# GR with Torsion: Definition of contorsion

I start doing some computations in manifolds with non vanishing torsion and things are getting a bit confused, basically because of notations and definitions. I understand that in presence of non vanishing torsion, one has two fundamental objects: \begin{align} \text{The metric} \quad g_{\mu\nu} & & \text{The torsion tensor} \quad T{^\rho}_{\mu\nu} \end{align} The Torsion tensor is defined as $$$$T{^\rho}_{\mu\nu} \equiv \tilde{\Gamma}{^\rho}_{\mu\nu}-\tilde{\Gamma}{^\rho}_{\nu\mu} = 2\tilde{\Gamma}{^\rho}_{[\mu\nu]}$$$$ where $$\tilde{\Gamma}{^\rho}_{\mu\nu}$$ is the affine connection so that the covariant derivatives $$\nabla_{\mu}$$ of any vector $$A^{\rho}$$ and any form $$a_{\nu}$$ are respectively \begin{align} \nabla_{\mu}A^{\rho} = \partial_{\mu}A^{\rho} +\tilde{\Gamma}{^\rho}_{\mu\nu}A^{\nu} & & \nabla_{\mu}a^{\nu} = \partial_{\mu}a^{\nu} -\tilde{\Gamma}{^\rho}_{\mu\nu}a^{\rho} & & \end{align} In Shapiro's article (2001) it says (page 8, equations (2.8) and 2.9)):

The metricity condition enables one to express the connection through the metric and torsion in a unique way as $$$$\tilde{\Gamma}{^\rho}_{\mu\nu} = \Gamma{^\rho}_{\mu\nu}+K{^\rho}_{\mu\nu}$$$$ where $$K{^\rho}_{\mu\nu}$$ is the contorsion tensor define as $$$$K{^\rho}_{\mu\nu} = \frac{1}{2}\left(T{^\rho}_{\mu\nu}-T{_\mu}{^\rho}{_\nu}-T{_\nu}{^\rho}{_\mu}\right) \tag{1}$$$$

I tried to derive this last formula, but I find a different result. Here's my derivation: \begin{align} \nabla_{\mu}g_{\rho\nu} & = \partial_{\mu}g_{\rho\nu}-\tilde{\Gamma}{^\sigma}_{\mu\rho}g_{\sigma\nu}-\tilde{\Gamma}{^\sigma}_{\mu\nu}g_{\rho\sigma} = 0 \\ \nabla_{\nu}g_{\rho\mu} & = \partial_{\nu}g_{\rho\mu}-\tilde{\Gamma}{^\sigma}_{\nu\rho}g_{\sigma\mu}-\tilde{\Gamma}{^\sigma}_{\nu\mu}g_{\rho\sigma} = 0 \\ -\nabla_{\rho}g_{\mu\nu} & = -\partial_{\rho}g_{\mu\nu}+\tilde{\Gamma}{^\sigma}_{\rho\mu}g_{\sigma\nu}+\tilde{\Gamma}{^\sigma}_{\rho\nu}g_{\mu\sigma} = 0 \end{align} Summing up these three equations one gets \begin{align} {\color{blue}{\Big(\partial_{\mu}g_{\rho\nu}+\partial_{\nu}g_{\rho\mu}-\partial_{\rho}g_{\mu\nu}\Big)}} +g_{\nu\sigma}\Big(\tilde{\Gamma}{^\sigma}_{\rho\mu}-\tilde{\Gamma}{^\sigma}_{\mu\rho}\Big)& +g_{\mu\sigma}\Big(\tilde{\Gamma}{^\sigma}_{\rho\nu}-\tilde{\Gamma}{^\sigma}_{\nu\rho}\Big)+\\ & {\color{red}{-g_{\rho\sigma}\Big(\tilde{\Gamma}{^\sigma}_{\mu\nu}+\tilde{\Gamma}{^\sigma}_{\nu\mu}\Big)}} = 0 \tag{2} \end{align} Using the fact that $$\tilde{\Gamma}{^\sigma}_{\nu\mu} = \tilde{\Gamma}{^\sigma}_{\mu\nu}+T{^\sigma}_{\nu\mu}$$, and introducing the Christoffel symbols $$$$\Gamma{^\rho}_{\mu\nu} = \frac{1}{2}g^{\rho\sigma}\left(\partial_{\mu}g_{\sigma\nu}+\partial_{\nu}g_{\mu\sigma}-\partial_{\sigma}g_{\mu\nu}\right)$$$$ We can rewrite (2) as $$$${\color{red}{2g_{\rho\sigma}\tilde{\Gamma}{^\sigma}_{\mu\nu}}} = {\color{blue}{2g_{\rho\sigma}\Gamma{^\sigma}_{\mu\nu}}}{\color{red}{-g_{\rho\sigma}T{^\sigma}_{\nu\mu}}}+g_{\nu\sigma}T{^\sigma}_{\rho\mu}+g_{\mu\sigma}T{^\sigma}_{\rho\nu}$$$$ Hence we get \begin{align} \tilde{\Gamma}{^\rho}_{\mu\nu} & = \Gamma{^\rho}_{\mu\nu} -\frac{1}{2}\Big(T{^\rho}_{\nu\mu}-T{_\mu}{^\rho}{_\nu}-T{_\nu}{^\rho}{_\mu}\Big) = \\ & = \Gamma{^\rho}_{\mu\nu} +\frac{1}{2}\Big(T{^\rho}_{\mu\nu}+T{_\mu}{^\rho}{_\nu}+T{_\nu}{^\rho}{_\mu}\Big) = \Gamma{^\rho}_{\mu\nu}+K{^\rho}_{\mu\nu} \tag{3} \end{align}

