Mixed Einstein Tensor Calculations

In summary, the conversation discusses the problem of showing that G_\alpha^0 and G_0^\alpha can be expressed in terms of the metric tensor, its first derivatives, and the Ricci tensor when certain conditions are met. The equations and attempts at solutions are provided, and there is uncertainty about whether the new index should be a summation over all four dimensions when changing the tensors to covariant form.
  • #1
PhyPsy
39
0

Homework Statement


Show that [itex]G_\alpha^0= g^{00}M_{0\alpha}+ g^{0\beta}M_{\alpha\beta}[/itex] where Greek letters indicate summation over 3 space coordinates (M consists of terms involving the metric and its first derivatives; see equations below)
Also, when Ricci tensor [itex]R_{\alpha\beta}=0[/itex], show that [itex]G_\alpha^0= g^{00}R_{0\alpha}[/itex] and [itex]G_0^\alpha= g^{0\alpha}R_{00}+ g^{\alpha\beta}R_{0\beta}[/itex]

Homework Equations


[itex]R_{00}= -\frac{1}{2}g^{\alpha\beta}g_{\alpha\beta,00}+ M_{00}[/itex]
[itex]R_{0\alpha}= \frac{1}{2}g^{0\beta}g_{\alpha\beta,00}+ M_{0\alpha}[/itex]
[itex]R_{\alpha\beta}= -\frac{1}{2}g^{00}g_{\alpha\beta,00}+ M_{\alpha\beta}[/itex]
[itex]G_{ab}= R_{ab}- \frac{1}{2}g_{ab}R[/itex]

The Attempt at a Solution


The problem also includes calculations for [itex]G_0^0[/itex] and [itex]G_\alpha^\beta[/itex], but I got those correct. For [itex]G_\alpha^0[/itex], this is what I'm getting:
[itex]g^{0a}G_{\alpha a}= g^{0a}R_{\alpha a}- \frac{1}{2}g^{0a}g_{\alpha a}R[/itex], where English letters indicate summation over time and space coordinates (4 dimensions); I'm not entirely sure when changing the tensor to covariant form that the new index should be a summation over all four dimensions, but it gave me the right answer for [itex]G_0^0[/itex] and [itex]G_\alpha^\beta[/itex].

[itex]G_\alpha^0= g^{0\beta}R_{\alpha\beta}+ g^{00}R_{\alpha0}- \frac{1}{2}\delta^0_\alpha g^{ab}R_{ab}[/itex]
[itex]=g^{0\beta}R_{\alpha\beta}+ g^{00}R_{\alpha0}- \frac{1}{2}\delta^0_\alpha(g^{00}R_{00}+ 2g^{0\beta}R_{0\beta}+ g^{\alpha\beta}R_{\alpha\beta})[/itex]; breaking out the Ricci scalar into terms involving the metric tensor and the Ricci tensor is another spot where I'm not totally confident.

[itex]=g^{0\beta}R_{\alpha\beta}+ g^{00}R_{\alpha0}- \frac{1}{2}(g^{00}R_{\alpha0}+ 2g^{0\beta}R_{\alpha\beta}+ g^{0\beta}R_{\alpha\beta})[/itex]
[itex]=\frac{1}{2}(g^00R_{\alpha0}- g^{0\beta}R_{\alpha\beta})[/itex]
[itex]=\frac{1}{4}g^{00}g^{0\beta}g_{\alpha\beta,00}+ \frac{1}{2}g^{00}M_{0\alpha}+ \frac{1}{4}g^{0\beta}g^{00}g_{\alpha\beta,00}- \frac{1}{2}g^{0\beta}M_{\alpha\beta}= \frac{1}{2}g^{00}g^{0\beta}g_{\alpha\beta,00}+ \frac{1}{2}g^{00}M_{0\alpha}- \frac{1}{2}g^{0\beta}M_{\alpha\beta}[/itex]

When [itex]R_{\alpha\beta}=0[/itex], [itex]G_\alpha^0= \frac{1}{2}g^{00}R_{\alpha0}[/itex]

When I perform a similar calculation for [itex]G_0^\alpha[/itex], I come up with:
[itex]g^{\alpha a}R_{0a}= g^{\alpha0}R_{00}+ g^{\alpha\beta}R_{0\beta}- \frac{1}{2}\delta^\alpha_0(g^{00}R_{00}+ 2g^{\beta0}R_{\beta0}+ g^{\alpha\beta}R_{\alpha\beta})[/itex]
When [itex]R_{\alpha\beta}=0[/itex], [itex]G_0^\alpha= \frac{1}{2}g^{\alpha0}R_{00}[/itex]

Could someone check my math?
 
Physics news on Phys.org
  • #2
Specifically, when changing the tensors to covariant form, is it correct to assume that the new index should be a summation over all four dimensions? Any advice would be appreciated.
 

What is the Mixed Einstein tensor?

The Mixed Einstein tensor is a mathematical object used in the field of general relativity to describe the curvature of spacetime. It is a combination of the Ricci tensor and the scalar curvature.

How is the Mixed Einstein tensor calculated?

The Mixed Einstein tensor is calculated by taking the Einstein tensor, which is a combination of the Ricci tensor and the scalar curvature, and then contracting one of the indices with a metric tensor. This results in a tensor with one upper index and one lower index, hence the name "mixed."

What is the significance of the Mixed Einstein tensor in general relativity?

The Mixed Einstein tensor is an important quantity in general relativity as it is used to describe the gravitational field and the dynamics of matter in spacetime. It is also used in the Einstein field equations, which relate the curvature of spacetime to the distribution of matter and energy.

Can the Mixed Einstein tensor be used to describe other types of forces besides gravity?

No, the Mixed Einstein tensor is specific to general relativity and cannot be used to describe other types of forces, such as electromagnetism or nuclear forces. These forces are described by different mathematical objects and equations.

Are there any alternative theories of gravity that do not use the Mixed Einstein tensor?

Yes, there are alternative theories of gravity that do not use the Mixed Einstein tensor, such as f(R) gravity and scalar-tensor theories. These theories propose modifications to general relativity in order to address certain observed phenomena, such as the accelerated expansion of the universe.

Similar threads

  • Advanced Physics Homework Help
Replies
2
Views
470
Replies
3
Views
620
Replies
13
Views
640
  • Special and General Relativity
Replies
4
Views
283
  • Special and General Relativity
Replies
5
Views
1K
  • Special and General Relativity
Replies
1
Views
1K
  • Special and General Relativity
Replies
2
Views
850
  • Special and General Relativity
Replies
16
Views
968
  • Introductory Physics Homework Help
Replies
4
Views
1K
  • Differential Geometry
Replies
29
Views
1K
Back
Top