What is group velocity? I have found a few small definitions, but for some reason I'm not quite understanding how it works. Here's one bit I have read: "Recent experimental evidence shows that it is possible for the group velocity of light to exceed c. One experiment made the group velocity of laser beams travel for extremely short distances through caesium atoms at 300 times c. However, it is not possible to use this technique to transfer information faster than c; the product of the group velocity and the velocity of information transfer is equal to the square of the normal speed of light in the material. Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people in a distantly spaced line of people, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the last person shouting." Another bit: 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 accurate, 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. Basically, what I am drawing from this is that group velocity is basically setting up events to happen, and measuring them as if they were dependent upon eachother, when they really arent. I have this feeling that I am really far off.