Talk:Feynman diagram

Feynman won a nobel prize for this. Seems relevant. I will add if no one objects. — Preceding unsigned comment added by Ftrotter (talk • contribs) 17:36, 25 January 2024 (UTC)

short story wrong
there is no vertical line in the diagram as cited in the 'short story'

The description is the most clear part of the article.
The description is very clear.

It would be really helpful to an interested lay reader to work the formulation with the electron-positron annihilation example. The article has a nice clear example. Now, build on that clarity.

Thanks for including more Feynman diagrams in the article. It does help.

MiddleSchoolTeacher (talk) 03:44, 29 June 2014 (UTC)

First Feynman diagram caption
The only direction of time consistent with the labelling of this Feynman diagram is from left to right, if you have time from bottom to top as you seem to want to the electron and quark are miss-labelled and should be a positron and an anti-quark. The interaction in your description is not what is depicted, a positron does not turn into an electron when it interacts with a quark through a virtual photon. Quogle (talk) 12:31, 9 March 2015 (UTC)


 * Fair enough. Give me a minute, I'll fix the diagram so that it's consistent.--OCCullens (talk) 07:03, 22 March 2015 (UTC)


 * That is unnecessary, it is accurate as it stands, the description is entirely consistent with the diagram. Just to check you do know that the arrows for antiparticles point against the direction of time/propagation? It is possible that this is the source of confusion. Quogle (talk) 22:22, 22 March 2015 (UTC)

monolectics

 * φαϋνόγραμμα
 * φαϋνογράφημα
 * φαϋνοδιάγραμμα

periphrastic, polylectic

 * διάγραμμα Φάυνμαν

Motivation and History kaon diagram
No arrows to indicate time direction Len loker (talk) 08:44, 7 July 2016 (UTC)


 * This makes it one of only two diagrams on the page that are correct. Feynman diagrams don't have space/time axes. Dukwon (talk) 14:20, 22 November 2016 (UTC)


 * By convention time is usually read left to right, or bottom to top. 'M'&and;Ŝc2ħεИτlk 17:04, 22 November 2016 (UTC)

Dyson quote
I removed this: "However, in 2006 Dyson himself confirmed that the diagrams should be called Feynman diagrams because "he taught us how to use them".". It was added in 2011 but has never been sourced and I cannot verify it. Fences &amp;  Windows  09:09, 14 February 2017 (UTC)


 * I adduced the solid precise quote from Schwebber's book (op.cit), in the Alternative Names section. Feynman's heavy computational use of his eponymous spacetime diagrams as graphs of particle QED was firmed up, pedagogically, by Dyson's re-exression into QFT, and the purely symbolic abstract book-keeping device of today. Nevertheless, I personally know that Gell-Mann's jocular usage cited was not really a focussed historiographical statement. Cuzkatzimhut (talk) 19:16, 10 April 2017 (UTC)

Basic vertex explanation missing
There are six basic vertexes. Based on one vertex with an arrow pointing to the vertex, an arrow pointing out, and a wavy line from the vertex. This one vertex can be oriented in six ways giving the six basic vertexes. The vertexes are joined with others to describe interactions. It should be at the top of the article. Without this basic representation of the graphical context of Feynman Diagrams the other graphical diagrams are meaningless. The whole article becomes technobabble. By the way the usual representation is time is the vertical axis, and space is the horizontal. The article should use this convention because most source articles use it. Other phrases in source material are also based on this convention. For instance "an arrow pointing down the time axis is an anti particle" is meaningless if the time axis isn't vertical. 98.164.66.166 (talk) 04:00, 23 April 2020 (UTC)

Critique of the article
there is a old critique of this article by Arnold Neumaier

Reference: https://www.physicsforums.com/insights/misconceptions-virtual-particles/

”In this Feynman diagram, an electron and a positron annihilate, producing a photon (represented by the blue sine wave) that becomes a quark-antiquark pair, after which the antiquark radiates a gluon (represented by the green helix).”

Physical electron-positron annihilation never produces a single virtual photon as drawn in the diagram and explained in the caption – but always two real photons (or more, with much lower probability).

(The latter is described later on that page as a correct scattering process – except that they forget to say there that the intermediate particles and processes are virtual, not something that actually happens, and that they also add spurious, misleading space and time axes. In the center of mass frame, the in-particles would come in from opposite directions, and the out-particles would leave in another opposite direction, using a semiclassical picture of the scattering process appropriate for tree diagrams.)

The correct description of the diagram is that it gives one of the two possible terms in the tree approximation of a scattering process in which an electron and positron collide and decay into a quark-antiquark pair and a gluon. One cannot talk meaningfully about the claimed intermediate events in the temporal ordering suggested by the time coordinate underneath the diagram itself.

Similarly, a single photon cannot decay into a quark-antiquark pair.

Later in that entry, the section ”Particle-path interpretation” contains in the first and fourth paragraph pure nonsense – not at all justified by all the preceding information given.

”A Feynman diagram is a representation of quantum field theory processes in terms of particle paths. The particle trajectories are represented by the lines of the diagram, which can be squiggly or straight, with an arrow or without, depending on the type of particle. A point where lines connect to other lines is an interaction vertex, and this is where the particles meet and interact: by emitting or absorbing new particles, deflecting one another, or changing type.”

Feynman diagrams don’t describe particle paths – but integration processes.

Real collisions never happen at a well-defined point – but always in a region of significant extension.

"Feynman diagrams are often confused with spacetime diagrams and bubble chamber images because they all describe particle scattering. Feynman diagrams are graphs that represent the trajectories of particles in intermediate stages of a scattering process. Unlike a bubble chamber picture, only the sum of all the Feynman diagrams represent any given particle interaction; particles do not choose a particular diagram each time they interact. The law of summation is in accord with the principle of superposition—every diagram contributes to the total amplitude for the process."

Feynman diagrams don’t describe particle scattering – but partial contributions to S-matrix elements.

Feynman diagrams don’t represent the trajectories of particles in intermediate stages of a scattering process – but integrals arising in their perturbative computation.

The sum of all the Feynman diagrams represents not a given particle interaction but (if the in-and out-momenta are fixed) a particular S-matrix element.

Superposition of states is not related to the summation of the diagrams – but to taking linear combinations of S-matrix elements with variable in-and out-momenta.

The remainder of the entry is technical and reasonably accurate (I didn’t check all details since they are irrelevant to the present thread), with a few notable exceptions – though these mistakes have nothing to do with virtual particles, they show the amount of care given to the content of the page:

In the section ”Path integral formulation”, the concept of Wick rotation is misapplied; it doesn’t mean ”a little bit rotated into imaginary time”.

In the section ”Scattering”, it is erroneously claimed that Wigner’s definition of a single-particle state through an irreducible representation of the Poincaré group doesn’t apply to quarks. (The claimed reason – confinement – has nothing to do with their claim.) Instead, they promote an erroneous ”modern point of view” that a particle is anything whose interaction can be described in terms of Feynman diagrams, which have an interpretation as a sum over particle trajectories. Given the content of reference [3] of the page (that perturbation theory for classical field theory produces Feynman diagrams), this would turn classical field theories into theories of particles

Reference: https://www.physicsforums.com/insights/misconceptions-virtual-particles/#Critique-of-some-Wikipedia-pages-on-virtual-particles

Despite the article being almost 10 years old, much of what the article criticized is still present in the page. 2603:7000:A400:BEA:1003:FD0:7F88:1C1C (talk) 01:09, 7 June 2024 (UTC)