Obviously the contorsion tensors in (1) and (3) do not agree. Actually the definition of $$K{^\rho}_{\mu\nu}$$ that I got can be written as $$$$K{^\rho}_{\mu\nu} = \frac{1}{2}\Big(T{^\rho}_{\mu\nu}-T{_{\mu\nu}}{^\rho}+T{_\nu}{^\rho}{_\mu}\Big)$$$$ which is the definition that I found in some others references. What is going on? Ultimately what should I take as definition of the contorsion?

Edit The main problem here is that (if I did things correctly) using the conventions of Shapiro (2001) for the torsion and the contorsion tensors, I eventually get a result different from his.

• Take nothing as definition, you should be able to derive it. But beware that different authors may have indices shifted and this can affect relative signs. I've derived it before, don't have time now, but offer that advice. Try doing everything with lower indices and see if you get a match. remember you cannot switch an upper and lower index without first putting both in the same place, swapping, then applying a metric. – ggcg Dec 14 '18 at 12:34
• Yes actually on papers I did all computation with lowered indices, here I raised one of the indices to follow Shapiro's notation. I also realized that different authors have different indices order for the contorsion and the torsion; however in my question I used the same convention used by the article linked, and I get a different result. That's why I am confused, and that's why it's hard to me to compare this with other references. – M. M. R. Dec 14 '18 at 13:34
• But Thanks @ggcg, I edited the question because maybe it wasn't completely clear – M. M. R. Dec 14 '18 at 13:40
• I recall going through all the derivations 20 years ago, eventuality go 'em to work. I am sure if I tried again I might make the same mistakes. If I get to it before you or someone else I'll post. – ggcg Dec 14 '18 at 14:34

I decided to answer my own question because, hopefully, I found at least some coherent results.

In Shapiro's(2001) something doesn't work: I say this because according to his definition \begin{align} \tilde{\Gamma}{^\rho}_{\mu\nu} = \Gamma{^\rho}_{\mu\nu} + K{^\rho}_{\mu\nu} & & K{^\rho}_{\mu\nu} = \frac{1}{2}\left(T{^\rho}_{\mu\nu}-T{_\mu}{^\rho}{_\nu}-T{_\nu}{^\rho}{_\mu}\right) \end{align} Of course the affine connection must satifies the metricity condition $$\nabla_\rho g_{\mu\nu}=0$$. Therefore we get the equation $$$$\nabla_\sigma g_{\mu\nu}=\partial_\sigma g_{\mu\nu}-\Gamma{^\rho}_{\sigma\mu}g_{\rho\nu}-\Gamma{^\rho}_{\sigma\nu}g_{\mu\rho}-K{^\rho}_{\sigma\mu}g_{\rho\nu}-K{^\rho}_{\sigma\nu}g_{\mu\rho}$$$$ Substituting the expression of the Christoffel symbols, the first three terms of the above equations cancel and one is left with (pulling all indices down for simplicity) $$$$-K_{\nu\sigma\mu}-K_{\mu\sigma\nu} = -\left(T_{\nu\sigma\mu}-T_{\sigma\nu\mu}-T_{\mu\nu\sigma}\right)\neq 0$$$$ Hence the metricity condition is not satisfied; hence, either I did something wrong in these computations, either something is not well define in the paper.

Then I found two (of course equivalent) order-of-indices conventions in which torsion and contorsion are often introduced:

