A Philosopher Looks at Quantum

Information Theory*

Amit Hagaryz

Recent suggestions to supply quantum mechanics (QM) with realistic foundations by

reformulating it in light of quantum information theory (QIT) are examined and are found

wanting by pointing to a basic conceptual problem that QIT itself ignores, namely, the

measurement problem. Since one cannot ignore the measurement problem and at the same

time pretend to be a realist, as they stand, the suggestions to reformulate QM in light of QIT

are nothing but instrumentalism in disguise.

1. Introduction. The last century ended with the rise of the new and
fascinating science of quantum information theory (QIT) which has
developed from a visionary idea into a lively and fashionable domain of
research, combining physics, computer science, and information theory.
The results have been more than promising: physicists have succeeded in
harnessing the special features of the sub-atomic world to devise fast
algorithms, to decipher unbreakable codes, and to ‘‘teleport’’ quantum
states.1 The success of the theory has been such that lately Christopher
Fuchs, one of the rising stars of this new field, has suggested reformulating
the conceptual foundations of quantum mechanics (QM) in purely
information-theoretic terms (Fuchs 2001a, 2001b, 2002a, 2002b). Fuchs
believes that such reformulation gives us better understanding of the

*Received July 2002; revised April 2003.

yDepartment of Philosophy, University of British Columbia, Vancouver, BC, V6T 1Z1,
Canada, email: ahagar@interchange.ubc.ca.

zI thank Meir Hemmo for encouragement and discussions, Chris Fuchs for stimulating
conversations, and William Demopoulos and Steven Savitt for a careful reading of earlier

drafts of this paper. I also thank two anonymous referees for helpful comments and

suggestions.

1. For an accessible textbook on QIT see Nielsen and Chuang 2000.

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Philosophy of Science, 70 (October 2003) pp. 752–775. 0031-8248/2003/7004-0006$10.00

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quantum world than any of those that have emerged after so many con-
ferences and debates on the interpretation of QM.

Fuchs is motivated by a view shared by many practicing physicists
today that QM needs no interpretation (Fuchs and Peres 2000, 70–71).
QIT is thus an opportunity for him to reformulate the foundations of QM
in purely pragmatic, or instrumental, terms which are devoid of ‘‘self
referential paradoxes’’ and metaphysical obscurities.

Fuchs’ suggestion is interesting and challenging, and yet it epitomizes a
dominant trend among physicists to dismiss decades of discussions on the
foundations and interpretation of QM. At stake here is the old and simple
question of what our theories are about. Fuchs believes that the way the
world is constrains what can be known about it and that these epistemo-
logical constraints warrant viewing QM in part as a theory about infor-
mation; hence nothing better than QIT is appropriate to supply foundations
for it. Though I am sympathetic to Fuchs’ observation about the relation
between ontology and epistemology in QM, I argue that nevertheless
QM—indeed scientific theories in general—are about the world around us,
and so interpretational questions are part and parcel of the scientific enter-
prise. This paper is thus aimed to rehabilitate and defend the conceptual
foundations of QM in light of the development of QIT.

There are many ways one can object to the reconstruction of the foun-
dations of QM on the basis of QIT. First, one can approach the issue from a
methodological perspective. For example, if one is motivated by the idea
that theoretical concepts should not be invented only for pragmatic
purposes but to understand what is really going on, then one can claim
that the interpretative ‘‘flow of information’’ between QM and QIT should
be from the former to the latter and not vice versa. Second, one can
elaborate on an old philosophical aphorism and say that pragmatism in
science—the idea that the scientific method, as far as it is a method, ‘‘is
nothing more than doing one’s damnedest with one’s mind, no holds
barred’’ (Bridgman 1945)—is perfectly fine if you are an engineer, whose
income depends on constructing machines that work, but is rather more
like junk food if you are a philosopher whose livelihood depends on
analyzing conceptual difficulties.

I am afraid, however, that both forms of defence—no matter how plau-
sible—are bound to convince no one but the already converted. My strat-
egy is thus not to defend but to attack. After all, if engineers usually solve
well-defined problems, then philosophers usually find problems in what
previously were thought well-settled solutions. My argument is simple.
Fuchs offers us what he calls a new basis for the foundations of QM while
denying that in so doing he adopts instrumentalism. And yet this new basis
is infected with the very problem that interpretations of QM originally set
out to resolve, namely, the quantum measurement problem. This situation

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in itself is harmless if one simply ignores the problem; yet ignoring the
problem is tantamount to admitting instrumentalism. Thus Fuchs’ dilemma
is as follows: either he is a realist, but has not solved the measurement
problem (which is fatal for the project) or he is not. In both cases Fuchs’
original proposal is simply inconsistent.

The rest of the paper is organized as follows. In Section 2 I present
Fuchs’ suggestion to reformulate the foundations of QM in light of the
developments in QIT. In Section 3 I show how QIT ignores the measure-
ment problem. Section 4 demonstrates how this problem can be solved
within the foundations of QM itself. The conclusion is spelled out in
Section 5.

2. Are There no Sundays in a Quantum Engineer’s Life? QM is infa-
mous for its measurement problem, i.e., the problem that any attempt to
account with the quantum formalism for the simple fact that measurements
have results requires an arbitrary ‘‘shifty split’’ between the classical and
the quantum world. At the heart of the measurement problem lies the
mutual inconsistency of the three following claims: (1) The wave function
of a system is complete; the wave function specifies all the physical
properties of a system; (2) the wave function always evolves in accord with
a linear dynamical equation (the Schrödinger equation); (3) measurements
of, e.g., the spin of an electron, always (or at least usually) have deter-
minate outcomes, i.e., at the end of the measurement the measuring device
is either in a state which indicates spin up (and not down) or spin down
(and not up).

The founding fathers, Dirac and von-Neumann, insisted on (1) and (3),
and so they had to reject (2) and to postulate a collapse of the wave
function due to the measurement. It was claimed that after the measure-
ment the superposition captured by the wave function ‘‘shrinks’’ to one of
the eigenvalues of the eigenstate of the observable measured. As John Bell
(1990, 35) notes, the orthodox collapse postulate involves two quantum
jumps, one of the classical apparatus to the eigenstate of its ‘reading’ and
one of the system to the eigenvalue of that eigenstate.

But the orthodox view is widely held to be unsatisfactory. In Bell’s own
words:

It would seem that the theory is exclusively concerned about ‘results
of measurements’ and has nothing to say about anything else. What
exactly qualifies some physical systems to play the role of the ‘‘mea-
surer’’? Was the wave function of the world waiting to jump for
thousands of millions of years until a single-celled living creature ap-
peared? Or did it have to wait a little longer, for some better-qualified
system—with a Ph.D.? (Bell 1990, 34)

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Bell’s (1987, 201) insight was that in order to solve the measurement
problem one must either admit that the wave function is not everything
(and reject (1)) or admit that Schrödinger’s equation is not always right
(and reject (2)). In other words, in order to solve the measurement problem
either the kinematics of orthodox QM or its dynamics must be modified.
The first alternative leads to Bohmian mechanics and to modal inter-
pretations;2 the second, to spontaneous localization theories, e.g., GRW or
CSL (Ghirardi et al. 1986; Pearle 1997). Agreed, one can always reject (3)
but then one must supply an explanation how measurements appear to
have definite results although they actually do not. This alternative leads to
Everettian theories, also known as many-worlds interpretations.3

