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{{wiktionary|EP|ep}}
'''Semiclassical gravity''' is the approximation to the theory of [[quantum gravity]] in which one treats [[Field (physics)|matter fields]] as being quantum and the [[Gravitation|gravitational field]] as being classical.
'''EP''', '''Ep''', '''The EP''' or '''<math>E_p</math>''' may refer to:
<!-- THIS PAGE IS ONLY MEANT TO LIST THINGS THAT CAN BE _CALLED_ "EP",
    NOT EVERYTHING THAT HAPPENS TO HAVE THOSE INITIALS!
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{{TOC right}}


==Music ==
In semiclassical gravity, matter is represented by quantum matter fields that propagate according to the theory of [[quantum field theory in curved spacetime|quantum fields in curved spacetime]]. The spacetime in which the fields propagate is classical but dynamical. The curvature of the spacetime is given by the ''semiclassical Einstein equations'', which relate the curvature of the spacetime, given by the [[Einstein tensor]] <math>G_{\mu\nu}</math>, to the expectation value of the [[Stress–energy tensor|energy–momentum tensor]] operator, <math>T_{\mu\nu}</math>, of the matter fields:
*[[Extended play]], a musical recording that contains more than a single, but less than a full album
*[[Electric piano]]
*[[EP (The 77s EP)|''EP'' (The 77s EP)]], 1999
*[[EP (Crystal Antlers EP)|''EP'' (Crystal Antlers EP)]]
*[[EP (Childish Gambino EP)|''EP'' (Childish Gambino EP)]], 2011
*[[EP (The Fiery Furnaces album)|''EP'' (The Fiery Furnaces album)]], 2003
*[[EP (The Format EP)|''EP'' (The Format EP)]], 2002
*[[EP (Hope&Social EP)|''EP'' (Hope&Social EP)]], 2008
*[[EP (Luna EP)|''EP'' (Luna EP)]], a 1996 indie pop/rock album
*[[EP (Matchbox Twenty EP)|''EP'' (Matchbox Twenty EP)]], 2002
*[[EP (Means End album)|''EP'' (Means End album)]], 2012
*[[EP (Mogwai EP)|''EP'' (Mogwai EP)]], 1999
* [[The EP (Deepspace5 EP)|''The EP'' (Deepspace5 EP)]], an 1998 EP by [[Deepspace5]]
* [[The EP (Frank n Dank album)|''The EP'' (Frank n Dank album)]], 2007
*''[[Subseven: the EP]]'', a 2004 EP by Subseven
*''[[Singles: $100 EP]]'', a 1996 EP by Alien Ant Farm
*[[Entity Paradigm]], a Pakistani rock band


==Arts and entertainment==
:<math> G_{\mu\nu} = \frac{ 8 \pi G }{ c^4 } \left\langle \hat T_{\mu\nu} \right\rangle_\psi </math>
*[[Edgar Allan Poe]], an American storywriter
*[[En passant]] (''e.p.''), a chess maneuver
*[[Europa-Park]], a theme park in Germany
*[[Executive producer]], non-technical producer of an entertainment production
*[[The Electric Playground]], a news show on the American G4 television channel
*Ep, an abbreviation of [[Episode]]s


== Companies and organizations ==
where ''G'' is [[Gravitational constant|Newton's constant]] and <math>\psi</math> indicates the quantum state of the matter fields.
*[[Editorial Photographers]], a membership organisation for photojournalists
*[[El Paso Corp.]] (NYSE stock symbol: ''EP''), a US natural gas producer
*[[Emerson Preparatory School]], a small private high school in Washington, D.C.
*[[Iran Aseman Airlines]] (IATA code: ''EP'')
*[[Europa Press (news agency)]], a privately held news agency, in Madrid, Spain


==Government, military, and politics==
==Stress–energy tensor==
*[[English Partnerships]], the former English urban regeneration agency
There is some ambiguity in regulating the stress–energy tensor, and this depends upon the curvature. This ambiguity can be absorbed into the [[cosmological constant]], [[Newton's constant]], and the [[f(R) gravity|quadratic couplings]]<ref>See Wald (1994) Chapter 4, section 6 "The Stress-Energy Tensor".</ref>
*[[Estradas de Portugal]], the Portuguese roads agency
:<math>\int d^dx \,\sqrt{-g} R^2</math> and <math>\int d^dx\, \sqrt{-g} R^{\mu\nu}R_{\mu\nu}</math>.
*[[European Parliament]], the European Union's lower house
There's also the other quadratic term
:<math>\int d^dx\, \sqrt{-g} R^{\mu\nu\rho\sigma}R_{\mu\nu\rho\sigma}</math>,
but (in 4-dimensions) this term is a linear combination of the other two terms and a surface term. See [[Gauss–Bonnet gravity]] for more details.


