With Just the Right Measures of Space and Time, We’ve Got a Perfect World

Why is our universe composed of exactly three dimensions of space and one of time?

That may be the only kind of universe that’s habitable.

Space of less than three dimensions would not allow enough complexity to biological molecules or the beings constructed from them. Imagine a two-dimensional, cardboard being. Any path through it that could serve as a digestive track would also cut the creature in half.

More fundamentally, there’s not enough room in two dimensions for all the intricate connections that go into making systems of nerves and blood vessels. Consider the problem of linking up three houses on a two-dimensional surface to the gas company, the water company and the electric company. There’s no possible way of linking each house to each utility without cutting a hole right through (and therefore severing) some of the lines.

In three dimensions, utility lines, like blood vessels, can go over and under each other, so more complex structures are possible.

Finally, gravity couldn’t exist in two-dimensional space, according to physicist Max Tegmark of the Institute for Advanced Studies. Gravity is the curvature of space-time into a dimension that wouldn’t exist in this hypothetical two-dimensional universe.

In contrast, four or more spatial dimensions would give so much room for things to move around that planetary systems couldn’t stay intact; atoms would unravel, their pieces drifting off into the airy spaces of the fourth dimension.

As for time, an extra dimension would add infinite complexity to the problem of predicting even such simple things as where your car is likely to be after five minutes of driving on the freeway. (It could also lead to disturbing social situations--say, chatting with someone who suddenly drifts off into another dimension of time.)

Although universes with any number of dimensions might exist, argues Tegmark, any universe but ours is likely to “correspond to dead worlds, devoid of observers.”