his book's main question-Is quantum mechanics non-local?-stilllooms large.But now we at least know that there can be no hope for a simple and unqualified "yes" or "no" answer to this riddle.There is such an enormous variety of interpretations of what constitutes a violation of the locality intuition, not to mention miscellaneous "degrees of severity" of these violations, that it would be utterly hopeless to expect a quick, onceand-for-all resolution to this problem.Yet we have made some progress.Along the quest to find the best counterfactual formulation of the condition of locality we have uncovered two possible interpretations of counterfactual conditionals which seem to be appropriate for describing quantummechanical phenomena that are in the center of our analysis.We should now take an important step forward and utilize these alternative interpretations to propose a precise formulation of the locality condition that could serve as a tenable explication for various intuitions associated with this notion.As we have already seen throughout this book, philosophers of physics and physicists alike often content themselves with pretty nebulous characterizations of the locality condition, of which the following are a representative sample:An event cannot be influenced by events in space-like separated regions.In particular, the outcome obtained in a measurement cannot be influenced by measurements performed in space-like separated regions; and analogously, possessed elements of physical reality referring to a system cannot be changed by actions taking place in space-like separated regions (Ghirardi & Grassi 1994).Since at the time of measurement the two systems no longer interact, no real change can take place in the second system in consequence of anything that may be done to the first system (Greenberger et al. 1990, p. 1132) For many purposes these descriptions may be considered sufficient, but without a thorough investigation into the possible meanings associated with terms such as "influence", "changed by", "in consequence of", etc. we T
his book's main question-Is quantum mechanics non-local?-stilllooms large.But now we at least know that there can be no hope for a simple and unqualified "yes" or "no" answer to this riddle.There is such an enormous variety of interpretations of what constitutes a violation of the locality intuition, not to mention miscellaneous "degrees of severity" of these violations, that it would be utterly hopeless to expect a quick, onceand-for-all resolution to this problem.Yet we have made some progress.Along the quest to find the best counterfactual formulation of the condition of locality we have uncovered two possible interpretations of counterfactual conditionals which seem to be appropriate for describing quantummechanical phenomena that are in the center of our analysis.We should now take an important step forward and utilize these alternative interpretations to propose a precise formulation of the locality condition that could serve as a tenable explication for various intuitions associated with this notion.As we have already seen throughout this book, philosophers of physics and physicists alike often content themselves with pretty nebulous characterizations of the locality condition, of which the following are a representative sample:An event cannot be influenced by events in space-like separated regions.In particular, the outcome obtained in a measurement cannot be influenced by measurements performed in space-like separated regions; and analogously, possessed elements of physical reality referring to a system cannot be changed by actions taking place in space-like separated regions (Ghirardi & Grassi 1994).Since at the time of measurement the two systems no longer interact, no real change can take place in the second system in consequence of anything that may be done to the first system (Greenberger et al. 1990, p. 1132) For many purposes these descriptions may be considered sufficient, but without a thorough investigation into the possible meanings associated with terms such as "influence", "changed by", "in consequence of", etc. we T
This book uses the formal semantics of counterfactual conditionals to analyze the problem of non-locality in quantum mechanics. Counterfactual conditionals (subjunctive conditionals) enter the analysis of quantum entangled systems in that they enable us to precisely formulate the locality condition that purports to exclude the existence of causal interactions between spatially separated parts of a system. They also make it possible to speak consistently about alternative measuring settings, and to explicate what is meant by quantum property attributions. The book develops the possible-world semantics of quantum counterfactuals using David Lewis’s famous approach as a starting point but modifying it significantly in order to achieve compatibility with the demands of the special theory of relativity as well as quantum mechanics. There have been several attempts to use counterfactual semantics to strengthen Bell’s theorem and its cognates such as the GHZ and Hardy theorems. These are critically evaluated in the book. Finally, a counterfactual reconstruction of the EPR argument and Bell’s theorem is proposed that sheds a new light on their philosophical consequences regarding the relations between realism and local causation.
