  In the theory of modern physics, such as in relativity and quantum mechanics,
the three-dimensionality of space is introduced as a presupposed fact. The
three-dimensionality of particle motion, that is, the three-dimensionality of
particle momentum, is a result of this fact. In this paper, I make two
assumptions on the spin properties of photons, and then deduce a relation
between the direction of photon momentum and the spin plane to which the photon
spin state belongs. Based on this relation, the three-dimensionality of photon
momentum is taken as the photon's own property. Moreover, this relation allows
us to better understand some of the well-known theoretical results, such as the
change rules of the spin states of spin-1 particles under space rotation as
well as the relation between the momentum direction of the classical
electromagnetic wave and the directions of the electric and magnetic fields.
