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. S

    Feyman Diagram - Represent QED model(s)?

    Hi - could someone briefly explain a Feyman Diagram (example?). Does this replace one or more complex QED model(s)? Thank you.
  2. P

    QED Lagrangian lead to self-interaction?

    Hi, I'm not very good at QFT, but starting from Dirac's QED Lagrangian (look for example at: http://en.wikipedia.org/wiki/Quantum_electrodynamics" ) L=\bar{\psi}\left(\gamma^{\mu}\left(i\hbar\frac{\partial}{\partial\mu}-q(A^{ext}_{\mu}+A_{\mu})\right)+ mc\gamma^0...
  3. S

    Feynman's QED Lectures: Questions about partial reflection

    In Feynman's lectures he describes partial reflection as occurring because the "little arrows" cancel out when the glass is certain thickness. I realize that there isn't an actual model of how QED works but what exactly does he mean by "cancel out"? Also when do you shrink and turn the little...
  4. M

    Explore New Horizons: Good Books to Study in QED for Beginners

    Suggest some good books in QED for a beginner ? I have holidays going on after my exams so thought if i could study something new on my own (i don't know how much will i be able to grasp ) although its not in my course .
  5. Q

    Two questions concerning scattering in QED

    1. Are there possible (4th order)process of "decay of photon" \gamma\rightarrow 3 \gamma with one-photon initial state and 3-photon final state? Intuition tells me there are no such processes. But because of crossing symmetry of scattering amplitude one can argue that such processes should be...
  6. M

    Have you heard about QED coherence in matter?

    Hi, I would like to know what do you think about the Giuliano Preparata theory on QED coherence in matter. He studied the problem of the ground state in condensed matter under the hypotesis that the standard perturbative vacuum is unstable with respect to a new coherent vacuum, whose...
  7. K

    Exercises and problems on QED and QFT ?

    I'm trying to study QFT using P&S book. Now I'm on the chapter 13 but I feel that I'm rather bad at solving practical problems on QFT and QED. Could anyone suggest a good source of problems (preferably with solutions) on QFT and QED (near the level of P&S ch. 1-13) to practise ? Thanks in advance !
  8. R

    QED Path Integrals: Weinberg QFT Book Explained

    Does anyone understand how weinberg in his QFT book does QED via path integrals: specifically, how does he integrate out the adjoint dirac spinor?
  9. O

    Mathematica QED: Uncovering the Meaning Behind Math Proofs

    What does QED stand for behind every mathematical proof? i can't seem to find out what it stands for thanks.
  10. Q

    Why Does the First Order Term in QED Scattering Matrix Give No Contribution?

    Can some explain me why first order term in perturbation expansion of scattering matrix gives no contribution for every possible IN and OUT states? It is said that this is connected with the fact that condition of energy-momentum conservation cannot be satisfied for real photons and electrons...
  11. C

    QED Confinement: Will Charge be Confined if Alpha is Larger?

    What would happen if the fine structure constant alpha was larger? Will charge be confined, as colour is in QCD? A simple argument, based on non-relativistic quantum mechanics, is that the binding energy of an electron in a hidrogen atom is given by E= - 1/2 (alpha)² m_e c². If alpha=2...
  12. T

    Why Does the Measurement Problem Terrify Physicists?

    I recently watched a program called 'Atom' on bbc4 that said something along the lines of: "If you want to strike fear into the eyes a physicist; mention the measurement problem" - What is the measurement problem and why is it such a problem?
  13. O

    QED Calculations: Progress Made in Last 25-30 Years

    I'm not sure if this is the right forum for this topic, so apologies if I got it wrong. I've been reading the Feynman Lectures on Physics. In it, Feynman states that though Quantum Electrodynamics is highly successful, it is still extremely difficult to evaluate the equations to obtain a...
  14. S

    Comparing Coumlomb's Force Law & QED for Electromagnetic Forces

    In textbooks I’ve read that the Coulomb’s force law is accurate up to 10^-12 m. Below this limit the force between charged particles should rather be calculated according to QED. But I’ve found no answer what are in fact the differences… So, are the electromagnetic forces between charged...
  15. Q

    The role of wave function in QED

    I am just learning QED and could not understand the role of wave function. Is the basic equation in QED the Schrodinger Equation? Is the difference between Quantum mechanics and QED just they have different Hamiltonians. I have tried to read the original paper of Tomonaga in 1946 Progress of...
  16. S

    Quantum Mechanics: QM, QCD, QED Explained

    Hi, I was wondering, what does the realm of quantum mechanics cover? I mean, I always hear a talk about how it would be a holy grail of physics if we could somehow combine GR and QM. But what about QCD, QED, and other theories. Why aren't they mentioned? Is it because QM encompasses them?
  17. E

    QED Feynman (coupling number called j)

    On page 92 (Figure 58) Feynman states: "The amplitude for a coupling is a number that I will call j; it is about negative .1 for the electron (this number is sometimes called the "charge"). Charge?? I don't get it. Would appreciate an explanation?
  18. N

    Finding "R. Gastmans, Ubiquitous Photon: Helicity Method for QED and QCD"

    i am encoutering a problem i use helicity method to caculate six photon amplitude in scalar QED but i don't know explicit form of wave fuction QED . And i need a book with title " R. Gastmans, Ubiquitous photon: Helicity method for QED and QCD" where can i find it? thank you
  19. C

    QED Courses at the Graduate Level: Standard or Optional?

