Physics and Our View of World 0521476801

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'

physics and our view of the

world •

edited

by

•

JAN HILGEVOORD •

contributors

•

John Barrow Paul Davies Dennis Dieks

Willem Drees Paul Feyerabend Bas van Fraassen

Mary Hesse Gerard

't

Hooft Ernan McMullin

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One

of the central questions of physics

Everything

is

possible.

is

Many physicists

be attainable, a belief which has lead

whether or not a Theory of

believe that such a theory might

to speculation that

we might one

day 'know the mind of God'. But what would be the philosophical implications of having a blueprint for the Universe? tell

us about reality?

room

for the existence of

distinguished physicists

modern

What does

physics

Does possession of the Theory of Everything leave

God?

In this fascinating book, a group of

and philosophers examine not only the claims of

physics, but also the impact these claims have

on our view of the

world.

Based on

talks given at the

Third Erasmus Ascension Symposium

the Netherlands, the book contains contributions from

in

John Barrow,

Paul Davies, Dennis Dicks, Willem Drees, Paul Feyerabend, Bas van

Fraassen,

Mary

Hesse, Gerard

't

Hooft and Ernan McMullin. Also

included, however, are the discussions which followed the talks,

characterized by a frank exchange of views and

many clear insights

into

these difficult issues.

At a time when many people view science with deep suspicion,

book

will

be of great interest

to

anyone wishing

relationships that exist between physics ideology.

to

this

explore the complex

and philosophy, theology and

Physics and our view of the world edited by Jan Hilgevoord Emeritus Professor of the Foundations of Physics University of Utrecht

|

Cambridge UNIVERSITY PRESS

Published by the Press Syndicate of the University

The

Building,

Pitt

40 West 20th

Trumpington

New

Street,

York,

Street,

NY

f

Cambridge

Cambridge CB2 1RP

10011-4211,

USA

10 Stamford Road, Oakleigh, Melbourne 3166, Australia

©Cambridge

University Press 1994

First published

1994

Printed in Great Britain at the University Press. Cambridge

A catalogue record for this book is available from

the British Library

Library of (Congress cataloguing in publication data Physics and our view of the world

'This

book

/

edited by Jan Hilgevoord.

cm.

p. is

a result of the third

Erasmus Ascension Symposium organized by the

Praemium Erasmianum Foundation

in

Amsterdam' - Foreword.

Includes index.

ISBN 1

.

3.

521 45372

Physics - Philosophy - Congresses. Religion

and science- Congresses.

Erasmianum Foundation. QC5.56.P486 530'.

01-dc20

1995

93-46713 CIP

ISBN

521 45372

ISBN

521 47680

wv

hardback 1

paperback

2. I.

Science - Philosophy - Congresses.

Hilgevoord, Jan.

II.

Praemium

6

Contents

Foreword

ix

Introduction

1

Jan Hilgevoord

Gerard

't

(lA^uestioning the answers or Stumbling upon good and bad Theories of Everything HooTt 1 (^g^Jheories of Everything 38

John D. Barrow

J_

[3/The

Dennis

scientific

view of the world: introduction 61

Dietcs

4 Enlarging the

known world

79

Ernan McMulLin /^0( 5 !The world of empiricism BasC. I. van Fraassen t-

114

I

6 /Has the scientific view of the world a special status compared with other views? 135 Paul Feyeraberra 3-

auantum theory and our view of the world

149

:

8 Interpretation of science; science as interpretation

Bas

C.

169

van Fraassrn 3-

\tyProblems in debates about physics and religion 188 Willem B. Drees 10

Paul

y

Mary

B.

The mind

of

God

226

DavW % 1 1

He^Se

The sources

of models for

~

Discussion 255

Notes on contributors 295 Bibliography 297

Index 299

God metaphysics or metaphor? :

239

1

Foreword

Erasmus Ascension Symposium organized by the Praemium Erasmianum Foundation in Amsterdam. The Praemium Erasmianum is known primarily for the Erasmus Prize This book

is

the result of the third

which, since 1958, has been awarded annually to individuals or organizations in

Europe that have exceptionally distinguished themselves

in

the field of European culture. In addition to this activity, every two

years the Foundation also organizes a symposium in the Netherlands

concerning a specially chosen multi-disciplinary topic of current interest.

