Talk:Quantum electrodynamics/Archive 1

Quantum Field Theory for the Electroweak Force ???

I saw it briefly mentioned in this article (with a one-liner) that electrodynamics and the weak force are more or less the same thing. I think the history of the search for a quantum field theory for the weak force should be presented in this article, and an explanation that QED today encompasses both electromagnetism and the weak nuclear force (if that is in fact the case). Also, QED and QCD are mentioned. Somewhere we should mention the ongoing search for a quantum field theory for gravity and that these three quantum field theories would cover everything (Or the opposite, if this is not the case). —Preceding unsigned comment added by OrganicSolar (talk • contribs) 21:44, 2 August 2010 (UTC)

Perhaps a reference to Quantum flavordynamics and maybe even the hyperweak force? OrganicSolar (talk) 21:55, 2 August 2010 (UTC)

QED does definitely not include the weak force! It describes the interactions of photons with matter based on a U(1) gauge theory. It is appropriate to link to the Standard Model as the quantum field theory which contains in a nontrivial way QED, weak and strong force, and which seems to be correct to very high level of accuracy, but QED is only a subset. Aknochel (talk) 07:37, 4 August 2010 (UTC)

Untitled

Page on the Nobel Prize official site http://www.nobel.se/physics/laureates/1965/feynman-bio.html shows this as the 1965 Prize in Physics

I didn't initiate the attention notice, but the guidelines state that it is alright to put the notice on the talk page. Interested parties will note that the category will alphabetize this entry correctly. Ancheta Wis 22:38, 31 Dec 2004 (UTC)

Expansion request

A lot of information on the history of this theory should be pulled in here. See e.g. Quantum_mechanics#History. -- Beland 05:50, 14 August 2005 (UTC)

Expansion request

The Dirac Adjoint isn't discussed here or on the adjoint page. The adjoint page simply has \psi bar = \gamma_0 \psi dagger. psi dagger isn't defined or discussed there except to say that it's not the hermitian adjoint, and that links to a page on matrices rather than 4 spinors. The property that psi dagger psi is a scalar rho is mentioned, but that's it. This page assumes that as long as we know how to differentiate the lagrangian with the adjoint, with zero discussion about the adjoint's relation to psi.

From PNA/Physics

• Quantum electrodynamics - I fixed the broken LaTex formula to conform to the author's apparent intent. I'm not sure if the "D" in the sentence following the formula should also be slashed. Could someone who knows QED double-check this please? --Ortonmc 23:22, 13 Sep 2003 (UTC)

cleaned up some Latex, but it still needs a lot of work lethe

I moved this from the "mathematics" section. Paul August ☎ 18:33, Jun 5, 2005 (UTC)

</math>

Persistent... idontknowhattocallit

The same anon that's been going around vandalizing other theoretical phyisics related articles, has apparently settled on this page, and while I wouldn't call its edits outright vadalism, I would say that its past history makes me doubt its seriousness, seems more like it's picked an article where vandalism wouldn't be very likely to be noticed, and stuck with it

• I'd like to get an outside opinion on this--Aolanaonwaswronglyaccused 03:20, 17 December 2005 (UTC)

My opinion is that you should look up the information in the citation before accusing the person of vandalism. Edit: ...though it seems that the user has not made a strict citation or reference, making a check quite difficult to do. I'll see what I can dredge up. In the meantime, let's not treat something as vandalism until we see genuine reasons to do so. Lucidish 01:42, 5 January 2006 (UTC)

The citation for the Feynman quote that's floating around is James Gleick's "Genius: The life and science of Richard Feynman", p348. Lucidish

The editor was a notrious POV-pusher, Licorne. I have re-removed the quote, because as it stands it sounds like Feynman repudiated QED in his later life, which is, as best I know, not true. –Joke 16:43, 14 March 2006 (UTC)

He didn't repudiate it, but even in his Nobel acceptance speech he called the theory he'd won the prize for a bit of a cheat or something to that effect. This quote doesn't seem to be inappropriate to me, I'm going to reinsert it, but with a small amount of explanation.WolfKeeper 17:21, 14 March 2006 (UTC)

Fair enough. I'm happy with your version. –Joke 17:28, 14 March 2006 (UTC)

Every Possible Path?

From article "In fact, according to QED, it takes EVERY possible path between the start and end points."

I think that the claim that it takes EVERY possible path is Feynman's Many World's interpretation of quantum mechanics and not a necessary component of QED.