Of course, this way of classifying the possible solutions to the mea-
surement problem is not unique. Clifton and Bub, for example, have
proved a uniqueness theorem for no-collapse interpretations with which
one can distinguish further between no-collapse theories according to what
counts as a ‘‘beable,’’ or as an observable with a definite value, in each
theory (Bub 1997, chap. 4)4. Presented as they are here, however, the
possible solutions to the measurement problem can be easily distinguished
according to the way each of them answers the question ‘‘what are the
quantum probabilities—the probabilities which are calculated according to
Born’s rule—probabilities for?’’5 Thus, for example, in Bohmian mechan-
ics these probabilities are for particles to have certain configurations, and
we understand them similar to the way we understand the probabilities of
statistical mechanics. In spontaneous collapse theories these probabilities
are for the wave function to evolve in a certain way, and we understand
them as purely chancy dynamical transition probabilities. How to under-
stand quantum probabilities in an Everettian context is still an open
question. Recent work by Saunders (1998), Deutsch (1999), and Wallace
(2002) suggests how to interpret them within a decision-theoretic scheme. In
this framework the quantum probabilities are probabilities for rational
observers to make decisions with respect to their own ‘branching,’ and
further proofs are given in order to show how the ‘weights’ on each branch
are equal to the probabilities given by Born’s rule.

2. For an accessible exposition of Bohmian mechanics see Albert 1992, chap. 7. Further

technical details are found in Goldstein et al. 1992 and in Holland 1993. Modal

interpretations are presented in van Fraassen 1991, chap. 9. See also Bacciagaluppi and

Dickson 1999.

3. For a classification of this type of theories see, e.g., Wallace 2002, 1–2.

4. Interestingly, when matters are put in these terms the instrumentalism of the Copenhagen

interpretation in which the ‘‘beable’’ varies with the experimental set-up becomes even more

evident.

5. For a lucid presentation of this idea see Maudlin 2001, 283–285.

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And yet, a dominant attitude among physicists is to ignore the mea-
surement problem, arguing, as Fuchs and Peres (2000) do, that ‘‘quantum
theory needs no interpretation.’’ Physicists say that they use quantum
mechanics to calculate probabilities for results of experiments and that the
theory does not describe matters of fact in the world but rather provides an
algorithm for computing these probabilities. The probabilities of QM are
simply probabilities for finding the system in a certain state, and physicists
leave the ‘philosophizing’ to philosophers or to ‘‘physicists who have lost
their taste or ability for ‘real’ physics.’’ (Unruh 1986, 242). John Bell was
an example of the contrary, recognizing that although it was difficult to
point at any real discovery that arose out of concerns having to do with quan-
tum measurement theory in his time, denying its significance amounted to
denying the main reason why many physicists were what they were, namely,
the desire to understand the world. ‘‘I am a quantum engineer,’’ he said,
‘‘but on Sundays I have principles.’’6

The decade since Bell’s death has seen much progress in the founda-
tions of quantum mechanics. Conceptually improved alternatives to ortho-
dox QM were formalized and a better understanding of the disagreements
was achieved. But this decade has also seen the birth of the new science of
QIT. Suddenly quantum weirdness has become a resource for engineers
and the scientific focus has shifted from understanding this weirdness to
harnessing it.

One could think that with the rise of QIT the neglected measurement
problem would once again attract physicists’ attention, and that with the
improvement of technology and the possibility of creating macroscopic
superpositions metaphysics would become experimental again.7 Unfor-
tunately, the physics community has chosen otherwise.

In fact, instead of using QIT to gain better understanding of the inter-
pretation of QM, scientists who found a new playground for QM in the
domain of QIT are suggesting now that we reformulate the foundations of
the former on the basis of the conceptual framework of the latter (see, e. g.,
Wheeler 1989; Caves et al. 2000; Clifton et al. 2003). The most lucid
advocate of this approach is Christopher Fuchs, who presents the
motivation for his idea in the following way:

The issue at stake is when will we ever stop burdening the taxpayer
with conferences and workshops devoted—explicitly or implicitly—to
the quantum foundations? The suspicion is expressed that no end will

6. John Bell’s opening words in an ‘‘underground colloquium,’’ cited in Gisin 2002.

7. It was Abner Shimony who coined the phrase ‘‘experimental metaphysics’’ in the context

of Bell’s inequalities and Aspect’s experiments.

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be in sight until a means is found to reduce quantum theory to two or
three statements of crisp physical (rather than abstract, axiomatic)
significance. In this regard, no tool appears to be better calibrated for a
direct assault than quantum information theory. Far from being a
strained application of the latest fad to a deep-seated problem, this
method holds promise precisely because a large part (but not all) of the
structure of quantum theory has always concerned information. It is
just that the physics community has somehow forgotten this. (Fuchs
2001b, 1).

Fuchs (2002b) begins with an analogy, insinuating that the relation be-
tween QIT and the whole class of interpretations of QM is analogous to the
relation between Einstein’s special theory of relativity (STR) and Lorentz-
Fitzgerald contraction theory. He (rightly) claims that the principles
Einstein introduced in STR supplied an understanding to the abstract
mathematical structure of Lorentz transformations, but then he argues that
a similar move is required in QM, i.e., that we should substitute the old
axioms of QM with information-theoretic justifications in order to regain
better understanding of the abstract structure of Hilbert space.

The motivation for such a radical move, says Fuchs (2001b, 8–9),
comes from Einstein, who, given his separability principle, regarded the
quantum state as an incomplete description of reality.8 Einstein, of course,
saw the incompleteness of the quantum state as a feature of the theory, and
not of the world. Fuchs, however, along with most of physics community,
draws opposite conclusions:

The theory prescribes that no matter how much we know about a
quantum system—even if we have maximal information about it—
there will always be a statistical residue. (2001b, 9)

So far there is nothing new here; quantum ‘‘incompleteness’’ is indeed an
existent fact, and no ‘‘hidden variables’’ theory can complete it without
violating STR. But Fuchs goes further to declare that the mystery of that
existent fact cannot be removed even if we endow the quantum state with
an ontological status. Note that Fuchs is making a dangerous general-
ization here. There is a big difference between Bohmian mechanics, or any
other no-collapse interpretation which denies the completeness of QM, and

8. Einstein in a letter to Born in Born 1970, 170–171: ‘‘An essential aspect of this arrange-

ment of things in physics is that they lay claim, at a certain time, to an existence independent

of one another, provided these objects ‘are situated in different parts of space.’ Unless one
makes this kind of assumption about the independence of the existence (of ‘being thus’) of

objects which are far apart from one another in space—which stem in the first place from
everyday thinking—physical thinking in the familiar sense would not be possible.’’

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collapse interpretations which accept it; and, it is exactly this difference
which we are about to elaborate in what follows.

Fuchs proceeds to convince his reader of the necessity of reconstructing
the foundations of QM by referring again to Einstein, this time to the
general theory of relativity (GTR). Here the claim is that GTR geometrized
nature and detached it from coordinate charts. The idea is that we should
do the same in QM, i.e., make clear the distinction between the observer
and the observed in order to gain insight about Nature itself. An important
difference between the co-ordinate charts in GTR and the observer in QM,
however, is that the latter is an active agent, whose impact on Nature is
similar to the impact of matter on the geometry of spacetime:

Observers, scientific agents, a necessary part of reality? No. But do
they tend to change things once they are on the scene? Yes. If QM can
tell us something truly deep about nature, I think it is this. (Fuchs
2002b, 6f).

