## 1. Main Ideas


### 1.1 Values of physical variables



The starting point of RQM is that quantum mechanics is not about a
wave function (or a quantum state) \(\psi\); it is about values of
physical variables. The ontology assumed by RQM, accordingly, includes
only physical systems and variables that take values, as in classical
mechanics. For instance, a real fact is the position of a particle
having a certain value \(x\) at a certain time \(t\). Facts as this
one ("the particle is at \(x\) at time \(t\)") are called
"events", or "quantum events". Quantum theory
is about events. However, in classical mechanics there are two general
assumptions that are abandoned in quantum theory.



(a) In classical mechanics it is assumed that all variables of a
system have always a value at every time. RQM, on the contrary,
assumes that this is in not the case in nature in general (Heisenberg
1925; Kochen & Specker 1967). Rather, this is only an
approximation that holds when disregarding quantum phenomena. Physical
variables acquire values only at some times, and have no value at the
other times. Events, in other words, are discrete. This is the basic
intuition that led Heisenberg to find the key to quantum mechanics in
1925; that is, there are questions that make sense in classical
mechanics, but have no significance in nature. For instance, the
question "What is the \(y\)-component of the spin of an electron
when its \(z\)-component is \(\frac{1}{2} \hslash\)?" is
meaningless: neither it admits an operational definition, nor is it
needed for a realistic understanding of nature (see below, about
realism). When does then a generic variable \(A\) of a system \(S\)
acquire a value? When does an event happen? The RQM answer is: when
and only when the system \(S\) interacts with a second system \(S'\)
and the effect of the interaction on \(S'\) depends on the variable
\(A\). In RQM, this is what it means for a variable to have a
value.



(b) The second assumption of classical mechanics which is dropped in
RQM is that there are variables that take absolute values, namely
values independent from any other systems. Instead, RQM assumes that
*all* (contingent) physical variables are relational. (The
contingent variables are those with in classical theory are
represented by phase space functions.) Any value these variables take
is always (implicitly or explicitly) labelled by a second physical
system. If the variable \(A\) of a system \(S\) takes a value in the
interaction with a second systems \(S'\), the value it takes is only
*relative to* \(S'\). The actualisation of an event is always
relative to a system. The concrete meaning of this is the postulate
above, according to which future ways the system \(S\) affects \(S'\)
depend (probabilistically) on the values that variables of \(S\) have
taken with respect to \(S'\), but *future ways the system \(S\)
affect a third system \(S''\) do not*. The ensemble of all events
relative to an arbitrary system \(S'\), together with the
probabilistic predictions these entail, is called the
"perspective" of the "observer" \(S'\).



The central claim at the basis of RQM is therefore that:
"different observers can give different accounts of the same set
of events" (Rovelli 1996: 1643). Textbook quantum mechanics is a
complete description of the perspective of a single observer, but it
disregards the effect of this observer system on the perspective of
other systems. RQM emphasises the fact that the observer itself
behaves as a quantum system, when acting on other systems. The
relation between perspectives is discussed below.


### 1.2 Relative variables: "Different observers can give different accounts of the same set of events"



Relative variables are variables whose value does not depend on a
single system, but rather on two systems. A well known example is the
velocity of an object in classical mechanics. The velocity is always
implicitly or explicitly relative to a second object. There is no
"velocity of a single object", irrespectively on any other
object, in classical mechanics. Other well-known examples are the
electric potential (only the potential of a conductor with respect to
another conductor has physical meaning) and position (only position
with respect to some other object has physical meaning). Relational
quantum mechanics takes a further long step in this direction,
assuming that we can make sense of quantum mechanics by assuming that
*all* physical variables are relational in this sense.



