# Does Science need Intersubjectivity? The Problem of Confirmation in Orthodox
Interpretations of Quantum Mechanics

Emily Adlam The Rotman Institute of Philosophy, 1151 Richmond Street, London
N6A5B7<EMAIL_ADDRESS>
What must an interpretation of quantum mechanics do in order to be considered
viable? We suggest that one vitally important criterion is the following: _Any
successful interpretation of quantum mechanics must explain how our empirical
evidence allows us to come to know about quantum mechanics._ That is, an
interpretation of quantum mechanics must be able to tell a sensible story
about how empirical confirmation works in the context of quantum-mechanical
experiments, otherwise the whole project will be self-defeating: we cannot
rationally believe an interpretation of a theory which tells us we have no
good reason to believe that the theory itself is right, because our only
reason for believing the interpretation is our belief that the theory is
right!

The ‘probability problem’ and the related problem of confirmation has been
much-discussed in the context of the Everett interpretation[1, 2, 3, 4, 5],
but it is evident that other interpretations which also postulate major
changes to our usual ideas about the relationship between observers and
reality will be vulnerable to similar objections. In this article, we will
focus on the class of interpretations sometimes known as ‘orthodox’
interpretations[6] or alternatively ‘Copenhagenish’ interpretations[7]. These
are interpretations which posit nothing but ‘ _unitary dynamics to
characterize the dynamical evolution of a state vector_ ’ and which ‘ _deny
that we should think of a system as having an observer-independent state._
’[6] Orthodox interpretations differ from the Everett interpretation because
they tell us that measurements have unique outcomes, but those outcomes are
relativized to an observer. Examples of orthodox interpretations include
Copenhagen interpretations[8, 9, 10], neo-Copenhagen interpretations[11, 12,
13, 14, 15, 16, 17], QBism[18], pragmatic interpretations[19] and some
versions of relational quantum mechanics or RQM[20] (specifically, those which
postulate that unitary quantum mechanics is ‘complete’).111 A companion paper
(ref [21]) sets out a proposal to update relational quantum mechanics to
address the problems raised in this article. Moreover, in a sense this paper
is just making explicit ideas which were implicit in RQM all along, so one
might contend that even earlier versions of RQM are not really vulnerable to
our criticisms either. However, this depends on the way in which they are
formulated - in particular, we will see that the assumption that ‘unitary
quantum mechanics is complete’ is the source of much of the problem, and there
are earlier versions of RQM which explicitly postulate that unitary quantum
mechanics is complete, so our criticisms do apply to those formulations.
Certain forms of the ‘It from Bit’ hypothesis may also fall into this
category, insofar as they conceive of ‘information’ in an epistemic, agent-
relative way rather than as a part of physical reality[22, 23]222For reasons
of space we will assume readers are familiar with the interpretations we
discuss in this paper, and thus we will not provide any detailed explication
of them beyond our characterisation of general features of operational
interpretations, but readers who wish to know more are invited to consult the
texts we have referenced here.. We emphasize that the class of interpretations
we are concerned with in this paper does not include instrumentalist
approaches in which one simply chooses not to ask questions about the reality
underlying quantum mechanics - our criticisms pertain specifically to orthodox
interpretations which claim to be revealing a deep truth about reality, e.g.
that it is intrinsically perspectival or that there is no observer-independent
description of it.

It has been observed that interpretations of this kind challenge the standard
scientific doctrine of _intersubjectivity_ about measurement outcomes and
other macroscopic events[24, 25, 26, 27, 28], i.e. they imply that
measurements and other macroscopic events will not typically have the same
outcome for all macroscopic observers. Proponents of orthodox interpretations
have historically taken a rather cavalier attitude toward this fact - for
after all, every interpretation of quantum mechanics has some features which
appear strange to our classical intuition, so it might seem that orthodox
interpretations are no worse off than other approaches in this regard.
However, what this discussion overlooks is the fact that intersubjectivity
plays a vitally important role in the process of empirical confirmation, and
thus the failure of intersubjectivity in orthodox interpretations means that
these approaches have serious difficulties in fulfilling the crucial criterion
of showing how our empirical evidence could allow us to know about quantum
mechanics. This is not just a matter of learning to accept something which
clashes with our classical intuitions: if no reasonable account of empirical
confirmation can be given within these approaches, it would be irrational to
believe in the picture of the world that they present to us, regardless of how
appealing or unappealing we might otherwise find it.

We will begin in section 1 by demonstrating how intersubjectivity fails within
orthodox interpretations of quantum mechanics, and then we will present two
related argument aiming to show that these failures of intersubjectivity
undermine the process of empirical confirmation. First, in section 2, we will
argue that observers in the kind of reality postulated by the orthodox
interpretations would be unable to escape their own perspective in order to
learn anything about the perspectives of other observers, and thus they would
be unable to empirically confirm quantum mechanics unless the theory is
regarded as nothing more than a description of the contents of a single
perspective. Second, in section 3, we will argue that since there is no
natural criterion of identity for ‘observers’ in orthodox interpretations, we
cannot assume that observers in this kind of reality persist over time, and
thus these observers would not even be able to confirm quantum mechanics as a
description of their own personal experiences. We will conclude in section 4
by discussing what could be done to save the orthodox interpretations,
concluding that it cannot be rational to believe these sorts of
interpretations unless they are supplemented with some observer-independent
structure which underwrites intersubjective agreement between different
observers or at least between successive ‘observers’ associated with what we
would normally take to be the same person; and we suggest how this can be
achieved in the case of RQM.

To be clear, our concern in this article is not about whether or not it’s
possible to find empirical evidence that favours an orthodox interpretation
over other interpretations of quantum mechanics - roughly speaking, we accept
that if epistemic concerns of the kind raised in this article can be cleared
up, then all interpretations of quantum mechanics will make the same
predictions at least within the domain of non-relativistic quantum mechanics.
Our concern is rather that if we believe one of the orthodox interpretations,
then quantum mechanics itself will lose its status as empirically confirmed,
and then there can be no possible reason to believe in the orthodox
interpretation in question, or indeed to believe in any interpretation of
quantum mechanics at all. That is, we are worried about the very _possibility_
of empirical confirmation within the kind of universe postulated by the
orthodox interpretations.

Note that in this text we will largely address the orthodox interpretations
together, aiming our arguments at key shared elements which we take to be
responsible for the main epistemic problems. In some cases we will make
digressions to discuss the example of a specific orthodox approach, but we
will not work through the arguments separately for every different orthodox
interpretation - there are simply too many of them for this to be practical.
We consider that this level of detail is adequate to demonstrate that all of
these interpretations have some questions to address, but we acknowledge that
there may be relevant distinctions which entail that our arguments do not land
in quite the same way in the context of some specific orthodox
interpretations, so we would invite proponents of orthodox interpretations to
consider these matters in more detail.

##  1 Intersubjectivity in Orthodox Interpretations

We define the class of interpretations we are concerned with - ‘orthodox
interpretations’ - by their adherence to the following principles (we note
that Pienaar uses a very similar set of principles to define ‘Copenhagenish’
interpretations[29]):

  * •

Relative to each observer, every measurement has a unique outcome, and the
probabilities for these unique outcomes are given by the Born rule - i.e.
there are no Everett-style branches.

  * •

Unitary quantum mechanics is universal and complete - i.e. nothing needs to be
added to the unitary formalism, there is no ‘collapse of the wavefunction,’
there are no hidden variables, and there is no explicit mechanism for
selecting and actualising a single measurement outcome either relative to an
observer or absolutely.

  * •

Quantum mechanics does not describe an observer-independent external reality -
i.e. there is no observer-independent fact of the matter about the true
quantum state of a given system at a given time.

All orthodox interpretations have different ways of characterising what they
take quantum mechanics to be about, given that it is not about an observer-
independent reality, but the general picture is that it describes facts
relative to an observer, agency, consequences, effects, experiences or similar
- the key point is that the content of quantum mechanics is understood to be
relativized to an observer or agent, and is often regarded as being epistemic
in some way. For example, ‘ _A QBist takes quantum mechanics to be a personal
mode of thought – a very powerful tool that any agent can use to organize her
own experience … quantum mechanics itself does not deal directly with the
objective world; it deals with the experiences of that objective world that
belong to whatever particular agent is making use of the quantum theory._
’[30] Similarly, Brukner writes of his neo-Copenhagen interpretation that ‘
_measurement records … can have meaning only relative to the observers; there
are no “facts of the world per se,”_ ’[13], Zeilinger emphasizes that ‘ _it is
information about possible measurement results that is represented in the
quantum states,_ ’[12] and Timpson summarizes the view of Zeilinger as the
statement that ‘ _a physical system literally is nothing more than an
agglomeration of actual and possible sense impressions arising from
observations_ ’[31]. Note that in this article we will use the general term
‘perspectives’ to refer indiscriminately to all of these similar ontologies -
we understand the ‘perspective’ of an observer to contain facts relative to
the observer, experiences, consequences, measurement outcomes, effects, or
whatever it is that a given orthodox interpretation takes the subject of
quantum mechanics to be.

We note that while some orthodox interpretations (e.g. the neo-Copenhagen
approaches of Brukner[13] and Zeilinger[11] and some versions of RQM[26]) deny
that there is any observer-independent reality, or ‘view from nowhere,’ at
all, others (e.g. QBism[18]) maintain that there is an external reality but
deny that quantum mechanics functions as a third-person description of it.
Orthodox approaches of the latter kind necessarily insist that the nature of
external reality is unspeakable and ineffable, in the sense that we cannot
have information about it, obtain evidence about it or meaningfully form
hypotheses about any of its features other than those revealed to us directly
in our own experience in the form of measurement outcomes. For example,
Timpson suggests we should understand QBism as motivated by the view that ‘
_once one seeks to go beyond a certain level of detail, the world simply does
not admit of any straightforward description or capturing of by theory_
’[32]333We note in passing that if this is the motivation for QBism, then one
might think that QBism should be adopted only as a last resort when all
attempts to describe an underlying reality have conclusively failed, and we
are not convinced that this level of despair is justified by the current state
of the field.. Thus QBists cannot use their putative observer-independent
reality to insist that, contra the discussion above, there is some observer-
independent fact about which outcome of a measurement has really occurred,
because if that were the case then there would be a fact of the matter about
which quantum state assignment were right in this scenario, and QBism insists
firmly that there is never any fact of the matter about which quantum state
assignment is right. Fuchs emphasizes this point in his discussion of Wigner’s
friend: ‘ _Who has the right state of information? The conundrums simply get
too heavy if one tries to hold to an agent-independent notion of correctness
for otherwise personalistic quantum states._ ’[33]

Admittedly QBists do hold out the hope that ‘ _when the subjective Bayesian
conception of quantum states and related structures is adopted it will be
possible to ‘see through’ the quantum formalism to the real ontological
lessons it is trying to teach us_ ’[32] but with these kinds of claims they
are walking a fine line - they must maintain that as a matter of principle
this underlying ontology can be grasped only dimly and we will never be able
to describe it or say anything concrete about it, since otherwise QBism would
just amount to a hidden-variable approach with the stipulation that we don’t
yet have a good model of the hidden variables. If there are QBists out there
who believe that the underlying reality is not ineffable and we can
meaningfully form hypotheses about it and/or come up with a theory describing
it, our arguments would not apply to their position - but we would ask QBists
of that genre why they are content to stop at QBism rather than trying to
understand the nature of the underlying reality.

Thus although certain orthodox interpretations admit the existence of an
observer-independent reality, if they are to remain orthodox interpretations
it is crucial that they should forbid us from articulating any concrete
hypotheses about the nature of that observer-independent reality, and
therefore the putative existence of the observer-independent reality doesn’t
significantly change the epistemic status of these approaches as compared to
orthodox interpretations which deny observer-independent reality altogether.

