because the theory uf quantum mechanics could explain
all of chemistry and he various properdes of substances,

it wus a tremendous success. But still here was the problem
of the interaction of fight and matter. That is. Maxwell’
theory of electncity and magnetism had w be changed 0
bein accord with the new principles of quantum mechanics,
that had bea developed. So a wew theoy, dhe quantaca