What is Qed: Definition and 339 Discussions

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. Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 34: Vertex Correction 3

    Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 34: Vertex Correction 3

    Copyright reserved to Prof. Tripathy and NPTEL, Govt. of India. Duplication prohibited. Lectures: http://nptel.ac.in/courses/115106065/ Syllabus: http://nptel.ac.in/syllabus/syllabus.php?subjectId=115106065
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  2. Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 35: Vertex Correction 4

    Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 35: Vertex Correction 4

    Copyright reserved to Prof. Tripathy and NPTEL, Govt. of India. Duplication prohibited. Lectures: http://nptel.ac.in/courses/115106065/ Syllabus: http://nptel.ac.in/syllabus/syllabus.php?subjectId=115106065
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  3. Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 36: Electron Self-energy

    Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 36: Electron Self-energy

    Copyright reserved to Prof. Tripathy and NPTEL, Govt. of India. Duplication prohibited. Lectures: http://nptel.ac.in/courses/115106065/ Syllabus: http://nptel.ac.in/syllabus/syllabus.php?subjectId=115106065
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  4. Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 37: Photon Self-energy 1

    Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 37: Photon Self-energy 1

    Copyright reserved to Prof. Tripathy and NPTEL, Govt. of India. Duplication prohibited. Lectures: http://nptel.ac.in/courses/115106065/ Syllabus: http://nptel.ac.in/syllabus/syllabus.php?subjectId=115106065
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  5. Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 38: Photon Self-energy 2

    Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 38: Photon Self-energy 2

    Copyright reserved to Prof. Tripathy and NPTEL, Govt. of India. Duplication prohibited. Lectures: http://nptel.ac.in/courses/115106065/ Syllabus: http://nptel.ac.in/syllabus/syllabus.php?subjectId=115106065
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  6. Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 29: Compton Scattering 3

    Quantum Field Theory by Dr. Prasanta Tripathy (NPTEL):- Lecture - 29: Compton Scattering 3

    Copyright reserved to Prof. Tripathy and NPTEL, Govt. of India. Duplication prohibited. Lectures: http://nptel.ac.in/courses/115106065/ Syllabus: http://nptel.ac.in/syllabus/syllabus.php?subjectId=115106065
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  7. P

    A Are vacuum EM modes circularly polarized according to QED?

    I'm talking of the virtual EM modes of vacuum...
  8. F

    I How can electrostatic fields be composed of photons?

    I know little about QED, QCD, and whatever the corresponding theory for the weak force is, and of course virtually nothing about the quantized theory of the gravitational force, which mostly doesn’t exist, so the following arguments and questions may be somewhat wrongly based where they refer...
  9. D

    A Renormalisation: what are the physical observables?

    I'm trying to understand renormalisation properly, however, I've run into a few stumbling blocks. To set the scene, I've been reading Matthew Schwartz's "Quantum Field Theory & the Standard Model", in particular the section on mass renormalisation in QED. As I understand it, in order to tame the...
  10. LarryS

    I A Closer Look at the Randomness of Quantum Measurements in QED

    In all Quantum Physics experiments, the sequence of measurement results is inherently random. Consider just the position observable. In the Schrodinger picture of non-relativistic QM, in each measurement-event, nature steps in and randomly selects one of the observable's eigenvalues/vectors to...
  11. J

    I Feynman's Thesis, 3 Principles

    Hi, I've just started studying Feynman's thesis and am in need of some discussion regarding the three principles he put forward on the development of his 'Principle of least action in quantum mechanics'. The three principles are 1) The acceleration of a point charge is due to the sum of its...
  12. D

    A Infrared Divergences in QED Revisited

    What do you think of the following paper about QED? https://journals.aps.org/prd/pdf/10.1103/PhysRevD.96.085002 Infrared divergences in QED revisited Daniel Kapec, Malcolm Perry, Ana-Maria Raclariu, and Andrew Strominger Phys. Rev. D 96, 085002 (2017) – Published 10 October 2017 It has been...
  13. C

