#jsDisabledContent { display:none; } My Account | Register | Help
 Flag as Inappropriate This article will be permanently flagged as inappropriate and made unaccessible to everyone. Are you certain this article is inappropriate?          Excessive Violence          Sexual Content          Political / Social Email this Article Email Address:

# Euclidean quantum gravity

Article Id: WHEBN0000912982
Reproduction Date:

 Title: Euclidean quantum gravity Author: World Heritage Encyclopedia Language: English Subject: Collection: Quantum Gravity Publisher: World Heritage Encyclopedia Publication Date:

### Euclidean quantum gravity

In theoretical physics, Euclidean quantum gravity is a version of quantum gravity. It seeks to use the Wick rotation to describe the force of gravity according to the principles of quantum mechanics.

## Contents

• Introduction in layperson's terms 1
• The Wick rotation 1.1
• Application in quantum mechanics 1.2
• More rigorous treatment 2
• Relation to ADM Formalism 3
• References 4

## Introduction in layperson's terms

### The Wick rotation

In physics, a Wick rotation, named after Gian-Carlo Wick, is a method of finding a solution to dynamics problems in n dimensions, by transposing their descriptions in n+1 dimensions, by trading one dimension of space for one dimension of time. More precisely, it substitutes a mathematical problem in Minkowski space into a related problem in Euclidean space by means of a transformation that substitutes an imaginary-number variable for a real-number variable.

It is called a rotation because when complex numbers are represented as a plane, the multiplication of a complex number by i is equivalent to rotating the vector representing that number by an angle of \pi/2 about the origin.

For example, a Wick rotation could be used to relate a macroscopic event temperature diffusion (like in a bath) to the underlying thermal movements of molecules. If we attempt to model the bath volume with the different gradients of temperature we would have to subdivide this volume into infinitesimal volumes and see how they interact. We know such infinitesimal volumes are in fact water molecules. If we represent all molecules in the bath by only one molecule in an attempt to simplify the problem, this unique molecule should walk along all possible paths that the real molecules might follow. Path integral formulation is the conceptual tool used to describe the movements of this unique molecule, and Wick rotation is one of the mathematical tools that are very useful to analyse an integral path problem.

### Application in quantum mechanics

In a somewhat similar manner, the motion of a quantum object as described by quantum mechanics implies that it can exist simultaneously in different positions and have different speeds. It differs clearly to the movement of a classical object (e.g. a billiard ball), since in this case a single path with precise position and speed can be described. A quantum object does not move from A to B with a single path, but moves from A to B by all ways possible at the same time. According to the principle of superposition (Richard Feynman's integral of path in 1963), the path of the quantum object is described mathematically as a weighted average of all those possible paths. In 1966 an explicitly gauge invariant functional-integral algorithm was found by DeWitt, which extended Feynman's new rules to all orders. What is appealing in this new approach is its lack of singularities when they are unavoidable in general relativity.

Another operational problem with general relativity is the difficulty to do calculations, because of the complexity of the mathematical tools used. Integral of path in contrast has been used in mechanics since the end of the 19th century and is well known. In addition Path integral is a formalism used both in mechanics and quantum theories so it might be a good starting point for unifying general relativity and quantum theories. Some quantum features like the Schrödinger equation and the heat equation are also related by Wick rotation. So the Wick relation is a good tool to relate a classical phenomenon to a quantum phenomenon. The ambition of Euclidean quantum gravity is to use the Wick rotation to find connections between a macroscopic phenomenon, gravity, and something more microscopic.

## More rigorous treatment

Euclidean quantum gravity refers to a Wick rotated version of quantum gravity, formulated as a quantum field theory. The manifolds that are used in this formulation are 4-dimensional Riemannian manifolds instead of pseudo Riemannian manifolds. It is also assumed that the manifolds are compact, connected and boundaryless (i.e. no singularities). Following the usual quantum field-theoretic formulation, the vacuum to vacuum amplitude is written as a functional integral over the metric tensor, which is now the quantum field under consideration.

\int \mathcal{D}\bold{g}\, \mathcal{D}\phi\, \exp\left(\int d^4x \sqrt{|\bold{g}|}(R+\mathcal{L}_\mathrm{matter})\right)

where φ denotes all the matter fields. See Einstein–Hilbert action.

## Relation to ADM Formalism

Euclidean Quantum Gravity does relate back to ADM formalism used in canonical quantum gravity and recovers the Wheeler–DeWitt equation under various circumstances. If we have some matter field \phi, then the path integral reads

Z = \int \mathcal{D}\bold{g}\, \mathcal{D}\phi\, \exp\left(\int d^4x \sqrt{|\bold{g}|}(R+\mathcal{L}_\mathrm{matter})\right)

where integration over \mathcal{D}\bold{g} includes an integration over the three-metric, the lapse function N, and shift vector N^{a}. But we demand that Z be independent of the lapse function and shift vector at the boundaries, so we obtain

\frac{\delta Z}{\delta N}=0=\int \mathcal{D}\bold{g}\, \mathcal{D}\phi\, \left.\frac{\delta S}{\delta N}\right|_{\Sigma} \exp\left(\int d^4x \sqrt{|\bold{g}|}(R+\mathcal{L}_\mathrm{matter})\right)

where \Sigma is the three-dimensional boundary. Observe that this expression vanishes implies the functional derivative vanishes, giving us the Wheeler-DeWitt equation. A similar statement may be made for the Diffeomorphism constraint (take functional derivative with respect to the shift functions instead).

## References

• Arundhati Dasgupta, "The Measure in Euclidean Quantum Gravity." Eprint arXiv:1106.1679.
• Arundhati Dasgupta, "The gravitational path integral and trace of the diffeomorphisms." Gen.Rel.Grav. 43 (2011) 2237–2255. Eprint arXiv:0801.4770.
• Bryce S. DeWitt, Quantum Theory of Gravity - The Manifestly Covariant Theory, Phys. Rev. D 162, 1195 (1967).
• Bryce S. DeWitt, Giampiero Esposito, "An introduction to quantum gravity." Int.J.Geom.Meth.Mod.Phys. 5 (2008) 101–156. Eprint arXiv:0711.2445.
• Richard P. Feynman, Lectures on Gravitation, Notes by F.B. Morinigo and W.G. Wagner, CalTech 1963 (Addison Wesley 1995).
• Gary W. Gibbons and Stephen W. Hawking (eds.), Euclidean quantum gravity, World Scientific (1993)
• Herbert W. Hamber, Quantum Gravitation - The Feynman Path Integral Approach, Springer Publishing 2009, ISBN 978-3-540-85293-3.
• Stephen W. Hawking, The Path Integral Approach to Quantum Gravity, in General Relativity - An Einstein Centenary Survey, Cambridge U. Press, 1977.
• James B. Hartle and Stephen W. Hawking, "Wave function of the Universe." Phys. Rev. D 28 (1983) 2960–2975, eprint. Formally relates Euclidean quantum gravity to ADM formalism.
• Claus Kiefer, Quantum Gravity. Oxford University Press, second ed.
• Emil Mottola, "Functional Integration Over Geometries." J.Math.Phys. 36 (1995) 2470–2511. Eprint arXiv:hep-th/9502109.
• Martin J.G. Veltman, Quantum Theory of Gravitation, in Methods in Field Theory, Les Houches Session XXVIII, North Holland 1976.
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.

Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.

By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.