RQM is sometimes wrongly assumed to be an interpretation where
subjects, or agents, play a role. The source of the misunderstanding
is the confusion between relative and subjective. When we say that the
our speed is 11km/second with respect to the sun, we are not assuming
that the sun has subjectivity. When we say that the distance between a
signpost and a road intersection is 100 meters, we are not thinking
that a road's intersection is an agent. In a naturalistic
perspective, a person, an agent, a subject, are physical systems. On
the other hand, the world this person, agent, or subject, relates to
is described by the value of the variables *with respect to*
the physical system she is. This is like saying that a subject
dwelling on the Earth sees the Cosmos rotating because the Earth is
spinning. To say that RQM requires subjects or agents is the same
mistake as saying that our explanation of the daily rotation of sun
moon and stars around the Earth requires to take agency or
subjectivity into account: an obvious nonsense. There is nothing
subjective, idealistic, or mentalistic, in RQM.


### 1.3 Observer and measurement



In textbook presentations, quantum mechanics is about measurement
outcomes performed when an "observer" makes a
"measurement" on a quantum system. What is an observer, if
all physical systems are quantum? What counts as a measurement? Common
answers invoke the observer being macroscopic, onset of decoherence,
irreversibility, registration of information, or similar. RQM does not
utilise anything of the sort. Any system, irrespectively of its size,
complexity or else, can play the role of the textbook's quantum
mechanical observer. The "measurement outcomes" of a given
observer, however, refer only to values of the variables of the
quantum system relative to that system. In particular, they do not
affect events relative to other systems. In the same spirit, textbook
presentations of quantum mechanics refer to "measurement
outcomes". In the relational interpretation, any interaction
counts as a measurement, to the extent one system affects the other
and this influence depends on a variable of the first system. Every
physical object can be taken as defining a perspective, to which all
values of physical quantities can be referred.



According to RQM, therefore, quantum mechanics is not a theory of the
dynamics of an entity \(\psi\), from which the world of our experience
somehow emerges. It is instead a theory about the standard world of
our experience, described by values that conventional physical
variables take at interactions, and about the transition probabilities
that determine which values are likely to be realized, given that
others were.


### 1.4 The wave function



The wave function, and more in general the quantum state \(\psi\), are
interpreted realistically in several presentations of quantum theory.
From the perspective of RQM, this is precisely what generates the
confusion about quantum theory (Rovelli 2018). RQM circumvents the
theorems for the reality of the wave function (Leifer 2014; Pusey,
Barrett, & Rudolph 2012) because it is not a strongly realist
theory--in the sense defined below--which is an implicit
assumption of these theorems. The interpretation of the wave function
in the context of RQM is akin the interpretation of the
Hamilton-Jacobi functional in classical mechanics: a theoretical tool
to facilitate the computation of the probabilities of future events on
the basis of certain given knowledge.



The relation between the wave function \(\psi\) and the
Hamilton-Jacobi functional \(S\) is more than an analogy, because in
the semiclassical approximation the later approximates the first
\((\psi \sim \exp iS/\hslash)\). This fact can be taken as an argument
against a realistic interpretation of \(\psi\) for the following
reason. A way to clarify the interpretation of a quantity in the
mathematical apparatus of a physical theory is to study what this
quantity reduces to in an approximation where the interpretation is
clear. The Hamilton-Jacobi functions of, say, a classical particle has
no realistic interpretation. Notice that if we gave it a realistic
interpretation, we would generate mysterious collapses and jumps as we
have for the wave function. Voiding the quantum states of a realistic
interpretation avoids jumps and collapses. (Another argument against
the realistic interpretation of \(\psi\) is given in Rovelli
2016.)



In classical mechanics we can dispense of the Hamilton-Jacobi
functional. This testifies for its lack of ontological weight. We can
equally dispense with \(\psi\) in quantum theory. Evidence of this is
that the full early development of the quantum mechanical formalisms
(Heisenberg 1925; Born & Jordan 1925; Dirac 1925; Born,
Heisenberg, & Jordan 1926) predates the work where \(\psi\) was
introduced (Schroedinger 1926)! Quantum mechanics can be formulated
without reference to the quantum state, as a theory of probabilities
for sequences of events. For instance, quantum theory can be
formulated by giving the non-commutative algebra of observables and a
one-parameter group of automorphisms of the algebra representing time
evolution, and a single positive linear function on the algebra.
Equivalently, it can be formulated in terms of transition
probabilities between ensembles of values of variables; these can be
directly computed, for instance with path integral methods. The state
\(\psi\) is a convenient, not a necessary tool.



