We have seen that three spatial dimensions are not just the stage on which physics happens.
They help determine what kinds of physics can happen at all. Change the number of large spatial dimensions, and familiar structures begin to strain. Gravity and electric forces behave differently. Stable orbits become harder to arrange. Atoms and chemistry may lose the reliable bound states they need. Even topology changes: knots, entanglements, and certain kinds of persistent structure belong very naturally to three dimensions.
So the question becomes sharper.
Does this explain why space is three-dimensional?
Or does it only explain why beings like us could only find themselves in a universe where something like three-dimensional space was already in place?
That is a different kind of question. It is not mainly about calculating orbits or atoms anymore. It is about what counts as an explanation.
There are at least two ways an explanation of three-dimensional space might work.
One would be a physical necessity explanation.
That kind of explanation would say: the deeper laws of reality require exactly three large spatial dimensions. Space is 3D because, at a more fundamental level, it could not have been otherwise. Some hidden principle, symmetry, consistency condition, or deeper theory would force this structure into place.
In schematic form:
deeper law → three large spatial dimensions
