Local realism has been the subject of much discussion in modern physics, partly because our deepest theories of physics appear to contradict one another in regard to whether reality is local. According to general relativity, it is, as physical quantities (perceptible or not) in two spacelike separated regions cannot affect one another. Yet, in quantum theory, it has traditionally been thought that local realism cannot hold and that such effects do occur. This apparent discrepancy between the two theories is resolved by Everettian quantum theory, as first proven by Deutsch & Hayden (2000). In this paper, I will explain how local realism is respected in quantum theory and review the advances in our understanding of locality since Deutsch & Hayden's work, including the concept of local branching and the more general analysis by Raymond-Robichaud (2021)
Local realism has been the subject of much discussion in modern physics, partly because our deepest theories of physics appear to contradict one another in regard to whether reality is local. According to general relativity, it is, as physical quantities (perceptible or not) in two spacelike separated regions cannot affect one another. Yet, in quantum theory, it has traditionally been thought that local realism cannot hold and that such effects do occur. This apparent discrepancy between the two theories is resolved by Everettian quantum theory, as first proven by Deutsch & Hayden (2000). In this paper, I will explain how local realism is respected in quantum theory and review the advances in our understanding of locality since Deutsch & Hayden's work, including the concept of local branching and the more general analysis by Raymond-Robichaud (2021)
A growing body of literature supports Deutsch and Hayden’s (2000) claim that in the Heisenberg picture, quantum entanglement can be characterised locally using objects they call descriptors. I argue that their notion of locality rests, in part, on a qualified version of the principle of separability that I call spatial separability. I show that spatial separability is not sufficient for separability simpliciter, and that the more general version, spatiotemporal separability, reveals that their claim is false. Deutsch and Hayden’s approach also has a novel implication: if descriptors represent actual physical properties, they are non-qualitative properties, which are profoundly different from the qualitative properties we have come to expect from other branches of physics.
Bell's theorem establishes that a local and spatially separable physical theory with no superdeterminism or retrocausality must be a theory of many local worlds. The local many worlds formalism of quantum mechanics is a transparent separable physical theory in spacetime with a Lorentz covariant causal structure. Unlike the Everett many worlds interpretation, there is no nonseparable universal wavefunction in configuration space, but like Everett, there is only local unitary evolution which conserves probability (fluid) current, and no wavefunction collapse. Each event in spacetime has its own local wavefunction which is built up using only the local wavefunctions and local unitary evolution operators within its past light cone, and which is entirely disconnected from spacelike separated local wavefunctions. We call this the local Schrödinger picture. Local many worlds can also be described in the Heisenberg picture by ascribing Deutsch-Hayden (DH) descriptors to each individual system, each of which evolves under local unitary operations. Both the local wavefunctions and DH descriptors contain records of past interactions, and while the local wavefunctions provide a clear and intuitive physical narrative, the descriptors contain all of the same information in a much more compact form. Unfortunately, the compact form is also somewhat cryptic, which has made the local Heisenberg picture relatively inaccessible. The purpose of this article is to show that the local wavefunctions for any N-qubit system can always be constructed directly from the DH descriptors, allowing us to combine the benefits of both pictures.
In a previous paper, Hayden and I ( Deutsch & Hayden 2000 Proc. R. Soc. Lond. A 456 , 1759–1774. ( doi:10.1098/rspa.2000.0585 )) proved, using the Heisenberg picture, that quantum physics satisfies Einstein's criterion of locality. Wallace and Timpson ( Wallace & Timpson 2007 Found. Phys. 37 , 951–955. ( doi:10.1007/s10701-007-9135-7 )) have argued that certain transformations of the Heisenberg-picture description of a quantum system must be regarded as leaving invariant the factual situation being described, and that taking this into account reveals that Einstein's criterion is violated after all. Here, I vindicate the proof and explain some misconceptions that have led to this and other criticisms of it.
It has been more than 20 years since Deutsch and Hayden proved the locality of quantum theory, using the Heisenberg picture of quantum computational networks. Of course, locality holds even in the face of entanglement and Bell’s theorem. Today, most researchers in quantum foundations are still convinced not only that a local description of quantum systems has not yet been provided, but that it cannot exist. The main goal of this paper is to address this misconception by re-explaining the descriptor formalism in a hopefully accessible and self-contained way. It is a step-by-step guide to how and why descriptors work. Finally, superdense coding is revisited in the light of descriptors.
I distinguish two senses in which one can take a given physical theory to be `complete'. On the first, a complete physical theory is one that, in principle, completely describes physical reality. On the second, a complete physical theory is one that provides all of the conceptual resources one needs to describe any (in general probabilistic) physical phenomenon to any level of detail one likes, in principle. I argue that while the (neo-)Everettian approach to interpreting quantum mechanics aims to show that it is complete in the first sense, the (neo-)Bohrian approach begins from an understanding of quantum mechanics as being complete in the second sense. I then discuss some of the essential differences between how classical and quantum theory describe phenomena, and the way in which the quantum description can be thought of as a ``natural generalisation'' (to use Bohr's phrase) of the classical description. Finally, I elaborate upon the two visions of physics from which one can motivate the first and the second sense of completeness, respectively: \emph{metaphysical realism}, on the one hand, and what I will call \emph{methodological realism}, on the other -- and discuss what one can say about the significance of the differences between quantum and classical description from each of these points of view. I suggest that there is a sense in which the views of (neo-)Everett and the views of (neo-)Bohr can be understood to be mutually supporting positions, from their respective perspectives, even though they are diametrically opposed.
Local realism reduces to the proposition that every physical effect arises exclusively at the spacetime locality where the determinate properties of physical reality determine that it be so. The causal model of quantum theory, however, and thus all prospects for quantum computers, require that local realism is false. The essential argument favoring quantum beliefs is that local realism cannot explain empirical Bell violations, whereas quantum theory can. By contrast, the article cited below offers compelling evidence to the contrary. Furthermore, such alleged refutations of local realism are predicated on an interpretation of the Bell inequality that many researchers reject (the cited article provides specific references). Meanwhile, the causal model of quantum theory reduces to the quantum formalism itself, which distributes outcomes across separate localities without providing enough information to determine what those outcomes are. More specifically, depending on subtleties of interpretation, the quantum formalism requires either: (A) There exists a bit of physical information such that this bit exists as mutually exclusive states, or (B) there exists a bit of physical information such that this bit is not physically informed. They are both contradictions. Pick your poison. Or reconsider local realism. See ** http://vixra.org/abs/1704.0078 ** Local Realism Explains Bell Violations (author Andrew P. Yake) - for a demonstration that all empirical evidence taken to support quantum theory over local realism plausibly does the reverse. The article comprises 8 pages, 4 figures, 6 equations, 32 references, 1 graph of testable predictions, and 2 paragraphs that purport to expose how the Bell inequality misrepresents the local realistic predictions for the EPR experiment. Thoughtful feedback appreciated (apyake@gmail.com).