In 1992, in conjunction with a day that

select

to

was open

to the public, a

group of 40 young Dutch scholars once again had the opportunity

engage

with a

in a penetrating

number

and in-depth discussion on the chosen

of specialists during the three-day

topic

symposium.

The choice of the theme for the 1992 symposium, Physics and Our View of the World, was prompted by the great interest that exists for this topic, as

is

evident from the appearance of so

publications in this

field.

many semi-popular

In this way, the Foundation

a forum for the useful exchange of views

hoped

to provide

and ideas about the

sophical and religious implications of recent developments in physics.

We

the exact in

hoped

thus

and the

to help bridge the

philo-

modern

gap between the realms of

spiritual sciences, a division

which

is still

so apparent

our time.

We the

are pleased that Professor Jan Hilgevoord, faculty

Department

of the History

member

of

and Foundations of Mathematics and was able to organize this sym-

Science at the University of Utrecht,

posium for the Foundation.

we hope, make the valuable symposium accessible to a much wider public.

The publication butions to this

of this book, will,

H. R. Hoetink Director,

IX

Praemium Erasmianum Foundation

contri-

Introduction

Jan Hilgevoord

Until quite recently physicists rarely speculated about relations

between

the laws of fundamental physics, the physics of elementary particles

and the Universe, and our existence on earth as conscious beings. They admitted that no physical reason for our existence was known, and that man was an alien in the physical world, but they did not perceive any

conflict

A first ics

was

between our existence and the basic laws of physics.

attempt to connect humankind and elementary particle phys-

Capra's The Tao of Physics (1975). In

Fritjof

tried to relate the so-called 'bootstrap' theory of to

this

book Capra

elementary particles

Eastern mysticism by pointing to similarities between the picture

that bootstrap physics gives of the behaviour of elementary particles

and the utterances ies

of certain Eastern mystics.

To Capra

these similarit-

suggested a deep analogy between the world at the level of elemen-

tary particles

and the world

of personal experience. Such a relation

would lessen the deplorable alienation of advanced abstract physics

from what can be directly and personally experienced. The Tao of Physics was a great success. The book was welcomed by supporters of holism and of the translated into

New Age Movement.

many languages and has been

It

has been

followed by related

was conspicuously ignored by the physicists, who found it at best wishful thinking and a lot of nonsense and at worst pure deception. But the book caused a marked increase books by Capra and others, but

in public interest in the results of

has led physicists

and cosmology

who

it

modern

physics. This interest, in turn,

are specialists in elementary particle physics

to try and explain these results to the general public; some also gave their views on what this remote world of strange particles and abstract theory might imply for our perspective

in so doing,

Jan Hilgevoord

of the world

wave

ourselves. Probably best

of publications

God and

like

and the

is

New

theory, a that

Physics and The

Theory of Everything, and

and

second

A

Mind

titles

of God, and, of course,

Brief History of Time. In this

seriously considers the possibility of a unifying physical

when we have such

scientists,

this

the series of books by Paul Davies bearing

Stephen Hawking's all-time bestseller

book Hawking

known among

just

paragraph he writes

in the final

a theory: 'Then

we

shall

philosophers,

all,

ordinary people, be able to take part in the dis-

cussion of the question of

why

it is

that

we and

the Universe exist.

If

would be the ultimate triumph of human know the mind of God.' Whereas Capra, at the time when he wrote The Tao of Physics, was a relatively unknown physicist, Hawking is a leadingfigure in cos-

we

find the

answer

to that,

it

reason - for then we would

mology, and whatever he says Nevertheless,

I

don't think that the majority of scientists take seriously

his statements relating

compelled

to the final physical theory.

such arrogance and to

felt

testify to their belief

God. Hawking's view on God may not be representa-

may be

attainable

is

shared by

many

experts in the

field.

hard, then, to avoid thinking about the consequences such a theory

would have for our view of the world and our own

who were It

Some even

the attitudes of physicists in general, but his belief that a Theory

of Everything It is

God

to protest against

in a less abstract tive of

of interest to the physics community.

is

capable of discovering

was the purpose

of the Third

role in

this ultimate truth

it

as creatures

about the world.

