# Einstein's General Theory of Relativity | Lecture 8 | Summary and Q&A

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March 27, 2009
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Stanford
Einstein's General Theory of Relativity | Lecture 8

## Summary

In this video, the concept of curvature and the derivation of the curvature formula are discussed. The curvature of a vector is represented by the commutator of two covariant derivatives. The Riemann tensor is introduced as the expression for the curvature, and it is shown how to calculate the Riemann tensor using the formulas for covariant derivatives and the Christoffel symbols.

### Q: What is the curvature of a vector and how is it represented mathematically?

The curvature of a vector is a measure of how the vector changes when going around a closed loop. Mathematically, it is represented by the commutator of two covariant derivatives.

### Q: What is a covariant derivative and how does it relate to curvature?

A covariant derivative is an operator applied to a vector that takes into account the curvature of the space. It combines derivative operations and matrix multiplication. The commutator of two covariant derivatives gives the curvature of a vector.

### Q: What are some examples of linear operators?

Some examples of linear operators are derivatives with respect to the coordinates and multiplication by matrices or functions of the coordinates.

### Q: How does the commutator of derivatives with respect to the coordinates relate to curvature?

The commutator of derivatives with respect to the coordinates gives the change in a function when going around a closed loop. It is equal to the derivative of the function multiplied by the derivative of the function.

### Q: What is the covariant derivative of a vector and how is it calculated?

The covariant derivative of a vector is an operator applied to the vector that takes into account the curvature of the space. It is calculated by taking the derivative of the vector with respect to the coordinates and adding the matrix multiplication with the Christoffel symbols.

### Q: How does the covariant derivative of a vector relate to the Riemann tensor?

The covariant derivative of a vector can be expressed in terms of the Riemann tensor, which represents the curvature of the space.

### Q: What are the properties of the Riemann tensor?

The Riemann tensor has four indices and is anti-symmetric under the interchange of two indices. It is also anti-symmetric under the interchange of two other indices and symmetric under the interchange of the remaining two indices.

### Q: How can the Riemann tensor be calculated from the Christoffel symbols?

The Riemann tensor can be calculated by taking the commutator of two covariant derivatives and substitution of the Christoffel symbols.

### Q: What are the components of the Riemann tensor made up of?

The components of the Riemann tensor are made up of second derivatives and quadratic combinations of first derivatives of the metric tensor.

### Q: How can the Riemann tensor be expressed in a more elegant form?

The Riemann tensor can be expressed as the commutator of two covariant derivatives, without explicitly writing out the matrix indices.

## Takeaways (in one paragraph)

The concept of curvature and the calculation of the Riemann tensor were discussed in this video. The Riemann tensor represents the curvature of a space and is calculated using the commutator of two covariant derivatives. The Riemann tensor has several properties, including anti-symmetry and symmetry under index interchange. The components of the Riemann tensor are made up of second derivatives and quadratic combinations of first derivatives of the metric tensor. Understanding and calculating the Riemann tensor is essential in understanding the curvature of a space and its implications in general relativity.