**Herong's Notes on Physics** - Version 3.21, by Dr. Herong Yang

What Is Light Cone?

This section provides a quick introduction of light cone, which is a 4-dimensional surface in a spacetime frame where all events on the surface can be connected to a central event by light.

**What Is Light Cone?**
A light cone is a 4-dimensional surface in a spacetime frame where
all events on the surface can be connected to a central event by light.

If we look at it from the central event point of view, a light cone can be divided into 2 parts:

- Future Light Cone - The surface where all future events that can be connected to the central event by light.
- Past Light Cone - The surface where all past events that can be connected to the central event by light.

The future light cone can also be illustrated using the following example:

- Ignite a fire and consider it as the central event.
- At the next moment, the frontline of the light wave from the fire forms a sphere.
- As time goes by, this sphere grows very fast at the speed of light.
- The trace of this growing sphere is the future light cone of the event of igniting a fire.

Again, drawing a fast growing sphere in 4 dimensions on paper is very difficult. But if we remove the z-Axis in spacetime, the light cone becomes a 3-dimensional cone shape surface as shown below:

*Last update: 2014.*

Table of Contents

Introducion 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