alt Hacker NewsTo be more perfectly precise, it is only in a vacuum with an infinitely repeating regular signal that "phase velocity of light" = "group velocity of light".
The deviation caused by uncertainty in the package arrival time is typically extremely short, but not zero, for finite signals.
The actual "entity" that is restricted to be <=c is "information velocity". An infinitely repeating pattern would have travel at (phase velocity)=c ... but would ironically transmit no actual information (the bit value is restricted to 1, not 0 or 1).
Nice. Trying to read/understand "RF Systems and Modulation" is what got me interested in this subject in the first place.
So in my mind i map your comment as; the "carrier signal" frequency (analog) is the infinitely repeating regular signal traveling at v(p) while the "information signal" frequency (analog/digital) imposed on top of it creates the pulse/wave packet traveling at v(g).
Add in different media and you have a difference between v(g) and v(p) - https://news.ycombinator.com/item?id=49988070