We emphasize that our arguments in this paper are not intended to apply to
approaches which merely hold that quantum mechanics has _some_ epistemic or
relational aspects. We agree with Jaynes that ‘ _[O]ur present [quantum
mechanical] formalism … is a peculiar mixture describing in part realities of
Nature, in part incomplete human information about Nature - all scrambled up
by Heisenberg and Bohr into an omelette that nobody has seen how to
unscramble,_ ’[34] and therefore we agree that at least some parts of the
formalism should be regarded as ‘epistemic’ and/or ‘relative to an observer.’
We also recognise that one might reasonably believe that some parts of quantum
mechanics are epistemic while being uninterested in the nature of the
underlying non-epistemic reality, so we have no problem with the orthodox
interpretations if they are regarded as merely ‘good enough for now’ or ‘good
enough for most current applications’ - e.g. we would not object to a
deflationary form of QBism which suggests that we should interpret quantum
mechanics as a normative principle for making predictions based on what we
currently know, with the proviso that one day it will hopefully be supplanted
by a description of what is really going on underneath. We object only to
views which insist that there is no observer-independent reality at all, or
that observer-independent reality is in principle unknowable and ineffable;
and in particular we object to the corollary that science should not even
attempt to describe an observer-independent reality which could be associated
with quantum mechanics. For it is our contention that the rationality of
scientific confirmation depends implicitly on the assumption that there exists
an observer-independent reality which regulates and coordinates different
perspectives, and therefore one cannot consistently conclude on the basis of a
scientific theory that there is no observer-independent reality or that
nothing whatsoever can be known about it444We note that in some of the
examples we have listed above as orthodox interpretations, it is not entirely
obvious whether their proponents intend to say a) there is no observer-
independent reality, b) observer-independent reality is ineffable or c)
observer-independent reality isn’t ineffable, but quantum mechanics isn’t a
description of observer-independent reality, so we can apply it sensibly even
if we don’t have an understanding of the nature of the underlying reality. The
arguments of this article apply to these views only if they are interpreted as
saying either a) or b) - we have no complaint about c)..

###  1.1 Failures of Intersubjectivity

The point of relativizing the content of quantum mechanics to an observer, as
proposed by the orthodox interpretations, is that this makes it possible to
sidestep some of the problems of standard quantum mechanics by insisting that
‘ _in quantum mechanics different observers may give different accounts of the
same sequence of events._ ’[20] This is supposed to help answer the questions
raised by the ‘Wigner’s friend’ scenario and other similar cases. For example,
suppose Bob knows that his friend Alice is performing a measurement on a
system S which probes the variable $V$ of $S$. When Alice performs the
measurement, she witnesses some measurement outcome $M_{A}$ and thus learns
the value of $V$. But since Bob describes the whole interaction unitarily,
from his point of view the interaction has only caused Alice and S to become
entangled; it has not selected any one measurement result. Orthodox
interpretations resolve the tension between these two descriptions by
insisting that Alice’s measurement has a definite outcome relative to Alice
but not relative to Bob.

We reinforce that in order for this strategy to work, the orthodox
interpretations must maintain that the observer-dependence of the content of
quantum mechanics applies even to macroscopic events like measurement
outcomes, as this is ‘ _the very injunction that keeps the potentially
conflicting statements of (Bob) and (Alice) in check’_[33]. Indeed, since
orthodox interpretations tell us that quantum mechanics is universal, they
cannot possibly insist that measurement outcomes are observer-independent
while microscopic quantum events are observer-dependent - that would be to
postulate an in-principle split between the microscopic and the macroscopic,
which is exactly what orthodox interpretations seek to avoid. Thus such
interpretations must accept that different observers can assign different
states to the apparatus after a measurement, and thus observers can disagree
about which measurement outcome has occurred, or about whether any definite
outcome has occurred at all - and crucially, the orthodox interpretations
insist that there is no fact of the matter about which observer is right. So
in orthodox interpretations, the fact that some observer has seen some
measurement outcome does not entail that this outcome has occurred in any
objective, observer-independent sense, or that it will be reflected in anyone
else’s perspective. And by and large, proponents of orthodox interpretations
seem to accept this consequence: Healey observes that ‘ _QBists and others
have come to question and even deny the principle that a well-conducted
quantum measurement has a definite, objective, physical outcome_ ’[35] and
proponents of orthodox interpretations including Brukner[36] and Healey[35]
have presented several no-go theorems leading to the conclusion that ‘ _the
universal applicability of unitary quantum theory is inconsistent with the
assumption that a well-conducted measurement always has a definite physical
outcome_ ’[35]555As detailed in refs [35, 37] there are reasons one might
object to the arguments of both Brukner and Healey. We mention them here not
because we endorse them but simply as evidence that proponents of orthodox
interpretations typically accept that measurements do not always have definite
outcomes..

Having acknowledged this, we have some further questions to resolve about the
scenario of Alice and Bob. Suppose that Bob measures S in the $V$ basis, and
hence he obtains a measurement outcome $M_{B}^{S}$ which he will interpret as
providing information about the result of Alice’s measurement of $V$ on S.
Suppose that Bob also ‘measures’ Alice herself (e.g. perhaps he simply asks
her about her measurement result) and obtains a measurement outcome
$M_{B}^{A}$ for the value of some pointer variable which is supposed to be a
record of her measurement result. So in this scenario we have three different
measurement outcomes $M_{A},M_{B}^{S},M_{B}^{A}$ all supposedly providing
information about the value of the same variable. What should an orthodox
interpretation say about the relationships between these three measurement
results?

We present two different postulates which could be employed to answer this
question. We have drawn these postulates from discussions in the literature on
RQM (Shared facts is the version proposed by ref [24], while Internally
consistent descriptions is the version which appears in [38]) but it is clear
that other orthodox interpretations will also face a similar dilemma about how
to describe this scenario.

######  Definition 1.1.

Shared facts (SF): If Bob measures Alice to ‘check the reading’ of a pointer
variable, the value he finds is necessarily equal to the value that Alice
recorded in her earlier measurement of S (i.e. $M_{A}=M_{B}^{A}$)

######  Definition 1.2.

Internally consistent descriptions (ICD): If Bob measures Alice then also
measures S, the two values found are in agreement (i.e. $M_{B}^{A}=M_{B}^{S}$)

We note that ICD actually follows directly from the formalism of unitary
quantum mechanics. For when Alice measures S, from Bob’s point of view she
becomes entangled with S in a state of the form
$\sum_{i}c_{i}|i\rangle_{S}|i\rangle_{A}$ where $\\{|i\rangle_{S}\\}$ is the
set of possible values for the variable $V$, and $|q\rangle_{A}$ represents
the state of Alice in which she has seen the outcome $V=q$. Clearly then if
Bob measures S and gets outcome $|q\rangle_{S}$, when he measures Alice he
must get outcome $|q\rangle_{A}$, and vice versa, so $M_{B}^{S}$ and
$M_{B}^{A}$ must always agree - in particular, if Bob ‘measures’ Alice by
asking her what her measurement outcome was, necessarily Bob will experience
Alice as reporting a measurement result which matches the result that Bob
himself obtained in his measurement on S. Thus since orthodox interpretations
make no changes to the standard framework of unitary quantum mechanics, they
must all predict that ICD is true, i.e. they all entail that
$M_{B}^{S}=M_{B}^{A}$. (We note that the case of QBism is a little more
complicated since QBism tells us that the probabilities in quantum mechanics
are not objective - we will return to this in a moment).

We now have a further question about whether SF is also true. If ICD is true
but SF is not true, then it could be the case that $M_{B}^{S}=M_{B}^{A}$ but
$M_{A}\neq M_{B}^{S}$ and hence $M_{A}\neq M_{B}^{A}$: in that case, whatever
Alice tells Bob about the result of her experiment will be perceived by Bob as
being consistent with Bob’s other observations, but what Bob perceives Alice
as saying may be different from what Alice experienced herself as observing
and what she perceives herself as saying.

And in fact, a short argument shows that SF cannot be reliably true within any
orthodox interpretation:

  1. 1.

Orthodox interpretations tell us that there are no observer-independent facts
about events which can be described quantum-mechanically.

  2. 2.

Orthodox interpretations insist that quantum mechanics is universal, so all
macroscopic events can be described quantum-mechanically; thus orthodox
interpretations tell us that there are no observer-independent facts about
_macroscopic_ events.

  3. 3.

Since there are no observer-independent facts about macroscopic events in the
context of an orthodox interpretation, then in order to say that SF is
reliably true in such an interpretation, we will need the interpretation to
explicitly provide some structure which coordinates measurement outcomes for
different observers in the way specified by SF.

  4. 4.

Unitary quantum mechanics does not have the resources to ensure that the
measurement outcomes for different observers will reliably match in the way
specified by SF, since it does not even have an explicit mechanism to select
and actualise a single measurement outcome relative to any one observer.

  5. 5.

Orthodox interpretations do not add any structures or mechanisms to unitary
quantum mechanics, so they don’t have the resources to ensure that the
measurement outcomes for different observers will reliably match in the way
specified by SF.

  6. 6.

So orthodox interpretations cannot tell us that SF is reliably true.

Let us consider points three and four in more detail. First, evidently in an
interpretation which allows the existence of observer-independent facts, we
can have an explicit mechanism similar to a wavefunction collapse which
selects and actualises a unique observer-independent measurement outcome, and
that outcome can play a coordinating role which ensures that if observers use
their perceptual equipment and communicate properly, their outcomes will match
in the way specified by SF. But orthodox interpretations do not postulate any
observer-independent facts, so they must appeal to some other kind of
structure if they are to uphold SF. Since the orthodox interpretations insist
that unitary quantum mechanics is universal and complete, the only structures
they endorse are those inherent in the unitary dynamics of quantum mechanics.
But the unitary dynamics do not even provide any mechanism to select and
actualise a single measurement outcome relative to one observer, so they
certainly cannot provide a mechanism to select and actualise a single
measurement result in a global way that holds for all observers, so there is
no structure available within the orthodox interpretations which could ensure
that SF is upheld. In an interpretation like the Everett interpretation we can
get around this problem by relativizing SF to a branch of the wavefunction and
showing that the measurement results of all observers within a single branch
will always match. However, since orthodox interpretations insist that
measurement have unique outcomes, this route is not open to them either. Thus
it would seem in an orthodox interpretation there is simply nothing that could
possibly play the role of making sure that $M_{B}^{S}$ matches $M_{A}$, and
for the same reasons there is nothing that could possibly play the role of
making sure that $M_{B}^{A}$ matches $M_{A}$; thus SF cannot be reliably true
within an orthodox interpretation. This is also the conclusion reached by Van
Fraassen, who notes that ‘ _we have no basis or law (within unitary quantum
mechanics) on which to connect the outcomes of measurements by different
observers, no matter how intimately they may be related._ ’[27] Similarly,
Muc̃ino et al note that standard quantum mechanics as used in RQM ‘
_explicitly lacks such a single unified description, i.e., it does not contain
a mathematical object encoding all the different perspectives._ ’ [28]

We recognise that it may be responded by some proponents of orthodox
interpretations that questions about the relation between $M_{A}$ and
$M_{B}^{A}$ are illegitimate - i.e. it is meaningless to ask if Alice and
Bob’s perspectives agree, because ‘ _“facts” can only exist relative to the
observer’_[13]. It will presumably be argued that within the context of an
orthodox interpretation, the closest we could manage to such a comparison
would be to have some third observer measure Alice and Bob separately and
compare the results, and of course due to ICD these results will always be
found to agree666Again, we acknowledge that QBism does not necessarily entail
that ICD holds so QBism does not necessarily tell us that these results will
always be found to agree., so from this point of view there can never be any
genuine disagreement between Alice and Bob. However, the results of a
measurement by a third observer tells us nothing about the relationship
between the subjective experiences of Alice and Bob, and subjective
experiences cannot be disregarded, because it is our subjective experiences
which form the basis for the empirical confirmation of a scientific theory
like quantum mechanics. In fact we fully concur that questions about the
relation between relationships between the subjective experiences of Alice and
Bob are probably meaningless if the world is the way that the orthodox
interpretations say it is - but that is precisely the problem! As we will
shortly argue, it is necessary to be able to pose and answer such questions if
we are to carry out empirical confirmation for a scientific theory like
quantum mechanics, so if the orthodox interpretations say that such questions
can’t meaningfully be posed, then the orthodox interpretations necessarily
tell us that quantum mechanics is not empirically confirmed. Our complaint is
not simply that the orthodox interpretations do not have answers to questions
that we would like answered - our complaint is that if these questions really
can’t be posed then most forms of scientific confirmation will be severely
undermined.

