Why we have to definte covariant derivative?

In summary, the conversation discusses the importance of defining the covariant derivative when writing down differential equations on fields on a manifold. The reason for this is that the simple partial derivative of a tensor is not always a tensor, and using the covariant derivative ensures that the resulting vector is intrinsic to the surface. This is demonstrated through the example of a curve on a surface with a vector field tangent to the surface. The projection of the derivative onto the manifold is the covariant derivative.
  • #1
HeilPhysicsPhysics
16
0
Why we have to definte covariant derivative?
 
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  • #2
I don't know what "definte" means, but if you mean "define," then, well, you have to define things in order to know what they are!
 
  • #3
I think the question is probably why, when writing down differential equations on fields on a manifold we can't we just use partial derivitives. And now I'm trying to think why the Lie derivitive won't work.
 
  • #4
Because the simple partial derivative of a tensor is not, in general, a tensor.
 
  • #5
A helpful way to think about it is that if you had a curve with parameter t on a surface and vector field V along along the curve and tangent to the surface, dV/dt would not be intrinsic to the surface, but it's projection onto the manifold would be, and this is precisely the covariant derivitive. See

http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/Sec8.html
 
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Related to Why we have to definte covariant derivative?

1. Why do we need to define a covariant derivative?

The covariant derivative is a mathematical concept used in differential geometry and tensor calculus. It allows us to differentiate vector fields and tensors on curved manifolds, where the usual notion of differentiation from calculus does not apply. This is important because many physical systems, such as general relativity, require a curved space-time to accurately describe their behavior. The covariant derivative allows us to perform calculus on these curved spaces, making it a crucial tool in understanding and solving physical problems.

2. How is a covariant derivative different from a regular derivative?

A covariant derivative takes into account the curvature of the space in which a vector field or tensor is defined, while a regular derivative does not. This means that the covariant derivative takes into consideration how a vector or tensor changes as it moves along a curved path, whereas a regular derivative only considers changes along a straight line. Additionally, the covariant derivative is designed to be consistent with the rules of tensor algebra, making it a more suitable tool for working with tensors on curved spaces.

3. Can the covariant derivative be applied to any type of tensor?

Yes, the covariant derivative can be applied to any type of tensor, including scalars, vectors, and higher-order tensors. It is a generalization of the regular derivative, which only applies to scalar fields. However, the calculation of the covariant derivative may vary depending on the type of tensor being differentiated, as each type has its own transformation rules.

4. What is the main advantage of using a covariant derivative?

The main advantage of using a covariant derivative is that it allows us to perform calculus on curved spaces, which is necessary for understanding and modeling many physical systems. It also ensures that the laws of physics remain consistent, even in curved spaces. Without the covariant derivative, we would not be able to accurately describe and solve problems involving curved manifolds, making it an essential tool for scientists and mathematicians.

5. How is the covariant derivative related to the metric tensor?

The covariant derivative is defined using the metric tensor, which encodes the curvature of a space. The metric tensor provides a way to measure distances and angles on a curved manifold, and it is used to define the connection coefficients that are necessary for calculating the covariant derivative. In this way, the metric tensor plays a crucial role in the definition and application of the covariant derivative.

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