Entropic gravity, also known asemergent gravity, is a theory in modern physics that describesgravity as anentropic force—a force with macro-scale homogeneity but which is subject toquantum-level disorder—and not afundamental interaction. The theory, based onstring theory,black hole physics, andquantum information theory, describes gravity as anemergent phenomenon that springs from thequantum entanglement of small bits ofspacetime information. As such, entropic gravity is said to abide by thesecond law of thermodynamics under which theentropy of a physical system tends to increase over time.

The theory has been controversial within the physics community but has sparked research and experiments to test its validity.

At its simplest, the theory holds that when gravity becomes vanishingly weak—levels seen only at interstellar distances—it diverges from its classically understood nature and its strength begins to decaylinearly with distance from a mass.

Entropic gravity provides an underlying framework to explainModified Newtonian Dynamics, or MOND, which holds that at agravitational acceleration threshold of approximately1.2×10⁻¹⁰ m/s², gravitational strength begins to vary inverselylinearly with distance from a mass rather than the normalinverse-square law of the distance. This is an exceedingly low threshold, measuring only 12 trillionthsgravity's strength at Earth's surface; an object dropped from a height of one meter would fall for 36 hours were Earth's gravity this weak. It is also 3,000 times less than the remnant of Earth's gravitational field that exists at the point whereVoyager 1 crossed the solar system'sheliopause and entered interstellar space.

The theory claims to be consistent with both the macro-level observations ofNewtonian gravity as well as Einstein'stheory of general relativity and its gravitational distortion of spacetime. Importantly, the theory also explains (without invoking the existence of dark matter and tweaking of its newfree parameters) whygalactic rotation curves differ from the profile expected with visible matter.

The theory of entropic gravity posits that what has been interpreted as unobserved dark matter is the product of quantum effects that can be regarded as a form ofpositivedark energy that lifts thevacuum energy of space from its ground state value. A central tenet of the theory is that the positive dark energy leads to a thermal-volume law contribution to entropy that overtakes the area law ofanti-de Sitter space precisely atthecosmological horizon.

Thus this theory provides an alternative explanation for what mainstream physics currently attributes todark matter. Since dark matter is believed to compose the vast majority of the universe's mass, a theory in which it is absent has huge implications forcosmology. In addition to continuing theoretical work in various directions, there are many experiments planned or in progress to actually detect or better determine the properties of dark matter (beyond its gravitational attraction), all of which would be undermined by an alternative explanation for the gravitational effects currently attributed to this elusive entity.

The thermodynamic description of gravity has a history that goes back at least to research onblack hole thermodynamics byJacob Bekenstein andStephen Hawking in the mid-1970s. These studies suggest a deep connection betweengravity and thermodynamics, which describes the behavior of heat. In 1995,Theodore Jacobson demonstrated that theEinstein field equations describing relativistic gravitation can be derived by combining general thermodynamic considerations with theequivalence principle.^[1] Subsequently, other physicists, most notablyThanu Padmanabhan andGinestra Bianconi, began to explore links between gravity andentropy.^[2][3][4]

In 2009,Erik Verlinde proposed a conceptual model that describes gravity as an entropic force.^[5] He argues (similar to Jacobson's result) that gravity is a consequence of the "information associated with the positions of material bodies".^[6] This model combines the thermodynamic approach to gravity withGerard 't Hooft'sholographic principle. It implies that gravity is not afundamental interaction, but anemergent phenomenon which arises from the statistical behavior of microscopicdegrees of freedom encoded on a holographic screen. The paper drew a variety of responses from the scientific community.Andrew Strominger, 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."^[7]

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.^[8]

Verlinde's article also attracted a large amount of media exposure,^[9][10] and led to immediate follow-up work in cosmology,^[11][12] thedark energy hypothesis,^[13]cosmological acceleration,^[14][15]cosmological inflation,^[16] andloop quantum gravity.^[17] Also, a specific microscopic model has been proposed that indeed leads to entropic gravity emerging at large scales.^[18] Entropic gravity can emerge from quantum entanglement of localRindler horizons.^[19]

