Visualizing relativity

The principle of relativity

Suppose Alice is standing on a train platform waving her friend Bob goodbye as the train he is on moves away from the platform. From Alice's perspective, she is standing still on the platform while the train is moving away from her. But from Bob's perspective, he is standing still on the train while the platform is moving away from him.

We suppose that Alice and Bob are describing the same situation, as governed by the same laws of nature, but just from a different vantage point. This idea is captured by the principle of relativity:

Principle of relativity: The laws of nature are the same in every frame of reference

In the train example, Alice and Bob each represent a frame of reference. Those reference frames are relative to each other, hence the name of the principle. And the train scenario is governed by natural laws that are invariant across those reference frames.

This series of posts will show various applications of this principle and how to visualize their effects:

• Galilean relativity
• Special relativity
• General relativity