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The Schwarzchild metric is the most general spherically symmetric, vacuum solution of the Einstein field equations.

I was wondering if there was a simple argument to explain why the Schwarzchild metric is diagonal in the spherical coordinate system, i.e. of the form

$$ds^2 = dt^2 + \cdots d\theta^2 + \cdots d\phi^2 + \cdots dr^2.$$

This Wikipedia article gives a really simple explanation which seems false.

(When you write the transformation law for $ g_{\mu 4} $, it should be understood: $$ g'_{\mu 4} (x') = \frac{\partial x^\alpha}{\partial x'^\mu}\frac{\partial x^\beta}{\partial x'^4}g_{\alpha\beta}(x)= -g_{\mu 4}(x) ,$$ while at the same time the invariance tells you that $$ g'_{\mu 4}(x') = g_{\mu 4}(x') .$$ This leads to the conclusion that $$ g_{\mu 4}(x') = -g_{\mu 4}(x) ,$$ but I fail to see how to go further in the reasoning without any aditionnal assumption.)

Other derivations either start from the diagonal form or are much more complicated.

It's probably a dumb question, but I fail to see a simple argument.

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    $\begingroup$ I'm ok with the general transformation law part, but there's no reason to say that $g_{\mu,\nu}$ is invariant under those symmetries. $\endgroup$ – Jeannette Oct 17 '16 at 10:07
  • $\begingroup$ - I was wondering if there was a simple argument to explain why the Schwarzchild metric is diagonal in the spherical coordinate system - I may be missing something here, but isn't the fact that it just is diagonal an explanation in itself? And about spherical coordinates: it is rather due to the fact that the metric is of this particularly simple form in these coordinates that we call them spherical, not the other way around. $\endgroup$ – Solenodon Paradoxus Oct 21 '16 at 12:14
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OP is right: For a generic pseudo-Riemannian manifold $(M;g)$, there does not necessarily exist an open coordinate neighborhoods $U\subseteq M$, where the metric $g_{|U}$ is on diagonal form. Fermi normal coordinates always ensure a diagonal form along a geodesic $\gamma$ (but not necessarily in the ambient spacetime outside the geodesic). However, the Schwarzschild geometry has Killing symmetries that ensure that diagonal metrics exist in open neighborhoods.

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