🏷️ Tag: central forces

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

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