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Finding temporal asymmetry within the many, many worlds of Everettian quantum mechanics

Richard Corry

Published Jul 25, 2025• 0 citations
Publisher

Abstract

Abstract Our experience of the world is temporally asymmetric: coffee spontaneously cools, glasses smash, people age and die, while the reverse processes never occur. However, our best theories of fundamental physics are temporally symmetric, making no distinction between past and future directions. This creates a puzzle: how can our asymmetric experiences emerge from a symmetric foundation? The usual solution is to posit a special initial condition for the universe. This solution, however, amounts to putting in the asymmetry by hand, and one may be left looking for a further explanation of this asymmetric boundary condition. In this paper I explore another possible solution, namely that the asymmetry of experience can be explained anthropically, via an observer selection effect. In particular, I use the formalism of decoherent histories to argue that such an explanation is available in the Everettian, or “many worlds”, interpretation of quantum mechanics. Along the way, we will discover that Everettian quantum mechanics implies the existence of many more worlds than are traditionally recognised.

DOI
10.1007/s11229-025-05166-5

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Abstract Our experience of the world is temporally asymmetric: coffee spontaneously cools, glasses smash, people age and die, while the reverse processes never occur. However, our best theories of fundamental physics are temporally symmetric, making no distinction between past and future directions. This creates a puzzle: how can our asymmetric experiences emerge from a symmetric foundation? The usual solution is to posit a special initial condition for the universe. This solution, however, amounts to putting in the asymmetry by hand, and one may be left looking for a further explanation of this asymmetric boundary condition. In this paper I explore another possible solution, namely that the asymmetry of experience can be explained anthropically, via an observer selection effect. In particular, I use the formalism of decoherent histories to argue that such an explanation is available in the Everettian, or “many worlds”, interpretation of quantum mechanics. Along the way, we will discover that Everettian quantum mechanics implies the existence of many more worlds than are traditionally recognised.
abstract: This thesis attempts to explain Everettian quantum mechanics from the ground up, such that those with little to no experience in quantum physics can understand it. First, we introduce the history of quantum theory, and some concepts that make up the framework of quantum physics. Through these concepts, we reveal why interpretations are necessary to map the quantum world onto our classical world. We then introduce the Copenhagen interpretation, and how many-worlds differs from it. From there, we dive into the concepts of entanglement and decoherence, explaining how worlds branch in an Everettian universe, and how an Everettian universe can appear as our classical observed world. From there, we attempt to answer common questions about many-worlds and discuss whether there are philosophical ramifications to believing such a theory. Finally, we look at whether the many-worlds interpretation can be proven, and why one might choose to believe it. (abstract)
The Heisenberg uncertainty principle is conventionally interpreted as a constraint on the precision of future measurement outcomes. We argue that this temporal asymmetry is not justified by the underlying mathematics of quantum mechanics, which is fundamentally time-symmetric. We propose that uncertainty applies equally in both temporal directions: the past has no greater ontological determinacy than the future. Between collapse events, prior states were no more determined than future states are now. Only the present moment of collapse constitutes anything approaching a definite physical state, and that moment is immediately receding into a past that was never fully determined before collapse occurred. We formalize this interpretation through a timesymmetric reading of the existing uncertainty relations and discuss its implications for the ontological status of the past, the measurement problem, and the foundations of physical determinism.
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.
Abstract This paper shall explore the conjunction of eternalism and Everettian quantum mechanics. It shall be argued that there is a strong analogy between these two views. In case there is an indefinite number of worlds and observers that are all equally real, there should be an indefinite number of local times which are all also equally real. Whereas Everettianism, specifically the diverging version, treats actuality indexically, relativistic eternalism treats tense indexically. All times exist analogously to all isolated Everettian worlds. There is no unique ‘now’ that cuts throughout all that physically exists. Instead, as eternalism propounds, all times exist. The paper concludes that eternalism and the many-worlds interpretation are not only compatible but complement each other, providing a coherent framework for understanding the nature of temporal reality.
We argue that the many worlds of quantum mechanics and the many worlds of the multiverse are the same thing, and that the multiverse is necessary to give exact operational meaning to probabilistic predictions from quantum mechanics. Decoherence---the modern version of wave-function collapse---is subjective in that it depends on the choice of a set of unmonitored degrees of freedom, the environment. In fact decoherence is absent in the complete description of any region larger than the future light cone of a measurement event. However, if one restricts to the causal diamond---the largest region that can be causally probed---then the boundary of the diamond acts as a one-way membrane and thus provides a preferred choice of environment. We argue that the global multiverse is a representation of the many worlds (all possible decoherent causal diamond histories) in a single geometry. We propose that it must be possible in principle to verify quantum-mechanical predictions exactly. This requires not only the existence of exact observables but two additional postulates: a single observer within the Universe can access infinitely many identical experiments; and the outcome of each experiment must be completely definite. In causal diamonds with a finite surface area, holographic entropy bounds imply that no exact observables exist, and both postulates fail: experiments cannot be repeated infinitely many times; and decoherence is not completely irreversible, so outcomes are not definite. We argue that our postulates can be satisfied in hats (supersymmetric multiverse regions with vanishing cosmological constant). We propose a complementarity principle that relates the approximate observables associated with finite causal diamonds to exact observables in the hat.
Hugh Everett III was an American physicist best known for his many-worlds interpretation of quantum mechanics, which formed the basis of his PhD thesis at Princeton University in 1957. Although counterintuitive, Everett's revolutionary formulation of quantum mechanics offers the most direct solution to the infamous quantum measurement problem--that is, how and why the singular world of our experience emerges from the multiplicities of alternatives available in the quantum world. The many-worlds interpretation postulates the existence of multiple universes. Whenever a measurement-like interaction occurs, the universe branches into relative states, one for each possible outcome of the measurement, and the world in which we find ourselves is but one of these many, but equally real, possibilities. Everett's challenge to the orthodox interpretation of quantum mechanics was met with scorn from Niels Bohr and other leading physicists, and Everett subsequently abandoned academia to conduct military operations research. Today, however, Everett's formulation of quantum mechanics is widely recognized as one of the most controversial but promising physical theories of the last century. In this book, Jeffrey Barrett and Peter Byrne present the long and short versions of Everett's thesis along with a collection of his explanatory writings and correspondence. These primary source documents, many of them newly discovered and most unpublished until now, reveal how Everett's thinking evolved from his days as a graduate student to his untimely death in 1982. This definitive volume also features Barrett and Byrne's introductory essays, notes, and commentary that put Everett's extraordinary theory into historical and scientific perspective and discuss the puzzles that still remain.