Space has three large dimensions: left-right, forward-back, and up-down.
We rarely experience that as a remarkable fact. Three dimensions do not seem like one property of the universe among others. They seem like the basic format in which properties can exist at all.
A table has length, width, and height. A bird can fly above a road while a car passes beneath it. Your hand can move around an obstacle rather than only toward it or away from it. Ordinary objects occupy volumes, possess interiors, and can be approached from many directions.
Nothing about this feels exotic. Our bodies learned the geometry of three-dimensional space before our minds had words for it.
Yet physics gives us good reason to treat dimensionality as more than a neutral backdrop.
Change the number of spatial dimensions, and forces spread differently. The stability of orbits changes. Atomic bound states change. Topological possibilities, including ordinary knots, change. A universe with a different number of dimensions would not merely provide the same physics with more or fewer directions. It could support a substantially different inventory of durable structures.
This leaves us with two related questions:
Physics does not yet have a settled answer to the first.
It has much more to say about the second.
