It is my understanding the the phase velocity of a wave can be greater then the speed of light. So imagine we had a wave packet consisting of a single sinusoidal wave; $$y=\sin(\omega t-kx)$$ Then from the definition of phase velocity, this wave will (if I am not mistaken) travel with the phase velocity, which as stated above can be greater then $c$. But in this case information is been passed greater then the speed of light, is it not? If we start the wave packet of at a point $A$ say and let it propagate to another point $B$ it will travel to $B$ faster than the speed of light and pass of the information of its frequency and wavelength. Why is this not breaking special relativity?
Answer
The simple answer is that the wave packet travels at the group velocity not the phase velocity, and the group velocity is always less than or equal to $c$.
You might argue that you aren't using a wave packet. For example you might argue that you are just turning the light on and waiting for it to get to the point $B$. However any modulation of the wave intensity, including turning it on and off, will propagate at the group velocity.
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