This paper systematically discusses the essence of the four fundamental interactions, and proposes a chain transmission and superposition mechanism of gravity to explain the formation and stability of the Milky Way’s spiral arms. The four fundamental forces are direct, instantaneous, non-contact interactions without force-carrying particles. On the galactic scale, a central celestial body still exerts direct gravity on distant stars, but it is extremely weak due to long distance and cannot maintain the galactic structure alone. The stable spiral arms are formed by the overall gravitational field generated by the step-by-step chain transmission and layer-upon-layer superposition of short-range gravity between celestial bodies. Computational difficulties in physics are objective; mathematical models can be used as calculation tools, but constructing fictitious matter and particles with mathematics is firmly opposed.
Jiaqing Yan (Thu,) studied this question.