WAVES / OSCILLATORY TRANSMISSION
WAVES / OSCILLATORY TRANSMISSION
System notes
The word 'wave' hides a mathematical split between two distinct phenomena: linear waves (d'Alembert) that scale continuously and superpose, and excitable pulses (Hodgkin-Huxley) that operate via threshold, fixed amplitude, and annihilation on collision.
Linear waves obey d'Alembert's equation and hold across thirty-three orders of magnitude, from gamma rays (10^-12 m) to radio waves (10^4 m) to gravitational waves (10^21 m), because they are the simplest second-order linear PDE and emerge under small-amplitude approximation.
Neurons, cardiac tissue, and chemical oscillations (Belousov-Zhabotinsky) propagate excitable pulses, not linear waves: fixed amplitude, all-or-none threshold, refractory period creating directionality, and annihilation on collision.
Population cycles (Lotka-Volterra) and chemical oscillations (Turing reaction-diffusion) are limit cycles in phase space, not propagating waves; they are temporal oscillations governed by nonlinear ODEs, not spatial transmission governed by the wave equation.
All real waves become nonlinear at high amplitude; the linear wave equation is a convenience and small-amplitude approximation, not a deep truth, as Whitham (1974) established that shock waves, solitons, and turbulence emerge when linearity breaks down.
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