# Any such equation to relate spacetime to energy/mass

professor
this may be quite a usefull euation indeed by my standards... and think its worth looking into, though i do not suspect one would exist

## Answers and Replies

professor
by this i have no definite meaning, just anything you have that fits the catigory even a little let me know... (this sure would help with quantum gravity if there is even an indirect relation of some sort...atleast i suspect it would)

Crosson
Einstein's General Relativity is the theory of the interaction between matter and spacetime. There is a system of equations, the Einstein Field Equations, that uniquely determines the 4-dimensional curvature in a region given the distribution of matter and energy.

The EFE are a system of 16 highly nonlinear coupled partial differential equations in 16 unknowns. Unless you have a truly exceptional backround in physics, it is unlikely that you have encountered anything like this. Without the aid of specail notations, it is essentially impossible to express the tens of thousands of individual terms (all quadratic differentials). Here is an example of one of the 16 components of the Reimann curvature tensor (which is highly simplified due to symmetry) which is just one a part of EFE:

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Gold Member
Crosson said:
Here is an example of one of the 16 components of the Reimann curvature tensor (which is highly simplified due to symmetry) which is just one a part of EFE:
That would make a great wallpaper pattern! The whole set would probably be enough to do my living room.

professor
could you explain a bit more about theis EFE, you are right i have never heard of it... and am fairly surprised to hear it exists...but then again what exactly is it, or its point? i could use a more in depth explination... and while im not a physics genious just yet, i can handle all the equations you throw at me. so in such a manner direct me to anything that may help (tensor calc more likely then not, i believe i should be able to atleast determine a good amount of the reason for such equations with the equations themselves and defined variables)

professor
in that thumbnail just about all i can make out is some wave functions, and what may be some sigmas... i relate sigma with bonding generaly, what is the use for it here... and what are those reallly blurry smaller symbols :P

Crosson
Long before Einstein, Reimann found that distance on any curved (or not) space could be expressed by a metric. Here are some examples:

$$ds^2=dx^2+dy^2$$

$$ds^2 = dr^2 +r^2 d\theta ^2$$

Hopefully, these two metrics look familiar to you Professor. They both represent flat space, one is in cartesian coordinates and one is in polar coordinates. It is noteworthy that although the two metrics look different, they both represent the same thing. Here is a metric that expresses distance on the surface of a sphere:

$$ds^2 = R^2d\phi ^2 + R^2 Sin^2(\phi) d\theta ^2$$

This surface now has a curvature (the first two metrics haves zero curvature they are flat). It is certainly not obvious by looking that this metric belongs to a curved surface where as the flat metric in polar coordinates does not. How can we tell the curvature of a space using the metric? Reimann and Gauss found that all you have to do is take a lot of derivatives!

Here is Reimann's general form of the metric:

$$ds^2 = \sum_{\mu,\nu} g_{\mu,\nu} dx^{\mu}dx^{\nu}$$

Hopefully you can see that this is:

$$ds^2 = g_{0,0} (dx^0)^2 + g_{1,0}dx^1 dx^0 +....$$

Notice that the metric coeffecients $g_{i,j}$ (there are 16 of them in 4-d space time because we have four coordinates $x^i where i = 0,1,2,3$) contain all the information, for the reason we put them in 4x4 matrix called the metric tensor. Now like I said, once we have the metric tensor, we are only a few thousand derivatives away from finding the curvature.

From the metric tensor we define the affine connection:

$$\Gamma_{\sigma,\mu,\nu} = \frac{1}{2}\sum_{\mu,\nu} g^{\mu,\nu}(\partial _{\nu} g_{\mu,\sigma}-\partial_{\sigma}g_{\mu,\nu} + \partial_{\mu}g_{\sigma,\nu})$$

As you can see, any particular affine connection (Ex. $\Gamma_{1,1,1}$) contains 48 terms. So we already have 64 affine connections with 48 terms each.

From the affine connection we find the rank 4 reimann curvature tensor:

$$Reimann_{\lambda,\mu,\nu,\kappa} = \frac{1}{2}(\partial _{\kappa,\mu} g_{\lambda,\nu} +\partial_{\kappa,\lambda}g_{\mu,\nu} +\partial_{\nu,\lambda} g_{\kappa,\mu} + \sum _{\sigma,\epsilon} g_{\sigma,\epsilon}(\Gamma_{\epsilon, \nu, \kappa}\Gamma{\sigma,\mu,\kappa}-\Gamma_{\epsilon,\kappa,\lambda}\Gamma_{\sigma,\mu,\nu}$$

For General Relativity we contract this to the Ricci Tensor:

$$R_{\mu,\kappa} = \sum_{\nu,\lambda} g^{\lambda,\nu} Reimann_{\lambda, \nu,\mu,\kappa}$$

And the Ricci Scalar:

$$R = \sum_{\nu,\lambda} g^{\lambda,\nu} R_{\lambda, \nu}$$

In the most general case, computation of the Ricci Scalar involves over 15,000 terms. But at least now we have reached a point where we can express the EFE:

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

You should be able to interpret the left side of the equation as the curvature of space time. The right side is called the stress energy tensor, and it is basically energy density (and momentum densities in different directions).

The dependent variables in the system of PDEs are the metric coefficients, the independent variables are the four coordinates of spacetime; the components of the stress energy tensor are the inhomogeneous terms. A straightfoward method of approach would be to choose a stress energy tensor and solve the system of PDEs for the metric coefficients; but, in all but the simplest cases, the equations are so ultra-complex that nothing on earth is capable of this method. A more much more tractable approach is to input a particular metric and stress energy tensor, and solve the EFE for the time evolution of that metric.

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professor
i feel im on the verge of some sort of mental break though...dont worry i doubt i means much though, you have been a great help-thx for the info

It's actually RIEMANN and the comma between sub/superscripts means a differentiation wrt "x"... 