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arXiv:1109.6223 · Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences

Vindication of quantum locality

David Deutsch

Published Oct 26, 2011• 19 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1098/rspa.2011.0420

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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.
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.
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.
From gravity to electromagnetism, apparent action at a distance has always been resolved by deeper, local explanations. Yet today, Bell's theorem is widely interpreted as the death knell for local reality. In this chapter, I present the theorem in accessible terms, examine the three main strategies that attempt to preserve hidden variables, and argue that they share a common defect: the attempt to explain the quantum from the classical rather than the other way around. When quantum mechanics is applied universally, classicality itself is given a quantum account; and when the Bell scenario is formulated in the Heisenberg picture, a strictly local explanation emerges. This chapter serves as a non-technical front-end to Explaining Bell Locally (Proc. R. Soc. A).