Erasmus Ascension Symposium

to

provide first-hand information regarding the status of such theories

and

their philosophical implications.

To

that

end a number

of promi-

nent physicists and philosophers of science were invited to discuss these problems with each other

and with the

participants of the

symposium. It is

rare for physicists

and philosophers

to

engage

in direct dis-

cussions; they speak different languages. But the philosophers at this

symposium all had a very sound knowledge of physics and some of them had even started their careers as physicists before they turned to the philosophy of science. So, on this occasion there was no language problem. To me this seemed a tremendous advantage. Many scientists, especially physicists, do not have a high opinion of philosophy. To them

Introduction

the fantastic

amount

of highly non-trivial

have acquired over the

knowledge that

with the seemingly endless squabbles about ever

no

practical importance that philosophers

course, this

is

scientists

few hundred years bears no comparison

last

being unfair

more

subtle points of

have spent their time on. Of

to philosophers. Philosophical questions

cannot be answered by empirical inquiry and the pace of progress in philosophy, therefore,

interested in

much slower than

is

what physics

tells

asking philosophical questions.

in physics.

But anyone

us about the world can hardly avoid

Many

great physicists have occupied

themselves with philosophy; the names of Einstein, Bohr, Schrodinger

and Heisenberg come

to

mind. Einstein said that physics without

osophy was not interesting enough

The main question

when we know

in the

to

spend a

philosophy of physics

physics?' This

is

is:

the question to ask

we know when confronted

'what do

was become evident from

with the remarkable recent claims of physicists and basic question of the symposium. tributions in this

answer

as

book

It will

phil-

lifetime on.

it

also the

the con-

that questions like this are at least as hard to

any questions

in physics.

The symposium was held on 27-31 May 1992, in Leiden and Oosterbeek in the Netherlands. There were three main themes: (1)

What do

the results of

(2)

Does the

scientific

modern

physics

tell

us about reality?

view of the world have a special status compared to

other views? (3)

Physics and theology.

The theoretical physicist Gerard 't Hooft, the astrophysicist John Barrow^and the philosopher Ernan McMullin were asked to talk on theVfirst^theme. The philosophers Paul Feyerabend and Bas van Fraasserrwere invited to speak on the second theme, and the theoretical physicist Paul Davies and the philosopher Mary Hesse agreed to talk

on the^third theme. The physicists were askedlxTgrve particular

attention to the idea of a

Theory of Even-thing, but otherwise the

speakers were free to develop their contributions as they wished. There

was no preliminary

consultation.

The

result

was a very

interesting col-

lection of lectures; clearly, the subject stimulated the speakers to give

the coherence of the symposium,

rather personal views.

To enhance

an introduction

second theme was given by Dennis Dieks

to the

who

/

'

Jan Hilgevoord

is

a philosopher of science,

who

The

and

theme by Willem Drees

to the third

a philosopher of religion.

is

discussion

was an

symposium and helped

essential part of the

considerably to clarify the lectures and to obtain specific answers from the speakers. is

A

selection of questions

replies

from the discussion

shall say a

few words about the

and

included in this book. In the remainder of this introduction

contents of each of the lectures.

main purpose,

of course,

The contributions

is

I

I

and connect them. But my

shall also try

to stimulate the

reader to read the book.

book are ordered according

in this

to the three

themes mentioned above, but there are many overlaps. Gerard

't

Hooft

begins the series with an admirably clear and thought-provoking cussion of Theories of Everything:

and what they

can't.

