This chapter starts with a revision, from the viewpoint of objective reality, of some physical, logical, and conceptual inconsistencies in the description of the photon in free space. Then, in the context of a four-dimensional ether, the author introduces the novel concept of dynamic rest mass as a signed flow of ether fluid. Here, particles (antiparticles) are extended objects formed by a momentum flow along the positive (negative) direction of the normal to the 3D surface of Dirac's sea of energy. The photon is modeled as a semiclassical particle–antiparticle doublet, which can explain the meaning of frequency and rest mass of photon. For the photon's ground state, it predicts two values of spin (±1) and de Broglie's energy equation. In the excited state, the photon has three possible values of spin: 0, ±2. It also leads to solutions of Maxwell's equations containing both advanced and retarded components.