Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A generalization of momentum in fourdimensional spacetime
Etymologies
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Examples

This reflects two possible solutions of the fourmomentum vector in the theory of relativity. (iii) Thus, one must distinguish between a passive time reversal operation and an active time reversal operation.

If we put the energy and the treedimensional momentum together into the fourmomentum we see that it is not constrained by the masslessness condition, we say that the photon is virtual and consequently has a (virtual) mass.

We can thus interpret the process above as either a certain photon going from particle 1 to particle 2 with a certain fourmomentum or as one from particle 2 to particle 1 with the opposite fourmomentum.

I have personally checked that if you take a massive particle, and make its rest mass tend to zero and its speed tend to c while keeping its energy constant, its fourmomentum approaches that of a photon.

Fourmomentum is the special relativity analogue of the familiar momentum from classical mechanics, with the property that the time coordinate of a particle's fourmomentum is simply the energy of the particle; the other three components of fourmomentum are the same as in classical momentum.

"propagator"  duh , a function of the particle's fourmomentum, which has a pole (a value where it becomes infinite) for a specific value of fourmomentum squared.
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