Fresnel's Mechanical Legacy Recovered: How Bubble Acoustics Unifies Partial Drag, Velocity Addition, and Atomic Polarization

Sound waves in bubbly water and light in special relativity obey identical first-order transport laws. This equivalence is not approximate or analogical but mathematically identical to first order in $U/v$, sharing the same form discovered by Fresnel in 1818 for light in moving transparent media. We demonstrate that all three systems-bubble acoustics, optical drag, and relativistic velocity addition-are described by a universal partial entrainment equation where wave coupling to compliant components determines the drag coefficient. In bubbly liquids this physics is directly observable: waves couple to compressible bubbles rather than rigid liquid. Since bubble dynamics reproduces the relativistic result, velocity addition itself admits mechanical interpretation. Von Laue's 1907 derivation abstracted mechanics into kinematics; we reverse this, showing relativistic effects preserve mechanical content in abstract form. Beyond first-order equivalence, we show Fresnel's dispersive term encodes group velocity (energy transport) versus phase velocity (wave-crest motion)-a distinction mechanical models naturally capture but pure kinematic velocity addition cannot address. The rigidity-based interpretation of dispersion, established in acoustic metamaterials, provides cross-domain insights for materials design and suggests testable predictions linking acoustic compliance to optical drag. This three-way equivalence reduces independent phenomena to a single principle: waves riding compliant inclusions. Fresnel drag, Lorentz contraction, and atomic polarization all emerge as aspects of this one mechanism, traced through the historical density-versus-rigidity fork that shaped aether theory's trajectory. The compliant-inclusion principle shows striking quantitative agreement with isotope mass-dependence and resonance structure in existing spectroscopic data.