# Relational quantum mechanics

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Relational quantum mechanics (RQM) is an interpretation of quantum mechanics which treats the state of a quantum system as being observer-dependent, that is, the state is the relation between the observer and the system. This interpretation was first delineated by Carlo Rovelli in a 1994 preprint, [1] and has since been expanded upon by a number of theorists. It is inspired by the key idea behind special relativity, that the details of an observation depend on the reference frame of the observer, and uses some ideas from Wheeler on quantum information. [2]

## Contents

The physical content of the theory has not to do with objects themselves, but the relations between them. As Rovelli puts it:

"Quantum mechanics is a theory about the physical description of physical systems relative to other systems, and this is a complete description of the world". [3]

The essential idea behind RQM is that different observers may give different accurate accounts of the same system. For example, to one observer, a system is in a single, "collapsed" eigenstate. To a second observer, the same system is in a superposition of two or more states and the first observer is in a correlated superposition of two or more states. RQM argues that this is a complete picture of the world because the notion of "state" is always relative to some observer. There is no privileged, "real" account. The state vector of conventional quantum mechanics becomes a description of the correlation of some degrees of freedom in the observer, with respect to the observed system. The terms "observer" and "observed" apply to any arbitrary system, microscopic or macroscopic. The classical limit is a consequence of aggregate systems of very highly correlated subsystems. A "measurement event" is thus described as an ordinary physical interaction where two systems become correlated to some degree with respect to each other.

The proponents of the relational interpretation argue that this approach resolves some of the traditional interpretational difficulties with quantum mechanics. By giving up our preconception of a global privileged state, issues around the measurement problem and local realism are resolved.

In 2020, Rovelli published an account of the main ideas of the relational interpretation in his popular book Helgoland.

## History and development

Relational quantum mechanics arose from a comparison of the quandaries posed by the interpretations of quantum mechanics with those resulting from Lorentz transformations prior to the development of special relativity. Rovelli suggested that just as pre-relativistic interpretations of Lorentz's equations were complicated by incorrectly assuming an observer-independent time exists, a similarly incorrect assumption frustrates attempts to make sense of the quantum formalism. The assumption rejected by relational quantum mechanics is the existence of an observer-independent state of a system. [4]

The idea has been expanded upon by Lee Smolin [5] and Louis Crane, [6] who have both applied the concept to quantum cosmology, and the interpretation has been applied to the EPR paradox, revealing not only a peaceful co-existence between quantum mechanics and special relativity, but a formal indication of a completely local character to reality. [7] [8]

## The problem of the observer and the observed

This problem was initially discussed in detail in Everett's thesis, The Theory of the Universal Wavefunction . Consider observer ${\displaystyle O}$, measuring the state of the quantum system ${\displaystyle S}$. We assume that ${\displaystyle O}$ has complete information on the system, and that ${\displaystyle O}$ can write down the wavefunction ${\displaystyle |\psi \rangle }$ describing it. At the same time, there is another observer ${\displaystyle O'}$, who is interested in the state of the entire ${\displaystyle O}$-${\displaystyle S}$ system, and ${\displaystyle O'}$ likewise has complete information.

To analyse this system formally, we consider a system ${\displaystyle S}$ which may take one of two states, which we shall designate ${\displaystyle |{\uparrow }\rangle }$ and ${\displaystyle |\downarrow \rangle }$, ket vectors in the Hilbert space ${\displaystyle H_{S}}$. Now, the observer ${\displaystyle O}$ wishes to make a measurement on the system. At time ${\displaystyle t_{1}}$, this observer may characterize the system as follows:

${\displaystyle |\psi \rangle =\alpha |{\uparrow }\rangle +\beta |{\downarrow }\rangle ,}$

where ${\displaystyle |\alpha |^{2}}$ and ${\displaystyle |\beta |^{2}}$ are probabilities of finding the system in the respective states, and obviously add up to 1. For our purposes here, we can assume that in a single experiment, the outcome is the eigenstate ${\displaystyle |{\uparrow }\rangle }$ (but this can be substituted throughout, mutatis mutandis, by ${\displaystyle |{\downarrow }\rangle }$). So, we may represent the sequence of events in this experiment, with observer ${\displaystyle O}$ doing the observing, as follows:

${\displaystyle {\begin{matrix}t_{1}&\rightarrow &t_{2}\\\alpha |{\uparrow }\rangle +\beta |{\downarrow }\rangle &\rightarrow &|{\uparrow }\rangle .\end{matrix}}}$

