Gravity as Entropy: Is Gravity Emergent from Thermodynamics? A Quanta Magazine Podcast

Short Summary

In this episode of the Quanta Podcast, host Samir Patel speaks with journalist George Musser to explore entropic gravity, the provocative idea that gravity is not a fundamental force but could arise from thermodynamic disorder. The conversation threads through Newtonian gravity, Einsteinian curvature, and the thermodynamic links that connect entropy to space-time geometry, drawing on the work of Jacobson and Verlinde. The discussion then turns to a concrete toy model in which hypothetical qubits form a surrounding haze that aligns in the presence of mass, producing an emergent attraction between bodies. The episode closes with the potential for cosmological and quantum tests, and with a candid look at the challenges ahead in reconciling gravity with quantum theory.

Introduction: The Gravity-Entropy Question

The Quanta Magazine podcast episode opens by framing gravity as a problem that has challenged the best minds for centuries. Gravity is familiar, but a complete description remains elusive, especially when quantum mechanics enters the picture. The host and guest frame entropic gravity as a concept in which gravity is not a fundamental interaction but a derivative consequence of deeper, thermodynamic processes. The central premise is that gravity might arise from the way the universe excepts disorder, or entropy, to evolve, linking a familiar force to the physics of information and state counting. This sets the stage for a detailed exploration of how entropy, energy, and geometry may weave together to produce the gravitational phenomena we observe.

Quote 1

"Entropic gravity is a concept positing that gravity is not fundamental, but a derivative from deeper, entropic processes." - George Musser

Foundations: Newton, Einstein, and Thermodynamics

The episode then reviews gravity's two long-standing pillars: Newton's universal gravitation and Einstein’s general relativity. Newton provides a universal law that governs motions of planets and projectiles alike, while Einstein recasts gravity as the curvature of space-time, removing the need for an action-at-a-distance explanation. The guest emphasizes that General Relativity describes how energy and matter curve space-time, a viewpoint that challenges the view of gravity as a fundamental force in its own right. The discussion naturally moves to a thermodynamic lens on gravity, where entropy and geometry are tightly linked through ideas that energy, entropy, and the curvature of space-time are interdependent. The entropy perspective invites thinking about gravity as an emergent property rather than a basic interaction, a shift that helps bridge gravity with quantum physics.

Quote 2

"Entropy increases in a closed system, and gravity, in extreme regimes like black holes, shows similar irreversible behavior tied to geometry." - George Musser

Entropic Gravity: Jacobson and Verlinde’s Roadmap

The middle portions of the interview trace key historical milestones. Ted Jacobson’s thermodynamic derivation connects energy, entropy, and geometry, suggesting Einstein’s field equations can be seen as thermodynamic relations. The guest explains Verlinde’s entropic gravity as a related but distinct line of thought, where gravity emerges as an effective force from informational or entropic constraints rather than a fundamental pull. Verlinde’s rubber-band metaphor—gravity as an elastic-like response arising from the statistical behavior of deeper ingredients—illustrates how a seemingly fundamental force might be a macroscopic manifestation of underlying microphysics. The conversation also notes that Verlinde’s approach reframes gravity as an emergent phenomenon rather than a standalone law, a view that continues to provoke debate in the physics community.

Quote 3

"Gravity is like a rubber band, not because elasticity is built into nature, but because the polymers in the underlying substrate produce an emergent force." - Eric Verlinde

A Concrete Model: Cubits as the Hidden Medium

Musser describes a concrete toy model proposed by Dan Carney and colleagues, in which a sea of cubits—quantum-like degrees of freedom surrounding space—exhibit directionality (spins or magnetic-like orientations). When a mass is introduced, the cubits align toward it, creating a low-entropy configuration in the surrounding region. The attraction between two masses emerges because bringing masses closer reduces the number of aligned cubits, a gain in entropy for the overall system. The model suppresses a simple, direct cause–effect mechanism in favor of a thermodynamic explanation: an extremely small, local force arises from the aggregated effect of countless microscopic degrees of freedom, producing the familiar gravitational pull. The guest clarifies that while this provides a compelling thermodynamic narrative, the mechanism remains speculative and must be reconciled with the full framework of general relativity to account for observed gravitational phenomena like light bending and planetary precession.

Quote 4

"The qubits align around masses and create a low-entropy condition that pushes masses together." - Dan Carney

The discussion closes by outlining how entropic gravity could be tested. Potential cosmological tests consider gravity in the weak-field regime, where deviations from general relativity might surface in the outskirts of galaxies, potentially offering an alternative lens on dark matter or dark energy. There’s also the prospect of laboratory-scale and tabletop quantum experiments probing whether quantum-measurement processes interact with gravity in ways that align with entropic gravity expectations. The guest stresses that, to be a viable replacement for Einstein’s theory, entropic gravity would need to reproduce all well-tested gravitational phenomena, including gravitational lensing, Mercury’s orbit precession, and precise timekeeping. While the theory remains speculative, its ability to generate testable predictions and its roots in deep questions about emergence, entropy, and space-time make it a persistent, worthwhile line of inquiry in fundamental physics.

Quote 5

"This theory admits of new experimental tests, including in the outskirts of galaxies and in quantum-measurement experiments." - Eric Verlinde