What is then the quantum state in the relational interpretation? It is
a mathematical device that refers to two systems, not a single one. It
codes the values of the variables of the first that have been
actualised in interacting with the second (Groenewold 1957); it
therefore codes anything we can predict regarding the future values of
these variables, relative to the second system. The state \(\psi\) ,
in other words, can be interpreted as nothing more than a compendium
of information assumed, known, or gathered through measurements,
determined entirely by a specific history of interactions: the
interactions between the system and a second 'observing'
system. Understood in this manner the quantum state is always and only
a *relative state* in the sense of Everett (1957). In this
sense RQM is "Everettian"; it is so in a different sense
than the Many Worlds interpretations, which are based on a realistic
interpretation of the universal wave function, rejected in RQM.


### 1.5. Quantum superposition: can a cat be half dead, half alive?



If \(\psi'\) and \(\psi''\) are two (orthogonal) quantum states of a
system, quantum mechanics predicts that the system can also be in the
state \(\psi = (\psi' +\psi'')/\sqrt{2}\) . This is the superposition
principle, a cornerstone of the theory. For instance, if \(\psi'\) is
the state of a live cat and \(\psi''\) the state of a dead cat, then
\(\psi\) is a state which the cat is in a quantum superposition of
dead and alive; the theory predicts that this is a possible state of a
cat. Why then don't we ever see cats that are half alive and
half dead? That is: why don't we see macroscopic objects in
quantum superpositions?



The answer is that the existence of states like \(\psi =(\psi'
+\psi'')/\sqrt{2}\) does not mean that we should "see
superpositions": what we "see", namely what we
measure, according to textbook quantum theory, are eigenvalues of
self-adjoint operators, not quantum states. Measured eigenvalues are
always univocal, never "superimposed".



What does it mean then for a state to be a superposition? First, it
means that if an observable has value \(a'\) in \(\psi'\) and value
\(a''\) in \(\psi''\) then any observation of the system will give
either \(a'\) or \(a''\), each with probability 1/2. Second, the
probability distribution of the outcomes of the measurement of any
observable that is not diagonal in the \((\psi'\), \(\psi''\)) basis
will be affected by interference: that is, it will not be the mean of
the average values of the observable in \(\psi'\) and \(\psi''\). This
and nothing else is the meaning of being in a quantum superposition.
Hence, the answer to the question "Why don't we ever see
cats that are half alive and half dead?" is: because quantum
theory predicts that we never see this sort of things. It predicts
that we see cats either alive or dead. It also predicts that in
principle we should be able to predict interference effect between the
two states. These interference effects are strongly suppressed by
decoherence in the case of macroscopic systems (like cats), hence the
theory actually predicts that they are extremely hard to observe, in
agreement with our experience.



A problem, however, appears in quantum mechanics if we ask what the
cat itself would perceive. Say the brain of the cat measures whether
its heart is beating or not. The theory predicts that the brain will
find either that it does or that it does not. In textbook quantum
mechanics, this implies a collapse of \(\psi\) to either \(\psi'\) or
\(\psi''\). In turns, this implies that no further effects of
interference between these two states will happen. And *this*
contradicts the conclusion that interference effects, although small
due to decoherence, are nevertheless real. *This* problem is
resolved by RQM by the postulate above: the way the cat, as a quantum
system, affects an external system, is not affected by the specific
way the heart of the cat has affected its brain. That is, the state of
the cat with respect to the external world does not collapse when a
part of the cat interact with another.


## 2. Related Issues


### 2.1 Information



The early presentations of RQM were formulated in the language of
information theory (Rovelli 1996). The quantum state is a way of
coding the information that an observing system \(S'\) may have about
a quantum system \(S\), relevant for predicting future ways \(S\) can
affect \(S'\). This information is determined by the ways \(S\) has
affected \(S'\) in the past. The hope was raised in Rovelli (1996)
that a full reconstruction of the quantum formalism on the basis of
simple informational postulates was possible. Two main postulates were
proposed:


* (i) relevant
information is finite for a system with compact phase space,
* (ii) new information
can always be acquired.