#### ICD

We note that ICD has been presented by some proponents of orthodox
interpretations as a solution to the problem of intersubjectivity: so for
example Laudisa and Rovelli write, ‘ _this means that a third system measuring
X and $F_{X}$ will certainly find consistent values. That is: the perspectives
of F and W agree on this regard, and this can be checked in a physical
interaction._’[39] But in fact, ICD is really the source of the problem! For
if quantum mechanics is indeed universal, as the orthodox interpretations
insist, it follows that when one observer asks another observer what outcome
they obtained in some measurement, that is just another kind of quantum
measurement - it is a physical interaction in which one observer ‘measures’
the other observer by asking questions about their measurement results. And
thus since we have already granted that in orthodox interpretations there is
no guarantee that different observers will agree on the outcome of a quantum
measurement, it follows that they may also disagree about what is communicated
in this sort of conversation. Furthermore, not only is it possible that they
will disagree, ICD tells us that they _must_ disagree any time they compare
notes about the result of a measurement where their respective perspectives
say different things about the outcome of the measurement. For example, we
have seen that in the case of Alice and Bob, unitary quantum mechanics cannot
provide a guarantee that $M_{B}^{S}$ matches $M_{A}$; and since ICD entails
that Bob’s two outcomes match, it follows that if Alice’s result is different
from Bob’s, then when Bob asks Alice about the result of her measurement (i.e.
when he performs the measurement whose result we have labelled $M_{B}^{A}$),
necessarily Alice and Bob will disagree about the very words that Alice says
during their conversation! And presumably, since the orthodox interpretations
insist that everything is always relativized to an observer, the same is true
no matter how Alice and Bob attempt to communicate with one another: there is
no way for them to get outside their own perspectives to achieve
intersubjective agreement, and ICD will always prevent them from telling one
another about the ways in which their relative descriptions disagree. So Alice
and Bob seem to be permanently stuck in incommensurate realities - they can
never actually compare their descriptions of reality because all possible
means of communication are also part of the relative description. Thus
orthodox interpretations are saddled with ubiquitous failures of
intersubjectivity that can’t be rectified by any amount of communication and
comparison. Pienaar poetically describes this scenario as an ontology of ‘
_island universes._ ’[24]

The situation is slightly different in QBism (and in any other orthodox
interpretation which tells us that the probabilities associated with quantum
mechanics are purely subjective). Like the other orthodox interpretations
QBism says that quantum mechanics is complete and universal, and Pienaar
emphasizes that this means ‘ _it can be used upon any part of reality
(including other agents),_ ’ so it remains true that in the context of QBism
Bob must necessarily describe Alice after her measurement using the entangled
state $\sum_{i}c_{i}|i\rangle_{S}|i\rangle_{A}$. It then seems natural to
conclude that Bob’s measurements must match and thus ICD must hold in QBism,
meaning that agents in a QBist universe will likewise be prevented from
communicating their experiences to each other. However, because the
probabilities in QBism are purely subjective probabilities, it does not follow
from Bob’s assignation of the state $\sum_{i}c_{i}|i\rangle_{S}|i\rangle_{A}$
that the results of Bob’s measurements must actually match, because Bob’s
assignation of this state only describes his own subjective beliefs and does
not place any constraints on what will actually happen. So although it is the
case that QBism, like all orthodox interpretations, must _predict_ that ICD is
true, in the sense that any individual observer in a QBist world will always
assign probabilities in accordance with ICD, nonetheless in QBism reality is
not obliged to conform to the probabilities predicted by an agent and
therefore it need not be the case that ICD is obeyed by the actual outcomes
that are obtained.

However, this doesn’t help QBism much, because although it doesn’t insist that
ICD is always true, it also can’t possibly tell us that SF is reliably true.
For QBism is very clear about the point that _‘measurement outcomes are
personal_ ’ and ‘ _My measurement outcomes happen right here, to me, and I am
talking about my uncertainty of them_ ,’ which entails that there can be no
automatic expectation of any kind of agreement between outcomes of different
observers. And since the QBist tells us that quantum probabilities are purely
subjective and do not describe or constrain anything in reality, we cannot use
those probabilities to draw conclusions about the relationships between
outcomes of different observers. Nor does the Bayesian part of QBism help with
this, as by definition the QBist’s coherence requirements apply only to the
credences of a single observer, so they tell us nothing about the
relationships between the perspectives of different observers. So although
QBism does not necessarily have to obey ICD, nor is there any reason for
QBists to think that SF is generally true, so QBist agents are also trapped in
their own ‘island universes.’

#### Perspectives on SF

We note that many proponents of orthodox interpretations by and large seem to
agree that their approaches are not compatible with SF. For example, Dascal
writes that in a neo-Copenhagen interpretation, ‘ _there need not be any
causal arrow between what Alice observes and WignerA’s outcome_ ’ - i.e. we
should not assume that Alice’s outcome has any causal effect on the outcome of
the measurement of WignerA (who plays the role of Bob in our scenario).
Similarly, Fuchs tells us that according to QBism, ‘ _What we learn from
Wigner and his friend is that we all have truly private worlds in addition to
our public worlds.’_[33]. And Healey presents several no-go theorems which aim
to show that, under certain assumptions, if quantum mechanics is universal
than quantum measurement outcomes cannot always be objective in the sense that
‘ _(an observer’s) outcome is accessible to other observers by consulting her
records_ ’[35].

However, some proponents of orthodox interpretations have tried to argue that
SF actually does hold in their approaches. For example, in the neo-Copenhagen
approach of Brukner, it is argued that although in general the information
about Alice’s outcome (in this case, the outcome of a coin toss) is not
available to Bob, nonetheless ‘ _Were he to measure in a basis that instead
corresponded to Alice’s coin toss (i.e. a basis whose vectors corresponded to
‘Alice having observed heads’ and ‘Alice having observed tails’), then the
theorist may explain, perhaps, that his information frame grows or changes to
encompass Alice’s as well._[13] While we agree that this seems sensible, we
emphasize that it cannot possibly follow from unitary quantum mechanics. In
fact in making this kind of suggestion the orthodox interpretations are
illegitimately piggybacking on intuitions developed in the context of quantum
mechanics _with collapse_. For if we take it that the wavefunction collapses
when Alice performs a measurement, it follows that after the measurement Alice
will be in an eigenstate of the form $|heads\rangle$, and hence clearly when
Bob measures her he will obtain the result $|heads\rangle$. But orthodox
interpretations specifically deny that the wavefunction collapses relative to
Bob when Alice performs a measurement (they insist, for example, that it is
still possible for Bob to observe interference between the ‘heads’ and ‘tails’
branch of Alice’s superposition) and therefore after the measurement Alice is
not in an eigenstate relative to Bob, so unitary quantum mechanics gives us no
reason to expect that Bob’s measurement will have an outcome that matches
Alice’s. So if the proponents of orthodox interpretations want to insist that
Bob’s measurement outcome will in fact match Alice’s, they owe us some account
of how this is supposed to come about: if all facts are relative to
perspectives and measurement outcomes are just ‘personal experiences,’ what is
the justification for assuming that in the specific case where Bob and Alice
use the same basis there is suddenly a guarantee of agreement between them?
Orthodox interpretations which try to insist on SF are trying to have it both
ways: they are sneaking in some observer-independent facts which are not part
of unitary quantum mechanics to ground their expectation of agreement between
perspectives in certain cases, while still insisting that everything is
observer-dependent and nothing needs to be added to unitary quantum mechanics.

In a similar vein, some QBists have suggested that although SF is not true in
QBism, nonetheless we can achieve intersubjective agreement after the fact: ‘
_An agent-dependent reality is constrained by the fact that different agents
can communicate their experience to each other … Bob’s verbal representation
of his own experience can enter Alice’s, and vice-versa. In this way a common
body of reality can be constructed, limited only by the inability of language
to represent the full flavor … of personal experience_ ’[30]. However, we have
already seen that this is not the case, because QBism can provide no guarantee
that $M_{A}$ matches $M_{B}^{A}$, and $M_{B}^{A}$ could represent an instance
of verbal communication, e.g. Bob asking Alice what her measurement result
was. If we are really to believe that quantum mechanics is universal then it
must describe verbal utterances as well as other possible methods by which Bob
could ‘measure’ Alice, and therefore verbal utterances cannot magically ensure
intersubjective agreement. Moreover, if we _did_ allow that verbal utterances
were somehow privileged over all other types of macroscopic events, such that
that Alice and Bob can disagree about readings on measuring instruments but
they will never disagree about the content of verbal communication, then QBism
would be empirically inadequate, because this would imply that
experimentalists should frequently find themselves inspecting measuring
instruments and then in subsequent conversations being told that their
colleagues have seen a completely different reading on the same measuring
instrument at the same time - and to our knowledge this is not something which
regularly happens! Thus we must conclude that in a QBist world, Bob has no way
to know whether or not the words he is hearing from Alice are the same as the
words she imagines herself to be speaking; QBism puts us all within our own
‘truly private worlds,’ and because of its own determined reticence about
observer-independent reality it can’t allow us to say anything at all about
the way in which those private worlds are connected together via verbal
communication or otherwise. Thus in QBism there is no way to build up a
‘common body of reality,’ because the theory gives us no reason to expect that
communication via language will work properly when observers try to
communicate disagreements in their perspectives.

Finally, some proponents of orthodox interpretations seem to accept that SF
fails to be true in special cases, but maintain that in most circumstances it
will be true. For example, after presenting his thought-experiment aiming to
show that quantum measurement outcomes are not always objective, Healey
observes that ‘ _the circumstances of the Gedankenexperiment in the third
argument are so extreme as forever to resist experimental realization. There
are no foreseeable circumstances in which the argument would require us to
deny that a well-conducted quantum measurement has a definite, physical
outcome. The arguments considered in this paper give us no reason to doubt the
sincerity or truth of experimenters’ reports of definite, physical outcomes._
’[35] But this conclusion gets the burden of proof the wrong way round. Healey
seems to assume here that observers are guaranteed to agree on measurement
outcomes except in the special cases where it is possible to prove a no-go
theorem to the effect that intersubjective agreement is impossible: but the
orthodox interpretations specifically deny the existence of any external
reality or structure which could coordinate the perspectives of different
observers, so they are not entitled to take intersubjective agreement for
granted in any scenario, even if there aren’t any no-go theorems for that
scenario. No-go theorems of this kind, if accepted, simply demonstrate in a
more explicit way that the orthodox interpretations are committed to failures
of intersubjectivity, but there is no reason to think that the failures of
intersubjectivity will be confined to the cases described by the no-go
theorems.

#### Decoherence

Some proponents of orthodox interpretations believe that decoherence has no
foundational significance; for example, in QBism, ‘ _decoherence does not come
conceptually before a “selection,” but rather is predicated on a time $t=0$
belief regarding the possibilities for the next quantum state at time $t=\tau$
… In this sense the decoherence program of Zeh and Zurek … regarded as an
attempt to contribute to our understanding of quantum measurement, has the
story exactly backward._’ On the other hand, some proponents of orthodox
interpretations seem to suggest that the problem of intersubjectivity can be
solved by decoherence. For example, Brukner uses an approach based on a
‘Q-function’ characterising probabilities for measurements of coarse-grained
variables777The Q-function is an approach to classicality which has some
similar features to decoherence, although it is not dynamical in character and
thus differs conceptually in various ways. to argue that for measurements on
macroscopic systems ‘ _the Q-function before and after a coarse-grained
measurement is approximately the same. It therefore becomes possible for
different observers to repeatedly observe the same macroscopic state. The
result is a certain level of intersubjectivity among them._ ’

However, Brukner’s approach, like the others we have considered, regards
quantum states as relative to an observer, which entails that when Alice
performs a coarse-grained measurement, the macroscopic state of the system
becomes definite relative to Alice, but relative to other observers this
interaction simply leads Alice to become entangled with the relevant system as
in the example we saw earlier. So when Bob performs another coarse-grained
measurement on the same system, he will find a definite macroscopic state, but
since this approach adds nothing to unitary quantum mechanics it cannot
provide us with a guarantee that the macroscopic state found by Bob is the
same one found by Alice. So although both observers experience a stable
macroscopic reality, there is no reason to expect that both observers
experience the _same_ macroscopic reality. Similar issues would be expected to
arise within other orthodox interpretations, and therefore it seems that
decoherence does nothing to solve the problem of intersubjective agreement in
orthodox interpretations.

