  The concept of Lorenz invariant mass and mean propagation velocity have been
investigated in detail for various multiphoton wave-packet states of light.
Based on photodetection theory and straightforward kinematics, we presented a
physically reasonable and at the same time rigorous proof that mean propagation
velocity is consistent with the Lorentz-invariant mass concept for an arbitrary
multiphoton wave-packet state. We argued that mean propagation velocity is less
than the speed of light constant in vacuum and is governed by geometric
properties of state's amplitude in wave-vector space for arbitrary wave-packet
states. To classify states with different fixed values of Lorentz-invariant
mass, we introduced a specific set of modes that allow us to describe the
wave-packet state in its rest frame.
