Question #283318

Find killing equations for spherical polar coordinates


1
Expert's answer
2021-12-29T16:30:21-0500

Killing vector field is a vector field on a Riemannian manifold that preserves the metric.

Killing equation:

μXv+vXμ=0\nabla_{\mu}X_{v}+\nabla_{v}X_{\mu}=0

Killing fields on a 2-sphere:

The conventional metric on the sphere is

g(Ω)=dθ2+sin2θdϕ2g(\Omega)=d\theta^2+sin^2\theta d\phi^2

Rotation about the pole should be an isometry. The infinitesimal generator of a rotation can then be identified as a generator of the Killing field. This can be

U=ϕU=\partial_ \phi

The surface of the sphere is two-dimensional, and there is another generator of isometries; it can be taken as

V=θV=\partial_ \theta


conventional 3-space coordinate system is given by

x=sinθ,y=sinθsinϕ,z=cosθx=sin\theta,y=sin\theta sin \phi,z=cos\theta

first generator, rotation about the  z-axis:

R=xyyx=sin2θϕR=x\partial _y-y\partial_x=sin^2\theta \partial \phi

second generator, rotation about the  x-axis:

S=zyyzS=z\partial _y-y\partial_z

third generator, rotation about the  x-axis:

T=zxxzT=z\partial _x-x\partial_z


Normalizing this, and expressing these in spherical coordinates:


R=Rsin2θ=ϕR'=\frac{R}{sin^2\theta}=\partial_ \phi


S=Ssin2θ=sinϕθ+cotθcosϕϕS'=\frac{S}{sin^2\theta}=sin\phi \partial_ \theta +cot\theta cos\phi \partial _\phi


T=Tsin2θ=cosϕθ+cotθsinϕϕT'=\frac{T}{sin^2\theta}=cos\phi \partial_ \theta +cot\theta sin\phi \partial _\phi


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