Qed Definition and 340 Threads

QED: Question, Explore, Discover (also called QEDcon or simply QED) is an annual skeptical conference held in Manchester, England. QED is organised by North West Skeptical Events Ltd (NWSE), a volunteer-owned non-profit organisation originating from a collaboration between the Merseyside Skeptics Society and the Greater Manchester Skeptics Society.

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  1. LightPhoton

    A QED and photon path experiments

    I was reading Optics by Hecht where he states that the law of reflection is only valid statistically and some photons might reach the observation point (P in image, S being the source) by following different paths, that is, the paths for which the angle of reflection is not equal to the angle of...
  2. Frigorifico9

    I Is my interpretation of the g-factor correct?

    I'm trying to make sure I understand the g-factor of the electron, so if my question is flawed please don't just point out my flaws, but help me correct my understanding If I understand correctly the magnetic moment of an object depends on it charge, its mass and its momentum $$ \mu =...
  3. topsquark

    A Derivation of free fields... coefficient question

    Editing... Please be patient! I came across this difficulty comparing several versions of the Feynman rules for QED. I traced the problem back to a statement of Weinberg's in "The Quantum Theory of Fields (Vol 1)", Chapter 5, page 195. Weinberg is busy setting up for deriving free field...
  4. D

    What is the Role of Physics in Understanding the World Around Us?

    Hi, Thanks for having me as a member. I've had a lifelong interest in science, all types of science but particularly in physics. It's the basis of everything isn't it? Biology is based on chemistry, chemistry is based on physics, physics is based on quantum physics and that is based...
  5. T

    I QED Renormalization Counterterm Confusion

    Hey all, When looking at the renormalization conditions for QED (see page 332, equation 10.40 from Peskin), there is a condition that requires the photon propagator at ##q^2 = 0## to evaluate to 0. But looking at the expression for the photon propagator counterterm: ##-i(g^{\mu\nu}q^2 -...
  6. haushofer

    A Something is wrong in the state of QED...?

    Dear all, recently I came across this paper by one Oliver Consa, https://arxiv.org/abs/2110.02078 The recap is I'm curious whether experts think this is historically right. To me it seems that the author is mainly rephrasing critical sounds from the past regarding renormalization, before...
  7. S

    A Covariant four-potential in the Dirac equation in QED

    Under the entry "Quantum electrodynamics" in Wikipedia, the Dirac equation for an electron is given by $$ i\gamma^{\mu}\partial_{\mu}\psi - e\gamma^{\mu}\left( A_{\mu} + B_{\mu} \right) \psi - m\psi = 0 ,\tag 1 $$ or $$ i\gamma^{\mu}\partial_{\mu}\psi - m\psi = e\gamma^{\mu}\left( A_{\mu} +...
  8. anorlunda

    I Feynman's QED; 36,000 revolutions per inch for red light.

    In Feynman's book, QED, The Strange Theory of Light and Matter, page 27 he says, "As long as the photon moves, the stopwatch hand turns (about 36,000 times per inch for red light); when the photon end up at the photomultiplier, we stop the watch." I've been puzzling about the significance of...
  9. physicsclaus

    A Can anyone give me the details of creating a cat state in Circuit QED?

    I am so new to circuit quantum electrodynamics. As far as I know, there are few things I could manipulate, like resonator, qubit resonance frequencies, Hamiltonian, coupling strength, Hilbert-space cutoff, dissipation rate, but they do not make sense to me and I do not how they can relate to my...
  10. N

    A Noether and the derivative of the Action

    I know that the Action has units Energy·time or Momentum·position. A second fact is that the derivative of the action with respect to time is Energy and similar with momentum-position, consistent with a units ie. dimensions check.Is it a coincidence that both are Noether conserved quantities...
  11. M

    A QED Formulation with Massive Photon Fields

    I was reading Diagrammatica by Veltman and he treats the photon field as a massive vector boson in which gauge invariance is disappeared and the propagator has a different expression than in massless photon. After some googling, I found that this is one way to formulate QED which has the...
  12. E