IRC Feynman claimed only the consistent histories interpretation of quantum mechanics which is a stronger claim than many worlds, since it makes no claim for non observable universes, only those that reconnect back with ours.WolfKeeper 13:07, 23 February 2006 (UTC)

However, in order to do some of the calculations in QED, which, according to the book "QED" cited on the page, can be done by drawing pictures on a 2d access of space and time, Feynman diagrams. Now in order to do the calculations you need to know every possible path it can take before you compute the probability of it taking a certain path (which is the experimental evidence of QED), but that is not the same as it taking every possible path.--MobyDikc 23:43, 8 February 2006 (UTC)

Yeah, it is, due to destructive/constructive interference. If the particle didn't go every which way you wouldn't get interference like that.WolfKeeper 13:07, 23 February 2006 (UTC)

The particle itself does not travel all possible paths. The wavefunction propagates through space, and upon reaching an aperture, splits into a continuous source of spherically propagating wavefunctions (Huygens-Fresnel). The particle itself is not at every point of the aperture - such an assertion is simply ridiculous, and would result in the aperture essentially containing an infinite amount of energy. If we were capable of doing so, we could map out the definite path traveled by the particle, however the uncertainty principle does not allow us to do so. For example, imagine the double slit experiment using electrons where a single electron is shot through the slits at a regular interval (1 every tenth of a second, let's say). As you undoubtedly know, an interference pattern will arise. However, as you also likely know, if the slit that each electron passes through is recorded, the interference pattern collapses (except for the interference due to diffraction). The electron is not passing through both apertures - it's only passing through one. When we do not record the slit that each electron passes through, each one is STILL passing only through one of the slits, it's simply that the wavefunction representing this particle passes through both. The finite scale of time with which particle propagation occurs prevents it from taking all possible paths, but does not prevent the wavefunction from doing so. (144.92.123.60 (talk) 17:47, 17 February 2009 (UTC))

We Need a Picture of the Basic Vertex!

One of the simple beauties of QED is that the trilinear term in the Lagrangian corresponds to a vertex in a Feynman diagram where an electron absorbs a photon (or emits one, or etcetera depending on how one views the vertex). This should be explained, and there should be a nice picture of this basic vertex. Right now only some fancier Feynman diagrams are shown... the simplicity of the theory is not being explained!

I would improve this article right now but I'm busy preparing a talk and I just wanted a picture of this vertex...

John Baez 10:32, 23 February 2006 (UTC)

John, it is very good to see someone of your calibre take a close interest in this important article on the "jewel of physics." Although I am no physicist (but do own a copy of Feynman 1985), forgive me for making the following possibly rash and uninformed remarks:

• Someone please do a yeoman's job of communicating the essential "simplicity" of QED.
• A subject this rich and this important deserves a longer entry, period.
• Beautiful physics (GR, Maxwell's EMF, Boltzmann) often culminates in beautiful equations, especially when these are cast in Planck units. Why does no beautiful equation command pride of place in expositions of QED?
• Wikipedia should include a killer intro to Feynman diagrams, a splendid instance of science done by semiotic means.
• The last word re renormalization has yet to be written. One day the physics and math will reach a state that Feynman would applaud.202.36.179.65 19:33, 25 March 2006 (UTC)

I have just added a detailed but simple account of QED, based on Feynman's book, which includes an image of the basic vertex. Does this please?--Dylan1946 11:28, 6 January 2010 (UTC) —Preceding unsigned comment added by Dylan1946 (talk • contribs)

Feynman diagrams

There are some nice illustrations of Feynman digrams with loops but no accompanying explanatory text. Could someone please remedy this ommission?

"See Also" list

The "See also" list is very long; in my opinion there is a lot there that isn't directly relevant to quantum electrodynamics. E.g.: Basics of quantum mechanics, Photon dynamics in the double-slit experiment, Schrödinger equation, and Theoretical and experimental justification for the Schrödinger equation. Paring down this list might help focus people on the more relevant items. (I'm not confident enough to be bold here.) HEL 02:30, 8 October 2006 (UTC)

I agree, as well with the pages on Standard Model, Positronium, and Photon polarization. Watchayakan 05:14, 14 November 2006 (UTC)

Accuracy

Predictions of QED agree with experiments to an extremely high degree of accuracy: currently about 10⁻¹² (and limited by experimental errors)

What does this 10⁻¹² mean exactly? Significance? This seems unclear to me. --Jaapkroe 11:40, 28 October 2006 (UTC)

This means "Theory and experiment agree with each other to within one part in a million million." This is a completely standard scientific terminology, but a translation into words might be nice for a broader audience.