This is observer ${\displaystyle O}$'s description of the measurement event. Now, any measurement is also a physical interaction between two or more systems. Accordingly, we can consider the tensor product Hilbert space ${\displaystyle H_{S}\otimes H_{O}}$, where ${\displaystyle H_{O}}$ is the Hilbert space inhabited by state vectors describing ${\displaystyle O}$. If the initial state of ${\displaystyle O}$ is ${\displaystyle |{\text{init}}\rangle }$, some degrees of freedom in ${\displaystyle O}$ become correlated with the state of ${\displaystyle S}$ after the measurement, and this correlation can take one of two values: ${\displaystyle |O_{\uparrow }\rangle }$ or ${\displaystyle |O_{\downarrow }\rangle }$ where the direction of the arrows in the subscripts corresponds to the outcome of the measurement that ${\displaystyle O}$ has made on ${\displaystyle S}$. If we now consider the description of the measurement event by the other observer, ${\displaystyle O'}$, who describes the combined ${\displaystyle S+O}$ system, but does not interact with it, the following gives the description of the measurement event according to ${\displaystyle O'}$, from the linearity inherent in the quantum formalism:

${\displaystyle {\begin{matrix}t_{1}&\rightarrow &t_{2}\\\left(\alpha |{\uparrow }\rangle +\beta |{\downarrow }\rangle \right)\otimes |{\text{init}}\rangle &\rightarrow &\alpha |{\uparrow }\rangle \otimes |O_{\uparrow }\rangle +\beta |{\downarrow }\rangle \otimes |O_{\downarrow }\rangle .\end{matrix}}}$

Thus, on the assumption (see hypothesis 2 below) that quantum mechanics is complete, the two observers ${\displaystyle O}$ and ${\displaystyle O'}$ give different but equally correct accounts of the events ${\displaystyle t_{1}\rightarrow t_{2}}$.

## Central principles

### Observer-dependence of state

According to ${\displaystyle O}$, at ${\displaystyle t_{2}}$, the system ${\displaystyle S}$ is in a determinate state, namely spin up. And, if quantum mechanics is complete, then so is this description. But, for ${\displaystyle O'}$, ${\displaystyle S}$ is not uniquely determinate, but is rather entangled with the state of ${\displaystyle O}$  note that his description of the situation at ${\displaystyle t_{2}}$ is not factorisable no matter what basis chosen. But, if quantum mechanics is complete, then the description that ${\displaystyle O'}$ gives is also complete.

Thus the standard mathematical formulation of quantum mechanics allows different observers to give different accounts of the same sequence of events. There are many ways to overcome this perceived difficulty. It could be described as an epistemic limitation  observers with a full knowledge of the system, we might say, could give a complete and equivalent description of the state of affairs, but that obtaining this knowledge is impossible in practice. But whom? What makes ${\displaystyle O}$'s description better than that of ${\displaystyle O'}$, or vice versa? Alternatively, we could claim that quantum mechanics is not a complete theory, and that by adding more structure we could arrive at a universal description (the troubled hidden variables approach). Yet another option is to give a preferred status to a particular observer or type of observer, and assign the epithet of correctness to their description alone. This has the disadvantage of being ad hoc , since there are no clearly defined or physically intuitive criteria by which this super-observer ("who can observe all possible sets of observations by all observers over the entire universe" [9] ) ought to be chosen.

RQM, however, takes the point illustrated by this problem at face value. Instead of trying to modify quantum mechanics to make it fit with prior assumptions that we might have about the world, Rovelli says that we should modify our view of the world to conform to what amounts to our best physical theory of motion. [10] Just as forsaking the notion of absolute simultaneity helped clear up the problems associated with the interpretation of the Lorentz transformations, so many of the conundrums associated with quantum mechanics dissolve, provided that the state of a system is assumed to be observer-dependent  like simultaneity in Special Relativity. This insight follows logically from the two main hypotheses which inform this interpretation:

• Hypothesis 1: the equivalence of systems. There is no a priori distinction that should be drawn between quantum and macroscopic systems. All systems are, fundamentally, quantum systems.
• Hypothesis 2: the completeness of quantum mechanics. There are no hidden variables or other factors which may be appropriately added to quantum mechanics, in light of current experimental evidence.

Thus, if a state is to be observer-dependent, then a description of a system would follow the form "system S is in state xwith reference to observer O" or similar constructions, much like in relativity theory. In RQM it is meaningless to refer to the absolute, observer-independent state of any system.

### Information and correlation

It is generally well established that any quantum mechanical measurement can be reduced to a set of yes/no questions or bits that are either 1 or 0.[ citation needed ] RQM makes use of this fact to formulate the state of a quantum system (relative to a given observer!) in terms of the physical notion of information developed by Claude Shannon. Any yes/no question can be described as a single bit of information. This should not be confused with the idea of a qubit from quantum information theory, because a qubit can be in a superposition of values, whilst the "questions" of RQM are ordinary binary variables.

