Qed Definition and 324 Threads

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

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...

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...

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...

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...

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}...

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...

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?

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?

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...

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!

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

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...

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.

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...

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...

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...

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...

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...

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...

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

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?

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...

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

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...

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...

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...

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 =...

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...

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...

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...

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...

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...

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...

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

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

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...

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...

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 .

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...

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...

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 !

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

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

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...

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...

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?

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...

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...

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...