Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitude (known as the envelope of the wave) progagates through space. It is often thought of as the velocity at which energy or information is conveyed along a wave. In most cases this is true, and the group velocity can be thought of as the signal velocity of the waveform.It is however possible to design experiments where the group velocity of laser light pulses sent through specially prepared materials significantly exceeds the signal velocity, and even exceeds the speed of light.
It is also possible to stop the laser pulse.
See dispersion for a full discussion of wave velocities.