Any quantum measurement is fundamentally a physical interaction between the system being measured and some form of measuring apparatus. By extension, any physical interaction may be seen to be a form of quantum measurement, as all systems are seen as quantum systems in RQM. A physical interaction is seen as establishing a correlation between the system and the observer, and this correlation is what is described and predicted by the quantum formalism.

But, Rovelli points out, this form of correlation is precisely the same as the definition of information in Shannon's theory. Specifically, an observer O observing a system S will, after measurement, have some degrees of freedom correlated with those of S. The amount of this correlation is given by log2k bits, where k is the number of possible values which this correlation may take  the number of "options" there are.

### All systems are quantum systems

All physical interactions are, at bottom, quantum interactions, and must ultimately be governed by the same rules. Thus, an interaction between two particles does not, in RQM, differ fundamentally from an interaction between a particle and some "apparatus". There is no true wave collapse, in the sense in which it occurs in some interpretations.

Because "state" is expressed in RQM as the correlation between two systems, there can be no meaning to "self-measurement". If observer ${\displaystyle O}$ measures system ${\displaystyle S}$, ${\displaystyle S}$'s "state" is represented as a correlation between ${\displaystyle O}$ and ${\displaystyle S}$. ${\displaystyle O}$ itself cannot say anything with respect to its own "state", because its own "state" is defined only relative to another observer, ${\displaystyle O'}$. If the ${\displaystyle S+O}$ compound system does not interact with any other systems, then it will possess a clearly defined state relative to ${\displaystyle O'}$. However, because ${\displaystyle O}$'s measurement of ${\displaystyle S}$ breaks its unitary evolution with respect to ${\displaystyle O}$, ${\displaystyle O}$ will not be able to give a full description of the ${\displaystyle S+O}$ system (since it can only speak of the correlation between ${\displaystyle S}$ and itself, not its own behaviour). A complete description of the ${\displaystyle (S+O)+O'}$ system can only be given by a further, external observer, and so forth.

Taking the model system discussed above, if ${\displaystyle O'}$ has full information on the ${\displaystyle S+O}$ system, it will know the Hamiltonians of both ${\displaystyle S}$ and ${\displaystyle O}$, including the interaction Hamiltonian. Thus, the system will evolve entirely unitarily (without any form of collapse) relative to ${\displaystyle O'}$, if ${\displaystyle O}$ measures ${\displaystyle S}$. The only reason that ${\displaystyle O}$ will perceive a "collapse" is because ${\displaystyle O}$ has incomplete information on the system (specifically, ${\displaystyle O}$ does not know its own Hamiltonian, and the interaction Hamiltonian for the measurement).

## Consequences and implications

### Coherence

In our system above, ${\displaystyle O'}$ may be interested in ascertaining whether or not the state of ${\displaystyle O}$ accurately reflects the state of ${\displaystyle S}$. We can draw up for ${\displaystyle O'}$ an operator, ${\displaystyle M}$, which is specified as:

${\displaystyle M\left(|{\uparrow }\rangle \otimes |O_{\uparrow }\rangle \right)=|{\uparrow }\rangle \otimes |O_{\uparrow }\rangle }$
${\displaystyle M\left(|{\uparrow }\rangle \otimes |O_{\downarrow }\rangle \right)=0}$
${\displaystyle M\left(|{\downarrow }\rangle \otimes |O_{\uparrow }\rangle \right)=0}$
${\displaystyle M\left(|{\downarrow }\rangle \otimes |O_{\downarrow }\rangle \right)=|{\downarrow }\rangle \otimes |O_{\downarrow }\rangle }$

with an eigenvalue of 1 meaning that ${\displaystyle O}$ indeed accurately reflects the state of ${\displaystyle S}$. So there is a 0 probability of ${\displaystyle O}$ reflecting the state of ${\displaystyle S}$ as being ${\displaystyle |{\uparrow }\rangle }$ if it is in fact ${\displaystyle |{\downarrow }\rangle }$, and so forth. The implication of this is that at time ${\displaystyle t_{2}}$, ${\displaystyle O'}$ can predict with certainty that the ${\displaystyle S+O}$ system is in some eigenstate of ${\displaystyle M}$, but cannot say which eigenstate it is in, unless ${\displaystyle O'}$ itself interacts with the ${\displaystyle S+O}$ system.

An apparent paradox arises when one considers the comparison, between two observers, of the specific outcome of a measurement. In the problem of the observer observed section above, let us imagine that the two experiments want to compare results. It is obvious that if the observer ${\displaystyle O'}$ has the full Hamiltonians of both ${\displaystyle S}$ and ${\displaystyle O}$, he will be able to say with certainty that at time ${\displaystyle t_{2}}$, ${\displaystyle O}$ has a determinate result for ${\displaystyle S}$'s spin, but he will not be able to say what${\displaystyle O}$'s result is without interaction, and hence breaking the unitary evolution of the compound system (because he doesn't know his own Hamiltonian). The distinction between knowing "that" and knowing "what" is a common one in everyday life: everyone knows that the weather will be like something tomorrow, but no-one knows exactly what the weather will be like.

