Monday, 25 November 2019

general relativity - Difference of connections in the Killing vector equation


For the Killing vector equation, I sometimes see it written in terms of spin connection ω and other times in terms of the affine connection Γ.


More clearly μVν+νVμ=0=μVν+ΓμνλVλ+νVμ+ΓνμλVλ


Another time, you find it satisfying this AVB+BVA=ωA,CBVC+ωB,CAVC



What is the difference between the two descriptions?



Answer



Comments to the question (v3):




  1. The vielbein eAμ in the Cartan formalism is an intertwiner gμν = eAμ ηAB eBν

    between the curved (pseudo) Riemannian metric gμν and the corresponding flat metric ηAB. Here Greek indices μ,ν,λ,, are so-called curved indices, while capital Roman indices A,B,C,, are so-called flat indices. See also this related Phys.SE post.




  2. The vielbein eAμ translates the vector field V = Vμμ

    with curved components Vμ into flat components VA := eAμVμ.





  3. The christoffel symbols Γλ,μν and the spin connection ωABμ can be written in terms of the vielbein eAμ, see Wikipedia for details.




  4. TL;DR: Using the vielbein eAμ as an intertwiner, it is possible to show that OP's various conditions for a Killing vector field are equivalent.




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