  Recently, Kocsis et al. reported the observation of "average trajectories of
single photons" in a two-slit interference experiment [Science 332, 1170
(2011)]. This was possible by using the quantum weak-measurements method, which
implies averaging over many events, i.e., in fact, a multi-photon limit of
classical linear optics. We give a classical-optics interpretation to this
experiment and other related problems. It appears that weak measurements of the
local momentum of photons made by Kocsis et al. represent measurements of the
Poynting vector in an optical field. We consider both the real and imaginary
parts of the local momentum, and show that their measurements have been
realized in classical optics using small probe particles. We also examine the
appearance of "anomalous" values of the local momentum: either negative
(backflow) or exceeding the wavenumber (superluminal propagation). These
features appear to be closely related to vortices and evanescent waves.
Finally, we revisit a number of older works and find examples of
photon-trajectories and anomalous-momentum measurements in various optical
experiments.
