The quantum universe: essays on quantum mechanics, quantum cosmology, and physics in general
The author discusses the Spacetime Approach to Quantum Mechanics, in the context of the quantum field theory of closed systems, by generalising the sum-over-histories quantum mechanics formulation to deal with spacetime alternatives that are not at definite moments in time.
Abstract
This fine compendium of essays is a fascinating compilation of articles written by Professor James B. Hartle over a period of thirty years. The volume is well presented, organised in 29 essays addressing current challenges, open problems, unresolved issues, discussing fundamental theoretical work on quantummechanics, quantum cosmology and fundamental physics. The level of knowledge and interest in each essay is different but accessible to graduate students in the field or early career researchers with an interest in theoretical physics in general. The book addresses the problemof computability in physics and prediction by means of algorithm use, as a criterion for acceptable physical theories, as well as the sources of predictability in the basic laws of physics in theoretical context. The essay Excess Baggage discusses the general ideas that have to be discarded to reach a general theory of quantum cosmology as a fundamental theory of the universe’s quantum initial state. Hartle discusses the possibility of developing a theory of everything that has to be independent of a quantum mechanical perspective on the universe. Some of the essays are written in collaboration with M. Srednicki, R. Geroch and T. Hertog on the observer problem in quantum mechanics, the very large structure of the universe in cosmology and on computability and predictability in physical theories. Chapter 5 discusses the fundamental limits to scientific knowledge from the perspective of quantum cosmology and examines these limits in general theoretical context. Hartle introduces us to the Physics of Now describing the origin of the division between past, present and future in a four-dimensional space–time as well as the basic laws of physics governing it. The book continues with a pedagogical introduction into the quantum mechanics of closed systems and the universe as a whole, containing both observer and observed, exploring the probabilities of alternative coarse-grained time histories of a closed system, their interference, decoherence dynamics in post-Everett quantum mechanics and the origins of decoherence in our Universe. Hartle describes how classical predictability of everyday experience emerges from a quantum theory of the universe’s state and dynamics. The author discusses the Spacetime Approach to Quantum Mechanics, in the context of the quantum field theory of closed systems, by generalising the sum-over-histories quantum mechanics formulation to deal with spacetime alternatives that are not at definite moments in time. The result is a generalised sum-over-histories form of quantum mechanics of spacetime, offering a solution to the problem of time. Hartle also explores the link between quantum physics and the human language, containing excess baggage that must be qualified, discarded or reformed to give a clear account in the context of fundamental physics and everyday phenomena that languages have evolved to describe. The book is elegant, well written and addresses a number of contemporary pressing quantum physics and cosmology issues and questions, from the nature of spacetime and the arrow of time, computability in physical theories, the very large structure of the universe and the quantummultiverse, predictability and comprehensibility in fundamental physics, the problem of the observer in quantummechanics or the problem of time in quantum gravity.