Everettian quantum mechanics is often said to lack coherent probability because every possible measurement result occurs. If all of them exist globally, what is probability supposed to be a probability of? This paper defends an axiomatic answer: normalized Born measure is postulated as objective probability over the records borne by an observer's future continuations. A collapse law assigns the same probabilities to possible global outcomes. Neither theory derives the probabilities from its ontology. I relate this minimal proposal to earlier axiomatic accounts, separate it from their accompanying theories of personal persistence, and defend it against recent arguments that the axiomatic route cannot succeed. At the level of recorded results, the Everettian law supports the same predictions and empirical inferences as the collapse law, despite the theories' different ontologies.
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14 pages. v3: added references to Ismael's work on Everettian chance and first-person thought; streamlined the discussion of personal persistence
Everettian quantum mechanics is often said to lack coherent probability because every possible measurement result occurs. If all of them exist globally, what is probability supposed to be a probability of? This paper defends an axiomatic answer: normalized Born measure is postulated as objective probability over the records borne by an observer's future continuations. A collapse law assigns the same probabilities to possible global outcomes. Neither theory derives the probabilities from its ontology. I relate this minimal proposal to earlier axiomatic accounts, separate it from their accompanying theories of personal persistence, and defend it against recent arguments that the axiomatic route cannot succeed. At the level of recorded results, the Everettian law supports the same predictions and empirical inferences as the collapse law, despite the theories' different ontologies.
Ultimately, any explanation of quantum measurement must be extendable to relativistic quantum mechanics (RQM), since many precisely confirmed experimental results follow from quantum field theory (QFT), which is based on RQM. Certainly, the traditional "collapse" postulate for quantum measurement is problematic in a relativistic context, at the very least because, as usually formulated, it violates the relativity of simultaneity. The present paper addresses this with a relativistic model of measurement in which the state of the universe is decomposed into decoherent histories of measurements recorded within it. The approach is essentially Everettian, in the sense that it uses the unmodified, unitary quantum formalism of RQM. But it addresses the difficulty with typical "many worlds" interpretations on how to even define probabilities over different possible ``worlds''. To do this, Zurek's concept of envariance is generalized to the context of relativistic spacetime, giving an objective definition of the probability of any one of the quantum histories, consistent with Born's rule. It is then shown that the statistics of any repeated experiment within the universe also tend to follow the Born rule as the number of repetitions increases. The wave functions that we actually use for such experiments are local reductions of very coarse-grained superpositions of universal eigenstates, and their "collapse" can be re-interpreted as simply an update based on additional incremental knowledge gained from a measurement about the "real" eigenstate of our universe.
Everett's interpretation is best known under the name "many worlds", a name Everett never used in his published writings and which has concentrated the criticism directed at it: how many worlds are there? when do they split? what can probability mean if everything happens? We argue that these questions target the popularization of the theory rather than the theory itself, and that the last of them can now be retired on a precise ground. The probability problem divides into a quantitative question (why the Born weights?) and a question of meaning (what is there to be uncertain about?). The quantitative question is closed by a theorem, T*: the Born rule is the unique weighting of alternatives that is independent of the chosen grain of description, convergent among observers sharing the same records, and stable under branching. T* is established in a companion article by reduction to two published theorems, Gleason (1957) and Short (2023); non-contextuality, the traditionally contested premise of Gleason-type arguments, is derived there from grain coherence rather than postulated; convergence turns out to be a corollary; and the central theorem is verified end to end in Lean 4 with Mathlib, in full generality, with Gleason's theorem as the single declared axiom. The question of meaning is answered by internality: an observer who is part of the state is, by theorem, ignorant of their own location in it, and self-locates on accessible records. What then remains is one posited principle, the bridge between an objective measure and rational expectation, and we show step by step that it is the same principle a deterministic coin already exacts from classical physics. Everett's interpretation, properly formulated, therefore has no special problem of probability: it carries the normative residue that all probabilistic physics carries, and one postulate fewer, since the Born measure is derived and nothing collapses.
Quantum mechanics lacks a physical definition of measurement, observer, probability, and experienced time. These notions are instead treated as primitive or interpretation-dependent, leading to persistent foundational problems including the measurement problem, the origin of the Born rule, observer disagreement in Wigner’s Friend scenarios, the arrow of time, and apparent information loss in black hole evaporation. In this work, we show that these problems share a single missing ingredient: the absence of an operational definition of observers as bounded physical systems. We define observers as physically realizable systems capable of forming irreversible internal records under unitary dynamics. Measurement is identified with record formation, probability with record redundancy, experienced time with record accumulation, and irreversibility with computational inaccessibility rather than non-unitarity. Using decoherence and quantum Darwinism, we show that the number of accessible, decoherence-stable records associated with an outcome scales with the squared amplitude, yielding the Born rule from record counting without invoking collapse, subjective probability, or additional axioms. Observer-relative paradoxes dissolve once outcome definiteness is indexed to record formation rather than to the global quantum state. Reconstruction of global quantum information is shown to require exponential resources, rendering measurement and black hole evaporation operationally irreversible despite global unitarity. The resulting framework closes quantum mechanics conceptually by eliminating undefined primitives, without modifying its mathematical structure or introducing new dynamical postulates.
The Born rule assigns probabilities to quantum measurement outcomes using only the local quantum amplitudes immediately before measurement. Yet the physical consequences of those outcomes may differ dramatically in their subsequent evolution, producing spacetime histories with very different entropies, information content, computational complexity, entanglement structure, and ultimately even spacetime curvature. These global properties appear to play no role in determining quantum probabilities. This paper argues that, viewed through the relativistic block-universe picture, this separation between local probability and global physical structure raises a natural conceptual question. The Many-Worlds Interpretation (MWI) preserves the complete branching structure of these histories, whereas collapse interpretations retain only a single realized history. The purpose of this paper is not to challenge the Born rule, but to highlight a conceptual asymmetry that may deserve further investigation.
Objective probability in quantum mechanics is often thought to involve a stochastic process whereby an actual future is selected from a range of possibilities. Everett's seminal idea is that all possible definite futures on the pointer basis exist as components of a macroscopic linear superposition. I demonstrate that these two conceptions of what is involved in quantum processes are linked by two alternative interpretation of the mind-body relation. This leads to a fission, rather than divergence, interpretation of Everettian theory and to a novel explanation of why a principle of indifference does not apply to self-location uncertainty for a post-measurement, pre-observation subject, just as Sebens and Carroll claim. Their Epistemic Separability Principle is shown to arise out of this explanation and the derivation of the Born rule for Everettian theory is thereby put on a firmer footing.