When I see harmonic coordinates used somewhere, what should my association be?
Is there some general use or need to consider the harmonic coordinate condition?
I don't really see what's behind all the things said in the wikipedia article and I've seen these coordinates used in some papers but don't really understand their significance, other than that one can consequently algebraically use the relation Δxμ=0 in computations. I don't see what that means though, do the full set of these coordinates have some general geometrically visible properties, maybe characterized with the associated base of distributions?
Moreover, do these coordinates have some special role in geometric quantization?
Answer
The well known property of the harmonic coordinates is that the covariant divergence of a vector field and the d'Alambertian of a scalar field take a particularly simple form: DμAμ→gμν∂μAν,gμνDνDμϕ→gμν∂μ∂νϕ.
The weak-field expansion of the Einstein-Hilbert action with respect to hμν-field (2) has the form: S=116πGN∫d4x√−gR=12κ2∫d4x[∂αhμν∂αhμν−∂αh∂αh−2∂μhμν(∂αhαν−∂νh)+O(h3)],
Using the gauge condition (1) and vertices extracted from the weak-field expansion of the Einstein-Hilbert action and utilizing the QFT perturbation theory with respect to hμν, one can find, for example, the gravitational field of a static spinless source. The result will be no more than the rg/r-expansion of the Schwarzschild metric in the harmonic coordinates (see, e.g., S. Weinberg, Gravitation and Cosmology, eq. (8.2.15)): ds2=1−rg/(2r)1+rg/(2r)dt2−1+rg/(2r)1−rg/(2r)r2g4r4(r⋅dr)2−(1+rg2r)2dr2.
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