    As far as I know QED is taught at the graduate level. However, how many physics students end up taking courses on it? Is it standard to take courses on QED or is it only out of interest, or "need"?
  20. dextercioby

    Exploring the Unitarity Problem in Non-Renormalizable Theories

    Using this convention A \overleftrightarrow{\partial }_{\mu} B =:A \overrightarrow{\partial}_{\mu} B - A \overleftarrow{\partial}_{\mu} B one can write the QED Lagrangian density simply as \mathcal{L}_{QED} =\frac{i}{2} \bar{\Psi}_{\alpha} \left(\gamma^{\mu}\right)^{\alpha}{}_{\beta}...
  21. B

    QED vs Point Charge of the Electron

    If the electron is a point charge, then its' charge to mass ratio approaches infinity. How does the Standard Model (QED) deal with this?
  22. M

    Learn QCD & QED: What you need know

    wat do u need to learn to learn qcd and qed
  23. S

    QED & Static Electricity: Is There a Connection?

    so i was reading about QED, and it said that when 2 charged particles repelled each other, it was because of virtual photons. is it true then that static electricity is caused by virtual photons, or is it an entirely different force all toghether?
  24. N

    QED: science meets science fiction

    IF QED (quantum electrodynamics) is not a hoax, then matter truly does not exist and all we have is logic-math. The electron according to QED is an infinite set of interacting virtual particles described and understood only in terms of their feynman diagrams which are nothing more than integrals...
  25. E

    What is the connection between Unruh radiation and quantum electrodynamics?

    I am interested in answer in the following questions. Where in quantum electrodynamics Unruh radiation is hidden? How Feynmans graphs assign Unruh radiation? Of course, it is clear that acceleration in quantum electrodynamics is not uniform. But there are some photons that arise. Examples of...
  26. K

    QED: virtual photons -spin conservation?

    I'm talking of quantum electro dynamics where we try to explain the electromagnetic "force" with virtual photons. Now an electron can "send" a photon that has more energy then it self (this photon is virtual because it can only exist because of the HUP). But spin must always be conserved. But it...
  27. K

    QED : Momentum of the photon-field

    Greetings, I have to show that \vec{P}=\frac{1}{c^2} \int d^3 x N \left(\dot{A}^{\mu} \nabla A_{\mu}\right) is equivalent to \vec{P}= \sum_{\vec{k}} \hbar \vec{k} N(\vec{k}) N in the first expression denotes the normal product, and N(\vec{k}) is the usual number operator...
  28. V

    QED could precisely predict the magnetic moment of electron

    I have a little question about QED.. How come QED could precisely predict the magnetic moment of electron and muon, but can't predict the lifetime of neutron with the same accuracy?
  29. A

    Energy conservation in vertices in QED

    An elementary electron-> electron + gamma process can not occur by itself in the vacuum because if we assume the photon invariant mass zero and the electron invariant mass positive, energy and momentum can not be conserved at the same time. The delta E may be computed and from the energy time...
  30. R

    QED vs Three Roads to Quantum Gravity: Which is Better?

    is QED a good book? i was originally looking at three roads to quantum gravity, but on amazon.com someone said qed was better. are they both on the same topic? if anyone has any insight i would appreciate it.
  31. R

    Gravity's Effect on Electric Charges in a Gravitational Field

    There are two protons in the gravitational field of the Earth, one directly above the other.They experience the electric force through exchange of virtual photons.A virtual photon traveling vertically upwards from the lower proton towards the higher proton would be redshifted and so exert a...
  32. E

    Why is the Electromagnetic Field Tensor in the QED Lagrangian?

    The QED Lagrangian is given by \mathcal{L}_{\hbox{QED}} = \bar{\psi}(i\partial - m)\psi - \frac{1}{4}(F_{\mu\nu})^2 - e\bar{\psi}\gamma^\mu\psi A_\mu What is the purpose of the middle term. I know that it represents the energy of the E and B fields. However is that due to the external...
  33. S

    Trying to learn QED are my assumptions correct?

    When mass is constant, momentum is a function of velocity and velocity is a function of time. So would it be incorrect to say that the more precise your time measurement (or the smaller timeframe you are looking at) the more uncertainty in the position of the particle a la Heisenberg...
  34. S

    Question regarding Feymann Diagrams and other QED things

    I have been going through many books on QED, trying to get a general non-mathematical feel for it before I take a course in it and I have come across some questions that I have not been able to answer. Nearly all of the sources I have gone to refer to Feymann Diagrams as a easy way to depict...
  35. T

    Exploring QED: A DIY Diffraction Grating Experiment for a Physics Presentation

    Hello, I have purchased, read, and understood the concepts in Feynman's book QED: The Strange Theory of Light and Matter. I would like to reproduce the diffraction grating experiment (on pg. 50 for those who have it) for a physics presentation at school in order to prove that light doesn't...
  36. A

    Why light diffracts and was directed to QED by Feynman

    A while ago I made a post asking why light diffracts and was directed to QED by Feynman. I have now read the book (or the relevant part at least) and it explains diffraction by the number of possible routes of photons through an aperture as being reduced when the is gap made smaller, and...
  37. M

    Can Feynman's QED Explain Partial Reflection and Light Communication Speeds?

    Hope this is the right forum. Rather daunting task to pick the right forum when you don't understand what differentiates each topic... Anyhow, I've been reading Feynman "QED The Strange Theory of Light and Matter" (lectures). Actually, I've read it about 3 times, cover to cover. I get some of...
  38. dextercioby

    Why can people learn about QED and QCD and not about: QGD

    Why can people learn about QED and QCD and not about:QGD(i.e.Quantum gravitodynamics)?What are then the difficulties (both physically and mathematically speaking)that prevented theoretical physicists to develop such a theory??
  39. A

    QED question to field experts (like Tom, etc)

    Kinda continuing discussing of how classic electrodynamics follows from quantum one. I was always wondering myself what is more fundamental of the "trinity" (=charge, field, virtual carrier)? My feeling is that electric charge is the one which is "responsible" for virtual photons (bunch...
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