But, let us imagine that ${\displaystyle O'}$ measures the spin of ${\displaystyle S}$, and finds it to have spin down (and note that nothing in the analysis above precludes this from happening). What happens if he talks to ${\displaystyle O}$, and they compare the results of their experiments? ${\displaystyle O}$, it will be remembered, measured a spin up on the particle. This would appear to be paradoxical: the two observers, surely, will realise that they have disparate results.[ dubious ]

However, this apparent paradox only arises as a result of the question being framed incorrectly: as long as we presuppose an "absolute" or "true" state of the world, this would, indeed, present an insurmountable obstacle for the relational interpretation. However, in a fully relational context, there is no way in which the problem can even be coherently expressed. The consistency inherent in the quantum formalism, exemplified by the "M-operator" defined above, guarantees that there will be no contradictions between records. The interaction between ${\displaystyle O'}$ and whatever he chooses to measure, be it the ${\displaystyle S+O}$ compound system or ${\displaystyle O}$ and ${\displaystyle S}$ individually, will be a physical interaction, a quantum interaction, and so a complete description of it can only be given by a further observer ${\displaystyle O''}$, who will have a similar "M-operator" guaranteeing coherency, and so on out. In other words, a situation such as that described above cannot violate any physical observation, as long as the physical content of quantum mechanics is taken to refer only to relations.

### Relational networks

An interesting implication of RQM arises when we consider that interactions between material systems can only occur within the constraints prescribed by Special Relativity, namely within the intersections of the light cones of the systems: when they are spatiotemporally contiguous, in other words. Relativity tells us that objects have location only relative to other objects. By extension, a network of relations could be built up based on the properties of a set of systems, which determines which systems have properties relative to which others, and when (since properties are no longer well defined relative to a specific observer after unitary evolution breaks down for that observer). On the assumption that all interactions are local (which is backed up by the analysis of the EPR paradox presented below), one could say that the ideas of "state" and spatiotemporal contiguity are two sides of the same coin: spacetime location determines the possibility of interaction, but interactions determine spatiotemporal structure. The full extent of this relationship, however, has not yet fully been explored.

### RQM and quantum cosmology

The universe is the sum total of everything in existence with any possibility of direct or indirect interaction with a local observer. A (physical) observer outside of the universe would require physically breaking of gauge invariance, [11] and a concomitant alteration in the mathematical structure of gauge-invariance theory.

Similarly, RQM conceptually forbids the possibility of an external observer. Since the assignment of a quantum state requires at least two "objects" (system and observer), which must both be physical systems, there is no meaning in speaking of the "state" of the entire universe. This is because this state would have to be ascribed to a correlation between the universe and some other physical observer, but this observer in turn would have to form part of the universe. As was discussed above, it is not possible for an object to contain a complete specification of itself. Following the idea of relational networks above, an RQM-oriented cosmology would have to account for the universe as a set of partial systems providing descriptions of one another. The exact nature of such a construction remains an open question.

## Relationship with other interpretations

The only group of interpretations of quantum mechanics with which RQM is almost completely incompatible is that of hidden variables theories. RQM shares some deep similarities with other views, but differs from them all to the extent to which the other interpretations do not accord with the "relational world" put forward by RQM.

### Copenhagen interpretation

RQM is, in essence, quite similar to the Copenhagen interpretation, but with an important difference. In the Copenhagen interpretation, the macroscopic world is assumed to be intrinsically classical in nature, and wave function collapse occurs when a quantum system interacts with macroscopic apparatus. In RQM, any interaction, be it micro or macroscopic, causes the linearity of Schrödinger evolution to break down. RQM could recover a Copenhagen-like view of the world by assigning a privileged status (not dissimilar to a preferred frame in relativity) to the classical world. However, by doing this one would lose sight of the key features that RQM brings to our view of the quantum world.

### Hidden variables theories

Bohm's interpretation of QM does not sit well with RQM. One of the explicit hypotheses in the construction of RQM is that quantum mechanics is a complete theory, that is it provides a full account of the world. Moreover, the Bohmian view seems to imply an underlying, "absolute" set of states of all systems, which is also ruled out as a consequence of RQM.

We find a similar incompatibility between RQM and suggestions such as that of Penrose, which postulate that some process (in Penrose's case, gravitational effects) violate the linear evolution of the Schrödinger equation for the system.

### Relative-state formulation

The many-worlds family of interpretations (MWI) shares an important feature with RQM, that is, the relational nature of all value assignments (that is, properties). Everett, however, maintains that the universal wavefunction gives a complete description of the entire universe, while Rovelli argues that this is problematic, both because this description is not tied to a specific observer (and hence is "meaningless" in RQM), and because RQM maintains that there is no single, absolute description of the universe as a whole, but rather a net of interrelated partial descriptions.

