The intuition runs deep: space is the stage upon which physics unfolds, the container within which objects exist and events occur. Newton formalized this as absolute space—a fixed, three-dimensional arena independent of its contents. Even Einstein, while revolutionizing our understanding by weaving space and time into dynamic spacetime, preserved its fundamental status. Spacetime curves and warps, but it remains the basic fabric of reality.

Yet developments in quantum gravity increasingly suggest this picture inverts the true order of explanation. From holographic dualities in string theory to quantum information approaches to gravity, a striking consensus is emerging: spacetime itself may be derivative, emerging from more fundamental structures that possess no spatial character whatsoever. The three dimensions we navigate, the distances we measure, the very notion of 'here' versus 'there'—all may be approximate descriptions valid only at macroscopic scales, dissolving into something radically different at the Planck scale.

This possibility represents more than a technical revision to physics. If spacetime emergence proves correct, it demands a fundamental reconceptualization of locality, causation, and the relationship between quantum mechanics and general relativity. The hard problem isn't just deriving Einstein's equations from non-spatial primitives—it's understanding what kind of world we inhabit if space is genuinely illusory. What follows examines the strongest theoretical evidence for this proposal and confronts its profound metaphysical implications.

The holographic principle emerged from an unlikely source: black hole thermodynamics. Bekenstein and Hawking's discovery that black hole entropy scales with horizon area rather than volume suggested something profound. If entropy measures information, and black holes represent maximal entropy for a given region, then the information content of any spatial volume might be bounded by its surface area. The universe's information, in some sense, lives on boundaries rather than in bulk.

This intuition found precise mathematical expression in Juan Maldacena's 1997 discovery of AdS/CFT correspondence—arguably the most important development in theoretical physics of the past three decades. The correspondence establishes an exact equivalence between a theory of quantum gravity in (d+1)-dimensional anti-de Sitter space and a conformal field theory without gravity living on the d-dimensional boundary. A fully quantum gravitational theory in the bulk is mathematically identical to a non-gravitational quantum theory with one fewer spatial dimension.

The implications are vertiginous. Consider a string theory with gravity in five-dimensional AdS space. Every gravitational phenomenon in this bulk—black hole formation, gravitational waves, spacetime curvature—has a precise translation into the four-dimensional boundary theory, which contains no gravity and no extra dimension. The fifth dimension, along with its gravitational dynamics, is entirely encoded in correlations and entanglement patterns of the boundary theory. The bulk emerges from the boundary; the spatial dimension is constructed, not fundamental.

Critics note that AdS/CFT concerns anti-de Sitter space—negatively curved spacetime quite unlike our approximately flat, positively-curved (de Sitter) universe. This is legitimate: we lack an analogous dS/CFT correspondence, though not for want of trying. Yet the existence of even one rigorous example where space demonstrably emerges from non-spatial degrees of freedom constitutes proof of concept. If holographic emergence is possible in any consistent theory, the burden shifts to explaining why our universe would differ.

Moreover, holographic reasoning extends beyond AdS/CFT. The covariant entropy bound, Bousso's generalization of Bekenstein's original insight, applies to arbitrary spacetimes. The holographic principle appears to be a feature of quantum gravity as such, not merely of specific solutions. This suggests that whatever fundamental theory ultimately describes our universe, spatial information will be boundary-encoded—space will emerge from lower-dimensional structures.

Takeaway

When a complete theory of quantum gravity in higher-dimensional space proves mathematically equivalent to a non-gravitational theory in fewer dimensions, we must take seriously that spatial dimensions are emergent rather than fundamental—even if the specific correspondence doesn't directly describe our universe.

Mark Van Raamsdonk's 2010 thought experiment crystallized a remarkable implication of AdS/CFT. Consider the boundary CFT in its vacuum state—a highly entangled quantum state where different spatial regions share quantum correlations. This boundary state corresponds, via holographic duality, to empty AdS space in the bulk. Now imagine gradually reducing entanglement between two halves of the boundary theory. Van Raamsdonk demonstrated that the corresponding bulk geometry responds dramatically: a spatial bridge connecting the two regions pinches off and separates. Removing entanglement literally disconnects spacetime.

The conclusion inverts our usual understanding of physics and geometry. We typically assume spatial geometry exists first, then ask how quantum fields behave upon this geometric stage. Van Raamsdonk's analysis suggests the opposite: quantum entanglement is primary, and spatial connectivity is a macroscopic manifestation of microscopic correlations. Two regions are 'near' each other in emergent space precisely because their underlying quantum degrees of freedom are highly entangled. Distance, in this picture, measures inverse entanglement.

