Why Is Spacetime Four-Dimensional?

The Physical Society of Japan has presented the question "Why is spacetime four-dimensional?" among its "70 Mysteries of Physics." This paper addresses that question from the standpoint of the Nakaza New Relativity Theory (NNRT), arguing that the puzzle arises from identifying real spacetime with measurement spacetime. In NNRT, the real foundation consists of one-dimensional universal time and three-dimensional universal space, and real motion among all inertial systems is described by the Galilean transformation (GT). Here, universal spacetime does not imply an absolute rest frame or an ether. By contrast, electromagnetic measurements spanning different inertial systems exhibit Doppler shifts, while gravitational fields produce gravitational redshifts and blueshifts. The quantities directly observed in the Pound–Rebka experiment and Gravity Probe A are frequency differences of electromagnetic radiation or atomic clocks, and GPS satellites are in fact assigned a frequency offset before launch to compensate for such differences. Because differences in clock rate are realized as differences in frequency rate in atomic clocks, the GPS fact that the clock rate can be matched by pre-adjusting only the frequency scale, without changing the source or metric, exposes an internal contradiction in the conventional interpretation that a single curved real spacetime itself determines the rate of clocks. NNRT interprets these effects not as deformation of real time itself, but as changes in the measurement scales by which time and distance are constructed. Moreover, the terms conventionally separated in GPS into a "general-relativistic gravitational effect" and a "special-relativistic velocity effect" are a weak-field decomposition; for a circular-orbit clock in the Schwarzschild metric they are integrated into a single proper-time expression. NNRT inherits the successful predictive mathematics of the Lorentz transformation, the Schwarzschild solution, Einstein-type equations, and the experimental values reproduced by them. The theoretical foundation, however, is reconstructed by restoring universal time, universal three-dimensional space, GT, and Newton's laws of motion to the layer of real motion, placing an electromagnetic observation map above that layer, and reassigning the Lorentz transformation and the four-dimensional metric to the measurement layer. This reconstruction is not a mere alteration of the foundations; it changes the character of the entire theory and establishes NNRT as a new theory of relativity that incorporates the successful mathematics of conventional relativity. Furthermore, resolving the confusion between the real and measurement layers removes not only the mystery of four-dimensional spacetime but also the common source of conventional relativistic paradoxes concerning time, length, and simultaneity.