# Questions tagged [quantum-electrodynamics]

Quantum electrodynamics (QED) is the quantum field theory believed to describe electromagnetic interaction. It is the simplest example of a quantum gauge theory, where the gauge group is abelian, U(1).

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### Quantum mechanical explanation of electrostatic force [duplicate]

Feynman diagram explains repulsion of electrons by means of photon exchange. Similarly can we explain repulsion of protons and attractions between protons and electrons?
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### A question on Feynman's book “QED- the strange theory of light and matter” - on the junction number $j$

On page 91 in the Penguin edition, in the chapter "Electrons and Their Interactions", Feynman writes: There is no complicated formula for an electron to emit or absorb a photon: it doesn't depend ...
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### In QED why is the electron a point particle? [duplicate]

I read Feynman's book but this still unclear to me.
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### What does it mean to measure a photon?

In a lab at the university where I'm studying, there's a detector of gamma rays. We bring a radioactive caesium source near the detector, and the detector clicks now and then. The closer the source is ...
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### Frame-independent probability amplitude for Compton scattering in scalar QED

In scalar QED, the matrix element for compton scattering is the sum of three diagrams: the s-channel, t-channel and the seagull diagram. After simplifcation, making use of the orthogonality between ...
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### Why do the Euler-Mascheroni constant $\gamma$ and $\ln 4\pi$ not show up in observables (renormalisation of electric charge)?

