Wave-Packet DispersionToss a stone in a pond and the ripple that leaves is a little bundle of waves. Look closely: each crest sprints forward through the bundle, peaks in the middle, and dies at the front, while the bundle as a whole crawls along at half that speed.In deep water a wave's speed depends on its wavelength (c = √(g / k)). Add up a spread of wavelengths and the crests move at the “phase” speed but the energy travels at the slower “group” speed — exactly twice as slow for water, the reason ship wakes look the way they do.