### Consistent histories approach

In the consistent histories approach to QM, instead of assigning probabilities to single values for a given system, the emphasis is given to sequences of values, in such a way as to exclude (as physically impossible) all value assignments which result in inconsistent probabilities being attributed to observed states of the system. This is done by means of ascribing values to "frameworks", and all values are hence framework-dependent.

RQM accords perfectly well with this view. However, the consistent histories approach does not give a full description of the physical meaning of framework-dependent value (that is it does not account for how there can be "facts" if the value of any property depends on the framework chosen). By incorporating the relational view into this approach, the problem is solved: RQM provides the means by which the observer-independent, framework-dependent probabilities of various histories are reconciled with observer-dependent descriptions of the world.

## EPR and quantum non-locality

RQM provides an unusual solution to the EPR paradox. Indeed, it manages to dissolve the problem altogether, inasmuch as there is no superluminal transportation of information involved in a Bell test experiment: the principle of locality is preserved inviolate for all observers.

### The problem

In the EPR thought experiment, a radioactive source produces two electrons in a singlet state, meaning that the sum of the spin on the two electrons is zero. These electrons are fired off at time ${\displaystyle t_{1}}$ towards two spacelike separated observers, Alice and Bob, who can perform spin measurements, which they do at time ${\displaystyle t_{2}}$. The fact that the two electrons are a singlet means that if Alice measures z-spin up on her electron, Bob will measure z-spin down on his, and vice versa: the correlation is perfect. If Alice measures z-axis spin, and Bob measures the orthogonal y-axis spin, however, the correlation will be zero. Intermediate angles give intermediate correlations in a way that, on careful analysis, proves inconsistent with the idea that each particle has a definite, independent probability of producing the observed measurements (the correlations violate Bell's inequality).

This subtle dependence of one measurement on the other holds even when measurements are made simultaneously and a great distance apart, which gives the appearance of a superluminal communication taking place between the two electrons. Put simply, how can Bob's electron "know" what Alice measured on hers, so that it can adjust its own behavior accordingly?

### Relational solution

In RQM, an interaction between a system and an observer is necessary for the system to have clearly defined properties relative to that observer. Since the two measurement events take place at spacelike separation, they do not lie in the intersection of Alice's and Bob's light cones. Indeed, there is no observer who can instantaneously measure both electrons' spin.

The key to the RQM analysis is to remember that the results obtained on each "wing" of the experiment only become determinate for a given observer once that observer has interacted with the other observer involved. As far as Alice is concerned, the specific results obtained on Bob's wing of the experiment are indeterminate for her, although she will know that Bob has a definite result. In order to find out what result Bob has, she has to interact with him at some time ${\displaystyle t_{3}}$ in their future light cones, through ordinary classical information channels. [12]

The question then becomes one of whether the expected correlations in results will appear: will the two particles behave in accordance with the laws of quantum mechanics? Let us denote by ${\displaystyle M_{A}(\alpha )}$ the idea that the observer ${\displaystyle A}$ (Alice) measures the state of the system ${\displaystyle \alpha }$ (Alice's particle).

So, at time ${\displaystyle t_{2}}$, Alice knows the value of ${\displaystyle M_{A}(\alpha )}$: the spin of her particle, relative to herself. But, since the particles are in a singlet state, she knows that

${\displaystyle M_{A}(\alpha )+M_{A}(\beta )=0,}$

and so if she measures her particle's spin to be ${\displaystyle \sigma }$, she can predict that Bob's particle (${\displaystyle \beta }$) will have spin ${\displaystyle -\sigma }$. All this follows from standard quantum mechanics, and there is no "spooky action at a distance" yet

. From the "coherence-operator" discussed above, Alice also knows that if at ${\displaystyle t_{3}}$ she measures Bob's particle and then measures Bob (that is asks him what result he got)  or vice versa  the results will be consistent:

${\displaystyle M_{A}(B)=M_{A}(\beta )}$

Finally, if a third observer (Charles, say) comes along and measures Alice, Bob, and their respective particles, he will find that everyone still agrees, because his own "coherence-operator" demands that

${\displaystyle M_{C}(A)=M_{C}(\alpha )}$ and ${\displaystyle M_{C}(B)=M_{C}(\beta )}$

while knowledge that the particles were in a singlet state tells him that

${\displaystyle M_{C}(\alpha )+M_{C}(\beta )=0.}$

Thus the relational interpretation, by shedding the notion of an "absolute state" of the system, allows for an analysis of the EPR paradox which neither violates traditional locality constraints, nor implies superluminal information transfer, since we can assume that all observers are moving at comfortable sub-light velocities. And, most importantly, the results of every observer are in full accordance with those expected by conventional quantum mechanics.

