# Goldberg–Sachs theorem

Goldberg–Sachs theorem General relativity {displaystyle G_{mu nu }+Lambda g_{mu nu }={kappa }T_{mu nu }} IntroductionHistory Mathematical formulation Tests show Fundamental concepts show Phenomena show EquationsFormalisms show Solutions show Scientists Physics portal Category vte The Goldberg–Sachs theorem is a result in Einstein's theory of general relativity about vacuum solutions of the Einstein field equations relating the existence of a certain type of congruence with algebraic properties of the Weyl tensor.

More precisely, the theorem states that a vacuum solution of the Einstein field equations will admit a shear-free null geodesic congruence if and only if the Weyl tensor is algebraically special.

The theorem is often used when searching for algebraically special vacuum solutions.

Contents 1 Shear-Free Rays 2 The Theorem 3 Importance and Examples 4 Linearised gravity 5 See also 6 References Shear-Free Rays A ray is a family of geodesic light-like curves. That is tangent vector field {displaystyle l^{a}} is null and geodesic: {displaystyle l_{a}l^{a}=0} and {displaystyle l^{b}nabla _{b}l^{a}=0} . At each point, there is a (nonunique) 2D spatial slice of the tangent space orthogonal to {displaystyle l^{a}} . It is spanned by a complex null vector {displaystyle m^{a}} and its complex conjugate {displaystyle {bar {m}}^{a}} . If the metric is time positive, then the metric projected on the slice is {displaystyle {tilde {g}}^{ab}=-m^{a}{bar {m}}^{b}-{bar {m}}^{a}m^{b}} . Goldberg and Sachs considered the projection of the gradient on this slice.

{displaystyle A^{ab}={tilde {g}}^{ap}{tilde {g}}^{bq}nabla _{p}l_{q}=z{bar {m}}^{a}m^{b}+{bar {z}}m^{a}{bar {m}}^{b}+{bar {sigma }}m^{a}m^{b}+sigma {bar {m}}^{a}{bar {m}}^{b}.} A ray is shear-free if {displaystyle sigma =0} . Intuitively, this means a small shadow cast by the ray will preserve its shape. The shadow may rotate and grow/shrink, but it will not be distorted.

The Theorem A vacuum metric, {displaystyle R_{ab}=0} , is algebraically special if and only if it contains a shear-free null geodesic congruence; the tangent vector obeys {displaystyle k_{[a}C_{b]ijc}k^{i}k^{j}=0} .[1] This is the theorem originally stated by Goldberg and Sachs. While they stated it in terms of tangent vectors and the Weyl tensor, the proof is much simpler in terms of spinors. The Newman-Penrose field equations[2] give a natural framework for investigating Petrov classifications, since instead of proving {displaystyle k_{[a}C_{b]ijc}k^{i}k^{j}=0} , one can just prove {displaystyle Psi _{0}=Psi _{1}=0} . For these proofs, assume we have a spin frame with {displaystyle o^{A}} having its flagpole aligned with the shear-free ray {displaystyle l^{a}} .

Proof that a shear-free ray implies algebraic specialty: If a ray is geodesic and shear-free, then {displaystyle varepsilon +{bar {varepsilon }}=kappa =sigma =0} . A complex rotation {displaystyle o^{A}rightarrow e^{itheta }o^{A}} does not affect {displaystyle l^{a}} and can set {displaystyle varepsilon =0} to simplify calculations. The first useful NP equation is {displaystyle Dsigma -delta kappa =0} , which immediately gives {displaystyle Psi _{0}=0} .

To show that {displaystyle Psi _{1}=0} , apply the commutator {displaystyle delta D-Ddelta } to it. The Bianchi identity gives the needed formulae: {displaystyle DPsi _{1}=4rho Psi _{1}} and {displaystyle delta Psi _{1}=(2beta +4tau )Psi _{1}} .[3] Working through the algebra of this commutator will show {displaystyle Psi _{1}^{2}=0} , which completes this part of the proof.

Proof that algebraic specialty implies a shear-free ray: Suppose {displaystyle o_{A}} is a degenerate factor of {displaystyle Psi _{ABCD}} . While this degeneracy could be n-fold (n=2..4) and the proof will be functionally the same, take it to be a 2-fold degeneracy. Then the projection {displaystyle o^{B}o^{C}o^{D}Psi _{ABCD}=0} . The Bianchi identity in a vacuum spacetime is {displaystyle nabla ^{AA'}Psi _{ABCD}=0} , so applying a derivative to the projection will give {displaystyle o_{A}o_{B}nabla ^{AA'}o^{B}=0} , which is equivalent to {displaystyle kappa =sigma =0.} The congruence is therefore shear-free and almost geodesic: {displaystyle Dl_{a}=(varepsilon +{bar {varepsilon }})l_{a}} . A suitable rescaling of {displaystyle o^{A}} exists which will make this congruence geodesic, and thus a shear-free ray. The shear of a vector field is invariant under rescaling, so it will remain shear-free.

Importance and Examples In Petrov type D spacetimes, there are two algebraic degeneracies. By the Goldberg-Sachs theorem there are then two shear-free rays which point along these degenerate directions. Since the Newman-Penrose equations are written in a basis with two real null vectors, there is a natural basis which simplifies the field equations. Examples of such vacuum spacetimes are the Schwarzschild metric and the Kerr metric, which describes a nonrotating and a rotating black hole, respectively. It is precisely this algebraic simplification which makes solving for the Kerr metric possible by hand.

In the Schwarzschild case with time-symmetric coordinates, the two shear-free rays are {displaystyle l^{mu }partial _{mu }=pm left(1-{frac {2M}{r}}right)^{-1}partial _{t}+partial _{r}.} Under the coordinate transformation {displaystyle (t,r,theta ,varphi )rightarrow (tmp r^{*},r,theta ,varphi )} where {displaystyle r^{*}} is the tortoise coordinate, this simplifies to {displaystyle l^{mu }partial _{mu }=partial _{r}} .

Linearised gravity It has been shown by Dain and Moreschi[4] that a corresponding theorem will not hold in linearized gravity, that is, given a solution of the linearised Einstein field equations admitting a shear-free null congruence, then this solution need not be algebraically special.

See also Geodesic Optical scalars References ^ Goldberg, J. N.; Sachs, R. K. (1962). "A theorem on Petrov types (republished January 2009)". General Relativity and Gravitation. 41 (2): 433–444. doi:10.1007/s10714-008-0722-5.; originally published in Acta Phys. Pol. 22, 13–23 (1962). ^ Penrose, Roger (1984). Spinors and space-time Volume 1 two-spinor calculus and relativistic fields. Cambridge University Press. ISBN 0-521-24527-3. ^ Newman, Ezra (1962). "An Approach to Gravitational Radiation by a Method of Spin Coefficients". Journal of Mathematical Physics. 3 (3): 566. doi:10.1063/1.1724257. S2CID 121898444. ^ Dain, Sergio (2000). "The Goldberg-Sachs theorem in linearized gravity". Journal of Mathematical Physics. 41 (9): 6296–6299. arXiv:gr-qc/0203057. doi:10.1063/1.1288249. Categories: Theorems in general relativity

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