# Christoffel symbols in Kruskal-Szekeres coordinates [closed]

What are the Christoffel symbols for the Schwarzschild spacetime, expressed in the Kruskal-Szekeres coordinates?

I worked these out recently, and it was a fair amount of computation to get them, so I thought they might be useful to others.

Throughout the following, let $$m=1/2$$, so the horizon is at $$r=1$$, i.e., $$r$$ is in units of the Schwarzschild radius. The exterior regions of the maximal extension of the Schwarzschild spacetime are at $$r>1$$, which is regions I and III. The interior is $$r<1$$, regions II and IV.

The version of the Kruskal-Szekeres coordinates I'll use are null coordinates $$(V,U)$$, equivalent to Hawking and Ellis's $$(v'/\sqrt2,w'/\sqrt2)$$.

Even when working in the Kruskal-Szekeres coordinates, it's convenient to express some things in terms of the Schwarzschild $$r$$ coordinate, which can be found using $$\begin{equation} r = 1+W(-VU/e). \end{equation}$$ Here the function $$W$$ is the principal real branch of the Lambert W function, and $$e$$ is the base of natural logarithms. It's also convenient to define $$\begin{equation} B = \frac{4}{re^r}. \end{equation}$$

The metric is $$\begin{equation} ds^2 = B dVdU-r^2 d\Omega^2. \end{equation}$$

The Christoffel symbols are as follows: \begin{align} \Gamma^V_{VV} &= (r^{-1}+r^{-2})Ue^{-r} \\ \Gamma^U_{UU} &= (r^{-1}+r^{-2})Ve^{-r} \\ \Gamma^\theta_{V\theta} = \Gamma^\phi_{V\phi} &= -UB/4r \\ \Gamma^\theta_{U\theta} = \Gamma^\phi_{U\phi} &= -UB/4r \\ % \Gamma^V_{\theta\theta} &= -Vr/2 \\ \Gamma^U_{\theta\theta} &= -Ur/2 \\ \Gamma^V_{\phi\phi} &= -(Vr/2) \sin^2\theta \\ \Gamma^U_{\phi\phi} &= -(Ur/2) \sin^2\theta \\ % \Gamma^\theta_{\phi\phi} &= -\sin\theta \cos\theta \\ \Gamma^\phi_{\theta\phi} &= \cot\theta \\ \end{align} I got these by calculating them in the computer algebra system Maxima and then cleaning up the resulting expressions by hand. I checked my cleaned-up versions numerically against the raw output from Maxima to make sure they were right. They are implemented in an open-source software project called karl.

The metric and the Christoffel symbols misbehave at the $$r=0$$ singularities, and also at the coordinate singularities at $$\theta=0$$ and $$\pi$$.

• @BenCrowell: Could you, please, rename your null coordinate from $W$ (to, say, $U$) so that it does not clash with Lambert function? Because, even after your warning, $W(\ldots W)$ is confusing. May 14, 2018 at 4:27