Relative Motion

  • All uniform motion is relative. There is no absolute motion.
  • The speed of light is a constant. 299 792 458 m/s


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

299 792 458 m/s

relative to the observer. It is a fundamental constant. So fundamental that the length of the metre was changed in 1983 to make the speed of light, in metres per second, an exact value. The length of the metre is now defined in terms of the speed of light. In this exploration of relativity we shall use the standard letter c as a symbol for the speed of light.

c = 299 792 458 m/s

When studying the animation observe the following.

  • you can view the scene from either a yellow, blue or purple spacecraft by clicking on a spacecraft. Each spacecraft provides a different reference frame.
  • The relative velocities of the spacecraft change as you move from one reference frame to the another. For example, if you are in a yellow spacecraft all the yellow spacecraft are stationary, when you move to a purple spacecraft the yellow spacecraft are moving.
  • The photons always have the same speed relative to you irrespective of the spacecraft you are in. Quickly flick between different spacecraft and note that the photons above and below the spacecraft are travelling at the same speed. This should seem odd.
  • In each of the spacecraft you will feel stationary. The other spacecraft and photons have a velocity relative to you.

Imagination is more important than knowledge

( Albert Einstein )

Now that you have knowledge about the animation, use your imagination. It is fundamental when exploring the theory of relativity to visualise a dynamic system from many different frames of reference, and question what you would observe. For example

  • How fast would a photon travel if it were fired out the front of a spacecraft?
  • Imagine viewing that photon from all the different coloured spacecraft.
  • With the velocity of light a constant for all the spacecraft, is it possible for a spacecraft to travel at the speed of a photon?


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 as illustrated in the animation
  • there is no Doppler effect illustrated
  • there is no aberration of light illustrated
  • there is no length contraction, though this effect would be very small and not really noticeable.

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