When

what they

are,

writing for a general public on a subject in

handicapped: they cannot

theoretical physics, physicists are greatly

speak their own mathematical language. They are thus forced

up pictures and analogies that convey the characteristics of cal theories,

't

dis-

what they can do

Hooft uses Conway's

Game

of Life, a

to think

their physi-

computer game,

model of a Theory of Everything. The game shows how simple rules may lead to surprisingly complex outcomes, 't Hooft believes that a Theory of Everything is possible in principle, although we may never as a

be able

to find

it

or

make

would mean that Nature

practical use of

it.

such a theory exists

If

and

it

fast computer.

But a

theory of this type will not be a Theory of Everything in the

literal

sense:

it

will

to explain.

is like

a very big

not enable us to explain just anything that

Because the reader should not be

left

we should want

with the impression

that the rules of present-day physics are as simple as those of

game,

't

Hooft

briefly discusses the

shortcomings as a Theory of Everything. Of the

ticles,

as well as

many

things that are touched

its

Conway's

Standard Model of elementary par-

upon in this rich article I mention the author's dissenting view on quantum mechanics and his astounding speculations regarding the nature of time and the merging of theoreti-

cal physics with

pure mathematics.

In the second contribution the astrophysicist

John Barrow looks

Theories of Everything from a cosmologist's perspective. Like

't

at

Hooft,

Introduction

he stresses the limitations that are inherent

Theory of Everything. He also points

physicists' concept of a

formidable problems one theory.

we

It

may

is

likely to

We

to the

encounter in searching for such a

well be, for example, that the part of the Universe that

are able to get information from

a whole.

elementary particle

in the

will

is

very atypical of the Universe as

then have a hard time finding the underlying Theory

of Everything.

Barrow

discusses the notion of the compressibility of information.

law of physics in

may be

seen as a

an abbreviated form. Belief

the belief that

all

way in a

to

A

compress physical information

Theory of Everything amounts

to

physical processes can be rendered by one short

formula, a formula 'you could wear on your T-shirt', as Paul Davies puts

It is like

it.

representing

the rules of chess. 't

is

games by just stating more modest here than is

possible chess

certainly

who would have the theory also fix exactly which game is being played in Nature. Among the various

Hooft

chess

particular

aspects of

Barrow discusses I mention the notion symmetry'. Symmetry amounts to saying that things that 'broken

modern candidate of

Barrow

all

theories that

look different are actually the same. Physicists are fond of symmetries, for two reasons:

first,

second, because

it

is

because symmetry compresses information, and beautiful.

Beauty

but very dubious notion about which

in physics I

will

is

a very important

have more

to say later.

Unfortunately, the kind of symmetry that theorists would like their

fundamental theories cal fact.

to

The symmetry

have does not always manifest

is

then said to be 'broken'

:

it is

itself in

physi-

present in the

underlying theory, in the mind (of God?) so to speak, but not in our

compromise between what you want and what you way in which theoretical physicists manage to eat cake and have it, and they sometimes get away with it surprisingly

actual world. This

have their

is

typical of the

well.

After the reviews by structure of

and go

't

Hooft and Barrow of the aims, problems and

modern fundamental

physics, the philosophers take over

right into the old debate

empiricists (van Fraassen).

It

between

realists

(McMullin) and

appears that the recent developments in

The and has become

phvsics are of no great importance to their outlook on physics.

realism-empiricism debate has a very long history

k

/Z

Jan Hilgevoord

enormously complicated

^JxM,

almost as

many

in

As McMullin puts it, there are same can presumably

our time.

realisms as realists, and the

be said of the anti-realisms. The general overview by Dennis Dieks therefore most welcome. Roughly speaking, scientific realism belief that science deals with

and

the entities

'really' exist in

other,

the external world. Scientific realism, in one

part of their motivation.

deal of time

an independent external world and that

structures that occur in our best physical theories

the position taken by

is

is

the

is

A

working

'all'

scientist

scientists. It is

would be unlikely

to

and energy on discovering the properties

particles unless convinced

form or

an essential

spend a great of elementary

that they really existed 'out there'.