    I (QED) The initial mass and the correction cancelling out

    The prefix is a bit irrelevant This is on renormalisation. How do they cancel out? Isn't it adding? So the mass experimental = m + (c2correction) so how do you cancel out the m and correction? I'm new to this area (just finished watching lectures by Richard Feynman, specifically a 4 lecture...
  14. lalo_u

    A QED propagator in Coulomb gauge

    My aim is to derive the photon propagator in an Coulomb gauge following Pokorski's book method. In this book the photon propagator in Lorenz gauge was obtained as follows: 1. Lorenz gauge: ##\partial_{\mu}A^{\mu}=0## 2. It's proved that ##\delta_{\mu}A^{\mu}_T=0##, where...
  15. G

    A Divergent Diagrams in the Standard Model

    It is my understanding that the task of enumerating all of the divergent diagrams in a quantum field theory can be reduced to analyzing a hand full of diagrams (well, at the moment I know that this is at least true for QED and phi^4 theory), and that all other divergent diagrams are divergent...
  16. L

    A How the g factor comes from QFT?

    I'm reading the book Quantum Field Theory and the Standard Model by Matthew Schwartz and currently I'm studying the chapter 17 titled "The anomalous magnetic moment" which is devoted to computing the corrections due to QFT to the g factor. My main issue is in the beginning of the chapter, where...
  17. G

    B How Precise Can QED Predict the Motion of Charged Particles?

    It is claimed that QED could predict certain quantities, like the lamb shift and magnetic moment of electron with astonishing precision of 12 digits. My question is the following: does QED predict only a few things to this precision, or does it predict, this precisely, the entire arbitrary...
  18. SonnetsAndMath

    I Effective Mass-to-Charge Ratio of Electroweak or QED Vacuum

    Apologies if my question is confused, because I'm beginner in field theory: adding up all corrections, can we talk about an effective (at least approximate) overall mass-to-charge ratio for the electroweak or QED vacuums? Suppose we want to talk about the relative strength of...
  19. L

    Ward-Takahashi identities at tree level in scalar QED

    Homework Statement Let \Gamma^\mu be the three-point vertex in scalar QED and \Gamma^{\mu\nu} be the four-point vertex. Use Feynman's rule at tree level and verify that the Ward-Takahashi identities are satisfied: q^\mu \Gamma_\mu(p_1,p_2)=e[D_F^{-1}(p_1)-D_F^{-1}(p_2)],\\...
  20. tom.stoer

    A Challenges to QED from hyperfine measurements

    Does anybody have more insights regarding https://www.nature.com/articles/ncomms15484 High precision hyperfine measurements in Bismuth challenge bound-state strong-field QED Does this really challange QED? Or does this mean that we miss certain contributions to QED calculations? The authors...
  21. Sophrosyne

    I Why can't we detect virtual photons?

    I have heard a virtual phton as used in QED defined as being forced carriers between two fermions which last for very short periods of time. A couple of questions about this: 1) how do we know these are photons and not some other force carrier if we cannot detect them directly? 2) can...
  22. N

    I Common interaction vacuum for QED + QCD?

    Hello, I know QED and QCD as isolated theories but now I thought about particle interactions with QED and QCD processes (like fpr proton-antiproton scattering). But I'm not sure how to interpret this mathematically. As I understood my Feynman diagrams are nothing more like pictures for the...
  23. davidge

    How far am I from understanding this thread: "Is QED nonrelativistic?"

    Hi. I'm at the second year to get my bachelor degree in physics. I actually know the fundamentals of physics. I was looking some threads on PF, then I found this one https://www.physicsforums.com/threads/is-qed-nonrelativistic.900451 My question is, given that I have approximately the knowledge...
  24. M

    I A question from reading Feynman's book on QED

    I am reading Feynman's book QED: The Strange Theory of Light and Matter". What I am getting out of this is that a photon is like a tiny harmonic oscillator traveling through space. Each harmonic oscillator has frequency and phase. If two photons have the same source and start out nearly in...
  25. S

    A Feynman rule for closed fermion loop in QED

    One of the Feynman rules of QED is the following: For a closed fermionic loop, the Feynman rule is to start at an arbitrary vertex or propagator, follow the line until we get back to the starting point, multiply all the vertices and the propagators in the order of the line, then take the trace...
  26. J

    I A few questions about Feynman's QED Lecture

    Hello, Apologies if this has been asked before. After watching Feynman’s QED lectures on the probabilities of different paths that photons can take towards a photomultiplier, a few questions came up in me. Let’s take the simple scenario in which a photon emitter is faced straight towards a...
  27. A. Neumaier

    A Does QED Originate from Non-Relativistic Systems?