##  2 Accessing Other Perspectives

Now, the ‘ontology of island universes,’ might seem unappealing - for example
Pienaar objects that ‘ _This proliferation of disjoint universes is not
motivated by observations, nor does it serve any explanatory purpose; it
therefore seems to be (paraphrasing Wheeler) ontological ‘excess baggage’.
Moreover, it is difficult to see what ‘objectivity’ could possibly mean in
such a universe._ ’[24] But by and large the proponents of orthodox
interpretations seem willing to accept the failure of intersubjectivity.
Indeed, they regard the notion that science describes an external reality on
which we could hope to reach intersubjective agreement as a naive idea that
must be discarded; ref [18] criticizes it as ‘ _a profound misconception in
our general view of science, which led us into major confusion in the 20th
century,_ ’ and ref [6] suggests that ‘ _Orthodox interpreters can counter …
that the ideal of an observer-independent reality is not methodologically
necessary for science and … modern physics (especially, but not only, quantum
theory) has taught us … that there is a limit to the usefulness of pursuing
this ideal.’_

But these responses, we suggest, misunderstand the issues at stake. The
problem with the ontology of island universes is not just that it is
unappealing or in tension with our naïve classical ideas; the problem is that
intersubjectivity plays a crucial role in the process by which we arrive at
and confirm scientific theories, so if intersubjectivity fails, the status of
quantum mechanics as empirically confirmed is endangered. And if quantum
mechanics can no longer be regarded as empirically confirmed, then evidently
we will have no reason to attach any credence to an orthodox interpretation,
or indeed to any interpretation of quantum mechanics at all. Thus the whole
approach starts to look self-undermining: no adequate interpretation of
quantum mechanics can have the consequence that we should not believe in
quantum mechanics in the first place, so this is an issue which should be
treated with the utmost seriousness by the proponents of orthodox
interpretations.

Intersubjectivity is important to the scientific process because science is a
collaborative endeavour: typically we put our faith in a theory based not only
on our own observations but also the observations that have been made and
reported by a multitude of other scientists across the globe. For example, the
proponents of the orthodox interpretations clearly have a high level of
confidence that quantum mechanics is right, and yet many of them are
theoreticians who have not spent much time performing laboratory tests of
quantum-mechanical effects, and therefore their own observations certainly do
not provide enough data to empirically confirm quantum mechanics as a whole.
Even a physicist more experimentally inclined who has engaged in a variety of
experimental tests of quantum mechanics will necessarily rely on the testimony
of others to verify that quantum mechanics has worked in roughly the same way
across most of the Earth for at least the last 100 years, and to reproduce her
experiments to make sure that she is using correct experimental technique and
her observations are not just some kind of fluke.

Obviously, even in classical physics intersubjectivity does not hold with
complete reliability. Other scientists could lie about their results;
observers could have visual impairments or hallucinations or false memories
which cause them to disagree about the result of a measurement. But
nonetheless in this case there is no barrier _in principle_ to
intersubjectivity: the observations of different observers are connected to
each another by the unique observer-independent reality which they are all
trying to observe, so provided that everyone is honest and everyone’s
perceptual systems and memories are working correctly, everyone will agree on
the results of measurements and other such macroscopic events. Moreover,
classical physics tells us that it is possible to verify the accuracy of our
observations by comparing notes with other observers, so if something has gone
wrong it is straightforward to find out and repeat the experiment.

The failure of intersubjectivity in orthodox interpretations of quantum
mechanics is significantly more severe, as even in the ideal case where
everything is working perfectly, observers may still disagree. And we have
seen that not only will observers disagree in such cases, in general they
_will not even realise that they disagree._ So for example if Alice and Bob in
the scenario described above are both trying to use the outcome of Alice’s
measurement on $S$ to confirm a scientific theory, both of them will add
different pieces of data to their set of recorded observations and will
proceed with their scientific labours in blissful ignorance of the fact that
they’re both working with different data. And presumably, if they repeat this
kind of experiment multiple times, it seems likely that they will ultimately
come up with different scientific theories to match their different datasets -
but any time they try to have conversations about the theories they have come
up with, ICD tells us that both will always be convinced that the other is
agreeing with them!888As we observed earlier, ICD does not necessarily hold in
QBism so QBism doesn’t guarantee that observers will always perceive other
observers as agreeing with them - but as we noted, QBism also cannot provide
any reason to think that verbal communication is accurately conveying the
content of experience, so the same problem arises.

Furthermore, intersubjectivity is actually _more_ important in orthodox
interpretations than it is in the case where we regard quantum mechanics as a
description of an observer-independent reality. For orthodox interpretations
tell us that quantum mechanics is specifically _about_ the structure of a
network of perspectives, not about external reality, so when we are trying to
obtain empirical confirmation for the theory, the only kind of information
that matters is information about the structure of this network of
perspectives. Moreover, clearly information about just one perspective will
not tell us much about the features of the whole network, and yet we will
shortly see that it is important for orthodox interpretations to maintain that
quantum mechanics is a theory about the whole network, not just a single
perspective. So in order for quantum mechanics to have any empirical
confirmation in this context, it is vital that we should have access not only
to our own perspective but also to the perspectives of some other observers -
exactly what the orthodox interpretations apparently forbid us from having!

Orthodox interpretations thus face a stark dilemma: either there are no
grounds for expecting intersubjective agreement between observers about
measurement results, in which case it will be impossible to gather the kind of
evidence that is needed for quantum mechanics to be empirically confirmed, or
there _are_ grounds for expecting intersubjective agreement between observers
in at least some relevant subset of cases, in which case it’s simply not true
that quantum mechanics does not describe an observer-independent reality - the
observer-independent reality is precisely what one gets from combining the
perspectives of observers in the cases in which they typically agree. It is
therefore clear that intersubjectivity cannot be discarded too lightly: if we
_are_ going to postulate that intersubjectivity fails in the way posited by
these orthodox interpretations, it is vital that we should be able to offer a
good story about how it is that quantum mechanics could come to be empirically
confirmed in a world like this.

###  2.1 Confirmation

Let us now take a closer look at how the process of empirical confirmation
might work in the context of an orthodox interpretation. We begin with the
idea that in order for me to be able to arrive at and empirically confirm
scientific theories, at minimum I must be able to to look at a measurement
outcome and use that observation to update my beliefs about the world as a
whole - that is, it must be the case that when I make an observation, I will
sometimes update my beliefs in such a way that the probabilities I assign to
various different possible worlds will change. Moreover, we take it that a
defining feature of a scientific theory is that it is generalisable - it does
not just describe a set of observations, but also allows us to extrapolate
beyond the observations that we have used to arrive at it. So in order for me
to be able to empirically confirm scientific theories, it must be the case
that when I make an observation, I will sometimes update my beliefs about
something other than the observation I have just made - i.e. the probabilities
I assign over events in parts of the world outside of the region of my
observation must change.

Now, in the case of orthodox interpretations this is already complicated by
the fact these approaches typically insist that ‘ _there is no description of
the universe “in toto,” only a quantum-interrelated net of partial
descriptions._ ’[20] This makes it a little hard to understand what exactly we
are trying to confirm when we seek empirical confirmation within an orthodox
interpretation. However, these approaches are mostly intended as realist views
and thus they do postulate something real: in general they agree that we have
a set of observers whose experiences are subject to reliable probabilistic
relationships as predicted by quantum mechanics. So we will take it that in
this context the role of a ‘possible world’ is played by a possible _network
of perspectives_ , i.e. a set of perspectives together with the observations
occurring within each of the perspectives. We emphasize that the orthodox
interpretations tell us that quantum mechanics is universal, i.e. it correctly
describes all the perspectives in the network, so what we are trying to
confirm is not just the content of a single perspective but the regularities
obtaining in many different perspectives. Thus in this section we will suppose
for the sake of argument that we believe ourselves to live in a world in which
there are no observer-independent facts (except possibly ineffable ones), but
there are some facts about the regularities that obtain within different
perspectives, and we are trying to confirm the hypothesis that the
regularities that obtain across the whole network of perspectives are
approximately the ones specified by quantum mechanics. We will then ask what
empirical confirmation could look like in such a scenario.

We pause to note that the role of confirmation may seem less clear in the case
of a view like QBism, which holds that quantum mechanics is normative rather
than descriptive - i.e. it simply tells agents how to assign probabilities.
One might wonder if it makes sense to seek empirical confirmation for a purely
normative theory, and indeed, Fuchs has argued that as far as possible we
should seek to derive the mathematical framework of quantum mechanics from
pure information-theoretic principles. But plausible as those information-
theoretic principles might be, they are not tautologies, so we still have to
rely on empirical evidence to find out if they hold in our actual world - for
example, although the Born rule may have some Bayesian justification,
empirical input is also needed to arrive at it. Indeed, if we were to say that
quantum mechanics actually has nothing whatsoever to do with the empirical
evidence that has led us to arrive at it, it would be an amazing and
inexplicable coincidence that the empirical evidence has led us to a theory
which the QBists believe to be correct, even though that theory does not
actually have any connection to the evidence! So it seems that the QBists must
accept that quantum mechanics is at least to some degree susceptible to
empirical confirmation. And indeed, most QBists seem to accept that their
normative principles are ‘ _empirically based (as quantum theory itself is)_
’[33] so we think it is reasonable to demand that QBism should allow for
empirical confirmation, in the sense that we should be able to use empirical
observations to gain support for the QBist’s proposed empirically motivated
norm of rationality.

###  2.2 Minimal conditionalisation

One common model for belief-updating based on empirical evidence is minimal
conditionalisation, which ‘ _simply amounts to setting one’s credence to zero
on the proposition whose falsity one has just learnt, and renormalising._ ’[2]
For example, if we perform an experiment at time t and observe outcome E, we
learn that the proposition ‘E does not occur at time t,’ is false, and we
should therefore eliminate all possible worlds in which E does not occur at t.
But it is hard to see how we could apply minimal conditionalisation in the
context of an orthodox interpretation, because we simply don’t seem to be
dealing with the right sort of objects. When I perform an experiment at time t
and observe outcome E, I learn that the proposition ‘E does not occur at time
t,’ is false _relative to me_ , and therefore I can eliminate all possible
descriptions of the world _relative to me_ in which E does not occur at t,
then renormalise my credences across descriptions of the world _relative to
me_. There does not seem to be any point at which I can drop the ‘relative to
me’ modifier and say that I have learned something about the network of
perspectives as a whole: for ICD entails that even when other observers report
their results to me, I will hear the content of their report _relative to me_
, which may be different from what they themselves imagine the report to be
saying! So it is clear that minimal conditionalisation is not adequate for
empirical confirmation in the kind of world described by an orthodox
interpretation, since it will never allow agents to update their beliefs about
regularities obtaining in perspectives other than their own.

###  2.3 Centered Worlds

Now, one might hope that these problems could be addressed by moving to an
alternative account of belief-updating. For example, Lewis offers an approach
designed to deal with the case of self-locating beliefs, i.e. beliefs about
where or when one is located within a given possible world. He postulates a
set of ‘centered worlds,’ where ‘ _roughly, a centered world is an ordered set
of a possible world and a perspective within it,_ ’[40] and he claims that
Bayesian conditionalisation can be applied straightforwardly to self-locating
beliefs by replacing possible worlds with centred worlds[41]. On this
approach, the credence assigned to a possible world is the sum of the
credences assigned to all of the centred worlds associated with it, so the
credences assigned to a possible world as a whole may be altered by a change
in our credences for the centred worlds associated with that possible world,
and therefore learning purely self-locating information can alter our beliefs
about reality as a whole.

We might hope to make use of this strategy in the context of orthodox
interpretations by defining a centered world as a perspective relative to a
certain observer. That is, let a centered world be an ordered set of a network
of perspectives (which plays the role of a ‘possible world’ in the orthodox
interpretation context) and a ‘perspective’ (in the sense of an orthodox
interpretation, i.e. a set of facts relative to an observer or similar). Then
when I make an observation I can rule out all the centered worlds which are
incompatible with my observations, which may result in changes in the number
of centered worlds associated with some possible networks of perspectives, and
thus overall the probabilities I assign to various different networks of
perspectives may change, making it possible to perform empirical confirmation
in the orthodox context after all.

