Relational quantum mechanics (RQM) is an interpretation of quantum mechanics which treats the state of a quantum system as being observer-dependent, i.e., 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.^[1] An interpretation of quantum mechanics is an attempt to answer the question, What exactly is quantum mechanics talking about?. Although quantum mechanics is widely considered the most precisely tested and most successful theory in the history of science (Jackiw and Kleppner, 2000), many feel that in spite of this the... Carlo Rovelli is an Italian-born physicist who now works at the University of the Mediterraneum and the Centre de Physique Theorique in Marseille, France. ... 1994 (MCMXCIV) was a common year starting on Saturday of the Gregorian calendar, and was designated as the International Year of the Family and the International Year of the Sport and the Olympic Ideal by United Nations. ... A preprint is a draft of a scientific paper that has not yet been published in a peer-reviewed scientific journal. ... The special theory of relativity was proposed in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. Some three centuries earlier, Galileos principle of relativity had stated that all uniform motion was relative, and that there was no absolute and well-defined state of rest... For the medical use of the term observation, see watchful waiting. ... A frame of reference in physics is a set of axes which enable an observer to measure the aspect, position and motion of all points in a system relative to the reference frame. ... John Archibald Wheeler (born July 9, 1911) is an eminent American theoretical physicist. ... In quantum mechanics, quantum information is physical information that is held in the state of a quantum system. ...

The physical content of the theory is thus to do not with objects themselves, but the relations between them. As Rovelli puts it:^[2]

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 accounts of the same series of events: for example, to one observer at a given point in time, a system may be in a single, "collapsed" eigenstate, while to another observer at the same time, it may appear to be in a superposition of two or more states. Consequently, if quantum mechanics is to be a complete theory, RQM argues that the notion of "state" describes not the observed system itself, but the relationship, or correlation, between the system and its observer(s). 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. However, it is held by RQM that this applies to all physical objects, whether or not they are conscious or macroscopic (all systems are quantum systems). Any "measurement event" is seen simply as an ordinary physical interaction, an establishment of the sort of correlation discussed above. The proponents of the relational interpretation argue that the approach clears up a number of traditional interpretational difficulties with quantum mechanics, while being simultaneously conceptually elegant and ontologically parsimonious. In linear algebra, the eigenvectors (from the German eigen meaning inherent, characteristic) of a linear operator are non-zero vectors which, when operated on by the operator, result in a scalar multiple of themselves. ... Quantum superposition is the application of the superposition principle to quantum mechanics. ... In mathematics and related technical fields, a mathematical object is complete if nothing needs to be added to it. ... Linear correlations between 1000 pairs of numbers. ... Quite literally, quantum state describes the state of a quantum system. ... The phrase degrees of freedom is used in three different branches of science: in physics and physical chemistry, in mechanical and aerospace engineering, and in statistics. ... Consciousness is a quality of the mind generally regarded to comprise qualities such as subjectivity, self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and ones environment. ... Macroscopic is commonly used to describe physical objects that are measurable and observable by the naked eye. ...

Sections 1 to 5 of this article describe the history and interpretation of the idea and analyse its logical and philosophical nature, while the last two sections describe the formal background to the theory and the mathematics of its construction, and also provide an analysis of its application to the EPR paradox. Logic (from ancient Greek λόγος (logos), meaning reason) is the study of arguments. ... Philosophy of science is the branch of philosophy that studies the philosophical assumptions, foundations, and implications of science, including the formal sciences, natural sciences, and social sciences. ... Theoretical physics employs mathematical models and abstractions, as opposed to experimental processes, in an attempt to understand Nature. ... For other meanings of mathematics or math, see mathematics (disambiguation). ... In quantum mechanics, the EPR paradox (Einstein-Podolsky-Rosen) is a thought experiment which challenged long-held ideas about the relation between, on the one hand the observed values of physical quantities and on the other, the values that can be accounted for by a physical theory. ...

