When Worlds Collide

Abstract

Perhaps the key untested idea in all of physics is that extra spatial dimensions might exist. Once we admit the possibility of extra dimensions several avenues for exploring theories of unification open up. Without extra dimensions there simply does not seem to be room for unification of the various forces. But where are these extra dimensions? For six decades after the pioneering work of Klein, the belief was that extra spatial dimensions — if they exist — are unobservable because they are tiny. The assumption was that any extra dimensions must be curled up on the Planck length scale of 10−35 meters. Eventually, that assumption began to be challenged. In 1983, for example, two physicists at Moscow’s Institute for Nuclear Research, Valery Rubakov and Misha Shaposhnikov, suggested there could be large extra dimensions that we have not yet observed. And in 1990, Ignatios Antoniadis, who was based at CERN, gave a serious proposal for how some of the extra dimensions of string theory could be 10−19 meters in length — still small, for sure, but much larger than previously thought. Such proposals were not without drawbacks, however, and in any case the ideas were not motivated by any deep underlying principles. Then came M-theory.