🏷️ タグ: complexity

Why does the universe have three large spatial dimensions?

That question can sound decorative at first, almost philosophical in the loose sense. We are used to treating space as the given background, the silent room in which physics happens. But dimensionality is not neutral scenery. It changes how influences spread, how forces weaken, and what kinds of structures can remain intact through time.

So the real question is sharper than it first appears:

Would anything familiar survive if space had fewer dimensions, or more?

Not just geometry in the abstract. Not just whether equations could still be written down. Would there still be long-lived orbits? Would there still be atoms? Would there still be the kind of persistent, layered complexity that gives you stars, chemistry, planets, and stable environments?

The striking possibility is that three-dimensional space is not an arbitrary stage. It may be one of the conditions that makes a durable world possible.

Space changes the law, not just the picture

A first way to see this is to notice that forces do not merely exist in space. They spread through it.

Imagine a source sending influence outward equally in all directions. In ordinary three-dimensional space, that influence spreads over the surface of a sphere, whose area grows like \( r^2 \). As the same total influence is smeared over more and more area, the strength falls like

$$ F(r) \propto \frac{1}{r^2}. $$