UC San Diego

Unifying distinct domains of phenomena is one of the most important non-empirical virtues of scientific theories. However, what counts as unification and what makes it important are philosophically controversial. I canvass two positions toward unification (the enthusiasts and the dissenters) as well as two methods to approach unification : the general approach and the specific approach based on case studies. Some philosophers take unification to be truth conducive (Friedman), others to be central to scientific explanation (Kitcher) and still others find it to be typically neither (esp. Morrison). To make progress on these questions, attention should be paid to concrete, historical episodes. In my dissertation I tackle one of the most significant episodes in the history of physics, an episode that--oddly given how important the theory is now in the context of String Theory--has escaped historical and philosophical investigations or it has been under-investigated. That episode is the early attempt to unify gravity and electromagnetism within a five- dimensional spacetime by Kaluza (1921) and Klein (1926). This theory is philosophically interesting in its own light, but as the ancestor to current attempts to unify gravity with matter fields, it is rich with consequences for the contemporary foundations of physics. Morrison (2000) argues that many instances of unification are trivial, spurious or related neither to explanatory power, nor to scientific realism. Others have recently argued that unification in general is neither necessary, nor sufficient for explanation, although there may be some (weak) correlations between unification and explanation. Against this background I emphasize the novelty of my approach by making room for a new type of unification illustrated by Kaluza, and especially by Klein, in which unification is strongly related to explanation. Although some aspects of my case study are suggested in the philosophical literature, they are never fully discussed. I argue that, as a two-stage process from Kaluza to Klein, the Kaluza-Klein theory brings about an increased unificatory and explanatory power and becomes less ad-hoc. Kaluza's theory is interesting because it is, arguably, almost a real-life case of a spurious unification (save his speculations about quantum mechanics). Klein improves significantly on Kaluza and proposed a curled fifth axis (a procedure called "compactification"), explains the quantization of electrical charge, uses fewer brute facts and fewer types of symmetry, and solves problems Kaluza could not. As the five-dimensional theory became more unified with Klein, I argue that it has a greater explanatory power. In addition, I show how the sense in which Klein's theory is unificatory is interestingly different than in some other unificatory theories (in contrast to e.g. electromagnetism). Unlike Kaluza, Klein employed an extrinsic factor: the behavior on the fifth dimension of a wave-function--present neither in gravity nor in electromagnetism--which has had its own interesting history. Kaluza-Klein offers a novel type of unification; Klein's unification, in particular, constitutes a type of unification which is neither reductive, nor synthetic. In opposition to some dissenters, I show in greater detail how unification works in the practice of science and how it relates to explanation, simplicity, theory validation, etc. I claim that the recurrent skeptical positions are rooted in a misunderstanding of both the concept of unification and the concept of scientific explanation. Finally, I stress the importance of the Kaluza-Klein type of unification for recent attempts to explore extra- dimensions of spacetime (related mainly to String Theory)