Conclusion: Underdetermination in Quantum Mechanics
We are thus facing a real-life situation of empirical equivalence that is robust enough to make hopes that the decision procedures defended in the philosophical literature will before too long lead to an objectively best choice are utterly unrealistic.
This diagnosis turns out to be in conformity with the attitude of the scientific community. On the one hand, physicists have no qualms whatsoever to speak—in general terms—about the unobservable parts of physical reality. Indeed, it is hard even to imagine what the practice of quantum physics without continuous reference to elementary particles, fields, strings, membranes, etc., would look like. These terms are not meant as mere mental tools without reference, but as denoting actual parts of reality. However, on the other hand, there is also a wide-spread conviction that very different views can be upheld about the precise details of the associated physical ontology. Are quantum objects living in one unique world, the only one in existence, or do they have doppelgangers in other worlds? Difficult to be sure! Are quantum particles entities a la Bohr, with contextual properties obeying complementarity and uncertainty relations? There is certainly something to be said for this option. But then again, is it not possible that the Bohm picture is right, so that particles possess definite locations and velocities, while interacting non-locally? Probably there are less enthusiasts for this alternative, but its scientific tenability is generally accepted. It is true that many find the non-local Bohmian interactions strange, but then, quantum mechanics displays strange features of non-locality anyway, and there seems to be a consensus that it is a matter of personal preference rather than of an objectively justified scientific stand which side one chooses.[125]A number of diverse viewpoints about the nature of the quantum world thus coexist, each one with its own limited circle of convinced devotees.
Many more physicists know about at least a number of these interpretations, without committing themselves to any of them—they refer in realist fashion to quantum objects, but refrain from becoming specific about their exact nature. Not infrequently, physicists make use of one picture or another depending on the kind of question that is being discussed. For example, sometimes it is easy to think of localized particles following trajectories while influencing each other superluminally, then again it seems to provide more insight to think of a system of “particles” as one undivided whole.This situation should not be mistaken for one of “anything goes”. In fact, the uncontested and shared core of empirical consequences of quantum mechanics imposes conditions on possible interpretations that make each of them necessarily distinctly nonclassical. Although the predictions of quantum mechanics do not fix a unique description of the physical world, they do give us a delineated field of possibilities. Indeed, the mathematical formalism and its empirical predictions exclude interpretations that, e.g., do not in some way reflect non-locality and contextuality. This has been shown in a long tradition of so-called “no-go theorems” harking back to the work of von Neumann in the early nineteen thirties (Von Neumann (1932), see also Dieks (2017)), in which the results of Bell (1964) and Kochen and Specker (1967) are some of the highlights. These theorems identify a number of characteristic quantum features that set the theory apart from its classical counterparts, and are differently represented in different interpretations. For example, in Bohm’s theory Bell’s theorem—excluding local realism—has the repercussion that interactions must be non-local and instantaneous, but in other interpretations (Copenhagen, modal interpretations) the theorem’s conclusion is reflected in a feature of holism according to which properties of a composite system cannot be reduced to the properties of its components.
In this way the various interpretations open up different perspectives, within different conceptual frameworks, of what it means to be a quantum theory.This finally brings us back to the questions of rationality and objectivity of theory choice. As we have seen in Sect. 2, Laudan and Leplin warned against the dangers of empirical equivalence and underdetermination, which in their eyes are odious doctrines that have “wreaked havoc” in the philosophy of science and have encouraged deplorable forms of pluralism of belief and practice. How can science be rational if no objective evidence-based choices can be made between different descriptions of physical reality? The practice of modern physics supplies an answer to this rhetorical question.
This practice shows us that there is no harm in the coexistence of a plurality of world pictures that all do equal justice to what is established in experiments. Different world views of this kind may all in their own way bring out salient features of what physical reality is like, each making use of its own conceptual framework. There is no need for an objective decision between such interpretative options and it is no problem that individual scientists may feel drawn more to one of them than to another, on subjective grounds. Indeed, congenial ideas facilitate reasoning and make it easier to fathom the consequences of a theory and to achieve understanding—it is an an unnecessary impediment for scientific understanding and rationality to require one unique conceptual framework Regt and Dieks (2005). Rather than hampering rational discourse, the competition of different points of view furthers it.