# Tensors: Bar Symbol Over Functions or Indices?

• I
• kent davidge
In summary: So in summary, it seems that most people prefer to use the symbol ##g_{\mu \nu}## instead of ##\bar g_{\mu \nu}## or ##g_{\bar \mu \bar \nu}## when referring to the metric coefficients in a coordinate transformation.

#### kent davidge

When dealing with any tensor quantity, when making a coordinate transformation, we should put a bar (or whatever symbol) over the functions or over the indices? For exemple, should the metric coefficients ##g_{\mu \nu}## be written in another coord sys as ##\bar g_{\mu \nu}## or as ##g_{\bar \mu \bar \nu}##?

I think this is more of a notational preference. There might be a clearer choice depending on the type of problem you’re working, though. Personally, if I’m referring to the same tensor field simply in different coordinates, it usually makes more sense to me to put the bars over the indices.

kent davidge and Orodruin
Pencilvester said:
Personally, if I’m referring to the same tensor field simply in different coordinates, it usually makes more sense to me to put the bars over the indices.
I fully agree. The symbol itself is the field that does not care about coordinates and what actually is basis dependent are the components. Unfortunately, many people don't agree ...

Pencilvester and kent davidge
Pencilvester said:
I think this is more of a notational preference. There might be a clearer choice depending on the type of problem you’re working, though. Personally, if I’m referring to the same tensor field simply in different coordinates, it usually makes more sense to me to put the bars over the indices.
Orodruin said:
I fully agree. The symbol itself is the field that does not care about coordinates and what actually is basis dependent are the components. Unfortunately, many people don't agree ...
Thank you. For this to be valid, we should consider the same region of the manifold before and after the parametrization, correct? Because otherwise the domain of the functions would be different, what in turn makes the functions different.

kent davidge said:
Thank you. For this to be valid, we should consider the same region of the manifold before and after the parametrization, correct? Because otherwise the domain of the functions would be different, what in turn makes the functions different.
It sounds like you’re thinking about overlapping charts on a manifold, in which case, yes, you should be looking at where the domains of the coordinate functions overlap. However, for many problems, the issue is not getting from one patch of spacetime to another, it’s what happens simply when we transform the coordinates we’re using (think of almost all problems in SR, where most reasonable choices of coordinates for inertial observers cover the entire manifold).

kent davidge
Pencilvester said:
It sounds like you’re thinking about overlapping charts on a manifold, in which case, yes, you should be looking at where the domains of the coordinate functions overlap. However, for many problems, the issue is not getting from one patch of spacetime to another, it’s what happens simply when we transform the coordinates we’re using (think of almost all problems in SR, where most reasonable choices of coordinates for inertial observers cover the entire manifold).
Thanks!

## 1. What are tensors?

Tensors are mathematical objects that describe linear relations between vectors, scalars, and other tensors. They are used in physics and engineering to represent physical quantities and their transformations.

## 2. What does the bar symbol over a tensor function or index mean?

The bar symbol over a tensor function or index indicates that the quantity is a covariant tensor, meaning that it transforms in the same way as the coordinate system. It is also known as a contravariant index.

## 3. How are tensors different from scalars and vectors?

Scalars are quantities that have only a magnitude and no direction, while vectors have both magnitude and direction. Tensors, on the other hand, not only have magnitude and direction but also describe how they change under different coordinate systems.

## 4. What is the significance of using tensors in physics?

Tensors are used in physics to describe the laws of nature, such as the laws of motion, electromagnetism, and general relativity. They provide a mathematical framework for understanding and predicting physical phenomena.

## 5. How are tensors used in engineering applications?

In engineering, tensors are used to model and analyze complex systems, such as stress and strain in materials, fluid dynamics, and structural mechanics. They also play a crucial role in computer vision and machine learning for image and signal processing tasks.