# On playing gods: The fallacy of the many-worlds interpretation

Luis C. Barbado  Institute for Quantum Optics and Quantum Information (IQOQI),
Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria.
Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna,
Austria  Flavio Del Santo  Group of Applied Physics, University of Geneva,
1211 Geneva 4, Switzerland; and Constructor University, Geneva, Switzerland

(August 28, 2024)

###### Abstract

We present a methodological argument to refute the so-called many-worlds
interpretation (MWI) of quantum theory. Several known criticisms in the
literature have already pointed out problematic aspects of this
interpretation, such as the lack of a satisfactory account of probabilities,
or the huge ontological cost of MWI. Our criticism, however, does not go into
the technical details of any version of MWI, but is at the same time more
general and more radical. We show, in fact, that a whole class of theories–of
which MWI is a prime example–fails to satisfy some basic tenets of science
which we call facts about natural science. The problem of approaches the likes
of MWI is that, in order to reproduce the observed empirical evidence about
any concrete quantum measurement outcome, they require as a tacit assumption
that the theory _does in fact_ apply to an arbitrarily large range of
phenomena, and ultimately to all phenomena. We call this fallacy the holistic
inference loop, and we show that this is incompatible with the facts about
natural science, rendering MWI untenable and dooming it to be refuted.

##  I Introduction

Quantum mechanics is about to celebrate its first centenary and yet, as D.
Mermin pointed out, the “disagreement about the meaning of the theory is
stronger than ever. New interpretations appear every day. None ever disappear”
[1].

One of the most popular interpretations of quantum theory seems today to be
the so-called many-worlds interpretation (MWI) [2, 3, 4, 5, 6, 7, 8, 9]. This
interpretation supposedly solves the measurement problem by refuting the fact
that quantum experiments yield single outcomes (among mutually exclusive
possibilities), but rather all results are obtained in different worlds. John
Bell pointed out some sociological origin in the rising consensus of this
exotic interpretation: “The MWI […] seems to attract especially quantum
cosmologists, who wish to consider the world as a whole, and as a single
quantum system, and so are particularly embarrassed by the requirement, in the
pragmatic approach, for a ‘classical’ part outside the quantum system… i.e.,
outside the world.” ([10], p. 193.).

The MWI has been already criticized on several different accounts [11, 12, 13,
14, 15, 16, 17, 18, 8, 19, 20], especially for the ontological cost of its
enormous inflation of worlds [21], the need of a preferred basis [22, 14], and
its inability to account in a satisfactory way for the probabilistic nature of
quantum predictions [11, 18, 19] (see also Sect. III). In this paper we put
forward a criticism that is at the same time more general (in fact, it applies
to a whole class of theories of which the MWI is only an example) and more
radical. Indeed, we will show that adopting an approach like the one
propounded by the MWI leads to a fallacy that we call the holistic inference
loop (HIL). We will show that, in turn, HIL is incompatible with some basic
facts about how natural science, thereby rendering theories which enter the
HIL untenable.

In a nutshell, we show that, to connect the MWI ontology with the observed
measurement outcomes, it is necessary to assume that the theory is indeed
strictly valid for an undefined, arbitrary set of phenomena, and ultimately
for all phenomena in nature, a statement which can never be empirically
proven. We notice critically how such statement on the universal validity of
the theory is not taken just as a “not-yet-falsified hypothesis”, for which
certain limitations may be empirically found in the future. Rather, the
statement that the theory _does hold universally_ constitutes a necessary
assumption in the explanation of each an every single empirical verification
of the theory. This sort of reasoning is what we identify with the
aforementioned HIL.

Our arguments lead us to label MWI as an untenable physical theory. Such a
sharp conclusion is a natural consequence of our criticism going to the roots
of the methodological justification of the theory, unlike other criticisms
that focus on specific elements within MWI. Therefore, if our arguments are
accepted there is no possible redemption for MWI with future amendments: MWI
should be rejected once and for all.

##  II Natural science as a human endeavour

###  II.1 Facts about natural sciences

We begin by highlighting three quite evident, but important facts about
science, which are a direct consequence of it being a purely human activity.
Due to the hardly debatable nature of the following three statements about
science, we will simply refer to them as _facts about natural science:_

  * (i)

_Empirical data are limited by the range of observations._
The empirical data at our disposal to test our theories are ultimately
obtained from the observation of natural phenomena,111We are not here taking
any radical antirealistic stance by claiming that _only_ sensorial experiences
are meaningful. We merely claim that, as a matter of fact, all the available
data in science are acquired through observation, i.e., through human senses,
often aided by instruments purposely built, in turn, by humans. and hence they
are obviously limited in several different ways. Among the many limitations,
we focus on the necessarily limited range of natural phenomena covered by
human observations. No matter how much we strive for enlarging such range, at
any point in time there will always be phenomena that lie beyond current
experimental reach.

  * (ii)

_Scientific theories are human creations._
The scientific theories that we use to organize our data and conceptually
explain (our sensorial experience of) the natural phenomena are purely a human
construction. It does not matter how astonishingly well a theory seems to fit
with a huge range of empirical evidence, or even with all evidence at disposal
at a certain stage of the development of science, there is always a
fundamental distinction between the concepts of the theory and our available
empirical data. The former may _match_ the latter, but they _are not_ the
latter.222A similar position to the content of (i) and (ii) was upheld by D.
Bohm, perhaps even more radically because he gave to the limitations a more
ontological character: “Any given set of qualities and properties of matter
and categories of laws that are expressed in terms of these qualities and
properties is in general applicable only within limited contexts, over limited
ranges of conditions and to limited degrees of approximation, these limits
being subject to better and better determination with the aid of further
scientific research. Indeed, both the very character of the empirical data and
the results of a more detailed logical analysis show that beyond the above
limitations on the validity of any given theory, the possibility is always
open that there may exist an unlimited variety of additional properties,
qualities, entities, systems, levels, etc., to which apply correspondingly new
kinds of laws of nature.” [23], p. 133.

