In the previous essay, I asked what it means to say that spacetime curves. General relativity provides extraordinarily precise equations describing how curvature behaves, predicting planetary motion, gravitational lensing, black holes, and gravitational waves with remarkable accuracy (Einstein 1916; Abbott et al. 2016). Yet they remain silent on a deeper question: what, if anything, is actually curving?
General relativity already encourages a physical way of thinking about spacetime. In Einstein’s theory it is not simply a passive background against which events unfold, but something that changes in response to energy and momentum and, in turn, influences how matter and light move. Physicists therefore treat spacetime as more than a coordinate system. It behaves as something capable of transmitting influence and participating in physical processes. Gravitational waves, for example, are disturbances that travel through spacetime and carry measurable energy across the universe, a prediction confirmed directly by observation (Abbott et al. 2016).
Other familiar phenomena reinforce this picture. Cosmological expansion reflects an evolving spacetime in which distances between galaxies change with time, while black hole mergers release enormous energy through rapid rearrangements of curvature itself. In practice, spacetime is modeled as something that can change, respond, and interact, all qualities we normally associate with physical systems rather than purely mathematical descriptions. Yet despite these features, most physicists remain reluctant to describe spacetime as a material physical system or substance.
Despite these dynamical features, the reluctance to describe spacetime as a substance has deep roots in both experiment and philosophy. In the nineteenth century, light was thought to propagate through a physical medium called the luminiferous ether. When the Michelson–Morley experiment failed to detect Earth’s motion through such a medium (Michelson and Morley 1887), and when Einstein’s special relativity eliminated the need for a medium (Einstein 1905), the lesson seemed decisive: do not introduce unseen substrates when the phenomena can be described without them.
There are also conceptual concerns. If spacetime were treated as a material substance, what kind of substance would it be? A solid suggests elasticity and stress, yet elasticity usually implies microscopic structure. A fluid suggests flow and vortices, but fluids introduce preferred frames of motion, something relativity was designed to avoid (Landau and Lifshitz 1975). Moreover, quantum field theory defines fields within spacetime as a background framework. Treating spacetime itself as a field-like medium raises an additional question: what contains it, inviting a potential regress problem (Weinberg 1995).
Philosophers of physics have long debated whether spacetime should be regarded as something that exists independently (substantivalism) or merely as a network of relations among physical events (relationalism). Modern discussions suggest that this question remains unsettled (Earman 1989; Maudlin 2012). General relativity deepens the debate because spacetime is neither a fixed arena nor simply a bookkeeping device: its geometry evolves alongside matter and energy. The theory therefore blurs traditional distinctions between background and participant, leaving open the question of whether spacetime should ultimately be understood as an entity in its own right or as an emergent description of deeper physical organization.
General relativity therefore leaves us in an unusual position. Spacetime behaves like something capable of change, influence, and interaction, yet the language available to describe it remains uncertain. Physicists avoid calling it a substance, partly because past experience cautions against introducing unseen media, and partly because no familiar category seems to fit. Whether spacetime represents a fundamental entity, a network of relations, or something belonging to a category not yet clearly understood remains an open question.
I invite your comments.
Robert J. Conover
References
Abbott, B. P., et al. (LIGO Scientific Collaboration and Virgo Collaboration). 2016. Observation of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters, 116, 061102.
https://doi.org/10.1103/PhysRevLett.116.061102
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(English translation widely available online.)
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https://doi.org/10.1103/PhysRevLett.75.1260
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Open access version: https://arxiv.org/abs/1001.0785
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