It is tempting to think of planets as the main test.
Suppose, somehow, a universe with a different number of spatial dimensions managed to have something like gravity, stars, planets, and long-lived orbit-like motion. That would already be a remarkable achievement. It would mean the large-scale structure problem had passed one of its hardest exams.
But chemistry has its own exam.
Atoms are not tiny solar systems in any simple sense. They are quantum systems. An electron does not merely need to be pulled toward a nucleus. It needs to form a stable bound state with it: a durable arrangement with a definite size, definite energy levels, and enough individuality for one kind of atom to differ meaningfully from another.
Attraction is not enough. A trap can be too weak. It can also be too steep.
That is where dimensionality returns, quietly holding the tuning fork.
In ordinary three-dimensional space, a negatively charged electron is attracted to a positively charged nucleus. That much is familiar.
But if attraction alone guaranteed atoms, the story would be much simpler than it is. The electron would just fall inward, and the atom would be a collapse, not a structure.
Quantum mechanics prevents that simple fall. Confining an electron to a smaller region makes its momentum more uncertain, which raises its kinetic energy. So the atom exists through a balance: electrical attraction pulls inward, while quantum localization resists being squeezed too tightly.