  * (iii)

_Valid scientific theories are self-consistent and empirically adequate._
Necessary conditions for the validity of a theory in natural science are its
internal consistency _and_ its empirical adequacy–namely, the degree to which
it fits and predicts empirical data– _within_ its purposed range of
applicability.333Clearly empirical adequacy is endorsed by both realists (who
take this as a sign that a theory is approaching truth) and empiricists (for
whom this is the main, and perhaps the only necessary, standard of science);
see, e.g., [24]. Within natural science, a theory should be devoted, besides
yielding predictions, to provide satisfactory explanations for the empirical
observations. This means to be able to formulate consistent stories that
account for the observed phenomena. Note that this may involve introducing
into a theory metaphysical and mathematical elements (such as particles,
forces, fields, quantum states, etc.), as well as correspondence rules between
the observed data and these terms (see, e.g., [25]).444A disclaimer is here in
order. In quantum foundation it is customary to distinguish between theories
and interpretations. What we mean here by theory encompasses not only the
empirical content and the mathematical structure, but also the whole
collection of postulated metaphysical entities. Therefore, in this paper we
will use theory and interpretation interchangeably unless differently
specified.

Without commitments regarding metaphysical positions, we claim that the
previous facts about natural science are compatible with a wide variety of
views about science. In fact, independently of whether one has a strong
realistic or a more empiricist view (or even a solipsistic view, for that
matter), who is in the position to refute that we only have limited empirical
data and that we will never have a “complete” collection thereof? Does anyone
have a collection of empirical data that covers all natural phenomena we can
ever experience? Does anyone visualize a concrete plan that will provide us at
some point with such a collection? The same applies for the human nature of
scientific theories and concepts: Who can refute that any scientific theory we
know is a creation of humankind? As a matter of fact, it was not flies, dogs,
or monkeys that created scientific theories, but humans did, in the same way
that they dominated fire, developed natural languages, invented religions, or
built the Pyramids. Scientific theories do not get revealed to us in any
Tablets of Stone: All we have done and can do, _it is a fact,_ is to create
our own theories and test them against our _limited_ empirical evidence.
Finally, the fact that the validity of a theory is given by its capacity to
reproduce the empirical evidence is simply a desideratum of natural science by
definition, to distinguish it from different kinds of knowledge.

###  II.2 Tenability of scientific theories

Having in mind the facts about natural science that we have stated, let us now
go into the philosophical discussion on what implications these facts have
when critically analyzing any scientific theory. In this sense, we propose the
following elementary statement:

_Necessary condition of tenability (NCT) for a scientific theory: Any theory
that enters in contradiction with the facts about natural science, should be
deemed untenable._555For our argument, we do not need to engage into the
notorious debate on the problem of induction and demarcation (see [26]),
either based on verifiability or falsifiability. We merely take the compliance
of a theory with the facts about natural science as a minimal necessary
condition of tenability.

As radical as it may sound, we believe that it is a completely fair requisite
for any scientific theory. Any theory whose validity relies on assumptions
that contradict the evidence about how science is indeed constructed is
untenable. This reasoning is based on a descriptive argument: It would be a
theory that is _pretending to do something that it is not doing._

A theory may enter in conflict with the facts about natural science in many
ways. We wish to bring here one specific way, which we shall call the
_holistic inference loop,_ on which we will base our criticism of the many-
worlds interpretation (Sect. III). Let us call some scientific theory $T$,
some specific range of known phenomena $P$ that we wish to scientifically
explain with said theory,666$P$ needs not to be the set of all phenomena that
$T$ aims to account for. It could be also a subset thereof, but in any case it
must be of course limited, in the sense discussed in fact (i). Even in the
case that $T$ aims to be an “universal theory”, in whatever sense, it is
always only tested against the set of all experienced phenomena, which is
necessarily limited. and the empirical data we have collected about such
phenomena $E_{P}$. Then one can formulate the following:

_Holistic inference loop (HIL): A theory $T$ enters the HIL if the inferential
connection between $E_{P}$ and $T$, which allegedly justifies the validity of
$T$ for $P$, necessarily relies on the assumption that $T$ applies to some
other arbitrarily large range of phenomena $P’$._

We contend that

_HIL implies untenability: A theory $T$ that enters the HIL cannot meet the
NCT, and should therefore be rejected._

Indeed, meeting the NCT means that $T$ is compatible with the simultaneous
fulfilment of the three facts about natural sciences (i), (ii) and (iii)
(Subsect. II.1). According to (iii), for a theory $T$ to explain the set of
phenomena $P$ means to be able to fit and predict the available observed data
$E_{P}$. However, if a theory enters the HIL, such capacity is conditioned on
the fulfilment of a tacit hypothesis, which we shall call the _holistic
hypothesis,_ namely that $T$ also applies to other arbitrarily large set of
phenomena $P^{\prime}$. In other words, $T$ explains $P$ only if it applies to
an arbitrarily set of phenomena $P^{\prime}$. However, in agreement with (ii),
we understand the contingent nature of the theory $T$ as a human creation, and
this implies that we cannot assume its applicability to some arbitrary
phenomena $P^{\prime}$ as granted. One can only assume this as a provisional
hypothesis, but then the only possibility to check the hypothesis would be,
again according to (iii), to empirically show that $T$ does indeed apply to
$P^{\prime}$. According to (i), however, this is not possible due to the
arbitrary nature of $P^{\prime}$: We can never empirically exhaust an
arbitrarily large set of phenomena. Therefore, the connection between our
theory $T$ and the empirical data $E_{P}$ not only remains dependent on a
hypothesis, but rather on a hypothesis that is impossible to corroborate while
fulfilling the NCT! It is therefore also impossible to justify the theory $T$
from $E_{P}$, and thus according to (iii) the theory $T$ has not been
validated. In conclusion, if $T$ enters the HIL we cannot justify its validity
for $P$ while fulfilling the NCT, and $T$ is therefore an untenable theory.

