What does ''4-momenta'' mean, and what context is it used?
Special relativity can be more easily handled by using Minkowski geometry in 4D. That means that we represent spacetime as a 4D surface. So vectors now have 4 components. Furthermore, lengths of vectors are defined by slightly different function than the usual one in Euclidean 3D of "square everything, sum and square root". Various quantities are then combined into one geometrical object. For example, we can talk about the distance (in both space and time) between two spacetime points as one 4-vector -- the advantage being that thing like the length of the vector, the "invariant interval", is the same in every frame. Just like a normal vector's length doesn't depend on the direction you look at it, 4-vectors don't change length because you're travelling at a different speed. 4-momentum is just the momentum and energy combined into one geometrical object.
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