Jonathan Walgate

Bell theorems show how to experimentally falsify local realism. Conclusive falsification is highly desirable as it would provide support for the most profoundly counterintuitive feature of quantum theory—nonlocality. Despite the preponderance of evidence for quantum mechanics, practical limits on detector efficiency and the difficulty of coordinating space-like separated measurements have provided loopholes for a classical worldview; these loopholes have never been simultaneously closed. A number of new experiments have recently been proposed to close both loopholes at once. We show these novel designs fail in the most basic way, by not ruling out local hidden variable models, and we provide an explicit classical model to demonstrate this. These experiments share a common flaw, which reveals a basic misunderstanding of how nonlocality proofs work. Given the time and resources now being devoted to such experiments, theoretical clarity is essential. Our explanation is presented in terms of simple logic and should serve to correct misconceptions and avoid future mistakes.

We examine the phenomenon of emergence, referring particularly to Arthur Peacocke's ideas on emergence, the self, and spirituality. He believes that the whole of an emergent structure influences the way its parts cohere and that emergent structures (including minds and persons) and their effects are very important. He thereby hopes to remove the reductionist challenge that seeks to understand a whole fully in terms of its parts. We argue that emergent phenomena are not influential in the above sense. The holistic completeness of these structures at their own theoretical level does not substitute for the causal independence Peacocke suggests by the idea of influence. Some computer simulations that generate emergent complexity follow simple and self‐contained sets of rules. Peacocke also adheres to a hierarchical account of reality as a series of levels into which matter is organized, running from atoms through molecules to cells and eventually to whole ecosystems. But influential behavior does not respect this ordering. Further, Peacocke's opposition to reductionism is unnecessary; any “completeness” of lower‐level models does not imply the redundancy of higher‐level descriptions. Emergence transforms reductionism into a constructive and positive principle.

Entanglement is an useful resource because some global operations cannot be locally implemented using classical communication. We prove a number of results about what is and is not locally possible. We focus on orthogonal states, which can always be globally distinguished. We establish the necessary and sufficient conditions for a general set of 2x2 quantum states to be locally distinguishable, and for a general set of 2xn quantum states to be distinguished given an initial measurement of the qubit. These results reveal a fundamental asymmetry to nonlocality, which is the origin of ``nonlocality without entanglement'', and we present a very simple proof of this phenomenon.

We consider one copy of a quantum system prepared in one of two orthogonal pure states, entangled or otherwise, and distributed between any number of parties. We demonstrate that it is possible to identify which of these two states the system is in by means of local operations and classical communication alone. The protocol we outline is both completely reliable and completely general - it will correctly distinguish any two orthogonal states 100% of the time.