In recent decades there has been a resurge of interest in the foundations of quantum theory, partly motivated by new experimental techniques, partly by the emerging field of quantum information science. Old questions, asked since the seminal article by Einstein, Podolsky and Rosen (EPR), are being revisited. The work of John Bell has changed the direction of investigation by recognising that those fundamental philosophical questions can have, after all, input from experiment. Abner Shimony has aptly termed this new field of enquiry "experimental metaphysics". The objective of this Thesis is to contribute to that body of research, by formalising old concepts, proposing new ones, and finding new results in well-studied areas. Without losing from sight that the appeal of experimental metaphysics comes from the adjective, every major result is followed by clear experimental proposals for quantum-atom optical setups.
In recent decades there has been a resurge of interest in the foundations of quantum theory, partly motivated by new experimental techniques, partly by the emerging field of quantum information science. Old questions, asked since the seminal article by Einstein, Podolsky and Rosen (EPR), are being revisited. The work of John Bell has changed the direction of investigation by recognising that those fundamental philosophical questions can have, after all, input from experiment. Abner Shimony has aptly termed this new field of enquiry "experimental metaphysics". The objective of this Thesis is to contribute to that body of research, by formalising old concepts, proposing new ones, and finding new results in well-studied areas. Without losing from sight that the appeal of experimental metaphysics comes from the adjective, every major result is followed by clear experimental proposals for quantum-atom optical setups.
In recent decades there has been a resurge of interest in the foundations of quantum theory, partly motivated by new experimental techniques, partly by the emerging field of quantum information science. Old questions, asked since the seminal article by Einstein, Podolsky and Rosen (EPR), are being revisited. The work of John Bell has changed the direction of investigation by recognising that those fundamental philosophical questions can have, after all, input from experiment. Abner Shimony has aptly termed this new field of enquiry experimental metaphysics. The objective of this Thesis is to contribute to that body of research, by formalising old concepts, proposing new ones, and finding new results in well-studied areas. Without losing from sight that the appeal of experimental metaphysics comes from the adjective, every major result is followed by clear experimental proposals with detailed analysis of feasibility for quantum-atom optical setups. After setting the appropriate terminology and the basic concepts, we will start by analysing the original argument of Einstein, Podolsky and Rosen. We propose a general mathematical form for the assumptions behind the EPR argument, namely those of local causality and completeness. That formalisation entails what was termed a Local Hidden State model by Wiseman et al., which was proposed as a formalisation of the concept of steering first introduced by Schrodinger in a reply to the EPR paper. Violation of any consequences that can be derived from the assumption of that model therefore implies a demonstration of the EPR paradox. We will show how one can then re-derive the well-known EPR-Reid criterion for continuous-variables correlations, and derive new ones applicable to the spin setting considered by Bohm. The spin set-up of the EPR-Bohm was used by Bell to derive his now famous theorem demonstrating the incompatibility of the assumption of local causality and the predictions of quantum mechanics. The inequalities which bear his name can be derived for any number of discrete outcomes, but so far there has been no derivation which can be directly applied to the continuous-variables case of the original EPR paradox. We close the circle by deriving a class of inequalities which make no explicit mention about the number of outcomes of the experiments involved, and can therefore be used in continuous-variables measurements with no need for binning the continuous results into discrete ones. Apart from that intrinsic interest, these inequalities could prove important as a means to perform an unambiguous test of Bell inequalities since optical homodyne detection can be performed with high detection efficiency. The technique, which is based on a simple variance inequality, can alsobe used tore-derive alarge class of well-known Bell-type inequalities and at the same time find their quantum bound, making explicit from a formal point of view that the non-commutativity of the local operators is at the heart of the quantum violations. Finally, we address the issue of macroscopic superpositions originally sparked by the infamous cat paradox of Schrodinger. We consider macroscopic, mesoscopic and ‘S -scopic’ quantum superpositions of eigenstates of an observable, and develop some signatures for their existence. We define the extent, or size S of a (pure-state) su-perposition, with respect to an observable X, as being the maximum difference in the outcomes of X predicted by that superposition. Such superpositions are referred to as generalised S -scopic superpositions to distinguish them from the extreme superpo-sitions that superpose only the two states that have a difference S in their prediction for the observable. We also consider generalised S -scopic superpositions of coherent states. We explore the constraints that are placed on the statistics if we suppose a system to be described by mixtures of superpositions that are restricted in size. In this way we arrive at experimental criteria that are sufficient to deduce the existence of a generalised S -scopic superposition. The signatures developed are useful where one is able to demonstrate a degree of squeezing.
