Qed Definition and 340 Threads

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...

As I understand it, the fundamental unit of interaction in QED is a term with a pair of (spinor) electron factors and a (vector) photon factor, represented in a Feynman diagram as two (anti-)electron lines and one photon line meeting at a vertex. I get the case where the photon and electron...

To my understanding, QED is about electromagnetic interaction, and in a solid system, the interaction is only electromagnetic So why there are stills lots of research in solid state physics ? Is there something not explained by QED ?

First I would like to say that I'm sorry if this question has been asked before- I'm new here. I was reading QED by Richard Feynman, and he mentioned that any given antiparticle is just it's regular particle counterpart moving backwards in time. How is this possible? I thought that it was only...

Hi, In QED it is stated that an EM field can be written as a sum of quantized oscillators (the photons). In "classical" Electrodynamics, it can also be shown that the EM field decomposes into normal modes. But both the quantized oscillators (in the Heisenberg picture) and the classical normal...

The measured energy density of the vacuum has a disturbing discrepance with the one theorized by imposig Poincare invariance in QFT, usually referred to as the "vacuum catastrophe". On the other hand the Heisenberg indeterminacy principle leads to a nonzero vacuum expectation value for the...

In the process $$e^+e^- \rightarrow \gamma \gamma$$ for which the amplitude can be written as: $$M= \epsilon^*_{1\nu}\epsilon^*_{2\mu}(A^{\mu\nu}+\tilde{A}^{\mu\nu})$$, where $$\epsilon_i$$ is the polarization vector of a photon. How can one find the tensors $$A^{\mu\nu}$$ and...

My book says that in this case $$e^+e^- \rightarrow \gamma \gamma $$ gauge invariance requires that $$k_{1\nu}(A^{\mu\nu} + \tilde{A}^{\mu\nu})=0=k_{2\mu}(A^{\mu\nu} + \tilde{A}^{\mu\nu})$$ Please see attachment. My question is how does this statement hold?

Homework Statement Write the momentum space integral representation for the following diagram in QED Homework EquationsThe Attempt at a Solution [/B]...

1. The problem statement, all variables and given/known I have to prove an equation for the differential cross section of compton scattering of an electron and a photon (electron (P) + photon(K) ⇒ electron(P') + photon (K') ) where P and so on are the inital and final four momenta. Given is...

Homework Statement First of all, Happy New Year! I have to solve the following exercise (xmas gift :P) and some things are a bit vague..Here is it: For the ee--->ee scattering process, draw all amputated and connected graphs that would contribute. The hint is that one should find 10 different...

Feynman's first topic in his second lecture on QED is the nature by which light reflects off of a mirror. We work in ##\mathbb{R}^2##. Suppose we have a light source sitting at ##(-1,1)## and a photomultiplier sitting at ##(1,1)##, with a mirror along the x-axis. We also place a block between...

Homework Statement I am trying to calculate the following quantity: $$<0|T\{\phi^\dagger(x_1) \phi(x_2) exp[i\int{L_1(x)dx}]\}|0>$$ where: $$ L_1(x) = -ieA_{\mu}[\phi^* (\partial_\mu \phi ) - (\partial_\mu \phi^*)\phi] $$[/B] I am trying to find an expression including the propagators...

Hi all. Consider a UV cutoff regulator ##\Lambda## with an effective QED lagrangian ##\mathcal{L}_{\Lambda} = \bar{\psi}_{\Lambda}(i\not \partial - m_{\Lambda})\psi_{\Lambda} - \frac{1}{4}(F^{\mu\nu}_{\Lambda})^2 - e_{\Lambda}\bar{\psi}_{\Lambda}\not A_{\Lambda}\psi_{\Lambda}##. One can of...

What is the difference between Yang-Mills and QED theories? Yukawa and QCD? specially in terms of the lagrangians. I really want to get into this subject with a previously first sight.

See the passage attached below. Consider the 1-loop vertex correction (c.f. p.2 of http://bolvan.ph.utexas.edu/~vadim/classes/2012f/vertex.pdf) and vacuum polarization diagrams in QED. A very simple UV regulator that makes the integrals for the amplitude very simple is the prescription that we...

