The special theory of relativity postulates that the speed of light is always constant in a vacuum. However, recent experiments [5, 6, 7] indicate that the photon has a very small but nonzero mass, which will affect the speed of light. According to quantum theory, the photon is a massless particle, and it is purely energy. But the photon having mass will contradict the photon's definition in quantum theory, because in the special theory of relativity, there is a relationship between mass and energy. In this paper, I will discuss the meaning of the photon's mass and how we can unify the definitions of the photon in quantum theory (pure energy) and in special relativity (a particle). At first, I will discuss the relativistic mass and then derive new relativistic equations regarding time and length. As we will see, these new equations are useful for the interpretation of the truth, which says: Why can only massless particles spread over an infinite range? However, we have to introduce a new model using these new equations. Finally, we will show that the origin of the elementary particles' masses can be interpreted in this model. |