History and development

Relational Quantum Mechanics arose from a historical comparison of the quandaries posed by the interpretation of quantum mechanics with the situation after the Lorentz transformations were formulated but before Special Relativity. Rovelli felt that just as there was an "incorrect assumption" underlying the pre-relativistic interpretation of Lorentz's equations, which was corrected by Einstein's derivation of them from Lorentz covariance and the constancy of the speed of light in all reference frames, so a similarly incorrect assumption underlies many attempts to make sense of the quantum formalism, which was responsible for many of the interpretational difficulties posed by the theory. This incorrect assumption, he said, was that of an observer-independent state of a system, and he laid out the foundations of this interpretation to try to overcome the difficulty.^[3] Since then, the idea has been expanded upon by Lee Smolin^[4] and Louis Crane,^[5] 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.^{[6] [7]} An interpretation of quantum mechanics is an attempt to answer the question, What exactly is quantum mechanics talking about?. Although quantum mechanics is widely considered the most precisely tested and most successful theory in the history of science (Jackiw and Kleppner, 2000), many feel that in spite of this the... The Lorentz transformation (LT), named after its discoverer, the Dutch physicist and mathematician Hendrik Antoon Lorentz (1853-1928), forms the basis for the special theory of relativity, which has been introduced to remove contradictions between the theories of electromagnetism and classical mechanics. ... The special theory of relativity was proposed in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. Some three centuries earlier, Galileos principle of relativity had stated that all uniform motion was relative, and that there was no absolute and well-defined state of rest... Albert Einstein, photographed in 1947 by Oren J. Turner. ... ‹The template below has been proposed for deletion. ... The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness. In metric units, c is exactly 299,792,458 metres per second (or 1,079,252,848. ... In physics, an inertial frame of reference, or inertial frame for short (also descibed as absolute frame of reference), is a frame of reference in which the observers move without the influence of any accelerating or decelerating force. ... One of the remarkable characteristics of the mathematical formulation of quantum mechanics, which distinguishes it from mathematical formulations of theories developed prior to the early 1900s, is its use of abstract mathematical structures, such as Hilbert spaces and operators on these spaces. ... Lee Smolin at Harvard Lee Smolin is a theoretical physicist who has made major contributions to the quantum theory of gravity. ... Louis Crane is a theorist in quantum gravity. ... In theoretical physics, quantum cosmology is a young field attempting to study the effect of quantum mechanics on the earliest moments of the universe after the Big Bang. ... In quantum mechanics, the EPR paradox (Einstein-Podolsky-Rosen) is a thought experiment which challenged long-held ideas about the relation between, on the one hand the observed values of physical quantities and on the other, the values that can be accounted for by a physical theory. ... In physics, the principle of locality is that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. ...

The problem of the observer observed

Observer O measures a system S; what information does O' have?

This problem was initially discussed in detail in Everett's thesis, The Theory of the Universal Wavefunction. Consider the diagram to the right. Observer O measures the state of the quantum system S (represented by a Feynman diagram). We assume that he has complete information on the system, and that he can write down the wavefunction describing this particle. At the same time, there is another observer O'-, who is interested in the state of the entire Feynman diagram-physicist-experiment system, and he likewise has complete information. Hugh Everett III (November 11, 1930 – July 19, 1982) was an American physicist who first proposed the many-worlds interpretation of quantum physics, which he called his relative state formulation. ... // The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that rejects the non-deterministic and irreversible wavefunction collapse associated with measurement in the Copenhagen interpretation in favor of a description in terms of quantum entanglement and reversible time evolution of states. ... The framework of quantum mechanics requires a careful definition of measurement, and a thorough discussion of its practical and philosophical implications. ... A quantum state is any possible state in which a quantum mechanical system can be. ... Fig. ... In this Feynman diagram, an electron and positron annihilate and become a quark-antiquark pair. ... Information as a concept bears a diversity of meanings, from everyday usage to technical settings. ... This article discusses the concept of a wavefunction as it relates to quantum mechanics. ...

To analyse this system formally, we consider a system S which may take one of two states, which we shall designate and , ket vectors in the Hilbert space H_S. Now, the observer O wishes to make a measurement on the system. At time t₁, this observer may characterize the system as follows: Bra-ket notation is the standard notation for describing quantum states in the theory of quantum mechanics. ... In mathematics, a Hilbert space is a generalization of Euclidean space which is not restricted to finite dimensions. ...

where | α | ² and | β | 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 (but this can be substituted throughout, mutatis mutandis, by ). So, we may represent the sequence of event in this experiment, with observer O doing the observing, as follows: In linear algebra, the eigenvectors (from the German eigen meaning inherent, characteristic) of a linear operator are non-zero vectors which, when operated on by the operator, result in a scalar multiple of themselves. ...

