Interpreting Quantum Theory

A Therapeutic Approach

S. Friederich

• Publisher: Palgrave Macmillan
• Year: 2014
• ISBN:
  9781137447159
• Topic: Philosophie
• Subtopic: Philosophie & Ethik in der Wissenschaft

Is it possible to approach quantum theory in a 'therapeutic' vein that sees its foundational problems as arising from mistaken conceptual presuppositions? The book explores the prospects for this project and, in doing so, discusses such fascinating issues as the nature of quantum states, explanation in quantum theory, and 'quantum non-locality'.

Part I

Introduction and Background

1

Introduction

1.1 Quantum foundations

Quantum theory is perhaps the theory with the greatest predictive and explanatory success in all the history of physics. To name just a few of its countless achievements: it explains the stability of the stable atoms and nuclei and predicts the decay rates of the unstable ones; it accounts for the different manifestations of matter such as gaseous, liquid and solid, metallic and insulating, magnetic, superfluid and superconducting; it forms the basis of our chemical knowledge; and it provides the conceptual framework of all contemporary models for the fundamental constituents of matter. In addition, some of its predictions, for example that of the ‘electron spin g-factor’ in quantum electrodynamics, are perhaps the most accurate ones ever made in the history of science. Some of its predictions concern matter at extremely high energy and interactions between bits of matter at extremely short distances; others are about matter at extremely low energy and close to the zero of absolute temperature. As far as its technological applications are concerned, its insights are at the heart of the overwhelming technological progress in information technology in the past few decades, it explains why nuclear fission and fusion work as they do, and it forms the basis of the rapidly growing field of nanotechnology.

However, all of quantum theory’s stupendous predictive and explanatory achievements notwithstanding, the debates about its foundations are more hotly contested than ever. In fact, the majority view among philosophers of physics, accurately summarised by David Wallace, is that quantum theory ‘is the great scandal of physics; despite [its] amazing successes, we have no satisfactory physical theory at all – only an ill-defined heuristic which makes unacceptable reference to primitives such as “measurement”, “observer” and even “consciousness”’ (Wallace [2008], p. 16). Most philosophers of physics would agree that quantum theory either needs to be modified and replaced by a different theoretical framework, or that it requires an interpretation which leads to a picture of the world that is radically at odds with our everyday views. The most prominent examples of theories designed to replace quantum theory are pilot wave theory (‘Bohmian mechanics’) and GRW theory. The most famous speculative interpretations, which lead to a radical breaking with our everyday views, are the variants of the Everett interpretation, including the so-called many-worlds and many-minds interpretations. Their core idea is that our world is (or our minds are) constantly subject to ubiquitous branching processes, and that what we take to be the actual world (or our actual mind) is just one among countless many world (or mind) branches.

A few decades ago, the debate about the foundations of quantum theory was still shaped by widespread commitment to the so-called Copenhagen interpretation. Whatever exactly this interpretation actually says, one of its few clear-cut tenets (perhaps the only one) is that quantum theory is complete as an internally sound physical theory and requires neither modification nor re-interpretation in speculative terms. Almost all philosophers of physics would now reject this view.¹ Their main reason for regarding the Copenhagen interpretation as untenable (whatever exactly it says) can be summarised roughly as follows:

The Copenhagen interpretation must either embrace the ‘collapse of the wave function’ as one of quantum theory’s crucial constituents (for introductory comments on the collapse of the wave function see Section 2.2) or reject it.² If it rejects collapse, the Copenhagen interpretation cannot account for the manifest – and empirically trivial – fact that measurement processes always result in determinate outcomes. It is then empirically inadequate in the most blatant way possible. If, in contrast, the Copenhagen interpretation accepts collapse, it must specify under which conditions collapses occur, but without resorting to such vague, ambiguous and anthropocentric notions as ‘measurement’ and ‘observer’. These are inadequate in the context of supposedly fundamental physical theories and may therefore not be used to specify the criteria for the occurrence of collapse. Since neither Bohr nor Heisenberg nor Pauli nor Dirac nor any other supposed adherent of the Copenhagen interpretation seems to have proposed a coherent alternative to this fatal dilemma, it follows that the Copenhagen interpretation – whatever it actually says – is deeply unsatisfying; or so contemporary philosophers of physics essentially seem to agree.

