Einstein's Theory of Special Relativity is based on two principles:

• The laws of physics are constant, even when objects move at constant speeds relative to each other.
• The speed of light is the same for everyone, regardless of how they move relative to the light source.

One fundamental problem with Einstein's theory of relativity is that relative spacetime is not true. Special relativity also challenges our intuitive understanding of simultaneity. According to the theory, two events that are simultaneous in one frame of reference may not be simultaneous in another. Firstly we see that at any point filled with matter there exists a preferred state of motion, namely that of the substance at the point con- sidered. Our problem starts however with the question whether physically preferred states of motion exist in reference to extensive regions. This ebook contains a Lecture delivered to the Nordic Assembly of Naturalists at Gothenburg, July 11, 1923 by A.Einstein