>>13479630All particles are allowed momentum. In the same sense, all particles are allowed position. The knowability of either is about inversely proportional to each other, i.e., knowing more precisely where a particle is located means being less able to determine the particle's momentum (and thus direction of movement), and vice versa. This applies to all QM conjugate variables (like Energy and Time).
Momentum can be calculated through the Energy-mass-momentum equation.
With zero momentum, we get einstein's equation
With zero mass, we get
For light, we know that
and
which means
