Wave Equation for a Massless Particle with Arbitrary Spin

A wave equation for a noninteracting particle with zero mass and arbitrary spin $s$ is given in this paper. The Hamiltonian is proportional to the inner product of the momentum and spin operators so that the wave function has $2s+1$ components. As an auxiliary condition, solutions with spins not parallel or anti-parallel to the momentum are discarded. With this condition the theory is Lorentz-covariant. The energy, momentum, and angular momentum are defined in terms of expected values of the usual type of displacement operator. The specialization $s=\frac{1}{2}$ is the two-component neutrino theory and $s=1\mathrm{}$ gives Maxwell's equations for the photon.