However, note that we can used centered worlds to arrive at meaningful
empirical confirmation for scientific theories only if we start out with some
priors such that in some cases, our priors assign _different_ probability
distributions over the content of the ‘rest of the world’ conditional on
subjectively different centred worlds. That is to say, let the ‘rest of the
world’ be the content of some possible world minus the content of some
particular centered world within that possible world. Let us form equivalence
classes of ‘subjectively identical’ centered worlds, i.e. centered worlds or
perspectives which belong to different possible worlds but are
indistinguishable from the point of view of the observer to whom they belong;
when we make an observation, we will rule out all the equivalence classes of
centered worlds which are inconsistent with our observation and renormalise.
Thus if our priors assign the same probability distribution over the ‘rest of
the world’ conditional on every possible equivalence class of subjectively
identical centered worlds, if follows that the probability distribution we
assign over the ‘rest of the world’ will not change during this updating
process. Thus under these circumstances, an observation does not allow me to
update my beliefs about anything other than the observation that I have just
made: I learn that the actual world contains a centered world with certain
subjective features, but I learn nothing at all about what other centered
worlds feature in the actual world or how these centered worlds are related,
so I can’t obtain confirmation for any beliefs about the world which
extrapolate beyond my direct observations, and as we have already noted, in
order for me to be able to arrive at and empirically confirm a theory like
quantum mechanics it must sometimes be possible to extrapolate beyond direct
observations.

In ordinary cases of empirical confirmation, our priors do indeed assign
different probability distributions over the content of the ‘rest of the
world’ conditional on subjectively different centred worlds. For example, this
is often achieved by choosing priors which assign a higher weight to possible
worlds which look ‘simple’ in some way, e.g. worlds in which regularities
typically persist across time. But we do not seem to have this option in a
setting where we are not allowed to postulate any knowable observer-
independent facts. In particular, we saw in section 1.1 that orthodox
interpretations do not allow us to postulate any that there should be any
systematic relations between observations made by different observers, so it
would seem that if we believe ourselves to be in a universe of the kind
described by an orthodox interpretation, we cannot select prior probabilities
which favour networks of perspective exhibiting certain sorts of relations
between perspectives: we can’t select priors which encode beliefs like
‘regularities typically persist across different perspectives’ since those
beliefs, if true, would constitute knowable, observer-independent facts.
Indeed it’s hard to see how we could formulate any prior probabilities at all
under this limitation (even the uniform distribution is not really a neutral
choice, as it still involves choosing _some_ measure over the relationships
between perspectives).

###  2.4 Adding more assumptions

It seems, therefore, that neither minimal conditionalisation nor Lewisian
centered-world conditionalisation is going to lead to any meaningful empirical
confirmation in the context of an orthodox interpretation. Of course, it is
possible that a new account of confirmation could be proposed specifically for
this scenario, and indeed we would invite the proponents of orthodox
interpretations to take up this challenge. But there is an overarching problem
which seems likely to inflict any account of confirmation in the context of
orthodox interpretations: if we are to understand how quantum mechanics (or
any other scientific theory) could be empirically confirmed in this context,
we must be provided with some bridging principle telling us how to get from
the facts about events within a single perspective to facts about the rest of
the network of perspectives, but because these interpretations do not allow us
to postulate knowable, observer-independent relations between perspectives no
such bridging principle can exist. Thus it seems that in these kinds of
approaches there is simply no way to get outside an individual perspective and
say something about reality which is not relativized to one’s current
perspective.

Proponents of orthodox interpretations might hope to solve these problems by
slightly relaxing the requirement that there are no knowable observer-
independent facts, in order to allow the assumption (A1) that most or all of
the perspectives in the actual network of perspectives exhibit similar kinds
of regularities, even if they disagree on the specific outcomes of certain
measurements. This assumption could perhaps be grounded on the principle that
I am not in any way special as a physical system, and therefore the
probabilistic relationships exhibited in my own evidence should also hold for
other observers. The proponent of orthodox interpretations would then
presumably hope to employ this assumption in the same way as in ordinary cases
of empirical confirmation we typically employ an assumption to the effect that
(A2) regularities persist through time.

However, these two assumptions are not analogous. For assumption (A2) is
susceptible to empirical enquiry: we do experiments to test _which_
regularities persist, we test out systems in new regimes to see if the usual
regularities still hold, we try to come up with explanations for regularities
in terms of common causal factors or underlying mechanisms. Moreover,
observing regularities persisting in the short term provides some grounds for
the assumption that they will continue to persist - at least we know that
these regularities _sometimes_ persist. So although we do ultimately have to
make an unprovable assumption that the future will be in some ways like the
past, the parameters of that assumption are not ad hoc: we use empirical
evidence to decide the specific ways in which we expect the future will be
like the past.

But in the case of assumption (A1) no such empirical enquiry is available. We
will never have access to any perspective other than our own, so we can’t do
experiments to test out the ways in which other perspectives are similar to
our own, or check to see if there are regimes in which those similarities
break down, or offer any explanation in terms of common causal factors or an
underlying mechanism. And since we have never accessed _even one_ other
perspective, we don’t have any empirical grounds at all for our assumption of
similarity between perspectives - we don’t even know that different
perspectives _sometimes_ look similar. So necessarily, the parameters of the
assumption that other perspectives are like our own are completely ad hoc: no
empirical evidence can possibly tell us anything about the ways in which
different perspectives are similar to ours. So how do we know where to stop
with our assumptions about similarity of perspectives? The principle that ‘I
am not special’ does not tell us much: after all one way to satisfy this
principle would be to say that all perspectives are identical, and another
would be to say that they are all completely random. In fact, obviously where
proponents of the orthodox interpretations would like to stop is precisely at
the limits of quantum mechanics: we are supposed to take it that all
perspectives obey quantum mechanics, but they differ in other ways. Yet there
seems to be no real justification for this assumption, so the approach taken
by the orthodox interpretations amounts to transferring the conclusions
reached in our original enquiries about the persistence of regularities over
time to a completely different question about similarities of perspectives,
even though there is no reason to think the former has any relevance to the
latter.

Of course, it will be responded that even in classical physics we have no way
to be sure that other people’s subjective perspectives are like our own. But
the crucial difference is that orthodox interpretations, unlike classical
physics, are _about_ perspectives: they tell us that quantum mechanics is
nothing more than a characterisation of a network of perspectives associated
with different observers. In the case of classical physics, because our goal
is to empirically confirm a theory which is about the structure of mind-
independent reality, it doesn’t really matter that we don’t know for sure what
subjective experiences other people are having: in order to perform empirical
confirmation we only need to assume that our _own_ subjective experiences
gives us information about the structure of mind-independent reality. But in
order to confirm a theory which is explicitly about a network of perspectives,
our experiences must give us information about the network of perspectives,
including information about perspectives other than our own. Yet the orthodox
interpretations specifically tell us that our experience cannot ever give us
such information, and thus they do not allow that we can have empirical access
to the very objects (perspectives) which are the subject of the theory - or
rather, we can have access to only one such object, which clearly will not
provide us with enough data to confirm a universal theory about the properties
of these objects.

To make matters worse, not only do the orthodox interpretations hinder us from
making hypotheses about how different perspectives relate to one another, they
often don’t seem to have the means to tell us which perspectives will exist in
the first place. The ontology of an orthodox interpretation consists of a set
of observers together with their perspectives: in RQM any system whatsoever
can be an ‘observer,’ whereas in QBism and most Copenhagen and neo-Copenhagen
views it seems that only conscious beings count as observers. But _which_
observers are there? In the usual case where we have an observer-independent
objective reality, we can look at some observer-independent theoretical model
of reality and pick out the objects in it which play the role of observers.
But in an orthodox interpretation there is no observer-independent reality, or
at any rate observer-independent reality is ineffable, so where should we get
our set of observers from? My own perspective will presumably include a a set
of objects which might count as observers, but it is an important part of the
motivation for these views that no observer is special, so the set of
observers can’t be defined by which observers exist relative to me - if
observers can only have their existence in virtue of existing in a set of
facts relative to some other observer, it would seem that we would end up with
an infinite regress which will not give rise to a well-defined set of
observers. So does every logically possible observer exist, seeing every
logically possible description of reality, and thus every possible set of
relative frequencies? Or is some set of observers simply picked out of thin
air? Either way, if we don’t have a well-defined set of observers it seems
difficult to even formulate the claim that most perspectives exhibit similar
kinds of regularities - in order to make sense of this claim we’d have to say
something about the proportion of observers who see relative frequencies which
exhibit the same kind of regularities, and we can’t make sense of such a claim
without first having some concrete idea of what our set of observers looks
like.

##  3 Persistence over time

One possible response to our concerns would be to argue that although an
observer in a world described by an orthodox interpretation may not be able to
learn about observations made by others, she can at least perform empirical
confirmation using her own observations and her memories of the measurement
results that other observers have reported to her (while accepting she can’t
be sure that her memories of those reports correspond to what the other
observers experienced themselves as perceiving) and thus she can at least
confirm that her own experiences are as predicted by quantum mechanics.
Evidently the theory that this observer can confirm is of much more limited
scope than quantum mechanics as it is usually understood, but nonetheless it
would seem that there is _something_ that she can confirm, and perhaps one
might argue that this is enough to justify the practice of empirical science.

However, this argument relies crucially on the question of whether or not
‘observers’ can be understood to persist through time in orthodox
interpretations. If observers do persist and the set of facts relative to a
given observer remains the same throughout their existence, then indeed these
observers may be able to empirically confirm quantum mechanics as a
description of their own experiences, but this will clearly not work if we are
forced to say that an ‘observer’ exists only at a single instant, because then
later versions of the same person must be treated as distinct ‘observers’ who
may have a completely different set of facts.

And unfortunately for the proponents of orthodox interpretations, it seems
quite difficult for them to insist that observers persist over time. For in
order to do this, the orthodox interpretation would have to provide some
criterion of identity over time so we can track which temporal parts of a
continuant are supposed to count as the same observer and thus have the same
set of relative facts, and it seems hard to do this in any precise way. Do I
cease to be the same observer when all the cells or particles in my body have
been replaced? Perhaps only the particles in my brain are relevant, or the
particles in some particular part of my brain? What about ‘ship of Theseus’
examples[42], where an observer is slowly deconstructed and reassembled
elsewhere: when does the observer in the process of deconstruction cease to be
identical with the original observer, is the reassembled observer ever
identical with the original system, and can there be some time at which _both_
observers are identical to the original? If we want to claim that there is
some objective notion of continuity that provides answers to these questions,
we would essentially need to postulate something akin to a disembodied soul
which tracks ‘the same person’ over time, and this will surely be unacceptable
to most proponents of orthodox interpretations, particularly those motivated
by broadly physicalist and naturalist intuitions. But if we accept that these
questions are essentially just semantic ones to be answered by appeal to our
linguistic conventions, then surely the account of the content of reality
given by an interpretation of quantum mechanics cannot depend in any crucial
way on the answers to them.

QBism might perhaps be more friendly to the ‘disembodied soul’ approach, as by
construction QBism represents ‘ _a radical departure from the strictly
physicalist ontology endorsed by nearly all other quantum interpretations,_.’
However, note that according to QBism, what it is to be an ‘observer’ is to be
a decision-theoretic agent whose beliefs are regulated by a principle of
coherence. And the principle of coherence does not allow us to make
comparisons between observers at different times: ‘ _Coherence is a condition
about an agent’s_ current beliefs, _including her beliefs about her future
probability assignments._ ’ Thus QBism does not postulate any direct
connections between the beliefs of observers at different times: it demands
that an agent’s current beliefs about what beliefs they will hold in the
future must be coherent, but it does not allow any diachronic rationality
constraints. This means that QBists cannot postulate any prima facie
connections between observers at different times, meaning that QBism likewise
cannot offer any guarantee that an agent’s beliefs about what has happened in
the past will remain stable over time.