Figure 1: Schematic depiction of a) a standard validation procedure of the
theory $T$, and b) an untenable validation of the theory $T$ that enters the
holistic inference loop.

In Fig. 1 we show in a) a correct validation of the theory $T$ as applicable
to the phenomena $P$, due to its capacity to reproduce and predict the
empirical evidence $E_{P}$ of such phenomena; and in b) the scheme for a
theory $T$ entering the HIL. Let us pay attention to the two crucial aspects
of such scheme:

  1. I.

The range of phenomena $P^{\prime}$ within the holistic hypothesis must be an
_arbitrary_ range. If $P^{\prime}$ is a well-defined finite range of
phenomena, the theory is not entering the HIL. The reason is that, in such
case, the validity of $T$ for $P^{\prime}$, and with it the validity of $T$
for $P$, could be proven while fulfilling the NCT. Usually, one would simply
be in a _legitimate_ holistic scenario, i.e., a scenario where the
applicability of $T$ to several phenomena $P,\leavevmode\nobreak\
P^{\prime},\ldots$ (provided that the total range remains well-defined and
finite) cannot be understood separately, but rather _altogether._
777Paradigmatic examples of holistic scenarios appear for example in ecology.
Consider that we are studying whether a theory $T$ applies to the feeding
behaviour $P$ of a certain species of predator in a given ecosystem. In order
to do so, we make use of some collected empirical data $E_{P}$ about the
ecosystem. However, since all the different elements in an ecosystem are
interconnected, $E_{P}$ is always highly dependent on many other phenomena in
the ecosystem. So we may also decide to assume certain hypothesis about other
phenomena $P^{\prime}$ in it; for example, we may assume that the theory $T$
also applies to the feeding behaviour of other competing predators, _even when
we do not have full evidence for it._ If we prove this way that $T$ applies to
$P$, the validity of such conclusion will be conditioned to out hypothesis
that $T$ applies to $P^{\prime}$. However, this is a well-defined hypothesis
that could be proven right or wrong with further empirical research.

  2. II.

The holistic hypothesis must systematically appear as a necessary condition to
invoke when justifying the theory $T$ from the empirical data $E_{P}$. This is
perhaps the most distinctive aspect of the theories entering the HIL, so let
us stress the point: _The formulation of a holistic hypothesis alone is not
sufficient for entering the HIL._ The hypothesis must also constitute an
omnipresent necessary argument any time one wishes to experimentally justify
the theory for some concrete phenomena.888Classical mechanics is a clear
example of a theory that presumes an arbitrary applicability and yet does not
enter the HIL. We deepen on this idea in Subsect. III.4.

Finally, let us clarify the meaning of the curve that closes the loop in Fig.
1, which we actually labelled ‘HIL’ in the diagram. This represents the
“psychological” fallacy that leads to propose the holistic hypothesis in the
first place: Namely, the belief that there is already strong enough empirical
evidence supporting $T$ (as applicable to certain phenomena $P$) as to suggest
the extrapolation of its validity to an arbitrary set of phenomena
$P^{\prime}$; that is, as to formulate the holistic hypothesis. However, as we
already argued, there is no empirical evidence that can ever fully validate
the holistic hypothesis. Such hypothesis will either be falsified or remain
empirically unsupported. But then, we stress again: This is _not_ the full
problem of the HIL. The _full_ problem is that the whole theory $T$, including
any concrete explanations of experienced phenomena that “inspired” the
holistic hypothesis, relies on such hypothesis being strictly true. The
circular argument is manifest! Hence, the whole theory $T$ remains as devoid
of a validation as the holistic hypothesis itself.

The construction of theories that do not fulfil the NCT leads, as an
inevitable consequence, to fatal consistency problems in such theories and in
their relation with the empirical evidence. In particular, for the theories
entering the HIL we identify major problems both for the robustness of their
statements and, at the same time, for the prospects to overcome these theories
with better theories in the future. We will further discuss these problems in
details through the concrete case of the MWI, which we address in the next
section as a paradigmatic example of a theory that enters the HIL.

##  III The Many-worlds Interpretation

###  III.1 Interlude: The empirical evidence for quantum mechanics

At the level of empirical evidence, what we find for whatever range of quantum
phenomena $P$ is: 1) That each time a quantum measurement takes place a single
outcome is found, 2) that for a measurement repeated under empirically
identical conditions, the outcomes of each repetition are in general
different, and their long run statistics match the probabilities predicted by
the Born rule for the given conditions, and 3) that the relation between the
statistics of successive measurements for isolated physical systems can always
be reproduced by the unitary evolution given by the Schrödinger equation.
These are the three (interrelated) parts in which we can organize the set of
available empirical evidence $E_{P}$ for quantum mechanics. Any theory or
interpretation $T$ of quantum mechanics must, first and foremost, reproduce
and provide an explanation for _all three_ parts, at least to the accuracy to
which they have been tested.

###  III.2 How MWI falls into the HIL

The Many-worlds interpretation (MWI) was introduced by H. Everett in 1957
(initially called relative-state interpretation) [3, 7]. It maintains that
“every time a quantum experiment with different possible outcomes is
performed, all outcomes are obtained, each in a different newly created world”
[6]. As we mentioned, the MWI has received a profuse collection of criticisms
[11, 12, 13, 14, 15, 16, 17]. Most of these can be broadly simplified into
questions, such as: Where are all those worlds supposed to live? When, where
and under what circumstances the branching happens? Which is the preferred
basis in which such branching occurs? [14]. And, most of all, how do the
(observed) probabilities arise from the unitary (deterministic) dynamics? [18,
9, 8, 19, 27]. Although we share these critical views, the argument against
MWI we propose here, as we already advanced, does not address any of these
problems within the theory. So, in what follows, we can as well assume that
these known criticisms can be somehow overcome.