The interplay between non-relativistic quantum theory and metaphysics has generated radically opposed interpretations for quantum theory: Niels Bohr's 'orthodox' interpretation, and Einstein's 'realist' one. Out of this debate emerged the classical first-generation paradoxes of quantum theory: Schrodinger's Cat and the Einstein-Podolsky-Rosen paradoxes. And more recently a range of new paradoxes has emerged from the work of J.S. Bell which questions the `local' nature of quantum theory, prefiguring a return either to an action-at-a-distance metaphysics or to a radically holistic conception of matter. This book outlines the contours of these debates, and presents an interpretation of quantum theory which, while metaphysically realist, resolves most of the paradoxes. This interpretation takes quantum systems to be fuzzy particles, piloted by fields of propensities which are represented by density operators on Hilbert space; and constitutes, in part, a return to an earlier view of quantum systems advanced by De Broglie, Schrodinger, Born, and Einstein. A book for philosophers and physicists who are interested in the philo sophy o f science.
Instead of having to rely on gedanken (thought) experiments, it is possible to base this discussion of the foundations of quantum physics on actually performed experiments because of the enormous experimental progress in recent years. For reasons of space, the author discusses mainly experiments related to the Einstein-Podolsky-Rosen paradox and Bell's theorem, that is, to quantum entanglement. Not only have such fundamental experiments realized many historic proposals, they also helped to sharpen our quantum intuition. This recently led to the development of a new field, quantum information, where quantum teleportation and quantum computation are some of the most fascinating topics. Finally the author ventures into a discussion of future prospects in experiment and theory.
Although the title of this focus issue is ‘The Possibility of Metaphysics’ the first part has as its focus not only metaphysics in general but Buddhist metaphysics in particular. This is because the motivation for this focus issue was sparked by an email from a colleague who asked for my opinion of a book written by Robert Ellis. The aspect of the ‘experimental metaphysics’ of quantum theory is examined in detail in the first article "The Matter of Mindnature" The Buddhist metaphysical viewpoint tells as the nature of ultimate reality is best understood as a fundamentally interrelated and interpenetrating field of Mind-like energy, or Mindnature, and such a view is clearly supported by the quantum violation of Bell’s inequalities. In this article I examine Ellis’s notion of the impossibility of metaphysics in the light of both philosophical considerations and the implications of the quantum evidence. The next article "Taking the‘Meta’ Out of Physics" is Ellis’s response to my criticisms of his work. I leave it to readers to come to conclusions without further comment from me. It is my hope that there will be feedback concerning the issues raised as I am personally convinced that Ellis’s position is untenable but am curious to know whether my viewpoint is widely held. Certainly the last two articles from James Kowall, ‘What is Reality in a Holographic World?’, and Brian Whitworth, ‘Introducing The Virtual Reality Conjecture’, seem to support my position.
