# 6. The principle of equivalence. | Summary and Q&A

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August 26, 2020
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6. The principle of equivalence.

## TL;DR

This content discusses the use of curvilinear coordinates and covariant derivatives in the geometric approach to physics, specifically focusing on the Christoffel symbols and their relationship with the metric tensor.

## Key Insights

• ❓ Curvilinear coordinates introduce differences in basis vectors, displacement elements, and derivatives compared to Cartesian coordinates.
• 🤬 The Christoffel symbols relate derivatives of basis vectors to the vectors themselves and play a role in the covariant derivative.
• ❓ The covariant derivative is important in studying tensors and their behavior under coordinate transformations.
• 🖼️ Gravity breaks the existence of global inertial frames and requires the use of local inertial frames.

## Transcript

[SQUEAKING] [RUSTLING] [CLICKING] SCOTT HUGHES: So we're just picking up where we stopped last time. So we are beginning to discuss how we are going to sort of do a geometrical approach to physics, using a more general set of coordinates now. So we began talking about how things change when I discuss special relativity, so for the moment keeping ou... Read More

### Q: How are curvilinear coordinates different from Cartesian coordinates?

Curvilinear coordinates, such as plane polar coordinates, involve a transformation from Cartesian coordinates in the spatial sector. The basis vectors, displacement elements, and derivatives in curvilinear coordinates have different forms compared to Cartesian coordinates.

### Q: What are the Christoffel symbols and how are they related to the curvilinear coordinates?

The Christoffel symbols are mathematical objects that relate derivatives of basis vectors to the vectors themselves. They have different components depending on the choice of curvilinear coordinates and play a role in the covariant derivative.

### Q: What is the covariant derivative and why is it important?

The covariant derivative is a mathematical operation that generalizes the derivative in a way that preserves the tensor transformation law. It is important in studying tensors and their behavior under coordinate transformations.

### Q: How does gravity affect the existence of global inertial frames?

Gravity breaks the existence of global inertial frames, which are inertial reference frames that accurately describe the entire spacetime manifold. Instead, local inertial frames, which cover a specific region of spacetime, are used to account for the effects of gravity.

## Summary & Key Takeaways

• The content explores the use of curvilinear coordinates in the geometric approach to physics, starting with a discussion on special relativity and the need to work in more general coordinate systems.

• The concept of curvilinear coordinates, including plane polar coordinates, is introduced and compared to Cartesian coordinates.

• The content focuses on the differences in basis vectors, displacement elements, and derivatives in curvilinear coordinates, leading to the introduction of the Christoffel symbols.

• The Christoffel symbols are derived through the gradient of the metric tensor and are shown to be related to the covariant derivative.