has anybody done any thinking on the properties of a universe with four "normal" spatial dimensions? i've done a bit and some interesting properties have surfaced. I've discovered that rivers don't need bridges, sponges and screens can pass through each other, and there are several types of wheels. I created a webpage a while back with an introduction to the fourth dimension. It discusses some properties of a world with a fourth spatial dimension: http://tetraspace.alkaline.org i'd be grateful if people could visit the site and give me comments on it. I have the best collection of links to other fourth dimension pages, plus a forum for discussing the fourth dimension.