Manipulation within the Einstein Tensor in Einstein field equations

In summary: Your summation convention is incorrect. You should use the summation convention to write the equation as:R\mu\nu - \frac{1}{2}g\mu\nuR\mu\nu = ((8\piG)/c4)T\mu\nu
  • #1
space-time
218
4
Hello everybody. I was recently brainstorming ways to make the Einstein field equations a little easier to solve (as opposed to having to write out that monstrosity of equations that I started on some time ago) and I got an interesting idea in my mind.

Here, we have the field equations:

R[itex]\mu[/itex][itex]\nu[/itex] - [itex]\frac{1}{2}[/itex]g[itex]\mu[/itex][itex]\nu[/itex]R = ((8[itex]\pi[/itex]G)/c4)T[itex]\mu[/itex][itex]\nu[/itex]

Now, notice that the curvature scalar R is apart of the equations. Now we know that:

R = g[itex]\mu[/itex][itex]\nu[/itex]R[itex]\mu[/itex][itex]\nu[/itex]

This means that we can express the equations as:

R[itex]\mu[/itex][itex]\nu[/itex] - [itex]\frac{1}{2}[/itex]g[itex]\mu[/itex][itex]\nu[/itex]g[itex]\mu[/itex][itex]\nu[/itex]R[itex]\mu[/itex][itex]\nu[/itex] = ((8[itex]\pi[/itex]G)/c4)T[itex]\mu[/itex][itex]\nu[/itex]

Now, the g[itex]\mu[/itex][itex]\nu[/itex] and the g[itex]\mu[/itex][itex]\nu[/itex] cancel each other out.

This just leaves:

R[itex]\mu[/itex][itex]\nu[/itex] - [itex]\frac{1}{2}[/itex]R[itex]\mu[/itex][itex]\nu[/itex] = ((8[itex]\pi[/itex]G)/c4)T[itex]\mu[/itex][itex]\nu[/itex]

This reduces to:

[itex]\frac{1}{2}[/itex]R[itex]\mu[/itex][itex]\nu[/itex] = ((8[itex]\pi[/itex]G)/c4)T[itex]\mu[/itex][itex]\nu[/itex]

Finally, you should be able to then multiply both sides of the the equation by 2 to get:

R[itex]\mu[/itex][itex]\nu[/itex] = ((16[itex]\pi[/itex]G)/c4)T[itex]\mu[/itex][itex]\nu[/itex]

Now, you have the Ricci tensor derived and then you can derive the metric tensor from here (thus solving the equations).

Would my logic in this process be accurate or is there some mathematical error within my logic that would render this invalid?
 
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  • #2
You have made a mistake in doing your indices. You have overloaded the ##\mu,\nu## indices. The expression in terms of the Ricci tensor is:

$$R_{\mu\nu}-\frac{1}{2}g_{\mu\nu}g^{\rho\tau}R_{\rho\tau}=\frac{8\pi G}{c^4}T_{\mu\nu}$$

There IS a trace reversed version of the equations, and they look like this:

$$R_{\mu\nu}=\frac{8\pi G}{c^4}\left(T_{\mu\nu}-\frac{1}{2}Tg_{\mu\nu}\right)$$

Where here similarly in the previous case ##T\equiv T_{\mu\nu}g^{\mu\nu}##.

Basically you take the trace of the EFE's and then multiply it by ##g_{\mu\nu}/2## and add it back to the EFE's to get this equation. This may make things somewhat easier sometimes (especially useful in the vacuum case for when ##T_{\mu\nu}=0##), but you're really still just solving the same equation.
 
  • #3
space-time said:
R[itex]\mu[/itex][itex]\nu[/itex] - [itex]\frac{1}{2}[/itex]g[itex]\mu[/itex][itex]\nu[/itex]g[itex]\mu[/itex][itex]\nu[/itex]R[itex]\mu[/itex][itex]\nu[/itex] = ((8[itex]\pi[/itex]G)/c4)T[itex]\mu[/itex][itex]\nu[/itex]

Your indices are all messed up here. It should be ##g_{\mu\nu}g^{\alpha\beta}R_{\alpha\beta}## which won't give you anything useful beyond the canonical form of the EFEs.