This is observer 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 , where H_O is the Hilbert space inhabited by state vectors describing O. If the initial state of O is , some degrees of freedom in O become correlated with the state of S after the measurement, and this correlation can take one of two values: or where the direction of the arrows in the subscripts corresponds to the outcome of the measurement that O has made on S. If we now consider the description of the measurement event by the other observer, O'-, who describes the combined S + O system, but does not interact with it, the following gives the description of the measurement event according to O'-, from the linearity inherent in the quantum formalism: In mathematics, the tensor product, denoted by , may be applied in different contexts to vectors, matrices, tensors, vector spaces, algebras and modules. ... Quite literally, quantum state describes the state of a quantum system. ... Degrees of freedom is a general term used in explaining dependence on parameters, and implying the possibility of counting the number of those parameters. ... The word linear comes from the Latin word linearis, which means created by lines. ...

Thus, on the assumption (see hypothesis 2 above) that quantum mechanics is complete, the two observers O and O'- give different but equally correct accounts of the events .

Central principles

Observer-dependence of state

According to O, at t₂, the system S is in a determinate state, namely spin up. And, if quantum mechanics is complete, then so is his description. But, for O'-, S is not uniquely determinate, but is rather entangled with the state of O — note that his description of the situation at t₂ is not factorisable no matter what basis he chooses. But, if quantum mechanics is complete, then the description that O'- gives is also complete. Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated. ... This article is about the mathematical concept. ... In linear algebra, a basis is a minimum set of vectors that, when combined, can address every vector in a given space. ...

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 O's description better than that of 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 descripion — the much vilified, and some would even say discredited, 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" ought to be chosen. One of the remarkable characteristics of the mathematical formulation of quantum mechanics, which distinguishes it from mathematical formulations of theories developed prior to the early 1900s, is its use of abstract mathematical structures, such as Hilbert spaces and operators on these spaces. ... Epistemology or theory of knowledge is the branch of philosophy that studies the nature and scope of knowledge. ... In mathematics and related technical fields, a mathematical object is complete if nothing needs to be added to it. ... In physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenomena beyond quantum mechanics are needed to explain an individual event. ... Ad hoc is a Latin phrase which means for this [purpose]. It generally signifies a solution that has been tailored to a specific purpose, such as a tailor-made suit, a handcrafted network protocol, and specific-purpose equation and things like that. ...

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.^[8] 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 conundra associated with quantum mechanics dissolve if we assume that the state of a system is observer-dependent — like simultaneity in Special Relativity. This insight follows logically from the two main hypotheses which inform this interpretation: Absolute simultaneity is a hypothetical coincidence of two or more events in different points in space for all observers in the universe. ... The Lorentz transformation (LT), named after its discoverer, the Dutch physicist and mathematician Hendrik Antoon Lorentz (1853-1928), forms the basis for the special theory of relativity, which has been introduced to remove contradictions between the theories of electromagnetism and classical mechanics. ... Relativity of simultaneity means that events that are considered to be simultaneous in one reference frame are not simultaneous in another reference frame moving with respect to the first. ... The special theory of relativity was proposed in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. Some three centuries earlier, Galileos principle of relativity had stated that all uniform motion was relative, and that there was no absolute and well-defined state of rest... A hypothesis is a suggested explanation of a phenomenon or reasoned proposal suggesting a possible correlation between multiple phenomena. ...

• 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 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. There is no meaning in RQM in the "absolute", observer-independent state of any system. A priori is a Latin phrase meaning from the former or less literally before experience. In much of the modern Western tradition, the term a priori is considered to mean propositional knowledge that can be had without, or prior to, experience. ... In physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenomena beyond quantum mechanics are needed to explain an individual event. ...

