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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,
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f
Cambridge
Cambridge CB2 1RP
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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