Say I have a large spacetime lattice set up on a supercomputer where I calculate the scattering cross section of two spinless electrons of equal and opposite momentum via lattice QED. To get the right results we must add the amplitudes for every possible "path" the field can evolve from initial...

Is it possible to derive the speed of light from quantum electrodynamics (like it can be done from Maxwell's equations) or is the fact that the speed of light in vacuum is constant and has a certain value an assumption in the theory? My understanding is that QED assign a mass zero to photons and...

I have personally been deeply fascinated by the processes and principles that underlie the QED and have done some recent research myself. However, recently my teacher posted me a question that stunned me. He went,"Why do like charges repel and unlike attract?" In QED I understand that this is...

Homework Statement To calculate a certain Dirac bracket I need to calculate this Poisson bracket (Weinberg QTF 1 p.349 first eq.) $$[F,\Pi_i(\mathbf{z})]_P$$ where F is any functional of matter fields and their conjugates and pi is the conjugate to the vector potential. It should be zero...

I looked QED theory but I don't understand anything can someone explain me What is a QED theory ?

Hello, I read the Feynman's QED book, where I learned that a photon has a intrinsic property called frequency. This property affect, for example, the interference profile when we have a lot of photon together. Ok. Now, thinking on an conventional antenna. When we have a 100kHz signal on...

Hi everyone. I have a question about the calculation of Rutherford cross section in the context of QED. I know how to compute it using the usual four potential: $$ A_\mu(q)=(\frac{e}{q^2},0,0,0) $$ and taking the matrix element to be: $$ \mathcal{M}=\bar u_{s'}(p')\gamma_\mu...

Does the measurement problem ("wave function collapse") or something similar somehow manifest itself in QED and other quantum field theories? Is it somehow built-in into the propagators etc. "away from sight"? If so, how does it affect the theories and is this a problem, which needs to be...

Hello everyone, I was hoping to ask you for a good textbook reference on the subject of Non-Relativistic QED. I am attending a graduate course which is now covering this topic, but the adopted textbook (G. Baym - Lectures in Quantum Mechanics) is just extremely unsatisfactory! I also...

I have read some fairly vague descriptions of charge that say it can be looked at as the amplitude for a particle to exchange a photon. For example, when two electrons repel, it is because a photon is emitted from one too the other, which would change the direction of both equally and...

I was curious about two things. I just started reading Feynman's QED book this week. He starts by using an analogy of a clock spinning and that determines the direction (which I think is being analogous to the amplitude of the frequency?) My first question is, as a layperson, what causes...

I understand that the QED Lagrangian is invariant under the CPT transformation. Is it also invariant under C, P or T separately?

In scalar QED, there are two noether currents ##J_{global}## and ##J_{local}##corresponding to the global and local gauge transformations respectively. In QED, the two currents are exactly the same. But in scalar QED, they are totally different. $$J_{global}^\mu=i e (\phi^\dagger...

Can the classical electromagnetic force between two arbitrarily moving electrons, described by the Lienard-Wiechert EM fields and the Lorentz force law, be derived from a "simple" tree-level QED calculation? Can anyone point to a paper where this has been done?

I might be asking a question who's answer is way beyond my level but ill give it a shot. In QED, an electric or magnetic field is described in terms of exchanged photons, right? But if a photon itself is made of electric and magnetic fields... a paradox to my ill-informed brain. :mad:

I read recently in my old ugrad 2012 solid state physics textbook, in its discussion of the attractive force between electrons in cooper pairs (in superconductors), that the attractive force between cooper pairs is explained mostly by the exchange of phonons (a quasiparticle associated with the...

About photons, in qed, I have not understood if there is difference between "the excitation modes of the EM field" or "the energy difference between these excitation modes". Sorry if I make a mess with these concepts. -- lightarrow

Hi all, I've been playing around with spin 1/2 Lagrangians, and found the very interesting Fierz identities. In particular for the S x S product, (\bar{\chi}\psi)(\bar{\psi}\chi)=\frac{1}{4}(\bar{\chi} \chi)(\bar{\psi} \psi)+\frac{1}{4}(\bar{\chi}\gamma^{\mu}\chi)(\bar{\psi}\gamma_{\mu}...