Let us show how the MWI enters the HIL. This theory, in fact, elects the
unitary evolution as _the_ fundamental law of nature, which applies to
literally everything, i.e., to the whole state of the Universe. So it
automatically reproduces, just by definition, the part of the empirical
evidence that corresponds to the relations between statistic outcomes of
different measurements, given by the proposed unitary evolution. But what
about the fact that only one outcome is empirically found each time a
measurement takes place, and about the statistics of those outcomes
themselves? In this case, the reasoning that connects these empirical
evidences $E_{P}$, for whatever quantum measurement $P$ we wished to describe,
with the theory $T$, necessarily assumes that all such quantum mechanical
concepts not only satisfactorily describe $P$, but also other phenomena
$P^{\prime}$. The latter include the evolution of the measurement apparatuses,
our brains, consciousness, and ultimately the entire Universe. This can be
explicitly found in the words of Everett himself [2]: “[A]ssume the universal
validity of the quantum description, by the complete abandonment of [the Born
rule plus the state-update rule]. The general validity of pure wave mechanics,
_without any statistical assertions_ , is assumed for all physical systems,
including observers and measuring apparata. […] [This alternative
interpretation] has the virtue of logical simplicity and it is complete in the
sense that it is applicable to the entire universe.” The HIL is thus here
striking because, in order for the theory $T$ to account for any specific
$E_{P}$ in a consistent way, one needs to assume the universal applicability
of the theory, i.e., $P^{\prime}$ must be the whole Universe.

We emphasize the critical point: The problem is not only the assumption that
MWI applies universally, the problem is the use, or rather abuse, made of that
statement. Other well-established physical theories also make formal claims on
their universal applicability. However, when contrasting those theories with
the experimental evidence, one does not need to invoke such universality as
something strictly true. Rather, one only requires that the theory applies to
the finite range of phenomena which is significantly involved in the specific
experiment under consideration. On the contrary, for MWI strict universality
is a tacit necessary condition appearing in each and every single account of
whatever quantum experiment.

For the self-consistency of MWI, universality must strictly mean that the
theory does apply to _any natural phenomena that we may ever access
empirically,_ which is definitely an arbitrary range of phenomena
$P^{\prime}$. One could at first glance hesitate whether this is really a
strict necessity of the theory to account for each concrete measurement
outcome we perceive. After all, consider that we rephrase our empirical
evidence on “the outcomes of measurements” as, say, “the perception by our
consciousness of the outcomes of measurements”. Would it then be enough if we
could just prove that “our consciousness” (if we ever manage to define it
accurately enough) evolves unitarily according to quantum mechanics? We argue
that this is not the case, by simply noticing that any phenomena $P^{\prime}$
we may ever access in nature must possibly become affected by results of
quantum measurement outcomes just as our consciousness is. Therefore, any
depart from quantum mechanics by $P^{\prime}$ would equally compromise the MWI
account, not only of $P^{\prime}$, but of the result of the quantum
measurement. In other words, any possible phenomena we may ever access
empirically can play the role of the observer in a quantum measurement, in the
sense of becoming affected by its result. Therefore, if the requisite that
_any_ observer is governed by quantum mechanics is a necessary ingredient in
the understanding of the measurement process, and of the consistency of the
empirical evidences on the individual outcomes of it, then the validity of
such understanding becomes conditioned to quantum mechanics governing all
phenomena.

For example, in a more recent work [28] on a variant of the MWI, the following
is stated on the perception of the trajectory of an alpha-particle emitted in
a cloud chamber: “The reason for the fact that we only see one trajectory at a
time is that even though everything is a q[uantum]-wave within which things
exist at the same time, when we interact with this q-wave we can only reveal
some of its aspects, one at a time (this is where Heisenberg’s Uncertainty
comes from). We ourselves are also a collection of q-waves and it is when our
q-waves correlate with the q-waves of the alpha-particle that
c[lassical]-numbers emerge. These correlations between q-waves are called
quantum entanglement and so the classical world owes its own existence to
quantum entanglement.” Here we find explicitly how the experimental evidence
on the alpha-particle trajectory can only be accounted for due to the quantum
nature of the observer. But then, either strictly _everything_ in nature is
truly a quantum wave, or making something which is not to play the role of the
observer would ruin the given MWI explanation, also for the alpha-particle and
for our perception of it. We will explore this idea in further detail, and its
fatal consequences for the MWI, in the next subsection.

The analysis of MWI as a theory entering the HIL reveals as glaring the
circular, self-justified nature of its supposed validity. Indeed, without the
_a priori_ assumption that any phenomena can be accounted for with an unitary
evolution, the supporters of many-worlds have absolutely no way to justify the
empirical adequacy of quantum mechanics, because they cannot reproduce the
statistic outcomes of measurements. Following the psychological fallacy of the
HIL that we mentioned in the previous section, in order to elevate quantum
mechanics to its universal status, MWI supporters resort to the fact that
there is an outstanding empirical evidence for quantum mechanics, indeed the
best ever achieved for any physical theory. But such empirical adequacy of
quantum theory, i.e. the success of its empirical predictions, was allowed by
testing it against theories that account for probabilistic outcomes at the
fundamental level, predominantly the Copenhagen Interpretation. These are all
theories that consider a transition between the quantum and the classical
domain, i.e., that display a Heisenberg cut.999These theories can be divided
into two main families: those that take the cut to be epistemic and
arbitrarily movable (such as more or less subjective variants of the
Copenhagen interpretation [29, 30, 31, 32, 33]), or the objective collapse
models ([34, 35, 36]). These theories manage to separately account for the
empirical evidence of each limited quantum phenomena one may wish to explore,
without needing to tacitly involve the whole Universe in each an every
experiment. The accumulation of an overwhelming amount of successes of those
individual experiments, the self-consistency among them, and the lack of any
failure to date, may suggest distinguishing the theory with some honour of
“universality” (as of today). But such honour is only a late award for a
flawless career of well-established and coherent achievements. In contrast,
for MWI the medal of being a _strictly universal_ theory is the initial
(unaffordable) deposit that the theory demands in advance, before gathering
its very first successful empirical prediction.

