Hi there, last week I got stuck on a paper by Randall and Karch, addressing the question why we live in 3 dimensions Relaxing to Three Dimensions http://xxx.lanl.gov/abs/hep-th/0506053 We propose a new selection principle for distinguishing among possible vacua that we call the "relaxation principle". The idea is that the universe will naturally select among possible vacua through its cosmological evolution, and the configuration with the biggest filling fraction is the likeliest. We apply this idea to the question of the number of dimensions of space. We show that under conventional (but higher-dimensional) FRW evolution, a universe filled with equal numbers of branes and antibranes will naturally come to be dominated by 3-branes and 7-branes. We show why this might help explain the number of dimensions that are experienced in our visible universe. Though I wanted to put the question aside, it kept running through my head the whole weekend (okay, among one billion other things). Now I wonder whether anyone has some further references or remarks on the question. (The question of why 3 as well as why Lorentzian.) More about the paper see also Why do we live in 3+1 dimensions If possible, don't go anthropic Best, B.