from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A generalization of momentum in four-dimensional spacetime
Sorry, no etymologies found.
This reflects two possible solutions of the four-momentum 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 tree-dimensional momentum together into the four-momentum 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 four-momentum or as one from particle 2 to particle 1 with the opposite four-momentum.
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 four-momentum approaches that of a photon.
Four-momentum is the special relativity analogue of the familiar momentum from classical mechanics, with the property that the time coordinate of a particle's four-momentum is simply the energy of the particle; the other three components of four-momentum are the same as in classical momentum.
"propagator" - duh -, a function of the particle's four-momentum, which has a pole (a value where it becomes infinite) for a specific value of four-momentum squared.