Baggott J. The Meaning of Quantum Theory: A Guide for Students of Chemistry and Physics

SCIENCE

JIM

BAGGOTT

The

Meaning

of

Quantum

Theory

A

Guide

for

Students

of

Chemistry

and

Physics

Jim Baggott

OXFORD

NEW

YORK

TOKYO

OXFORD

UNIVERSITY

PRESS

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Publication

Data

Boggofl,

J.

E.

The meaning

oj

Quantum theory

.-

IJ guide

for

s(udf'nl5

of

chemistry

and

physics I Jim Baggoft.

Includes bibliographical refe-rences and index.

1.

Quantum

theory. 2.

QuanfUm

chemisfry. f. Title.

QCJ74,12.834

1992

503.1'2-dclO

91-)4937

ISBN

O-19-855575-X

(Pbk)

Pdnfed

in

Harrisonburg,

Va.

by

R.R.Donnelley

&:

Sons Company

To

Timothy

KERNER:

Now

we

come

to

the exciting

par!.

We

will

watch

the

bullets

of

light

to

see

which

way they

go.

This

is

not

difficult,

the

apparatus

is

simple.

So

we

look

carefully

and

we see the

bullets

one

at

a lime,

and

some

hit

the

armour

plate

and

bounce

back,

and

some

go

through

one

slit,

and

some

go

through

the

other

slit,

and,

of

course,

none

go

through

both

slits.

B

LA

i R: I knew

that.

KERNER:

You

knew

that.

Now

we

come

to

my

favourite

bit.

The

wave

pattern

has

disappeared!

It

has

become

particle

pattern,

jusllike

with

real

machine-gun

bullets.

BLAIR:

Why?

KERNER:

Because

we

looked.

So,

we

do

it

again,

exactly

the

same

except

now

without

looking to see which way

the

bullets go;

and

the

wave

pattern

comes

back.

So

we try

again

while

looking,

and

we get

particle

pattern.

Every time we

don't

look

we get wave

pattern.

Every

time

we

look

to

see how we get wave

pattern,

we get particle

pattern.

The

act

of

observing

determines

the reality.

Tom

Stoppard,

Hapgood

Preface

Why have I written this book?

Perhaps

a more

burning

question for YOll

is: Why should you read it? .

.

i'

I

wrote

this

book

because in August 1987 I

made

a discovery that

~

shocked me. If,

before

this

date,

you

had

asked me

at

what stage in the

process

of

emission and subsequent detection

of

a

photon

its state

of

polarization

is

established, I would have answered:

At

thc

moment

of

emission,

of

coursel Imagine then

that

two

photons

emitted in rapid suc-

cession from

an

excited calcium

atom

are

obliged, by

the

laws

of

atomic

physics,

to

be emitted in opposite states

of

circular polarization: one left

circularly polarized

and

one

right circularly polarized. Surely, they set

off

from the

atom

towards

their respective detectors already in those

states

of

circular polarization. Yes?

Well,

...

no. I have since learned

that

this

view-the

assumption

that

the

physical states

of

quantum

particles like

photons

are

'real'

before they

are

measured -

is

called (rather disparagingly, I sometimes think) naive

realism. Now in the

J92O$

and 19305, some

of

the most famous figures

in

twentieth century physics were involved in a big debate

about

the

meaning

of

the new

quantum

theory and

its

implications for physical

reality. In August 1987 I knew a lillIe bit

about

this debate. But I had

assumed that

it

had

the

status

of

a philosophical

debate,

with little or no

, relevance

to

practical matters

that

could be settled in the laboratory. I

had been trained as a scientist,

and

although I enjoyed reading

about

philosophy (like I enjoyed listening

to

musk),

I was

too

busy with more

important

matters

to

dig deeply

into

the subject.

In

July

and

August

1987, I

made

a short study visit

to

the University

of

Wisconsin

at

Madison, where I bought a

book

(always dangerous)

from

the

University

bookstore.

