Just as Einstein claimed, "Light is always propagated in empty space with a definite velocity \(c\) which is independent of the state of motion of the emitting body", which has been widely approved. However, physics textbooks do not tell us the dynamic reason why the speed of light remains constant in a vacuum. By re-examining the simultaneity (measured by light and sound) within different inertial frames, the electrostatic field dragged by moving charges, the gravitational field (Hill sphere) dragged by rotating and translating celestial bodies, and the Casimir effect of a vacuum (not empty), this paper presents that all non-zero-mass objects possess their respective static vacuum fields, and reintroduces the vacuum medium model in which the constancy of light speed in a vacuum follows continuum mechanics. This result shows that a simply modified classical theory can more intuitively explain everything special relativity can explain, and that a slightly revised Newtonian cosmic model can exactly predict the cosmological redshift, the cosmic microwave background, and the relevant cosmological constants, thereby affording the theoretical possibility to quantize the vacuum and unify quantum-level and macroscopic systems.

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