  We study theoretically the collective orbital excitation named orbital wave
in the orbital ordered manganites.The dispersion relation of the orbital wave
is affected by the static spin structure through the coupling between spin and
orbital degrees of freedom. As a probe to detect the dispersion relation, we
propose two possible methods by utilizing resonant inelastic x-ray scattering.
The transition probability of the orbital wave scattering is formulated, and
the momentum and polarization dependences of the structure factor are
calculated in several types of the orbital and spin structures. The elastic
x-ray scattering in the L-edge case to observe the orbital ordering is also
discussed.