Information and correlation

It is generally well established that any quantum mechanical measurement can be reduced to a set of yes/no questions. 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. The framework of quantum mechanics requires a careful definition of measurement, and a thorough discussion of its practical and philosophical implications. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Claude Elwood Shannon (April 30, 1916 _ February 24, 2001) has been called the father of information theory, and was the founder of practical digital circuit design theory. ... BIT is an acronym for: Bangalore Institute of Technology Bilateral Investment Treaty Bhilai Institute of Technology - Durg Birla Institute of Technology - Mesra Battles in Time (Doctor Who magazine) Category: ... To meet Wikipedias quality standards and make it more accessible, this article needs a better explanation of technical details or more context regarding applications or importance to make it more accessible to a general audience, or at least to technical readers outside this specialty. ... Quantum information science is a field of research at the interface of quantum mechanics and computer science. ... The term superposition can have several meanings: Quantum superposition Law of superposition in geology and archaeology Superposition principle for vector fields Superposition Calculus is used for equational first-order reasoning This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... The binary numeral system (base 2 numerals) represents numeric values using two symbols, typically 0 and 1. ...

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. Linear correlations between 1000 pairs of numbers. ...

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. The phrase degrees of freedom is used in three different branches of science: in physics and physical chemistry, in mechanical and aerospace engineering, and in statistics. ...

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 the Copenhagen interpretation. In certain interpretations of quantum mechanics, wavefunction collapse is one of two processes by which quantum systems apparently evolve according to the laws of quantum mechanics. ... The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ...

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

Taking the model system discussed above, if O'- has full information on the system, he will know the Hamiltonians of both S and O, including the interaction Hamiltonian. Thus, the system will evolve entirely unitarily (without any form of collapse) relative to O'-, if O measures S. The only reason that O will perceive a "collapse" is because he has incomplete information on the system (specifically, he does not know his own Hamiltonian, and the interaction Hamiltonian for the measurement). In physics, Hamiltonian has distinct but closely related meanings. ... In quantum mechanics, the Interaction picture (or Dirac picture) is an intermediate between the Schrödinger picture and the Heisenberg picture. ...

Consequences and implications

Coherence

In our system above, O'- may be interested in ascertaining whether or not the state of O accurately reflects the state of S. We can draw up for O'- an operator, M, which is specified as: In mathematical formulations of quantum mechanics, an operator is a linear transformation from a Hilbert space to itself. ...

with an eigenvalue of 1 meaning that O indeed accurately reflects the state of S. So there is a 0 probability of O reflecting the state of S as being if it is in fact ,and so forth. The implication of this is that at time t₂, O'- can predict with certainty that the S + O system is in some eigenstate of M, but cannot say which eigenstate it is in, unless P itself interacts with the S + O system. In mathematics, a number is called an eigenvalue of a matrix if there exists a nonzero vector such that the matrix times the vector is equal to the same vector multiplied by the eigenvalue. ...

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 O'- has the full Hamiltonians of both S and O, he will be able to say with certainty that at time t₂, O has a determinate result for S's spin, but he will not be able to say what 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. For a system with internal state (also called stateful system), time evolution means the change of state brought about by the passage of time. ...

But, let us imagine that O'- measures the spin of 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 O, and they compare the results of their experiments? 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.

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 O'- and whatever he chooses to measure, be it the S + O compound system or O and 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 O'', who will have a similar "M-operator" guaranteeing coherency, and so on out. In other words, a situation such as that desribed 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. In special relativity, a light cone is the pattern describing the temporal evolution of a flash of light in Minkowski spacetime. ...

RQM and quantum cosmology

It is a sound observation, from both a philosophical and physical perspective, that one should avoid evaluating cosmological scenarios in terms of an observer (whether "real" or "imaginary") outside of the system — which in this case, of course, is the universe. Philosophically, this is because the universe is the sum total of all that is in existence. Physically, a (physical) observer outside of the universe would require the breaking of gauge invariance,^[9] and a concomitant alteration in the mathematical structure of the 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, and as was discussed above, it is impossible for an object to give 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. // Cosmology, from the Greek: κοσμολογία (cosmologia, κόσμος (cosmos) order + λογια (logia) discourse) is the study of the Universe in its totality, and by extension, humanitys place in it. ... The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field. ... Gauge theories are a class of physical theories based on the idea that symmetry transformations can be performed locally as well as globally. ...

Ontology and epistemology

The implications of RQM for philosophy are far-reaching. Perhaps foremost among them is the nominalism implied by the absence of the observer-independent state of a system. A state, after all, is nothing but a compound of properties that a system possesses, and RQM implies that properties of a system are better ascribed to the relationship between the system and a particular observer. Thus properties do not inhere in the objects themselves, and exist as binary relations between objects, not unary relations as is the more typical view. In philosophy, nominalism is the theory that abstract terms, general terms, or universals do not represent objective real existents, but are merely names, words, or vocal utterances (flatus vocis). ... The word property, in philosophy, mathematics, and logic, refers to an attribute of an object; thus a red object is said to have the property of redness. ... In mathematics, the concept of binary relation, sometimes called dyadic relation, is exemplified by such ideas as is greater than and is equal to in arithmetic, or is congruent to in geometry, or is an element of or is a subset of in set theory. ...