The rampant success of quantum theory is the result of applications of the 'new' quantum \nmechanics of Schrödinger and Heisenberg (1926-7), the Feynman-Schwinger-Tomonaga Quantum \nElectrodynamics (1946-51), the electro-weak theory of Salaam, Weinberg, and Glashow (1967-9), \nand Quantum Chromodynamics (1973-); in fact, this success of `the' quantum theory has depended \non a continuous stream of brilliant and quite disparate mathematical formulations. In this carefully \nconcealed ferment there lie plenty of unresolved difficulties, simply because in churning out fabulously \naccurate calculational tools there has been no sensible explanation of all that is going on. It is even \nargued that such an understanding is nothing to do with physics. A long-standing and famous \nillustration of this is the paradoxical thought-experiment of Einstein, Podolsky and Rosen (1935). \nFundamental to all quantum theories, and also their paradoxes, is the location of sub-microscopic \nobjects; or, rather, that the specification of such a location is fraught with mathematical inconsistency. \nThis project encompasses a detailed, critical survey of the tangled history of Position \nwithin quantum theories. The first step is to show that, contrary to appearances, canonical quantum \nmechanics has only a vague notion of locality. After analysing a number of previous attempts at a \n`relativistic quantum mechanics', two lines of thought are considered in detail. The first is the work \nof Wan and students, which is shown to be no real improvement on the usual `nonrelativistic' theory. \nThe second is based on an idea of Dirac's - using backwards-in-time light-cones as the hypersurface \nin space-time. There remain considerable difficulties in the way of producing a consistent scheme here. \nTo keep things nicely stirred up, the author then proposes his own approach - an adaptation \nof Feynman's QED propagators. \nThis new approach is distinguished from Feynman's since the propagator or Green's function \nis not obtained by Feynman's rule. The type of equation solved is also different: instead of an \ninitial-value problem, a solution that obeys a time-symmetric causality criterion is found for an \ninhomogeneous partial differential equation with homogeneous boundary conditions. \nTo make the consideration of locality more precise, some results of Fourier transform theory are \npresented in a form that is directly applicable. \nSomewhat away from the main thrust of the thesis, there is also an attempt to explain the \nmanner in which quantum effects disappear as the number of particles increases in such things as \nexperimental realisations of the EPR and de Broglie thought experiments.
By questioning the validity of some of our basic concepts, such as space, object, and causality, quantum physics contributes quite decisively to the dramatic changes now taking place in our world picture.This book is addressed not only to physicists at an early stage in their careers (the first or second year graduate student) but also to philosophers, as well as to all the senior physicists interested in the interpretation problem. Beginning with a chapter that could be described as ?philosophy for physicists,? it presents an in-depth analysis of present-day quantum mechanical concepts, an analysis of physicists and philosophers alike. Specifically, it first offers an extensive critical analysis of such topics as the Einstein, Podolsky, Rosen reality criterion, nonseparatability, the quantum measurement riddle, decoherence theory, consistent histories approaches and ontologically interpretable theories. All this then naturally leads to philosophical questions concerning, in particular, intersubjective agreement and the limit of realism. And a thorough examination of this whole material finally leads to the view that distinguishing between empirical reality and a veiled man-independent reality yields an acceptable answer to the perplexing question of how to interpret quantum physics. Veiled Reality offers nonspecialists, including students in physics, philosophy and the history of science, an accessible perspective on basic problems in the foundations of physics.
