Talk:Feynman diagram

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This article states that antiparticles aren't really normal particles travelling backwards in time, but the antiparticle article seems to say that seeing them as travelling backwards in time is a good interpretation. This is inconsistent.

The antiparticle article is correct, however in practice it is more useful to consider it an anti-particle moving forward in time
JeffBobFrank 02:24, 27 Mar 2004 (UTC)

Contents

[edit] WikiTex

Man, that WikiTeX Feynman diagram thing at Wikisophia is the bee's knees. We need it over here! --Matt McIrvin 01:40, 10 Oct 2004 (UTC)

[edit] This article is too technical

This article would be fine for someone who already knows quite a lot about physics, and would thus would understand the explanations given, but I have very little physics knowledge and I can't understand the explanations given in this article of exactly what a Feynman diagram is. It would be great if an explanation would be added to the article in layman's terms to explain what one of these things actually is in terms a non-physicist would understand. Would some kind soul with physics knowledge please simplify and/or append a simpler explanation to this article? --NicholasTurnbull 00:03, 28 Dec 2004 (UTC)

[edit] Picture

Maybe a picture of an actual diagram would be appropriate in this article? Anyone? --Ignignot 20:52, Feb 15, 2005 (UTC)

[edit] Penguin diagram picture?

I got redirected to "Feynman diagram" from "Penguin diagram". However, the article is missing any picture of penguin diagrams... I think a picture (or even a separate article on the subject) could be appropriate. I found a nice picture at http://www.kek.jp/intra-e/info/archive/belle2-e.html -- I am not sure about copyrights though... Anyone experienced in checking/posting that?

  • Surely penguin diagram should be in a separate article; it is hardly an important enough fact about feynman diagrams to be mentioned where it is, its prominence in the article is very misleading. the penguin is a fact about gauge theory really rather than feynman diagrams. Notjim 22:09, 1 March 2007 (UTC)

[edit] Expert needed

This article has problems with its writing, particularly in the "Mathematical details" section. Some of the statements don't even make sense:

Lemma: The reduced graph associated with a connected feynman graph is a tree. Otherwise, it is a forest.

Otherwise to what?

The paragraph immediately below it is incoherent, with extremely long sentences sprinkled liberally with parentheses. And so on.

I did a thorough copyedit and rephrasing some while ago. My version is now at Feynman graph. Septentrionalis 16:16, 28 July 2005 (UTC)

Someone who understands the theory should do this article a huge favor and clean it up. RSpeer 05:11, Mar 20, 2005 (UTC)

It has been assigned to me... I'll see what I can do, but its not my particular bit of expertise. Gimme a few weeks to read the background, and I might be able to improve it some more... DoomBringer 04:49, 12 July 2005 (UTC)
I'm an expert, and this article is way too math-oriented. I'll see what I can do in my spare time when I'm not, you know, solving Feynman diagrams. Nubby 02:02, 19 July 2005 (UTC)

[edit] Diagram shows

Surely it shows an electron and a positron annihilating? --Richard Clegg 18:19, 24 April 2006 (UTC)


The top diagram in the article seems to have the arrows on the wrong way round. For the electron and positron to approach one another the arrows should point towards the collision point. Likewise, the quark arrows should point away from their collision point -- or am I missing something deep and meaningful? -- R. Carter —Preceding unsigned comment added by 172.143.221.192 (talk) 11:00, 4 December 2007 (UTC)

Not true. Antiparticles, like positrons and antiquarks, are drawn backwards in time. Please read the article, it contains all necessary information. --Quilbert (talk) 14:42, 5 December 2007 (UTC)

[edit] Graph vs Diagram

For me it seems that this kind of Feynman diagram vs Feynman graph distinction is misleading. First of all one can observe the intention to distinguish physical and mathematical usage, that would be OK, but actually it is not what only happens. "Diagram" better speaks about "noncovariant perturbation theory diagram"s (see "time story") and "graph" is about the mathemathical and the covariant (what IS actually the Feynman-graph or diagram of physics that are interchangable). Hidaspal 11:49, 10 May 2006 (UTC)

[edit] short story wrong

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

[edit] Intro to page

A small point: but it seems a pity that the verbal description has time increasing bottom -> top but the example diagram has time increasing left -> right. It would seem trivial to change one or the other to bring them into correspondence and avoid this extra minor hurdle to understanding - any reason why not? And would someone in the field like to say which is the commonest convention? (Perhaps in some sense it doesn't matter because you can rotate the axes depending of the frame of reference - but since the elementary description at the start does not assume this, it would still be best to have the words and pictures in harmony.)

David Young, Department of Informatics, University of Sussex

My experience is that theorists tend to use the time goes from bottom to top convention and experimentalists tend to use the left to right convention. Josh Thompson 23:39, 16 February 2007 (UTC)
I would vote for going with bottom to top, as that is what Feynman typically used (see his lectures on the concept at http://www.vega.org.uk/video/subseries/8. Most people browsing Wikipedia would tend to have more exposure to theorists than experimentalists, so this makes sense to me. Of course, we should add a caveat that some physicists prefer time moving from left to right, which is equivalent in all respects. Wikipedia brown 14:37, 28 February 2007 (UTC)
I've done some general rewriting in (what are now) the first two paragraphs. There is now an explanation of the fact that time sometimes is drawn on one axis and sometimes on the other. IMHO, the article could now use some rewriting (or at least rearrangement) below the first two paragraphs. There introduction of cross section and discussion of perturbation theory seems out of place. I may do it eventually. Josh Thompson 19:13, 30 March 2007 (UTC)

[edit] General Comment

This article is terrible. There is far too much whimsy and technicality. I'd remove the whole buisness of Penguin diagrams from here - they don't warrant a mention right in the intro. Leave the maths till later, drop the "little story", give some better pedagogical insight to the lay reader.