The two postulates are not in contradiction with each other because
when new information is gathered some previous relevant information
becomes irrelevant. "Relevant" here means that it affects
future probabilities. A moment of reflection shows that first
postulate implies the characteristic discreteness of quantum theory,
the second implies Heisenberg's uncertainties. Very similar
postulates were independently proposed by Zeilinger and Bruckner
(Zeilinger 1999; Brukner & Zeilinger 2003).



As later emphasized in Dorato (2017), information is best not
understood as a primary notion. It must be defined physically in terms
of something else; as such, it can play an important notion in
"theories of principle" in the sense of Einstein (1919).
In RQM, the information is defined relationally as relative
information (in the sense of Shannon) that a physical system has about
another system. Relative information is physical correlation between
two systems (see Rovelli 1996), namely a measure of the difference
between the possible number of states of the combined system and the
product of the number of states of the two systems, due to the
existence of physical constraints. Thus, we say that a variable
\(O\_A\) of a system has information about a variable \(A\) of another
system if the values that \(A\) and \(O\_A\) can take are correlated.
In the spirit of Shannon, this is a very weak definition of
information that has no mentalistic, semantic, or cognitive aspects.
The strong informational perspective of the early work in RQM has
influenced several later development of information theoretical
approaches to the foundations of quantum theory (see below).


### 2.2 Discreteness



Discreteness is not an accessory aspect of quantum theory: it is its
most characteristic feature (and it gives the theory its name).



Discreteness appears in two related ways in quantum theory. First, the
amount of information that can be gathered regarding the state of a
system which is in a finite region R of its phase space is finite. It
is given by the Liouville measure of R divided by the Planck constant,
per degree of freedom. This is what causes discrete spectra.
Continuous spectra require infinite phase spaces, and can be seen as
effects of idealisations. The discreteness of quantum mechanics is
captured by the first of the two informational postulates.



Second, quantum mechanics describes the world in terms of values of
variables at specific discrete times. This second aspect of
discreteness is directly accounted for by the sparse (or
"flash") ontology of RQM. The history of a quantum
particle, for instance, is neither a continuous line is spacetime (as
in classical mechanics), nor a continuous wave function on spacetime.
Rather, with respect to any other system it is a discrete set of
interactions, each localized in spacetime.



The flash ontology of RQM seems to raise a difficulty: what determines
the *timing* for the events to happen? The problem is the
difficulty of establishing a specific moment when, say, a measurement
happens. The question is addressed in Rovelli (1998), observing that
quantum mechanics itself does give a (probabilistic) prediction on
when a measurement happens. This is because the meaning of the
question whether or not a measurement has happened is to ascertain
whether of not a pointer variable \(O\_A\) in the observing system
\(S\) has become properly correlated with the measured variable \(A\)
of the system \(A\). In turn, this is a physical question that makes
sense because it can be posed empirically by measuring \(A\) and
\(O\_A\) and checking if they are consistent.


### 2.3 Comparison with other interpretations



A way to clarify an interpretation of quantum mechanic is to compare
it with the most discussed alternatives.



*Textbook Copenhagen*: To a good extent RQM is a completion of
the standard textbook interpretation. The difference is that the
latter assumes the existence of a classical world, or a classical
observer, and describes the way quantum systems affect it in an
interaction. The relational interpretation, on the contrary, assumes
that this description is valid with respect to any physical system.
Thus, RQM is a sort of "democratised" Copenhagen, where
the role of the single observer can be assumed by any physical
system.



*Many Worlds*: Both RQM and the Many Worlds interpretation (see
Vaidman 2002 [2018]) are rooted in the work of Everett (1957). Both
attempt to solve the mystery of quantum theory by adding a level of
indexicality. In RQM, variables have values with respect to other
physical systems. In Many Worlds, variables have values with respect
to branches of the universal wave function. In neither interpretations
there is an a priori special role for measurement, or observers. The
difference is the profoundly distinct ontological commitment: the Many
Worlds interpretation is based on a realist interpretation of the
universal wave function, which obeys a deterministic evolution law.
This implies that the Many Worlds interpretation must then work hard
to recover Heisenberg uncertainty (via branch indexicality),
probabilities (via subjective interpretation of probability), and
discreteness. RQM has all this easily in its foundation. On the other
hand, the Many Worlds interpretation is based on a (according to some,
inflated, but) no-nonsense realist metaphysics, which is precluded to
RQM. The two can perhaps made closer by the simple observation that
modality can always be transformed into multiple world realism
*a la* Lewis (1986), trading actuality for
indexicality.