First way: Having the metric $$g_{\mu\nu}$$ and the affine connection $$\tilde{\Gamma}{^\rho}_{\mu\nu}$$, one define the torsion as $$$$T{^\rho}_ {{\color{green}{\mu\nu}}} \equiv 2\tilde{\Gamma}{^\rho}_{[\mu\nu]} = \tilde{\Gamma}{^\rho}_{\mu\nu} -\tilde{\Gamma}{^\rho}_{\nu\mu}$$$$ I decide to write in green indices that in a tensor are antisymmetric. Hence in this first convention the torsion tensor is antisymmetric in the last two indices. One then can again compute \begin{align} \nabla_{\mu}g_{\rho\nu} & = \partial_{\mu}g_{\rho\nu}-\tilde{\Gamma}{^\sigma}_{\mu\rho}g_{\sigma\nu}-\tilde{\Gamma}{^\sigma}_{\mu\nu}g_{\rho\sigma} = 0 \tag{1.a}\\ \nabla_{\nu}g_{\rho\mu} & = \partial_{\nu}g_{\rho\mu}-\tilde{\Gamma}{^\sigma}_{\nu\rho}g_{\sigma\mu}-\tilde{\Gamma}{^\sigma}_{\nu\mu}g_{\rho\sigma} = 0 \tag{1.b}\\ -\nabla_{\rho}g_{\mu\nu} & = -\partial_{\rho}g_{\mu\nu}+\tilde{\Gamma}{^\sigma}_{\rho\mu}g_{\sigma\nu}+\tilde{\Gamma}{^\sigma}_{\rho\nu}g_{\mu\sigma} = 0 \tag{1.c} \end{align} Summing up these three equations one gets $$$$\tilde{\Gamma}{^\rho}_{\mu\nu} = \Gamma{^\rho}_{\mu\nu}+\frac{1}{2}\left(T{^\rho}_{{\color{green}{\mu\nu}}}-T_{\mu{\color{green}{\nu}}}{^{\color{green}{\rho}}}+T_{\nu}{^{\color{green}{\rho}}}_{{\color{green}{\mu}}}\right)$$$$ It's easy to verify that the combination in round brackets is antisymmetric in the exchange of $$\nu$$ and $$\rho$$. Then one can define the contorsion to be $$$$K_{\mu{\color{green}{\nu}}}{^{\color{green}{\rho}}} \equiv \frac{1}{2}\left(T{^\rho}_{{\color{green}{\mu\nu}}}-T_{\mu{\color{green}{\nu}}}{^{\color{green}{\rho}}}+T_{\nu}{^{\color{green}{\rho}}}_{{\color{green}{\mu}}}\right)$$$$

Second way: Having again the metric $$g_{\mu\nu}$$ and the affine connection $$\tilde{\Gamma}{^\rho}_{\mu\nu}$$, one can define the torsion switching the order of the indices as follows $$$$T_ {{\color{green}{\mu\nu}}}{^\rho} \equiv 2\tilde{\Gamma}{^\rho}_{[\mu\nu]} = \tilde{\Gamma}{^\rho}_{\mu\nu} -\tilde{\Gamma}{^\rho}_{\nu\mu}$$$$ Therefore in this second convention the torsion is antisymmetric in the first two indices. Again one can write the three equations (1.a) (1.b) and (1.c); their sum gives $$$$\tilde{\Gamma}{^\rho}_{\mu\nu} = \Gamma{^\rho}_{\mu\nu}+\frac{1}{2}\left(T_{{\color{green}{\mu\nu}}}{^\rho}-T_{{\color{green}{\nu}}}{^{\color{green}{\rho}}}_{\mu}+T{^{\color{green}{\rho}}}_{{\color{green}{\mu}}\nu}\right)$$$$ Again the quantity in round brackets is antisymmetric in $$\nu$$ and $$\rho$$, and the contorsion can be define as $$$$K_{\mu{\color{green}{\nu}}}{^{\color{green}{\rho}}} \equiv \frac{1}{2}\left(T_{{\color{green}{\mu\nu}}}{^\rho}-T_{{\color{green}{\nu}}}{^{\color{green}{\rho}}}_{\mu}+T{^{\color{green}{\rho}}}_{{\color{green}{\mu}}\nu}\right)$$$$

Of course one can change the order of the indices also in the contorsion tensor. For istance in the second way (or analogously for the first case) one can say $$$$K{^{\color{green}{\rho}}}_{{\color{green}{\nu}}\mu} \equiv \frac{1}{2}\left(T_{{\color{green}{\mu\nu}}}{^\rho}-T_{{\color{green}{\nu}}}{^{\color{green}{\rho}}}_{\mu}+T{^{\color{green}{\rho}}}_{{\color{green}{\mu}}\nu}\right)$$$$ This of course doesn't change the order of indices on the combination on the right hand side.

Finally, following the answer of @Saksith Jaksri in Torsion tensor: definition, I stress that for each paper one should be careful on how the "covariant derivative is define", namely \begin{align} \nabla_{{\color{red}{\mu}}}A^{\rho} & = \partial_{\mu}A^{\nu} +\tilde{\Gamma}{^\rho}_{{\color{red}{\mu}}\nu}A^{\nu} \tag{A}\\ \nabla_{{\color{red}{\mu}}}A^{\rho} & = \partial_{\mu}A^{\nu} +\tilde{\Gamma}{^\rho}_{\nu{\color{red}{\mu}}}A^{\nu} \tag{B} \end{align} In my answer I used the convention (A). Of course in case (B) is used things slightly change. So when torsion start to play a role, one has to keep track of all order-of-indices convention of each author.

final edit: Actually it seems to me that if one uses convention (B) in Shapiro(2001), everything seems to work, also for some other equations written later in the paper, even though in equation (2.1) (page 6) it clearly introduce the covariant derivative following (A). However I have to say that I didn't read the entire paper, so I can't be completely sure of this possible solution.