Global InformationQuantum gravity information
Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics. It deals with environments in which neither gravitational nor quantum effects can be ignored,[1] such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars[2][3] as well as in the early stages of the universe moments after the Big Bang.[4]
Three of the four fundamental forces of nature are described within the framework of quantum mechanics and quantum field theory: the electromagnetic interaction, the strong force, and the weak force; this leaves gravity as the only interaction that has not been fully accommodated. The current understanding of gravity is based on Albert Einstein's general theory of relativity, which incorporates his theory of special relativity and deeply modifies the understanding of concepts like time and space. Although general relativity is highly regarded for its elegance and accuracy it has limitations: the gravitational singularities inside of black holes, the ad hoc postulation of dark matter, as well as dark energy and its relation to the cosmological constant are among the current unsolved mysteries regarding gravity;[5] all of which signal the collapse of the general theory of relativity at different scales and highlight the need for a gravitational theory that goes into the quantum realm. At distances close to the Planck length, like those near the center of the black hole, quantum fluctuations of spacetime are expected to play an important role.[6] The breakdown of general relativity at galactic and cosmological scales also points out the necessity for a more robust theory. Finally, the discrepancies between the predicted value for the vacuum energy and the observed values (which, depending on the considerations, can be of 60 or 120 orders of magnitude)[7] highlight the necessity for a quantum theory of gravity.
The field of quantum gravity is actively developing, and theorists are exploring a variety of approaches to the problem of quantum gravity, the most popular being M-theory and loop quantum gravity.[8] All of these approaches aim to describe the quantum behavior of the gravitational field, which does not necessarily include unifying all fundamental interactions into a single mathematical framework. However, many approaches to quantum gravity, such as string theory, try to develop a framework that describes all fundamental forces. Such a theory is often referred to as a theory of everything. Some of the approaches, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces. Other lesser-known but no less important theories include Causal dynamical triangulation, Noncommutative geometry, and Twistor theory.[9]
One of the difficulties of formulating a quantum gravity theory is that direct observation of quantum gravitational effects is thought to only appear at length scales near the Planck scale, around 10−35 meters, a scale far smaller, and hence only accessible with far higher energies, than those currently available in high energy particle accelerators. Therefore, physicists lack experimental data which could distinguish between the competing theories which have been proposed.[n.b. 1][n.b. 2]
Thought experiment approaches have been suggested as a testing tool for quantum gravity theories.[10][11] In the field of quantum gravity there are several open questions – e.g., it is not known how spin of elementary particles sources gravity, and thought experiments could provide a pathway to explore possible resolutions to these questions,[12] even in the absence of lab experiments or physical observations.
In the early 21st century, new experiment designs and technologies have arisen which suggest that indirect approaches to testing quantum gravity may be feasible over the next few decades.[13][14][15][16] This field of study is called phenomenological quantum gravity.
- ^ Rovelli, Carlo (2008). "Quantum gravity". Scholarpedia. 3 (5): 7117. Bibcode:2008SchpJ...3.7117R. doi:10.4249/scholarpedia.7117.
- ^ Overbye, Dennis (10 October 2022). "Black Holes May Hide a Mind-Bending Secret About Our Universe - Take gravity, add quantum mechanics, stir. What do you get? Just maybe, a holographic cosmos". The New York Times. Archived from the original on 16 November 2022. Retrieved 16 October 2022.
- ^ Starr, Michelle (16 November 2022). "Scientists Created a Black Hole in The Lab, And Then It Started to Glow". ScienceAlert. Archived from the original on 15 November 2022. Retrieved 16 November 2022.
- ^ Kiefer, Claus (2012). Quantum gravity. International series of monographs on physics (3rd ed.). Oxford: Oxford University Press. pp. 1–4. ISBN 978-0-19-958520-5.
- ^ Mannheim, Philip (2006). "Alternatives to dark matter and dark energy". Progress in Particle and Nuclear Physics. 56 (2): 340–445. arXiv:astro-ph/0505266. Bibcode:2006PrPNP..56..340M. doi:10.1016/j.ppnp.2005.08.001. S2CID 14024934.
- ^ Nadis, Steve (2 December 2019). "Black Hole Singularities Are as Inescapable as Expected". quantamagazine.org. Quanta Magazine. Archived from the original on 14 April 2020. Retrieved 22 April 2020.
- ^ Koksma, Jurjen; Prokopec, Tomislav (2011). "The Cosmological Constant and Lorentz Invariance of the Vacuum State". arXiv:1105.6296 [gr-qc].
- ^ Penrose, Roger (2007). The road to reality : a complete guide to the laws of the universe. Vintage. p. 1017. ISBN 9780679776314. OCLC 716437154.
- ^ Rovelli, Carlo (2001). "Notes for a brief history of quantum gravity". arXiv:gr-qc/0006061.
- ^ Bose, S.; et al. (2017). "Spin Entanglement Witness for Quantum Gravity". Physical Review Letters. 119 (4): 240401. arXiv:1707.06050. Bibcode:2017PhRvL.119x0401B. doi:10.1103/PhysRevLett.119.240401. PMID 29286711. S2CID 2684909.
- ^ Marletto, C.; Vedral, V. (2017). "Gravitationally Induced Entanglement between Two Massive Particles is Sufficient Evidence of Quantum Effects in Gravity". Physical Review Letters. 119 (24): 240402. arXiv:1707.06036. Bibcode:2017PhRvL.119x0402M. doi:10.1103/PhysRevLett.119.240402. PMID 29286752. S2CID 5163793.
- ^ Nemirovsky, J.; Cohen, E.; Kaminer, I. (5 November 2021). "Spin Spacetime Censorship". Annalen der Physik. 534 (1). arXiv:1812.11450. doi:10.1002/andp.202100348. S2CID 119342861.
- ^ Hossenfelder, Sabine (2 February 2017). "What Quantum Gravity Needs Is More Experiments". Nautilus. Archived from the original on 28 January 2018. Retrieved 21 September 2020.
- ^ Experimental search for quantum gravity. Cham: Springer. 2017. ISBN 9783319645360.
- ^ Carney, Daniel; Stamp, Philip C. E.; Taylor, Jacob M. (7 February 2019). "Tabletop experiments for quantum gravity: a user's manual". Classical and Quantum Gravity. 36 (3): 034001. arXiv:1807.11494. Bibcode:2019CQGra..36c4001C. doi:10.1088/1361-6382/aaf9ca. S2CID 119073215.
- ^ Danielson, Daine L.; Satishchandran, Gautam; Wald, Robert M. (2022-04-05). "Gravitationally mediated entanglement: Newtonian field versus gravitons". Physical Review D. 105 (8): 086001. arXiv:2112.10798. Bibcode:2022PhRvD.105h6001D. doi:10.1103/PhysRevD.105.086001. S2CID 245353748. Archived from the original on 2023-01-22. Retrieved 2022-12-11.
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