Sunday, 2 August 2020

general relativity - Finding the metric tensor from the Einstein field equation?


I have have set my self a challenge to learn all the maths behind the Einstein field equation (EFE), and from reading it seems that the Metric tensor is the thing we are trying to find (from the 10 equations that forms the EFE). but since nearly all the terms in it are functions of this metric tensor, this seems very hard. What type of maths do we use to do this? (I have copied and pasted the EFE just for reference):


$$R_{\mu \nu}-\frac{1}{2}g_{\mu \nu}R+g_{\mu\nu}\Lambda=\frac{8 \pi G}{c^4}T_{\mu \nu}$$


Edit: Thanks for your comments. Just as I have no one else to ask, is this the equation that we try to solve:


$$\left(\frac{\partial}{\partial x^{\lambda}}\Gamma^{\lambda}_{\mu \nu}- \frac{\partial}{\partial x^{\nu}}\Gamma^{\lambda}_{\lambda \mu}+\Gamma^{\lambda}_{\lambda \rho}\Gamma^{\rho}_{\nu \mu}-\Gamma^{\lambda}_{\nu \rho}\Gamma^{\rho}_{\lambda \mu}\right)-\frac{1}{2}g_{\mu \nu}g^{ab}\left(\Gamma^c_{\enspace ab,c}-\Gamma^c_{\enspace ac,b}+\Gamma^d_{\enspace ab}\Gamma^c_{\enspace cd}-\Gamma^d_{\enspace ac}\Gamma^c_{\enspace bd} \right)+g_{\mu \nu}\Lambda=\frac{8 \pi G}{c^4}T_{\mu \nu}$$


with the $R_{μν}$ and $R$ terms expanded and where $g_{μν}$ and $T_{μν}$ are the μν th components from the related tensors? (a yes or no answer will be fine, thanks).



Answer



This is really a comment, but it got a bit long for the comment field.


I'd guess that, like me, your experience in physics is from an area where solving differential equations is a routine part of the job. We're used to analysing a problem, writing down a differential equation that encapsulates the physics and solving it, analytically if we're lucky or in the worst case throwing it at a computer.


What struck me very forcefully when I started reading up on GR is that this is hardly ever the approach used. The equations are so hard that in almost every case the metric is obtained either by ingenious use of symmetry or just guessing answers until one fits. If you read the derivation of the Schwartzschild metric, which is probably the simplest one most of us meet, Schwarzschild obtained the answer by guessing at a basic form for the metric then using the high symmetry to eliminate all the possibilities but one. Kerr seems to have arrived as his result by inspired guesswork (though inspired by vast amounts of effort!!).



This all feels somehow unsatisfactory for us hedge physicists. It feels as if general relativists just cheat all the time and never do things methodically like we do. That's an unfair impression of course, and born out of ignorance, but nevertheless I'm willing to bet that's how you will feel as you start reading around the subject.


If you want to start learning GR in anger then I strongly recommend A first course in general relativity by Bernard Schutz. This will get you from a starting point of knowing basic calculus to the point where you're comfortable doing basic GR calculations. Note however that even after 376 pages you will still not have seen the Einstein equation written out in full.


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