Ricci Tensor: Covariant Derivative & Its Significance

In summary: But unfortunately, it is not true that the covariant divergence of the energy tensor is an implied result of the continuity equation. It is actually a consequence of the Einstein Field Equation.
  • #1
dsaun777
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I read recently that Einstein initially tried the Ricci tensor alone as the left hand side his field equation but the covariant derivative wasn't zero as the energy tensor was. What is the covariant derivative of the Ricci tensor if not zero?
 
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  • #2
dsaun777 said:
the covariant derivative wasn't zero as the energy tensor was

It is true that the covariant divergence (not derivative) of the Ricci tensor is in general not zero. However, it is not true that the covariant divergence of the stress-energy tensor is zero. More precisely, there is no way of showing it to be zero, independently of the Einstein Field Equation. In other words, in GR as it was finally formulated, we deduce that the covariant divergence of the SET is zero because we know the covariant divergence of the Einstein tensor is zero, not the other way around.

dsaun777 said:
What is the covariant derivative of the Ricci tensor if not zero?

The exact nonzero value of the covariant divergence of the Ricci tensor (in spacetimes where it is not zero) depends on the spacetime. In vacuum solutions, such as Schwarzschild spacetime, the Ricci tensor itself is identically zero (that's part of what it means to be a vacuum solution), so its covariant divergence is also zero.
 
  • #3
I thought the covariant divergence of energy tensor was an implied result of the continuity equation which led to him seeking a curvature term that had that also had the same result.
 
  • #4
dsaun777 said:
I thought the covariant divergence of energy tensor was an implied result of the continuity equation

Why do you think that? Where did you get the idea from?
 
  • #5
PeterDonis said:
Why do you think that? Where did you get the idea from?
My mom told me.
 
  • #6
dsaun777 said:
My mom told me.

Are you serious? Is your mom a physicist?
 
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  • #7
dsaun777 said:
My mom told me.
Your mom sounds awesome!
 

1. What is the Ricci tensor?

The Ricci tensor is a mathematical object used in the field of differential geometry to describe the curvature of a manifold. It is named after the Italian mathematician Gregorio Ricci-Curbastro and is a key component in Einstein's theory of general relativity.

2. How is the Ricci tensor calculated?

The Ricci tensor is calculated by taking the covariant derivative of the Riemann curvature tensor. This involves taking the partial derivative of the Riemann tensor with respect to each coordinate direction and then subtracting off terms involving the connection coefficients.

3. What is the significance of the Ricci tensor?

The Ricci tensor is significant because it encodes information about the curvature of a manifold, which is a fundamental concept in geometry and physics. It is also a key component in Einstein's field equations, which describe the relationship between matter and the curvature of spacetime in general relativity.

4. What is the role of the covariant derivative in the calculation of the Ricci tensor?

The covariant derivative is a mathematical tool used to differentiate vector fields on a manifold. In the calculation of the Ricci tensor, the covariant derivative is used to account for the fact that the coordinates on a manifold may not be orthogonal and may change as one moves along a curve.

5. How is the sign of the Ricci tensor determined?

The sign of the Ricci tensor is determined by convention and depends on the choice of metric signature. In general relativity, the convention is to use a mostly plus signature, which results in a positive sign for the Ricci tensor. However, in other areas of mathematics and physics, different conventions may be used.

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