Editing Propagator (section)

===Momentum space propagator===
The [[Fourier transform]] of the position space propagators can be thought of as propagators in [[momentum space]]. These take a much simpler form than the position space propagators.

They are often written with an explicit {{mvar|ε}} term although this is understood to be a reminder about which integration contour is appropriate (see above). This {{mvar|ε}} term is included to incorporate boundary conditions and [[causality]] (see below).

For a [[4-momentum]] {{mvar|p}} the causal and Feynman propagators in momentum space are:

:<math>\tilde{G}_\text{ret}(p) = \frac{1}{(p_0+i\varepsilon)^2 - \vec{p}^2 - m^2}</math>
:<math>\tilde{G}_\text{adv}(p) = \frac{1}{(p_0-i\varepsilon)^2 - \vec{p}^2 - m^2}</math>
:<math>\tilde{G}_F(p) = \frac{1}{p^2 -  m^2 + i\varepsilon}. </math>

For purposes of Feynman diagram calculations, it is usually convenient to write these with an additional overall factor of {{mvar|i}} (conventions vary).