The one-loop contribution of the vacuum polarisation of the photon after using dimreg is given by $$\Pi_2^{\mu\nu}= e^2 J(q) \left(\eta^{\mu\nu} - \frac{q^\mu q^\nu}{q^2}\right),$$ with the metric ...
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Here notation used functional matrix notation. The spacetime variables $x_1,x_2,...$ will be denoted by $1,2,...$. For example $\int dx_1$ will be denoted by $\int_1$ and the fermion propagator $S(... 1answer 65 views ### Are all macro reflections caused by electrons? In everyday life, almost everything is at least somewhat reflective. Metals are the best (or mirrors, which are covered with metals), next go plastics per say, then glass, painted/varnished wood, and ... 0answers 18 views ### Vacuum Birefringence with alternating EM waves I read  that in the presence of strong static electric or magnetic fields in vacuum the refractive index changes in such a way that there is a difference of refractive index between parallel and ... 0answers 43 views ### What are some good textbooks on learning Quantum Field Theory? [duplicate] In particular i am interested in Quantum Electrodynamics. It’s been a bit more difficult to find information on quantum field theory rather than ordinary quantum mechanics. Textbooks like Griffiths ... 2answers 48 views ### Does annihilation differs from a pair production from two interacting particles? By reading this: "Can two photons annihilate?" and answers to that question, I have came to strange question - Does annihilation in principle is different than a pair production process ? My intuition ... 0answers 19 views ### Approximating the wave functions of multiple electrically charged elementary particles as a function of time and position As I understand it the wave function of a free particle in natural units is defined by the equation $$\Psi(\vec{r},t)=\left(\frac{a}{a+it/m}\right)^\frac{3}{2}e^{-\frac{{\vec{r}}^2}{2(a+it/m)}}$$ with ... 0answers 63 views ### QED electron self energy in effective action The electron self energy at one loop is given by the one particle irreducible graph I know how to calculate it using the Feynman rules but I was wondering how this diagram appears in the QED ... 2answers 65 views ### Electron muon scattering unpolarised cross-section calculation with traces Following my class on RQM, we wanted to evaluate the unpolarised cross section for the following process $$e^+ e^- \rightarrow \mu^+ \mu^-$$ In doing so, whenever summing over spinor indices, a ... 2answers 181 views ### Why does a photon have spin 1? Are we taking the photon spin to be one to describe electromagnetic force or there is any equation (is it relativistic Schrodinger or Dirac equation?) with a solution that tells us that its value is ... 0answers 57 views ### Feynman self-energy diagrams In the Feynman picture, I don't understand how virtual photons in the self-energy diagram for a rest-frame electron can have energies that exceed$2m_e$. Aren't negative energy states of the electron ... 0answers 19 views ### Is correlation in electronic structure theory ‘a bin’ that accounts for all the behaviour that we do not understand? I have been trying to learn about exchange and correlation. I have a vague understanding of exchange. It is a repulsion due to equal direction spins, that Hartree-Fock captures exactly. But what ... 1answer 79 views ### Topological photon mass and QED in (2+1)-D My question was: how to compute topological photon mass in (2+1) QED and why it comes? 0answers 20 views ### Conceptual Problems in Understanding Thermal Field States in nonrelativistic QED I have conceptual problems understanding the notion of thermal states in the context of nonrelatistic (cavity) QED. My main problem is the definition of temperature associated to the (quantized) ... 1answer 72 views ### Is it possible to increase the orbital angular momentum (OAM) of a photon? When the photon is emitted, can you change the angular speed? This has been experimented with vortex beams but I don’t quite understand how it works. Also, an optional follow up question, would a ... 1answer 90 views ### Mass Dimension 6 QED-Lagrangian Consider the QED Lagrangian $$\mathcal{L}_{\text{QED}}=-\frac{1}{4} F^{\mu \nu} F_{\mu \nu} + \bar{\psi}(i D_{\mu} \gamma^\mu -m) \psi.$$ I need to extend the Lagrangian up to mass dimension 6, of ... 0answers 19 views ### Stern-Gerlach: would QED treat it as many discrete virtual-photon interactions? If yes, how many? And in the region of few interactions, does that correspond to a “weak measurement”? (I come to this question while learning about decoherence. We usually think of entanglement for ... 0answers 42 views ### The path that light takes [duplicate] How can I explain how the probabilistic rules for light propagation are consistent with law of reflection from ray optics? 1answer 87 views ### Mathematical formalism to show that an atom casts a small shadow in the photon field that illuminate it This questions regards the relationship between photon absorption and the spatial mode of light. In the question I have some physical intuition which I think I understand and which is born out by ... 1answer 325 views ### Do photons cast shadows Large objects, for instance a tree, can cast shadows. Do elementary partices such as photos cast shadows. And can this be proven? 1answer 57 views ### Feynman rule for 4-point fermion vertex Suppose we have a Lagrangian with a fermion of mass$m$and a 4-point interaction of the form $$\mathcal L=\bar{\psi}(i\partial\!\!\!/-m)\psi-g\bar{\psi}\partial\!\!\!/\psi\bar{\psi}\partial\!\!\!/\... 1answer 142 views ### Can one construct a non-zero (conditional) probability in this situation? Background Inspired by this video I'd like to ask the following question: I'm not an expert at QED or optics. But this is my 2 cents on the problem: "Do we see the light or the object?" Problem ... 2answers 975 views ### Why don't atoms emit more than one photon during an energy level transition? If you look at the emission spectrum of an atom, there are sharp lines corresponding to the different energy level transitions. That's because the single photon emitted during each transition carries ... 1answer 72 views ### Vacuum energy from the quantization of the electromagnetic field I'm studying the quantization of the free electromagnetic field and at a certain point there is the Hamiltonian operator as:$$\hat{H}^f_{em}=\sum_\lambda\frac 1 2 (\hat{P}^2_\lambda+\omega_\lambda^2\... 1answer 2k views ### How do physicists deal with fields at the location of charges? In the Feynman Lectures Vol 1, Chapter 28 (at the end of section "28–1 Electromagnetism"), it is mentioned: For those purists who know more (the professors who happen to be reading this), we should ... 1answer 66 views ### Regulating a divergent integral in QED When we try to regulate a divergent integral, we introduce another parameter, say$\lambda$and then compute the integral. We finally take a limit (either$\lambda \rightarrow 0, \infty $) to restore ... 1answer 119 views ### Why is the gauge-fixing condition squared in the QED Lagrangian? Consider the free Maxwell Lagrangian: $$L= -\frac{1}{4}F_{\mu\nu}F^{\mu\nu}.$$ As we know, the gauge symmetry$A_{\mu} \rightarrow A_{\mu}+\partial_\mu \lambda$must be fixed when quantizing the ... 1answer 66 views ### Momentum conservation in spontaneous pair creation and annihilation I know that in free space, photon cannot decay into an electron and a positron since momentum is frame dependent for massive particles while invariant for a photon. Given this, how is spontaneous ... 1answer 88 views ### Why does$\zeta(3)$appear in the expression for the anomalous magnetic moment? The anomalous magnetic moment, to fourth order in$\alpha$, contains$\zeta(3)$. Is there a simple explanation for the appearance of this value of the Riemann zeta function? 0answers 44 views ### What light “with no apparent material source” is Feynman referring to at the start of his Nobel lecture? Near the beginning of his 1965 Nobel lecture (end of page 2 in this pdf), Feynman justifies trying to remove the degrees of freedom of the electromagnetic field since in the end the field now is ... 0answers 54 views ### N00b path integral formulation question I have a question about the path integral formulation used in QED etc. The path integral formulation implies to me that index of refraction (and reflection or anything also calculated using this ... 1answer 89 views ### Feynman’s QED: Photons travelling faster than$c\$

I was reading though a Phys.SE thread on Feynman’s QED and FTL photon travel over short distances where the most popular answer explained how Feynman was not implying photons can travel faster than ...
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### What happens to photon in photoelectric effect

In photoelectric effect, when the photon gives energy to the electron, then what happens to itself? Where does it go? For me the photon must take the place of electron after the electron escapes the ...