My complaint with this statement is with the "and limited by experimental errors". In QED most of the experiments are performed in particle accelerators which pin down particle-particle interactions to a very high level. In MANY cases the experiments are far more accurate that the QED predictions because to get better and better predictions, more and more higher order possible interaction pathways have to be considered. Eventually you just give up because your computer isn't big enough to work them all out. You just say "I've got the most imporatnt ones and I'll just ignore all the rest".

So I'd very much like the slur on experimental physicists removed. Please.

131.111.8.96 16:31, 6 November 2006 (UTC) Will (an experimental physicist)

Huhsaywhat?

I chanced upon this subject researching a midterm essay for science... can't say I very well know half of what it means. Any clue where to start, cause obviously this isn't exactly 1st grade science class. 68.239.57.153 16:41, 24 January 2007 (UTC)

The best place to start for non-physicists is by watching the 4 lectures that Feynman gave in 1979. They are available here: http://www.vega.org.uk/video/subseries/8 It'll take you 5 hours to watch all of them, but he explains QED (and a lot of other physics concepts) in a novel way without getting technical, and there's almost zero math involved. I'm not a physicist and I picked up a good understanding of what QED is all about from these. Just make sure you pay attention to what's going on. And feel free to ask questions at my talk page if you get confused. Wikipedia brown 02:32, 25 February 2007 (UTC)

Rewriting Request

After reading some of the text on the page I realise it is more like an essay than a encyclopedic entry. I wonder if someone can do something about it. 220.238.162.141 12:11, 21 March 2007 (UTC)

Mathematical Consistency of QED?

I think the article could be improved by providing the current expert's view (or range of views perhaps) on the question raised by Feynman (and many others!) in his 1985 popular level QED book (already referenced in the current version of the article; the book is based on lectures given 82 or 83 by the way?) which is (quoting Feynman): "It's surprising that the theory still hasn't been proved self-consistent one way or the other by now; I suspect that renormalization is not mathematically legitimate." In fact I came to this article looking for an answer to this question. What's confusing me is that somewhere or another I think I recently (last couple of years) read that QED had been shown to be inconsistent (perturbation series eventually diverges); however when I try to track this down all I can find is Dyson's 1952 article which as far as I can tell (as a non-expert) simply suggests that it is very likely that the series (as it was formulated at the time; has there been further progress?) almost certainly eventually diverges (and this would of course have been known to Feynman, having appeared 30+ years earlier). While this is probably of no importance for doing physics, it is (I think) extremely interesting mathematically, and also interesting in terms of philosophy of physics. So ... what's the story regarding mathematical self-consistency of QED? Jtrbnsn 16:38, 14 October 2007 (UTC)

I see (belatedly) that this issue is discussed in the article on renormalization (although I still can't seem to find a definitive statement that the perturbation series in QED are known to diverge) - apparently the current attitude is that it doesn't matter whether QED is mathematically consistent, since it just an "effective" field theory. Jtrbnsn (talk) 23:07, 9 January 2009 (UTC)

From my point of view, the solution of the Dirac equation can be expressed in 3-dimension momentum space. The only part involving time is : exp(-ita)-1-(-ita)-(-ita)*(-ita)/2-...-(-ita)^n/(n!), in which n is the order the perturbation series. Since the scattering matrix describe the relation between the initial state (t→-∞) and final state (t→+∞), the time interval between the two states is obviously infinitly large: ∆t→+∞, therefore exp(-ita)-1-(-ita)-(-ita)*(-ita)/2-...-(-ita)^n/(n!) ≈-(-ita)^n/(n!) →∞. We now know the divergences appear because of the infinite time interval between initial state (t→-∞) and final state (t→+∞). If we can calculate all orders of perturbation series, we will find out that these divergences cancel each other. Take an example: exp(-x^2)=1-x^2+x^4/2-x^6/6+…. When x→∞ all terms in the right side of the equation is divergent except the first term (1), but the total sum up result exp(-x^2)→0, that’s because these divergences cancel each other. However, we can easily cut off the divergences that we don’t like. Simly ignore the part -1-(-ita)-(-ita)*(-ita)/2-...-(-ita)^n/(n!) and keep the exponential function part, then we can get finite result at any given time. The zeroth order of perturbation series is exp(-ita); The 1st order is exp(-ita)-1; The 2nd order is exp(-ita)-1-(-ita); … That explains why the divergences begin to appear in the 2nd order of perturbation series.