The law of gravitation is derived from classical statistical mechanics applied to theholographic principle, that states that the description of a volume of space can be thought of as${\displaystyle N}$ bits of binary information, encoded on a boundary to that region, a closed surface of area${\displaystyle A}$. The information is evenly distributed on the surface with each bit requiring an area equal to${\displaystyle {\ell }_{\text{P}}^2}$, the so-calledPlanck area, from which${\displaystyle N}$ can thus be computed:$${\displaystyle N=\frac{A}{{\ell }_{\text{P}}^2}}$$where${\displaystyle {\ell }_{\text{P}}}$ is thePlanck length. The Planck length is defined as:$${\displaystyle {\ell }_{\text{P}}=\sqrt{\frac{\hslash G}{c^3}}}$$where${\displaystyle G}$ is theuniversal gravitational constant,${\displaystyle c}$ is the speed of light, and${\displaystyle \hslash }$ is the reducedPlanck constant. When substituted in the equation for${\displaystyle N}$ we find:$${\displaystyle N=\frac{A{c}^3}{\hslash G}}$$

The statisticalequipartition theorem defines the temperature${\displaystyle T}$ of a system with${\displaystyle N}$ degrees of freedom in terms of its energy${\displaystyle E}$ such that:$${\displaystyle E=\frac{1}{2}N{k}_{\text{B}}T}$$where${\displaystyle k_{\text{B}}}$ is theBoltzmann constant. [Note, though, that, according to the sameequipartition theorem, this only applies to the quadratic degrees of freedom, that is, to those degrees of freedom${\displaystyle Q}$ whose contribution to the total internal energy is of the form${\displaystyle Q^2}$. This means that one is assuming a model of matter as formed by a collection of independent harmonic oscillators.] This is theequivalent energy for a mass${\displaystyle M}$ according to:$${\displaystyle E=M{c}^2.}$$

The effective temperature experienced due to a uniform acceleration in avacuum field according to theUnruh effect is:$${\displaystyle T=\frac{\hslash a}{2\pi c{k}_{\text{B}}},}$$where${\displaystyle a}$ is that acceleration, which for a mass${\displaystyle m}$ would be attributed to a force${\displaystyle F}$ according toNewton's second law of motion:$${\displaystyle F=ma.}$$

Taking the holographic screen to be a sphere of radius${\displaystyle r}$, the surface area would be given by:$${\displaystyle A=4\pi r^2.}$$

From algebraic substitution of these into the above relations, one derivesNewton's law of universal gravitation:$${\displaystyle F=m\frac{2\pi c{k}_{\text{B}}T}{\hslash }=m\frac{4\pi c}{\hslash }\frac{E}{N}=m\frac{4\pi c^3}{\hslash }\frac{M}{N}=m4\pi \frac{GM}{A}=G\frac{mM}{r^2}.}$$

Note that this derivation assumes that the number of the binary bits of information is equal to the number of the degrees of freedom.$${\displaystyle \frac{A}{{\ell }_{\text{P}}^2}=N=\frac{2E}{k_{\text{B}}T}}$$

Entropic gravity, as proposed by Verlinde in his original article, reproduces theEinstein field equations and, in a Newtonian approximation, a${\displaystyle \frac{1}{r}}$ potential for gravitational forces. Since its results do not differ from Newtonian gravity except in regions of extremely small gravitational fields, testing the theory with Earth-based laboratory experiments does not appear feasible. Spacecraft-based experiments performed atLagrangian points within theSolar System would be expensive and challenging.