Fuchs’ analogies are shrewdly (not to say diplomatically) contrived.
The first analogy to STR drives home the lesson that the interpretations of
QM are nothing but metaphysical flim-flam: just as STR and Lorentz-
Fitzgerald theories are empirically indistinguishable, so are QM and its
interpretations; and since in the former case the physics community has
chosen the less metaphysically extravagant option, we should do the same
in the latter. The second analogy to GTR allows Fuchs to acknowledge the
existence of an external world independent of an observer but to insist
that the ontology of this world is such that it puts constraints on the
information-gathering capacities of the observer, i.e., on epistemology.
Thus, if the world is contrived in a way that it conceals information from
us then the science of this world should be a science about the sort of
information we can have and how this information can be moved around,
hence QIT.

It is important not to read Fuchs as saying that QM is only about knowl-
edge or information since such a reading misses the point. The analogies
above make it clear that Fuchs follows the well-known Kantian idea of the
relation between epistemology and ontology, but ‘‘with a twist.’’ The
ontology of the world we live in constrains our epistemology, thus pre-
cluding the possibility of knowledge of the world ‘‘in itself,’’ independent
of an observer. Classical physics is an example of an ‘‘observer-free’’
science in which physical systems are endowed with independent physical
properties. If one wants such an ‘‘observer-free’’ description in QM, one
ends up with Bohmian mechanics (see Bub 1997). Of course, this theory
violates Lorentz invariance, but it also incorporates a mechanism for
hiding this violation. In this sense Bohmian mechanics is to QM what
Lorentz-Fitzgerald theory is to STR. But if the observer’s epistemology is

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inevitably constrained, i.e., the reality is always ‘‘veiled,’’ to borrow
d’Espagnat’s term, (although not in the conspiratorial Bohmian sense of
concealing undetected violations of laws), then from a philosophical point
of view, science must capture this feature, which is an objective feature of
the world. Since ‘‘philosophy is too important to be left to the philos-
ophers’’ (Fuchs (2002b, 7) cites J.A. Wheeler), the foundations of science
should be left to the quantum information theorists.

The philosophical tradition behind the idea that the peculiar features of
QM cannot be attributed to the ‘‘things in themselves’’ goes back to
Berkeley, Kant, and Mach, and many of the founding fathers of the theory
accepted it either deliberately or opportunistically. Fuchs, as we have seen,
is more sophisticated than that. He takes pains to convince us that his claim
is exactly the opposite: the peculiar features of QM are indeed a part of
reality, but in order to grasp them crisply one should avoid quantum
interpretations and instead strive to justify the original QM axioms all
interpretations start with by simple information-theoretic principles.

Fuchs emphasizes throughout his writings that he is neither a Kantian
nor an idealist, but despite the diplomatic apologetics, the main thrust of
his argument cannot be disguised. Kant was shrewdly silent about the
world ‘‘in itself’’ and never gave his critics the joy of catching him in anti-
realistic statements. Nevertheless, Kant was no realist, and the picture of
man dictating Nature its laws is nothing but (some might say distorted)
Kantian legacy. In a similar vein Fuchs says:

This attempt to be absolutely frank about the subjectivity of some of
the terms in quantum theory is a part of a larger program to delimit the
terms that can be interpreted as objective in a fruitful way. (2002b, 7f)

Agreed, that the ‘‘surface’’ terms of QM describe knowledge or infor-
mation does not automatically entail anti-realism about the world. But the
realistic approaches to QM are exactly the ones Fuchs mocks:

Go to any meeting, and it is like being in a holy city in great tumult.
You will find all the religions with all their priests pitted in holy war—
the Bohmians, the Consistent Historians, the Transactionalists, the
Spontaneous Collapseans, the Einselectionists, the Contextual Objec-
tivists, the outright Everettics, and many more beyond that. They all
declare to see the light, the ultimate light. Each tells us that if we will
accept their solution as our savior, then we too will see the light. . . .
Despite the accusations of incompleteness, nonsensicality, irrelevance,
and surreality one often sees one religion making against the other, I
see little to no difference in any of their canons. They all look equally
detached from the world of quantum practice to me. For, though each
seems to want a firm reality within the theory—i.e., a single God they

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can point to and declare, ‘‘There, that term is what is real in the uni-
verse even when there are no physicists about’’—none have worked
very hard to get out of the Platonic realm of pure mathematics to find
it. (2002b, 2–3)

His own rejoinder is to discover the underlying reality by way of rewriting
the foundations of QM with QIT. In itself this is an admirable task, yet it is
but a Pyrrhic victory if what is left of ‘‘objective reality’’ after such an
enterprise is quite ‘‘minuscule’’ (Fuchs’ (2002b, 6) own term). Two such
terms that can carry objective ontological weight which Fuchs is willing to
admit are the quantum system and the dimensionality of the Hilbert space
which accompanies it. Although the states of the quantum system repre-
sent only ‘‘a collection of subjective degrees of belief about something to
do with that system,’’ the latter ‘‘represents something real and inde-
pendent of us.’’ (Fuchs 2002b, 5)

Motivated in this way Fuchs (2002b, 42–43) then goes on to ‘‘trash’’
(Fuchs’ own description of his attempt to understand QM) about as much
of QM as he can: he argues that ‘‘quantum states—whatever they are—
cannot be objective entities’’; that ‘‘there is nothing sacred about the quan-
tum probability rule and that the best way to think of a quantum state is as
a state of belief about what would happen if one were to ever approach a
standard measurement device locked away in a vault in Paris’’; that ‘‘even
our hallowed quantum entanglement is a secondary and subjective effect’’;
and that ‘‘all a measurement is is just an arbitrary application of Bayes’
rule—an arbitrary refinement of one’s beliefs—along with some account
that measurements are invasive interventions into nature.’’ Finally he
argues that ‘‘even quantum time evolutions are subjective judgements;
they just so happen to be conditional judgements.’’

But what is ‘‘real’’ in QM According to Fuchs?

If one is looking for something ‘‘real’’ in quantum theory, what more
direct tack could one take than to look to its technologies? People may
argue about the objective reality of the wave function ad infinitum, but
few would argue about the existence of quantum cryptography as a
solid prediction of the theory. Why not take that or a similar effect as
the grounding for what quantum mechanics is trying to tell us about
nature? (Fuchs 2002b, 49)

It turns out that the result of Fuchs’ ‘‘realistic’’ enterprise, albeit an
enterprise covered with much apologetic sauce, ala Kant, is that the only
real property one can endow the world ‘‘in itself’’ is that it is sensitive to
our experimental interventions. This was, indeed, Fuchs’ only conclusion
in a predecessor to the paper discussed here (Fuchs 2001b), yet he himself
now admits that this conclusion was ‘‘singularly unhelpful to anyone who

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wanted to pursue the program further’’ (Fuchs 2002b, 51),9 and so his
humble rejoinder is that what is real ‘‘out there’’ is nothing but the dimen-
sion of the Hilbert space associated with the quantum system.10

Not surprisingly, if one identifies quantum theory as ‘‘law of thought’’
rather than a law of Nature, as Fuchs does, then less of the content of the
theory corresponds to reality. But Fuchs is convinced that the consequence
of his project—the admission that Nature is sensitive to our interven-
tions—counts as an achievement that warrants throwing away decades of
quantum foundational discussions.

As I have said in the introduction, I shall not try to convince the con-
verted and defend quantum interpretations against Fuchs’ rebuttal. Nor
shall I argue that Fuchs’ ‘‘realism’’ is a caricature. Instead in the next
section I shall launch an attack on another central thesis of Fuchs’ project.

