The proposition that our three-dimensional universe could not have been less than three-dimensional is based on our current understanding of physics and the observations we have made. While we cannot provide absolute proof in the strictest sense, there are several lines of evidence and theoretical arguments that strongly support the notion that our universe has three spatial dimensions.
Empirical observations: Our everyday experience and the scientific observations made so far consistently indicate that we live in a three-dimensional world. We perceive objects with three-dimensional shapes, and our measurements and experiments have confirmed this spatial arrangement. From macroscopic scales to the microscopic world, the behavior of matter and the fundamental forces are consistent with three-dimensional space.
Mathematical consistency: Three-dimensional space has proven to be mathematically consistent and compatible with the laws of physics as we understand them. The fundamental equations that describe our universe, such as the laws of gravity (General Relativity) and quantum mechanics, are formulated in three-dimensional spacetime. These mathematical frameworks accurately predict and explain a wide range of physical phenomena, providing further support for the notion of three spatial dimensions.
Fundamental forces and their behavior: The behavior of the fundamental forces, such as gravity and electromagnetism, suggests a three-dimensional spatial structure. The inverse square law, which describes how the strength of these forces diminishes with distance, is consistent with a three-dimensional geometry. If our universe had fewer dimensions, the behavior of these forces would likely be different, which would lead to observable deviations from what we actually observe.
Occam's Razor: Occam's Razor is a principle in science that suggests simpler explanations should be preferred over more complex ones when both can account for the available evidence. Assuming a three-dimensional universe is simpler and more intuitive compared to postulating additional hidden dimensions or a lower-dimensional starting point. Until there is compelling evidence to the contrary, the simplest explanation—that our universe is three-dimensional—is generally favored.
It is important to note that our understanding of the universe is continually evolving, and new discoveries could potentially challenge or refine our current understanding. However, based on the evidence and theories available to us at present, the idea that our universe has always had three spatial dimensions is the most widely accepted and supported perspective.