Explaining the fourth dimension is hard because we're three-dimensional creatures. We understand the world as a collection of matter distributed on three axes, because that's how things are. The point in a philosophy or physics class when the professor starts talking about the fourth spacial dimension is the part where about half of the brains in the room sieze up completely.
It's a tough story to tell in words, and pictures don't help much. But this, this works: the Four Dimensional App, designed by a math-obsessed programmer and former Dreamworks employee, is the best explanation of the fourth dimension you'll find anywhere.
Creator Drew Olbrich says the seed for the app was planted over 25 years ago. "In 1986 there was a Scientific American article which described how to write a program that could to represent a hypercube on a computer," he says. "I wrote a version in Basic, then another one later for friends. This is the third iteration."
It's more than an iPhone tesseract app: It's a step-by-step guide through the dimensions, starting at zero and ending at four. You start with a sizeless point, which you gradually build into a line, a plane, and a cube. Then: a hypercube. You spin it around. You fold it. You try to grasp it. You literally cross your eyes. Somewhere along the way, you get it.
Olbrich built the app in what little spare time he had over six months, mostly for fun. But he may have happened onto a whole new category of app: teaching tools that people actually want to download. The Fourth Dimension is to boring learning apps what Carl Sagan's Cosmos was to staid science documentaries.
He's already got a followup idea, and it's suitably Saganesque, and perfect for a multitouch device: "If i have time eventually, I'm interested in the concept of scale in space — the scale of things in the solar system."
You can download The Fourth Dimension here. It's three bucks, or 75 cents per dimension.