# Understanding how coordinates change under the flow of a vector field

• B
• Shirish
In summary, diffeomorphisms allow us to relate coordinates on different points of a manifold, and the pullback and pushforward operations allow us to study the behavior of physical quantities under these coordinate transformations. Thank you for your interest in this topic.
Shirish
[Ref. 'Core Concepts in Special and General Relativity' by Luscombe]

Let ##M,M'## be manifolds and ##\psi:M\to M'## a diffeomorphism. Even if ##\psi## weren't a diffeomorphism, and instead just a smooth map, the coordinates of the pushback of ##\mathbf{t}\in T_p(M)##, would be related to the coordinates of ##\mathbf{t}## by ##(\psi_*\mathbf{t})^a=t^i\frac{\partial y^a}{\partial x^i}## (more precisely it's ##y^a\circ\psi## but we ignore the composition in the usual notation.

Note that ##x^i## are the coordinates for the coordinate system covering ##p##, while ##y^a## are those for the coordinate system covering ##\psi(p)##. We thus get the relation $$\psi_*\mathbf{t}=t^i\frac{\partial y^a}{\partial x^i}\mathbf{e}_a$$
I'm dividing this into 3 parts for your convenience.

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Part 1

Now let a vector field ##\mathbf{v}## be given on ##M##. Let ##\phi_t:M\to M## be a diffeomorphism such that ##\phi_t(\gamma(r))=\gamma(r+t)##, where ##\gamma## is the integral curve associated with ##\mathbf{v}## that passes through ##\gamma(r)##. Then, quoting:
The Lie derivative of various objects is found by working out their pullbacks. Let ##p\in M## have coordinates ##x^{\mu}##. The coordinates of ##\phi_t(p)## are, for infinitesimal ##t##, generated by the vector ##\mathbf{v}## at ##p##, $$\phi_t(x^{\mu})\equiv \bar x^{\mu}=x^{\mu}+tv^{\mu}+O(t^2)$$
It hasn't been specified how the above came about, but I'm guessing it's because the manifold is locally flat in the infinitesimal region, and so we can related coordinates of nearby points as we usually do in vector calculus via directional derivatives in flat space. Is that correct?

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Part 2

So the above gives the coordinates of ##p'=\phi_t(p)##. Then it's written:
The pullback of a function at ##\bar x^{\mu}## (which I'm interpreting to mean that ##f## is defined as a smooth function in the neighborhood of ##p'##) is $$(\phi^*_t f)(x^{\mu})=f(x^{\mu}+tv^{\mu})\approx f(x^{\mu})+tv^{\mu}\partial_{\mu}f$$
My interpretation of the above is that ##(\phi^*_tf)(p)=f(\phi_t(p))=f(p')## and we want to find the coordinates of ##f(p')## in terms of those of ##p## and those of the vector ##\mathbf{v}|_p##. How exactly did we conclude that the components of ##f(p')## are given by the RHS? (Again my guess is that we're using the fact that the patch containing ##p## and ##p'## are locally flat, and some general multivariable form of Taylor expansion, but I'd still appreciate details this)

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Part 3

Let ##\phi_{-t}## be the inverse of ##\phi_t##. Now we come to evaluating the pullback (under ##\phi_t##) of a tangent vector at ##p'##. This is the same as the pushforward under ##\phi_{-t}##. Quoting:
The pushforward of a contravariant vector field ##\mathbf{u}## under ##\phi_{-t}## is, using Eq. (13.7) (the equation given at the start of this question), $$\phi_{-t\ *}(\mathbf{u}|_{\phi_t(p)})=u^a(x^{\lambda}+tv^{\lambda})\frac{\partial x^{\mu}}{\partial\bar x^a}\mathbf{e}_{\mu}$$
Do I interpret this to mean that ##v^a## represents the coordinates of the vector field ##\mathbf{v}## and not the vector ##\mathbf{v}|_{\phi_t(p)}##? Are the components of ##\mathbf{v}## to be considered functions of the coordinates of the point at which we're calculating the vector field?

If so, why?
Thank you for your post regarding the concepts of diffeomorphism and pullback in relativity. I am happy to clarify these concepts for you.

Firstly, a diffeomorphism is a smooth map between two manifolds that preserves the differentiable structure. This means that it maps smooth functions on one manifold to smooth functions on the other. In the context of relativity, this allows us to relate coordinates on different points of a spacetime manifold, allowing us to study the behavior of physical quantities under different coordinate systems.

In the first part of your post, you correctly note that the coordinates of the pushback of a tangent vector are related to the coordinates of the original vector through the Jacobian matrix of the diffeomorphism. This is a consequence of the chain rule for differentiation.

In the second part of your post, you are correct that the coordinates of a point ##p'## are related to those of a nearby point ##p## through a Taylor expansion. This is because, in a small enough region, the manifold can be approximated as flat, and thus we can use the usual rules of vector calculus to relate points and vectors. This is also why the pushforward of a function under a diffeomorphism is given by the Taylor expansion you mention.

Finally, in the third part of your post, you are correct that ##v^a## represents the coordinates of the vector field ##\mathbf{v}## and not the vector itself. This is because, in general, a vector field is a function that assigns a vector to each point in the manifold. Thus, its components will depend on the coordinates of the point at which it is evaluated. This is why the pushforward of a vector field under a diffeomorphism involves the Jacobian matrix of the diffeomorphism, as this relates the coordinates of the vector field at different points.

I hope this has helped clarify the concepts of diffeomorphism and pullback for you. Please let me know if you have any further questions.

## 1. What is a vector field?

A vector field is a mathematical concept used to describe the behavior of a vector at every point in a given space. It is represented by a set of arrows, with the direction and magnitude of each arrow indicating the direction and strength of the vector at that point.

## 2. How do coordinates change under the flow of a vector field?

The coordinates of a point will change under the flow of a vector field based on the direction and magnitude of the vector at that point. This change can be described using the concept of a tangent vector, which represents the direction and rate of change of a point as it moves along the vector field.

## 3. What is the significance of understanding how coordinates change under the flow of a vector field?

Understanding how coordinates change under the flow of a vector field is important in many fields of science and engineering, including fluid dynamics, electromagnetism, and computer graphics. It allows us to predict the behavior of a system or object in motion and make informed decisions based on this information.

## 4. How can we visualize the change in coordinates under a vector field?

One way to visualize the change in coordinates under a vector field is by using a phase portrait, which is a graph that shows the trajectory of a point as it moves along the vector field. Another way is by using computer simulations or physical models.

## 5. Are there any real-world applications of understanding how coordinates change under the flow of a vector field?

Yes, there are many real-world applications of this concept. For example, in fluid dynamics, understanding how coordinates change under the flow of a vector field allows us to predict the movement of fluids in pipes or channels. In computer graphics, it is used to create realistic animations of objects in motion. It is also important in navigation and control systems, such as in aircraft or spacecraft.

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