This framework has been dramatically extended through the Ryu-Takayanagi formula and its generalizations. The formula states that entanglement entropy of a boundary region equals the area of the minimal surface in the bulk that bounds that region, divided by 4G. Entanglement and geometry are not merely correlated—they are quantitatively identified. The geometric quantity (minimal surface area) literally equals the information-theoretic quantity (entanglement entropy). Such precise correspondence strongly suggests both quantities describe the same underlying reality.

The ER=EPR conjecture, proposed by Maldacena and Susskind, extends this reasoning. Einstein-Rosen bridges (wormholes) and Einstein-Podolsky-Rosen correlations (quantum entanglement) may be two descriptions of the same phenomenon. Entangled particles, however distant in emergent space, remain connected by non-traversable wormholes at the fundamental level. What appears as 'spooky action at a distance' in quantum mechanics and what appears as spatial connectivity in general relativity are unified: correlation is connection.

These developments suggest that quantum information theory provides the vocabulary for fundamental physics. Concepts like entanglement, mutual information, and quantum error correction aren't merely useful for describing certain phenomena—they may constitute the primitive ontology from which spacetime derives. The metric tensor of general relativity, specifying distances and angles, emerges from patterns in the entanglement structure of fundamental quantum states.

Takeaway

If entanglement constitutes spatial connection rather than merely correlating across it, then quantum information theory isn't just a tool for physics—it's the language in which fundamental reality is written, with geometry as a derived, approximate description.

Spacetime emergence poses severe challenges to locality—the principle that physical influences propagate through space, affecting nearby regions before distant ones. If space itself emerges from non-spatial structures, locality cannot be a fundamental constraint. What appears as local interaction in the emergent geometric description may reflect non-local entanglement patterns in the fundamental theory. This doesn't violate relativistic causality in practice—the emergent description remains predictively accurate at accessible scales—but it undermines locality's status as a basic principle.

Causation faces similar revision. We naturally understand causes as temporally prior events that influence effects through spatiotemporal connection. But if both space and time emerge from more fundamental structures (as many quantum gravity approaches suggest), causation cannot fundamentally work this way. The challenge isn't merely to derive effective causal relationships from non-spatiotemporal primitives—it's to understand what causation even means in a pre-geometric context. Some researchers propose that causal structure might be among the primitive features from which spacetime emerges, rather than being derivative. Others suggest replacing causation with purely information-theoretic dependencies.

The relationship between quantum mechanics and general relativity transforms dramatically under emergence. The standard framing treats quantum gravity as the problem of quantizing the gravitational field—applying quantum principles to spacetime itself. But if spacetime emerges from quantum entanglement, the relationship inverts. Quantum mechanics is fundamental; general relativity is an approximate, emergent description valid when entanglement patterns exhibit appropriate structure. The 'problem' of quantum gravity dissolves into the 'problem' of understanding how classical geometry arises from quantum information—difficult, but conceptually cleaner.

Consider the measurement problem through this lens. If spatial separation is itself entanglement-dependent, and measurement involves changes in entanglement structure, then measurement may play a more fundamental role than typically assumed. Some speculate that wavefunction 'collapse' might be reconceptualized in terms of entanglement dynamics in pre-geometric degrees of freedom. This remains highly speculative, but spacetime emergence opens conceptual space for novel approaches to quantum foundations.

Perhaps most fundamentally, emergence challenges substantivalism—the view that spacetime exists independently of its material contents. If space emerges from quantum states of matter, the container-contents distinction collapses. There is no stage prior to actors; the stage is constructed by the actors' relationships. This favors relationalist interpretations where spatial properties reduce to relations between physical entities—a vindication of Leibniz against Newton, achieved through physics Newton could never have imagined.

Takeaway

If spacetime genuinely emerges, we must abandon locality and causation as fundamental principles, reconceive quantum gravity as the problem of deriving geometry from entanglement, and accept that the container-contents distinction between space and matter dissolves at the deepest level.

The evidence for spacetime emergence—holographic dualities, entanglement-geometry correspondences, information-theoretic approaches to gravity—hasn't yet delivered a complete theory of quantum gravity. We cannot declare with certainty that space is illusory. But the theoretical pressure is substantial and growing. Multiple independent research programs converge on the conclusion that spatial structure derives from something more fundamental.

For philosophers, this demands serious engagement. If the physics community's direction proves correct, our most basic metaphysical categories require revision. Locality, causation, the space-matter distinction, the relationship between quantum and classical—all require rethinking. The opportunity is a reconceptualization of reality as thorough as any in history.

The question 'Is space an illusion?' may ultimately prove poorly framed. Space is real at the scales where we live and make measurements—as real as temperature or pressure. But like those quantities, it may be emergent, approximate, and reducible to more fundamental structures. What those structures are, and what it means to inhabit a world where space is constructed rather than given, remains physics and philosophy's deepest open problem.