    A Trace of Numerator in QED vacuum polarization

    Sorry I just typed out my query .For some reason I can't seem to find the buttons for attaching files on this thread. When writing the QED vacuum polarization loop, the numerator ,consisting momenta slashed + m from the fermion propagators and the two gamma matrices, has a trace over all of it...
  13. DaniV

    Proving modified Maxwell action is gauge invariant

    I want to show that the action staying the same action after taking ##A^\mu \to A^\mu + \partial ^\mu \chi##, for the first term I suceeded in showing the invariance using the fact ##[\partial ^ \mu , \partial ^\nu]=0## but for the second term I'm getting: ##\epsilon^{\alpha\mu\nu}A_\alpha...
  14. Pouramat

    QED, chapter 4 P&S page 125 (Coulomb Potential)

    $$\bar u(p') \gamma^i u(p) = u^\dagger(p') \gamma^0 \gamma^i u(p)$$ if ##p = p'## we can use $$u^\dagger(p) u(p) = 2m \xi^\dagger \xi$$ but how can we conclude the statement?
  15. joneall

    A Symmetry of QED interaction Lagrangian

    I am trying to get a foothold on QFT using several books (Lancaster & Blundell, Klauber, Schwichtenberg, Jeevanjee), but sometimes have trouble seeing the forest for all the trees. My problem concerns the equation of QED in the form $$ \mathcal{L}_{Dirac+Proca+int} = \bar{\Psi} ( i \gamma_{\mu}...
  16. thaiqi

    I What is the relation between the Dirac equation and QED?

    Hello, everyone. Need I understand Dirac equation if I plan to learn QED?
  17. Quarkman1

    Greetings from a 'late learner' and physics fan

    Hello! I am a 'mature' learner and am fascinated by all kinds of physics and math ideas. Learning is the key to enjoying science and keeping an open mind. I must admit, I am not very sharp on my physics skills and my calculus is pretty rusty now (I don't work in the science field, per se) so I...
  18. tomdodd4598

    I QED - running of coupling (beta function)

    Hey there, I am a little confused about the way most textbooks and notes I've read find the beta function for QED. They find it by looking at how the photon propagator varies with momentum ##q##, in particular in the context of a ##2\rightarrow2## scattering process which is proportional to...
  19. tomdodd4598

    I QED - ordering of Feynman diagram rules

    Hey there, In QED, we often have Feynman diagrams involving various numbers of non-commuting components, such as for the one-loop vertex correction (Peskin & Schroeder, page 189): The diagram this integral corresponds to is: How do we choose which order to put the propagators and vertex...
  20. JD_PM

    Quantum Books on electroweak and strong interactions (& QED)

    Hi, I am looking for a book about the unified electroweak and strong interaction theories, dealing with the following: Gauge Theories. Intermediate Vector Boson Theory. Gauge Theory of Weak Interactions. Spontaneous Symmetry Breaking. The Standard Electroweak Theory. I've basically quoted...
  21. S

    A Confusions on QED renormalization

    In many QFT textbooks, we usually see the calculations of vertex function, vacuum polarization and electron self-energy. For example, one calculates the vacuum polarization to correct photon propagator $\langle{\Omega}|T\{A_{\mu}A_{\nu}\}|\Omega\rangle$, where $|\Omega\rangle$ is the ground...
  22. W

    I QED on electromagnetic interactions in atoms

    I think my main question is pretty much summarised in the TL;DR. I have another related question: Is it possible for one to "create" two fermionic particles of the same charge but a different spin (using creation operators ## \hat{a}_{ \downarrow , + }(x,t)\hat{a}_{ \uparrow , + }(x,t) |0...
  23. L

    QED, Electron-Proton Scattering: Rosenbluth cross-section

    Hello, this problem is causing headaches, it would be very helpful if you could tell me if you know of a book where you can solve it. Maybe my English is lousy because I use a translator. Thank you very much.
  24. thaiqi

    Quantum How about Cohen-Tannoudji's QED book?

