Signal Velocity

Signal velocity is frequently equal to class velocity (the speed of a short “pulse” or of a wave-packet’s middle or “envelope”). However, in a few special situations (e. g., media made to amplify the front-most areas of a pulse and attenuate the back portion of the pulse), class velocity can go beyond the speed involving light in cleaner, while the signal velocity will still be less than or equal to the speed involving light in vacuum.