    [Mentor's note - this thread was split off from https://www.physicsforums.com/threads/a-skeptics-view-on-bohmian-mechanics.899967/ as it is interesting in its own right and a digression there] This is a distortion of facts. One looks at lattice QCD only because it is derived from the Poincare...
  28. mfb

    I Evidence of Light-by-light scattering by ATLAS

    For centuries, scientists argued whether light was waves or particles. Light scattering with other light would favor the particle concept. Today we know both models are wrong, but quantum electrodynamics also predicts this scattering - just with an incredibly tiny rate, so it has never been...
  29. S

    I Screening in QED: Electron, Positron, and Gauss's Law

    Hello! I read today about the screening in QED and vacuum polarization. And the image they used to explain it was an electron surrounded by a ring of positrons and then by a ring of electron, which makes sense. Now, the Gauss's law outside and inside the 2 rings will give the same result, as the...
  30. USeptim

    A Issue in the electron’s infinite self-energy

    Hello, Reading Richard Feynman’s book “Quantum Electrodynamics” (Edited by Advanced Book Classics), I read that the electron’s self-energy is infinite and that has been a trouble for QED during 20 years. Feynman proposed a solution based on a cut-off, but that’s not fully satisfactory and I...
  31. A

    Explaining the Photoelectric Effect: Why Can't Classical Electromagnetism Do It?

    I've read that the photoelectric effect cannot be explained according to Maxwell's equations and theory of electromagnetism. Classical EM theory treats light as a wave and states that the intensity of the light matters and not the frequency. According to Einstein, if we think of light as a...
  32. H

    A QED vs Scalar QED: Proving Divergence in P&S 10.1

    In Peskin and Schroeder problem 10.1 is about showing that superficially divergent diagrams that would destroy gauge invariance converge or vanish. We are supposed to prove it for the 1-photon, 3-photon, and 4-photon vertex diagrams. Does this change for scalar QED?
  33. M

    How to find Lagrangian density from a vertex factor

    Homework Statement This isn't a homework problem, per se, in that it's not part of a specific class. That being said, the question I would like help with is finding a Lagrangian density from the vertex factor, $$-ig_a\gamma^{\mu}\gamma^5.$$ This vertex would be identical to the QED vertex...
  34. LarryS

    I EM Wave Components for a Photon?

    Although mass-less, a photon, like any other quantum particle, has a frequency/wavelength associated with its energy/momentum. If we have a group of photons all with the same specific energy, then each photon can be represented by a little, complex-valued plane wave. Photons are also EM waves...
  35. A

    Other Comparing Lattice QCD vs QED Research Experience

    I have the opportunity to pursue a research project in either lattice QCD or QED. In the case of QED, I'd be writing the code myself, will be able to understand the whole thing, et cetera. There shouldn't be any mystery in this. It almost seems like a long exercise, and there isn't anything I'd...
  36. Spinnor

    Feynman's approach to precision QED

    Feynman's approach to precision QED calculations, see his book "Quantum Electrodynamics", was shown(?) to be equivalent to the method of quantum fields. Both methods get the right the get the right answers to some problems. What quantum problems does Feynman's method (there is a name for that I...
  37. zonde

    QED explanation of entanglement

    I would like to understand basic construction of entanglement "explanation" in QED. As I understand because we talk about coincidences QED explanation necessarily involves Fock states. And Fock states are expressed using annihilation and creation operators, right?
  38. A

    Radiation reaction in the classical limit of QED?