==Places==
Since the theory of quantum gravity is not yet known, it is difficult to say what is the regime of validity of semiclassical gravity. However, one can formally show that semiclassical gravity could be deduced from quantum gravity by considering ''N'' copies of the quantum matter fields, and taking the limit of ''N'' going to infinity while keeping the product ''GN'' constant. At diagrammatic level, semiclassical gravity corresponds to summing all [[Feynman diagram]]s which do not have loops of gravitons (but have an arbitrary number of matter loops). Semiclassical gravity can also be deduced from an axiomatic approach.
*[[Eden Prairie, Minnesota|Eden Prairie]], a suburb of Minneapolis, Minnesota, US
*[[El Paso, Texas]], a large city in Texas, United States
*[[Ep, Kentucky]]


== Science, medicine, and mathematics==
==Experimental status==
*[[Engineering physics]]
There are cases where semiclassical gravity breaks down. For instance,<ref>See Page and Geilker; Eppley and Hannah; Albers, Kiefer, and Reginatto.</ref> if ''M'' is a huge mass, then the superposition
*[[Electrophotography]]
:<math>\frac{1}{\sqrt{2}} \left( \left| M \text{ at } A \right\rangle + \left| M \text{ at } B \right\rangle \right)</math>
*[[Ectopic Pregnancy]]
where ''A'' and ''B'' are widely separated, then the expectation value of the stress–energy tensor is ''M/2'' at ''A'' and ''M/2'' at ''B'', but we would never observe the metric sourced by such a distribution. Instead, we [[decohere]] into a state with the metric sourced at ''A'' and another sourced at ''B'' with a 50% chance each.
* [[Electrically powered spacecraft propulsion|Electrically Powered Spacecraft Propulsion]]
*[[Electrophysiology]]
*[[EPDM rubber]], a synthetic ethylene-propylene-based rubber
*[[Erythropoietin]], a hormone
*[[Evoked potential]], a stimulation induced electrophysiological test
*[[Evolutionary psychology]], psychological approach that looks at traits such as memory
*[[European Pharmacopoeia]], listing of substances used to make pharmaceutical products in Europe
*[[Expectation propagation]], a technique in Bayesian machine learning
*Etoposide and platinum agent (cisplatin), a [[Chemotherapy regimens|chemotherapy regimen]]
*EP, the alias of a patient with extreme anterograde and retrograde amnesia, similar to [[HM (patient)]]
*<math>E_p</math>, [[potential energy]]
*[[Entry point]] (computer programming)
*Environmental Professional, as provided by [[ECO Canada]]


==Transportation==
==Applications==
*[[EP additive]], a lubricant additive (mainly used in automotive gear oils)
The most important applications of semiclassical gravity are to understand the [[Hawking radiation]] of [[black hole]]s and the generation of random gaussian-distributed perturbations in the theory of [[cosmic inflation]], which is thought to occur at the very beginnings of the [[Big Bang|big bang]].
*[[Estimated position]], a navigation term
*EP, an [[electric locomotive]] (Polish train designation: ''EP'')
*EP (English Premium) brake, an abbreviation for the [[electro-pneumatic brake system on British railway trains]]


{{disambiguation}}
==Notes==
[[Category:Two-letter disambiguation pages]]
{{Reflist}}
 
