Relational quantum mechanics

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, 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.
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".
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.
The idea has been expanded upon by Lee Smolin and Louis Crane, 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.
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 formula_1, measuring the state of the quantum system formula_2. We assume that formula_1 has complete information on the system, and that formula_1 can write down the wavefunction formula_5 describing it. At the same time, there is another observer formula_6, who is interested in the state of the entire formula_1-formula_2 system, and formula_6 likewise has complete information.
To analyse this system formally, we consider a system formula_2 which may take one of two states, which we shall designate formula_11 and formula_12, ket vectors in the Hilbert space formula_13. Now, the observer formula_1 wishes to make a measurement on the system. At time formula_15, this observer may characterize the system as follows:
where formula_17 and formula_18 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 formula_19 (but this can be substituted throughout, "mutatis mutandis", by formula_20). So, we may represent the sequence of events in this experiment, with observer formula_1 doing the observing, as follows:
This is observer formula_1'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 formula_24, where formula_25 is the Hilbert space inhabited by state vectors describing formula_1. If the initial state of formula_1 is formula_28, some degrees of freedom in formula_1 become correlated with the state of formula_2 after the measurement, and this correlation can take one of two values: formula_31 or formula_32 where the direction of the arrows in the subscripts corresponds to the outcome of the measurement that formula_1 has made on formula_2. If we now consider the description of the measurement event by the other observer, formula_6, who describes the combined formula_36 system, but does not interact with it, the following gives the description of the measurement event according to formula_6, from the linearity inherent in the quantum formalism:
Thus, on the assumption (see hypothesis 2 below) that quantum mechanics is complete, the two observers formula_1 and formula_6 give different but equally correct accounts of the events formula_41.
Central principles.
Observer-dependence of state.
According to formula_1, at formula_43, the system formula_2 is in a determinate state, namely spin up. And, if quantum mechanics is complete, then so is this description. But, for formula_6, formula_2 is "not" uniquely determinate, but is rather entangled with the state of formula_1 note that his description of the situation at formula_43 is not factorisable no matter what basis chosen. But, if quantum mechanics is complete, then the description that formula_6 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 formula_1's description better than that of formula_6, 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") 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. 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:
Thus, if a state is to be observer-dependent, then a description of a system would follow the form "system "S" is in state "x" "with 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. 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 log2"k" 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 formula_1 measures system formula_2, formula_2's "state" is represented as a correlation between formula_1 and formula_2. formula_1 itself cannot say anything with respect to its own "state", because its own "state" is defined only relative to another observer, formula_6. If the formula_36 compound system does not interact with any other systems, then it will possess a clearly defined state relative to formula_6. However, because formula_1's measurement of formula_2 breaks its unitary evolution with respect to formula_1, formula_1 will not be able to give a full description of the formula_36 system (since it can only speak of the correlation between formula_2 and itself, not its own behaviour). A complete description of the formula_67 system can only be given by a further, external observer, and so forth.
Taking the model system discussed above, if formula_6 has full information on the formula_36 system, it will know the Hamiltonians of both formula_2 and formula_1, including the interaction Hamiltonian. Thus, the system will evolve entirely unitarily (without any form of collapse) relative to formula_6, if formula_1 measures formula_2. The only reason that formula_1 will perceive a "collapse" is because formula_1 has incomplete information on the system (specifically, formula_1 does not know its own Hamiltonian, and the interaction Hamiltonian for the measurement).
Consequences and implications.
Coherence.
In our system above, formula_6 may be interested in ascertaining whether or not the state of formula_1 accurately reflects the state of formula_2. We can draw up for formula_6 an operator, formula_82, which is specified as:
with an eigenvalue of 1 meaning that formula_1 indeed accurately reflects the state of formula_2. So there is a 0 probability of formula_1 reflecting the state of formula_2 as being formula_19 if it is in fact formula_20, and so forth. The implication of this is that at time formula_43, formula_6 can predict with certainty that the formula_36 system is in "some" eigenstate of formula_82, but cannot say "which" eigenstate it is in, unless formula_6 itself interacts with the formula_36 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 formula_6 has the full Hamiltonians of both formula_2 and formula_1, he will be able to say with certainty "that" at time formula_43, formula_1 has a determinate result for formula_2's spin, but he will not be able to say "what" formula_1'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 formula_6 measures the spin of formula_2, 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 formula_1, and they compare the results of their experiments? formula_1, 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.
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 formula_6 and whatever he chooses to measure, be it the formula_36 compound system or formula_1 and formula_2 individually, will be a "physical" interaction, a "quantum" interaction, and so a complete description of it can only be given by a further observer formula_114, 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, 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 formula_15 towards two spacelike separated observers, Alice and Bob, who can perform spin measurements, which they do at time formula_43. 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 formula_117 in their future light cones, through ordinary classical information channels.
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 formula_118 the idea that the observer formula_119 (Alice) measures the state of the system formula_120 (Alice's particle).
So, at time formula_43, Alice knows the value of formula_118: the spin of her particle, relative to herself. But, since the particles are in a singlet state, she knows that
and so if she measures her particle's spin to be formula_124, she can predict that Bob's particle (formula_125) will have spin formula_126. 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 formula_117 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:
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
while knowledge that the particles were in a singlet state tells him that
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. 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:
We let formula_132 denote the set of all possible questions that may be "asked" of a quantum system, which we shall denote by formula_133, formula_134. We may experimentally find certain relations between these questions: formula_135, corresponding to {intersection, orthogonal sum, orthogonal complement, inclusion, and orthogonality} respectively, where formula_136.
Structure.
From the first postulate, it follows that we may choose a subset formula_137 of formula_138 mutually independent questions, where formula_138 is the number of bits contained in the maximum amount of information. We call such a question formula_137 a "complete question". The value of formula_137 can be expressed as an N-tuple sequence of binary valued numerals, which has formula_142 possible permutations of "0" and "1" values. There will also be more than one possible complete question. If we further assume that the relations formula_143 are defined for all formula_133, then formula_132 is an orthomodular lattice, while all the possible unions of sets of complete questions form a Boolean algebra with the formula_137 as atoms.
The second postulate governs the event of further questions being asked by an observer formula_147 of a system formula_2, when formula_147 already has a full complement of information on the system (an answer to a complete question). We denote by formula_150 the probability that a "yes" answer to a question formula_151 will follow the complete question formula_152. If formula_151 is independent of formula_152, then formula_155, or it might be fully determined by formula_152, in which case formula_157. There is also a range of intermediate possibilities, and this case is examined below.
If the question that formula_147 wants to ask the system is another complete question, formula_159, the probability formula_160 of a "yes" answer has certain constraints upon it:
The three constraints above are inspired by the most basic of properties of probabilities, and are satisfied if
where formula_165 is a unitary matrix.
This third postulate implies that if we set a complete question formula_172 as a basis vector in a complex Hilbert space, we may then represent any other question formula_173 as a linear combination:
And the conventional probability rule of quantum mechanics states that if two sets of basis vectors are in the relation above, then the probability formula_175 is
Dynamics.
The Heisenberg picture of time evolution accords most easily with RQM. Questions may be labelled by a time parameter formula_177, 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 formula_43 is isomorphic to the set of all possible questions at time formula_15. It follows, by standard arguments in quantum logic, from the derivation above that the orthomodular lattice formula_180 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 formula_181 that satisfies:
and
where formula_184 is the Hamiltonian, a self-adjoint operator on the Hilbert space and the unitary matrices are an abelian group.