There

is

a strange paradox here. For, although scientists regard realism as only

>"ii3 V

/:

7

Bas C. van Fraassen

the world

we

go

angles to the realist course;

at right

live in,

or to our experience. Empiricism cannot simply if it is

The

out of that plane of motion altogether.

to

work

at

all, it

realist sees

must step

our epistemic

enterprise as achieving a world-picture, something that purports to be the 'One True Story of the World'. Outright denial of that view would

push us simply of

life

into a rival world-picture.

without a world-view —

at least

I

want

to raise the possibility

without the sort of world-view

that metaphysical realists hold out for us as the

aim of science and

philosophy.

Classical empiricism

2

The

story

shall

I

tell

in this part

is

a

drama

in three acts. Aristotle

insisted that science aims not just to describe the

explain them.

He

phenomena

but to

then immediately went on to identify explanation

with description of something 'deeper'. This led to a view of science as describing necessity in nature (as opposed to the 'merely' actual), or

laws of nature (as opposed to 'mere' regularities). The nominalist/ empiricist rebellion of the late Middle

Ages challenged any such

enterprise which requires empirical science to reach for something far

beyond empirical ken. In the

third act the realists face the empiricists

with a tragic dilemma: either you resign yourselves to living in an utterly

meaningless world, or you must believe something not because

your experience leads you to

it,

but purely to escape this meaning-

lessness.

Of course you

I

admit what

will

am giving you a know my bias I want to

synopsis:

I

:

already be obvious to you from this

little

rather biased history of philosophy. But

be an empiricist, in some way that makes

sense for us today.

2.1

Aristotle's

view of science: the

What and

the

Why

We find him focussing moon. Looking back from the twentieth century, we

Let us begin with Aristotle's account of the eclipse.

on

eclipses of the

think immediately that the reason visible at the

eclipse

is

same time over

visible only in a small part.

116

is

not far to seek.

A lunar eclipse is

a large part of the earth, while the solar

Therefore quite accurate predictions

The world

of empiricism

were possible for lunar

we then check are not at

eclipses, but not for eclipses of the sun.

we

Aristotle's discussions,

concerned with

all

What does he

is

to

find to our surprise that they

this difference in predictability!

He

discuss then?

the aim of science

But when

discusses

what an

reach understanding, to

eclipse

know

is.

To him,

the reasons

why

things happen the way they do. Then it turns out that according to him, we understand such a phenomenon as the eclipse when we know what an eclipse is:

The question 'What moon's

light

question

.

.

.

is

eclipse?'

and

its

answer 'The privation of the

by the interposition of the earth' are identical with the

'Why does

the

moon

suffer eclipse?'

and the reply

'Because of the failure of light through the earth's shutting (Aristotle, Posterior Analytics

it

II, 2,

out'.

90, 7-18)

mind when he emphasizes the 'is' in 'what is'. Eclipses are many things - perhaps, for example, the eclipse is the one lunar phenomenon that has struck terror in the hearts of millions — but they don't all help to tell us why Persuasive. But he must have something quite special in

there are eclipses. So Aristotle envisages a sort of hierarchy or priority of properties:

The

some properties are

essential ones

essential, others

answer the Why-question

This hierarchy in what the thing explanation.

To take

a

If

A

modern example: The if

light

it is

B

in

explains A.

reaching us from distant galaxies

those galaxies are receding from us; and vice

versa: those galaxies are receding

Yet

comes from an asymmetry

explains B, you cannot also say that

exhibits a red shift

shift.

is,

merely accidental.

as well.

from us

if

their light has this red

the receding motion which explains the red shift -

and

not vice versa. Aristotle himself gave two examples: the planets do not twinkle (unlike the stars) because they are near; the

wanes

as

it

does because

it is

spherical. In each case,

moon waxes and we are disinclined

add 'and vice versa'. ]

had two

sorts of pictures,

two

sorts of models,

were mutually exclusive the wave picture and the :

which

particle picture. Dif-

ferent processes were modelled in these two different ways,

and no

process could be of both sorts. For a while in the twentieth century, scientists

though

were using both

sorts of pictures for the

in connection with different

same

processes,

experimental set-ups. Sometimes

the behaviour of light, for example, admitted representation as a

178

wave

Interpretation of science

in a

medium and sometimes

particles.

