This chapter reviews and discusses the essential aspects of three theories—or rather clusters of theories—that, however distinct, all lay special emphasis on more or less the same points. The theories discussed are the Everett relative state theory, the theory of environment-induced classical appearances and the Gell-Mann and Hartle cosmological theory. These points essentially are that if quantum theory is to be applicable to the world as a whole, this implies that it should be expressible with no reference to classical instruments lying outside the analyzed system and it should somehow account for emergence of classical appearances within this world. The fact that quantum mechanics works so remarkably well as a tool for making reliable predictions in all sorts of domains is of course felt by many physicists as a powerful incitement to the quest of a quantum theory of the universe as a whole.
This chapter reviews and discusses the essential aspects of three theories—or rather clusters of theories—that, however distinct, all lay special emphasis on more or less the same points. The theories discussed are the Everett relative state theory, the theory of environment-induced classical appearances and the Gell-Mann and Hartle cosmological theory. These points essentially are that if quantum theory is to be applicable to the world as a whole, this implies that it should be expressible with no reference to classical instruments lying outside the analyzed system and it should somehow account for emergence of classical appearances within this world. The fact that quantum mechanics works so remarkably well as a tool for making reliable predictions in all sorts of domains is of course felt by many physicists as a powerful incitement to the quest of a quantum theory of the universe as a whole.
Abstract This chapter provides a detailed explanation of Everett's three works — his thesis “On the Foundations of Quantum Mechanics”, a short paper entitled “‘Relative State’ Formulation of Quantum Mechanics”, and a long paper “The Theory of the Universal Wave Function”. It also discusses what was wrong with von Neumann's theory. It evaluates five alternatives presented by Everett for developing a satisfactory formulation of quantum mechanics. It provides an explanation for Everett's interpretation of pure wave mechanics and his principle of the relativity of states. Lastly, it evaluates the classical appearance and behaviour of macroscopic systems.
The classical world, governed by classical physics, might have felt more intuitive and certain. However, the 20th century’s revolutionary birth of quantum mechanics opened a way to the “quantum world”. This chapter investigates the evolving concept of “world”, especially exploring the special and general theories of relativity, with Wittgenstein’s philosophical view that the world is composed of facts, or the existence of states of things. Quantum mechanics has two essential principles: The Heisenberg uncertainty principle and the new Heisenberg uncertainty principle, where the former indicates the inherent uncertainty within the quantum world and the latter uncovers the certainty under some circumstances such as quantum entanglement. Quantum mechanics, while having multiple interpretations, is considered a relatively complete framework; however, its “counterfactuals”—scientific facts that defy classical expectations—can be challenging for the classical understanding. Additionally, the potential for “backward causality” under time symmetry conditions adds another level of complexity. These counterfactuals and the possibility of backward causality don’t contradict existing facts, but rather signal the expansion of our scientific scope and, potentially, a new scientific paradigm.
Two types of ontologies about the universe arise: the materialist ontologies that describe the universe (and everything in it) as composed of fluids and atoms that move through space and time, and the conventional materialist ontologies that look at nature as contained in intact particles. Even though Whitehead perceives incompatibilities with the interpretations of some physicists, this section provides discussions on Abner Shimony's five points implicitly supplied by the quantum theory, Bohr's utilization of interconnected information as nature's most basic element, the benefits of the Jamesian ontological accommodation from the Interpretation of Copenhagen, and the idea of a “closed system.” Also given in this chapter are equations and sample situations to support the propositions of Murray Gell-Mann and John Bell, Werner Heisenberg's uncertainty relations, and ontological conclusions from the quantum formalism specializing in the elaboration of potential facts.