We propose the MWI supporters the following simple thought experiment:
Convince a classical physicist, who knows nothing about quantum mechanics, of
the validity of MWI as accounting for the empirical evidence of some very
simple quantum experiment. Notice that we say the validity _of MWI_
specifically. That is, the whole explanation should be done _strictly without
ever mentioning any notions of collapse, measurement outcome, probabilities,
Heisenberg cut, etc.,_ at least at a fundamental level. It is not difficult to
realize how puzzled the classical physicist will be. But the astonishment will
not come from a fascination about the existence of those unfathomable parallel
universes. It will rather be much more worldly: She will need to accept that
this completely new theory, of which she has not been given any single
empirical evidence yet, does indeed apply, not only to the simple quantum
system that she is about to test in the laboratory, but also to other
arbitrary phenomena, including her own consciousness, _as a pre-condition_ to
possibly make any connection between the theory and the empirical evidence
that she will obtain from the experiment. That is, she needs _first_ to
accept, as an _act of faith,_ such arbitrary range of applicability _before_
she gets her very first empirical verification of MWI in, say, a double-slit
experiment. It is again not difficult to realize, following our previous
discussion, that within the MWI there will be no empirical verification she
will ever obtain that is not conditioned to further acts of faith.

We highlight what the previous discussion ultimately evinces: MWI can be
considered strictly as an _interpretation_ of quantum mechanics, with
identical empirical predictions to other interpretations, _only if_ one
illegitimately assumes that universality does hold, whereas other
interpretations do not require such assumption. We have thus revealed how the
MWI rhetoric has hidden its own empirical void behind the enormous empirical
success of _the other_ interpretations. The seemingly religious need of
unification and simplification of “many-worlders” leads them to take the
success of these theories, and add to them the unwarranted claim on the
universal domain of validity of unitary evolution, to finally try to relate
MWI with the empirical evidence about even the simplest quantum phenomenon:
They need to play gods to justify their theory.

###  III.3 Lack of robustness: The many-worlds interpretation is doomed to be
refuted

As we already discussed, entering the HIL leads to the untenability of a
theory. Here we address a trivial consequence of it, namely the lack of
robustness of the description of reality made by MWI.

Many-worlders claim that their theory is empirically testable because quantum
mechanics is empirically testable. Let us assume that. The point is that, if
the explanation of what happens with some _concrete_ phenomena $P$ relies on
the universality of the theory, then if one day we find _whatever_ phenomenon
that does not fit into quantum mechanics, it means that we don’t understand
$P$ with MWI any more. Actually, we suddenly don’t understand anything at all.
Our comprehension of the basic quantum phenomena such as the Stern-Gerlach
experiment, the nucleus decay, the structure of the atom, etc. just falls
apart!

Let us give some concrete example on how this would actually happen. Consider
a Stern-Gerlach experiment in which, depending on the trajectory in which the
spin particle is deflected by the magnetic field, a macroscopic device raises
either a yellow or a green flag. The many-worlds description of the experiment
is that the spin orientation, the particle trajectory and the colour of the
raised flag become all entangled during the process. The fact that we perceive
sometimes a yellow flag and sometimes a green flag is only a consequence of
our own (quantum) consciousness further entangling with all those elements.
But, in the many-worlds ontology, the reality of the flag is neither yellow
nor green. The reality of the flag, as that of the rest of the elements in the
experiment, is given by a wave function that describes a collection of worlds,
in each of which the flag and the other elements have different properties.
New physical theories to come may conceptualize the reality of this experiment
very differently, and eventually more accurately. But at least for experiments
describable by quantum mechanics, such as this one, they _must reproduce the
existence of these many-worlds_ as a very accurate and consistent description.
This would be the desired robustness of the theory that we will prove to be
nonexistent.

Now, a simple question can be made: Any time after the experiment took place,
can we use the colour of the raised flag that we perceive as an empirical
evidence for other experiments? Say, for example, that we wished to study the
colour perception of certain families of insects, or we wished to test several
brands of detergent on different colouring of clothes. Could we rely on the
colour of the flag that we perceive and use it, for such purposes, as an
_empirical fact?_ The answer within many-worlds, we believe, would be
affirmative. But notice that this is done even when our perception of the flag
and the reality of the flag are completely different things! The reason why
this is possible is that our own perception is telling us that we are in the
branch where the flag is, say, yellow; and _noticing that all the further
elements involved (the insects, the detergents…) are also quantum mechanical,_
a simple computation shows that the interference with the branch in which the
flag is green is completely negligible. So, for practical purposes, in order
to keep track of our own perception of the further experiments we want to
carry, we can take the colour of the flag as an empirical fact, and everything
remains consistent with the simultaneous existence of the rest of the wave
function.

But then, what about testing a potentially post-quantum phenomena? Can we use
the colour of the flag? Consider that we have found some new natural
phenomenon, which we wish to study experimentally in order to check whether we
can describe it with quantum mechanics or not. Notice that, on the one hand,
we already have a very accurate description of the flag by MWI, so we can
quite safely state that the flag is ultimately neither yellow nor green. But,
on the other hand, the phenomenon that will become affected by the reality of
the flag may not be describable with MWI, so we cannot rely anymore in the
fact that the overlapping between the realities with different colours will
stay negligible. Hence, we shall not trust anymore our own perception of the
colour and use it directly as an empirical evidence, since it is strictly a
subjective perception which consistency is only warranted for other quantum
systems. Maybe we need to be more careful and consider the full reality of the
flag that MWI provides us, namely the wave function of it.

