Wave Packet Derivation ★ Secure & Tested

Define: [ \omega_0 = \omega(k_0), \quad v_g = \omega'(k_0) \quad \text{(group velocity)} ] Let (k = k_0 + \kappa), where (\kappa) is small. [ \Psi(x,t) = \frac{1}{\sqrt{2\pi}} e^{i(k_0 x - \omega_0 t)} \int_{-\infty}^{\infty} A(k_0+\kappa) , e^{i\kappa (x - v_g t)} , e^{-i \frac{1}{2} \omega''(k_0) \kappa^2 t + \dots} , d\kappa ] 5. Neglect dispersion for short times / narrow packet If (\omega''(k_0) \approx 0) or (t) is small enough, we ignore the (\kappa^2) term (dispersion). Then:

We’ll start with the simplest 1D case. A single plane wave [ \psi_k(x,t) = e^{i(kx - \omega(k) t)} ] has definite momentum ( \hbar k ) but extends infinitely in space. To get a localized wave, we superpose many plane waves with different (k) values. 2. Wave packet definition Consider a continuous superposition: wave packet derivation

[ \Psi(x,t) = e^{i(k_0 x - \omega_0 t)} \cdot e^{-\sigma^2 (x - v_g t)^2} \cdot \text{(constant)} ] Define: [ \omega_0 = \omega(k_0), \quad v_g =

[ \Psi(x,t) \approx e^{i(k_0 x - \omega_0 t)} , F(x - v_g t) ] where [ F(X) = \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} A(k_0+\kappa) e^{i\kappa X} , d\kappa ] Then: We’ll start with the simplest 1D case