3. Trouble in Paradise. Fuchs’ project of supplying an information-
theoretic justification to the axioms of QM by way of rewriting the
foundations of QM in terms of QIT leads to realism which is quite ‘‘thin.’’
Thus, although Fuchs is at pains to make it clear he is not a full-fledged anti-
realist, the outcome of his efforts speaks for itself. Nevertheless, since there
is no point in getting into a debate about what reality is, I shall abandon this
line of defence, and move to a different, more active, one.

In this section we are about to discover that no matter how Fuchs makes
fun of the quantum measurement problem11—the core foundational
problem of QM—the problem continues to haunt him even in QIT. As a
result, Fuchs’ own characterization of his project in realist terms becomes
suspect.

The argument is simple. Fuchs offers us what he calls a new basis for
the foundations of QM while denying that in so doing he adopts anti-
realism. And yet this new basis is infected with the very problem the inter-
pretations of QM have originally set forth to resolve, namely, the quantum
measurement problem. This situation in itself is harmless if one simply

9. The similarity to Kant’s apologetics is striking. As mentioned before, Kant was reluctant

to attribute any property or structure whatsoever to the ‘‘thing in itself,’’ and yet, Kant’s

solution to the third antinomy signifies a rare moment in his philosophy, i.e., his awareness
of the dire consequences of this reluctance. It is also one of the exceptional places in Kant’s

philosophy where he does assign some property, e.g., freedom, to ‘‘the unconditioned

condition,’’ if only to save the human race from the doom of life without morality.

10. The dimension of the Hilbert space associated with a quantum system appears to be a

crucial physical resource in deciding whether a quantum computer can be physically realized

in a way that will ensure tractability of its computations. See Caves et al. 2002a.

11. As Fuchs (2002b, 13) puts it: ‘‘Hideo Mabuchi [another practicing physicist] once told

me, ‘The quantum measurement problem refers to a set of people.’ And though that is a bit

harsh, maybe it also contains a bit of the truth.’’

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ignores the problem; but ignoring the problem is tantamount to admitting
anti-realism. Thus Fuchs’ dilemma is as follows: either he is a realist, but
has not solved the measurement problem (which is fatal for his project) or
he is not. Both outcomes are inconsistent with Fuchs’ original proposal.

3.1. The Quantum State—It or Bit? One’s attitude towards the quantum
measurement problem depends on the way one treats the wave function. If
no interpretation is needed, as some physicists say, then one can adopt an
epistemic attitude in which the wave function simply supplies statistical
information for measurement results. The probabilities computed by the
standard Born rule are then probabilities of finding the system in a specific
state. Collapsing the wave function, under this account, is just an adjust-
ment of subjective probabilities, conditionalizing on newly discovered
results of measurement. No physical change takes place since the wave
function represents only what we know about the system. If quantum states
merely represent points of view of observers, then there is no genuine
measurement problem. The change in the quantum state as a result of a
measurement, dictated by the orthodox collapse postulate, is only a change
in the observer’s knowledge, or probability assignments, whereas the
Schrödinger equation describes the time evolution of these probabilities
when no measurement takes place.

The idea that the quantum state represents information of an observer is
a legacy of the ‘‘Copenhagen school.’’ It is also epitomized in Wheeler’s
idea of the participatory universe:

It from bit symbolizes the idea that every item of the physical world
has at the bottom—at the very deep bottom, in most instances—an
immaterial source and explanation; that which we call reality arises
in the last analysis from the posing of yes-no questions and the
registration of equipment-evoked responses; in short, that all things
physical are information-theoretic in origin and this is a participatory
universe. (Wheeler 1989, 5)

This epistemic view of the wave function is also central to Fuchs’ program,
and yet he insists that the latter should not be interpreted as mere idealism
or instrumentalism:

The point is that deep within quantum theory we hope to find an
ontological statement. But that direct ontological statement will refer
to our interface with the world: the world is wired in such an such a
way that surface terms of the theory (the density operators) can only
refer to our subjective state of knowledge. That does not negate that
there is a world out there and that we strive to say useful things about
it. (Fuchs 2001a, 99)

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We have seen that the amount of useful things Fuchs’ allows himself to
say about the world ‘‘out there’’ is ‘‘minuscule’’—an understatement in
light of the consequence of his reformulation. But in what follows I want to
focus on another central idea of Fuchs’ project, i.e., the idea that one can
maintain an epistemic view of the wave function and still declare oneself
non instrumentalist, or a realist of some sort. The claim I make here is that
the epistemic view can be rendered consistent only in the price of denying
even the weakest form of realism Fuchs alludes to.

3.2. The Story About Adam and Eve. In an attempt to demonstrate that
quantum mechanics does need an interpretation, Meir Hemmo (unpub-
lished) offers the following set up.12 Consider an experiment done in a lab,
and two separated observers, Eve—confined to the lab; and Adam—
outside the lab. Say that Eve is about to measure a spin 1/2 electron in
some direction. QM tells Eve what are the probabilities for the two pos-
sible results. When Eve performs the experiment her measuring device gets
entangled with the electron to yield a superposition that cannot be taken to
describe a single result of the measurement, and as a result the pure quan-
tum algorithm stands in flat contradiction to the empirical fact that mea-
surements have (or appear to have) results. Indeed, this is no more than the
quantum measurement problem re-construed.

Now if one takes an epistemic stance toward the wave function, one can
evade this problem by suggesting that the latter simply provides mathe-
matical short hand for information or knowledge available to an observer,
in order to compute probabilities for results of experiments. Thus, the
tension between the unitary Schrödinger evolution and the single deter-
minate result to be accounted for is relieved as long as one assumes that
quantum states which are encoded in the wave function are relative to the
information available to the observer, that is, that quantum states depend
on the knowledge of the observer and that these states of knowledge are
updated simply by applying the orthodox collapse postulate, which in this
case has no factual meaning. On this picture, after the experiment has been
performed, Eve updates her knowledge with one of the two possible
outcomes by applying the collapse postulate and replacing the super-
position with a state corresponding to the result obtained.

We now move outside the lab to consider the point of view of Adam.
Here we notice that Adam’s knowledge of the quantum state of the entire
lab remained the same superposed state that was available to Eve before
she updated her knowledge. Now if Adam wishes to compute the state of
the lab after Eve’s measurement, he can do so with Eve’s reduced state,

12. This set up is a variant of a thought experiment that was originally proposed by Wigner

and is commonly known as ‘‘Wigner’s friend’’.

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that is, the state confined to the observables of Eve alone, by tracing over
all the degrees of freedom other than the relevant ones of Eve. But since no
factual collapse ever occurred in the lab, this reduced state cannot be
considered to be a classical mixture; it must be an improper mixture.13

The epistemic view thus leads to a situation in which the same state is
described differently by two observers. From the point of view of Eve the
state is pure. From the point of view of Adam the state is an (improper)
mixture. Agreed, the two descriptions cannot be distinguished and infi-
nitely many experiments will yield matching subsequent computations.
Moreover, it can be shown that no measurement of observables pertaining
to the lab alone (i.e., to Eve, to her measuring apparatus, or to the electron)
can decide between them. The reason for this is that the probability
distribution for such measurements is completely fixed by the reduced
states of the relevant systems.14

And yet, QM itself tells us that there are experiments that can
distinguish between the two descriptions.15 Indeed, if no super-selections
rules are imposed, then there exists an observable of the lab for which the
entangled state of the electron and the measuring apparatus and Eve’s mind
(or the part of it that records the outcome) is an eigenstate with some
definite eigenvalue, and for which the ‘‘collapsed’’ state after the measure-
ment is not an eigenstate (since the superposition and one of its compo-
nents are not orthogonal). For these special observables, if Eve and Adam
compute the probabilities for subsequent measurements to be carried out at
some time on the basis of the information that is available to them now,
they will inevitably come up with different predictions: Adam’s prediction
will be determined by the definite eigenvalue of the observable, while
Eve’s prediction will be probabilistic since for her the state in the lab is not
an eigenstate of the observable. Hemmo concludes that this indicates that
the epistemic view is inconsistent, since it yields two different predictions
for one and the same experiment, no matter how complicated and difficult
the actual performance of the experiment will be.