But then, if we cannot use the colour of the flag as an empirical evidence, a
huge problem arises: Can we use _any_ outcome of a quantum measurement
whatsoever as empirical evidence in order to test some potentially post-
quantum phenomenon? We notice how the consequences of a negative answer to
this question are completely unacceptable: All our sensorial experience, even
the seemingly most stable one, can be described, to a great extent, as a
consequence of quantum phenomena; and thus its reality is, at least to such
extent, describable by MWI. Therefore, we can safely state that all reality
around us, meaning the wave function of it, and our sensorial perception of
it, are extremely different things. We cannot then rely on our sensorial
experience at all in order to do experiments beyond the quantum regime, and it
is thus completely impossible to empirically test any post-quantum
phenomenon.101010Any attempt to compute the “really existing wave-function” as
the correct evidence to use is also hopeless: Any computation of any wave
function must itself rely on the use of some other sensorial experiences that
cannot be trusted themselves either, for the same reasons. Such conclusion is
a natural consequence of MWI assuming the universality of quantum mechanics in
order to make sense of our sensorial experience. But this is of course
untenable, because it would mean that quantum mechanics cannot be refuted
experimentally.

If we want to avoid such blind alley, we have to accept that at least some
sensorial experiences can be considered as empirical evidences on their own
right, that we could involve into experiments testing potentially post-quantum
phenomena. For the sake of continuity, let us symbolically pick again the
colour of our flag as one of those evidences. We then make some parameters of
the potentially post-quantum experiments we are carrying to be dependent on
such colour. Consider now that we find some post-quantum phenomenon, something
which description does not fit into the quantum mechanics formalism. Of
course, the results of those post-quantum experiments will be consistent with
the colour of the flag. And notice that, by stating this, we are assuming
absolutely nothing about such post-quantum phenomenon. Nothing except that we
can use the colour of the flag as an empirical evidence.

But then, how is it that we find consistencies, even for the post-quantum
phenomenon, with the flag being yellow when we see it yellow and with it being
green when we see it green? Notice that the usual many-worlds explanation is
now not warranted. We cannot say that our post-quantum system got entangled
with the flag and that we got two copies of our system, one acting as if the
flag was yellow and the other as if the flag was green. That would truly mean
we are imposing properties to our post-quantum system that we have no right to
impose at all. Ultimately, in order to reconcile the situation with the many-
worlds reality of the flag that we were so sure about, we would need to impose
that our post-quantum phenomenon is actually a quantum one!

Again, we encounter ourselves with the need to enforce universality of quantum
mechanics so that the connection between the proposed realities _of the
quantum flag, particle trajectory and spin,_ and the empirical evidence we
have about them can survive at all. In conclusion, as we already advanced,
finding just a single post-quantum phenomenon, we would find out that MWI
describes absolutely no phenomenon of the reality we live in, in no specific
limit and to no reliable degree of accuracy.111111One may dismiss our argument
as an illegitimate criticism to MWI, as soon as it involves (potentially)
post-quantum phenomena to which MWI does not pretend to apply. However, it is
not us who make the nefarious decision of involving those post-quantum
phenomena. It is MWI itself that does so, as soon as its explanation of the
outcomes of quantum measurements makes statements about “the observer” of such
outcomes. Given such explanation, we just need to further notice that post-
quantum phenomena may exist and may play the role of that observer, and
tragedy unfolds on its own. As importantly, we remark that our argument is
_not_ requesting MWI to provide an _a priori_ recipe on how it should be
understood in coexistence with the next theory to come. We are just pointing
how it fails to account for the empirical evidence of a quantum measurement
outcome without assuming that whatever else that coexists with such outcome
must also be quantum.

Figure 2: Scheme with possible answers on whether to consider the _specific_
outcome of a quantum measurement as an empirical evidence for other phenomena.
‘QM’ stands for _quantum mechanics._

The given example is just a detailed staging of how entering the HIL
compromises the robustness of the statements about reality that any theory can
make. In Fig. 2, we provide the reasoning scheme of the example presented in a
generic way. While following it, one easily realizes how the problem arising
is always the same: The need to be consistent with an unjustified assumption
of universality, so that the full house of cards does not crumble. Therefore,
beyond the detailed follow-up of the reasoning, our core argument is precise
and simple: Making a theory completely dependent on its universality to
actually hold for anything concrete whatsoever is an _all in_ bet: Either one
“explains everything”, or nothing at all (see Fig. 2). But such bet in natural
sciences is doomed to lose, sooner or later, due to our always limited
experimental evidence, and the always existing possibility that new phenomena
defying the theory shows up. Therefore, MWI could at most survive as a mental
scheme in a “pure-quantum universe” of fantasy, but not as a valid theory of
any concrete phenomena in the natural world we happen to live in.121212We
bring at this point the following quote from the approach to MWI in [4]:
“Whether or not a theory is universal is a purely logical property of the
theory. This must be distinguished from the empirical question whether quantum
theory is universally true. The latter question is not addressed in this
paper.” It seems thus that some approaches to MWI are indeed explicitly
dedicated to build up (on paper) a self-sufficient and self-consistent pure-
quantum universe. Put like that, there is nothing really problematic in such
formal endeavour. This article simply shows how, if the self-consistency is so
fragile that it does not admit any interaction with a non-quantum ingredient
without fully breaking (equivalently, it presumes universality of quantum
mechanics as an omnipresent internal requisite), the price to pay for building
such beautiful universe is to strictly detach it from any tenable description
of any natural phenomena.