*Hidden variables* (Bohm): Hidden variable theories, of which
Bohm theory (Bohm 1952) is the best available example, provide a
realistic and deterministic interpretation of quantum mechanics. The
similarity between RQM and Bohm theory is in the realistic
interpretation of some variables, such as the position of a particle.
A difference is in the sparse ontology of RQM, compared to the
assumption of quantities unobservable in principle which is implicit
in Bohm theory.



*Physical collapse*: Physical collapse theories like Ghirardi,
Rimini, and Weber (1986) and Penrose (1996) are physically
distinguishable from standard QM, which is instead assumed correct up
to contrary empirical indications in RQM.



There are also current interpretation of quantum mechanics that are
close to the relational interpretation:



*Zeilinger Bruckner*: The Relational Interpretation is very
close to the view of quantum theory held developed by Zeilinger and
Bruckner; in particular, almost identical postulates to the original
ones of RQM were independently proposed in (Zeilinger 1999, Brukner
& Zeilinger 2003). These ideas generated some of the interesting
mathematical work aiming at making the derivation of the formalism of
quantum theory from information theoretical postulates precise. For
versions of this program strictly related to RQM see (Grinbaum 2005;
Hohn 2017; Hohn & Wever 2017).



*QBism*: The emphasis on information in Rovelli (1996)
influenced the birth of QBism (see Fuchs 1998: 3). There are
similarities between RQM and QBism (Fuchs 2001, 2002
 [OIR]).
 One similarity is the emphasis on dropping questions considered
meaningless. The second is the use of the language of information
theory (Spekkens 2014). The difference is mostly in the way the
subject holding information is treated. In RQM, this subject is fully
naturalised: it is itself considered a physical system that can be
described by quantum theory. This leads to a definitely stronger
version of realism that QBism, and to the emphasis on the relational
aspect of all variables. In QBism, instead, the emphasis is in the
information about the world held by a single subject, taken as
primary. In RQM, the information is relative information (in the sense
of Shannon) that a physical system has about another system; it is not
primary (see Dorato 2017): it is can be simply understood physically
as a correlation between the two systems that can be observed by a
third system (Rovelli 1996).



*Richard Healey*: Healey's pragmatist approach (Healey
1989) has in common with RQM the idea that the quantum state is not a
description of physical reality, not even incompletely. Its main
function is to be a (dispensable) tool for generating quantum
probabilities. The main difference is the emphasis on what quantum
states are relative to. For Healey's pragmatist view, a quantum
state ascription is relative *only* to the perspective of an
actual or potential agent (Healey 2012). In RQM, values are objective
and relative to any physical system. Restricting quantum theory to its
use by agents is not a concern for Healey's pragmatist
philosophy; it is more so in a naturalistic perspective searching for
an understanding of Nature that remains significative also where no
agents are around. This is the same difference as between RQM and
QBism, but Healey's position is closer to RQM than QBism because
while the QBism's quantum state ascriptions depend on the
epistemic state of the agent, for Healey the quantum state ascribed to
a system depends only on the physical circumstances defining the
perspective of the agent.


### 2.4 Representation



The issue of the interpretation of quantum mechanics is strictly
related to the issue of the possibility of offering a
*representation*: a description of what happens in the world.
It can be useful to give a simple-minded pictorial representation of
the images sustaining different interpretations. Imagine at time
\(t\_1\) a radioactive atom is surrounded by Geiger counters and at
time \(t\_2\) one of the counters clicks, having detected a product of
the decay. What has happened around the \(t\_1\)-\(t\_2\) interval?