admitted representation as a stream of

it

Bohr's quite revolutionary idea was that this could be

accepted as a normal and satisfactory state of affairs, that a theory

some prescription about when to switch from the use of one to the use of another. The idea was workable only, however, if that prescription itself was not equivocal, and hence only if the scientist's apparatus could be exempted from could simply offer two families of models, with

this,

said to have a univocal description. But the only

and could be

was

univocal description available

that of classical physics,

which

unfortunately predicted wrongly even at the macroscopic level of the apparatus.

Today there seems

little

hope

com-

of re-instituting

plementarity as the key to interpretation of physics.

While

this

worry seems today rather out of date with respect

our understanding of quantum theory, the crises did not cease the idea of complementarity

was relegated

to

when

to its proper, rather less

fundamental, place. Another very basic tension which brought along vacillation,

and hence suspicion that the theory cannot have a satiswas perhaps first made fully explicit in Whee-

factory interpretation, ler's is

commentary on

and

sciences as parts, of the world.

On

On

Everett."'

fundamental

putatively the

the one

hand quantum mechanics

science, in principle

in principle affording a

the other hand,

much

of

it

encompassing

all

complete description

developed in the form

- systems studied input from and output to

an

of a theory of partial systems

in relation to

environment, in terms of

that environment.

The question is then whether the title Q uantum Mechan ics ofjOgein__Systems

book

of E. B. Davies' really

describes the

JTTze

entire

theory (with every aspect of an environment being potentially part of a described system, as

open

to

which

will itself

however always be described

an environment), merely a sub-theory, or a proper exten-

sion of the theory. Interpretations of

available differ

on

this question;

quantum mechanics presently

and even when they agree, they

differ in other significant ways.

However, the question about value of ambiguity should be subdivided into two. First

we ask whether

ambiguity'

and openness have been

of value to science in practice, or instead have

Then we must ask how

179

hampered

different philosophical views rule

its

progress.

on whether

'

CftiS

1

Bas C. van Fraassen

ambiguity and openness are defects, or alternatively, can be valuable to science.

As

to the first. question,

no philosopher should prejudge the

history,

At every point in the history we and the two are inseparable. The and insight, see both blindness insight that Newtonian mechanics lent itself to being the mainstay of sociology, or psychology of science.

a deterministic world-view, blinded the Enlightenment to the possibilities

of indeterminism.

Prima

both that insight and

facie, at least

its

correlate blindness are to be credited with inspiring the spirit of

research which led to such triumphs, and also to the phenomenal

and ambiguities - became

previously undetected gaps, vaguenesses, visible as

it

was

limits,

unseen alternatives - the

of classical physics. But then the previously

realized that science did already have resources to

begin the study of discontinuity and chance in nature.

But the second question, in science

how the

presence of vagueness or ambiguity

must look sub specie different philosophical views of

science,

more probing. Philosophical views of science are interpretations this, to us very salient and indeed more and more pervasive, aspect is

our

Such interpretation of science takes

civilization.

its

of of

place at one

further remove from nature, after scientific theories interpret as well

phenomena, and those theories themselves are subjected to interpretation in foundational enquiry. Prominent in our discussions at the symposium were the two opposed views of the aim of as represent the

d^

V

science which

view

is

I

call

empiricism and

scientific realism.

The

they need only be accurate in their representation of the

and how they

interpret those

of this primary good, is

empiricist

that scientific theories need not be true (overall) to be good,

i.e.

phenomena

is

empirical adequacy.

instead that the defining aim of science

provide us with a true story of what

is

phenomena,

of value only in the pursuit

lies

The

scientific realist

with truth: the aim

really

view is

to

going on, behind the

scenes as well as on the stage, so to speak.

There are nuances, of course.

A

more or

(not likely to exist in reality, of course) guity,

vagueness, and gaps are

less

naive scientific realist

would have

all defects.

The

last

to say that

two

spell

ambi-

incom-

pleteness of achievement with respect to the literally true story of the

world.

