wave packet, dispersion, group velocity, Schrödinger equation, electromagnetic pulse, mechanical wave 1. Introduction A wave bundle (or wave packet) is a superposition of multiple sinusoidal waves with slightly different frequencies and wavenumbers, resulting in a spatially and temporally localized disturbance. From a stone dropped in water to a femtosecond laser pulse and an electron’s probability density, wave bundles are ubiquitous.

[ \psi(x,t) = \frac1\sqrt2\pi \int_-\infty^\infty A(k) , e^i(kx - \omega(k)t) , dk ]

For an ideal flexible string, ( \omega = v|k| ) (linear, nondispersive).