If some form of epistemological naturalism is granted, meaning that epistemic propositions are merely indicators of correlation of state between an individual and their environment, a form of coherentism is implied by RQM. One of the key results of the theory is that despite the lack of an "absolute, underlying state", two or more individuals reporting on their observations will agree as to what they have observed (see the "M"-operator above). A term for a range of philosophical positions that link the concept of epistemology to natural science. ... Coherentism - Wikipedia, the free encyclopedia /**/ @import /skins-1. ...

Epistemologically, RQM differs fundamentally from Bohr's understanding of the role of quantum mechanics as a theory: Bohr felt that quantum mechanics is not a theory about the world as it is, but a theory about what we (humans) can say about the world. On the other hand, while RQM is very much a theory about what can be said about the world (where "said" refers to any correlation of state between any two relata), it claims to be exhaustive in its description. There is nothing more to the world than the net of relational descriptions. Niels Bohr Niels Henrik David Bohr (October 7, 1885 – November 18, Danish physicist who made essential contributions to understanding atomic structure and quantum mechanics. ...

While the relational interpretation implies that nature is fundamentally local, the view should not be confused with certain brands of local realism. Properties of objects are not defined before they are observed, but recall that "observation", in RQM, is a very general term, referring to any interaction whatsoever, since there is no a priori split between conscious, macroscopic observers with PhDs in theoretical physics, and arbitrary interactions between particles. Indeed, the worldview suggested by RQM may be characterised as a weak form of anti-realism, inasmuch as unobserved properties are indeterminate, non-existent (or conversely, no property predicated of an object is necessarily a valid predication for all observers). In physics, the principle of locality is that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. ... In physics, the principle of locality is that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. ... In philosophy, the term anti-realism is used to describe any position involving either the denial of the objective reality of entities of a certain type or the insistence that we should be agnostic about their real existence. ...

If all properties are relational, then what can be said of the objects they relate? Nothing: for any description is a property, which, in RQM, is a relation. This idea is perhaps unusual, but not new to philosophy. Heraclitus held a similar view, of a world constituted entirely of relations between relata of an indeterminate nature. The perspectivism advocated by Nietzsche, too, espouses a fundamentally relational reality, constituted of the myriad descriptions offered by individual observers, without the existence of an "absolute", underlying reality. Heraclitus by Johannes Moreelse Heraclitus of Ephesus (Greek Herakleitos) (about 535 - 475 BC), known as The Obscure (Greek Ainiktin), was a pre-Socratic Greek philosopher from Ephesus in Asia Minor. ... Perspectivism is the philosophical view that all perception takes place from a specific perspective. ... Friedrich Nietzsche, 1882 Friedrich Wilhelm Nietzsche (October 15, 1844 - August 25, 1900) was a highly influential German philosopher. ...

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. In physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenomena beyond quantum mechanics are needed to explain an individual event. ...

Copenhagen interpretation

RQM is, in essence, quite similar to the Copenhagen interpretation, but with an important difference. In the Copenhagen interpretation, the world is assumed to be intrinsically classical in nature, and wave 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. The Copenhagen interpretation is an interpretation of quantum mechanics formulated by Niels Bohr and Werner Heisenberg while collaborating in Copenhagen around 1927. ... Classical mechanics is a branch of physics which studies the deterministic motion of objects. ... In certain interpretations of quantum mechanics, wavefunction collapse is one of two processes by which quantum systems apparently evolve according to the laws of quantum mechanics. ... In theoretical physics, a preferred or priveleged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different to those in other frames. ...

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, i.e. 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. The Bohm interpretation of quantum mechanics, sometimes called the Bohmian Mechanics or Ontological interpretation is an interpretation postulated by David Bohm in 1952, which was an extension of the de Broglie-pilot-wave theory of 1927. ...

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. Sir Roger Penrose, OM, FRS (born 8 August 1931) is an English mathematical physicist and Emeritus Rouse Ball Professor of Mathematics at the University of Oxford. ...

