relativity axioms

Relative Motion

All uniform motion is relative. There is no absolute motion.

The speed of light is a constant. 299792.458 km/s


The two axioms listed above are the foundations of relativity. The animation illustrates both. In this animation there are spacecrafts moving relative to each other. There are also photons, packets of light, traveling in opposite directions above and below each spacecraft. The speed of a photon in a vacuum is always

299792.458 km/s

relative to you. It is a fundamental constant. So fundamental that the length of the metre was changed in 1983 to make this velocity of light exactly this value. The length of the metre is now defined in terms of the speed of light.

When studying the animation observe the following.

  • you can view the scene from either a yellow, blue or purple spacecraft. Each spacecraft provides a different reference frame.
  • the relative velocities of the spacecrafts change as you move from one reference frame to the next. For example if you are in a yellow spacecraft all the yellow spacecrafts are stationary, when you move to a purple spacecraft the yellow spacecrafts are moving.
  • the photons always have the same speed relative to you irrespective of the spacecraft you are in. This should seem odd.
  • in each of the spacecrafts you will feel stationary. The other spacecrafts and light each have a velocity relative to you.


For the sake of illustrating the axioms I have deliberately introduced some effected that would not be visible and excluded some that may be a distraction.

  • photons are not visible as little packages like in the animation
  • there is no Doppler effect illustrated
  • there is no aberration effect illustrated
  • there is no length contraction

The effects that have been excluded will be dealt with later in the course.