The constancy of the speed of the light comes from the causality principle

It is well-known that Einstein's relativity theory begins with a principle, the constancy of the speed of the light. However, the principle does not meet our usual, Galilean perspectives. Then, we can ask : What is the origin of the principle? What makes the speed of the light constant to everyone? In this work, it is argued that, in mathematically rigorous terms, the principle is almost equivalent to the local causality principle and this can be interpreted as ‘Even if time machine exists, it depends not on the local structure but on the global structure, or equivalently, on the topology of our universe.’ In mathematical terms, we show that any diffeomorphism between spacetimes is conformal if the diffeomorphism preserves pointwise harmonicity of functions.