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. P

    Exploring Hot Research in Theoretical Quantum Optics/Cavity QED

    OK guys Can anyone tell me some hot research problems in theoretical quantum optics/cavity QED right now ?
  2. B

    New Feynman Rules in QED from Counterterms

    Hi, I have been looking at the renormalisation of QED and been using Peskin & Schroeder. I understand (I think) what is going on, but I am slightly confused over 2 issues: 1. In the new feynman rules from the counterterms, the feynman diagrams all have a small circle with a cross in...
  3. C

    Gauge invariance of QED action

    Hello, I don't understand two steps in solution to the problem: I. Homework Statement Show that QED action is invariant under gauge transformation. II. Relevant equations QED action: S= \int{d^{4} x \left[\overline{\Psi}\left(i\gamma^{\mu} D_{\mu} -m \right)\Psi...
  4. G

    QED: Electric Field - Can We Observe Photons?

    hi i heard some things about quantum electrodynamics and one thing was that they gave up the concept of actual fields and said that the electromagnetic information from one particle to the other is transported by photons. Now my question is: Can one observe these photons or is it just a...
  5. N

    SR and QED: How Particles Interact with Time Dilation

    SR and QED: how? how the particle moving at a speed near to that of light interact to it's surrounding,considering the very fact of it's time dilation? what i mean to ask probably is that will it b possible for this particle to create another one from it's energy and still manage to throw...
  6. P

    QED Renormalization: Feynman Diagrams with Counterterms on External Legs

    I have a question concerning renormalization of QED. I don't know if Feynman diagrams with counterterms on ecternal legs are allowed. Normally to find S matrix amputated green function is necesary and to find it one don't take into account all propagators on external legs - they are canceled...
  7. U

    Searching Book about QED; compton scattering of polarized photons

    does anybody know a book, where the compton-scattering with respect to the initial photon-polarisation is calculated? (beside landau-lifschitz-berestetzki)
  8. A

    Should QM or QED be used?

    Supposed external energy is added to the system such that the electron is rise to the next orbital releasing photon. Can QM still be used to model it or does one need Quantum Field Theory? How about additional external energy that increase say the molecular rotational or translational speed, is...
  9. S

    QED: the strange theory of light and matter. Light is made of particles ?

    QED: the strange theory of light and matter. " Light is made of particles " ? Over Christmas I ordered the book: QED: The strange theory of light and matter by Richard P feynamn 2006 edition. Note: This book was written in 1985. "We know that light is made of particles because we can't...
  10. S

    Runaway Solution in QED: A Discussion with Lorenzo

    HI everybody. I'm here with a question that I would like to discuss with you. As you all probably know better than I do, in classical electromagnetism there exists the so called "Runaway solution", i.e. the auto-acceleration of a charged particle. It is not clear to me how this problem can...
  11. E

    Variable speed of light in QED

    In QED calculations are made with variable c. But Duff says that c does not exists, because it is only a transformation factor. So if we put c=1 how QED is changed at small distances?
  12. T

    QED: Vertex renormalisation; Please have Peskin and Schroeder book handy

    Hi guys, I have a question regarding a point in the QFT book of peskin and schroeder. Iv been working through renormalisation in QED, The 2-point corrections are simple enough, however in this book the 3-point function is a little more involved and I have some issues. Essentially id to...
  13. H

    Can I Calculate Traces and Cross Sections in QED Using Gamma Matrices?

    hi how to calculate the traces of product of Dirac matrices in QED. i want caculate crossection of process scattering in QED. a program to calculate it
  14. S

    Dyson-Wick formalism in second-order QED - trouble with derivation

    I have in front of me Quantum Field Theory, Mandl & Shaw. Chapter 7 deals with the theoretical basis of Feynman Diagrams using the Dyson-Wick formalism. The chapter begins with applying Wick's Theorem to produce six S-Matrix components with a variety of no-equal-time contractions. It then...
  15. J

    QED: Lagrangian, and Action principle

    I'm probably making a mistake, but looking at the free field lagrangian for QED \mathcal{L} \propto (-F^{\mu\nu}F_{\mu\nu}) \propto (\mathbf{E}^2 - \mathbf{B}^2) it appears to me that the action is not bounded from above, nor from below. Does that mean the equations of motion we obtain by...
  16. P

    Why is QED being blamed for the discrepancy in proton radius measurements?

    Radius of proton due to QED?? In one account of the new findings of smaller proton radius, the claim was repeated several times that the radius of a proton was predicted by QED, in conflict with the new measurement. Why QED? I would think, within a proton, QCD is orders of magnitude more...
  17. E

    Attraction between two charges in QED

    Attraction between nuclei and electron in QED is the most simply described with Feynman's diagram and calculation, when an electron fastly flow close to a nuclei. (?? scattering) 1. But how, in QED, to describe attraction between almost standstill electron and nuclei. In classical physics...
  18. B

    QED - correctly expressing a sum over spin-polarizations

    Homework Statement You have spinors that you designed in a computer algebra system. You’re trying to verify Casimir’s trace-trick for a single anti-spinor fermion-line...which you don’t understand all that well. You successfully verify the trace to be equal to: {\mathop{\rm Tr}\nolimits}...
  19. snoopies622

    Basic QED Question: Are a & b Dirac Matrices?

