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I believe this is the same as group velocity. Should we redirect? -- Pgabolde 20:25, 1 September 2005 (UTC)
- Actually I think this is usually called the signal velocity. It's not quite the same thing as the group velocity, in the cases where v_g > c. -- DrBob 17:59, 7 September 2005 (UTC)
No, all three are different. Signal velocity is usually defined as the velocity measured from when a pulse reaches half its maximum intensity. Clearly this is not the same as front velocity (which can also be defined as the speed at which a sudden change in intensity travels). These distinctions were thrashed out by Sommerfeld and Brillouin a long time ago. --Tdent 12:11, 4 October 2005 (UTC)
- Reading Brillouin's "Wave Propagation and Group Velocity", the way he defines the front velocity, it is always equal to c (for light, of course). That's the speed that at which the Sommerfeld precursor (or "first forerunner", as he calls it) travels. Your definition seems closer to his "velocity of energy propagation" (U_1). Is there a difference in terminology here? --Bob Mellish 20:35, 4 October 2005 (UTC)