This is perhaps a stupid question, but are the field equations, for example, for QED useful for anything? By field equations (equations of motion) I mean the equations, which are obtained by inserting the Lagrangian into the Euler-Lagrange equation. In the case of QED, one gets a Dirac-like...

Homework Statement Problem 7.15 from Aitchison and Hey, Volume I, 3rd Edition. Verify the forum (7.139) of the interaction Hamiltonian \mathcal{H_{S}^{'}}, in charged spin-0 electrodynamics. Equation 7.139 is \mathcal{H_{S}^{'}}= - \mathcal{L}_{int} - q^2 (A^0)^2 \phi^{\dagger} \phi...

Binding energy equations can be derived for the main spherical orbitals of hydrogen. If derived correctly, the energies computed by the equations are negative in value, for example, the well known approximate -13.6 eV for hydrogen's spherical ground state. In QED, corrections are added to the...

I'm not sure whether to post this here or not, but since the book is about quantum i figured it was as good a place as any. I don't know too much about this, so I am just trying to understand exactly what the heck Feynman was talking about. I understood most of it but I have a few questions I...

Hi pf, i have been wondering what differentiates QED Vacuum from QCD Vacuum? How would u explain its implications? I mean, how can u define pure vacuum in 2 ways?

I've been reading a bit around Quantum Electrodynamics (QED) which says that when you shine monochromatic light onto a thin film, the fact of whether it produces a dark or bright band is purely a probability function. It seems to say you use Feynman arrows, one for the first surface and one for...

Let say I want to study electron-proton scattering (without considering proton's quarks, i.e. no QCD), which is the Lagrangian? I've seen two different answers to this question :confused: First one: L=\bar{ψ}e(i∂-me)ψe+\bar{ψ}p(i∂-mp)ψp-\frac{1}{4}Fμ\nuFμ\nu-e\bar{ψ}eγμψeAμ+e\bar{ψ}pγμψpAμ...

For a phi-four theory the LSZ reduction formula, as stated in peskin and schroeder essentially boils down to $$\langle \vec{p'_1},\vec{p'_2}, \ldots ,\vec{p'_m}| S| \vec{p_1}, \vec{p_2}, \ldots, \vec{p_n}\rangle = Z^{(n+m)/2} \mathcal{M}_{\text{on shell}}$$ where we have n incoming and m...

(PS: this post was also posted at the quantum mechanics/field theory forum, but I did not get any replies there) Often when one speaks about the effective QED coupling one defines it as $$e = \frac{Z_2 Z_3^{1/2}}{Z_1} e_0 \ \ \ \ (*)$$ when ##Z_1 = Z_2## by the Ward identity this...

Often when one speaks about the effective QED coupling one defines it as $$e = \frac{Z_2 Z_3^{1/2}}{Z_1} e_0 \ \ \ \ (*)$$ when ##Z_1 = Z_2## by the Ward identity this turns out to be ##Z_3^{1/2}e_0## and some authors just define the coupling to be this right away. So why do some make a...

It is often stated that quantum mechanics is able to explain the stability of atoms. I think most explanations are cheating b/c the compare apples and oranges. There are two reasons in classical theory which indicate that atoms should be unstable: A) there is no minimum for the orbit; the...

Three related questions on which I could use some help: a] In another thread, Bill_k posted something like: "Electrons, being charged, could exchange energy by exchanging a virtual photon." ok, yet I thought electrons usually interacted via the EM field ['real' photons]... So how...

Any Conceptual Underpinning for Partial Reflection of Light (QED)? I recently partially read Feynman's QED. At one point, he says "The situation today is, we haven't got a good model to explain partial reflection by two surfaces;..." (page 24--can visit Amazon "Look Inside" to read) My...

The results of QED calcualtions are in the form of an asymptotic series in the fine structure constant. This means that if one adds too many terms, the result diverges. So what is the meaning of "18 digits accuracy" of certain QED calculations, if they can be spoiled by adding MORE loops to the...

Hi, I am interested on learning QED on my own. I have a good background in QM. What are the best textbook that would you recommend? C

Using classical (non-quantum, that is) electrodynamics, one can predict that a charged particle accelerated by a nonuniform electric field will radiate. This can be modeled (although not without problems, such as unphysical runaway solutions) by the Abraham–Lorentz force, which is a dissipative...