The

first

180

also obstructs, sabotages, such achievement as

it

drives

Interpretation of science

thought into several different directions at once - reaching for the aimed-for achievement continues only with the etimination of

conflict-

ing interpretations.

But what

if

the empirical predictions remain invariant under

of resolving the ambiguity

completion over

Indeed, the only true the empiricist,

throw new

way

to

on the theory, by showing that

light

good way

respond

to

infinity

and

how

such

this is

new is

how

light

is

the

valu-

potentially a

as yet unexpected or even

by ambiguity,

like the

paradoxes

modern mathematics,

infinitesimals that plagued

well be the crucial clues to creative development.

6

As

defect.

science, for

be chosen among, but valued and appreci-

ated. Indeed, the tensions created

about

Any

new phenomena

to

unimagined, they are not

may

it.

each of those ways of seeing the world

to

ways

could be resolved. Each tenable interpretation

it

world could be as the theory describes able. Since

no

at least sees

enhance our understanding of

not to resolve such ambiguity, but to find out in

is

many different ways will

ways Then empiricism

(all

rivals)?

its

all

of opting for one interpretative

a concrete case,

ation of

quantum mechanics

Interpretation of I

will

now

discuss

some

quantum mechanics, subsequent

recent

work

in the interpret-

to the episodes related in the

previous section.

An

interpretation of a theory

which that theory

left

To check whether the theory (the

some extent

is

an attempt

open3 This presupposes

that

is so,

'official'

arbitrary.

presupposes in

1 theory). -

We

to

answer questions

some are left open. turn that we can identify that

recognize that the division

The models provided

is

to

for us by the theory are the

Hilbert space models equipped with Hermitean operators to represent

observables and states.

statistical

operators ('density matrices') to represent

This leaves some indefiniteness, for which of the Hermitean

operators do represent observables? All, or only some? selection rules

may be

introduced?

selection rules, then not ables,

all

(When

181

super-

there are non-trivial super-

Hermitean operators represent observ-

so these questions are connected.)

Schrodinger equation there

What

is

some

Even

in the case of the

indefiniteness:

must

H

be an

lias

van Fraassen

(

observable? This

amounts

to the question

operators of

connected with the preceding two questions:

is

whether

H commutes

with

all

it

superselection

which case dynamic evolution cannot move a system out

(in

one coherent subspace into another). These questions may be among the more abstruse, but everyone

agrees,

I

some topics were rightly controversial and subject to what exactly happens in a measurement, significance of distant correlations, and the extent to which

think, that

dispute from the beginning: the physical

the 'correspondence principle' bestows a classical character on nature at

^

the macroscopic level, to mention but three of the most obvious.

But granted that there are open questions, and hence interpretation is

needed, what attitude are

is

to

ation.

It is

in

may

do not stand

X

One

critics,

be entirely clear and

likely to

possible attitude

now,

that

is

explicit at the begin-

emerge only along the way. But

well

One

at the beginning.

years ago

quantum

if

in this case,

great difference between

someone proposes an

quantum theory

of

measurement

no interpretation can survive unless

it

we

now and

interpretation of

theory, they will face a large set of stony-faced

with a real arsenal of possible objections and

specifically, the

that

take to this?

In any such inquiry as this one, the criteria

its rivals.

adequacy are not

ning; they

fifty

to

general not easy to give a verdict on the consequences of

such an attitude or of

we

maintain the conviction: there must be a unique right interpret-

hard-headed

difficulties.

Today,

so well-developed

is

obeys very high standards

of rigor.

Yet there are a

which appear into

to

number

two groups. The

first

concerning

interpretation

means: an observable value (one of

its

first

ation, with

its

the

is

in

under study today

We

agreement with von Neumann's

pertaining to a given system if

can divide them

'Eigenstate-Eigenvalue

and only

if

X is

in

consists of interpretations in

some group we

in at least

In the

A

group

eigenvalues)

The second group broken

of rival interpretations

have a good chance of surviving.

That

Link'.

