  D. Giovannini et al. (Reports, 20 February 2015, p. 857) reported that they
measured spatially structured photons travelling in free space slowing down
even in vacuum. Here we present a simple quantum mechanical consideration which
shows that even in these cases photons travel with the speed of light (c), and
this measurement provided experimental results on the "projection" of this
velocity to the axis of symmetry/beam propagation.