* According to *textbook quantum theory*, the wave function
of a particle classically trapped in the nucleus leaks out of the
nucleus symmetrically, filling the space surrounding the nucleus. At
the moment of the detection this wave function magically disappears
everywhere except at the particular detector that clicks.
* According to the the *Many Worlds* interpretation, all
detectors click. In fact, each detector clicks at every moment of
time, but the wave function of the universe branches continuously into
innumerable branches: we, ourselves, happen to be in one particular
branch in which one particular detector clicks at one particular
time.
* According to the *Bohmiam interpretation*, the wave
function equally leaks uniformly in space, but at the same time the
associated particle, guided by this wave function, zigzags around,
until hitting a particular detector.
* According to *Physical Collapse* interpretations, the wave
function also leaks out uniformly, but when it effect on the heavy
Geiger detectors begins to displace too much matter, the wave function
collapses as in the textbook interpretation, but driven by a
hypothetical dynamical process which has not yet been explicitly
observed (Ghirardi 2002 [2018]).
* What about *RQM*? In the spirit of Heisenberg, there is no
actual wave function in nature, nor there is any fact of the matter
about *the position of the particle respect to the Geiger
counter* at any moment between t1 and t2. However, there may be
other facts of the matter. For instance, *the position of the
particle respect to some air molecule along the way*. These, on
the other hand, have no bearing on the the position of the particle
respect to the Geiger counter, which actualises at time t2 and whose
probabilistic distribution does not depend on the position of the
particle with respect to the air molecules.


### 2.5 Frauchiger-Renner experiment and locality



The Frauchiger-Renner thought-experiment (Frauchiger & Renner
2018) can be viewed as an indirect support to RQM, since it makes
concrete the idea that "different observers can give different
accounts of the same set of events", as in the original RQM
slogan (Rovelli 1996: 1463). The experiment is discussed in the
conceptual framework of RQM by Waaijer and van Neerven (2019
 [OIR]).



The application of RQM to the EPR context and the problem of quantum
non locality has been initially discussed in Smerlak and Rovelli
(2007) and (Laudisa 2001). Some of the claims of the earlier
discussion about RQM being "local" have been questioned,
pointing out that RQM should in any case be "forced to accept
*some* form of non-locality in quantum phenomena"
(Laudisa 2019: 227). A more recent discussion is in Martin-Dussaud,
Rovelli, and Zalamea (2019), where a specific sense in which quantum
theory is non-local from the RQM perspective is specified, and
downplayed. See also Pienaar (2018
 [OIR])
 for comments.


### 2.6 Solipsism?



Prima facie, RQM may seem to imply a form of perspective solipsism, as
the values of variables realized in the perspective of some system
\(S'\) are not necessarily the same as those realized with respect to
another system \(S''\). This is however not the case, as follows
directly from quantum theory itself. The key is to observe that any
physical comparison is itself a quantum interaction. Suppose the
variable \(A\) of \(S\) is measured by \(S'\) and stored into the
variable \(A'\) of \(S'\). This means that the interaction has created
a correlation between \(A\) and \(A'\). In turn, this means that a
third system measuring \(A\) and \(A'\) will certainly find consistent
values. That is: the perspectives of \(S'\) and \(S''\) agree on this
regard, and this can be checked in a physical interaction.



For instance: imagine experimenter \(S'\) measures the spin of the
electron \(S\), and writes the value of this spin on a piece of paper.
In principle, experimenter \(S''\) can devise an experiment where she
can detect an effect due to interference between the two branches
where the spin of the electron (and the text) have one or the other
value. But if \(S''\) measures the spin and reads the piece of paper,
she will find that experimenter \(S'\) has seen the same spin as
herself.



Why? Because quantum theory predicts so, as can be seen from the
following: with respect to \(S''\), the first interaction yields a
quantum state of the form

 

\[ \begin{align} & \ket{\text{spin up}} \times \ket{\text{paper
with text } \lsq\text{spin up}\rsq} \\ & + 
\ket{\text{spin down}} \times
\ket{\text{paper with text } \lsq\text{spin down}\rsq} \end{align} \]


Measuring the spin projects the state on one single branch of the two,
and both branches lead to consistency. Therefore, as long as we do not
chase subtle interference phenomena hidden behind decoherence, RQM
implies that we all 'see the same world'.