In your example of a Feynman diagram you have a diagram showing two electrons colliding and the arrows giving the result of two quarks along with a gluon and they are all labelled EXCEPT WHAT DOES THE "Y" STAND FOR IN THE DIAGRAM!!!! That's what I want to know since the diagrams explanation says everything about the reaction EXCEPT WHAT THE "Y" REPRESENTS.Please tell me what the "Y" represents in the diagram, then maybe I will better understand the wiki article explaining Feynman diagrams.TY.

24.226.10.98 08:06, 29 September 2006 (UTC)gf

The "Y" is a γ (gamma) and is the standard symbol for a photon, just as "e" is the standard symbol for an electron. Perhaps the diagram should use a typeface that actually shows the loop at the bottom of the gamma, though. --Strait 14:22, 29 September 2006 (UTC)

TY! 216.221.81.98 09:55, 2 October 2006 (UTC)gf

[edit] Physorg

PhysOrg used the penguin diagram from this article. Particle decay may point to New Physics Discussion at PhysOrgForum -Ravedave (help name my baby) 21:33, 12 October 2006 (UTC)

[edit] Can someone add Order?

What is the order of a Feynman diagram? Can someone add it to the article? It is alluded to in the fine structure constant article. Thanks. Wikipedia brown 14:41, 28 February 2007 (UTC)

The "order" is the number of vertices that the diagram has. On the fine-structure constant page, there is some discussion of perturbation theory. I added another wikilink there. Josh Thompson 07:29, 8 March 2007 (UTC)

[edit] Real φ4

I really think the Real φ4 section needs to be removed, it doesn't help explain the diagrams at all, if anything, it just shrouds them in mystery! Joelholdsworth 17:01, 9 March 2007 (UTC)

[edit] Penguin diagram picture

The penguin diagram picture seems completely incorrect to me. The arrow directions don't match the labelling, and the lower vertex doesn't even conserve baryon number. Please correct this, whoever made the picture. Quilbert 13:55, 12 March 2007 (UTC)

I have inserted a new image. --Quilbert 20:25, 23 September 2007 (UTC)

[edit] Direction of anti-particles in diagram #1

  • deleted*

Cynical Jawa 10:05, 6 April 2007 (UTC) Sorry, this was total crap. I am an eejit.Cynical Jawa 10:12, 6 April 2007 (UTC)

[edit] Links to graph theory

Why does vertex in "they met each other (the two lines meet at a first point -- [vertex (graph theory]|vertex])," link to graph theory? As far as I can see the use of the word vertex in this article is different from that used in graph theory, ie. in graph theory a vertex can have zero edges. Taemyr 20:37, 12 May 2007 (UTC)

I don't know anything about graph theory, but you're probably right. Getting rid of the links is fine. Josh Thompson 00:56, 13 May 2007 (UTC)

[edit] The Diagram - can't find citation though

I heard that Feynman modestly refers to this as "The Diagram", with emphasis indicating that this is, essentially, the only diagram that matters, it being central to everything. I would add this to the "other names" section, only I can't find a citation. It may be mentioned in GEB by Hofstadter but I can't find my copy. — PhilHibbs | talk 14:37, 5 June 2007 (UTC)

[edit] Drawing Convention

Having recently been introduced to Feynman diagrams at school, it has been made clear to all on the course that drawing them completely horizontal, like the diagram presently at the top of the page, is incorrect and causes needless confusion. This makes sense, as according to the axis on that diagram, positron and antiquark travel backwards in time during the reaction, which is rubbish. The diagram is also incorrectly labelled, as stated previously, as it shows an electron changing into a positron (and causing, or being caused by, the interaction on the right side of the diagram) rather than an electron-positron collision, as evidenced by the arrows on the diagram.

Those on my Physics course have been assured that the correct way to draw a Feynman diagram is that of the diagram currently under "mathematical details". In this form any collisions between particles or changes from one particle to another can be seen clearly, and the order in which events occur (ie, something decays, releasing something, causing something else to change) is also shown clearly.

The Initial form of the diagram seems to remain fairly common, but perhaps the article should address these issues to avoid confusion? As it stands the corrected form of the diagram seems to be intended to be taken as a Feynman graph, which is in fact something different as evidenced in the article that wikipedia has on the subject.

If nothing else, the article needs clarification.

As long as the diagram is clear and self-consistent, I don't see the problem. All the diagrams in this article illustrate their time axis, except for the kaon decay, where the description of the diagram and the labelling of the two quarks as a "kaon" on the left side make things; so I think there's enough clarity, and certainly they all illustrate what they say their illustrating. Practicing physicists use both time conventions; I personally prefer left-to-right time because it fits better with how English-speakers are used to following words and diagrams.
I think you misunderstand the arrow conventions in Feynman diagrams, by the way. For antiparticles, the arrow points in the opposite direction of "forward in time." This makes more sense once you understand the mathematics of quantum field theory; mathematically, an antiparticle going forward in time would be the same as a particle going forward. -- SCZenz 14:40, 27 July 2007 (UTC)
As SCZenz says, there is nothing incorrect with drawing them left-to-right, that's what many theorists do. The notable exception is Peskin and Schroeder. (As an aside, it would be more logical to draw them right-to-left, considering how you write the factors from the Feynman rules.)
But maybe the confusion stems from the fact that the diagrams actually show momentum flows, not space-time pictures. --RE 16:36, 27 July 2007 (UTC)