General Relativity – Disentangling Coordinates and Curvature in Differential Geometry

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While trying to understand General Relativity, I'm struggeling with disentangling coordinates and curvature.

The metric tensor contains information on both: coordinates as well as curvature.

Curvature is a physical characteristic of spacetime, while the coordinates can be chosen completely arbitrary.

Is there a method to disentagle coordinates and curvature?

(Stephan Hawking must have had one as he managed to show that the singularity at the event horizon of a black hole is only a coordinate singularity, not a physical one…)

I fistly thought that one could simply use cartesian coordinates – and then would be left with only the curvature (in this question: Determinant of metric tensor in Cartesian Coordinates constant in vacuum)
However, I was taught there, that cartesian coordinates can only be used in flat spacetime. Now, I'm totally clueless again how to disentangle coordinates and curvature.

I'm looking for a tensor/measure/metric/field, where only the physical curvature is in – and no coordinate curvature. Where the coordinates are flat like in a cartesian grid.

Where the metric tensor is simply that of a flat spacetime in cartesian coordinates, if the spacetime is flat (Minkowski spacetime).

Best Answer

Neither the metric nor the curvature depend on the coordinates. They are tensors. They exist independent of coordinates and are invariant under any coordinate transformation. e.g. $\boldsymbol{\eta}$ is the metric tensor for Minkowski space no matter your choice of coordinates. However, you can project a tensor onto a particular coordinate basis to obtain its $\textit{coordinate-dependent}$ components.

In Cartesian coordinates, $\boldsymbol{\eta}$ has components $\eta_{\alpha\beta} = \text{diag}(-1, 1, 1, 1)$. In spherical coordinates, $\boldsymbol{\eta}$ has components $\eta_{\bar{\mu}\bar{\nu}} = \text{diag}(-1, 1, r^2, r^2\sin^2\theta)$. At the end of the day, these components still represent the same geometric object:

$$\begin{align} \boldsymbol{\eta} &= -1\tilde{dt}\otimes\tilde{dt}+1\tilde{dx}\otimes\tilde{dx}+1\tilde{dy}\otimes\tilde{dy}+1\tilde{dz}\otimes\tilde{dz} \;\;\;\;\text{(Cartesian)}\\ &= -1\tilde{dt}\otimes\tilde{dt}+1\tilde{dr}\otimes\tilde{dr}+r^2\tilde{d\theta}\otimes\tilde{d\theta}+r^2\sin^2\theta\tilde{d\phi}\otimes\tilde{d\phi} \;\;\;\;\text{(spherical)}\\ &=\eta_{\alpha'\beta'}\tilde{d}x^{\alpha'}\otimes\tilde{d}x^{\beta'} \;\;\;\;\text{(arbitrary coordinates)} \end{align}$$

Similarly the curvature tensor is the same in all coordinate systems. A straightforward calculation shows the curvature is the zero tensor in Minkowski space - no matter the coordinate system you choose, if any.

Nonetheless, we often subscribe to a particular coordinate system to help with calculations. Then the Riemann tensor would have coordinate-dependent components: $R_{\alpha\beta\mu\nu}$. To study the curvature without being fooled by coordinate artifacts, we can compute curvature invariants which are scalars and the same in every frame.

The Kretschmann scalar $K$ is a particular curvature invariant. In the Schwarzschild geometry,

$$K = R_{\alpha\beta\gamma\delta}R^{\alpha\beta\gamma\delta} = \frac{48M^2}{r^6}$$

where we've used Schwarzschild coordinates: $(t,r,\theta,\phi)$. So the curvature is finite at the event horizon ($r=2M$), and diverges as $r\rightarrow 0$.

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