One way of reading this book is as an epilogue-or (more precisely) as a denouement-to a book I wrote more than thirty years ago, which was called Quantum Mechanics and Experience.That earlier book was meant to do two things:1) To introduce its readers to the elementary nuts and bolts of the foundations of quantum mechanics-the idea of superposition, the basics of the standard mathematical formalism of quantum mechanics, the argument of Einstein and Podolsky and Rosen to the effect that that formalism, considered as a description of the fundamental real ity of the world, must be somehow incomplete, Bell's extension of that argument into a general and power ful theorem about locality, the elaboration (by figures like Erwin Schrödinger, and John von Neumann, and Eugene Wigner) of the quantum-mechanical mea surement prob lem, and vari ous proposals (theories of the spontaneous collapse of the wave-function, Bohmian mechanics, the so-called many-worlds interpretations of quantum mechanics, and so on) for solving it.2) To show how the above-mentioned proposals for solving the mea sure ment prob lem, taken together, amount to a
What does it mean to say that an event is physically possible?One might say, in the spirit of everyday usage, that an event is physically possible if its occurrence is consistent with the laws of physics.But which laws of physics, starting from when?Here things get tricky.In a Laplacian universe governed by the fully-deterministic laws of classical electrodynamics and mechanics, exactly one future is consistent with any complete and precise specification of initial conditions.Given an incomplete, imprecise specification -the best that human observers can achieve, even in principle -many futures may appear to be physically possible, but only one of them actually is.In this kind of universe, our intuitive notion that many different outcomes of an experiment, for example, are physically possible is just an ignorancedriven delusion.Philosopher of physics Ruth Kastner's new book, The Transactional Interpretation of Quantum Mechanics, subtitled The Reality of Possibility, takes on this question of the meaning of physical possibility in the theoretical context defined by relativistic quantum field theory.Lest this seem daunting, be assured that this is a philosophical book; aside from Chapters 5 and 6 dealing with technical objections, it emphasizes ontological questions and employs the physics -all of which is clearly presented for the non-expert -as a way to raise them.The ontological questions that it raises, not just 'what is possibility?' but also 'what is actuality?' and 'what are space and time?' underlie not just physics but any theory of action.The Transactional Interpretation of Quantum Mechanics is, therefore, not just about the interpretation of quantum mechanics; it is about how 'nature makes its choice' in any circumstance, and how 'the actual arises from the potential' (205) as a result.After briefly introducing quantum theory and its various interpretations in Chapter 1, Kastner addresses, in Chapter 2, the essential preliminary question of what is to count as an acceptable scientific explanation.The divide between realists and instrumentalists about quantum theory -or between 'ontic ' and 'non-ontic' approaches
When investigating various interpretations of quantum mechanics one notices that each interpretation contains an element which escapes a complete and full description. This element is always associated with the stochasticity of the individual event in the quantum measurement process. It appears that the implications of this limit to any description of the world has not been sufficiently appreciated with notable exceptions of, for example, Heisenberg, Pauli and Wheeler. If we assume that a deeper foundation of quantum mechanics is possible, the question arises which features such a philosophical foundation might have. It is suggested that the objective randomness of the individual quantum event is a necessity of a description of the world in view of the significant influence the observer in quantum mechanics has. It is also suggested that the austerity of the Copenhagen interpretation should serve as a guiding principle in a search for deeper understanding. In recent years, a significant increase of interest in the foundations of quantum mechanics can be observed. This increase is certainly related to the immense progress made during the last two decades in experiments with individual quantum systems. This technological progress has made it possible to really perform more and more of the famous gedanken experiments which were so important in the early days of the theory. Not only did these experiments confirm the theory in every detail, they also opened up avenues for new experimental directions which might even lead some day to new technology. While such applications are certainly still far away, areas like quantum cryptography[1], quantum computation[2] and interaction-free measurement[3], just to name a few examples, certainly deserve the attention they receive. All this activity has also drawn more attention to the problems of understanding and interpreting quantum mechanics. It seems that there, a definitive consensus has not yet been reached. As witnesses quote authorities like Feynman as saying[4]: I think can safely say that nobody today understands quantum mechanics, whereby he apparently included himself, or like Roger Penrose[5], who describes his opinion remarking that while the theory agrees incredibly well with experiment and while it is of profound mathematical beauty, it makes absolutely no sense. Why did even physicists who have contributed so significantly to quantum theory Feynman was even rewarded the Nobel Prize for one of its mathematical formulations choose such emphatic and strong formulations? Why is it that the average physicist hears very little about such problems during his/her education? Why is the comprehension of the theory very often focused on formalism while questions that probe for deeper meaning are usually not tackled? To take a closer look at the situation, let us turn to a brief analysis of the interpretation of quantum mechanics. We note that there are at least two levels of interpreting a formalism, a physical theory. The first, basic, level supplies the rules that determine which element of the formalism corresponds to which measurable quantity or to which observable fact in a concrete experimental situation. Theses rules are a large, mostly not explicated but only implicit, set of instructions. These instructions concern the manner of working with the statements of the theory in order to obtain predictions for future experiments or events, and they concern the instructions on how to proceed in experiment in order to demonstrate or test a theoretical