==References==
* Birrell, N. D. and Davies, P. C. W., ''Quantum fields in curved space'', (Cambridge University Press, Cambridge, UK, 1982).
* Don N. Page, and C. D. Geilker, "Indirect Evidence for Quantum Gravity."  ''Phys. Rev. Lett.'' '''47''' (1981) 979–982. DOI:[http://dx.doi.org/10.1103/PhysRevLett.47.979 10.1103/PhysRevLett.47.979]
* K. Eppley and E. Hannah, "The necessity of quantizing the gravitational field." ''Found. Phys.'' '''7''' (1977) 51–68. [[Digital object identifier|doi]]:[http://dx.doi.org/10.1007/BF00715241 10.1007/BF00715241]
* Mark Albers, Claus Kiefer, Marcel Reginatto, "Measurement Analysis and Quantum Gravity." ''Phys.Rev.D'' '''78''' 6 (2008) 064051, [http://dx.doi.org/10.1103/PhysRevD.78.064051 DOI:10.1103/PhysRevD.78.064051]. Eprint [http://arxiv.org/abs/0802.1978 arXiv:0802.1978] [gr-qc].
* Robert M. Wald, ''Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics''. University of Chicago Press, 1994.
*[http://xstructure.inr.ac.ru/x-bin/theme3.py?level=1&index1=-43587 Semiclassical gravity on arxiv.org]
 
{{theories of gravitation}}
{{quantum gravity}}
 
[[Category:Theories of gravitation]]
[[Category:Quantum field theory]]
[[Category:Quantum gravity]]

Revision as of 03:41, 13 August 2014

Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical.

In semiclassical gravity, matter is represented by quantum matter fields that propagate according to the theory of quantum fields in curved spacetime. The spacetime in which the fields propagate is classical but dynamical. The curvature of the spacetime is given by the semiclassical Einstein equations, which relate the curvature of the spacetime, given by the Einstein tensor Gμν, to the expectation value of the energy–momentum tensor operator, Tμν, of the matter fields:

Gμν=8πGc4T^μνψ

where G is Newton's constant and ψ indicates the quantum state of the matter fields.

Stress–energy tensor

There is some ambiguity in regulating the stress–energy tensor, and this depends upon the curvature. This ambiguity can be absorbed into the cosmological constant, Newton's constant, and the quadratic couplings[1]

ddxgR2 and ddxgRμνRμν.

There's also the other quadratic term

ddxgRμνρσRμνρσ,

but (in 4-dimensions) this term is a linear combination of the other two terms and a surface term. See Gauss–Bonnet gravity for more details.

Since the theory of quantum gravity is not yet known, it is difficult to say what is the regime of validity of semiclassical gravity. However, one can formally show that semiclassical gravity could be deduced from quantum gravity by considering N copies of the quantum matter fields, and taking the limit of N going to infinity while keeping the product GN constant. At diagrammatic level, semiclassical gravity corresponds to summing all Feynman diagrams which do not have loops of gravitons (but have an arbitrary number of matter loops). Semiclassical gravity can also be deduced from an axiomatic approach.

Experimental status

There are cases where semiclassical gravity breaks down. For instance,[2] if M is a huge mass, then the superposition

12(|M at A+|M at B)

where A and B are widely separated, then the expectation value of the stress–energy tensor is M/2 at A and M/2 at B, but we would never observe the metric sourced by such a distribution. Instead, we decohere into a state with the metric sourced at A and another sourced at B with a 50% chance each.

Applications

The most important applications of semiclassical gravity are to understand the Hawking radiation of black holes and the generation of random gaussian-distributed perturbations in the theory of cosmic inflation, which is thought to occur at the very beginnings of the big bang.

Notes

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

References

  • Birrell, N. D. and Davies, P. C. W., Quantum fields in curved space, (Cambridge University Press, Cambridge, UK, 1982).
  • Don N. Page, and C. D. Geilker, "Indirect Evidence for Quantum Gravity." Phys. Rev. Lett. 47 (1981) 979–982. DOI:10.1103/PhysRevLett.47.979
  • K. Eppley and E. Hannah, "The necessity of quantizing the gravitational field." Found. Phys. 7 (1977) 51–68. doi:10.1007/BF00715241
  • Mark Albers, Claus Kiefer, Marcel Reginatto, "Measurement Analysis and Quantum Gravity." Phys.Rev.D 78 6 (2008) 064051, DOI:10.1103/PhysRevD.78.064051. Eprint arXiv:0802.1978 [gr-qc].
  • Robert M. Wald, Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics. University of Chicago Press, 1994.
  • Semiclassical gravity on arxiv.org

Template:Theories of gravitation Template:Quantum gravity

  1. See Wald (1994) Chapter 4, section 6 "The Stress-Energy Tensor".
  2. See Page and Geilker; Eppley and Hannah; Albers, Kiefer, and Reginatto.