3.3. What Price Consistency? If one is a sophisticated proponent of
the epistemic view, then one has at least two ways to oppose Hemmo’s
argument. The point I wish to make, however, is that in so doing one auto-
matically characterizes oneself as an instrumentalist, and this flatly contra-
dicts Fuchs’ original claim to supply QM with realistic foundations.

13. On the difference between the two see, e.g., d’Espagnat 1966.

14. The first to point at this feature of quantum composite systems is Furry 1936.

15. This idea goes back to Albert’s (1983; 1990) ‘‘self measurement.’’ See also Bub 1997,

207–212.

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The first objection one can raise in order to defend the consistency of
the epistemic view in light of Hemmo’s thought experiment is exactly that
the latter is a thought experiment, and that for all practical purposes
(FAPP) such re-interference experiments are impossible. The basic idea
behind this FAPP objection is that it is practically impossible to isolate
physical systems and hence due to environmental decoherence no such re-
interference experiments can actually be performed: the superposition in
the lab, which is the source of the alleged inconsistency, will rapidly
decohere.

More precisely, if we assume that decoherence conditions hold, i.e., that
the interaction Hamiltonian between the environment and the lab com-
mutes with the pointer observables of the lab in the position basis, and that
the Hilbert spaces of the lab and the environment form a tensor product at
the beginning of the interaction, then the state in the lab (including Eve’s
mind) would rapidly get entangled with the environment and the original
interference operator would become the wrong operator to measure.16 As a
result not only can we never distinguish in the lab between a collapsed
state and a reduced state, the latter being one in which interference terms
are delocalized into the environment, but also, since the recoherence time
scales are larger than the age of the universe, for all practical purposes the
state will never recohere.

And yet, decoherence by itself cannot and does not solve the measure-
ment problem in the way that collapse interpretations, no collapse inter-
pretations, or hidden variables theories, do. What it can do is to supply a
consistency proof for the appearance of a classical world with an under-
lying quantum dynamics. It can explain why we never see macroscopic
superpositions, not why they do not exist. Relying on decoherence alone,
the epistemic view renders QM a theory that can never be caught in a lie
rather than a theory that tells the truth. No realist can deny there is a differ-
ence here.17

The second objection that a proponent of the epistemic view can raise
hinges on the time-reversal-invariant character of orthodox QM.18 Let us
call it the Erasure objection. One might claim that even if interference
experiments could be performed, then they would ‘‘erase’’ Eve’s memory
since Adam—having as he does complete control of all the degrees of

16. A clear illustration of this situation is given in Barret 1999, chap. 8.

17. One can use decoherence to secure an effective state in the lab, but since this path leads
to no-collapse interpretations, following it is tantamount to surrendering the ‘‘no

interpretation’’ thesis which underlies the epistemic view of the wave function.

18. Here I adopt the common view that the dynamics in QM are invariant under temporal

reflection and complex conjugation, that is, y(x,t) is a solution of Schrödinger’s equation if
and only if y*(x,�t) is, for all y.

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freedom in the lab—could undo Eve’s observation. In other words, since
according to the epistemic view the collapse is non factual, Schrödinger’s
equation allows such reversal; and since Eve’s memory and the interfer-
ence observable do not commute, Eve will have no record whatsoever of
her ‘‘collapsed’’ state and consistency would be restored.

Hemmo’s response to the Erasure objection is to press on. The epistemic
view, he says, cannot escape contradiction since there must be a fact of the
matter whether a result of an experiment is deterministic or probabilistic,
irrespective of one’s knowledge of it.

Note, however, that this response presupposes a certain realistic stance
towards quantum probabilities. In saying that ‘‘there must be a fact of the
matter whether a result of an experiment is deterministic or probabilistic’’
Hemmo clearly regards quantum probabilities as objective propensities,
which is exactly what Fuchs takes great pains to deny. According to the epi-
stemic view, quantum probabilities are not objective propensities. Rather,
they are degrees of subjective belief, or gambling commitments, and as
such can co-vary with different observers, as long as the state-assignments
of the two observers are compatible, i.e., as long as the state-assignments
do not differ in an arbitrary way.19

Thus, a better response than Hemmo’s to the Erasure objection which
does not attribute to the epistemic view an assumption it denies would be
to show that Adam’s and Eve’s state-assignments are incompatible. It turns
out that according to one compatibility condition this is indeed the case;20

but according to another it isn’t.21 It seems that the discussion threatens to
be degraded to mere book-keeping, so let me quickly point out again that
the issue at stake is not whether the epistemic view is consistent, but what
price its consistency.22

As in the case of the FAPP objection, the epistemic view reclaims con-
sistency only at the price of instrumentalism, since relativizing quantum
probabilities to the observer (which is analogous to what Einstein did to

19. On compatibility conditions for state-assignments see Caves et al. 2002b and Brun

2002.

20. One such condition which appears in Caves et al. 2002b and goes back to Peierls 1991

is that both state-assignments (1) must be non orthogonal and (2) must commute. In

Hemmo’s thought experiments condition (2) is violated.

22. Note that nothing in the formalism of QM prevents us from viewing Adam and Eve as

the same system (with different degrees of freedom), and so notwithstanding the
compatibility of the state assignments, there is still no subjective consistent account of

QM probabilities which will explain all the statistics of actual experiments.

21. Brun 2002 strengthens Peierls’ condition in order to subsume the case where the two

observers start with different information which is exactly the case Hemmo’s thought
experiment describes.

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simultaneity in the special theory of relativity) is tantamount to stipulating
an arbitrary cut between the observer and Nature. And although it is true
that one can shift this cut according to whim, it is also true that according
to the epistemic view what counts as real, i.e., as having definite properties,
is now dependent on where this cut is made.

Taking stock, one can still argue that Hemmo’s thought experiment might
not give sophisticated proponents of the epistemic view such as Fuchs much
pause for thought. However, it does expose the fact that this view is
indifferent to the measurement problem. Since ignoring this problem is
tantamount to admitting instrumentalism, while the indifference of the
epistemic view to the measurement problem is in itself harmless, it is quite
at odds with Fuchs’ original goal to supply QM with realistic foundations.

Let me restate the reasons for this inconsistency. First, if in defending
the epistemic view against Hemmo’s attack one invokes practical exper-
imental capacities (the FAPP objection), then one must also accept that
human capabilities dictate Nature’s laws. In the eyes of the realist this is a
complete misunderstanding of the role of experiments. Experiments are no
more than a tool in the hand of science whose aim is to understand the
world around us (Bell 1990, 34), and although they are the highest court of
appeal for establishing the laws of nature, this does not mean that the
letters of any law of Nature depend on them. As Planck notes:

The limitation to the law, if any, must lie in the same province as its
essential idea, in observed Nature, and not in the observer. That man’s
experience is called upon in the deduction of the law is of no
consequence; for that is, in fact, our only way of arriving at knowledge
of a natural law. But the law once discovered must receive recognition
of its independence, at least in so far as Natural Law can be said to
exist independent of Mind. (Planck 1945, 106)

Physicists like Mach or Heisenberg could say that the last remark by
Planck is exactly the issue at stake and that the existence of natural law
independent of one’s mind is quite limited in its extent. A devoted realist
such as Fuchs would surely deny their claim.

