Group Velocity
Definition
The speed at which a pulse of waves — and its energy — actually travels, which can differ from the speed of individual wave crests.
Symbol: · Unit: meter per second (m/s)
Technical definition
v_g = dω/dk. For a wave packet, energy and information travel at the group velocity; the phase velocity v_p = ω/k may exceed c without violating relativity.
Example
In a dispersive fiber-optic cable, pulses spread because different frequency components have different group velocities — which limits data rates.
Common confusion
Phase velocity can exceed the speed of light, and that is fine — it carries no energy or information. Only the group (signal) velocity is capped at c.