Relative-state formulation

The many-worlds family of interpretations (MWI) shares an important feature with RQM, namely a relational view of the world. However, there is a tendency in some instances of this interpretation to overlook Everett's insight in this regard, which may give rise to problems similar to that of when wave collapse occurs: when does "branching" of the wavefunction happen? // The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that rejects the non-deterministic and irreversible wavefunction collapse associated with measurement in the Copenhagen interpretation in favor of a description in terms of quantum entanglement and reversible time evolution of states. ...

However, many proponents of the MWI do insist on a purely relational interpretation, by maintaining the relational nature of all value assignments (i.e. properties). RQM and MWI are very similar, then, except for one thing. Everett maintains that the universal wavefunction gives a complete description of the entire universe. Rovelli argues that this is problematic, not only because it provides a description which is not tied to a specific observer (which is a meaningless description in RQM); in RQM, there is no one, absolute description of the universal as a whole, but rather a net of inter-related partial descriptions. The Universal Wavefunction is a term introduced by Hugh Everett in his Princeton PhD Thesis[1], entitled The Theory of the Universal Wavefunction and forms a core concept in the relative state interpretation[2][3] or many-worlds interpretation[4][5] of quantum mechanics. ...

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. In quantum mechanics, the consistent histories approach is intended to give a modern interpretation of quantum mechanics, generalising the conventional Copenhagen interpretation and providing a natural interpretation of quantum cosmology. ...

RQM accords perfectly well with this view. However, where the consistent histories approach does not give a full description of the physical meaning of framework-dependent value (i.e. 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

The EPR thought experiment, performed with electrons. A radioactive source (center) sends electrons in a singlet state toward two spacelike separated observers, Alice (left) and Bob (right), who can perform spin measurements. If Alice measures spin up on her electron, Bob will measure spin down on his, and vice versa.

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. Download high resolution version (1173x444, 8 KB)EPR illustration from Xfig sources File links The following pages link to this file: EPR paradox Categories: GFDL images ... Download high resolution version (1173x444, 8 KB)EPR illustration from Xfig sources File links The following pages link to this file: EPR paradox Categories: GFDL images ... In theoretical physics, a singlet usually refers to a one-dimensional representation (e. ... In the context of special relativity, space-like separated points (or events) in spacetime have a spacetime interval less than 0 (see sign convention). ... In quantum mechanics, the EPR paradox (Einstein-Podolsky-Rosen) is a thought experiment which challenged long-held ideas about the relation between, on the one hand the observed values of physical quantities and on the other, the values that can be accounted for by a physical theory. ... In quantum mechanics, Bells Theorem states that a Bell inequality must be obeyed under any local hidden variable theory but can in certain circumstance be violated under quantum mechanics (QM). ...

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 t₁ towards two spacelike separated observers, Alice and Bob, who can perform spin measurements, which they do at time t₂. 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. In theoretical physics, a singlet usually refers to a one-dimensional representation (e. ... In the context of special relativity, space-like separated points (or events) in spacetime have a spacetime interval less than 0 (see sign convention). ...

The essence of the EPR paradox is that there is an apparent superluminal communication taking place between the two electrons. Orthodox quantum mechanics tells us that the state of each electron is only determined when it is measured: when the wavefunction is collapsed. We can assume a perfect experimental setup, and so the particles arrive at the observers at the same time. The problem is to explain how information can be transferred from one wing of the experiment to the other, as must surely happen if the states of the two particles are to obey the required constraints. Put simply, how does Bob's electron "know" what Alice measured on hers, so that it can adjust its own state accordingly, if the two measurements happen simultaneously? Superluminal communication is the term used to describe the hypothetical process by which one might send information at faster-than-light (FTL) speeds. ...

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' and Bob's light cones. Indeed, there is no observer who can instantaneously measure both electrons' spin. In mathematics, the intersection of two sets A and B is the set that contains all elements of A that also belong to B (or equivalently, all elements of B that also belong to A), but no other elements. ... In special relativity, a light cone is the pattern describing the temporal evolution of a flash of light in Minkowski spacetime. ... Relativity of simultaneity means that events that are considered to be simultaneous in one reference frame are not simultaneous in another reference frame moving with respect to the first. ...

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 t₃ in their future light cones.