Second, by interpreting quantum probabilities as degrees of belief (the
Erasure objection) one subordinates one’s ontology to one’s epistemology.
But while realists can accept that ontology constrains epistemology, they
also believe that the former is independent of the latter! Consequently, it is
difficult to understand how Fuchs can view himself a realist and yet ignore
the measurement problem.

3.4. Intermezzo. The upshot of the discussion so far is this. One can
grant Fuchs that QM indicates that the ontology of the world puts con-
straints on our epistemology (call this conjunct (1)) and as a result one

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must reject no-collapse theories such as Bohmian mechanics or modal
interpretations which aim to complete the quantum description. One can
also grant Fuchs the epistemic view of the wave function (call this conjunct
(2)). But as the objections to Hemmo’s thought experiment demonstrate,
one must accept that the conjunction of these two ideas is tantamount to
saying that epistemology dictates ontology. The latter, of course, is not a
position a realist can adopt; hence Fuchs’ apologetics and denial of the
accusations regarding his ‘‘Kantianism,’’ ‘‘idealism,’’ and ‘‘instrumen-
talism’’ turn out to be nothing but window-dressing.

Fuchs, along with many others,23 seems to think that any realistic ap-
proach to QM which says more about the world ‘‘in itself’’ than he allows
himself to say, must acknowledge pre-existing reality, i.e., must resurrect a
‘‘hidden variables’’ scheme, and hence is unwarranted:

. . . Information about what? . . . That information cannot be about pre-
existing reality (a hidden variable) unless we are willing to renege on
our reason for rejecting the quantum state’s objective reality in the first
place. (Fuchs 2002, 24)

As noted in Section 2, this generalization does not do justice to the current
state of affairs in the foundations of QM. So far the interpretations that
follow in some sense Einstein’s desiderata and acknowledge pre-existing
reality are hidden variables theories such as Bohmian mechanics and modal
interpretations. But there are other interpretations in which the quantum
state has ontological weight and these are nowhere close to claiming that the
statistical character of the theory is not a feature of the world, or that it
should be eliminated.

Simply put, Fuchs seems to believe that the only way to resist Einstein’s
idea of pre-existing reality is to hold to the conjunction of (1) and (2).
What Fuchs fails to appreciate, however, is that one can accept conjunct
(1)—that ontology constrains epistemology—and still deny conjunct (2),
i.e., reject the epistemic view of the wave function, without admitting a
‘‘hidden variables’’ scheme. In other words, one can be realistic about QM
and still hold that the wave function is complete yet supplies only
statistical information, and that there is nothing more to be known about
the world than is captured in the quantum state. In fact, this is exactly what
a certain collapse interpretation of QM amounts to.

4. Interpretations—the Forbidden Fruit. Quantum mechanics is un-
doubtedly weird. The question is what are we going to do about this weird-
ness. One easy way is to forget the attempts to understand what the theory

23. Apart from his influence on the physics community Fuchs has already succeeded in
converting philosophers such as J. Bub (personal communication).

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says and to harness its weirdness in order to construct sophisticated
machinery. There is nothing wrong in such an engineering endeavour.
To claim that QM weirdness can be explained in terms of this endeavour is,
however, something completely different. To claim further that one can
maintain that the engineering endeavour has an explanatory role—or even
that it is the only explanation needed—and to pose as a realist is simply
inconsistent.

I can understand how one might argue that any theory that admits
quantum information theoretic constraints, such the impossibility of
superluminal information transfer between systems by measuring one of
them, the impossibility of (perfectly) cloning the information contained in
an unknown physical state, and the impossibility of (unconditionally
secure) bit commitment, is ‘‘quantum mechanical’’ as far as the charac-
terization of its observables and state space are concerned (Clifton et al.
2003). But this simply means that underlying both QM and QIT is a
property structure with a non Boolean character which leads to interference
and entanglement. The problem of making sense of this non Boolean
character in light of the apparent Boolean character of the property
structure that underlies the macroscopic world (which lacks interference
and entanglement) remains to be solved.

Fuchs’ (2002b, 28–39) ‘‘solution’’ is to regard the transition from pure
states to mixtures, represented by ‘‘the collapse of the wave function,’’ as a
variant of Bayes’ rule for updating the subjective probabilities of the
observer as a result of a measurement. This is a ‘‘law of thought’’ that
stands over and above the details of physics. But the transition from pure
states to mixtures is not the explanans; it is the explanandum!

QM, a-la Fuchs, is mute about the physical transition from pure states to
mixtures since the theory does not describe states of affairs in the world; it
describes only our knowledge of them. The ‘‘collapse postulate’’ is thus an
adjustment of subjective probabilities in which no physical change takes
place. But here again one wonders what does Fuchs mean by denying he is
an instrumentalist, if, according to him, our best theory says nothing about
this feature of the world (apart, maybe, from that quantum mechanical
systems exist and that these systems are ‘‘sensitive to our touch’’). This
attitude is understandable in metaphysics, when one has no fixed
background of knowledge to rely on, but here we are dealing with a
fundamental physical theory, one which is empirically confirmed to a very
high degree. To say that this theory is silent about the world and to
subscribe to a realist position about this world are two things that simply
do not go together.

4.1. Towards a (Macro)realistic QM. Fuchs’ oversight is simple. Taking
as he does the epistemic stance with respect to the quantum state, i.e., seeing

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it as a useful epistemic construct for computing conditonalized proba-
bilities, he postulates an arbitrary ‘‘split’’ between the observed and the
observer: any attempt to apply the quantum formalism to the measurement
process itself will unavoidably lead to further conditional predictions. He
further assumes that by eliminating this ‘‘split’’ one admits pre-existing
reality and thus is bound to end up with hidden variables theories.

However, one can still accept the ‘‘split’’ between the observer and the
observed and yet eliminate its arbitrariness without postulating hidden
variables. In so doing one is led to a realistic interpretation of the quantum
state that, contrary to Fuchs’ assumption, views the quantum formalism as
complete but also incorrect. In this realistic interpretation, the spontaneous
localization scheme (SL), the collapse of the wave function is a physical
process; a natural consequence of the dynamics. Since a lot of ink has been
spilled on this interpretation (the interpretation that goes back to Ghirardi
et al. 1986), what I would like to emphasize here, in light of Fuchs’
oversight, is only the differences between this approach and the view that
regards the quantum formalism as incomplete—the view that Einstein was
referring to when he presented his EPR argument.

Hidden variables theories regard the quantum formalism as incomplete.
The quantum probabilities in these theories are epistemic probabilities that
signify our lack of knowledge, or ignorance, of the exact state the system is
in. Realism in this sense is better understood as applying to the objective
definiteness of the quantum state, and the statistical character of the theory
is due to the incompleteness of the formalism and not to any intrinsic
quantum indefiniteness. In Fuchs’ terminology, the quantum state supplies
in this case incomplete information on pre-existing reality.

