  It has been assumed for a century that the energy-momentum tensor of the
photon takes a symmetric form, with the renowned Poynting vector assigned as
the same density for momentum and energy flow. Here we show that the symmetry
of the photon energy-momentum tensor can actually be inferred from the known
difference between the diffraction patterns of light with spin and orbital
angular momentum, respectively. The conclusion is that the symmetric expression
of energy-momentum tensor is denied, and the nonsymmetric canonical expression
is favored.