## 3. General Comments


### 3.1 Realism and Relationality



The central move of RQM is to interpret all physical variables as
relational, namely as referring to two systems, not a single one, and
to view them as realised only in interactions. Relationality has been
playing an ever dominant role as our knowledge of the natural world
has increased. Examples are the relational nature of velocity in
classical mechanics, of locality in general relativity, of the
potential in electromagnetism, of the gauge invariant observables in
non-abelian gauge theories, and many others. RQM is a step further in
this direction. Taken seriously, the philosophical implications of
this overreaching relationally can be heavy. The main one is a
weakening of a strong version of realism.



If by realism we mean the assumption that the world is "out
there", irrespectively of our mental states, or perceptions,
there is nothing in RQM that contradicts realism. But if by realism we
mean the stronger assumption that each variables of each subsystem of
the world has a single value at each and every time, then this strong
version of realism is weakened by RQM. The ontology of RQM is a sparse
("flash") ontology of relational quantum events, taken as
primitive, and not derived from any "underlying"
representation.



This weakening of realism is in a direction similar to what happened
with Galilean or Einstein's relativity, which have shown that
there is no fact of the matter in the velocity of a single object, or
in the simultaneity of two space like separated events alone. But is a
more radical step in this direction. In Laudisa (2019) it is pointed
out that RQM gives no deeper justification or underlying dynamical
representation of the main process: the actualisation of quantum
events at interactions. This is the process which in textbook quantum
theory is called measurement and is accompanied by state reduction.
Quantum mechanics gives probabilities for quantum events to happen,
not a story representing how they happen. This core aspect of quantum
theory is not resolved in RQM: it is taken as a fact of the world.
What RQM does resolve is the question of when this happens: any time
one system affects another one, it happens relative to this other
system. What RQM does, is to show that this is not in contradiction
with the existence of interference effects. But the core discreteness
of the quantum event actualisation is not "explained" in
RQM: it is understood as the picture of how nature works according to
quantum theory.



The weakening of realism is the "price to pay" for the
relational interpretation of quantum mechanics. It can be compared
with the "price to pay" in other interpretations, such as
the inflated ontology and the distance between the ontology and the
world as we see it of the Many Worlds interpretation, the existence of
variables unobservables in principle and the loss of Lorentz
invariance of Bohm theory, and so on.



The other side of the coin of each "price to pay" is the
lesson we might gather from the empirical success of quantum theory:
for the Many Worlds interpretation, for instance, the lesson is the
real existence of other branches, for Bohm theory is the real
existence of unobservable variables that pick a preferred reference
frame, and so on. For RQM, the lesson of quantum theory is that the
description of the way distinct physical systems affect each other
when they interact (and not the way physical systems
'are') exhausts all that can be said about the physical
world. The physical world must be described as a net of interacting
components, where there is no meaning to 'the state of an
isolated system', or the value of the variables of an isolated
system. The state of a physical system is the net of the relations it
entertains with the surrounding systems. The physical structure of the
world is identified as this net of relationships. The notion of
substance that plays a major role in western philosophy might be
inappropriate to account for this science; perhaps the idea of a
"mutual dependency" [Nagarjuna 1995] may offer a
relevant philosophical cadre.


### 3.2 Reactions and criticisms



In van Fraassen (2010), Bas van Fraassen explores "the world of
quantum mechanics as RQM depicts it" (2010: 390), clarifying
what is and what is not relative to observers. He concentrates on the
apparently paradoxical aspects of RQM. The limits on information that
observers can have, which can only be acquired through physical
interaction, have surprising consequences for complex situations in
which an observer makes a measurement, a second observer makes
measurements on the first and its target, and even a third observer
comes in and observes a process involving the first two observers. Van
Fraassen concludes that all the consistency questions can be laid to
rest, when the situation's representation in RQM is properly
understood. On the other hand, he also observes that if in RQM what
the state of a system relative to an observer is, is not itself
relative to anything, then the question can be raised what
relationships there are between the state of a specific entangled
system or its components relative to different observers. He proposes
and additional postulate, weakly relating the description of the same
system as given by different observers, which forbids the possibility
of disturbing inconsistencies allowed by the multiplication of
perspectives.