###  III.4 Interlude: On refutations and overcomings

In this subsection we further stress that, once a single empirical evidence
escaping a theory that enters the HIL shows up, we ought to _complete refute_
it. The theory is not simply overcome by a more general one that encompasses
the previous theory as a limiting case.

As a paradigmatic example of a fully refuted theory, although for purely
experimental reasons, let us consider phlogiston theory. The different
formulations of this theory could apparently account for several aspects of
the combustion and rusting processes. However, all these supposed explanations
relied on an unproven hypothesis: The existence of a substance, namely
_phlogiston._ When further experiments and more rigorous analyses showed
evidences that were incompatible with such substance, the theory was fully
refuted: All the supposed explanations made by the theory were _strictly
wrong._ They became fairy tales.

This fate is in blatant contrast with, for example, that of classical
mechanics and Newton’s gravity. We still can understand and very accurately
explain the Solar System with their old concepts, regardless of quantum
mechanics, special and general relativity, or any broader theory that we will
find tomorrow. Indeed, all these new theories, far from fully refuting the old
ones, only reaffirm their validity within their range of applicability: Rather
than strictly wrong, they are _approximately correct_ theories. Notice in
particular, that it is not just specific results of the old theories that are
recovered: It is _the full theories_ that one can find in the appropriate
limits. Therefore, the old theories were _overcome,_ but _not_ fully refuted.

The reason that those theories survived as approximately correct is clear:
Their account of the empirical evidence did not depend on an unwarranted
hypothesis. In particular, and coming back to what matters for contrasting
them with theories entering the HIL: In describing, say, the Solar System, the
arguments connecting the theories with the empirical evidence do not make the
claim that “our consciousness”, or any _arbitrary_ phenomena that may be
affected by the motion of the planets, need to behave according to classical
laws or concepts.

We notice how some of the principles in classical mechanics may receive the
epithet of “universal”, famously Newton’s law of gravitation. But this is
because they are such _within_ the theory. However, there is no need to invoke
that this universality is strictly true in nature every time one applies the
principle to a concrete situation (indeed, we know today that Newton’s
specific formulation is not universal). Something similar can be said about
the concepts of absolute time or Euclidean space, which are assumed to “apply
universaly”. If we are describing what is happening on a billiard table, it is
enough for the theory if these concepts apply (very accurately) in that
regime, something of which we have astounding empirical evidence. Given that
_proven_ assumption, the theory already manages to reproduce the empirical
evidence on the motion of the billiard balls, and its duties are accomplished.
We refer to Subsect. II.2 to easily realize why, in view of these facts,
classical mechanics does _not_ enter the HIL.

An equally honorable future awaits the interpretations of quantum theory (and
here we mean _interpretations_ in the strict sense, see footnote 4) that are
provided with a Heisenberg cut. The latter limits itself to state that, in
certain contexts, one evolution (the unitary one) is more suitable than the
other (the “collapse”), without explaining why this is the case. And such a
statement, even if we may feel it incomplete or not fully satisfactory (for a
variety of reasons), will nonetheless remain approximately true and useful, no
matter what possible future theory or interpretation will revolutionize the
worldview of quantum mechanics one day.

On the contrary, with the assumption of MWI as the correct interpretation of
quantum mechanics, similarly to phlogiston theory, each and every supposed
empirical verification is yet pending on an unproven hypothesis: That the
theory is indeed strictly universal. And, worse off than with phlogiston
theory, this hypothesis is not only unproven, but also impossible to prove!
Therefore, the arguments exposed in this article show that the fate of MWI is
inescapably that of phlogiston theory: To be fully refuted.131313The rejection
of MWI is frequently compared with former naïve scepticism about currently
well-established theories (within their range of applicability), based on an
ingenuous embracing of our most immediate perception of reality, such as
scepticism about atomic theory [37] or even about Copernicus’ heliocentric
model [3]. We hope that the arguments exposed here make it clear, without need
of further discussion, how such comparisons are blatantly fallacious. We
simply suggest, as an amusing counterfactual story to think about, what would
have become today of Copernicus’ model if he had grounded it on whatever deep
statements about, say, the _ultimate_ functioning of our brain and the
emergence of our consciousness that he could manage to propose in the XVI
century.

###  III.5 MWI artificially obstructs the way to new theories of nature

As we have clarified in detail in the previous subsections, MWI relies on its
assumption of universality in order to explain the observed data, hence a
single post-quantum event would undermine the whole explanatory power of MWI.
Thus, believers of MWI would likely be more reluctant to accept new evidences
begging for an explanation outside of the theory itself. A theory that does
not suffer from the fallacy of the HIL, on the other hand, has already a well-
established (approximate) explanatory power over the proven finite domain of
applicability, without relying on an _a priori_ presumed universality;
therefore, it does not feel threatened by other theories applying to a larger
set of phenomena, or even outperforming it in accuracy for the same phenomena.
There is no true conflict between them, rather there must be complementarity
and agreement, and in any case they can co-live as the more general theory
should include the older one as a limiting case. But, as already argued, any
phenomena escaping MWI would make the explanatory power of the whole theory
collapse.

So, the very same tacit presumption of universality that ultimately
constitutes its lack of robustness, can be actually used by MWI as a more
conservative line of defence of the theory, slowing down the progress of
science. Indeed, theories that demarcate their finite range of applicability,
or at least _consistently_ admit that such range may at some point exist,
might find themselves in front of empirical evidence which, they must
acknowledge, are incapable of giving an explanation to, and react accordingly
just by humbly leaving the corresponding phenomena outside their scope.
However, in front of some empirical evidence that MWI may be struggling to
give a _concrete_ explanation, the tacit universality always provides a joker
to play: The theory is considered universally correct so the observed
phenomena must bend to it, one way or another.