We emphasize that we would not, under normal circumstances, expect a
scientific theory to provide an answer to questions about personal identity
over time; that kind of philosophical question is usually outside the realm of
science. But by making ‘observers’ so central to their approach, orthodox
interpretations impose on themselves a mandate to tell us in precise terms
what they mean by an observer, because that is a crucial step in defining the
empirical content of the theory. The criteria for tracking an ‘observer’ in
the technical sense used by the orthodox interpretations need not coincide
with the criteria which we would naturally employ to track personal identity
over time in the philosophical sense, but _some_ criteria must certainly be
provided if proponents of these interpretations want to insist that we can at
least use our own memories for empirical confirmation; and our concern is that
it seems far from clear that there is _any_ acceptable set of criteria
available for this purpose, even if we don’t demand that these criteria also
track the common-sense notion of personal identity.

Now, one might hope to avoid the problem by saying that the the ‘observers’
with which an orthodox interpretation is concerned are all individual
fundamental particles - that is to say, all ‘perspectives’ must actually be
relativized to a single particle - and therefore tracking identity over time
just comes down to following the trajectories of individual fundamental
particles over time. This is not an option within orthodox interpretations
like QBism which specifically require that their observers are conscious
beings, but it might be viable within something like RQM, which tells us that
every physical system can be an ‘observer’ in a general sense. However, we
probably do not want to make an interpretation of quantum mechanics contingent
on the claim that there exist fundamental particles following continuous
trajectories through spacetime, since this would likely cause problems when we
came to apply the interpretation to field theory, which tells us that
particles are simply excitations in fields and thus there is often no
meaningful notion of particle identity across time[43]. Moreover, the solution
of the measurement problem offered by orthodox interpretations depends
crucially on the claim that there is some unique set of facts relative to each
(human) observer - that is, a measurement always has a unique outcome relative
to the person who performs the measurement. This means that a person cannot
simply be regarded as the composite of all of the particles in their brain,
since we have already seen that there can be no guarantee that the
descriptions relative to all of these different systems will agree, and
therefore their composite would not give rise to a well-defined perspective.
Thus we do need to allow that conscious minds count as ‘observers,’ which are
distinct from all of their individual particles; and since there is no
unambiguous way to identify such things across time, it would seem that if the
orthodox interpretations wish to maintain their clean, unambiguous picture of
facts as relative to individual observers, they had better maintain that such
observers are defined only at a single time. And in fact, many proponents of
orthodox interpretations do indeed seem to accept that observers do not
persist over time. For example, Ruyant writes that in RQM, after a measurement
the observer ‘ _has just became a new observer with a new perspective,
relative to which objects have different properties._ ’[26]

Indeed, quantum mechanics itself provides concrete examples of cases where the
perspective of an observer cannot remain stable over time. For example, ref
[44] presents a no-go theorem shows that in a Wigner’s friend experiment we
cannot assign a joint probability distribution across the observations of the
friend before and after Wigner’s experiment unless we give up the assumption
that unitary quantum mechanics is universal or the assumption that the joint
probability of the friend’s perceived outcomes has a convex linear dependence
on the initial state of the system qubit. It seems likely that most proponents
of orthodox interpretations would wish to uphold these two assumptions, since
the universality of quantum mechanics is one of the basic principles of an
orthodox interpretation, and consequently they must accept that at least in
this case it isn’t possible that the observer retains a consistent perspective
over time. Of course examples like the Wigner’s friend experiment are
presumably quite rare, but nonetheless they serve to underline the point that
the orthodox interpretations have no sensible way to define the continuity of
a perspective over time.

###  3.1 Relative Frequencies

If orthodox interpretations cannot justifiably postulate that ‘observers’ in
their technical sense persist over time and thus retain the same set of
relative facts, this will clearly have serious consequences for empirical
confirmation, particularly with reference to the probabilities which feature
in quantum mechanics and which form an important part of its empirical
content.999There are a variety of positions on the nature of probability in
orthodox interpretations. Because they all insist that quantum mechanics is
complete, they all agree that the probabilities are ‘irreducible’ in the sense
used by Brukner[13], i.e. there are no hidden variables in the theory such
that conditioning on these variables would allow us to predict quantum
measurement outcomes more precisely than the Born rule. However, there are
some differences of opinion about the status of these irreducible
probabilities: in QBism they are purely subjective probabilities: ‘ _The
starting idea of QBism—which originally stood for “quantum Bayesianism”—is
that the probabilities delivered by quantum theory are to be understood as
degrees of belief along the lines of the subjective Bayesian approach to
probability_ ’[45], but in other contexts they may be regarded as objective,
as for example in Dorato’s proposal for a dispositional interpretation of the
probabilities in RQM[46]. In particular, we note that proponents of the
orthodox interpretations often wish to characterise probabilities as referring
specifically to the results of measurements yet to be made. But quantum
mechanics obtains its empirical confirmation in large part from observations
of relative frequencies - e.g. the pattern of dots on the screen from a two-
slit experiment, which constitutes a visual representation of relative
frequencies. A single incidence of a particle on the screen tells us virtually
nothing; it is the picture built up from recording many incidences on the
screen which allows us to draw conclusions about the interference processes
taking place. So if the theory is to have empirical confirmation, clearly the
probabilities assigned by the theory must be related not only to the results
of future measurements but also to relative frequencies observed in the past:
it must be the case that the structures postulated by the theory have some
part in the explanation for why we have seen these relative frequencies and
not others.

Now, in an interpretation of quantum mechanics which postulates a stable,
intersubjective macroscopic reality, if measurement outcomes typically _occur_
with frequencies matching the Born rule probabilities it is reasonable to
suppose that subsequent memories and records of those measurements will
exhibit relative frequencies which approximately match the Born rule
probabilities. But we have seen that in orthodox interpretations it can’t be
taken for granted that an observer’s perspective will be stable across time.
So in order for quantum mechanics to receive confirmation from relative
frequencies within an orthodox interpretation, we must say something more
about the relationship between memories and records and the probabilities
postulated by the theory. Thus let us once again suppose for the sake of
argument that we believe ourselves to live in a world in which there are no
observer-independent facts (except possibly ineffable ones), but there are
some observer-dependent facts about the measurement outcomes that appear in
different perspectives, and we will imagine that we are trying to confirm the
hypothesis that the correct probability distributions to describe at least the
events featuring in our own experiences are the ones specified by quantum
mechanics. Since we can’t assume that our memories represent what actually
happened in the past, in order to make progress on confirming this hypothesis
we have to make a decision about the status of memories and records in this
kind of world; in the following two sections we consider two possible options.

We pause to note that relative frequencies may seem to pose a particular
problem for QBism, since it denies that there are any objective facts about
the probabilities for quantum measurement outcomes, even relative to a single
observer: ‘ _the individual outcome of a quantum measurement is random and
lawless_ ’[33]. But as we have already seen, a hardline QBist position
maintaining that quantum mechanics has nothing whatsoever to do with the
empirical evidence would seem very hard to accept, so presumably the QBist
must hold that relative frequencies play some part in the justification for
the normative principles that they associate with the quantum formalism. It is
not entirely clear to us how relative frequencies can provide support for a
theory which explicitly tells us that all probabilities are purely subjective,
and Timpson also raises this as a significant concern: ‘ _if there are only
subjective probabilities … why does gathering data and updating our subjective
probabilities help us do better in coping with the world? … Why, that is,
should one even bother to look at data at all?_ ’ But we will leave that for
the QBists to explain; for now, we will simply take it that in QBism, and in
all other orthodox interpretations, the reasons for believing quantum
mechanics are at least partly based on relative frequencies, so it is
important that we can attach meaning to the relative frequencies that we have
observed.

###  3.2 Memories as measurements

One possibility is that consulting memories and/or records should be
understood as a _measurement_ on some kind of physical register (e.g. the
human brain), and hence the theory we are trying to confirm can be understood
as prescribing a probability distribution over measurements on memories and
records. Orthodox interpretations do not allow us to make the assumption that
these memories and records are an accurate reflection of what ‘really
happened,’ since what ‘really happened’ will in general be relative to a
different observer - i.e. they will either be relative to a past version of
ourself, or they will be relative to some other observer who performed the
measurements and passed on the records. Moreover, we can only arrive at
relative frequencies by consulting measurement and records, as we cannot
directly observe relative frequencies in a single measurement. Even if we
construct some kind of joint global measurement which is supposed to measure
the relative frequency of some property in some ensemble, a single measurement
is not enough to tell us that the result of this joint global measurement has
anything to do with the relative frequencies that we would have observed if we
had measured the members of the ensemble individually - that expectation could
only be based on the fact that quantum mechanics tells us how joint global
measurements are related to measurements on individual systems, but without
independent access to the individual measurements we have no reason to have
any faith in that part of quantum mechanics.

Thus it seems that under such circumstances, any theory we come up with on the
basis of observed relative frequencies can only be regarded as a theory of
_the results of measurements on memories and records_! For we only ever have
access to the sequence of outcomes recorded in our memories and records, not
the actual sequence of outcomes which originally occurred in the perspectives
of the observers who performed the measurements, and in the context of an
orthodox interpretation we can’t take for granted that these sets are the
same. Thus if we are to use our memories to empirically confirm quantum
mechanics, we must assume that (A3) the set of measurement outcomes derived
from a measurement on our memories and records will typically exhibit similar
_relative frequencies_ to the set of outcomes that originally occurred in the
perspectives of the observers who performed the measurements, even though for
any specific measurement outcome the recorded value may be different to the
value that originally occurred. But what grounds could we possibly have for
this assumption? There is no way we can ever check assumption (A3); it is
simply drawn verbatim from standard quantum mechanics, but remember that on
this construal quantum mechanics has empirical confirmation only as a theory
of memories and records, and we therefore can’t use it to infer (A3), since we
have no evidence that quantum mechanics is a correct theory of anything
_other_ than memories and records. Thus if consulting memories is to be
understood as a measurement which is governed by the theory we are trying to
confirm, it seems we cannot arrive at a theory which says anything about what
we will see the next time we do a measurement: we may be entitled to draw the
conclusion that, if we perform a sequence of measurements and then
subsequently inspect the records, we will find that these new relative
frequencies approximately match the ones we observed earlier, but we have no
grounds to make any predictions about what we will see at the time of the
measurement.

Moreover, on this construal we don’t even have enough data to justify coming
up with a theory which at least applies to measurements on memories and
records. For we will only ever have access to the result of _one_ measurement,
i.e. the measurement we have just made on our memories/records of the past. We
can’t measure our memories now and then later measure our memories again and
compare the two results, because we don’t have access to the result of the
earlier measurement except via the part of the outcome of the later
measurement which pertains to our memory of the earlier measurement, and as
usual there is no guarantee that the two agree. So we can never have access to
a sequence of outcomes of distinct, independent measurements on our memories
and records, and therefore we can’t obtain any relative frequencies in order
to arrive at a theory in the first place.

Of course, one might object that this predicament is not unique to the
orthodox interpretations - any scientific theory must in some sense be a
theory of memories and records rather than the events in and of themselves,
since theories can only ever be arrived at on the basis of memories and
records. But normally when we do science we start from the assumption that
there exists a stable intersubjective macroscopic reality and therefore we are
free to assume that (A4) our memories and records are mostly accurate
reflections of what actually took place in this stable intersubjective
macroscopic reality, and it is this assumption which allows us to regard
ourselves as having access to the result of more than one distinct
measurement. Of course, the proponents of orthodox interpretations might try
to insist that assumption (A4) is no different to assumption (A3) and thus the
orthodox interpretations are no worse off than any other scientific theory.
For example, this seems to be the approach taken by some proponents of QBism:
‘ _all these data and observations are obtained through experimental
conditions that are without exception instrumentally-mediated which in turn
means that we are already experiencing an interpreted reality._[30] But in
fact (A3) and (A4) are not equivalent, because assumption (A4) is independent
of any particular theory we might be trying to confirm: it is based on a pre-
existing philosophical commitment to the existence of an observer-independent
reality, which allows us to infer that when everything is working properly our
memories will reflect that observer-independent reality and thus will
accurately reflect what actually happened in the past. That is to say, if we
assume the existence of an observer-independent reality then the relative
frequencies exhibited in memories can be expected to match the relative
frequencies exhibited in the actual events because the latter are understood
to exist in an observer-independent sense and therefore they can plausibly be
the direct cause of the former. Whereas in the case of the orthodox
interpretations we are no longer allowed to postulate an observer-independent
reality (or at best we can only postulate an ineffable and unknowable one) and
therefore there can be no pre-theoretic reason to think that relative
frequencies exhibited in memories match the relative frequencies exhibited in
the actual events, since the latter do not exist in any observer-independent
sense and therefore they cannot be the cause of the former. The only
justification for assumption (A3) comes from appealing to QM itself, but we
cannot arrive at QM in the first place without making the assumption (A3), so
this looks like a vicious circularity. Evidently (A3) has just been tacked on
ad hoc because proponents of orthodox interpretations wish to be able to
arrive at standard quantum mechanics, but really the scientifically
responsible thing to do here would be to come up with a theory which describes
only the results of measurements on memories and records and which is silent
on the relationship between memories and actual events - and that theory would
certainly not be standard quantum mechanics.

