The Constancy of the Speed of Light

This section provides a thought experiment to help understanding the second assumption of the special theory of relativity: The constancy of the speed of light.

To help us understanding the second assumption of the special theory of relativity: "The Constancy of the Speed of Light - The speed of light has the same value c measured in all inertial frames", we can use the following thought experiment.

The goal of the experiment is to measure the speed of a single light blip from two inertial frames:

• The first inertial frame is on the ground, where two photocells are installed to measure the speed of a light blip passing by them.
• The second inertial frame is on a spaceship travelling at a constant speed of 0.5*c. Two photocells are also installed on the spaceship to measure the speed of a light blip passing by them.
• A single blip of light is released from a light source on the ground.
• The two photocells on the ground will report the speed of light is c (about 300,000,000 m/s).
• The two photocells on the spaceship will also report the speed of light is c (about 300,000,000 m/s). This is not ease to believe. But it is assumed to be true by the special theory of relativity.

Last update: 2014.