    In the Eric Weissteins's World of Physics entry on "Quantum Electrodynamics" he gives one of the governing equations as [ c \mathbf{a} ( -i \hbar \nabla - \frac {e}{c} \bf {A} ) + bmc^2 ] \psi = ( i \hbar \frac {\partial}{\partial t} - e \phi ) \psi but doesn't define a or b...
  20. M

    QED Proton Size: 4% Smaller Than Thought?

    http://www.nature.com/news/2010/100707/full/news.2010.337.html Apparently the proton is about 4% smaller than thought, which is somewhat challenging for QED. It could be new effects or experimental error of some sort. Any such guessing is speculative at this time, but what would you think?
  21. D

    QED Intro Books: Finding Exceptional Recommendations

    Despite the fact that QED is so popular, i have a hard time finding a good introductory book about it [ except feynman's of course;-) ]. Do you have any good recomendations?
  22. P

    How Does the Feynman Propagator Change When Multiple Photons Are Involved?

    In QED, Feynman talks about P(A to B), the probability that a photon starting at a particular point in space will travel to a different point in space (I've since learned that P(A to B) is actually the Feynman propagator, unless I was midled). What I'm wondering is - what happens if there are...
  23. Spinnor

    QED in 3D -> QED in 1D, what changes?

    QED in 3D --> QED in 1D, what changes? Can we have massive charged and massless fields which interact in one space dimension? What might the interaction term look like? Thanks for any help!
  24. P

    Interaction in QED: Electron/Proton Attraction & Repulsion

    Why according to QED two electrons repel each oteher while electron and proton attract. What are Feynman diagrams for these proceses. Thanks for help.
  25. M

    Why does QED work so well? - a question on EFTs

    Hi folks: I have a question about effective field theories. The magnetic moment of the electron calculated in renormalized QED gets it right to 12 decimal places. That suggests that QED gets things right all the way down to the order of 10^-12eV - that is, all the way to the TeV scale...
  26. R

    QED 1-Loop Renormalization Group Equation

    Please provide me the one loop renormalization group equation for massive QED. I would like to see how the beta functions for the coupling and the mass look like for QED. Thanks.
  27. T

    Are water waves indicative of QED?

    Maybe not. I'm just thinking, idly... If, under controlled conditions, you blow a steady gentle breeze across the surface of an otherwise perfectly calm body of water, I figure you'd make a ripple of (regular) waves. A steady-state analog input, yet quantized rhythmic output. Do you...
  28. S

    Confused in Chapter 3 of RF's QED

    Hello, I'm reading through Richard Feynman's QED (for the first time) and I'm at Chapter 3. At the bottom of page 102 (2006 edition), he says, "So the timer we used for the 'imaginary stopwatch' was the monochromatic source:--in reality, the angle of the amplitude for a given path depends on...
  29. J

    Feynman QED, Permanent Magnets, and Virtual Photons

    I'm not making any statements here, only asking questions. When I take two permanent magnets and I feel the repulsive force between them, Feynman QED says it is the momentum of the virtual photons "pushing" the magnets apart? Experiments have been done, forcing some of these virtual...
  30. U

    QED: Electron Spin & Magnetic Moment - Questions & Answers

    Hi, I've been doing Quantum Electrodynamics,and I've just finished the third chapter of QED-the strange theory of light and matter,by Richard Feynman. I have just three questions in regard to this... 1. what does Feynman mean when he says that any electron or photon can be polarized...
  31. S

    PhD in Experimental QED: AMO vs Superconductivity?

    Hi all I am looking for a phd in the field of cavity qed - quantum information (experimental). I have read some recent publications in the field from groups working with superconducting qubits and the superconductive approach seems to be more efficient/promising than the optical one. The...
  32. O

    How Do You Derive Feynman Rules for Scalar QED Using Functional Methods?

    Homework Statement Hello all, thanks for reading... I was assigned to calculate feynman rules for the scalar QED theory via functional methods. The fields are a scalar complex field \phi, \phi^* and gauge field A_\mu, and the lagrangean is \mathfrak{L} = (D_\mu \phi)^* (D^\mu \phi) - m^2...
  33. N

    QED photon vs photon as particle

    Quasi-classically, a photon is often considered as a particle with some momentum traveling across the space - for example, when describing experimental setups like Mach-Zender interferometer we often talk as if the photon was actually a particle moving along some possible paths, i.e. we treat it...
  34. maverick_starstrider

    Diffraction Grating Video (QED)

    I was just looking through Feynman's QED book (i'm surprised I'd never read it before) and I was wondering if anyone knew of any videos out there that actually experimentally demonstrate how light can be reflected by a diffraction grating even when the incident laser isn't actually POINTING at...
  35. Bob_for_short

    How does the dressed electron look like in QED?