This was a book published in celebration

of

the centenary

of

the birth

of

Niels Bohr. In it were lots'

of

articles

about

his

contribution

to

physics

and

his great

debate

with Einstein

on

the

meaning

of

quantum

theory.

One

of

these articles, written by

N. David Mermin, gave me a tremendous

shock. Mermin described the

results

of

experiments that

had

been carried

out

as recently as 198210 test

something called Bell's theorem using

two-photon 'cascade' emission

viii

Preface

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~~~

A

pictorial

history

of

the

Einstein-Podolsky-Rosen

paradox.

Montage

by

p,

J.

Kennedy.

Reprinted

by

permission

ot

the

publishers

from

French,

A.

P.

and

Kennedy,

P.

J.

{eds)

(1985).

Niels Bohr: 8

centenary

volume. Harvard

Univer-

sjty

Press.

MA.

Copyright

©

1985

by

The

International

Commission

on

Physics

Education.

I

Preface

ix

from excited calcium

atoms.

Put

simply, Bell's

theorem

says that my idea.

of

naive realism is in

conflict

with the predictions

of

quantum

theory

in

a way

that

can

be tested in the laboratory

in

spedal

experiments on

pairs

of

quantum

particles. These experiments had been done:

quantum

theory

had

been proved right

and

naIve realism wrong!

There

in a

montage was a pictorial history

of

the

debate

about

reality

and

the

experiments

that

had

been

done

to

test it (reproduced opposite).

This

work

struck

me

as desperately

important

to

my understanding

of

physical reality, something that as a scientist I felt I ought to know

about.

This

discovery also made

me

feel

rather

embarrassed. Here I was,

proud

of

my scientific qualifications

and

with almost 10 years' experi-

ence in chemical physics research at various prestiflious institutions

around

the world,

and

I had been going

around

with

a conception

of

physical reality

that

was completely wrong!

Why

hadn't

somebody

told

me

about

this

before?

I could not rest until I had sorted all this ouL How

can

it be

that

quan-

tum

particles

are

not '':91]' until they are detected?

Are

alternative inter-

pretation'

of

quantum

theory ·possible?

If

so,

what

arc they like? I

bought lots

more

books

(some very expensive) and spent hours

and

hours

trying

to

understand

what was going

on.

There

are

many excellently

wrilleu

popular

books

on

Ihis subject

that

are easy

to

understand,

but these left

me

dissatisfied. These books

just

told

me

that

there

is

a

problem, whereas I needed to know

why

there is a problem:

to

know

what

it

is

aboutlhe

mathematics

of

quantum

theory

that

leads to all these

difficulties .

. ,

The

trouble is

that

many

of

the most

important

works pUblished on

the

imerpretatiofl

of

quantum

theory

are

heavy

going

and

I (with a

mathematical background I will flatteringly

describe as 'poor')

made

heavy

wdther

of

them.

Nevertheless, I persevered

and

managed

to

arrive

at something

approaching

comprehension. I decided

to

write

it

all

out

in

a

book,

in such a way that undergraduate students

of

chemistry

and

physics should be able

to

comprehend the material without needing

to

spend hours poring over

more

advanced texts.

And

this,

of

course,

answers the second question: this

is

why

you

should read this

book.

Students

of

physical science are usually introduced to the subject

matter

of

quantum

theory at an early, sensitive period in their under-

graduate

sludies. Sensitive, because their earlier instruction will

not

have

taught them

the whoie

truth

about

the nature

of

scientific activity

and

the way in which scientific progress is made. Sensitive also because they

will

not

yet have been trained to question what they are told

or

what

they

read in textbooks.

Quantum

theory is unnerving. Not only

is

the theory mathematically

x Preface

complex,

it is

also

conceptually

challenging.

For

the

first time, students

arc

taught

about

a

theory

which they

have

to

accept

and

which they have

to

learn

how

to

apply,

but

for which Ihey

cannot

expect

to

be

told its

meaning.

Many

will

not

realize

that

their

inability

to

understand

the

theory

is

due

not

to

a failing

on

their

part,

but

to

the

fact

that

quantum

theory

in

its present

form

is inherently

non-understandable.

