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Thesis Format



Doctor of Philosophy




Smeenk, Christopher


The word “effective” has become the standard label attached to scientific theories these days. An effective theory allows us to make accurate predictions about a physical system at a certain (energy, length) scale while being largely ignorant of the details at more fundamental levels. One does not need to know anything about the deeper, quantum structure of water molecules to describe the macroscopic behaviour of waves or water in a glass. Although effective descriptions so broadly construed have been part of research in physics since the earliest stages of modern science, it is particle physics that has most clearly relied on and brought to the fore some of the most interesting and admittedly puzzling aspects of this way of looking at theories. Indeed, the effective field theory (EFT) program in QFT has established itself as the most natural way to understand renormalisation and dissipate initial reservations about the status of these techniques by treating higher-order processes as contributions suppressed at lower energy scales. QFT is thus treated as the “effective” framework par excellence with the decoupling of scales constituting its permeating tenet. The goal of this project is to attempt a philosophical appraisal of EFTs as currently used in high energy physics as well as assess the possibility that the whole program eventually breaks down, i.e. fails to apply when certain preconditions do not hold. Accordingly, the dissertation is logically divided into two parts with the first two chapters dedicated to discussion of the relation between EFTs and traditional questions in the philosophy of science concerning the structure of scientific theories, the formulation and defence of scientific realism as well as its connection to possible ontological readings of EFTs. The second part constitutes an analysis of two well-known problems that have been accorded the status of crises in the physics literature: the hierarchy problem and the cosmological constant problem. Our main focus will be to uncover those assumptions responsible for undermining the validity of the EFT techniques in their respective context. In light of this analysis, we will ultimately lean towards a more cautionary or “reserved” approach to EFTs.

Summary for Lay Audience

An effective theory allows us to make predictions about a physical system without possessing a complete description of all its microconstituents or without fully tracking their intricate interactions. For example, when we apply Newtonian mechanics in high school physics problems involving balls colliding with one another we typically abstract from their atomic and molecular structure and disregard negligible influences such as interactions with the surrounding air molecules. Including only a handful of input parameters for our models allows us to keep our equations tractable while extracting information up to a desired accuracy. Although effective theories understood as such approximating tools have been part of research in physics since its earliest stages of development (one just recall early applications of Newton’s theory of gravity to planetary motions], it is particle physics that has brought to the fore the most interesting and admittedly puzzling aspects of this approach to seeing theories. Perhaps the greatest conceptual insight obtained by an effective reading of QFT is that it was possible to put on a firmer physical ground the process of renormalisation, which, although indispensable in producing meaninful QFT calculations, was typically seen as a trick for sweeping the pathogenies of the theory under the rug. The advent of EFTs changed that. Unsurprisingly, the centrality of this new conception of understanding physical theories calls for key revisions to the way philosophers have traditionally thought about issues such as how is a scientific theory structured, how it represents entities in the world and how it is to be interpreted. One goal of this thesis is to contribute to this “revisionary” project. Another goal is to examine possible limitations of the effective framework, i.e. cases where its tools lead us to incorrect expectations. To this effect, two infamous open problems of modern physics, the hierarchy and cosmological constant problems, are examined in the latter half of the thesis. It is argued that they are signals of a breakdown of EFTs and are used as the source of extracting preconditions for the applicability of the effective framework.