A group of philosophers and physicists at Oxford have recently crafted, in light of developments in the theory of decoherence, a new version of the Everett or Many Worlds interpretation of quantum mechanics. I will focus on the work of David Wallace, who identifies the quasi-classical domains making up the various “worlds” or “branches” of the interpretation with macroscopic patterns that are functionally realized (to an approximate degree good enough for practical purposes) by the quantum wavefunction of the universe. I argue that Wallace’s reliance on Savage’s pragmatic account of probability fails, due to the unreasonableness of Savage’s Sure Thing Principle in a many-worlds setting. I also argue that Wallace’s program is vulnerable to a version of Putnam’s model-theoretic indeterminacy argument, with the result that almost any conceivable macroscopic “world” is in fact realized by that function and so is (by Wallace’s light) as real as our own. I offer a hylomorphic revision of the many-worlds model, the “traveling forms” interpretation of Alexander Pruss, and I argue that this revision solves both the indeterminacy problem and the problem of interpreting quantum probabilities.
A group of philosophers and physicists at Oxford have recently crafted, in light of developments in the theory of decoherence, a new version of the Everett or Many Worlds interpretation of quantum mechanics. I will focus on the work of David Wallace, who identifies the quasi-classical domains making up the various “worlds” or “branches” of the interpretation with macroscopic patterns that are functionally realized (to an approximate degree good enough for practical purposes) by the quantum wavefunction of the universe. I argue that Wallace’s reliance on Savage’s pragmatic account of probability fails, due to the unreasonableness of Savage’s Sure Thing Principle in a many-worlds setting. I also argue that Wallace’s program is vulnerable to a version of Putnam’s model-theoretic indeterminacy argument, with the result that almost any conceivable macroscopic “world” is in fact realized by that function and so is (by Wallace’s light) as real as our own. I offer a hylomorphic revision of the many-worlds model, the “traveling forms” interpretation of Alexander Pruss, and I argue that this revision solves both the indeterminacy problem and the problem of interpreting quantum probabilities.
The hidden variable theories are, more cumbersome and artificial—while the Copenhagen interpretation is hopelessly incomplete because of its a priori reliance on classical physics, as well as a philosophic monstrosity with a “reality” concept for the macroscopic world and denial of the same for microcosm. In recent work on the Everett (Many-Worlds) interpretation of quantum mechanics, it has increasingly been recognized that any version of the interpretation worth defending will be one in which the basic formalism of quantum mechanics is left unchanged. Properties such as the interpretation of wave-function as describing a multiverse of branching worlds, or the ascription of probabilities to the branching events, must be emergent from the unitary quantum mechanics rather than added explicitly to the mathematics. Deutsch’s strategy was to introduce a few assumptions from decision theory into the Everett interpretation and then to show how they work with quantum theory to create a natural value measure that is the same as the expectation value.
This thesis takes a look at the many-worlds interpretation (MWI) and discusses if it can become the leading interpretation of quantum mechanics. Firstly the Copenhagen interpretation is outlined and its issues relating to the measurement problem are discussed while taking recent insights in decoherence theory into account. Establishing that wave-function collapse still yields interpretational issues, I discuss the ideas of Hugh Everett, who proposed the many-worlds interpretation, and David Wallace who is a prominent advocate of the MWI. In particular I discuss and review Wallace's arguments in favor of viewing the wave-function and the many worlds realistically. The next part shortly focuses on the testability of the many-worlds interpretation and the philosophical and theoretical issues relating to probability in the MWI. I end by concluding that the no-collapse interpretations are preferable to the Copenhagen interpretation, and I argue in favor of a realistic view towards all of the worlds. Furthermore, I conclude that standard textbooks on quantum mechanics should be updated to introduce students to the insight that wave-function collapse isn't needed in forming a complete theory of quantum mechanics.
Recent years have seen a good deal of work done on one of the most mind blowing and conceptually radical views in quantum mechanics, namely the ‘many worlds’ interpretation of quantum mechanics. Many worlds was first proposed by Hugh Everett III in his long Ph.D. dissertation, which only some years later, and after significant shortening, came to be published as the ‘Relative State Formulation of Quantum Mechanics’ (Everett 1957). To commemorate the 50th anniversary of the first publication of Everett's ideas, two conferences were held at Oxford University and the Perimeter Institute, which brought together some of the most active researchers in the field. The works presented in these two conferences have now been compiled in the volume Many Worlds? Everett, Quantum Theory and Reality, edited by Simon Saunders, Jonathan Barrett, Adrian Kent and David Wallace.1 This is then a comprehensive collection of papers – 23 papers altogether – where the reader will find virtually anything that can be said about the many worlds interpretation today.
