My last essay noted that spacetime is often treated as a dynamical object yet is not considered a substance. General relativity describes gravity as geometry, but if spacetime curvature stretches, evolves, and propagates waves, what kind of physical reality does that geometry actually represent?

This question is not new. A century ago, Einstein confronted that puzzle. Having eliminated the Lorentz luminiferous ether as a preferred rest frame through special relativity, he later returned to the idea in an unexpected way in 1920. In a lecture at Leiden University, Einstein argued that general relativity assigns spacetime measurable physical qualities, famously remarking, “According to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether.” He also observed that “space without an ether is unthinkable,” since without it neither the propagation of light nor the very standards of space and time could exist. He made clear that this was not a return to the mechanical ether abandoned in the nineteenth century. The ether of general relativity possessed no motion, detectable flow, or constituent parts. Einstein’s argument was that geometric spacetime itself carries physical properties and therefore appeared to occupy an unfamiliar category between material medium and geometry, neither a classical substance nor a purely mathematical abstraction.

The use of the word ether by Einstein did not revive nineteenth-century physics, and the term gradually faded from common use. The underlying question, however, did not go away. As relativity and quantum theory matured, physicists encountered more situations in which spacetime appeared like an active participant in physical processes. The predicted gravitational waves that could carry energy across the universe were detected. Black holes revealed horizons with thermodynamic properties. Quantum theory assigned structure and fluctuation to the vacuum of space. In each case, spacetime exhibited properties normally associated with physical systems. Without returning to Einstein’s language, many researchers continued to explore whether the behavior described as spacetime curvature might reflect deeper organization.

Some physicists took an indirect approach to the puzzle. Rather than debating whether spacetime should be called a substance, they explored whether its geometric description might arise from something deeper. Physicist John Wheeler was among the first to pursue this possibility from within general relativity itself. He asked whether spacetime geometry might generate physical properties normally attributed to matter, asking whether particles themselves might arise as stable patterns within spacetime curvature. His graphic phrase “mass without mass” reflected the idea that geometry might do more than describe motion. Under certain conditions it may be instrumental in the creation of physical effects resembling matter and energy. Although many of these proposals remained investigative, they suggested that geometry could possess a kind of physical productiveness rather than serve merely as a mathematical description.

A different perspective emerged in 1967 when Andrei Sakharov suggested that gravity might not be fundamental at all. He argued that Einstein’s equations could arise as an induced effect of quantum vacuum fluctuations. Gravity would then resemble elasticity in a solid: a large-scale response emerging from microscopic processes rather than a primary interaction. General relativity would remain intact, but its equations would describe collective behavior rather than elemental structure. In this view, spacetime geometry would function as an effective but unseen description of deeper physical organization.

In 1995, Ted Jacobson took another approach. Drawing on the emerging connections between gravity, thermodynamics, and quantum theory, he showed that Einstein’s field equations could be derived from assumptions about entropy and heat flow. He argued that if spacetime obeys thermodynamic relations, then gravity may resemble a macroscopic equation of state rather than a fundamental interaction. Geometry then functions much like temperature or pressure, an extraordinarily accurate description of collective behavior whose microscopic origin remains unseen.

These ideas were reinforced from an unexpected direction. Studies of black holes revealed that horizons possess temperature (Hawking 1975) and entropy (Bekenstein 1973), linking gravity to thermodynamics in ways Einstein had not foreseen. Building on these insights, thermodynamic and informational approaches proposed that Einstein’s field equations might resemble macroscopic relations governing energy flow and information (Verlinde 2011). In this view, spacetime geometry could function as an effective description, much as pressure and temperature describe gases without revealing the motion of individual molecules.

Although these approaches never formed a consensus movement, and differed in method and conclusion, they reflected the same possibility. General relativity described nature with remarkable precision, yet hints kept appearing that geometry’s success might rest on an underlying organization not directly visible in the equations themselves.

Einstein did not propose a hidden medium beneath relativity. But he recognized that general relativity assigns spacetime physical properties while denying it the defining characteristics of ordinary matter. It could influence events and determine gravitational behavior, yet it possessed no motion or structure that could be observed independently. It seems he left us with a persistent puzzle.

General relativity already describes the universe with extraordinary precision. Its equations successfully predict planetary motion, the bending of light, the merging of black holes, and cosmic expansion without appealing to any hidden medium beneath spacetime. In that sense, nothing more may be required. Yet throughout the history of science, deeper physical pictures have often illuminated connections that mathematics alone left obscure. Einstein’s willingness to describe spacetime as possessing physical qualities reminds us that prediction and understanding are not always the same thing.

Does spacetime’s physical qualities suggest the existence of an unfamiliar category between a material medium and abstract geometry, as Einstein suggested? 

I would like to hear what you think.

Robert J. Conover

References:

Einstein, A. (1920). Ether and the Theory of Relativity. Leiden University Lecture. English translation available at: https://mathshistory.st-andrews.ac.uk/Extras/Einstein_ether/

Wheeler, J. A. (1962). Geometrodynamics. Academic Press. Internet Archive version of the book.

https://archive.org/details/geometrodynamics0000whee

Sakharov, A. D. (1967). Vacuum Quantum Fluctuations in Curved Space and the Theory of Gravitation. Soviet Physics Doklady, 12, 1040–1041: Vacuum quantum fluctuations in curved space and the theory of gravitation.

Jacobson, T. (1995). Thermodynamics of Spacetime: The Einstein Equation of State. Physical Review Letters, 75, 1260–1263.  https://arxiv.org/abs/gr-qc/9504004

Bekenstein, J. D. (1973). Black Holes and Entropy. Physical Review D, 7, 2333–2346.
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.7.2333

Hawking, S. W. (1975). Particle Creation by Black Holes. Communications in Mathematical Physics, 43, 199–220.  https://projecteuclid.org/euclid.cmp/1103899181

Verlinde, E. (2011). On the Origin of Gravity and the Laws of Newton. Journal of High Energy Physics, 2011(4), 29.  https://arxiv.org/abs/1001.0785