Does the Second Postulate Have Experiment Support? (2)

Currently, in the International System of Units, meter, the fundamental unit of length, is defined based on the speed of light. Does this guarantee that the second postulate is right?

The answer is no.

If one object follows the rule of relative motion, we only need to know its velocity in any one reference frame of uniform motion. Using the Galilean velocity addition rule, which has withstood the test of several hundred years, its velocity in any other reference frame can be obtained.

As light does not follow the rule of relative motion, we have no tested formula to use. In order to know the light speeds in several reference frames, we have to test the light speed in each of them.

Excluding the motion of the light source, and limiting the observer's motion to the direction of the light, three types of test are needed. The first has an observer at rest; the second has an observer approaching the light; and the third has an observer running away from the light.

The test of the first type has been completed with great accuracy, as can be seen from the currently accepted speed of light. Now only the second and third type are remaining, and both of them are facing the same technical difficulty.

At least the following difficulties are present in the moving observer test:

  1. All human made objects have relative low speed comparing with light.
  2. Any motion in the surroundings of the Earth is bumpy in nature.
  3. The techniques used in the first type of test may not work for a moving observer.

From the above difficulties, it is easy to see that results from such direct experiment still have to wait.