    Is there any solution ψ to understand how the real electron looks like after renormalization (dressing)?
  36. maverick_starstrider

    QM to QED: How Far Should I Self Study?

    Just out of curiosity, the last course I took (and the last one I'm probably ever going to take along that path) in QM ended with dirac equation, second quantization and the quantizing of the electric field. How far do I have to go (if I wanted to self study) to get to QED?
  37. R

    Path Integrals in QED: Solve by Substituting Classical Solution?

    When calculating a path integral, if the Lagrangian is quadratic in the field, then you can perform the path integral by just substituting in the classical solution for the field. So if you have free-field Lagrangians for electrons and photons, and add the standard QED interaction term - which...
  38. T

    What is an Eikonal Factor in QED and QCD?

    In theoretical physics talks involving QED and QCD, people speak of the 'Eikonal Factor?' Generically, what's an Eikonal factor? Thanks
  39. R

    Beta Function for QED: 20/9 or 8/3?

    The beta function for QED is given by: \beta=\frac{e^3}{16 \pi^2}*\frac{4}{3}*(Q_i)^2 where (Q_i)^2 represents the sum of the squares of the charges of all Dirac fields. For one generation, for the charge squared you have (2/3)^2 for the up quark, (-1/3)^2 for the down quark, but this is all...
  40. B

    From QED to Maxwell: Exploring Lagrangian Transformations

    I have some confusion as to how to get from the QED Lagrangian to ordinary electrodynamics. Beginning with the QED Lagrangian (density): \mathcal L = \bar{\psi} (i\gamma^{\mu} D_{\mu} - m) \psi - \frac14 F_{\mu\nu} F^{\mu\nu} I expand the covariant derivative D = \partial - iqA[/tex] to...
  41. C

    QED Photon Field: Exploring 4 Potential vs E & B Fields

    This may be a simple question, but I feel as though I don't clearly understand this fully. How come we can take the photon field to be represented by the four potential instead of the E and B fields? Is it equivalent and more convient to do? Also by using the 4 potential do you atomatically have...
  42. Orion1

    Can We Measure the Weak Nuclear Fine Structure Constant via Beta Decay?

    Penning trap upper limit for the electron particle radius: r_e \leq 10^{-22} \; \text{m} QED Compton wavelength electron radius: \boxed{r_e = \alpha_e \overline{\lambda}_c} QED Compton electron radius: \boxed{r_e = \frac{\hbar \alpha_e}{m_e c}} QED electron fine structure constant: \alpha_e =...
  43. X

    QFT, QED for condensed matter physics

    I don't have the time now to study QFT in depth but I need some basic knowledge about QED and quantum forces (like casimir). I want to study the role of casimir force to solid state devices (photonic and plasmonic devices). Which part of QFT is needed to study for now and which books are...
  44. R

    QED Vertex: Examining the Unexplainable G-Factor

    The QED vertex has two electrons lines joining with a photon line. However, 4-momentum conservation does not allow this process to occur. So why is this process studied? Does it really mean anything to calculate the g-factor of an electron for a process that cannot happen? Only the vertex...
  45. T

    Reduce QED to QM: A Restatement Based on Creation/Annihilation Operators

    I've been spending some time to think about a really conceptually and mathematically clean way of showing how QED reduces to ordinary QM in low energy limit. More specifically, I would like to consider a restatement of multi-particle QM (of electrons) based on creation and annihilation...
  46. E

    QED: reflection of light in glass

    Hello, This is my first post to physicsforums. I hope I've picked the right forum for this question, please correct me if not :blushing: To answer my question you'll have to have the book "QED, the strange theory of light and matter" in front of your eyes. I'm now reading Feynman's QED...
  47. malawi_glenn

    Why Is the Middle Term in the QED Field Strength Tensor Commutator Non-Zero?

    Hi, I was reading earlier today in Peskin about QED field strenght tensor: equation 15.15 and 15.16 [D_\mu, D_\nu ] \psi= [\partial _\mu , \partial _\nu ] \psi + ([\partial _\mu, A_\nu] - [\partial _\nu, A_\mu]) \psi + [A_\mu,A_\nu] \psi Where A is the gauge field... That part, I...
  48. G

    QED Loop Calculations: Questions and Solutions

    Hi, I'm currently doing some one-loop calculations for some bad-*** decays with vector final states, which yield me some tri-tensor threepoint functions. I know the Passarino-Veltmann reduction scheme, and also got the general reduction formulas from Denner-Dietmeier to simplify expressions...
  49. K

    Is the Correct Delta Function for a QED Vertex \(\delta(p+q-k)\)?

    say I have a QED vertex with an anti-fermion (p) going in and a fermion (q) and photon(k) coming out. Since the momentum flow is supposed to be along the direction of particle number flow, wouldn't this mean that the delta function at that vertex would be \delta(-p-q-k) ? that doesn't seem to...
  50. S

    Feyman Diagram - Represent QED model(s)?

    Hello - Trying the General Physics area. Question: Please explain how a Feyman Diagram represents or is conceived from a QED Model(s) i.e. IF it is the source of these diagrams? What is meant to be represented in them? Thank you
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