Now, the proponent of an orthodox interpretation might try to justify their
use of (A3) as some kind of bootstrapping procedure: one starts by assuming
(A3), one arrives at the theory of QM on this basis, and then one uses the
theory to justify the assumption (A3). But care must be taken with this kind
of reasoning, because obviously I could ‘justify’ more or less any assumption
I liked by first making the assumption, then using the assumption together
with some evidence to infer some conclusion, and then observing that the
conclusion implies the assumption that I used to derive it. So bootstrapping
procedures of this kind can be epistemically rational only when some further
conditions are met. For example, Glymour proposes the following criterion for
determining whether a bootstrapping procedure may be legitimate: ‘ _No Risk,
No Gain (NRNG): To test a hypothesis we must do something that could result in
presumptive evidence against the hypothesis._ ’[47] This criterion allows us
to see why (A4) is legitimate: there are certainly ways that my memories could
have been which would result in presumptive evidence against the hypothesis
(A4), for example if they represented to me a history full of such chaos that
it seems impossible that a conscious being could ever have arisen from such
conditions. But this reasoning doesn’t work in the case of (A3), because in
that case we are accepting that our memories do _not_ represent the actual
past, and asserting only that they match the actual past in one very specific
way. So even if my memories look utterly chaotic, (A3) is not disproved,
because the actual past could match the relative frequencies in my chaotic
memories while being very different from the representation in my memories in
many other ways. That is to say, obtaining a contradiction with (A4) requires
us only to determine that _one particular_ history makes the existence of a
conscious mind unlikely, while obtaining a contradiction with (A3) requires us
to determine that _every conceivable_ history exhibiting a certain set of
relative frequencies makes the existence of a conscious mind unlikely, and
given the enormous size of the possibility space, it’s hard to see how we
could ever establish the latter with any confidence. So it seems that there is
no possible way our memories could have been which would have been
incompatible with the hypothesis (A3), so NRNG does not apply here and thus
the boostrapping procedure can’t be regarded as providing justification for
believing (A3).

###  3.3 Transcendental access to the past

In view of these difficulties, it may seem tempting to say that consulting
memories and/or records should not be understood as a measurement whose
outcome can be predicted by the theory we are trying to confirm - rather
observers simply have some kind of transcendental access to facts about what
has happened in the past. But since we have agreed that orthodox
interpretations do not allow a view from nowhere from which different
perspectives can be compared, whatever that transcendental access constitutes,
it cannot guarantee that the memories that observers have about events in the
past match what previous versions of themselves observed - and thus, in
particular, it can’t offer any guarantee that observers will typically
remember relative frequencies which approximately match the probabilities
predicted by the theory we are trying to confirm, so without further details
on the nature of this ‘transcendental’ access we simply have no way to say
what observers will typically remember. Thus this way of understanding
memories and records does not allow us to attach any meaning to the relative
frequencies that we happen to remember at any given moment, since the
underlying theory plays no role in bringing it about that we remember these
relative frequencies rather than others.

Indeed, if this were the way memory really worked in an orthodox
interpretation, then what would be the chances that an observer in the world
correctly described by that orthodox interpretation would actually come up
with a theory that looks like quantum mechanics on the basis of their
memories? Practically zero, one might think - or rather, completely undefined,
since the ‘transcendental access’ model is too vague to provide a probability
distribution over what observers will remember - even the uniform distribution
doesn’t seem to be justified here. Thus if this is the right way to think
about memory in the kind of universe postulated by an orthodox interpretation,
it follows that if we believe that we live in such a universe, the fact that
our records of observed relative frequencies are consistent with quantum
mechanics cannot possibly count as evidence for anything at all.

Note also that in virtue of being _interpretations_ , the orthodox
interpretations are not just sets of models: they also include assertions
about what those models represent. That is to say, an orthodox interpretation
has semantic content, and therefore in order for an agent’s beliefs about an
orthodox interpretation to be true, presumably the central terms of their
beliefs must successfully refer to the network of perspectives that the
interpretation postulates. But since under the ‘transcendental access’ model
an agent in the type of world postulated by an orthodox interpretation can
have the right beliefs about this network of perspectives only by a lucky
accident, it’s unclear that reference can successfully be established under
such circumstances - for as noted by Putnam, it seems plausible to say that a
concept fails to refer to an actual entity if the resemblance between the
concept and the entity is merely accidental[48]. We will not develop this
argument further here, but we note that a very similar objection is discussed
in the context of the Everett interpretation in ref [1].

###  3.4 Other types of evidence

One way of responding to these concerns would be to observe that there are
features of the world other than relative frequencies which count as evidence
for quantum mechanics - for example, quantum mechanics is needed to explain
the stability of matter[49]. So maybe we can get empirical confirmation for
quantum mechanics just by observing the stability of matter - after all, this
is something that an observer can plausibly observe at a single moment and
thus it can be used in empirical confirmation even by agents who do not
persist over time. However, this approach also has problems. For a start, it
doesn’t seem plausible that quantum mechanics is the only possible explanation
for the stability of matter - we need evidence that is more specific to
quantum mechanics, such as relative frequencies, before we can have good
reason to choose it over other possible explanations. Moreover, the problems
we have pointed out for relative frequencies also apply to any other empirical
fact that we might hope to use as evidence - for if we can’t learn anything
about the perspectives of other observers, we can’t have any way to know that
matter is indeed stable from the point of view of other observers. Perhaps
matter is actually not stable relative to most other observers and you just
happen to be one of the few lucky observers such that at least momentarily
your perception of reality includes a stable macroreality.

One might perhaps try to argue that it is very unlikely we would find
ourselves in an apparently stable macroreality just by chance, and thus argue
that we should infer from our observations of a stable macroreality that
macroreality is stable for all observers. But this will not work either:
first, because there does not seem to be any way to postulate a probability
distribution over different perspectives in such a vaguely defined possibility
space, and second, because we can give an anthropic argument. For presumably
consciousness can exist for long enough to sustain extended thoughts about the
stability of matter only if matter is at least momentarily stable, so given
that you are having thoughts about the stability of matter, then necessarily
you are one of the (perhaps very few) observers relative to which matter is
stable for at least a moment, so the fact that you are such a observer doesn’t
licence any inferences about the stability of matter for other observers.
Indeed, one can make an orthodox interpretation analogue of the famous
Boltzmann brain argument[50]: if there is no knowable observer-independent
reality that could ground similarities between different perspectives,
wouldn’t it be most rational to suppose that the perceptions of each observer
are wildly different and completely unrelated, and thus you are not actually a
temporal part of a persisting entity at all, you are just a fully formed
perspective, complete with memories of a past that never really occurred, that
happens to exist due to a random fluctuation within the network of separate
and unrelated perspectives? Admittedly it’s not very clear how to evaluate the
probability of this possibility as compared to the probability that all
perspectives are similar in the way postulated by the orthodox
interpretations, but that is precisely the problem: if we don’t allow any
knowable observer-independent facts which are understood to give rise to
various different perspectives in some ordered way, there seems no reasonable
method to arrive at priors which say anything about the network of
perspectives at all. At the very least, the proponents of orthodox
interpretations owe us some account of how they have arrived at the set of
priors which justifies them in reaching the conclusion that all or most of the
perspectives are correctly described by quantum mechanics.

##  4 What now for orthodox interpretations?

One possible way for proponents of orthodox interpretations to respond to the
dilemma that we have posed would be by walking back the claim that quantum
mechanics is supposed to be a theory of ‘the whole world’ in any sense. That
is, they might choose to regard quantum mechanics as simply a convenient
codification of the regularities that appear in the facts which are true
relative to a single observer: other observers, and other versions of the same
observer at other times, will see different regularities and will thus arrive
at different theories which better describe the results that they have seen.
But actually, even this move is not really justifiable. For where did the idea
of postulating that everything is relativized to a perspective come from in
the first place? It came from treating quantum mechanics as a complete
description of the whole world and observing that on this construal, quantum
mechanics appears to say things about the relationships between observations
made by different observers which seem difficult to combine into a single
‘absolute’ reality. But if quantum mechanics is only a theory of the world
relative to me at this moment, then I am not allowed to attach any
significance to the part of quantum mechanics making claims about what other
observers will observe: thus if I believe that all facts are necessarily
relative to an observer, then I cannot have any grounds for believing in the
features of the theory which are the whole reason for postulating that all
facts are relative to an observer in the first place! Thus even retreating to
regarding quantum mechanics as a theory which is correct ‘relative to me now,’
will not solve the self-undermining problem.

So is this the death knell for orthodox interpretations? Well, although we
have criticized orthodox interpretations, we agree that they are based on a
correct insight: quantum states are not observer-independent. Quantum states
were postulated in the first place as a way of describing systems relative to
external observers, and once we start thinking about how observers themselves
should be incorporated into the picture, it becomes very plausible that
quantum states should indeed be regarded as relational descriptions in the
spirit of Everett’s relative state description[51]. The orthodox
interpretations combine this insight with the assumption that quantum
mechanics (or quantum field theory, or quantum gravity) is the correct and
final description of reality, and thus arrive at the conclusion that all facts
are relative to an observer. Indeed one is usually presented as the direct
consequence of the other, without even making the assumption of
$\psi$-completeness explicit: for example, Ruyant writes that the relational
interpretation proceeds by ‘ _positing that the states quantum mechanics
describes are relative to the observer rather than absolute. We should abandon
the idea that there is a “view from nowhere._ ’[26] But the proposition that
quantum states are relative rather than absolute certainly does not entail
that there is _no_ view from nowhere: it just entails that a description of of
the world ‘from nowhere’ would not contain quantum states as a fundamental
entity. There are many ways in which one could have observer-independent facts
without having observer-independent quantum states, or indeed states at all.
For example, one could have an ontology consisting entirely of pointlike
events, as in the Bell flash interpretation[52, 53].

Why, then, do proponents of the orthodox interpretations appear to ignore all
of these intermediate possibilities? In large part it is because they are
aiming to interpret the existing mathematical framework of unitary quantum
mechanics without adding anything new, and since the existing mathematical
framework in its most common formulation describes reality in terms of states,
it seems to follow that if they wish to eliminate observer-independent facts
about states, they will necessarily have to eliminate all observer-independent
facts whatsoever. But this motivation is flawed for several reasons. For a
start, there are other formulations of quantum mechanics which don’t describe
reality in terms of states - for example, the path integral formulation[54].
That formulation potentially offers a route to interpreting the existing
mathematical formulation in a way which does not postulate observer-
independent states, but does nonetheless postulate knowable observer-
independent _facts_ , i.e. facts about the path integral. Second, the drive to
maintain quantum mechanics in its standard formulation often seems to be
driven largely by an aesthetic preference for the pleasingly simple unitary
evolution, but we should not let aesthetic preferences blind us to all other
considerations, such as the serious epistemic problems discussed in this
article. Furthermore, there are ways to add something to the mathematical
framework of unitary quantum mechanics without losing the aesthetically
pleasing unitary evolution: for example, Kent’s solution to the Lorentzian
classical reality problem[55, 56, 57] involves allowing the wavefunction to
evolve unitarily until the end of time and then performing a final measurement
which selects and actualises a definite course of history, thus providing
‘observer-independent facts’ without compromising any of the mathematical
structure of unitary quantum mechanics.