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 M_A(α) the idea that the observer A (Alice) measures the state of the system α (Alice's particle).

So, at time t₂, Alice knows the value of M_A(α): the spin of her particle, relative to herself. But, since the particles are in a singlet state, she knows that

M_A(α) + M_A(β) = 0,-

and so if she measures her particle's spin to be σ, she can predict that Bob's particle (β) will have spin − σ. 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 t₃ she measures Bob's particle and then measures Bob (i.e. asks him what result he got) — or vice versa — the results will be consistent:

M_A(B) = M_A(β).-

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

M_C(A) = M_C(α)- and M_C(B) = M_C(β)-

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

M_C(α) + M_C(β) = 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.^[10] 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 2 empirical postulates are: In philosophy generally, empiricism is a theory of knowledge emphasizing the role of experience. ... In mathematical physics and quantum mechanics, quantum logic can be regarded as a kind of propositional logic suitable for understanding the apparent anomalies regarding quantum measurement, most notably those concerning composition of measurement operations of complementary variables. ... In philosophy generally, empiricism is a theory of knowledge emphasizing the role of experience. ...

• 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 denote the set of all possible questions that may be "asked" of a quantum system, which we shall denote by Q_i, . We may experimentally find certain relations between these questions: , corresponding to {intersection, orthogonal sum, orthogonal complement, inclusion, and orthogonality} respectively, where .

Structure

From the first postulate, it follows that we may choose a subset of N mutually independent questions, where N is the number of bits contained in the maximum amount of information. We call such a question a complete question. The value of can be expressed as an N-tuple sequence of binary valued numerals, which has 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 are defined for all Q_i, then is an orthomodular lattice, while all the possible unions of sets of complete questions form a Boolean algebra with the as atoms.^[11] In probability theory, to say that two events are independent intuitively means that knowing whether or not one of them occurs makes it neither more probable nor less probable that the other occurs. ... In mathematics, a tuple is a finite sequence of objects, that is, a list of a limited number of objects. ... In mathematics, a sequence is a list of objects (or events) arranged in a linear fashion, such that the order of the members is well defined and significant. ... This article is about permutation, a mathematical concept. ... In abstract algebra, a Boolean algebra is an algebraic structure (a collection of elements and operations on them obeying defining axioms) that captures essential properties of both set operations and logic operations. ...

The second postulate governs the event of further questions being asked by an observer O₁ of a system S, when O₁ already has a full complement of information on the system (an answer to a complete question). We denote by the probability that a "yes" answer to a question Q will follow the complete question . If Q is independent of , then p = 0.5, or it might be fully determined by , in which case p = 1. There is also a range of intermediate possibilities, and this case is examined below.

If the question that O₁ wants to ask the system is another complete question, , the probability 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 U^ij is a unitary matrix. In mathematics, a unitary matrix is an n by n complex matrix U satisfying the condition where In is the identity matrix and U* is the conjugate transpose (also called the Hermitian adjoint) of U. Note this condition says that a matrix U is unitary if it has an inverse...

• Postulate 3 If b and c are two complete questions, then the unitary matrix U_bc associated with their probability described above satisfies the equality U_cd = U_cbU_bd, for all b,c and d.

This third postulate implies that if we set a complete question as a basis vector in a complex Hilbert space, we may then represent any other question as a linear combination: In mathematics, a subset B of a vector space V is said to be a basis of V if it satisfies one of the four equivalent conditions: B is both a set of linearly independent vectors and a generating set of V. B is a minimal generating set of V... In mathematics, a Hilbert space is an inner product space that is complete with respect to the norm defined by the inner product. ... In mathematics, linear combinations are a concept central to linear algebra and related fields of mathematics. ...

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

Dynamics

The Heisenberg picture of time evolution accords most easily with RQM. Questions may be labelled by a time parameter , 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 t₂ is isomorphic to the set of all possible questions at time t₁. It follows, by standard arguments in quantum logic, from the derivation above that the orthomodular lattice 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. The Heisenberg Picture of quantum mechanics is also known as Matrix mechanics. ... Square with symmetry group D4 Symmetry is a characteristic of geometrical shapes, equations, and other objects; we say that such an object is symmetric with respect to a given operation if this operation, when applied to the object, does not appear to change it. ... In mathematics, an isomorphism (in Greek isos = equal and morphe = shape) is a kind of mapping between objects, devised by Eilhard Mitscherlich, which shows a relation between two properties or operations. ... In mathematical physics and quantum mechanics, quantum logic can be regarded as a kind of propositional logic suitable for understanding the apparent anomalies regarding quantum measurement, most notably those concerning composition of measurement operations of complementary variables. ... The concept of a linear subspace (or vector subspace) is important in linear algebra and related fields of mathematics. ...

