In the previous essay, I asked what current physics would require if spacetime geometry were emergent rather than fundamental. Observation placed surprisingly strict demands on any deeper organization. Whatever lies beneath geometry would need to transmit influence across distance, preserve conservation relationships, respond dynamically to matter and energy, remain stable under extreme conditions, be Lorentz invariant, and evolve alongside the expanding universe. If spacetime curvature does emerge from something deeper, the question becomes more precise: what kind of underlying organization could plausibly satisfy all of these requirements?

One possibility begins with something physics already recognizes as active: the vacuum itself. Modern quantum theory no longer treats empty space as simple nothingness. Even in the absence of particles, measurable effects appear. Closely spaced surfaces experience tiny forces arising from vacuum fluctuations, first described theoretically (Casimir 1948) and later confirmed experimentally (Lamoreaux 1997). Fields fluctuate, and energy briefly appears and disappears in ways experiments can detect. What we call “empty space” is therefore not entirely empty. Because of this underlying activity, some researchers have explored whether spacetime geometry might arise from the collective behavior of these fields (Weinberg 1995).

In this view, the vacuum would not simply provide a background within which events occur. Instead, large-scale structure could emerge from how countless small field interactions organize themselves over time. Curvature would not exist as an independent entity imposed from outside. Rather, geometry would reflect the way deeper field processes settle into stable patterns across vast regions of the universe.

This approach offers clear advantages. Fields already carry energy, transmit disturbances, and respond dynamically to interaction—many of the behaviors required by general relativity. But a difficulty remains. Quantum fields are normally described as existing within spacetime (Weinberg 1995). Their formulation often assumes the very geometric structure they are meant to explain. This raises the possibility of circular reasoning: expansion, causality, and gravitational behavior would need to emerge from entities that are themselves defined using spacetime geometry.

A different line of investigation approaches the same question from a thermodynamic perspective. Some researchers have suggested that the equations governing spacetime may reflect deeper statistical behavior. In a widely discussed result, Einstein’s field equations can be recovered from thermodynamic considerations if local regions of spacetime are assigned entropy and temperature associated with horizons (Jacobson 1995). In this view, gravity is not fundamental but instead reflects the large-scale behavior of underlying microscopic processes, much as pressure and temperature arise from the collective motion of molecules. Later work has extended this idea, suggesting that spacetime dynamics themselves may reflect deeper degrees of freedom, reinforcing the possibility that geometry is an emergent, large-scale description rather than a starting point (Padmanabhan 2010).

Physics already provides examples of systems whose large-scale behavior reflects deeper organization. To an observer at large scales, such systems can appear smooth and continuous, even though that behavior arises from underlying structure. In condensed-matter systems, many observed properties do not belong to individual components but emerge from collective interaction. The rigidity of a crystal, for example, does not come from a single atom, but from how many atoms settle into a repeating pattern that resists deformation. Similarly, in superconductors and superfluids, large numbers of particles move in coordinated ways, producing smooth currents or frictionless flow that no individual particle could sustain. In these cases, the familiar large-scale description does not reveal the underlying structure directly—it summarizes how that deeper organization behaves collectively (Anderson 1972; Leggett 2006).

If spacetime geometry is emergent, something similar may be occurring at a more fundamental level. Curvature and gravitational dynamics could represent the large-scale behavior of a deeper physical organization whose internal structure remains hidden, yet whose collective behavior reproduces the properties we attribute to spacetime itself (Volovik 2003).

Seen together, the requirements identified earlier begin to suggest more than a list of constraints. Transmission through local interaction, conservation that cannot fail, responsiveness without instability, and coherence across cosmic history are not independent demands. They can be read as clues pointing toward the kind of deeper organization capable of producing spacetime’s behavior.

What kind of physical organization could sustain such behavior without ever revealing itself directly?

I welcome any ideas you may have.

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 (6): 061102.

Anderson, Philip W. 1972. More Is Different. Science 177 (4047): 393–396.

Aspect, Alain, Philippe Grangier, and Gérard Roger. 1982. Experimental Realization of Einstein–Podolsky–Rosen–Bohm Gedankenexperiment: A New Violation of Bell’s Inequalities. Physical Review Letters 49 (2): 91–94.

Casimir, Hendrik B. G. 1948. On the Attraction Between Two Perfectly Conducting Plates. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen 51: 793–795.
(Open-access scan widely available via ADS and institutional archives.)

Einstein, Albert. 1920. Ether and the Theory of Relativity. Lecture delivered at the University of Leiden, May 5, 1920. Berlin: Springer.
English translation widely available online.

Hensen, Bas, Hannes Bernien, Anaïs E. Dréau, Andreas Reiserer, Norbert Kalb, Machiel S. Blok, et al. 2015. Loophole-Free Bell Inequality Violation Using Electron Spins Separated by 1.3 Kilometres. Nature 526: 682–686.

Jacobson, Ted. 1995. Thermodynamics of Spacetime: The Einstein Equation of State. Physical Review Letters 75 (7): 1260–1263.

Lamoreaux, Steven K. 1997. Demonstration of the Casimir Force in the 0.6 to 6 µm Range. Physical Review Letters 78 (1): 5–8.

Leggett, Anthony J. 2006. Quantum Liquids: Bose Condensation and Cooper Pairing in Condensed-Matter Systems. Oxford: Oxford University Press.

Padmanabhan, Thanu. 2010. Thermodynamical Aspects of Gravity: New Insights. Reports on Progress in Physics 73 (4): 046901.

Volovik, Grigory E. 2003. The Universe in a Helium Droplet. Oxford: Oxford University Press.

Weinberg, Steven. 1995. The Quantum Theory of Fields. Volume I: Foundations. Cambridge: Cambridge University Press.