Abstract What does realism about the quantum state imply? What follows when quantum theory is applied without restriction, if need be, to the whole universe? These are the questions which are debated in this book. All the chapters start from the point of realism, and based on the need, or the aspiration, for a theory that unites micro- and macroworlds, at least in principle. But the further claim argued by some is that if you allow the Schrödinger equation unrestricted application, supposing the quantum state to be something physically real, then this universe is one of countlessly many others, constantly branching in time, all of which are real. The result is the many worlds theory, also known as the Everett interpretation of quantum mechanics. The contrary claim sees this picture of many worlds as in no sense inherent in quantum mechanics, even when the latter is allowed unrestricted scope and even given that the quantum state itself is something physically real. For this picture of branching worlds fails to make physical sense, let alone common sense, even on its own terms. The status of these worlds, what they are made of, is never adequately explained. Ordinary ideas about time and identity over time become hopelessly compromised. The concept of probability itself is brought into question. This picture of many branching worlds is inchoate, it is a vision, an error. There are realist alternatives to many worlds, some even that preserve the Schrödinger equation unchanged. Twenty chapters, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They focus first on the question of ontology, the existence of worlds, second on the interpretation of probability, and third on alternatives or additions to many worlds. The introduction offers a helpful guide to the arguments for the Everett interpretation, particularly as they have been formulated in the last two decades.
Abstract Recent accounts of probability in the many worlds interpretation of quantum mechanics are vulnerable due to their dependence on probability theory per se. For this reason, the many worlds interpretation continues to suffer from the incoherence and quantitative problems. After discussing various theories of probability, I discuss the incoherence problem and argue that self-locating probabilities centered in time-extended worlds can solve it. I then discuss and refute various solutions to the quantitative problem. I argue that the only tenable way to ground these self-locating probabilities is to identify the mathematical form of the Born rule as a generic pattern in a time-extended wavefunction, and to distribute degrees of belief over the region of wavefunction occupied by this pattern. I then outline a time-symmetric version of quantum mechanics - the Fixed Point Formulation - which, interpreted within a time-symmetric Everettian framework, can provide the foundation for a theory of quantum probability.
David Wallace's ``The Emergent Multiverse" is the most recent complete presentation of\na version of Everett's theory of quantum mechanics that has attracted much scientific\nactivity in the past decade. I present a brief sketch of Wallace's solution to the measurement\nproblem, arguing that the many-worlds interpretation is not as far-fetched as it is\noften conceived to be. Taking the wavefunction as the fundamental ontology, it claims\nto solve the measurement problem by recognizing certain (quasi-classical) patterns of\nthe wavefunction in 3N-dimensional configuration space that functionally behave like\nthe (classical) configuration-space pattern of a N-particle system described by classical\nmechanics. In this sense, structures within the universal wavefunction are identified\nwith classical `worlds' at the coarse-grained level. I highlight two elements of this work:\nthe role of emergence and the functionalist framework that Wallace imports through\nwhat he calls `Dennett's criterion'. This criterion appeals to the virtue of usefulness\n(in the form of predictability and explanatory power) as a criterion for the reality of\n`patterns'. \n\nIt is shown that, due to decoherence, quasi-classical worlds emerge weakly, similar\nto that of the emergence of thermodynamic temperature from statistical physics, in the\nsense that they are autonomous and unexpected with respect to the lower-level domain\n(as opposed to strong emergence of some insuper et supra high-level ontology). However,\nthe use of Dennett's criterion obscures this result, laying bare some philosophical issues,\nwhich we address over three axes of distinctions: (i) the objective/subjective-axis, (ii)\nthe quantitative/qualitative-axis within the framework of intertheoretic reduction and\n(iii) the ontological/epistemological-axis.\n\nFirst, Daniel Dennett's `real patterns' are compared to Wallace approach to patterns.\nThen, I point out (i) an analogy with Bas van Fraassen's idea of causal patterns\nthat become salient due to pragmatic explanation, namely that in the context of a pragmatic\ngoal the quasi-classical pattern is made salient over other objectively existing,\nnon-classical, patterns. I conclude that in the absence of human goals there is no reason\nto regard the quasi-classical pattern as `more real' than other patterns. In analogy to\nDennett's `intentional stance', Wallace is committed to a `classical stance', equivalent\nto breaking Hilbert space democracy of bases. \n\nAlthough Wallace's version is a weaker one, his (ii) relations between theories\nbare resemblance to the reductionist program, and I argue that, next to quantitative\ndeduction, additional conceptual `bridge principles' à la Ernest Nagel are needed.\nThe appeal to usefulness as a criterion for reality (iii), I claim, is not necessary\nto solve the measurement problem itself, but has the further (unwarranted) goal of\nestablishing `real' worlds. I spell out a solution to the measurement problem, the\nmany-minds theory, which solves the problem along the same lines as The Emergent\nMultiverse, with the exception that the - although the quantum world itself is real\n- the classical worlds with definite properties are beliefs in the superposed brains of\nobservers.