The derivation of the 3-dimension momentum space expression is quite rigorous and boring, for details, see http://blog.sina.com.cn/u/1070440741

The convergence problems of QED (Even after renormalization) are not necessarily to be taken as proof that QED is inconsistent. It's merely a symptom of the mathematical technique used to solve the non-linear (interacting) equations. Dyson's series is an asymptotic series and has all the usual limitations of this kind of series. Dauto (talk) 16:31, 29 January 2009 (UTC)

I don't think divergences of QED is a symptom of the mathematical technique used to solve the non-linear (interacting) equations. It's because of the infinite time interval between intial states (t→-∞) and final states (t→+∞). If the time interval is finite(i.e. wave function transforms from t=-0.1 to t=+0.1), then divergences won't occur, the result is always finite. —Preceding unsigned comment added by Peter484 (talk • contribs) 09:33, 30 January 2009 (UTC)

You are talking about the divergence of loop Feynman diagrams used to compute each term in the Dyson series. You are right about that. I'm talking about the divergence of the Dyson series itself. Dauto (talk) 16:01, 1 February 2009 (UTC)

Free evolution of the electromagnetic field?

Quick question--at the end in the section 'In Pictures' it describes the second boxed equation as describing the free evolution ot the electromagnetic field, is this correct? As the equation involves the electron/positron fields is the term 'free' appropriate? Thanks, Ivanivanovich (talk) 15:18, 24 January 2008 (UTC)

Contradiction

On Precision tests of QED it says

Quantum electrodynamics (QED) is the most stringently tested theory in physics (after special relativity which currently is tested [1] to 10-21 - Michelson-Morley experiment: 10−4, Hughes-Drever experiment: 10−16, trapped atoms experiments: 3×10−22)

, whereas here it says

This makes QED the most accurate physical theory constructed thus far.

—Preceding unsigned comment added by Rangek (talk • contribs) 20:50, 25 January 2008 (UTC)

Discrepancy explained Omeganumber (talk) 03:13, 1 March 2008 (UTC)

This updated edit detracts from the importance of QED and looks to me almost blasphemous, however it was necessary to avoid confusion. Omeganumber (talk) 12:00, 1 March 2008 (UTC)

I've undone the edits because the situation remains muddy. What does "subjective precision" mean? How is it determined? Who determined it? Where is the reference for that?  --Lambiam 20:00, 1 March 2008 (UTC)

It is obvious the understanding of the indeterminacy of experimental accuracy at a quantum level can not be explained in simple terms to confirm QED is the most accurate physical theory constructed thus far (or refute ). I would revert the article, but will leave that to another user, as I was tempted to insert a well known acronym used by Feynman. Omeganumber (talk) 20:25, 1 March 2008 (UTC)

The problem has now been resolved; 80.192.42.32 has replaced "most accurate" by "one of the most accurate physical theories" (also at Precision tests of QED).  --Lambiam 11:01, 3 March 2008 (UTC)

Technically, isn't the electroweak theory more accurate, in addition to being more robust? I also feel it at least deserves a mention on this page; its absence bothers me. Eebster the Great (talk) 05:27, 29 January 2009 (UTC)

Mergefrom tadpole

Whoever proposed this merger has no idea of the complexity or inter-relationships. Please remove merger tag. Ouedbirdwatcher (talk) 20:20, 1 May 2009 (UTC)

Done. Headbomb - you ok with this? I did not see discussion anywhere, and it looks to me that there is enough to say about Tadpole (physics) to support a stand-alone article. - 2/0 (formerly Eldereft) (cont.) 20:31, 1 May 2009 (UTC)

Nonrelativistic quantum electrodynamics

Is there such a branch?--79.116.76.124 (talk) 20:05, 18 September 2010 (UTC)

"Theory is sick"

In his August 14, 2009 creation of the Quantum electrodynamics#Nonconvergence of series section, User:69.140.12.180 introduced language saying "the theory is sick...". Isn't there a better word to use here, such as "inadequate"? — Wdfarmer (talk) 06:35, 13 December 2010 (UTC)

Common slang would be "pathological", but I don't know if that's maybe too much jargon Aknochel (talk) 19:19, 14 December 2010 (UTC)

A superb article

This is an extraordinarily clear and insightful exposition, maybe the best non-technical discussion anywhere on the subject of QED. Of course I don't know who to thank for this gem -- but thank you anyway. Dratman (talk) 02:13, 12 September 2012 (UTC)

