The Second Postulate (1)

Here is the second postulate:

Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.

We have to make sure the second postulate is reasonable, just as we did with the first postulate.

We know that the speed of wave is not affected by the motion of the source. As light is also a kind of wave, the same should be true for light.

Source is at rest Source is moving

But we still have two questions to answer:

  • Which reference frame is stationary?
  • When talking about the motion of the light source, which reference frame are we using?

If we were talking about water waves, which has a medium, the situation would be very clear.

But as Einstein had refused the idea that light has a medium in the very beginning of his paper, it is really difficult for us to answer the above questions.

Let's have a look at Einstein's explanation.