Lie derivative and Riemann tensor

In summary, the conversation discusses the relationship between the curvature tensor and the Lie derivative in a spacetime with an observer at rest at the origin. The geodesics on the surface at t = 0 deviate from each other as they move away from the origin, with the deviation being determined by the proper time, position vector, and Riemann tensor at the origin. The conversation also mentions that the curvature tensor is related to the affine connection on the manifold, while the Lie derivative is not dependent on any connection.
  • #1
lark
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Suppose you have a spacetime with an observer at rest at the origin, and the surface at t = 0 going through the origin, and passing through the surface there are geodesics along increasing time. Then as you get a small ways away from the surface, the geodesics start to deviate from each other. Within a small region around the origin, the geodesic deviation is [tex]\tau x^b {R_{0b0}}^d[/tex], where [tex]\tau[/tex] is the proper time as measured by the observer, [tex]x^b[/tex] is the position vector on the surface at t = 0, and [tex]{R_{0b0}}^d[/tex] is the Riemann tensor at the origin. So the 4-velocity of points starting from rest at t = 0 is [tex]\tau x^b {R_{0b0}}^d + (1,0,0,0)[/tex].
So can you get the Riemann tensor, at least the [tex]{R_{0b0}}^d[/tex] components, as some kind of Lie derivative - what happens to the position vector of a point as it's carried along by the time flow vector field? I think it'd be a second Lie derivative, the points start out at rest so the first Lie derivative would be 0.
Laura
 
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  • #2
The curvature tensor is related to the affine connection on the manifold, which has to do with how nearby tangent spaces relate to each other. The Lie derivative on the other hand does not depend on any connection, only on the pullback using the flow of a local vector field. You therefore should generally not expect the two concepts to be related. While both the connection and the Lie derivative have several properties that you would typically associate with a derivative, only the connection is a natural directional derivative as ##\nabla_{fX} Y = f \nabla_X Y##, where ##f## is a scalar function. This does not hold for the Lie derivative.
 
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1. What is a Lie derivative?

The Lie derivative is a mathematical operation used in differential geometry to describe how a tensor field changes along a given direction or vector field. It allows for the comparison of tensor fields at different points in a manifold, and is closely related to the concept of parallel transport.

2. How is the Lie derivative defined?

The Lie derivative of a tensor field along a vector field is defined as the directional derivative of the tensor field along that vector field, minus the tensor field contracted with the Lie bracket of the vector field and the partial derivative of the tensor field with respect to the vector field.

3. What is the Riemann tensor?

The Riemann tensor, also known as the Riemann curvature tensor, is a geometric object used to describe the curvature of a manifold. It is a tensor field that encodes information about how a vector field changes as it is parallel transported around a closed loop in the manifold.

4. How is the Riemann tensor related to the Lie derivative?

The Riemann tensor is closely related to the Lie derivative, as it can be used to calculate the Lie derivative of a tensor field along a given vector field. The Riemann tensor encodes information about how the Lie derivative of a tensor field changes as the vector field is varied, making it an important tool in differential geometry.

5. What are the practical applications of Lie derivatives and the Riemann tensor?

Lie derivatives and the Riemann tensor have many practical applications in fields such as physics, engineering, and computer science. They are used to describe the curvature of spacetime in general relativity, to analyze the behavior of fluids in fluid dynamics, and to study the dynamics of systems in control theory, among many other applications.

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