However there is a very slick and elegant way to calculate certain components of ##R_{\mu\nu}## if you have Killing field(s) available. If ##\xi^{\mu}## is a Killing field then it is very easy to show that ##R_{\mu\nu}\xi^{\nu} = \frac{1}{\sqrt{g}}\partial_{\nu}(\sqrt{g}\nabla_{\mu}\xi^{\nu})##. This let's you calculate the components of ##R_{\mu\nu}## along ##\xi^{\mu}## almost effortlessly. It is infinitely quicker to calculate this way if you have Killing fields available. It won't necessarily get you all the components of the Ricci tensor but it's better than nothing.
 
  • #4
space-time said:
R = g[itex]\mu[/itex][itex]\nu[/itex]R[itex]\mu[/itex][itex]\nu[/itex]

This means that we can express the equations as:

R[itex]\mu[/itex][itex]\nu[/itex] - [itex]\frac{1}{2}[/itex]g[itex]\mu[/itex][itex]\nu[/itex]g[itex]\mu[/itex][itex]\nu[/itex]R[itex]\mu[/itex][itex]\nu[/itex] = ((8[itex]\pi[/itex]G)/c4)T[itex]\mu[/itex][itex]\nu[/itex]

Now, the g[itex]\mu[/itex][itex]\nu[/itex] and the g[itex]\mu[/itex][itex]\nu[/itex] cancel each other out.

Any time that your dummy indices are not paired, one up and one down and no other appearances, something has to be wrong. In this case, you'll see the problem if you try writing the summations out explicitly instead of using the summation convention.
 
  • #5
Friend

You made ​​a mistake. The rules of algebra notational not allow to have more than two equal indices in a monomial. The rule of the sum of Einstein and determining which are the free indices can not be applied:

Rμν - 1/2gμνgμνRμν = ((8πG)/c4)Tμν
 
  • #6
Please, it reads "notational algebra", read indicial algebra.
Excuse me.

VictorNeto
 
  • #7
Please, it reads "notational algebra", read indicial algebra.
Excuse me.

VictorNeto
 

1. What is the Einstein Tensor in Einstein field equations?

The Einstein Tensor, also known as the Einstein curvature tensor, is a mathematical object that describes the curvature of spacetime in Einstein's theory of general relativity. It is a key component in the Einstein field equations, which relate the curvature of spacetime to the distribution of matter and energy.

2. How is manipulation performed within the Einstein Tensor?

Manipulation within the Einstein Tensor involves applying mathematical operations, such as differentiation and integration, to the components of the tensor. These manipulations are used to derive the equations of motion for particles and fields within the framework of general relativity.

3. What role does the Einstein Tensor play in Einstein's theory of general relativity?

The Einstein Tensor is a fundamental component of Einstein's theory of general relativity, as it describes the curvature of spacetime caused by matter and energy. This curvature is what we experience as gravity, and the Einstein field equations allow us to calculate it based on the distribution of matter and energy.

4. Can the Einstein Tensor be manipulated to change the predictions of general relativity?

No, manipulation of the Einstein Tensor does not change the fundamental predictions of general relativity. The equations are derived from the principle of equivalence and have been extensively tested and confirmed by experiments. However, manipulation of the tensor can be used to explore different scenarios and solutions within the theory.

5. Are there any limitations to manipulating the Einstein Tensor?

As with any mathematical object, there are limitations to manipulating the Einstein Tensor. The equations are complex and can be difficult to solve in certain situations, and there are also limitations to our current understanding of the nature of spacetime and gravity. However, continued research and advancements in mathematics and physics may lead to new insights and techniques for manipulating the tensor.

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