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This chapter discusses three fundamental questions which the majority of the physics community believes are not worthy of attention and a minority believes are crucial and in urgent need of attention. The first concerns the “anthropic principle”: to what extent is it an “explanation” of basic physical data, such as the dimensionality of space-time, the values of the fundamental constants, etc., to observe that were they appreciably different, human life and consciousness could not have evolved to the point of asking the question? The second has to do with the “arrow of time”: how is the everyday sense of the “flow” of time from past to future consistent with the invariance of the laws of physics under time reversal? The third problem is how to incorporate the occurrence of definite outcomes within the framework of quantum mechanics (the “quantum measurement problem”).

This introductory chapter presents the book’s fourfold task, which consists of providing a historical account of the workings of multiple-world cosmologies; outlining contemporary standards of the multiverse in relation to their mythological, philosophical, and theological precedents; questioning why, how, and to whom the multiverse has become a particularly engaging theory; and finally, indicating multiverse cosmologies as the ground for reconfiguring the boundaries between science and religion. In addition, the chapter discusses the origin of the term multiverse, from its coinage by William James, to its usage in different cosmological interpretations—the Greek Atomists’ infinite worlds, the Stoics’ cyclical multiverses, quantum mechanics’ Many-Worlds Interpretation, Brian Greene’s ultimate multiverse, and Gottfried Leibniz’s possible worlds. There exists a nontheistic motivation behind the scientific turn to many-worlds scenarios, making the multiverse theory a sort of replacement for the God-as-creator argument, albeit with an equally perplexing article of faith.

Standard quantum theory is highly successful in all its applications, from photons and atoms, to semiconductors, superconductors, quarks, and so on. Yet the need to understand what the theory is really trying to tell us about the nature of the world beyond specific predictions forces us into the realm of philosophy, or, as some have dubbed it, quantum metaphysics: the study of the interpretation of quantum mechanics. There is a wide range of opinions regarding the philosophical foundations of quantum mechanics. This chapter focuses on two of the most prominent interpretations: the Copenhagen interpretation, and one alternative interpretation that has been gaining some support in recent years, the many-worlds interpretation (MWI), also known as the many universes interpretation. The central issues of contention are: firstly, that the measurement of some attribute of a quantum system generally involves the probabilistic collapse, or reduction, of the state vector where the probabilistic nature of the process is inherent—that is, not reducible to a more fundamental, deterministic explanation; and, secondly, the problem of bringing a measurement to a completion—a problem generally known as the measurement paradox.

This chapter discusses the possible existence of a multiverse, which stemmed from Hugh Everett’s Many-Worlds Interpretation (MWI) of quantum mechanics. According to the law of quantum mechanics, a particle’s position and momentum cannot be determined at the same time. The state of a subatomic particle can only be expressed in terms of a “wave function” that details the varying probabilities of its possible states; when the particle is being observed, the wave function “collapses” and the particle takes on a definitive place. This account prompted Everett to wonder what would happen if the wave function never collapses, explaining that if there is no collapse, then every possible outcome will happen—each in a different universe. Everett’s theory gave rise to numerous other works including Bryce Dewitt’s reinvention of the MWI, Stephen Hawking’s model-dependent realism, Laura Mersini-Houghton’s multiverse bath, Lee Smolin’s cosmological scenario, and Max Tegmark’s Mathematical Universe Hypothesis.

The interaction between Schrodinger’s cat and a radioactive atom is the sort of thing that happens in a quantum measurement. The “measurement problem” of quantum mechanics is to explain how the cat can be considered to end up either definitely alive or definitely dead if the final state is an entangled state of the radioactive atom and the cat. This chapter discusses Bohm’s theory and the Everett interpretation as solutions to the measurement problem. The Everett interpretation is fundamentally a proposal to solve what Pitowsky calls the “big” measurement problem: how a quantum measurement can produce a definite outcome. On the information-theoretic interpretation proposed here, the “big” measurement problem is a pseudo-problem. The chapter concludes with some remarks on the “small” measurement problem: how a classical probability distribution over macroscopic measurement outcomes emerges in a measurement process – how probabilities of “what you’ll obtain if you measure” become transformed to probabilities of “what’s there.”

Anaxagoras proposed “in everything there is a portion of everything,” a notion as bizarre as the most popular interpretation of quantum theory, the Copenhagen. A piece of gold, say, contains gold as well as everything else—copper, wheat—but appears as a distinct golden object because its gold portion is the greatest. But no part of the object is pure. Every part of the golden object is also simultaneously watery, milky (and all other materials), and black and white (and all opposite qualities). In the Copenhagen interpretation, before an observation, something (an electron, Schrödinger’s cat) is all opposite qualities simultaneously, too, with each quality described by a unique probability (“portion”) to actually occur. The cat is both dead and alive; the electron spins simultaneously both clockwise and counterclockwise. Only after the observation, the cat is found either dead or alive, and the object, as Anaxagoras would say, definitely golden, yellow, and dry.

The Quantum Cookbook shows that whilst quantum mechanics is mathematically challenging, some basic knowledge and a bit of effort will carry you a long way. It also explains how quantum mechanics was derived from the physics. The abstract formalism based on state vectors in Hilbert space was introduced only when it was deemed desirable to lend the theory greater mathematical consistency, and to reject some of its historical baggage. The best way to come to terms with this formalism is to understand how and why it came about. Debates about interpretation continue to this day and, by providing some historical context, you should get the impression that any lack of comprehension of its meaning on your part is absolutely not your fault. Quantum mechanics challenges our comprehension of what any (and all) scientific theories are meant to be telling us about the nature of reality. It’s okay to have doubts.

Chapter 10 reviews the writings of three prominent scientists who reject the hypothesis as the basis for scientific thinking and research. Stuart Firestein is virulently anti-hypothesis and champions Curiosity-Driven science, a free-form mode which is deliberately unstructured. David Glass’s program of Questioning and Model-Building is rigidly structured and inductivist in spirit; he rejects the hypothesis in favor of “models” that he distinguishes from hypotheses. Both Firestein and Glass accept the empiricist standard for assessing scientific truth and believe that a scientific investigation proceeds on the basis of asking testable questions. David Deutsch is a theoretical physicist whose advanced ideas are, in principle, empirically untestable and, since empirical testability is the key to scientific hypothesis testing, he also rejects the hypothesis. His program is called Conjectures and Criticism. Unlike the other two critics, Deutsch is sympathetic to the scientific thinking process that Karl Popper advanced, but feels that Popper’s program must be superseded for science to get beyond the constraints of empiricism. The chapter shows that the supposed incompatibilities of each of the alternative approaches with the hypothesis are largely based on misrepresentations or misapplications of the nature of hypothesis-based science. The counterproposals are not grounds for rejecting the hypothesis, which can in fact coexist comfortably with them.