Relational quantum mechanics
A "measurement event" is thus described as an ordinary physical interaction where two systems become correlated to some degree with respect to each other.
Rovelli criticizes describing this as a form of "observer-dependence" which suggests reality depends upon the presence of a conscious observer, when his point is instead that reality is relational and thus the state of a system can be described even in relation to any physical object and not necessarily a human observer.
By giving up our preconception of a global privileged state, issues around the measurement problem and local realism are resolved.
[6] The idea has been expanded upon by Lee Smolin[7] and Louis Crane,[8] 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.
[9][10] This problem was initially discussed in detail in Everett's thesis, The Theory of the Universal Wavefunction.
Note that the above scenario is directly linked to Wigner's Friend thought experiment, which serves as a prime example when understanding different interpretations of quantum theory.
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).
[citation needed] RQM makes use of this fact to formulate the state of a quantum system (relative to a given observer!)
But, Rovelli points out, this form of correlation is precisely the same as the definition of information in Shannon's theory.
's result is without interaction, and hence breaking the unitary evolution of the compound system (because he doesn't know his own Hamiltonian).
[dubious – discuss] 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.
The consistency inherent in the quantum formalism, exemplified by the "M-operator" defined above, guarantees that there will be no contradictions between records.
The universe is the sum total of everything in existence with any possibility of direct or indirect interaction with a local observer.
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.
[14] The only group of interpretations of quantum mechanics with which RQM is almost completely incompatible is that of hidden variables theories.
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.
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).
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.
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.
towards two spacelike separated observers, Alice and Bob, who can perform spin measurements, which they do at time
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).
Put simply, how can Bob's electron "know" what Alice measured on hers, so that it can adjust its own behavior accordingly?
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.
[15] 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?
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.
[16] 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.
is an orthomodular lattice, while all the possible unions of sets of complete questions form a Boolean algebra with the
is the Hamiltonian, a self-adjoint operator on the Hilbert space and the unitary matrices are an abelian group.
Rovelli limits the scope of this claim by stating that RQM relates to the variables of a physical system and not to constant, intrinsic properties, such as the mass and charge of an electron.