In contrast, the SL approach regards the quantum formalism complete.
But it also modifies this formalism and in this respect it is better under-
stood as a replacement—not as an interpretation—of contemporary QM.24

The quantum probabilities, according to this approach, are objective, i.e.,
they signify pure chance. The question is then what is so realistic about this
theory? The answer is threefold: the theory is realistic about (1) the wave
function, (2) its collapse, and (3) the quantum ‘‘weirdness,’’ i.e., non sepa-
rability. In the non-relativistic versions of the SL approach the physical
state of an isolated system at time t is described with its wave function.25

Everything else, e.g., particles and their positions in 3D space, supervenes

24. Note, moreover, that since the predictions of the SL theory differ from those of QM, there
exists in principle a way to distinguish between the two. Thus, following Gisin 2002, I find
the FAPP objection rather curious: there is nothing to be proud of when one cannot be proved

wrong, since this also means that (in a scientific relevant way) one cannot be proved right!

25. The issue whether a Lorentz invariant collapse theory is possible is beyond the scope of

this paper. See, e.g., Bell 1987, 206–208; Albert 2000; Myrvold 2002.

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on the wave function. This means that when the wave function is sharply
peaked in a volume of configuration space associated with a particle, then
it is located in that region.

The single dynamical law of the SL approach says that the wave func-
tion of an isolated system evolves in conformity with a probabilistic law
that specifies (depending on the wave function at t) the chances of various
wave functions at subsequent times. More precisely the wave function
evolves in accordance with Schrödinger’s deterministic equation except
that at any given time there is a chance that the function will collapse into a
narrower wave function. This collapse is a physical process, a ‘‘hit’’ or a
‘‘jump’’; a part of the wave function, not something else; it has a precise
measure and it occurs at a precise rate, one which is proportional to the
mass density of the system.

The wave function is also a mathematical construct which is composed
of expectation values, or probabilities. Probabilities of what, exactly? Nei-
ther of getting, or finding, a value when a quantity is measured, nor of the
quantity measured having the considered value, but rather the SL approach
has a different story, which draws on Schrödinger’s original interpretation
of the wave function as a density function:

In the beginning, Schrödinger tried to interpret his wavefunction as
giving somehow the density of the stuff of which the world is made.
He tried to think of an electron as represented by a wave packet—a
wave function appreciably different from zero only over a small region
of space. The extension of that region he thought of as the actual size
of the electron—his electron was a little bit fuzzy. At first he thought
that small wavepackets, evolving according to the Schrödinger equa-
tion, would remain small. But that was wrong. Wavepackets diffuse,
and with the passage of time become indefinitely extended, according
to the Schrödinger equation. (Bell 1990, 39)

Following Schrödinger, the SL approach views the wave function as giv-
ing the density of the ‘‘stuff’’ the world is made of in a multidimensional
configuration space. What the theory is about, what is real ‘‘out there’’ at a
given spacetime location x, is just the average mass density in the charac-
teristic volume around x.

In trying to make sense of the this interpretation the SL approach intro-
duces an accessibility criterion which simply states that a property
corresponding to a value of a certain variable is objectively possessed or
accessible when any experiment (or physical process) yielding reliable
information about the variable, would, if performed, give an outcome
corresponding to the claimed value (Ghirardi 1997). Now, the character of
the SL theory is such that the average mass density is not an accessible
property of both microscopic and macroscopic objects: a microscopic

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system, unless its wave function is localized better than a certain quantity
defined by the theory, has almost always an inaccessible average mass
density. A macrosystem, on the other hand, has almost always an acces-
sible average mass density, and hence can be described in classical terms.

Thus the quantum ‘‘weirdness,’’ or quantum non-separability, is, in this
case, ontologically construed. This is exactly the issue on which Einstein
and the SL approach disagree. Einstein objected to the Copenhagen world-
view: it only makes statements about what one will find if . . . , not what is.
The SL approach accepts Einstein’s position but restricts it to the classical
realm, i.e., along with John Bell it rejects the arbitrariness of the ‘‘shifty
split’’ between the classical and the quantum and offers not just vague
words but precise mathematics to tell us what is system and what is appa-
ratus, and which natural processes have ‘‘the special status of measure-
ments’’ (Bell 1990, 34). The idea here is that the collapse of the wave
function is a natural mechanism that permits ‘‘electrons and photons to
enjoy the cloudiness of waves, while allowing tables and chairs to be
described in classical terms’’ (Bell 1987, 190). Thus any embarrassing
macroscopic ambiguity is only momentary in the SL theories, and non-
separability of macrosystems is physically ruled out. The division of the
world, rather than being conventional or arbitrary, is a natural consequence
of the theory. For this reason the realism of the SL approach is best viewed as
‘‘macro-realism’’ (Ghirardi 2000), a position Einstein found hard to accept:

The macroscopic and the microscopic are so inter-related that it ap-
pears impracticable to give up this program [of a realistic description
in space and time] in the microscopic alone. (Einstein in Schilpp 1949,
688)

But perhaps this is the price of an acceptable realism.

5. A Joke Should Not Be Repeated Too Often. In this paper I have tried
to show that no matter how interesting they are, the suggestions to
reformulate the axioms of QM in information-theoretic terms cannot be
regarded as supplying a realistic foundation to QM. Fuchs’ ‘‘thin’’ realism,
and the entire ‘‘fog from the north’’ which inspires it,26 are nothing but
instrumentalism in disguise. Of course there would be no science without
scientists but this does not mean that the ultimate subject matter of science
is the scientist and his relation to the world, no matter how sensitive the
latter is to his touch.

26. Fuchs (2001b, 7) cites John A. Wheeler in a letter to the New York Times: ‘‘It may be, as

one French physicist [here Wheeler refers to de Broglie] put it, ‘the fog from the north,’ but

the Copenhagen interpretation remains the best interpretation of quantum mechanics that we

have.’’

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It is interesting that Einstein, having understood that the motivations
underlying his theories of relativity led to philosophical bankruptcy when
quantum mechanics was discovered—hence his famous EPR argument and
his antipathy to the anti-realist interpretations of Bohr, Heisenberg, and
almost all of the members of physics community —abandoned this ‘‘new
fashion’’ in favour of realism. Einstein’s colleagues, immersed as they
were in the Kantian state of mind, refused to understand how, having
started this fashion in 1905, Einstein could possibly reject it twenty years
later. Einstein’s words to his friend Philip Frank clearly reveal his insight:

Yes, I may have started it, but I regarded these ideas as temporary. I
never thought that others would take them much more seriously than I
did . . . A joke should not be repeated too often. (Clark (1971, 340) cited
in Bechler 1999, 352)

As shown here, there is no need to completely override Einstein’s insight
in light of QM or QIT. One can follow it up to a point, acknowledging
quantum incompleteness as a feature of the micro-world while maintaining
macro-realism and demonstrating how the latter arises from the former in a
precise, observer-independent, and natural way.

The standard measurement problem infects all quantum theories, be
they quantum field theories (see, e.g., Clifton and Halvorson 2002, 28),
quantum cosmology (see, e.g., Bell 1987, 117–138), or, as we have seen,
quantum information theory, and yet no one can plausibly argue that in
order to make progress in these fields the measurement problem must be
solved. Matters are quite different, however, when one ignores the mea-
surement problem and claims further to have made progress in foun-
dational issues. I am afraid the taxpayer will have to find someone else to
rely on in relieving himself from the burden of financing conferences on
quantum foundations.

references

Albert, David (1983), ‘‘On Quantum Mechanical Automata’’, Physics Letters A 98: 249–
252.

