What does it mean to be to space-time? The seemingly simple question gets to the essence of twentieth-century physics and has driven a hundred-year debate between philosophy and science. In Einstein’s general relativity, space and time combine to create a four-dimensional continuum whose shape depends on energy and matter. This is more than a mathematical nicety it properly calculates gravitational lensing, time dilation, and the acceleration of the universe with unprecedent accuracy. But the ontological status of this continuum remains open.
Space-time is occasionally described by physicists as a “fabric” or “block universe,” a complete four-dimensional map that contains all events past, present, and future. This photograph is consistent with eternalism, the view that all instants in time are on an equal plane, just as all instants in space are without hierarchy. Where the relativity of simultaneity holds, there is no such thing as “now” universally; what exists for one may be past or future for another. This experiment disproves presentism, the common sense that there is only the present, and confirms the eternalist model.
But eternalism muddles an essential difference between existing and occurring. There is something that exists as a transient three-dimensional cross-section of that elephant does not. The latter merely occurs it is part of a larger, longer-enduring whole. Carrying this over to the universe, the question becomes: does four-dimensional space-time exist in the same sense as the immortal elephant, or is it merely the sum of all that has transpired enumerated in a geometric framework? If the block universe is timeless in reality, then when is it? To respond, some have speculated on a five-dimensional framework, where a second dimension of time renders space-time itself “to exist” in a higher plane of being though this is beyond established physics.
The mathematical formalism of general relativity characterizes space-time as a differentiable manifold with a metric tensor, which determines distances and causality. This geometric framework has nothing to say regarding whether or not the manifold is a substance, a structure, or an abstraction. Substantivalists regard it as a vessel in which things happen; relationists see it as mere nexus of relations between things. The debate is a repetition of the ancient battles of absolute and relative space and time, from Newton’s bucket argument to Leibniz’s principle of sufficient reason.
Quantum theory adds further complexity. Microscopically, the continuous smoothness of relativity collides with the probabilistic, “chunky” character of quantum mechanics. Attempts at bringing the two together string theory, loop quantum gravity, and more esoteric concepts such as Craig Hogan’s “holometer” experiment raise questions about whether space-time is quantized. Hogan’s gadget is a sensor that measures mirror position to 10⁻¹⁸ meters, searching for jitter that would be the signature of a discrete spatial texture. If such granularity is found to exist, the continuum of general relativity would itself be an emergent approximation, and the “being” of space-time would be akin to the being of the surface of a fluid real at one level of description, illusory at another.
Lee Smolin, however, demands going back to Einstein’s relational concepts with the universe being one closed system unto itself without reference to an external clock. For him, time is fundamental and the physical laws can evolve. This view counteracts the timelessness block of eternalism and rather suggests that becoming is inherent in the texture of the universe. It also restates the question of the reality of space-time: if time is fundamental, then space-time may not be so much a frozen entity but an active history of encounters.
The metaphysical price is steep. If everything that transpires is as real, then free will and alteration are puzzles to be unraveled. If present moments alone are real, then relativity’s experimental foundation challenges our ontology. And if space-time is an emergent entity from more fundamental quantum structures, its “existence” could be no more fundamental than a rainbow’s dependent upon processes below, yet central to our description of reality.
