2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/51012Current attempts to find a unified theory that would reconcile Einstein's General Relativity and Quantum Mechanics, and explain all known physical phenomena, invoke the Kaluza-Klein idea of extra spacetime dimensions. The best candidate is M-theory, which lives in eleven dimensions, the maximum allowed by supersymmetry of the elementary particles. We give a non-technical account. An Appendix provides an updated version of Edwin A. Abbott's 1884 satire {\it Flatland: A Romance of Many Dimensions}. Entitled {\it Flatland, Modulo 8}, it describes the adventures of a superstring theorist, A. Square, who inhabits a ten-dimensional world and is initially reluctant to accept the existence of an eleventh dimension.Oskar Klein Professorship Inaugural Lecture, University of Michigan, 16 March 2001. 38 pages, Latex, 15 color figuresHigh Energy Physics - TheoryGeneral Relativity and Quantum CosmologyHigh Energy Physics - PhenomenologyHistory and Philosophy of Physicstext