In contrast to the above Gaussian wave packet, it has been observed that a particular wave function based on Airy functions, propagates freely without envelope dispersion, maintaining its shape. It accelerates undistorted in the absence of a force field: ψ ψ = Ai ⁡ ⁡ ( B ( x − − B 3 t 2 ) ) e i B 3 t ( x − − 2 3 B 3 t 2 ) . (For simplicity, ħ = 1, m = 1/2, and B is a constant, cf. nondimensionalization.)