Abstract

In this chapter I discuss the attempts by Theodor Kaluza [Kaluza, T., 1921. Zum Unitätproblem der Physik. Sitzungsber. der K. Ak. der Wiss. zu Berlin, 966–972] and by Oskar Klein [Klein, O., 1926a. Quantentheorie und fünfdimensionale Relativitätstheorie. Zeitschrift für Physik 37 (12), 895–906; Klein, O., 1926b. The atomicity of electricity as a quantum theory law. Nature 118, 516], respectively, to unify electromagnetism and general relativity within a five-dimensional Riemannian manifold. I critically compare Kaluza's results to Klein's. Klein's theory possesses more explanatory power and unificatory strength and uses less types of brute facts than Kaluza's. The characteristic feature of Klein's theory is that it relies on an extrinsic element of unification, i.e. the wavefunction behavior, which is not intrinsic to EM or GR. Finally, I compare and discuss Kaluza's and Klein's theories in the context of Tim Maudlin's [Maudlin, T., 1996. On the unification of physics. Journal of Philosophy 93 (3), 129–144] ranking of unification and I clarify in what sense they constitute counterexamples to some of Margaret Morrison's [Morrison, M., 2000. Unifying Scientific Theories: Physical Concepts and Mathematical Structures. Cambridge University Press] assertions about unification.