I too want to thank the article's writer. The Feynman part is extraordinarily clear. Perhaps it should be used as an exemplar, for what can be accomplished without adumbration with excess equations. I like math, but it's easy to forget it's a semantic tool; ideas come first.BrianMC — Preceding unsigned comment added by 208.80.117.214 (talk) 06:59, 6 November 2013 (UTC)

Much agreed. (Also I took the liberty of combining the "me too" section with this one.)TricksterWolf (talk) 21:09, 11 August 2014 (UTC)

“perturbation theory of the electromagnetic quantum vacuum”

What should that mean? Isn’t QED just a quantum field theory described by a special Lagrangian etc.? And it is usually treated using perturbational methods, but these methods are not an inherent property of it, and there are also lattice based QED computations? And what does “of the em quantum vacuum” mean? You only want to compute vacuum→vacuum transitions? --Chricho ∀ (talk) 23:39, 12 December 2012 (UTC)

Photons appear immobile in Feynman diagram elements image.

The image introducing Feynman diagram elements shows a particle moving less than c (deduced by the usual 45-degree angle for natural units in the example on the right), but the leftmost example illustrating what a photon looks like shows the photon stationary, which is mad. It wouldn't be so bad if it weren't right next to and parallel to the labeled "time" axis, but does anyone agree we should tilt the effer in a new picture?TricksterWolf (talk) 21:07, 11 August 2014 (UTC)

Units used in Lagrangian

I propose to include an comment on what units are used for the Lagrangian: Gauss, c=1, hbar =1, ...? Depending on the units, the factor in front of term with the electromagnetic field tensor can be 1/4, 1/4 pi or 1/16 pi... -- Arist0s (talk) 17:14, 25 February 2015 (UTC)

A bit too brief

The idea was simply to attach infinities to corrections of mass and charge that were actually fixed to a finite value by experiments.

Sounds interesting, but perhaps a slight shift in word choice would make this sentence comprehensible, at the heuristic-summary level. Why would attaching infinities fix anything? 178.38.100.30 (talk) 00:04, 28 April 2015 (UTC)

This is similar to the wording Feynman uses in his QED popularisation book/lectures. If I understood correctly, Feynman said that nobody knew why renormalisation worked. I agree that it deserves a couple of sentences of explanation, even if an ordinary reader like me doesn't understand the explanation. --Hroðulf (or Hrothulf) (Talk) 14:08, 29 April 2015 (UTC)

Is QED only Feynman diagrams?

The way the article begins, it sounds like QED is the theory of Dyson series and Feynman diagrams. However as I would understand it, QED refers to a very particular form of Lagrangian, and the popular use of perturbative techniques for calculations is just one mathematical approach. If I am not mistaken, there are also important results in QED that are strictly non-perturbative. --Nanite (talk) 12:01, 12 June 2015 (UTC)

The maths section vs. normal mathematical conventions

In the Mathematics / Equations of motion section, would it kill to explain the connection between $e{\bar {\psi }}\gamma ^{\mu }\psi \,$ and $J^{\mu }\,$ (for 4-current J), which would at least make clear how the normal version of maxwell's equations comes about ?

Also, in that section, the Lagrangian uses natural units, which should be made clearer - the version of the Lagrangian using SI units should be present as well, since SI units are what most scientists use and would assume is being used (only a small number of physicists, those specialising in particle theory, use natural units).

Particularly, and I can't emphasise this enough, since Maxwell's equations normally contain $\mu _{0}\,$ . (The gauss-ampere equations). Not including that quantity in the lagrangian makes its appearance in articles that describe maxwell's equations completely inexplicable to all but the most expert readers.

Derivation

I believe it will be helpful to have a brief "derivation" of the QED Lagrangian, starting with the Dirac Lagrangian, observing a global U(1) symmetry, and that (being a motivation to make it a "local" symmetry) giving rise to the covariant derivative with the gauge field (EM potential), and along with the Yang-Mills action, giving us the full Lagrangian. This line of argument does appear at Gauge_theory#An_example:_Electrodynamics. But I believe a brief discussion in this article will help in making it self-contained. - Subh83 (talk | contribs) 19:31, 30 March 2016 (UTC)

Why does the spin of the charge field matter?

A very naive question, but why does the spin of the source term (the four-current) fields matter? If instead of electrons, these were generated, say by mesons, would the equations differ? — Preceding unsigned comment added by 96.95.174.86 (talk) 05:48, 20 August 2017 (UTC)

Nonconvergence of series

The article state "Because the theory is 'sick' for any negative value of the coupling constant, the series do not converge, but are an asymptotic series." I don't believe the series has ever been shown to be an asymptotic series. Davidaedwards

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