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# Entropic gravity

Entropic gravity is a hypothesis in modern physics that describes gravity as an entropic force; not a fundamental interaction 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 hole thermodynamics by Bekenstein and Hawking 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 demonstrated that the Einstein equations describing relativistic gravitation can be derived by combining general thermodynamic considerations with the equivalence principle.[1] Subsequently, other physicists began to explore links between gravity and entropy.[2][3]

Erik Verlinde's theory

In 2009, Erik Verlinde disclosed a conceptual theory that describes gravity as an entropic force.[4] On January 6, 2010 he published a preprint of a 29 page paper titled On the Origin of Gravity and the Laws of Newton.[5] The paper was published in the Journal of High Energy Physics in April 2011.[6] 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's holographic principle. His theory implies that gravity is not a fundamental interaction, but an emergent phenomenon which arises from the statistical behavior of microscopic degrees 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] the dark energy hypothesis,[13] cosmological acceleration,[14][15] cosmological inflation,[16] and loop quantum gravity.[17] Also, a specific microscopic model has been proposed that indeed leads to entropic gravity emerging at large scales.[18]

Criticism and experimental tests

Verlinde’s theory is criticized by Archil Kobakhidze[19] on the basis that it fails to explain gravitational bound states of neutron observed in the experiments with ultracold neutrons.[20] This conclusion was disputed in a follow-up paper.[21] Another criticism says that if vacuum had a non-zero entropy density, then there would exist a privileged frame of reference, contrary to relativity.[22]

In a recent paper[23] 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[24] 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'" [25]

See also

Entropic elasticity of an ideal chain

Entropic force

Gravitation

Induced gravity

References

^ Jacobson, Theodore (4 April 1995). "Thermodynamics of Spacetime: The Einstein Equation of State". Phys.Rev.Lett.75:1260-1263,1995 75 (7): 1260–1263. arXiv:gr-qc/9504004. Bibcode 1995PhRvL..75.1260J. doi:10.1103/PhysRevLett.75.1260.

^ Padmanabhan, Thanu (26 November 2009). "Thermodynamical Aspects of Gravity: New insights". Rep. Prog. Phys. 73 (2010)01 73 (4): 6901. arXiv:0911.5004. Bibcode 2010RPPh...73d6901P. doi:10.1088/0034-4885/73/4/046901.

^ Mok, H.M. (13 August 2004). "Further Explanation to the Cosmological Constant Problem by Discrete Space-time Through Modified Holographic Principle". arXiv:physics/0408060 [physics.gen-ph].

^ van Calmthout, Martijn (12 December 2009). "Is Einstein een beetje achterhaald?" (in Dutch). de Volkskrant. Retrieved 6 September 2010.

^ Verlinde, Eric (6 January 2010). "Title: On the Origin of Gravity and the Laws of Newton". arXiv:1001.0785 [hep-th].

^ E.P. Verlinde. "On the Origin of Gravity and the Laws of Newton". JHEP 04, 29 (2011). Bibcode 2011JHEP...04..029V. doi:10.1007/JHEP04(2011)029.

^ Overbye, Dennis (July 12, 2010). "A Scientist Takes On Gravity". The New York Times. Retrieved 6 September 2010.

^ E. Verlinde, The Hidden Phase Space of our Universe, Strings 2011, Uppsala, 1 July 2011.

^ The entropy force: a new direction for gravity, New Scientist, 20 January 2010, issue 2744

^ Gravity is an entropic form of holographic information, Wired Magazine, 20 January 2010

^ Fu-Wen Shu; Yungui Gong (2010). "Equipartition of energy and the first law of thermodynamics at the apparent horizon". arXiv:1001.3237 [gr-qc].

^ Rong-Gen Cai; Li-Ming Cao; Nobuyoshi Ohta (2010). "Friedmann Equations from Entropic Force". Phys. Rev. D 8101(R) (2010) 81 (6). arXiv:1001.3470. Bibcode 2010PhRvD..81f1501C. doi:10.1103/PhysRevD.81.061501.

^ It from Bit: How to get rid of dark energy, Johannes Koelman, 2010

^ Easson; Frampton; Smoot (2010). "Entropic Accelerating Universe". Phys.Lett.B696:273-277,2011 696 (3): 273–277. arXiv:1002.4278. Bibcode 2011PhLB..696..273E. doi:10.1016/j.physletb.2010.12.025.

^ Yi-Fu Cai; Jie Liu; Hong Li (2010). "Entropic cosmology: a unified model of inflation and late-time acceleration". Phys.Lett.B 690:213-219,2010 690 (3): 213–219. arXiv:1003.4526. Bibcode 2010PhLB..690..213C. doi:10.1016/j.physletb.2010.05.033.

^ Yi Wang (2010). "Towards a Holographic Description of Inflation and Generation of Fluctuations from Thermodynamics". arXiv:1001.4786 [hep-th].

^ Lee Smolin (2010). "Newtonian gravity in loop quantum gravity". arXiv:1001.3668 [gr-qc].

^ Jarmo Mäkelä (2010). "Notes Concerning "On the Origin of Gravity and the Laws of Newton" by E. Verlinde (arXiv:1001.0785)". arXiv:1001.3808 [gr-qc].

^ Kobakhidze, Archil (15 January 2011). "Gravity is not an entropic force". Phys. Rev. D83: 021502,2011 83 (2): 021502(3 pages). arXiv:[hep-th arXiv:1009.5414 [hep-th]]. Bibcode 2011PhRvD..83b1502K. doi:10.1103/PhysRevD.83.021502.

^ Nesvizhevsky, V.V., et al. (17 January 2002). "Quantum states of neutrons in the Earth's gravitational field". Nature 415 (6869): 297–299. Bibcode 2002Natur.415..297N. doi:10.1038/415297a.

^ Chaichian, Masud; Oksanen, Markku and Tureanu, Anca. "On gravity as an entropic force". arXiv:1104.4650.

^ Why gravity can't be entropic, Luboš Motl Pilsen

^ Arxiv: Once more: gravity is not an entropic force

^ R. Colella, A. W. Overhauser and S. A. Werner, “Observation of gravitationally induced quantum interference,” Phys. Rev. Lett. 34, 1472 (1975)

^ Hossenfelder, Sabine. "Comments on and Comments on Comments on Verlinde's paper "On the Origin of Gravity and the Laws of Newton"". arXiv:1003.1015.

Further reading

It from bit - Entropic gravity for pedestrians, J. Koelman

Gravity: the inside story, T Padmanabhan

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