Even so, entropic gravity in its current form has been severely challenged on formal grounds.Matt Visser has shown^[20] that the attempt to model conservative forces in the general Newtonian case (i.e. for arbitrary potentials and an unlimited number of discrete masses) leads to unphysical requirements for the required entropy and involves an unnatural number of temperature baths of differing temperatures. Visser concludes:

There is no reasonable doubt concerning the physical reality of entropic forces, and no reasonable doubt that classical (and semi-classical) general relativity is closely related to thermodynamics [52–55]. Based on the work of Jacobson [1–6],Thanu Padmanabhan [7–12], and others, there are also good reasons to suspect a thermodynamic interpretation of the fully relativistic Einstein equations might be possible. Whether the specific proposals of Verlinde [26] are anywhere near as fundamental is yet to be seen – the rather baroque construction needed to accurately reproducen-body Newtonian gravity in a Verlinde-like setting certainly gives one pause.

For the derivation of Einstein's equations from an entropic gravity perspective, Tower Wang shows^[21] that the inclusion of energy-momentum conservation and cosmological homogeneity and isotropy requirements severely restricts a wide class of potential modifications of entropic gravity, some of which have been used to generalize entropic gravity beyond the singular case of an entropic model of Einstein's equations. Wang asserts that:

As indicated by our results, the modified entropic gravity models of form (2), if not killed, should live in a very narrow room to assure the energy-momentum conservation and to accommodate a homogeneous isotropic universe.

Cosmological observations using available technology can be used to test the theory. On the basis of lensing by the galaxy cluster Abell 1689, Nieuwenhuizen concludes that EG is strongly ruled out unless additional (dark) matter-like eV neutrinos is added.^[22] A team fromLeiden Observatory statistically observing thelensing effect of gravitational fields at large distances from the centers of more than 33,000 galaxies found that those gravitational fields were consistent with Verlinde's theory.^[23][24][25] Using conventional gravitational theory, the fields implied by these observations (as well as from measuredgalaxy rotation curves) could only be ascribed to a particular distribution ofdark matter. In June 2017, a study byPrinceton University researcher Kris Pardo asserted that Verlinde's theory is inconsistent with the observed rotation velocities ofdwarf galaxies.^[26][a][27] Another theory of entropy based on geometric considerations (Quantitative Geometrical Thermodynamics, QGT^[28]) provides an entropic basis for the holographic principle^[29] and also offers another explanation for galaxy rotation curves as being due to the entropic influence^[28] of the central supermassive black hole found in the center of a spiral galaxy.

In 2018, Zhi-Wei Wang andSamuel L. Braunstein showed that, while spacetime surfaces near black holes (called stretched horizons) do obey an analog of the first law of thermodynamics, ordinary spacetime surfaces — including holographic screens — generally do not, thus undermining the key thermodynamic assumption of the emergent gravity program.^[30]

In his 1964 lecture on the Relation of Mathematics and Physics,Richard Feynman describes a related theory for gravity where the gravitational force is explained due to an entropic force due to unspecified microscopic degrees of freedom.^[31] However, he immediately points out that the resulting theory cannot be correct as thefluctuation-dissipation theorem would also lead to friction which would slow down the motion of the planets which contradicts observations.

Another criticism of entropic gravity is that entropic processes should, as critics argue, breakquantum coherence. There is no theoretical framework quantitatively describing the strength of such decoherence effects, though. The temperature of the gravitational field in the earth's gravity well is very small (on the order of 10⁻¹⁹ K).

Experiments with ultra-cold neutrons in the gravitational field of Earth are claimed to show that neutrons lie on discrete levels exactly as predicted by theSchrödinger equation considering the gravitation to be a conservative potential field without any decoherent factors. Archil Kobakhidze argues that this result disproves entropic gravity,^[32] while Chaichianet al. suggest a potential loophole in the argument in weak gravitational fields such as those affecting Earth-bound experiments.^[33]

• It from bit – Entropic gravity for pedestrians, J. Koelman
• Gravity: the inside story, T Padmanabhan
• Experiments Show Gravity Is Not an Emergent Phenomenon

• Theories of gravity
• Information theory
• Thermodynamics
• Emergence

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