Einstein’s Second Postulate: Sufficient but Not Necessary

Traditional presentations of special relativity introduce Einstein’s second postulate, the constancy of light speed, as a fundamental axiom. This commonly creates student confusion, since synchronization procedures appear to rely on the very assumption they are meant to establish. This difficulty parallels student misconceptions documented in physics education research. A symmetry-based approach avoids this difficulty by deriving the Lorentz transformations from fundamental symmetry principles: Relativity, homogeneity, isotropy, reciprocity, and closure under composition. A four-step modular derivation leads to generalized k-Lorentz transformations with an invariant speed parameter k. The standard Lorentz form follows once experiment identifies this parameter with the measured signal speed, c. This framing clarifies that Einstein’s second postulate is sufficient but not logically necessary. The result separates logical structure from empirical identification, offering a compact method for classroom use. Students benefit from a clear progression, reduced circularity, and a structured pathway for understanding relativity as a consequence of symmetry principles.