There are two widely known interpretations of quantum theory: Bohr’s Copenhagen interpretation and Everett’s interpretation. The focus of Chapter 9 is to assess, within the context they provide, the interpretation-independent advances discussed in this book. We want to see whether the advances that include decoherence and quantum Darwinism fit these two established and widely known points of view. In fact, it is surprising that (with minor but significant adjustments) decoherence and quantum Darwinism fit very naturally, addressing questions that were recognized as open and important. We then discuss the existential interpretation. It can be seen as a continuation of the Copenhagen interpretation, with the Universe consisting of quantum and classical realms, but with classicality that is emergent, rather than preordained. It is also compatible with Everett’s interpretation, since quantum states and evolutions are all that is needed. However, unlike the Many Worlds interpretation (which regards the quantum state of the Universe as objectively existing, akin to a classical point in phase space or a classical electromagnetic field), the existential interpretation recognizes that quantum states combine information and existence—they are epiontic. The mix of existence and information they represent fits the relative states reading of Everett’s approach.

This chapter is a step towards understanding why quantum nonlocalityis a misleading concept. Metaphorically speaking, quantum nonlocality isJanus faced. One face is an apparent nonlocality of the state update basedon the Luders projection postulate. It can be referred as intrinsic quantumnonlocality. And the other face is subquantumnonlocality: by introducing a special model with hidden variables onederives the Bell inequality and claims that its violation implies the existenceof mysterious instantaneous influences between distant physical systems(Bell nonlocality). According to the Luders projection postulate, aquantum measurement performed on one of the two distant entangled physicalsystems, say on S1, modifies instantaneously the state of S2. Therefore, ifthe quantum state is considered to be an attribute of the individual physicalsystem (Copenhagen interpretation) and if one assumes thatexperimental outcomes are produced in a random way one arrives at the contradiction. It is a primary source of speculation about aspooky action at the distance. But Einstein had already pointed out that the quantum paradoxes disappear, ifone adopts the statistical interpretation.

This chapter surveys some of the ways in which the Copenhagen interpretation of quantum mechanics has led to a various views of the world with spiritual and moral implications; the perspective of this chapter is that most of these views are not demanded by the actual theory and experiments of quantum mechanics.

After having shown in previous chapters that wave-particle duality is not a fundamental problem for quantum mechanics, this chapter introduces the really strange effect of quantum mechanics, namely “nonlocal correlations” that appear to act over long distances faster than the speed of light. The “Copenhagen” interpretation of quantum mechanics is introduced, which puts human knowledge in a special role, and some of the philosophical objections to it.

We present a few of the gedanken- and real experiments that demonstrate the spookiness of quantum mechanics. We discuss the Einstein,Podolsky, and Rosengedankenexperiment that invokes hidden variables to create a paradox. We analyze Bell’s analysis of the paradox, which shows that the predictions of quantum mechanics are inconsistent with local hidden variable theories. We discuss the Schrödinger cat paradox, and the Copenhagen interpretation of quantum mechanics.