The Everett interpretation of quantum mechanics divides naturally into two parts: first, the interpretation of the structure of the quantum state, in terms of branching, and second, the interpretation of this branching structure in terms of probability. This is the second of two reviews of the Everett interpretation, and focuses on probability. Branching processes are identified as chance processes, and the squares of branch amplitudes are chances. Since branching is emergent, physical probability is emergent as well.

Foundational attitudes towards quantum theory have recently thrown off much of the old philosophical baggage largely associated with Niels Bohr to which Einstein famously objected, including the central ‘collapse of the wavefunction’ concept. A ‘neo-Copenhagen’ interpretation, it is suggested, has arisen. This development is placed in its historical context and contrasted to philosophical allegations of anti-realism. The neo-Copenhagen interpretation remains wedded to Heisenberg's uncertainty and observer-dependent values of particles. However a discussion of Nick Herbert's ‘rainbow analogy’ suggests that subatomic particles (...) are emergent from a ‘nonlocal’ level of reality outside the domain of space and time. Critical realism recognizes that emergent systems have an irreducibly ontologically character, and in its combination of epistemological relativism and ontological realism, provides a basis for the proposition that realists who support Einstein's objections must now recognize that their realism must be redefined. (shrink)

The Ollivier–Poulin–Zurek definition of objectivity provides a philosophical basis for the environment as witness formulation of decoherence theory and hence for quantum Darwinism. It is shown that no account of the reference of the key terms in this definition can be given that does not render the definition inapplicable within quantum theory. It is argued that this is not the fault of the language used, but of the assumption that the laws of physics are independent of Hilbert-space decomposition. All evidence (...) suggests that this latter assumption is true. If it is, decoherence cannot explain the emergence of classicality. (shrink)

The multiverse hypothesis is the leading alternative to the competing fine-tuning hypothesis. The multiverse dispels many aspects of the fine-tuning argument by suggesting that there are different initial conditions in each universe, varying constants of physics, and the laws of nature lose their known arbitrary values; thus, making the previous single-universe argument from fine- tuning incredibly weak. The position that will be advocated will be that a form of multiverse could exist and that any level of Tegmark's multiverse does not (...) circumvent the fine-tuning argument for the existence of a fine-tuner. An argument will be presented suggesting that the multiverse could strengthen the fine-tuning argument by narrowing down the parameters of the data to a more fundamental description needed for the multiverse to function as it does since it may be the case that some physical descriptions are not the same in every universe. That is, the task of explaining how the multiverse mechanism functions cannot be accounted for by the multiverse itself. Fine-tuning by a mind serves as a sufficient and best explanation for the data. (shrink)

The nature and topology of time remain an open question in philosophy. Both tensed and tenseless concepts of time appear to have merit. Quantum mechanics demonstrates that both are instantiated, as complementary aspects of the time evolution of physical systems. The linear dynamics is tenseless. It defines the universe probabilistically throughout space-time, and can be seen as the definition of an unchanging block universe. All properties of the universe are defined for the whole extent of the linear time dimension of (...) space-time. The collapse dynamics is the change to the linear dynamics: time evolution of the physical system in the quantum concept of time. This is tensed: different definitions of the probabilistic definition of the space-time universe exist from moment to moment in the quantum concept of time. Thus the linear time dimension of space-time is tenseless, while time in the quantum concept of time is tensed. Exercise of the linear dynamics is experienced as the passage of time, while the exercise of the collapse dynamics is experienced as change. (shrink)

As shown in The Quantum Mechanical Frame of Reference, Everett's formulation inherently defines idiosyncratic effective physical environments for each version of the functional identity of the observer, defined solely by observations, in the manner of Rovelli's Relational Quantum Mechanics. This accounts for determinate measurement records, and completes his resolution of the measurement problem. The remaining task is to make everyday sense of Everett's concept. He defines the functional identity as the record of sensory observations and machine configuration, which seems merely (...) an epiphenomenon of the body-mind of the observer. This functional identity is intensely familiar to each observer. It is simply the subjective reality, the known world, here the 'world hologram'. In Everett's formulation, the cut in the von Neumann chain is implicitly made at the level of conscious sensory awareness. Thus, not only is the effective physical environment defined solely by the record of observations, the self-identity synthesised from observations defines the sole determinacy of the body-mind of the observer. Everett's formulation is not addressing obscure issues of brain state, but the determinant of the effective physical reality of the observer, including the body-mind. Although highly counter-intuitive, it is nonetheless empirically coherent. On the 'inside view', as defined by Tegmark, Everett's relative state corresponds, directly and precisely, to human experience. (shrink)

The common practice of advancing arguments based on current physics in support of metaphysical conclusions has been criticized on the grounds that current physics may well be wrong. A further criticism is leveled here: current physics itself depends on metaphysical assumptions, so arguing from current physics is in fact arguing from yet more metaphysics. It is shown that the metaphysical assumptions underlying current physics are often deeply embedded in the formalism in which theories are presented, and hence impossible to dismiss (...) as mere motivational or interpretative speculation. It is then shown that such assumptions, when made explicit, can wreck havoc on otherwise-sensible philosophical arguments. It is argued in conclusion that this situation is both unlikely to be reparable just by being more careful, and unlikely to go away as further, presumably more subtle physical theories are developed. (shrink)

According to the Everett interpretation, branching structure and ratios of norms of branch amplitudes are the objective correlates of chance events and chances; that is, 'chance' and 'chancing', like 'red' and 'colour', pick out objective features of reality, albeit not what they seemed. Once properly identified, questions about how and in what sense chances can be observed can be treated as straightforward dynamical questions. On that basis, given the unitary dynamics of quantum theory, it follows that relative and never absolute (...) chances can be observed; that only on repetition of a large numbers of similar trials can relative probabilities be measured; and so on. The epistemology of objective chances can in this way be worked out from the dynamics. its curious features are thus explained. But one aspect of chance set-ups seems to resist this subsuming of chancing to branching: how is it that chance involves uncertainty? And if that is not possible, on Everettian lines, then the whole project is doomed. I argue that in fact there is no difficulty in making sense of uncertainty in the face of branching. Contrary to initial impressions, the unitary formalism is consistent with a well-defined notion of self-locating uncertainty. It is also consistent without: the mathematics under-determines the metaphysics in these respects. (shrink)