String theorists and reality Experiment, mathematics, and realism in particle theory (1968-1989)

This dissertation concerns the historical evolution of theoretical particle physics in the 1960s, 1970s, and 1980s, and its philosophical implications. The focus is the development of string theory, a mathematical framework potentially unifying all fundamental forces. Around 1985, string theory emerged as a widely studied approach in particle physics. To this day, however, there is no direct empirical evidence for it. This has led some to question its viability, while others contend that with string theory, physicists employed reliable forms of non-empirical reasoning. I argue that the rise of string theory can best be understood against the background of a shift from empiricism to realism in the broad philosophical outlook of particle theorists. Chapter 1 deals with particle physics in the 1960s and early 1970s, when the field was phenomenologically-oriented and string theory was first formulated as a purely mathematical approach to constrain scattering data. Chapter 2 concerns theorists’ formulation of the ‘Standard Model’, deemed empirically successful, and their efforts to construct unified theories superseding it. In this development, theorists already became primarily concerned with a ‘deeper’ explanation of the empirical world, in line with a traditional realist outlook. Chapters 3 and 4 then discuss the rise of string theory, demonstrating how this was partly due to a convergence of formal methods from different theoretical fields. Philosophically, I identify how with the consolidation of string theory, some physicists apparently embraced a position of mathematical realism, turning the subject into the study of ‘existing’ mathematical structures prior to the empirical phenomena.