## Derivation

A promising feature of this interpretation is that RQM offers the possibility of being derived from a small number of axioms, or postulates based on experimental observations. Rovelli's derivation of RQM uses three fundamental postulates. However, it has been suggested that it may be possible to reformulate the third postulate into a weaker statement, or possibly even do away with it altogether. [13] The derivation of RQM parallels, to a large extent, quantum logic. The first two postulates are motivated entirely by experimental results, while the third postulate, although it accords perfectly with what we have discovered experimentally, is introduced as a means of recovering the full Hilbert space formalism of quantum mechanics from the other two postulates. The two empirical postulates are:

• Postulate 1: there is a maximum amount of relevant information that may be obtained from a quantum system.
• Postulate 2: it is always possible to obtain new information from a system.

We let ${\displaystyle W\left(S\right)}$ denote the set of all possible questions that may be "asked" of a quantum system, which we shall denote by ${\displaystyle Q_{i}}$, ${\displaystyle i\in W}$. We may experimentally find certain relations between these questions: ${\displaystyle \left\{\land ,\lor ,\neg ,\supset ,\bot \right\}}$, corresponding to {intersection, orthogonal sum, orthogonal complement, inclusion, and orthogonality} respectively, where ${\displaystyle Q_{1}\bot Q_{2}\equiv Q_{1}\supset \neg Q_{2}}$.

### Structure

From the first postulate, it follows that we may choose a subset ${\displaystyle Q_{c}^{(i)}}$ of ${\displaystyle N}$ mutually independent questions, where ${\displaystyle N}$ is the number of bits contained in the maximum amount of information. We call such a question ${\displaystyle Q_{c}^{(i)}}$ a complete question. The value of ${\displaystyle Q_{c}^{(i)}}$ can be expressed as an N-tuple sequence of binary valued numerals, which has ${\displaystyle 2^{N}=k}$ possible permutations of "0" and "1" values. There will also be more than one possible complete question. If we further assume that the relations ${\displaystyle \left\{\land ,\lor \right\}}$ are defined for all ${\displaystyle Q_{i}}$, then ${\displaystyle W\left(S\right)}$ is an orthomodular lattice, while all the possible unions of sets of complete questions form a Boolean algebra with the ${\displaystyle Q_{c}^{(i)}}$ as atoms. [14]

The second postulate governs the event of further questions being asked by an observer ${\displaystyle O_{1}}$ of a system ${\displaystyle S}$, when ${\displaystyle O_{1}}$ already has a full complement of information on the system (an answer to a complete question). We denote by ${\displaystyle p\left(Q|Q_{c}^{(j)}\right)}$ the probability that a "yes" answer to a question ${\displaystyle Q}$ will follow the complete question ${\displaystyle Q_{c}^{(j)}}$. If ${\displaystyle Q}$ is independent of ${\displaystyle Q_{c}^{(j)}}$, then ${\displaystyle p=0.5}$, or it might be fully determined by ${\displaystyle Q_{c}^{(j)}}$, in which case ${\displaystyle p=1}$. There is also a range of intermediate possibilities, and this case is examined below.

If the question that ${\displaystyle O_{1}}$ wants to ask the system is another complete question, ${\displaystyle Q_{b}^{(i)}}$, the probability ${\displaystyle p^{ij}=p\left(Q_{b}^{(i)}|Q_{c}^{(j)}\right)}$ of a "yes" answer has certain constraints upon it:

1. ${\displaystyle 0\leq p^{ij}\leq 1,\ }$
2. ${\displaystyle \sum _{i}p^{ij}=1,\ }$
3. ${\displaystyle \sum _{j}p^{ij}=1.\ }$

The three constraints above are inspired by the most basic of properties of probabilities, and are satisfied if

${\displaystyle p^{ij}=\left|U^{ij}\right|^{2}}$,

where ${\displaystyle U^{ij}}$ is a unitary matrix.

• Postulate 3 If ${\displaystyle b}$ and ${\displaystyle c}$ are two complete questions, then the unitary matrix ${\displaystyle U_{bc}}$ associated with their probability described above satisfies the equality ${\displaystyle U_{cd}=U_{cb}U_{bd}}$, for all ${\displaystyle b,c}$ and ${\displaystyle d}$.