X has

a specific

an eigenstate of A. which

this

link

is

cases.

find of course

von Neumann's own interpret-

notorious 'acausal' transitions (collapse of the

wave

packet, Projection Postulate). There are also two other types in this

group, which explain those transitions in

182

some way

(or explain

them

Interpretation of science

if you like). One of them sees the clue to the 'collapse' in thermodynamics (Danieri, Loinger, and Prosperi; Hepp). The other finds

away,

the clue

Cassinelli).

(Kai-Kong Wan; Beltrametti and

superselection rules

in I

do not include here

the,

GRW

model) (Ghirardi, Rimini,

Weber) because, although they accept collapsesf they also accept an 'swerve'

indeterministic

Schrodinger's equation with calculable

in

(though extremely small) observable effects which diverge from those predicted by the

quantum

theory.

In the Second group, the interpretations deny the Eigenstate-Eigenvalue Link. In other words, they allow that an observable value

when

the system's state

Since an extra parameter

addition to the state, that interpretations.

Examples

id

The Ensemble

(2)

Pilot

(3)

Wave

a

to specify that value (extra in

these are

all

among

the 'hidden variable'

are:

interpretation (Rosen, Popper, Ballantine)

(de Broglie— Bohm)

'De-occamizing' in

is),

is

needed

is

may have

not an eigenstate of that observable.

(this is

Redhead's name for the sort of interpretation

which one operator can stand for more than one observable;

Gudder, the 'anti-Copenhagen variant' of the Modal interpretation)

Some

(4)

Many Worlds

(5i

Interaction algebra (Kochen, Bub)

of these

(Everett,

we can

the versions of the

De

Witt)

already be quite sure do not work; for example

Ensemble interpretation apparently favoured by

Rosen and by Popper. All of them have which may or may not threaten

show them

to

counter-intuitive consequences,

to turn into defects that will eventually

be untenable after

all.

The

Pilot

for example, are surprisingly resilient, but they yielding relativistically invariant pictures of

Wave interpretations, may be incapable of

what

is

going on in the

micro-world. In addition to the above there are the

Modal

interpretations, also in

this

group; for example those devised by Healey and by Dieks, and

also

my favorite,

the 'Copenhagen variant' of the

Modal

interpretation.

Needless to say, these too have their counter-intuitive (and possibly problematic) aspects. In general,

we cannot

than one genuinely mutually independent question

183

more open, and the

expect interpretation to be unique. is left

If

Bas C. van Fraassen

theory

is

not inconsistent, then necessarily, there will be

more than one

interpretation."

Among

philosophers especially

I

have encountered the attitude that

we don't really understand a theory if there is any point when we don't know what to say. And if any question can be answered in more than one way, i.e. if there is more than one tenable interpretation, then indeed, objectively, we don't know what to say. But I think this is a wrong view of understanding. It misunderstands what it is to understand. Every time we discover a new tenable interpretation, we understand more, we understand the theory better. Even if we find it is tenable only by some criteria, or if eventually it fails completely, we have gained a good deal more understanding from that. What we have seen then are possible ways the world could or could not be I

if

the world

is

as the theory says

it is.

submit respectfully that the quantum theory admits of a whole

spectrum of diverse, tenable interpretations, and that

this

(and in principle, characteristic) state for science to be

is

in.

a healthy

Every

covery of a tenable view increases our understanding. Each time find that limit the

an interpretation

is

dis-

we

not tenable, that reveals constraints which

range of tenable interpretation, and thereby also increases

our understanding. Tolerance of ambiguity

is

a

great virtue for

philosophers as well as for scientists, and helps rather than hinders

understanding.

Both when a new tenable interpretation

is found, and when a putashown wanting, the most obvious benefit is the refutation of common claims and received wisdom. Consider for example the claim that quantum theory implies tha^particles cannot have a definite spatial trajectory. Bohm's interpretation, despite some objections we may bring against it elsewhere, clearly shows at least that this is not logically implied by the theory. To discover that, after hearing the contrary claim so often, certainly makes for an increase in

tively

\-A