Abstract What does realism about the quantum state imply? What follows when quantum theory is applied without restriction, if need be, to the whole universe? These are the questions which are debated in this book. All the chapters start from the point of realism, and based on the need, or the aspiration, for a theory that unites micro- and macroworlds, at least in principle. But the further claim argued by some is that if you allow the Schrödinger equation unrestricted application, supposing the quantum state to be something physically real, then this universe is one of countlessly many others, constantly branching in time, all of which are real. The result is the many worlds theory, also known as the Everett interpretation of quantum mechanics. The contrary claim sees this picture of many worlds as in no sense inherent in quantum mechanics, even when the latter is allowed unrestricted scope and even given that the quantum state itself is something physically real. For this picture of branching worlds fails to make physical sense, let alone common sense, even on its own terms. The status of these worlds, what they are made of, is never adequately explained. Ordinary ideas about time and identity over time become hopelessly compromised. The concept of probability itself is brought into question. This picture of many branching worlds is inchoate, it is a vision, an error. There are realist alternatives to many worlds, some even that preserve the Schrödinger equation unchanged. Twenty chapters, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They focus first on the question of ontology, the existence of worlds, second on the interpretation of probability, and third on alternatives or additions to many worlds. The introduction offers a helpful guide to the arguments for the Everett interpretation, particularly as they have been formulated in the last two decades.
Core quantum postulates including the superposition principle and the unitarity of evolutions are natural and strikingly simple. I show that—when supplemented with a limited version of predictability (captured in the textbook accounts by the repeatability postulate)—these core postulates can account for all the symptoms of classicality. In particular, both objective classical reality and elusive information about reality arise, via quantum Darwinism, from the quantum substrate. This approach shares with the Relative State Interpretation of Everett the view that collapse of the wavepacket reflects perception of the state of the rest of the Universe relative to the state of observer’s records. However, our “let quantum be quantum” approach poses questions absent in Bohr’s Copenhagen Interpretation that relied on the preexisting classical domain. Thus, one is now forced to seek preferred, predictable, hence effectively classical but ultimately quantum states that allow observers keep reliable records. Without such (i) preferred basis relative states are simply “too relative”, and the ensuing basis ambiguity makes it difficult to identify events (e.g., measurement outcomes). Moreover, universal validity of quantum theory raises the issue of (ii) the origin of Born’s rule, pk=|ψk|2, relating probabilities and amplitudes (that is simply postulated in textbooks). Last not least, even preferred pointer states (defined by einselection—environment—induced superselection)—are still quantum. Therefore, unlike classical states that exist objectively, quantum states of an individual system cannot be found out by an initially ignorant observer through direct measurement without being disrupted. So, to complete the ‘quantum theory of the classical’ one must identify (iii) quantum origin of objective existence and explain how the information about objectively existing states can appear to be essentially inconsequential for them (as it does for states in Newtonian physics) and yet matter in other settings (e.g., thermodynamics). I show how the mathematical structure of quantum theory supplemented by the only uncontroversial measurement postulate (that demands immediate repeatability—hence, predictability) leads to preferred states. These (i) pointer states correspond to measurement outcomes. Their stability is a prerequisite for objective existence of effectively classical states and for events such as quantum jumps. Events at hand, one can now enquire about their probability—the probability of a pointer state (or of a measurement record). I show that the symmetry of entangled states—(ii) entanglement—assisted invariance or envariance—implies Born’s rule. Envariance also accounts for the loss of phase coherence between pointer states. Thus, decoherence can be traced to symmetries of entanglement and understood without its usual tool—reduced density matrices. A simple and manifestly noncircular derivation of pk=|ψk|2 follows. Monitoring of the system by its environment in course of decoherence typically leaves behind multiple copies of its pointer states in the environment. Only pointer states can survive decoherence and can spawn such plentiful information-theoretic progeny. This (iii) quantum Darwinism allows observers to use environment as a witness—to find out pointer states indirectly, leaving systems of interest untouched. Quantum Darwinism shows how epistemic and ontic (coexisting in epiontic quantum state) separate into robust objective existence of pointer states and detached information about them, giving rise to extantons—composite objects with system of interest in the core and multiple records of its pointer states in the halo comprising of environment subsystems (e.g., photons) which disseminates that information throughout the Universe.