Another major motivation for proponents of orthodox interpretations is the
desire to avoid certain features which are regarded as undesirable - such as
nonlocality, contextuality, retrocausality and so on. But this way of thinking
gets the priorities wrong. We can do without locality if we have to, but we
certainly cannot do without the possibility of empirical confirmation.
Nonlocality may present technical challenges, but it does not pose this kind
of existential threat to the status of the theory as empirically
confirmed.101010We also note that in some cases it is argued that we need
locality because otherwise the theory will be inconsistent with relativity,
and therefore we have to accept an orthodox interpretation if we want to have
any hope of a successful unification with gravity. But this is a misconception
arising from the fact that early nonlocal interpretations, like the GRW
collapse model[58] and the de Broglie-Bohm approach[59], required an
instantaneous wavefunction collapse or update on a spacelike hyperplane. Later
nonlocal interpretations, like the relatistic Bell flash model[52, 53] and
Kent’s Lorentzian quantum reality model[55, 56, 57] do not require a preferred
reference frame and are explicitly Lorentz-covariant, thereby demonstrating
that consistency with relativity does not force us to adopt an orthodox
interpretation.  So rationality demands that we prioritise securing the
evidential basis of the theory over trying to make it local - or non-
contextual, or non-retrocausal etc. Indeed, given that epistemic rationality
is a theoretic virtue which science must surely prioritise above all else, it
is hard to see how interpretations which undermine the evidential basis for
the theory could ever be preferred as long as there are any other viable
options.

###  4.1 Altering orthodox interpretations

So what options do we have if we want to come up with a version of an orthodox
interpretation which overcomes the problems set out in this article? An
obvious first step is to allow that there exist some mind-independent absolute
facts, albeit not facts about the physical states of systems. This seems to be
what Ruyant has in mind when he writes that ‘ _(RQM’s) general ontological
claims (“what the world is like” according to RQM) should be understood as
being absolutely true in order to avoid the self-refutation problems
associated with relativism, and the relativist stance should be reserved to
statements about particular entities or facts in the world._ ”[26] This
certainly seems like a step in the right direction - at least that way there
is something which could possibly be the subject of the theory we are aiming
to confirm! However, simply saying that the ‘general ontological claims’ of an
orthodox interpretation are an observer-independent fact is not adequate,
because typically the ontology of an orthodox interpretation consists simply
of observers plus perspectives relative to them, but we have seen in this
article that in order for these approaches to allow that quantum mechanics is
subject to empirical confirmation there must be something more - some kind of
unifying structure which underwrites intersubjective agreement in at least
certain cases.

Now, one of the motivations for the denial of observer-independent facts in
orthodox interpretations is the fact that any specific proposal for an
observer-independent reality will most likely need to have properties like
nonlocality which many people find unappealing, so one might naturally hope to
take the minimal route here by keeping one’s preferred orthodox interpretation
as it is and simply adding the hypothesis that there exists some kind of
unifying structure which guarantees intersubjective agreement in most cases,
without saying anything more about the nature of this unifying structure. For
example, QBists might perhaps try to argue that the unifying structure in
question is their ineffable ‘external reality’ - there exists something which
brings about intersubjective agreement but we are not allowed to ask how or
why. But this is not good enough. For a start, in order to verify that this
‘unifying structure’ can actually play the desired role in making empirical
confirmation possible, we need to know the limits of its promise of
intersubjective agreement: presumably this structure still cannot guarantee
intersubjective agreement in special cases like the Frauchiger-Renner
experiment, so what are the limits of the intersubjective agreement and how
reliable is it? Moreover, once we’ve added to our preferred orthodox
interpretation the stipulation that different observers can be expected to
agree most of the time, one might wonder why exactly we need to keep insisting
that everything is relative to an observer: it would seem simplest to just
accept that whatever it is they agree on is really the fundamental stuff of
reality. Thus in order to find a route which both achieves enough
intersubjective agreement to make empirical confirmation possible but also
maintains the relevance and usefulness of an observer-dependent approach, it
would be necessary to give a reasonably detailed account of the specific
circumstances in which perspectives do and do not agree. Something like the
QBist’s ineffable ‘external reality’ is not sufficient to solve the problem.

Rovelli has recently suggested a new postulate for RQM aimed at solving the
problem we have set out in this article:

######  Definition 4.1.

Cross-perspective links (CPL): In a scenario where some observer Alice
measures a variable V of a system S, then provided that Alice does not undergo
any interactions which destroy the information about V stored in Alice’s
physical variables, if Bob subsequently measures the physical variable
representing Alice’s information about the variable V, then Bob’s measurement
result will match Alice’s measurement result

That is to say, when Alice sees a definite value in a measurement, that piece
of knowledge is encoded in her physical state in a way that is accessible to
Bob. We agree with Rovelli that this is exactly what is needed in RQM to solve
the problems that we have raised in this article, and a companion article[21]
discusses the consequences of implementing this postulate. In particular, it
is important to note that adding this postulate requires us to make some
changes to existing accounts of the ontology of RQM: it is no longer the case
that quantum mechanics does not describe an observer-independent reality,
because it is now the case that the values of the variables of a system ($S$)
which become definite during an interaction with another system (Alice) are
observer-independent at least in the sense that that any other observer can in
principle find out these values by an appropriate measurement on either $S$ or
Alice. So although RQM can potentially solve the problem we have set out in
this article, it can only do so by ceasing to be what we have described as an
‘orthodox interpretation.’

Can other orthodox interpretations adopt something like CPL? This seems
unclear. The postulate works in RQM because Rovelli and co-authors have always
maintained that all physical systems count as possible ‘observers,’ and
therefore it is possible to to use interactions between observers and systems
in RQM to define an objective reality consisting of a set of mind-independent
events. On the other hand, approaches like QBism and neo-Copenhagen views are
more inclined toward the view that only conscious minds count as observers,
and therefore we don’t have the option of using interactions between observers
and systems to define a mind-independent set of events, since every such
interaction necessarily involves a conscious observer. Moreover, RQM has
always been associated with the naturalistic principle that ‘information is
physical’ so it is actually very much in line with the underlying motivations
of the approach to insist that information possessed by observers should
always be accessible to other observers by appropriate physical interactions.
Conversely, neo-Copenhagen approaches and similar views are often motivated by
the idea that information itself is fundamental and the physical world is in
some sense emergent from it, and therefore there is no physical world
available to ground the expectation of agreement between perspectives, so it
seems much harder in such cases to come up with any plausible justification
for something like CPL.

At any rate, even if these orthodox approaches are willing to adopt something
like CPL this will definitely involve some quite significant changes in the
metaphysics and ontology associated with these views. Proponents of orthodox
interpretations cannot simply postulate this coordination between perspectives
ad hoc without saying anything about the source of it - explaining the source
of regularities is, after all, at the heart of the scientific endeavour
according to most scientific realists! And it is particularly crucial in this
case because the intuitively obvious explanation for the coordination would be
to say that there exists an observer-independent underlying state of S and
thus the measurement results obtained separately by Alice and Bob agree
precisely because they are both probing the same underlying state. Indeed, in
the absence of any other concrete explanation for the coordination, inference
to the best explanation would seem to compel us to draw this conclusion. But
the nonexistence and/or inaccessibility of observer-independent reality is a
central principle of most of these orthodox interpretations - a version of
QBism or the neo-Copenhagen approach which allowed observers to probe a mind-
independent underlying state would be so far removed from the original that it
could hardly be said to be the same thing at all - so if proponents of
orthodox interpretations are to accept something like CPL, it will be vital to
come up with some equally compelling explanation for the coordination in order
to block this inference to the best explanation.

There would seem to be roughly three ways to provide such an explanation, each
corresponding to dropping or adjusting one of the three fundamental
characteristics of an orthodox interpretation set out in section 1. First, we
could drop the assumption of unique measurement outcomes, which would lead us
to the Everett interpretation or similar. Second, we could drop the
requirement that quantum mechanics is universal and argue that at least some
parts of macroscopic reality cannot be described quantum-mechanically, since
they exist in a single definite way for all observers. However, we would then
have to specify where the line between the quantum-mechanical and the non-
quantum-mechanical parts of reality falls, which is just the standard
measurement problem, and therefore we would presumably be forced to use some
other, non-orthodox interpretation of quantum mechanics to draw this line.
Third, we could drop the requirement that quantum mechanics does not describe
an observer-independent external reality and add in some kind of observer-
independent structure which coordinates perspectives in the required way.
However, once we move to this kind of picture we have a compelling alternative
ontology available to us: rather than insisting that everything is relative to
a perspective, we find ourselves with an ontology composed of events which are
observer-independent in the sense that any other observer can in principle
find out about the values of the variables realised in the events, together
with some kind of coordinating structure linking these events together. And
once we move to that kind of picture it becomes very natural to wonder if we
could perhaps find a way of describing the distribution of events all-at-once
using the coordinating structure, rather than using partial, relational
descriptions as in quantum mechanics - which is to say, it becomes less clear
that we can still maintain that standard quantum mechanics is complete and
universal. So this way of thinking leads us inevitably away from the kind of
picture associated with the orthodox interpretations and towards some other
kind of realism which deserves to be fully explored rather than hidden under
the veneer of an orthodox interpretation.

##  5 Conclusion

It is understandable that proponents of the orthodox interpretations wish to
insist that there are no knowable observer-independent facts: for if they
allow knowable observer-independent facts then they will have to say something
about what these observer-independent facts are, and various quantum no-go
theorems make it clear that it will be difficult to do so without postulating
something in the realm of hidden variables, nonlocality, retrocausality, or
superdeterminism - all things that proponents of orthodox interpretations
typically wish to avoid at all costs. But in order for it to be rational for
us to believe an orthodox interpretation, something must be done about
empirical confirmation, and therefore these approaches must be supplemented
with some mechanism for selecting and actualising measurement outcomes in such
a way as to provide at least some minimal level of intersubjective agreement
across different relative descriptions. Moreover the observer-independent
structure responsible for the coordination cannot simply be ‘the wavefunction’
since then the view would collapse into the Everett interpretation, and
therefore it would seem that something must be added to the existing formalism
if we are to make sense of empirical confirmation within these approaches.

Now, of course we do not mean to suggest that the formalism is unusable in its
current form: in most circumstances (i.e. outside of special cases like the
Frauchiger-Renner experiment) we know perfectly well how to use quantum
mechanics to make unambiguous predictions. So it is reasonable for a physicist
focused on applications to simply have no interest in the nature of the
underlying observer-independent reality, and we have no objection to that
pragmatic choice. However, we do object to the form of argument which involves
first proposing an orthodox interpretation and then using that orthodox
interpretation to dismiss all attempts to describe an observer-independent
underlying reality. And we particularly object to the claim that such attempts
are driven by attachment to naïve classical ideas - the commitment to an
observer-independent reality is not simply a pre-scientific prejudice, rather
it is a reasoned position based on the observation that some kind of
coordination between perspectives is necessary to ground the rationality of
science. Therefore there are good scientific reasons for being committed to
the existence of an observer-independent reality (and moreoever, one that is
not ineffable), and thus also good scientific reasons for trying to understand
the nature of that observer-independent reality.

We also do not mean to suggest that because orthodox interpretations make it
impossible for us to obtain confirmation for scientific theories like quantum
mechanics, the world can’t be the way they describe. Rather we claim that it
can’t be rational to _believe_ that the world is the way they describe _as an
interpretation of quantum mechanics_. Of course we accept that it could be the
case that really there are no observer-independent facts - indeed, there might
even be good reasons independent of physics to believe that to be the case.
Instrumentalism, antirealism, idealism, phenomenalism and the like have
certainly had many defenders over the centuries, and the concerns raised in
this article don’t threaten the traditional arguments for them, because people
do not usually argue for these positions on the basis of a specific scientific
theory. We simply contend that it is not rational to believe that the world is
the way the orthodox interpretations describe _because of quantum mechanics_ ,
since it is not rational to hold a belief which undermines its own evidentiary
basis. In order to do science we have to assume that there are appropriate
preconditions in place for us to successfully learn about reality by means of
empirical observations, and therefore we should insist that any interpretation
of a theory we thereby arrive at should at least approximately uphold those
preconditions and explain how it is possible for us to gain evidence for the
theory in question.

We finish by recalling that, as argued in ref [1], that a very similar problem
of confirmation may apply to the Everett interpretation, and in fact we would
suggest that problems connected to confirmation are likely to arise quite
generally in all interpretations of quantum mechanics which deny the common-
sense idea that macroscopic measurement events have unique outcomes which are