The Multiverse Interpretation of Quantum Mechanics (1105.3796v3)

Abstract: 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 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.

• The paper proposes that the many‑worlds interpretation and the cosmological multiverse are equivalent, offering a new approach to resolving quantum probabilistic challenges.
• It introduces the causal diamond concept to objectively anchor decoherence within finite spacetime regions, mitigating observer-dependent ambiguities.
• The authors advance hat complementarity, suggesting that finite observables have precise counterparts in eternal inflation regions, enabling infinite repetitions of experiments.

An Expert Perspective on "The Multiverse Interpretation of Quantum Mechanics"

In "The Multiverse Interpretation of Quantum Mechanics," authors Raphael Bousso and Leonard Susskind conceptualize an innovative link between the many-worlds interpretation of quantum mechanics and the cosmological construct of the multiverse. Their core thesis posits that both are fundamentally the same, and this synthesis provides a groundwork for resolving challenges in assigning operational meaning to probabilistic quantum predictions.

The paper critiques the conventional dichotomy of wave-function collapse versus decoherence. It highlights decoherence's subjective nature due to its dependency on unmonitored environmental degrees of freedom, which is context-dependent, typically defined by observer capability. Notably, decoherence is absent across large regions—anywhere larger than the future light-cone of a measurement event. This introduces complications in a cosmological model where predictions must be exact.

Bousso and Susskind propose the notion of a causal diamond, a finite spacetime region bound by an observer's causal limits, to introduce objectivity into decoherence. Within this framework, they attribute decoherence not to some external entity but to the boundary of the causal diamond itself, creating a self-contained unit for analysis without appealing to external infinities.

The paper ventures into an egalitarian view of quantum mechanics where the multiverse represents a unitary assembly of many-worlds, each an interplay of decoherent histories realized within causal diamonds. This thesis challenges the traditional unitary description of the global multiverse, suggesting instead a mosaic of decoherent causal histories—each history a manifestation of distinct causal diamond events. Here, a discrete, global spacetime emerges from juxtaposing these histories, recognized in steps of light-cone time which, in turn, correlates with causal patch measures.

A pivotal assertion in the discussion is that true tests of quantum mechanical predictions require more than convergence to classical probabilities. They necessitate infinite repetition of experiments and definite outcomes. The authors identify these qualitative features with the hypothetical "hat" regions of eternal inflation, characterized by supersymmetric conditions and vanishing cosmological constant, enabling infinite event repetition and unbounded entropy. The hat region permits exact observables and satisfies their proposed postulates, ensuring that experiments can occur an infinite number of times while outcomes entangle irreversibly with environments extending to infinity.

Moreover, the authors advance a hat complementarity conjecture, drawing parallels to black hole complementarity, suggesting that approximate observables in finite causal diamonds have precise counterparts in the hat region. This notion highlights how the information in finite diamonds may be replicated precisely and redundantly in hats, akin to the redundancy across multiple causal diamonds.

The paper's fundamental challenge to the global multiverse interpretation has robust implications for theoretical physics and cosmology. It delineates a cosmological picture that arguably better accommodates the probabilistic assertions of quantum mechanics within classical cosmology's stochastic framework. Future developments in AI and quantum computing might see these ideas furnish new approaches to algorithmic problem solving, leveraging the conceptual linking of probability, decoherence, and the ultimate computational efficiency across hypothetical multiversal states.

In summary, Bousso and Susskind deliver a compelling fusion of many-worlds quantum mechanics and multiverse cosmology, offering resolutions to outstanding questions in quantum interpretative frameworks. Their work motivates rigorous exploration into hat complementarity while challenging the community to reconcile quantum mechanics' inherent probabilistic nature with cosmological determinism.