I recently read a post that mentioned a physicist who accelerated a beam of light 300 times the speed of light...Does anyone know more about this? If this is indeed so, it would violate virtually every law of relativity and causality, because the light would arrive before it leaves...very strange it can't be true but i can't help wondering what if?

Group velocity, not phase velocity. Do some googling. There is no violation of known physical laws.

- Warren

Can chroot explain for me the difference between phase velocity and group velocity. I don't really distint them clearly.

LTK

In quantum mechanics for example, you get 2 waves describing the propogation of wave-packets. One is the group velocity which is always less than c, and is the velocity with which information is propogated. Phase velocity is attributed to the speed of propogation of the waves that superimpose to make up the wave-packet. Since the amplitude of these waves is zero outside the wave-packet, no information is propogated by them beyond the wave-packet.

Sorry ryans, you got them reversed!

- Warren

ryans' post is correct.

Thanks James.

Yes, I am correct, the group velocity is the velocity of information propogation.

Yes, I am correct, the group velocity is the velocity of information propogation


Ok, I undestand it clearly. but i want to know the formulation expressed by relations between them.

LTK

Phase velocity is

w/k where w is the angular frequency and k is the wavenumber

Group velocity is the derivative of w w.r.t. k. It's hard to show here. Go get a book called "Modern Physics" by Serway, it's all in there, and it's at a fairly easy level.