Four Vector Wikipedia

Four-momentum - Wikipedia.

In special relativity, four-momentum (also called momentum-energy or momenergy) is the generalization of the classical three-dimensional momentum to four-dimensional spacetime.Momentum is a vector in three dimensions; similarly four-momentum is a four-vector in spacetime.The contravariant four-momentum of a particle with relativistic energy E and ....

Wave vector - Wikipedia.

The direction in which the wave vector points must be distinguished from the "direction of wave propagation".The "direction of wave propagation" is the direction of a wave's energy flow, and the direction that a small wave packet will move, i.e. the direction of the group velocity.For light waves in vacuum, this is also the direction of the Poynting vector..