As Richard

Feynman

has said: '

...

I

think

I

can

safely

say

that

nobody

understands

quantum

mechanics"

(my

italics).

Most

undergraduate

courses

on

quantum

theory

never

touch

on

the

theory's

profound

conceptual

problems.

This is

because

the

theory

brings

us right

back

to

some

of

the

cenlral

questions

of

philosophy

and,

as we

know,

there

is

no

room

for

philosophy in a

modern

science degree.

I find

this

an

absurd

situation.

It

is my

opinion,

expressed in this

book,

that

quantum

theory

is philosophy.

011,

we

can

dress it up in grand

phrases littered with

jargon-state

vector,

hermitian

operator,

Hilbert

space,

projection

amplitude,

and

SO

on - we

can

make

it all very

mechanistic

and

mathematical

and

scientific,

b\Jt

this does not com-

pletely hide

the

truth.

Beneath the formalism

must

be

an

interpretation,

and

the

interpretation

is

pure

philosophy.

This

is the reason why

quantum

theory

is such a difficult subject. Until

they

reach

undergraduate

level,

students

of

chemistry

and

physics

are

brought

up

on

classical science

in

which

there

appears

10

be

no need for I

philosophy.

They

arc

consequently

ill

prepanod

10

come

10

terms with

quantum

theory.

And

be

warned:

students

are

rarely told the whole

truth

about

this theory. Instead they

are

fed

the

orthodox

interpretation

either

by design

or

default. It

is

perhaps

surprising

that

for

a theory

so

funda-

mental

to

our

understanding

of

much

of

chemistry

and

physics

our

teachers

do

not

find it necessary

to

explain

that

it has

many

alternative

interpretations.

This

is

a great pity,

Students

have

a right

10

know

the

truth,

even

if

it is

bizarre.

In

this

book

I have posed five

questions

which I believe students

might

have expected

to

be

provided

with answers. These five questions

are:

Why

is

quantum

theory necessary?

How

does

it

work?

What

does

it

mean?

How

can

it

be

tested?

What

are

the

alternatives?

I

have

tried

to

answer these questions,

one

in each

of

the

book's

five

chapters.

While I

am

no! in a position

to

tell

you

what

quantum

theory

means,

I

can

tell you why its meaning is

so

elusive,

I feynman,. R,

p,

(1%1). The

characfer

of

physical

law.

M,LT

Press, Cambridge,

MA.

Preface

Xl

Thanks

go

to

Mike Pilling, Ian Smith

and

Brian Elms for their con-

structive

comments

on

early drafts

of

this

book.

Spedal

thanks go to

Peter

Atkins,

not

only for the very helpful comments he made on the

draft

manuscript

in his role as reviewer for

Oxford

University Press,

but

also for his excellent textbook Molecular

quantum

mechanics from

which

much

of

my

knowledge

of

quantum

theory

and

its applications

has been derived. This

book

would

oot

have

been possible without the

encouragement

of

my

editor

at

OUP.

r will remain eternally grateful for

having

an

opportunity

to get this lot

off

my

chest.'

JOHN

S.

BELL

It

was with great sadness

that

I learned

of

the death

of

the physicist

John

S.

Bell during my writing

of

this

book.

I

had

never met Bell,

nor

heard him lecture,

but

in my reading

of

his scientific papers I have

developed a great

admiration

for him

and

his work. I have especially

admired

hrs

attempts

to dismantle

the

orthodox

Copenhagen interpreta-

tion

of

quantum

theory, written with such tremendous style and obvious

enjoyment. Although

in

this book I have tried

to

present a balanced

account

-arguing

one way and then

another-l

hope

that

I have done

justice to

Bell's superbly constructed criticisms.

The

debate Over the

meaning

of

quantum

theory will certainly

be

poorer

without him.

Reading

April

1991

J.

E. B.

Baggott J. The Meaning of Quantum Theory: A Guide for Students of Chemistry and Physics

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