    How about his two books: Photons & Atoms and Atom-Photon Interaction? Are they basic?
  25. G

    Checking Parity Invariance of the QED Lagrangian

    Hi, I'm trying to check that the QED Lagrangian $$\mathscr{L}=\bar{\psi}\left(i\!\!\not{\!\partial}-m\right)\psi - \frac{1}{4}F^{\mu\nu}F_{\mu\nu} - J^\mu A_\mu$$ is parity invariant, I'm using the general transformations under parity given by $$\psi(x) \rightarrow...
  26. G

    A Renormalization Conditions of QED

    Hello, I'm studying the renormalization of QED. I have the Lagrangian $$\mathscr{L}_{QED}=\mathscr{L}_{physical}+\mathscr{L}_{counterterms}$$ where $$\mathscr{L}_{physical}=-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}+\bar{\psi}(i\gamma^\mu\partial_\mu - m)\psi - e \bar{\psi}\gamma^\mu\psi A_\mu$$...
  27. F

    I QED & In-Ground Electrons: Why No Photon Emission?

    In ground state electron does not emit photon.Can we use QED to explain why in ground state electron does not emit any photons?
  28. M

    How Do You Evaluate Wick Contractions in Scalar QED with Mixed Fields?

    While writing out the Dyson series due to the time ordering above I encountered the two expressions $$T(\mathcal{L}_{int}(x))\quad \text{and}\quad T(\mathcal{L}_{int}(x)\mathcal{L}_{int}(y))$$ I was able to write out the first term in terms of contractions using Wick's theorem and then finally...
  29. D

    I QED question: Photons absorbed and emitted by electrons

    According to QED photons can be absorbed or emitted by electrons, and this process results in em forces via virtual photons. What determines the emitted photon frequency, and are they fully absorbed by electrons assuming photons are discrete and must be in quantized packets? How fast are...
  30. F

    B Is it true that QED explains the reflection of light?

    About a month or two ago I posted this question in the "Classical Physics" forum: if the light doesn't interact with an electromagnetic field, then which force explains light reflection in a mirror? I didn't get a clear answer for that (besides advice to buy a book from Feynmann), so I went on...
  31. C

    I Relationship between EM and matter fields in QED

    It seems that QED treats the matter and EM fields as independent yet coupled fields. On the other hand the EM field equations emerge immediately under local change of the phase of the matter field, exactly as required to reestablish local (gauge) invariance. From that perspective it almost seems...
  32. Jamister

    A QED Bound States: Weinberg's Explanation & Breakdown

    Weinberg writes in his book on QFT Vol1 that bound states in QED are problematic because perturbation theory breaks down. consider the case of hydrogen atom, electron+proton. Weinberg explains this case and I copy from the book: https://www.physicsforums.com/attachments/247655 what is time...
  33. A. Neumaier

    A Born's rule for the QED electron violates causality

    Summary: Born's rule for the QED electron violates causality. [Since the thread where some of this material was presented was closed for further discussion, I summarize here the main content relevant for the above topic.] The free QED electron can be described in terms of a non-local...
  34. D

    B What are Feynman Diagrams and how do they help explain Quantum Electrodynamics?

    I'm new to QED, so I want to have a general grasp of what's going on. I just want to understand it conceptually. Can anyone explain it in a way so a layman can understand?
  35. Jamister

    A How can infrared divergences in the fermion propagator be cured in QED?

    Summary: how to cure infrared divergences in fermion propagator in QED? In calculating the fermion propagator in QED, we identify Ultraviolet and Infrared divergences. the Ultraviolet divergences solved by regularization, but I don't understand how to treat the Infrared divergences. Infrared...
  36. Jamister

    A Electron Vertex Function in QED

    In Peskin book they calculate the QED Vertex Function named Gamma which depend on two scalar functions called form factors F1(q^2), F2(q^2). they are calculating F1(q^2) and they do not really finish calculating for what I understand. They are just showing the Infrared divergent is canceled by...
  37. Charles Link

    Quantum Feynman's QED: Mystery of Venus's 5 Cycles in 8 Yrs

    I started reading Feynman's "QED the Strange Theory of Light and Matter" (a paperback) , and an item that some of you might find of interest comes up in the first chapter. He basically says it is a mystery why Venus goes through five cycles in eight Earth years, but I can offer a simple...
  38. Jamister

    A How can we derive the results that we know about QM from QED?

