  In present paper we develop the description of massless fields on the basis
of space-time algebra of sixteen-component sedeons. The generalized sedeonic
second-order equation for unified gravitoelectromagnetic (GE) field describing
simultaneously gravity and electromagnetism is proposed. The second-order
relations for the GE field energy, momentum and Lorentz invariants are derived.
We consider also the generalized sedeonic first-order equation for the massless
neutrino field. The second-order relations for the neutrino potentials
analogues to the Pointing theorem and Lorentz invariant relations in
gravitoelectromagnetism are also derived.