It follows that there must be a unitary transformation that satisfies: A unitary transformation is an isomorphism (but not an antiisomorphism; that corresponds to an antiunitary transformation) between two Hilbert spaces or an automorphism of a single Hilbert space. ...

, and

where H is the Hamiltonian, a self-adjoint operator on the Hilbert space and the unitary matrices are an abelian group. In physics, Hamiltonian has distinct but closely related meanings. ... On a finite-dimensional inner product space, a self-adjoint operator is one that is its own adjoint, or, equivalently, one whose matrix is Hermitian, where a Hermitian matrix is one which is equal to its own conjugate transpose. ... In mathematics, an abelian group, also called a commutative group, is a group such that for all a and b in G. In other words, the order of elements in a product doesnt matter. ...

See also

• Interpretation of quantum mechanics
• Many-worlds interpretation
• Quantum information
• Philosophy of physics

An interpretation of quantum mechanics is an attempt to answer the question, What exactly is quantum mechanics talking about?. Although quantum mechanics is widely considered the most precisely tested and most successful theory in the history of science (Jackiw and Kleppner, 2000), many feel that in spite of this the... // The many-worlds interpretation (or MWI) is an interpretation of quantum mechanics that rejects the non-deterministic and irreversible wavefunction collapse associated with measurement in the Copenhagen interpretation in favor of a description in terms of quantum entanglement and reversible time evolution of states. ... In quantum mechanics, quantum information is physical information that is held in the state of a quantum system. ... Philosophy of physics is the study of the fundamental, philosophical questions underlying modern physics, the study of matter and energy and how they interact. ...

References

• Crane, L.: "Clock and Category: Is Quantum Gravity Algebraic?"; Journal of Mathematical Physics 36; 1993: 6180-6193; arXiv:gr-qc/9504038.
• Everett, H.: "The Theory of the Universal Wavefunction"; Princeton University Doctoral Dissertation; in DeWitt, B.S. & Graham, R.N. (eds.): "The Many-Worlds Interpretation of Quantum Mechanics"; Princeton University Press; 1973.
• Laudisa, F.: "The EPR Argument in a Relational Interpretation of Quantum Mechanics"; Foundations of Physics Letters, 14 (2); 2001: pp. 119-132; arXiv:quant-ph/0011016
• Laudisa, F. & Rovelli, C.: "Relational Quantum Mechanics"; The Stanford Encyclopedia of Philosophy (Fall 2005 Edition), Edward N. Zalta (ed.);online article.
• Mermin, N.D.: "What is Quantum Mechanics Trying to Tell us?"; American Journal of Physics, 66 (1998): 753-767, arXiv:quant-ph/9801057.
• Rovelli, C. & Smerlak, M.: "Relational EPR"; Preprint: arXiv:quant-ph/0604064.
• Rovelli, C.: "Relational Quantum Mechanics"; International Journal of Theoretical Physics 35; 1996: 1637-1678; arXiv:quant-ph/9609002.
• Smolin, L.: "The Bekenstein Bound, Topological Quantum Field Theory and Pluralistic Quantum Field Theory"; Preprint: arXiv:gr-qc/9508064.
• Wheeler, J. A.: "Information, physics, quantum: The search for links"; in Zurek,W., ed.: "Complexity, Entropy and the Physics of Information"; pp 3–28; Addison-Wesley; 1990.

Endnotes

1. ^ Wheeler (1990): pg. 3
2. ^ Rovelli (1996)
3. ^ Rovelli (1996): pg. 2
4. ^ Smolin (1995)
5. ^ Crane (1993)
6. ^ Laudisa (2001)
7. ^ Rovelli & Smerlak (2006)
8. ^ Rovelli (1996): pg. 16
9. ^ Smolin (1995), pg. 13
10. ^ Rovelli (1996): pg. 14
11. ^ Rovelli (1996): pg. 13