    Summary: QED describes all physics under certain energy level. So how we derive results that we know from QM from QED? I want to know how to get quantum mechanics from QED (Quantum Electrodynamics). In QED we showed that the potential between two electrons goes like 1/r as in quantum...
  39. A

    A Recent paper on QED using finite-dimensional Hilbert space - validity?

    I've been struggling with a somewhat-recent paper by Charles Francis, "A construction of full QED using finite dimensional Hilbert space," available here: https://arxiv.org/pdf/gr-qc/0605127.pdf But also published in...
  40. Wrichik Basu

    B From where did the ##ie\gamma## come into the picture? (QED)

    While reading the electromagnetic vertex function at one loop, the authors of the book I am reading, wrote down the following vertex function: corresponding to this Feynman diagram: The superscript in ##\Gamma## is the number of loops being considered. My problem is with the equation. I...
  41. Wrichik Basu

    I One vertex interactions in QED - error in the book?

    I am studying QFT from the book A First Book of QFT. I am currently in the chapter of QED. The authors have written down the interaction Lagrangian for QED. Thereafter, they have taken a special case of the electron, and written down the normal-ordered interaction Hamiltonian for this case...
  42. G

    A How does Lorentz invariance help evaluate tensor integrals?

    We're trying to reduce the tensor integral ##\int {\frac{{{d^4}k}}{{{{\left( {2\pi } \right)}^4}}}} \frac{{{k^\mu }{k^\nu }}}{{{{\left( {{k^2} - {\Delta ^2}} \right)}^n}}}{\rm{ }}## to a scalar integral (where ##{{\Delta ^2}}## is a scalar). We're told that the tensor integral is proportional...
  43. A

    A Do QED effects make a huge change to the position of the electrons?

    In https://en.wikipedia.org/wiki/Lamb_shift about the lamb shift, it's mentioned that the change in the electron's frequency due to QED effects (vacuum polarization and self-energy correction) is about 1 GHz, which would translate to an energy change of hf = 6.63E-25 J. This is 3E-7 times of the...
  44. A

    A Can we create a random variable using QED effects?

    Quantum Electrodynamics (QED) has some observable effects such as the lamb shift, which is mainly caused by the vacuum polarization and the electron self-energy. These effects contribute to the "smearing" of the electron in an unpredictable manner, other than the uncertainty we already have...
  45. P

    I Is it possible to detect an electrostatic field?

    If the electron creates an electric field around itself that can be detected,then the electrostatic field is real? So why was not the "virtual" photon found?
  46. P

    I Exploring Electron Splitting in Materials: Spinons and Orbitons in QFT

    What does it mean that an electron in a material can split into a spinon or an orbiton? Does QFT have a spinon or orbiton field?
  47. J

    Concerning QED and the Many Worlds theory.

    In QED the maths suggests that a photon takes all possible paths, and all but one of the paths cancel out leaving the path we observe. Using this as an analogy:- The maths of quantum mechanics suggests a vast number of possible universes concurrent with our universe. Perhaps all these...
  48. P

    I Electrons and the Pauli exclusion principle

    electrons are repelled using the Pauli exclusion principle?
  49. P

    I Electrostatic repulsion at a distance

    What does it mean ? "the virtual photon's plane wave is seemingly created everywhere in space at once, and destroyed all at once. Therefore, the interaction can happen no matter how far the interacting particles are from each other." As far as I know, the electrostatic force between two...
  50. J

    Computational Looking for books on computational QED

    I am looking for something similar to this one: https://www.amazon.com/dp/3319533355/?tag=pfamazon01-20 But this is for QCD, I want the same for QED.
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