Laura Candiotto (2017) argues that the best philosophical framework
for RQM is Ontic Structural Realism (OSR) (Ladyman& Ross 2007;
French & Ladyman 2011). Ontic structural realism is meant to be a
defensible form of scientific realism (Ladyman 2007 [2019]); it argues
for the priority of relations over substances, as self-subsistent
individual objects (Morganti 2011). For Candiotto, RQM is a realistic
theory that assumes the notion of relation (the physical interaction
between systems and instruments) as primitive; objects emerge as
relational "nodes" (French 2006), or intersections of
processes. The lack of observer-independence is not inability of
providing an account of the structure of matter, because there are no
intrinsic properties that can be assigned to systems independently of
their interactions, therefore this structure is itself relational,
hence in particular observer dependent. Relations via dynamical
processes of information exchange can be taken as the building blocks
of the universe.



The relation between RQM and ontic structural realism has been
emphasized also by Mauro Dorato (2016). Dorato gives an extensive
evaluation of RQM, pointing out its main characteristics. He
emphasises then two aspects that characterise RQM. The first is a
revisionary rather than a descriptive metaphysical account of quantum
theory; that is: central assumptions of common sense must go, if they
contradict contemporary physical theories. Here, what is abandoned is
the presupposition that quantum systems have a non-relational,
intrinsic nature. RQM's metaphysics is revisionary also for a
second reason. Analogously to the many-worlds interpretation, RQM does
not suggest changing the formalism of quantum theory--as
alternative formulations of the theory do--but rather, modifies
the conceptual schemes with which we can interpret the formalism, and
consequently, our metaphysics. Dorato observes that the relativisation
of values implies a relativisation of the very notion of object or
entity, if (i) having some intrinsic, non-purely dispositional
properties is essential to the identity of an object, and (ii) no
entity can exist if it does not have an intrinsic identity (see
Nagarjuna c. 2nd century CE [1995]). The only reality in RQM
is given by events, which are the result of interactions between
distinct quantum systems, but even these events can be described in a
different way by different physical systems. The interaction cannot be
described in a more precise way by a constructive theory in
Einstein's sense (Einstein 1919) that can explain the coming
into being of a definite outcome without just assuming it as a
fundamental fact. Dorato concludes that there is no measurement
problem in RQM because RQM is implicitly formulated as a theory of
principle. He also considers the issue of priority monism as defined
in Schaffer (2010): Shaffer claims that quantum mechanics'
entanglement is evidence that the whole universe has ontological
priority with respect to its parts. Dorato point out that the firm
advocacy of relationalism of RQM has instead radical anti-holistic
consequences.



The second characteristic aspect of RQM pointed out by Dorato is that,
consequently, the best way to capture the nature of not-yet
interacting quantum systems is to call into play a form of
dispositionalism: the only way to attribute some sort of intrinsicness
to the state-dependent properties of quantum systems is to attribute
them dispositions to manifest in a certain way according to the
interactions they are subject to. Dispositionalism is present in many
other views of quantum mechanics (Dorato 2006) but fits particularly
well in the context of RQM. Unlike Qbists interpretations of quantum
theory, which are agents-centered, in RQM the relation "\(S\)
manifest \(q\) relative to \(S'\)" is symmetric, and this is a
simple consequence of the hypothesis that in RQM quantum systems and
"observers" are on the same level. As a consequence of its
relational and dispositional aspect, Dorato stresses the fact that in
RQM there cannot be a universal flow of becoming, but only a local,
worldline-dependent and relational one. This still counts as a
relational form of becoming: no universal tide of coming into being,
but a crisscross of ripples. Since a physical system can exemplify a
given succession of events only relatively to another system and not
absolutely, in RQM there cannot be cosmic time, so that also in
general relativity temporal succession of events cannot be regarded as
a total order. In a sense, in RQM there is no quantum state of the
universe, or a God's eye point of view, since the cosmos can
only be described "from within a given perspective".