The difficulty of accounting for determinate measurement outcomes in quantum theory is an aspect of the famous measurement problem. Almost all suggested modifications and interpretations of quantum theory can be seen as attempts to solve the measurement problem. They are naturally categorised (as I will show in Chapter 3) according to how they react to this problem.

The measurement problem is not the only substantial foundational challenge in quantum theory. Another arises from what is widely known as ‘quantum non-locality’: the fact that quantum theory predicts correlations between measurement outcomes which obtain independently of the spatio-temporal distance between the outcomes and which it does not explain in terms of common causes. Bell’s theorem (for a sketch see Appendix A) shows that these correlations are incompatible with theories that respect a criterion that Bell takes to incorporate the idea of ‘local causality’: that causal influences cannot travel faster than light and probabilities depend only on what occurs in regions from which influences at velocities no larger than the velocity of light can arrive. Superluminal influences are widely regarded as problematic in the relativistic context, mainly due to the fact that they are backwards in time in some inertial frames.

Thus, in view of Bell’s result, quantum theory and relativity theory seem to clash at least in spirit (if not in letter) in a very profound and seemingly unavoidable way. Since the correlations predicted by quantum theory are experimentally well-confirmed, the pressure, according to many authors, is on relativity theory here. Some authors go as far as considering the revival of a space-time framework which contains a universally privileged inertial rest frame in terms of which an absolute simultaneity relation among space-time events would be defined.³ If this conclusion were the right one to draw, one of the main lessons of relativity theory – that all inertial frames are on an equal conceptual footing – would have to be unlearned, and the symmetries that are regarded as principled in relativity theory would be downgraded to merely apparent and emerging rather than fundamental.

In conclusion, it appears that reflections on the foundations of quantum theory have the potential to undermine our confidence not only in quantum theory itself, but also in relativity theory. So, these reflections shake our faith in the two main elements of our contemporary understanding of physics without providing any clear ideas as to what should replace them. Is there any way to avoid this devastating result?

1.2 The idea of a therapeutic approach

The aim of the present work is to probe a specific way of answering this question with a ‘yes, it can be avoided’. The hypothesis that I wish to explore is that the foundational problems require neither revising quantum theory or relativity theory nor any extravagant metaphysics. The idea which motivates this investigation is that both the measurement problem and the problem of quantum non-locality are mere artefacts of conceptual confusions that disappear once the actual role of the elements of the quantum theoretical formalism in its applications is properly taken into account. I refer to this type of approach as ‘therapeutic’, as it tries to ‘cure’ us from what it takes to be unfounded worries that arise from conceptual confusion.

The idea of philosophy as a form of intellectual therapy has a long tradition (one may trace it back to Epicurus in antiquity). In the more recent history of philosophy it is advocated in particular by the later Wittgenstein. The therapeutic approach to philosophical problems is a core element of the radically innovative conception of philosophy he develops and puts to work in his Philosophical Investigations (Wittgenstein [1958]). Wittgenstein’s suggested strategy for dissolving philosophical problems is to reflect on the actual context and mode of use of the concepts involved, i.e. their roles in our ‘language games’, and to investigate whether, in order to formulate the problems, they are perhaps used in ways in which they cease to make good sense. Typically, the Wittgensteinian diagnosis is that the source of a perceived philosophical problem is that one had naively – and mistakenly, as it turns out – assumed that the concepts involved in its formulation are used descriptively (or, more specifically, as more or less directly representing features of reality), which in fact they are not.⁴ Examples of Wittgensteinian therapeutic analyses include his account of mental concepts such as ...

Table of contents

• Cover
• Title
• Part I  Introduction and Background
• Part II  The Rule Perspective
• Part III  Objections
• Part IV  Non-locality, Quantum Field Theory, and Reality
• Appendices
• Notes
• Bibliography
• Index