The Essence of Spacetime: Space Is Light-String Structure, Time Is Frequency Propagation Sequence

Based on the complete axiom system of the First-Principle Unified Theory of Light Strings, starting from the only original physical entity—the light string, this paper presents the objective physical essence of space and time for the first time: Space is a three-dimensional evolutionary structure formed by the one-dimensional propagation, two-dimensional vibration, phase extension and coupling arrangement of light strings themselves; Time is an irreversible, ordered and serialized process of frequency propagation along light strings. This paper takes this set of original definitions as the core foundation, does not introduce extra dimensions, add free parameters, rely on mathematical fiction, or adopt ad hoc assumptions, and provides a unified interpretation of six long-standing core physical puzzles. Strictly adhering to the principles of logical closed-loop, causal completeness and physical reality, this paper constructs a global unified underlying physical description framework for spacetime, gravity and quantum laws.