four-momentum

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.

Backward Causation

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.

Forces

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.

Forces

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.

Conservapedia - Recent changes [en]

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.

Conservapedia - Recent changes [en]

"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.

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