——— (1990), ‘‘The Quantum Mechanics of Self-measurement’’, in Wojciech H. Zurek
(ed.), Complexity, Entropy and the Physics of Information. New York: Addison-Wes-
ley, 471–476.

——— (1992), Quantum Mechanics and Experience. Harvard: Harvard University Press.
——— (2000), ‘‘Special Relativity As an Open Question’’, in Heinz-Peter Breuer and

Francesco Petruccione (eds.), Relativistic Quantum Measurement and Decoherence.
New York: Springer, 1–14.

Bacciagaluppi, Guido, and Michael Dickson (1999), ‘‘Dynamics for Modal Interpretations’’,
Foundations of Physics 29: 1165–1201.

Barret, Jeffery (1999), The Quantum Theory of Worlds and Minds. New York: Oxford
University Press.

Bechler, Zeev (1999), Three Copernican Revolutions. Tel Aviv: Zemora-Bitan.
Bell, John S, (1987), Speakable And Unspeakable in Quantum Mechanics. Cambridge:

Cambridge University Press.

773a philosopher looks at qit

#03223 UCP: PHOS article # 700406



——— (1990), ‘‘Against ‘Measurement’’’, Physics World 8: 33–40.
Born, Max (1970), The Born-Einstein Letters. New York: Walker and Company.
Bridgman, Percy W. (1945), ‘‘The Prospect for Intelligence’’, Yale Review 34: 444–461.
Bub, Jeffery (1997), Interpreting the Quantum world. Cambridge: Cambridge
Caves, Carlton, Howard Barnum, Jerry Finkelstein, Christopher Fuchs, and Ruediger

Schack (2000), ‘‘Quantum Probabilities from Decision Theory?’’, Proceedings of the
Royal Society of London A 456: 1175–1182.

Caves, Carlton, Robin Blume-Kohout, and Ivan H. Deutsch (2002a), ‘‘Climbing Mount
Scalable: Physical-Resource Requirements for a Scalable Quantum Computer’’, Foun-
dations of Physics 32: 1641–1670.

Caves, Carlton, Christopher Fuchs, and Ruediger Schack (2002b), ‘‘Conditions for Compat-
ibility of Quantum State Assignments’’, Physical Review A 66: 062111.

Clark, Ronald (1971), The Life and Time of Albert Einstein. New York: The World Pub-
lication.

Clifton, Rob, Jeffery Bub, and Hans Halvorson (2003), ‘‘Characterizing Quantum Theory in
Terms of Information-theoretic Constraints’’, Foundations of Physics, forthcoming.

Clifton, Rob, and Hans Halvorson (2002), ‘‘No Place for Particles in Relativistic Quantum
Theories?’’, Philosophy of Science 69: 1–28.

Deutsch, David (1999), ‘‘Quantum Theory of Probability and Decisions’’, Proceedings of
the Royal Society of London A 455: 3129–3137.

d’Espagnat, Bernard (1966), ‘‘An Elementary Note about ‘Mixtures’’’, in Amos De Shalit
(ed.), Preludes in Theoretical Physics. Amsterdam: North Holland Publication Com-
pany, 185–191.

Fuchs, Christopher (2001a), ‘‘Notes on a Paulian Idea’’, pre-print. http://xxx.lanl.gov/abs/
quant-ph/0105039

——— (2001b), ‘‘Quantum Foundation in Light of Quantum Information’’, pre-print. http://
xxx.lanl.gov/abs/quant-ph/0106166.

——— (2002a), ‘‘The Anti-Växjö Interpretation of Quantum Mechanics’’, pre-print. http://
xxx.lanl.gov/abs/quant-ph/0204146.

——— (2002b), ‘‘Quantum Mechanics as Quantum Information (and a Little More)’’, pre-
print. http://xxx.lanl.gov/abs/quant-ph/0205039.

Fuchs, Christopher and Asher Peres (2000), ‘‘Quantum Theory Needs No Interpretation’’,
Physics Today 3: 70–71.

Furry, W.H. (1936), ‘‘Note on the Quantum Mechanical Theory of Measurement’’, Physical
Review 49: 393–399.

Ghirardi, GianCarlo (1997), ‘‘Quantum Dynamical Reduction and Reality’’, Erkenntnis 45:
249–265.

——— (2000), ‘‘Beyond Conventional Quantum Mechanics’’, in John Ellis and Daniel
Amati (eds.) Quantum Reflections. Cambridge: Cambridge University Press, 79–116.

Ghirardi, GianCarlo, Alberto Rimini, and Tulio Weber, (1986), ‘‘Unified Dynamics for
Microscopic and Macroscopic Systems’’, Physical Review D 34: 470–479.

Gisin, Nicolas (2002), ‘‘Sundays in a Quantum Engineer’s Life’’, in Reinhold A. Bertlmann
and Anton Zeilinger (eds.) Quantum (Un)speakables. New York: Springer, 199–208.

Goldstein, Sheldon, Detlef Dürr, and Nino Zanghi, (1992), ‘‘Quantum Equilibrium and the
Origin of Absolute Uncertainty’’, The Journal of Statistical Physics 67: 843–907.

Hemmo, Meir (unpublished), ‘‘Why Quantum Theory Needs an Interpretation — A Reply to
Fuchs and Peres’’.

Holland, Peter (1993), The Quantum Theory of Motion. Cambridge: Cambridge University
Press.

Maudlin, Tim (2001), ‘‘Interpreting Probabilities: What Interference Got To Do With It?’’, in
Jan Bricmont et al. (eds.) Chance in Physics: Foundations and Perspectives. New
York: Springer, 283–288.

Myrvold, Wayne (2002), ‘‘On Peaceful Coexistence: Is the Collapse Postulate Incompatible
With Relativity?’’, Studies in the History and Philosophy of Modern Physics 33: 435–
466.

Nielsen, Michael A., and Isaac L. Chuang (2000), Quantum Computation and Quantum
Information. Cambridge: Cambridge University Press.

774 amit hagar

#03223 UCP: PHOS article # 700406



Pearle, Philip (1997), ‘‘Tails and Tales and Stuff and Nonsense’’, in Robert. S. Cohen,
Michael Horne, and John Stachel (eds.), Experimental Metaphysics. Dordrecht: Reidel,
Boston Studies in the Philosophy of Science, 143–156.

Peierls, Rudolf (1991), ‘‘In Defence of Measurement’’, Physics World 4: 19–20.
Planck, Max (1945), Thermodynamics. New York: Dover Publications.
Saunders, Simon (1998), ‘‘Time, Quantum Mechanics, and Probability’’, Synthese 114:

235–266.
Schilpp, Paul. A. (ed.) (1949), Albert Einstein: Philosopher Scientist. Cambridge: Cam-

bridge University Press.
Unruh, William G. (1986), ‘‘Quantum Measurement’’, in Daniel Greenberger (ed.), New

Techniques and Ideas in Quantum Measurement Theory. New York: The New York
Academy of Science, 242–249.

van Fraassen, Bas C. (1991), Quantum Mechanics—an Empiricist View. Oxford: Oxford
University Press.

Wallace, David (2002) ‘‘Quantum Probability and Decision Theory Revisited’’, pre-print.
http://xxx.lanl.gov/abs/quant-ph/0211184.

Wheeler, John A. (1989), ‘‘Information, Physics, Quantum: The Search for Links’’, in
Wojciech. H. Zurek (ed.), Complexity, Entropy, and the Physics of Information. New
York: Addison Wesley, 3–28.

775a philosopher looks at qit

#03223 UCP: PHOS article # 700406