| entropic gravity | |
|---|---|
| Name | Entropic gravity |
| Field | Theoretical physics |
| Introduced | 2010 |
| Introduced by | Erik Verlinde |
| Related | Thermodynamics, General relativity, Quantum mechanics |
entropic gravity
Entropic gravity is a theoretical proposal that gravity is not a fundamental force but an emergent, entropic phenomenon arising from microscopic degrees of freedom. It was introduced to address the relationship between thermodynamics and gravitation, suggesting links between spacetime, information, and quantum degrees of freedom. The idea matters in the context of quantum gravity because it offers an alternative viewpoint to approaches like string theory and loop quantum gravity on how classical gravity might emerge from quantum constituents.
Entropic gravity was popularized in 2010 by Dutch theoretical physicist Erik Verlinde in a paper arguing that gravitational attraction can be understood as an entropic force associated with changes in information on holographic screens. The proposal built on prior work by Jacob Bekenstein on black hole thermodynamics and the Bekenstein–Hawking entropy relation, as well as Stephen Hawking's discovery of Hawking radiation. It also drew on the holographic principle articulated by Gerard 't Hooft and Leonard Susskind, and on insights from Rolf Landauer's information thermodynamics. Early reception was mixed: some researchers saw entropic gravity as a provocative bridge between statistical mechanics and general relativity; others treated it as speculative relative to established programs such as AdS/CFT correspondence.
The approach rests on a synthesis of concepts from statistical mechanics, information theory, and quantum field theory. Central is the notion that spacetime degrees of freedom encode information on lower-dimensional surfaces, echoing the holographic principle and concrete realizations like the AdS/CFT correspondence by Juan Maldacena. Entropic gravity leverages the Bekenstein bound and thermodynamic identities to derive effective forces. Quantum physics enters through the microphysics of the degrees of freedom, the role of quantum entanglement in generating area laws for entropy (as in work by Mark Van Raamsdonk and studies of entanglement entropy), and the expectation that a correct theory must reduce to quantum field theory in appropriate limits. Institutions and collaborations that have explored these ideas include research groups at Princeton University, University of Amsterdam, and CERN theorists who examine emergent spacetime scenarios.
Verlinde's heuristic derivation begins with an entropy change ΔS associated with moving a test mass near a holographic screen, combined with the thermodynamic relation FΔx = TΔS. Using the Unruh temperature experienced by an accelerated observer (from William Unruh's work) and associating energy on the screen with mass via E = Mc^2, one obtains Newton's law F = G m M / r^2 under assumptions about information density on the screen and the number of microscopic bits N ∝ A / ℓ_P^2, where A is area and ℓ_P is the Planck length. Extensions attempt to recover the Einstein field equations using variational or equipartition arguments analogous to those used by Ted Jacobson in deriving Einstein equations from thermodynamics of local Rindler horizons. Key relations referenced include the Bekenstein–Hawking entropy S = k_B A / (4 ℓ_P^2), the Unruh temperature T = ℏ a / (2π c k_B), and equipartition E = (1/2) N k_B T in semiclassical approximations.
Entropic gravity makes contact with astrophysical and cosmological observations primarily through proposed modifications at galactic or cosmological scales. Verlinde and others have suggested that emergent-gravity effects might account for phenomena attributed to dark matter or provide alternate explanations to Modified Newtonian Dynamics (MOND) proposals by Mordehai Milgrom. Empirical scrutiny has involved rotational curves of galaxies, gravitational lensing maps (including work tied to the Bullet Cluster), and constraints from cosmic microwave background data from missions such as Planck. To date, entropic gravity has not supplanted the Lambda-CDM model; competing fits to rotation curves and lensing often require additional assumptions. Precision tests in the Solar System and laboratory remain consistent with classical Newtonian mechanics and General relativity.
Critics argue that entropic gravity, in its original formulations, is heuristic and lacks a detailed microscopic model specifying the underlying degrees of freedom and dynamics. Key criticisms come from proponents of string theory and loop quantum gravity who question whether entropic arguments can recover full nonlinear dynamics of spacetime without extra structure. Specific debates center on reproducing observed gravitational lensing, the dynamics of galaxy clusters, and whether Verlinde's entropic corrections match precision cosmological data. Alternative emergent gravity frameworks include proposals by Ted Jacobson, Padmanabhan-style thermodynamic derivations, and more formal approaches via AdS/CFT and tensor network models developed by groups at Perimeter Institute and Caltech.
If gravity is emergent and entropic, it reshapes interpretation of cosmological issues like the nature of dark energy and entropy budget of the universe. Entropic scenarios emphasize the role of horizon entropy (cosmic and black hole horizons) and suggest reevaluating the second law of thermodynamics in cosmological settings. In black hole physics, the approach reinforces connections between horizon area, entropy, and quantum microstates sought in string theory microstate counts (e.g., work by Andrew Strominger and Cumrun Vafa). Entropic viewpoints feed into discussions of the information paradox and complement research into how entanglement structure may underlie spacetime connectivity, exemplified by the slogan "entanglement builds geometry" pursued in quantum gravity circles.
Future progress requires explicit microscopic models that produce the assumed entropy-area relations and dynamics while embedding within a quantum framework compatible with quantum field theory and cosmological observations. Ongoing directions include constructing candidate microscopic systems (e.g., network models, tensor networks, or condensed-matter analogues), refining connections to AdS/CFT correspondence, and confronting predictions with astronomical surveys (e.g., Dark Energy Survey) and gravitational-wave observations from LIGO and VIRGO. For the conservative ideal of intellectual cohesion, entropic gravity invites synthesis across thermodynamics, information theory, and established quantum gravity programs rather than replacement, aiming to preserve tested pillars of physics while exploring emergent descriptions of spacetime.