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Semiclassical gravity is the approximation to 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 matter fields propagate is classical but dynamical. The curvature of the spacetime given by the semiclassical Einstein equations, which relate the curvature of the spacetime, given by the Einstein tensor ${\displaystyle G_{\mu \nu }}$ to the expectation value of the energy-momentum tensor operator ${\displaystyle T_{\mu \nu }}$ operator of the matter fields:

${\displaystyle G_{\mu \nu }=8\pi G\left\langle {\hat {T}}_{\mu \nu }\right\rangle _{\psi }}$

where G is Newton's constant and ${\displaystyle \psi }$ indicates the quantum state of the matter fields.

Since the theory of quantum gravity is yet unknown, it is difficult to say which 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 be as well deduced from an axiommatic approach.

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• Gravity
• Quantum field theory