Gravity as an entropic force
Encyclopedia
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
in modern physics
Modern physics
The term modern physics refers to the post-Newtonian conception of physics. The term implies that classical descriptions of phenomena are lacking, and that an accurate, "modern", description of reality requires theories to incorporate elements of quantum mechanics or Einsteinian relativity, or both...
that describes gravity as an entropic force
Entropic force
In physics, an entropic force acting in a system is a phenomenological force resulting from the entire system's statistical tendency to increase its entropy, rather than from a particular underlying microscopic force.-Polymers:...
; not a fundamental interaction
Fundamental interaction
In particle physics, fundamental interactions are the ways that elementary particles interact with one another...
mediated by a particle, but a probabilistic consequence of physical systems' tendency to increase their entropy. The proposal has been intensely contested in the physics community.
Origin
The probabilistic description of gravity has a history that goes back at least to research on black holeBlack hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
thermodynamics by Bekenstein
Jacob Bekenstein
Jacob David Bekenstein is an Israeli theoretical physicist who has contributed to the foundation of black hole thermodynamics and to other aspects of the connections between information and gravitation.-Biography:...
and Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...
in the mid-1970s. These studies suggest a deep connection between gravity and thermodynamics, which describes the behavior of heat and gases. In 1995, Jacobson
Theodore Jacobson
Theodore A. "Ted" Jacobson is an American theoretical physicist. He is known for his work on the connection between gravity and thermodynamics. In particular, in 1995 Jacobson proved that the Einstein field equations describing relativistic gravity can be derived from thermodynamic...
demonstrated that the Einstein equations describing relativistic gravitation can be derived by combining general thermodynamic considerations with the equivalence principle
Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
. Subsequently, other physicists began to explore links between gravity and entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
.
Erik Verlinde's theory
In 2009, Erik VerlindeErik Verlinde
Erik Peter Verlinde is a Dutch theoretical physicist and string theorist. He is the identical twin brother of physicist Herman Verlinde. The Verlinde formula, which is important in conformal field theory and topological field theory, is named after him. His research deals with string theory,...
disclosed a conceptual theory that describes gravity as an entropic force. On January 6, 2010 he published a preprint
Preprint
A preprint is a draft of a scientific paper that has not yet been published in a peer-reviewed scientific journal.-Role:Publication of manuscripts in a peer-reviewed journal often takes weeks, months or even years from the time of initial submission, because manuscripts must undergo extensive...
of a 29 page paper titled On the Origin of Gravity and the Laws of Newton. The paper was published in the Journal of High Energy Physics
Journal of High Energy Physics
The Journal of High Energy Physics is a refereed scientific journal in the field of high energy physics, owned by the International School for Advanced Studies and published by Springer.The journal covers the following areas of theoretical and experimental physics:Collider PhysicsUnderground and...
in April 2011. Reversing the logic of over 300 years, it argued that gravity is a consequence of the "information associated with the positions of material bodies". This theory combines the thermodynamic approach to gravity with Gerardus 't Hooft
Gerardus 't Hooft
Gerardus 't Hooft is a Dutch theoretical physicist and professor at Utrecht University, the Netherlands. He shared the 1999 Nobel Prize in Physics with his thesis advisor Martinus J. G...
's holographic principle
Holographic principle
The holographic principle is a property of quantum gravity and string theories which states that the description of a volume of space can be thought of as encoded on a boundary to the region—preferably a light-like boundary like a gravitational horizon...
. If proven correct, it would imply that gravity is not a fundamental interaction
Fundamental interaction
In particle physics, fundamental interactions are the ways that elementary particles interact with one another...
, but an emergent phenomenon which arises from the statistical behavior of microscopic degrees of freedom
Degrees of freedom (physics and chemistry)
A degree of freedom is an independent physical parameter, often called a dimension, in the formal description of the state of a physical system...
encoded on a holographic screen. The paper drew a variety of responses from the scientific community. Andrew Strominger
Andrew Strominger
Andrew Eben Strominger is an American theoretical physicist who works on string theory and son of Jack L. Strominger. He is currently a professor at Harvard University and a senior fellow at the Society of Fellows...
, a string theorist at Harvard said “Some people have said it can’t be right, others that it’s right and we already knew it — that it’s right and profound, right and trivial."
In July 2011 Verlinde presented the further development of his ideas in a contribution to the Strings 2011 conference, including an explanation for the origin of dark matter.
Verlinde's article also attracted a large amount of media exposure, and led to immediate follow-up work in cosmology, the dark energy hypothesis
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
, cosmological acceleration
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
, cosmological inflation, and loop quantum gravity
Loop quantum gravity
Loop quantum gravity , also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the theories of quantum mechanics and general relativity...
. Also, a specific microscopic model has been proposed that indeed leads to entropic gravity emerging at large scales.
Criticism and experimental tests
Verlinde’s theory is criticized by Archil Kobakhidze on the basis that it fails to explain gravitational bound states of neutron observed in the experiments with ultracold neutrons. This conclusion was disputed in a follow-up paper. Another criticism says that if vacuum had a non-zero entropy density, then there would exist a privileged frame of reference, contrary to relativity.In a recent paper Kobakhidze argues that the counter to his argument against entropic gravity is invalid. Kobakhidze argues that an entropic origin of gravity would be in conflict with the observation of interference patterns in neutron two-slit experiments, when the slits are separated along the direction of the gravitational field. If there were a much larger number of microstates in the slit at the lower gravitational potential, he argues, this would destroy the interference pattern that was observed.
An example of the intense debate that Verlinde has provoked is "Comments on and Comments on Comments on Verlinde's paper 'On the Origin of Gravity and the Laws of Newton'"
See also
- Entropic elasticity of an ideal chain
- Entropic forceEntropic forceIn physics, an entropic force acting in a system is a phenomenological force resulting from the entire system's statistical tendency to increase its entropy, rather than from a particular underlying microscopic force.-Polymers:...
- GravitationGravitationGravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
- Induced gravityInduced gravityInduced gravity is an idea in quantum gravity that space-time background emerges asa mean field approximation of underlying microscopic degrees of freedom, similar to the fluid mechanics approximation of Bose–Einstein condensates...
Further reading
- It from bit - Entropic gravity for pedestrians, J. Koelman
- Gravity: the inside story, T Padmanabhan