The ontology of quantum field theory: Structural realism vindicated?
Introduction
Broadly construed, structural realism (SR) is the thesis that our best scientific theories tell us only about the structure of the world. This is to be contrasted with traditional scientific realism, which attempts to find a complete ontology of individual objects and properties in scientific theories. Cast in this way, SR is a form of selective skepticism toward scientific theories; we should regard theories as approximately correct about the structure of the world but have an attitude of skepticism toward everything they (apparently) attribute to the world beyond structure.1
In this paper I will be concerned with two main questions. In Section 2, I will ask what SR should lead us to expect. If SR is the correct view of scientific theories what follows? Here I will argue that SR should lead us to expect that future scientific theories (or at least theories of fundamental physics) will not provide an ontology of individual objects and properties as sought by traditional realists. If it is indeed structure alone which our best scientific theories are tracking, then we should expect ontological posits that go beyond structure to fall by the wayside.
In Section 3, the question at issue will be whether SR is vindicated. Is there reason to think that science is developing theories that fail to provide a traditional ontology? I will argue that there is good reason to think quantum field theory (QFT) fulfills this expectation. Despite a concerted effort by philosophers of physics, an ontology of individual objects and properties—of the sort sought by traditional realists—has not been forthcoming in QFT. This should be troubling for the traditional realist, but is just what the structuralist expects.
SR makes a positive prediction as well: theories should provide us with a structure of the world. Here the case of QFT is not so easily embraced by the structural realist. In fact, QFT seems just as unable to provide us with an unequivocal structure as a traditional ontology. After outlining several responses to this problem, I suggest that SR may embrace a pluralism about models to resolve the issue.
Section snippets
Structure and success
Following Worrall (1989), SR aims to be the “best of both worlds” by both accounting for theory change in science and providing an explanation for the instrumental success of individual scientific theories. Much work has focused on the first component: does a commitment to only structure allow the realist to sidestep antirealist arguments such as the “pessimistic metainduction”? But one may also wonder how SR is able to account for success in science.
Here the structural realist may adopt a
What we find
Quantum field theory (QFT) provides the framework for a great many instances of predictive success in the physics of elementary particles.5 Thus, QFT is just the sort of theory scientific realists claim provides a (roughly) faithful description of the world. Yet, it is not at all clear what
Conclusion
The testing of SR requires the consideration of not only historical cases, but also contemporary and future ones. I have argued that SR leads to the expectation of empirically successful theories, now or in the future, that do not posit a traditional ontology of objects and properties. The argument has two main steps. The first is to note that SR takes structure alone to be responsible for the empirical success of a scientific theory, and regards as surplus those aspects of theories that go
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