If You feel that scientific theories should reveal facts about the world, you will, of course, be profoundly dissatisfied with quantum mechanics! For, as it is generally interpreted, the formalism is a tool for making predictions about outcomes of possible experiments and nothing more. Quantum mechanics has nothing to say about the nature of the physical processes that bring about the very experimental results that it predicts, yet it is supposed to be the whole story of the phenomena it speaks of (i.e. it is supposed to be "complete").
If You feel that scientific theories should reveal facts about the world, you will, of course, be profoundly dissatisfied with quantum mechanics! For, as it is generally interpreted, the formalism is a tool for making predictions about outcomes of possible experiments and nothing more. Quantum mechanics has nothing to say about the nature of the physical processes that bring about the very experimental results that it predicts, yet it is supposed to be the whole story of the phenomena it speaks of (i.e. it is supposed to be "complete").
What does not satisfy me in that theory [quantum mechanics], from the standpoint of principle, is its attitude towards that which appears to me to be the programmatic aim of all physics: the complete description of any (individual) real situation (as it supposedly exists irrespective of any act of observation or substantiation). Albert Einstein, Reply to Criticisms [This] implies the impossibility of any sharp separation between the behaviour of atomic objects and the interaction with the measuring instruments which serve to define the conditions under which the phenomena appear . In fact, the individuality of the typical quantum effects finds its proper expression in the circumstance that any attempt of subdividing the phenomena will demand a change in the experimental arrangement introducing new possibilities of interaction between objects and measuring instruments which in principle cannot be controlled. Consequently, evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomena exhausts the possible information about the objects. Indeed the finite interaction between object and measuring agencies conditioned by the very existence of the quantum of action entails – because of the impossibility of controlling the reaction of the object on the measuring instruments, if these are to serve their purpose – the necessity of a final renunciation of the classical idea of causality and a radical revision of our attitude towards the problem of physical reality. […]
According to the Everett interpretation of quantum mechanics, the apparent paradoxes of quantum theory require no change either in our philosophical attitude toward scientific theories or in the formalism of quantum mechanics itself. Rather, the unaltered theory can be understood as a literal description of physical reality. But the physical reality thus described is one of many noninteracting, approximately classical worlds. These worlds enter the theory not at the fundamental level—they are nowhere to be found in the basic formalism—but at a higher, emergent level; in this sense they are (ontologically speaking) on a par with the ontology of the special sciences.
Despite scholarly consensus on the empirical facts, ever since its inception in the 1920s there has been deep disagreement about how to interpret quantum theory. “To interpret a physical theory is to say what the world would be like, if the theory were true.” All scientific theories require interpretation, since strictly speaking what they describe are our experiences of the world rather than the world itself. Thus, whether we are trying to explain or merely describe the world, we are always engaged in inference. To make inferences we need a context for interpretation. For any given claim this context is provided locally by related theories, more broadly by disciplinary paradigms, and ultimately by worldview claims about the nature of reality. The challenge in interpreting scientific theories is to integrate them into this hierarchy of knowledge. Usually this is not too difficult. New theories often require adjustments to nearby theories, but in doing so we can rely on paradigms to make them cohere. More rarely paradigmatic assumptions are themselves challenged, but in that case scientists can fall back on their worldview to make sense of the needed changes. By implication, the most difficult interpretive problems arise when our worldview is called into question, which leaves us without any frame of reference on which to fall back. Quantum theory poses a worldview problem. Prior to the quantum revolution the classical worldview could not be tested scientifically and so it was essentially metaphysical. Even though it corresponded to our experience of material objects, we lacked proof that it adequately represented the deep structure of reality. By giving us access to that deep structure for the first time, quantum mechanics made it possible to do such a test, or “experimental metaphysics.” Unfortunately, the results seemed to contradict classical assumptions, at least on the sub-atomic level.
When we are presented with apparently bizarre properties of microsystems governed by quantum mechanics (as in our discussion of the double-slit experiment in the previous chapter), we naturally ask whether it might be possible to find another theory, or at least another story to go with the equations, that would make more sense to us. We might seek a theory and a view of fundamental physical processes that would accord more with our classically based, everyday common sense. This typically takes the form of the question of the possibility of the existence of a more detailed description of atomic processes than that afforded by the wave function (that is, by the Copenhagen interpretation of the previous chapter). In this chapter we discuss some of the central interpretive problems of the Copenhagen version of quantum mechanics. Then, in the next two chapters, we look at the severe, general restrictions that are placed on any such completion of quantum mechanics and at one particular extension of quantum mechanics. First, though, we begin here with the struggle that ensued over the status of causality in quantum theory. THE COMPLETENESS OF QUANTUM MECHANICS Although the Heisenberg uncertainty principle (Section 20.4) and the lack of absolute predictive power are an inherent feature of quantum mechanics, there have been attempts to preserve an in principle completely deterministic structure for physics. Even classically, in a macroscopic sample of gas there are so many molecules that it would be hopeless, as a practical matter, to predict the future locations of all these molecules from Newton's laws of motion.
Despite its enormous practical success, quantum theory is so contrary to intuition that, even after 45 years, the experts themselves still do not all agree what to make of it. The area of disagreement centers primarily around the problem of describing observations. Formally, the result of a measurement is a superposition of vectors, each representing the quantity being observed as having one of its possible values. The question that has to be answered is how this superposition can be reconciled with the fact that in practice we only observe one value. How is the measuring instrument prodded into making up its mind which value it has observed?
This essay seeks to understand just what it is that modern science tells us about nature. For the longest time the story told by science appeared to be fully reflective of our common experience: nature was discovered as a collection of reciprocally influencing objects governed by laws which were consistent with that experience. And then, about a hundred years ago, the story suddenly became obscure. Science introduced into nature quantum objects which were supposed to look nothing like anything we had ever seen before and the laws governing them no longer appeared to make much sense to us. Thereafter, what science told us about nature was no longer quite as clear. This shift in the story is conspicuous and bespeaks of an earlier moment in the development of science when the project might have inadvertently taken a step which would eventually make her strange. The essay suggests that the scientific community had in fact made a fateful decision which inevitably led it to the strangeness of quantum mechanics and specifically identifies it as the decision to experimentally observe the constituents of matter as if they were real objects. It then proceeds to determine just what those constituents are if not objectively real and to specify the unique manner in which such objects are what they are. This focus on what constituents of matter are, rather than on how they constitute the inside of natural objects, classifies the essay as an investigation of the ontological side of modern science. Finally, the essay considers the alternative ontologies which could have been entertained for these objects at the time the decision was made to experimentally observe them and shows how 'skipping over' such alternatives generated the specific ways in which quantum mechanics, the theory describing the results of such experiments, had to be strange.