An example of how adopting MWI can be pernicious to the development of science
is provided in [38]. Therein, it is stated that observed events that are
believed to be anomalous (i.e. extremely unlikely in the space of possible
configurations)–such us the low entropy of the initial state of the Universe
to explain the thermodynamical arrow of time–do not beg for further
explanation. MWI can simply accept them by invoking the fact that we happen to
be in the decohered branch of the state of the Universe where such events
happen. In this way, any event expected to be rare or anomalous does not cry
for an explanation in MWI. This is a dangerous way of thinking about
scientific investigation itself, because the MWI seems to endorse the view
that, since all possible things _do happen_ in different worlds, we should
blindly accept anything without wonder (see [39] for a criticism). It’s just
_per accidens_ that we experience to live in this world.

We see in this example how MWI can appeal to the existence of different
universes within the theory, in order to automatically embrace any possible
experimental evidence within its domains. It is clearly just a rhetorical
solution, but it is quite distinctive from a theory entering the HIL: If,
consciously or not, we have already assumed that the theory applies to
arbitrary phenomena, we do not feel in the need to question whether a given
phenomenon can be accounted for with the theory or not. We may rather tend to
contort the arguments so that there is a way in which it can be accounted for,
especially if otherwise everything we believed in is suddenly lost forever.
But then, asking why things happen the way they do, i.e., asking for a
scientific explanation, becomes meaningless, since we are always blinded to
possible new explanations.

##  IV Discussion and outlook

In this paper we have put forward a novel argument aimed to show why the many-
worlds interpretation should be rejected. It ought to be noted that the
(numerous) already existing criticisms of MWI arise as conceptual problems
“inside” the theory, i.e., only once one has accepted the plausibility of the
interpretation in the first place. Our criticism is here more radical: It
shows that the claim of universality is the result of a vicious inference that
cannot be justified in the first place. Following a teleological view of
unification and universality, the many-worlds interpretation enters the
circular argumentation of the HIL, violating–like any other theory entering
the HIL–the necessary conditions of tenability for a scientific theory.

We have focused our criticism here on the MWI. It is our view that other
interpretations with claims of universality, such as “superdeterminism” [40]
or the “Lagrangian view” [41] may also enter the HIL, although the specific
reasons could greatly differ. We shall address them in future works.

Furthermore, some arguments we have raised in the present article may have
implications for other interpretations that share relevant features with MWI.
In particular, we would like to make an important remark on the implications
of assuming the quantum nature of the observer of a quantum measurement,
something which permeates several interpretations and is pivotal to some
discussions on quantum foundations, such as Wigner’s friend scenarios [42]. In
principle, the arguments exposed here do not openly confront the legitimacy of
such an assumption _per se._ The problem arises when such assumption becomes a
necessary ingredient in understanding why the observer does perceive a unique
outcome in each measurement, or in explaining the long run statistic of those
outcomes.

Almost a century of success of quantum mechanics has relegated the possibility
of its future overcomings to the corner of ideas that one shall gently give a
nod to, but then act as if they did not exist. Proposing that such envisioned
possible overcomings may have some implications in our current understanding
of the theory is felt as an unjustified attack to quantum mechanics. We would
like to overturn such view, and consider this work partly as a tribute to
quantum mechanics. The unblemished success of the theory in such ample range
of phenomena is really staggering. Precisely because of that, it is suicidal
to leave our best comprehension of such rounding success in hands of any
interpretation that, due to its soaring ambition, is incapable of building
itself on any concrete empirical ground, and therefore cannot but fall apart
sooner or later.

As we already mentioned, interpretations that admit a Heisenberg cut already
own their everlasting position as valid theories in the history of physics,
despite all the conceptual problems. In contrast, we cannot avoid noticing the
sort of irrational desperation that seems to animate certain interpretations,
including (but not only) MWI. By trying to “fill the gap” left by the
admittedly many conceptual problems of Copenhagen Interpretation, they jump to
unjustifiable constructions. Whatever unconventional and mind-blowing these
constructions may prima facie look, far from being revolutionary, they usually
pursue a rather conservative attitude in preserving some “philosophical
prejudices” [43] as something necessary and universal. Be them determinism,
locality, unitarity, linearity, relativistic causality, etc. We believe that
such an attitude can actually be harmful to scientific progress, since it
steers in the direction of building up more and more complicated conceptual
and mathematical constructions that make the desired “good idea” survive at
any price. Even at the unacceptable price of ignoring the reality of the
scientific process itself! One can uphold strong arguments on why any of those
ideas should be kept, and legitimately try to build reasonable interpretations
or theories that are coherent with such choice. But none of these arguments
can justify playing gods.

In the specific case of MWI, there seems to be an almost religious sentiment
that animates its supporters by believing that everything that exists is a
single, “simple”, immutable, elegant mathematical object, which supposedly
lives in an abstract Hilbert space. In this view, everything we observe and
experience, including the space in which we move and live, would just be
emerging from the only “real” entity–the universal wave function [44]. With
the arguments exposed in this article, we then join Heisenberg here who, to
similar claims put forward by Felix Bloch, once simply replied: “Nonsense,
space is blue and birds fly through it.” [45].

## Acknowledgements

We would like to thank Markus Müller, Thomas D. Galley, Esteban Castro-Ruiz,
Veronika Baumann, Philippe Allard Guérin, Časlav Brukner and Philip Goyal for
useful conversations during the elaboration of this paper. F.D.S. acknowledges
support from FWF (Austrian Science Fund) through an Erwin Schrödinger
Fellowship (Project J 4699s). L.C.B. acknowledges financial support by the
Austrian Science Fund (FWF) through BeyondC (F7103-N48). This publication was
made possible through the support of the ID 61466 grant from the John
Templeton Foundation, as part of The Quantum Information Structure of
Spacetime (QISS) Project (qiss.fr). The opinions expressed in this publication
are those of the authors and do not necessarily reflect the views of the John
Templeton Foundation.

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