🏷️ タグ: physics

Why should adding or removing a spatial dimension do anything so dramatic to gravity?

At first glance, it seems like it should only change the picture. Space would have a different geometry, certainly, but why should that alter the basic possibility of a world with stars, planets, and enduring motion?

The hidden hinge is that gravity does not merely happen in space. It spreads through space. Change the dimensionality, and you change the way influence thins out with distance. That means you change the force law itself. And once the force law changes, the architecture of possible motion changes with it.

This is why dimensionality is not just a background feature. It reaches directly into the question of whether bound orbital structure can persist.

Geometry comes first

Imagine a source whose influence spreads outward equally in all directions. In ordinary three-dimensional space, that influence is distributed over the surface of a sphere. The area of that sphere grows like $r^2$, so the same total influence is diluted over a larger and larger surface as distance increases.

That is why the force falls as

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

This is not a numerical accident. It is the three-dimensional expression of geometric spreading.

In $d$ spatial dimensions, the enclosing surface around a point source grows instead like $r^{d-1}$. So the corresponding central force typically scales as

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

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

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