    The classical problem of radiation reaction classical electrodynamics seems to be a huge mess with no good answer. There is no even consensus of the very form of the Newton law "predicted" theory. So, the question of this thread is: Does the classical limit of QED say something about this...
  39. Andreol263

    What are the key concepts needed to understand QED and QCD?

    Hi physicists , I'm in high-school yet, and since I've started study physics for myself my aim is to understand more complex fields of physics, and one of these fields in QED(Quantum Eletrodynamics) and QCD(Quantum Chromodynamics), could anyone tell me what the path to go for a good basis for...
  40. M

    What Are the Feynman Rules for Bremsstrahlung with Vector Particle Production?

    So this isn't a homework problem really, but based on the posting rules for the HEP section this seems to fit better here. My issue is that I'm trying to use a Feynman diagram to represent a Bremsstrahlung process in which a vector particle is produced: $$ Z + e^- \rightarrow Z + e^- + v. $$...
  41. T

    QED renormalization in Peskin's

    I think I have found a mistake/wrong formulation at Peskin’s, when he discusses the renormalization of QED. In particular, he defines the 1PI of the electron’s self-energy on page 331 as: –i\Sigma( \displaystyle{\not}p ) and the corresponding counterterm on page 332 as: i(...
  42. andrex904

    Euclidean Feynman rules for QED

    Hi, i have some trouble with feynman rules after wick's rotation. I don't understand how the propagators transform. In particular if i take the photon's propagator in minkowskian coordinates i don't understand where the factor "-i" goes after the transformation. ## \frac{-i\eta_{\mu\nu}}{p^2}...
  43. KiNGGeexD

    Exploring the Nature of Light: Is Light Made of Particles or Waves?

    I have been reading Richard Feynmans Quantum Electrodyamics and quite early in the first chapter he asserts that Photons are particles. His reasoning that as you decrease the intensity of light incident on a photomultiplier the clicks which the multiplier make become less frequent but equally...
  44. P

    How Does QED Explain the Electromagnetic Force

    From a conceptual point of view, how is it that Quantum Electrodynamics explains the interaction between two charged particles, (let's say two electrons)? I've heard people say that the electrons exchange photons, but how is it that an electron would know when to send off a photon and that it...
  45. JG1009

    Zero-point energy and QED vacuum state

    I'm relatively new to QFT and was wondering how the QED vacuum has a dormant zero average-field condition if there is a zero-point energy of the field as well? How is there a zero average and a zero point energy?
  46. Z

    What is the derivation for the vacuum density of states in the Purcell effect?

    The Purcell effect is when an atom placed inside a high finesse cavity with a very small mode volume gets an increase in the spontaneous emission rate. I've tried to find correct explanation for this effect, but it seems hard to find, except that it comes from an increase in the vacuum density...
  47. M

    Definition of charge of a free electron

    Hi there, I have a question about the definition of a charge of a free electron. Let's suppose that QED is the true theory of the interactions of charged particles. Presumably the charge on an (effectively) free electron, then, is the charge on an electron in which the electromagnetic...
  48. Naeem Anwar

    Can Muonium Be Produced and Observed in Electron-Positron Collider Experiments?

    Studying the production of the bound state of ##μ^+μ^- ## in the scattering experiments of ##e^+e^-## at electron positron colliders ##e^+e^-\toμ^+μ^- ## via ##e^+e^-\to 2γ\toμ^+μ^- ##. This bound state further can decay into ##e^+e^-## or ##2γ##. I am confused about the vertex ## 2γ\toμ^+μ^-...
  49. E

    Photon wave function and "QED: the strange theory of light "

    I read that a wave function of the photon does not exist in coordinate space. But, when I read Feynman's "QED: the strange theory of light and Matter", for instance, when the photons travel through the glass, it seems like wave functions of photons. How it is with this?
  50. itssilva

    When are quantum corrections significant for EM?

    As a rule of thumb, we might say that quantum theory becomes essential when we're analyzing systems at small distances (of the order of atomic sizes or less) and few enough particles (suppose particle number is conserved, as in QM); however, the world as a whole is quantum, and even a system...
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