**Physics Notes - Herong's Tutorial Notes** - v3.24, by Herong Yang

What Is World Line

This section provides a quick introduction of world line, which is the path of an object left in a spacetime frame of reference.

**What Is World Line?**
A world line is the path of an object left in a spacetime frame of reference.

For example, if we define a spacetime frame of reference fixed on Sun and uses only 2 dimension x and y on the Ecliptic plane, the world line of Sun will be a straight line along the t-Axis as shown on the right hand side of the picture below.

In the same spacetime frame, the world line of Earth will be a helix line.

As comparison, if we look at a space frame of reference fixed on Sun and uses only 2 dimension x and y on the Ecliptic plane, the trajectory of Sun will be a stationary point and the trajectory of Earth will be a circle. See the left hand side of the picture.

Table of Contents

Introduction of Frame of Reference

Introduction of Special Relativity

Time Dilation in Special Relativity

Length Contraction in Special Relativity

The Relativity of Simultaneity

Moving Frames and Galilean Transformation

Galilean Diagram of Moving Frames

Minkowski Spacetime and Diagrams

Introduction of Generalized Coordinates