**Ask a monkey a physics question thread**

[MadMojoMonkey]

For a wave that describes a particle, a higher frequency (shorter wavelength) means a faster phase velocity, but the opposite is true of classical waves (water or sound). Why is this?

I don't there's any dispersion in empty space. The change in speed for different frequencies is a matter of the dielectric properties of a medium through which the photon travels.

For classical waves the phase velocity may be faster or slower than the group velocity. It depends on the viscous properties of the material though which the wave travels.

For waves on a string the phase velocity is equal to the group velocity.
For water waves, the phase velocity is 2x the group velocity.
Unless I'm missing something, for quantum wave functions of free particles (not bound states), the phase velocity is 1/2 the group velocity.

[EDIT]The group velocity is 0 for bound states, which are standing waves.[/EDIT]

I at least appreciate now why a photon can move slower than c though. The phase velocity is c, but if several interfering waves have different wavelengths, the group velocity can be faster or slower. But this raises another question... if the sum of phase velocities can result in a slower photon, why not a faster photon?

Good fucking question. Wow. I never thought of that.

IDK. I'm asking around.