Higher dimensions can make separate forces of nature look like one unified geometry

Kaku traces the pioneering work of Theodor Kaluza and Oskar Klein in the 1920s, who showed mathematically that if you add a fifth dimension to Einstein's four-dimensional spacetime, Einstein's equations of gravity in that five-dimensional space naturally split apart into ordinary four-dimensional gravity plus Maxwell's equations describing electromagnetism. In other words, two forces that seem completely distinct in our everyday three-dimensional experience emerge as a single unified geometric structure once an extra dimension is added.

Kaku presents this as the founding insight behind the entire higher-dimensional program in physics: unification isn't achieved by finding some clever mathematical trick to glue different forces together, but by finding the right geometric setting in which forces that look separate are actually different faces of the same underlying structure. Modern theories extend this same logic to try to fold in the strong and weak nuclear forces as well.

Takeaway: what looks like separate forces of nature may just be gravity, viewed from too few dimensions.