Indeterminacy

In Everettian quantum mechanics, the universal quantum state is fundamental, non-contingent, and wholly determinate. By contrast, the parallel worlds of diverging EQM, and the contingency constituted by self-location amongst those worlds, are emergent and partly indeterminate. In particular, it is indeterminate both how many worlds there are, and what microscopic qualitative features those worlds have. This chapter discusses various ways to understand indeterminacy in the Everettian multiverse, and argues that the indeterminacies of EQM present no obstacle to the analytic ambitions of quantum modal realism. Everettians can understand quantum indeterminacy using models of indeterminacy that are familiar from the philosophical literature on vagueness.

Laws of Nature

One of the most attractive features of quantum modal realism is the unified and explanatory theory of laws of nature which it enables. Quantum modal realism incorporates a layered model of laws which share a common underlying structure; the model includes Fundamental laws of the multiverse, fundamental laws of Everett worlds, as well as a variety of types of non-fundamental laws of Everett worlds. This layered model of laws allows for a hitherto unexpected reconciliation of the Humean and anti-Humean conceptions of lawhood, and the model also enables powerful explanations both of why laws matter to us and of how we can come to know them. This chapter sets out the new quantum modal realist theory of laws, defends it from some objections, and draws out its most important consequences.

Analysing Modality

This chapter presents and defends the basic tenets of quantum modal realism. The first of these principles, Individualism, states that Everett worlds are metaphysically possible worlds. The converse of this principle, Generality, states that metaphysically possible worlds are Everett worlds. Combining Individualism and Generality yields Alignment, a conjecture about the nature of possible worlds that is closely analogous to Lewisian modal realism. Like Lewisian modal realism, Alignment entails that each possible world is a real concrete individual of the same basic kind as the actual world. These similarities render EQM suitable for grounding a novel theory of the nature of metaphysical modality with some unique properties. Also like Lewisian modal realism, quantum modal realism is a reductive theory: it accounts for modality in fundamentally non-modal terms. But quantum modal realism also has unique epistemological advantages over Lewisian modal realism and other extant realist approaches to modality.

Introduction: Explaining Contingency

The first part of this introduction sketches the main project of the book, and the structure of the arguments for my proposed quantum modal realism. The second part describes the unsatisfying present state of the metaphysics of modality, setting out what I take to be the most serious objections facing the best extant proposals. A naturalistic approach to metaphysics promises to resolve these objections by providing an account of modality that draws only on scientifically respectable theoretical resources. In the third part, I distinguish two big-picture approaches to the metaphysics of modality, and argue for the viability of an unfamiliar approach that takes the nature of contingency as the core phenomenon that a theory of modality needs to explain. In the fourth part, I explain my methodology and briefly defend the general project of naturalistic metaphysics.

Anthropic Contingency

Distinguish contingency in general from anthropic contingency. The former is what really could happen; the latter is what really could be observed to happen. Quantum histories which host no life cannot, as a matter of obvious necessity, be observed. This distinction generates an anthropic observation selection effect, which has been employed in response to the fine-tuning argument for the design hypothesis. This chapter argues that fine-tuning is a genuine phenomenon that cries out for explanation; that in one-world approaches to quantum theory a chancy determination of cosmological parameters would render the one universe we are in preposterously lucky; that no preposterous luck is required from the perspective of quantum modal realism; and that the correct interpretation of quantum mechanics turns out to have a significant evidential bearing on the design question.

Diverging Everettian Quantum Mechanics

This chapter sets out the version of quantum mechanics—diverging EQM—that constitutes the physics component of quantum modal realism. After a potted history of Everettian quantum mechanics, and an introduction to recent decoherence-based versions of the approach, I sketch a distinctive metaphysical understanding of Everettian quantum mechanics that involves diverging ‘parallel worlds’ instead of overlapping ‘splitting worlds’. I argue that the choice between divergence and overlap is underdetermined by the physics itself; I then adapt an argument of David Lewis’s that supports divergence over overlap, emphasizing the need to make room for objective probabilities in the Everettian picture. The chapter concludes by rejecting some unpersuasive motivations for overlap.

Conclusion

Quantum modal realism is a strange and unfamiliar doctrine. That is not by itself a reason to disbelieve it. Physical reality has continually turned out to be larger than we imagined possible; quantum modal realism expands reality one step further, beyond the actual world. In this short conclusion, I trace the history of human beliefs about the size of the cosmos, and compare the expansion of our horizons that results from quantum modal realism to previous revolutions in cosmology. I also observe that in some ways quantum modal realism is economical; while it may expand our conception of reality, it does not expand our conception of what is possible or of what is actual.

Emergent Chance

This chapter offers a theory of objective chance in the Everettian context, often seen as the main challenge facing EQM. By supplementing diverging EQM with quantum modal realist bridge principles connecting the physics of quantum mechanics with the metaphysics of modality, we obtain a package deal: Indexicalism. Indexicalist objective chance is an essentially self-locating phenomenon: chances are chances of self-location within the multiverse. I provide three arguments for Indexicalism: it establishes the right qualitative connections between chance and possibility, it establishes the right quantitative connection between chance and prediction, and it establishes the right epistemological story about how quantum mechanics is confirmed by empirical evidence. The resulting theory of chance is naturalistic and reductive; fundamental reality is deterministic, but chance arises at the non-fundamental level of Everett-worldbound perspectives. The theory provides unique resources for motivating an Everettian version of Lewis’s Principal Principle, helping to clarify at last the persistently mysterious connection between chance and rational credence.