This third postulate implies that if we set a complete question ${\displaystyle |Q_{c}^{(i)}\rangle }$ as a basis vector in a complex Hilbert space, we may then represent any other question ${\displaystyle |Q_{b}^{(j)}\rangle }$ as a linear combination:

${\displaystyle |Q_{b}^{(j)}\rangle =\sum _{i}U_{bc}^{ij}|Q_{c}^{(i)}\rangle .}$

And the conventional probability rule of quantum mechanics states that if two sets of basis vectors are in the relation above, then the probability ${\displaystyle p^{ij}}$ is

${\displaystyle p^{ij}=|\langle Q_{c}^{(i)}|Q_{b}^{(j)}\rangle |^{2}=|U_{bc}^{ij}|^{2}.}$

### Dynamics

The Heisenberg picture of time evolution accords most easily with RQM. Questions may be labelled by a time parameter ${\displaystyle t\rightarrow Q(t)}$, and are regarded as distinct if they are specified by the same operator but are performed at different times. Because time evolution is a symmetry in the theory (it forms a necessary part of the full formal derivation of the theory from the postulates), the set of all possible questions at time ${\displaystyle t_{2}}$ is isomorphic to the set of all possible questions at time ${\displaystyle t_{1}}$. It follows, by standard arguments in quantum logic, from the derivation above that the orthomodular lattice ${\displaystyle W(S)}$ has the structure of the set of linear subspaces of a Hilbert space, with the relations between the questions corresponding to the relations between linear subspaces.

It follows that there must be a unitary transformation ${\displaystyle U\left(t_{2}-t_{1}\right)}$ that satisfies:

${\displaystyle Q(t_{2})=U\left(t_{2}-t_{1}\right)Q(t_{1})U^{-1}\left(t_{2}-t_{1}\right)}$

and

${\displaystyle U\left(t_{2}-t_{1}\right)=\exp({-i\left(t_{2}-t_{1}\right)H})}$

where ${\displaystyle H}$ is the Hamiltonian, a self-adjoint operator on the Hilbert space and the unitary matrices are an abelian group.

## Notes

1. "www.phyast.pitt.edu/~rovelli/Papers/quant.mec.uu". 2 March 1994. Retrieved 13 May 2020.
2. Wheeler (1990): pg. 3
3. Rovelli, C. (1996), "Relational quantum mechanics", International Journal of Theoretical Physics, 35: 1637–1678.
4. Rovelli (1996): pg. 2
5. Smolin (1995)
6. Crane (1993)
7. Laudisa (2001)
8. Rovelli & Smerlak (2006)
9. Page, Don N., "Insufficiency of the quantum state for deducing observational probabilities", Physics Letters B, Volume 678, Issue 1, 6 July 2009, 41–44.
10. Rovelli (1996): pg. 16
11. Smolin (1995), pg. 13
12. Bitbol (1983)
13. Rovelli (1996): pg. 14
14. Rovelli (1996): pg. 13

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An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics "corresponds" to reality. Although quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily broad range of experiments, there exist a number of contending schools of thought over their interpretation. These views on interpretation differ on such fundamental questions as whether quantum mechanics is deterministic or stochastic, which elements of quantum mechanics can be considered real, and what is the nature of measurement, among other matters.

Wigner's friend is a thought experiment in theoretical quantum physics, first conceived by the physicist Eugene Wigner in 1961, and developed into a thought experiment by David Deutsch in 1985. The scenario involves an indirect observation of a quantum measurement: An observer W observes another observer F who performs a quantum measurement on a physical system. The two observers then formulate a statement about the physical system's state after the measurement according to the laws of quantum theory. However, in most of the interpretations of quantum theory, the resulting statements of the two observers contradict each other. This reflects a seeming incompatibility of two laws in quantum theory: the deterministic and continuous time evolution of the state of a closed system and the nondeterministic, discontinuous collapse of the state of a system upon measurement. Wigner's friend is therefore directly linked to the measurement problem in quantum mechanics with its famous Schrödinger's cat paradox.

In quantum mechanics, wave function collapse occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world. This interaction is called an "observation". It is the essence of a measurement in quantum mechanics which connects the wave function with classical observables like position and momentum. Collapse is one of two processes by which quantum systems evolve in time; the other is the continuous evolution via the Schrödinger equation. Collapse is a black box for a thermodynamically irreversible interaction with a classical environment. Calculations of quantum decoherence show that when a quantum system interacts with the environment, the superpositions apparently reduce to mixtures of classical alternatives. Significantly, the combined wave function of the system and environment continue to obey the Schrödinger equation. More importantly, this is not enough to explain wave function collapse, as decoherence does not reduce it to a single eigenstate.

Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two quantum states can be added together ("superposed") and the result will be another valid quantum state; and conversely, that every quantum state can be represented as a sum of two or more other distinct states. Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution.

The Greenberger–Horne–Zeilinger experiment or GHZ experiments are a class of physics experiments that may be used to generate starkly contrasting predictions from local hidden variable theory and quantum mechanical theory, and permit immediate comparison with actual experimental results. A GHZ experiment is similar to a test of Bell's inequality, except using three or more entangled particles, rather than two. With specific settings of GHZ experiments, it is possible to demonstrate absolute contradictions between the predictions of local hidden variable theory and those of quantum mechanics, whereas tests of Bell's inequality only demonstrate contradictions of a statistical nature. The results of actual GHZ experiments agree with the predictions of quantum mechanics.

Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. A definite phase relationship is necessary to perform quantum computing on quantum information encoded in quantum states. Coherence is preserved under the laws of quantum physics.

In quantum mechanics and computing, the Bloch sphere is a geometrical representation of the pure state space of a two-level quantum mechanical system (qubit), named after the physicist Felix Bloch.

The Bohr–Einstein debates were a series of public disputes about quantum mechanics between Albert Einstein and Niels Bohr. Their debates are remembered because of their importance to the philosophy of science, since the disagreements and the outcome of Bohr's version of quantum mechanics that became the prevalent view form the root of the modern understanding of physics. Most of Bohr's version of the events held in Solvay in 1927 and other places was first written by Bohr decades later in an article titled, "Discussions with Einstein on Epistemological Problems in Atomic Physics". Based on the article, the philosophical issue of the debate was whether Bohr's Copenhagen Interpretation of quantum mechanics, which centered on his belief of complementarity, was valid in explaining nature. Despite their differences of opinion and the succeeding discoveries that helped solidify quantum mechanics, Bohr and Einstein maintained a mutual admiration that was to last the rest of their lives.

In quantum mechanics, a triplet is a quantum state of a system with a spin of quantum number s =1, such that there are three allowed values of the spin component, ms = −1, 0, and +1.

LOCC, or local operations and classical communication, is a method in quantum information theory where a local (product) operation is performed on part of the system, and where the result of that operation is "communicated" classically to another part where usually another local operation is performed conditioned on the information received.

The Born rule is a key postulate of quantum mechanics which gives the probability that a measurement of a quantum system will yield a given result. In its simplest form, it states that the probability density of finding a particle at a given point, when measured, is proportional to the square of the magnitude of the particle's wavefunction at that point. It was formulated by German physicist Max Born in 1926.

In theoretical physics, quantum nonlocality refers to the phenomenon by which the measurement statistics of a multipartite quantum system do not admit an interpretation in terms of a local realistic theory. Quantum nonlocality has been experimentally verified under different physical assumptions. Any physical theory that aims at superseding or replacing quantum theory should account for such experiments and therefore must also be nonlocal in this sense; quantum nonlocality is a property of the universe that is independent of our description of nature.

Quantum walks are quantum analogues of classical random walks. In contrast to the classical random walk, where the walker occupies definite states and the randomness arises due to stochastic transitions between states, in quantum walks randomness arises through: (1) quantum superposition of states, (2) non-random, reversible unitary evolution and (3) collapse of the wave function due to state measurements.

In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution in time exhausts all that can be predicted about the system's behavior. A mixture of quantum states is again a quantum state. Quantum states that cannot be written as a mixture of other states are called pure quantum states, while all other states are called mixed quantum states. A pure quantum state can be represented by a ray in a Hilbert space over the complex numbers, while mixed states are represented by density matrices, which are positive semidefinite operators that act on Hilbert spaces.

In physics and the philosophy of physics, quantum Bayesianism is a collection of related approaches to the interpretation of quantum mechanics, of which the most prominent is QBism. QBism is an interpretation that takes an agent's actions and experiences as the central concerns of the theory. QBism deals with common questions in the interpretation of quantum theory about the nature of wavefunction superposition, quantum measurement, and entanglement. According to QBism, many, but not all, aspects of the quantum formalism are subjective in nature. For example, in this interpretation, a quantum state is not an element of reality—instead it represents the degrees of belief an agent has about the possible outcomes of measurements. For this reason, some philosophers of science have deemed QBism a form of anti-realism. The originators of the interpretation disagree with this characterization, proposing instead that the theory more properly aligns with a kind of realism they call "participatory realism", wherein reality consists of more than can be captured by any putative third-person account of it.

In quantum mechanics, weak measurements are a type of quantum measurement that results in an observer obtaining very little information about the system on average, but also disturbs the state very little. From Busch's theorem the system is necessarily disturbed by the measurement. In the literature weak measurements are also known as unsharp, fuzzy, dull, noisy, approximate, and gentle measurements. Additionally weak measurements are often confused with the distinct but related concept of the weak value.

Many-body localization (MBL) is a dynamical phenomenon which leads to the breakdown of equilibrium statistical mechanics in isolated many-body systems. Such systems never reach local thermal equilibrium, and retain local memory of their initial conditions for infinite times. One can still define a notion of phase structure in these out-of-equilibrium systems. Strikingly, MBL can even enable new kinds of exotic orders that are disallowed in thermal equilibrium --- a phenomenon that goes by the name of localization protected quantum order (LPQO) or eigenstate order

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