1. Quantum Mechanics, Reality and Separability: Physical Developments of the Einstein-Podolsky-Rosen Argument.- Realism in Quantum Mechanics and a New Version of the EPR Experiment.- Discussion: M. Cini, F. De Martini, K. Kraus, T.W. Marshall, K.R. Popper, H. Rauch, M.C. Robinson, F. Selleri, J. Six, G. Tarozzi, J.-P. Vigier..- The Physical Origin of the Einstein-Podolsky-Rosen Paradox.- Does the Bell Inequality Hold for All Local Theories?.- Is it Possible to Save Causality and Locality in Quantum Mechanics?.- Einstein-Podolsky-Rosen Experiments and Macroscopic Locality.- On the Compatibility of Local Realism with Atomic Cascade Experiments.- Is the Einstein-Podolsky-Rosen Paradox Demanded by Quantum Mechanics?.- Infinite Wave Resolution of the EPR Paradox.- On Non Local Influences.- Einstein Locality for Individual Systems and for Statistical Ensembles.- Can Nondetected Photons Simulate Nonlocal Effects in Two-Photon Polarization Correlation Experiments?.- 2. The Stochastic Interpretation of Quantum Processes.- Quantum Theory of Measurement without Wave Packet Collapse.- A Causal Fluidodynamical Model for the Relativistic Quantum Mechanics.- On the Nonlinear Schrodinger Equation.- Recent Developments in de Broglie Nonlinear Wave Mechanics.- The Role of the Quantum Potential in Determining Particle Trajectories and the Resolution of the Measurement Problem.- When is Statistical Theory Causal?.- Radiation Damping and Nonlinearity in the Pilot Wave Interpretation of Quantum Mechanics.- Stochastic Electrodynamics and the Bell Inequalities.- Nonlocal Quantum-Potential Interpretation of Relativistic Actions at a Distance in Many-Body Problems.- 3. The Realistic Interpretation of the Wave Function: Experimental Tests on the Wave-Particle Dualism.- Third Kind Measurements and Wave-Particle Dualism.- Quantum Effects in the Interference of Light.- Tests of Quantum Mechanics by Neutron Interferometry.- A Unified Experiment for Testing both the Interpretation and the Reduction Postulate of the Quantum Mechanical Wave Function.- Epilogue.- Evolutionary Epistemology.
The Nobel Prize in Physics 2022 was awarded to three experiment physicists: Alain Aspect, John F. Clauser and Anton Zeilinger, for their contributions to “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. In this paper, the history, basics, and applications related with the Bell inequalities are reviewed. The discussion of fundamental issues on quantum mechanics dated back to the year 1927 when the fifth Solvay conference was held, highlighting intense arguments between Albert Einstein and Niels Bohr. At the time, Einstein expressed, publicly for the first time, his critical attitude towards quantum mechanics. While the arguments did not reach a decisive conclusion, Bohr’s probabilistic interpretation of quantum mechanics, the Copenhagen interpretation, had since been well circulated among the community. Einstein’s arguments with Bohr lasted many years after the conference. The arguably most famous one was the Einstein-Podolsky-Rosen (EPR) paradox, proposed in a 1935 paper written by the three authors. They stated, by flawless reasoning, that quantum mechanics is ‘incomplete’. Bohr replied immediately. So did Erwin Schrödinger, who, in particular, coined the term ‘entangled’ for the first time to describe the bizarre quantum mechanical property. Nevertheless, the EPR paper received few citations at its publication, until 1990’s when quantum information science started blossoming. From today’s perspective, the paper resembles the ‘sleeping beauty’ who slumbered for decades before fully awakening. It has been widely accepted that it was John S. Bell who eventually resolved the EPR paradox. But many important figures before him should not go unnoticed, including John A. Wheeler, Chien-Shiung Wu, David Bohm, to name a few. In 1964, Bell wrote down a mathematical inequality which satisfies local hidden variable theory but which can be violated in quantum mechanics. The significance is that Bell’s inequality brings the rather philosophic debates on the EPR paradox down to be experimentally testable. Hence, an experiment is here appealing, as is the case in all fields of physics. To the end, John F. Clauser, alongside Stuart J. Freedman, conducted an experiment in 1972 to test a Bell inequality, the Bell-Clauser-Horne-Shimony-Holt (CHSH) inequality, proposed by him and his collaborators in 1969. The results, however, leaves loopholes open. One of the loopholes was closed in a later experiment led by Alain Aspect in 1982. The Bell-CHSH inequality was later tested under a much stricter condition by Anton Zeilinger’s team. Two most important loopholes were both closed by three teams in 2015 independently. Zeilinger’s team was among them. Almost all experiments to date have confirmed quantum mechanical predictions. Zeilinger’s team also performed the first teleportation of unknown quantum states, the first entanglement swapping, along with other contributions pioneering the quantum information science. In retrospect, the foundational study of quantum theory has been developed over a century. It once drew much of quantum physics founders’ attention, went virtually unnoticed while other physical fields advanced, had for quite a while been considered to have very weak relationship with major physics, and finally revived in recent decades leading to rapid progress in quantum information science and quantum technology. We believe that the Nobel Prize in Physics this year is not only an award and recognition to past achievements, but also heralds a new era of science in future.