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).

Filter by
Sorted by
Tagged with
0
votes
0answers
33 views

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?
1
vote
0answers
46 views

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 ...
0
votes
1answer
64 views

In QED why is the electron a point particle? [duplicate]

I read Feynman's book but this still unclear to me.
3
votes
1answer
68 views

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 ...
8
votes
3answers
328 views

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 ...
0
votes
0answers
48 views

Coulomb QED in terms of creation/annihilation operators

I am working with QED in Coulomb gauge. I am trying to write the instantaneous Coulomb interaction term in QED in terms of creation and annihilation operators. The term is given by $$\frac{1}{8\pi}\...
0
votes
0answers
84 views

Is the cut-off energy in the calculation of the electron's self-energy associated with an electron size?

I have got two questions on the self energy/point particle concept of the electron: What do physicists exactly mean when they say the elementary particles (e.g. the electron) are point particles? ...
1
vote
0answers
21 views

Force-carrying particles for electrostatic force [duplicate]

As I understand, Nuclear Fusion occurs when the nuclear forces overcome the electrostatic forces that act repulsively. But, what are these electrostatic forces? Are there force-carrying particles just ...
1
vote
1answer
91 views

How do magnets attract (QED)? [duplicate]

I'm starting my graduation in physics and it may sound like a dumb question, but it's haunting my mind for days. It is known that the scattering of two electrons can be represented by the following ...
0
votes
0answers
43 views

Electric field outside a standing electron wave

If an electron were confined in a cavity with perfectly reflecting ends, its wave function would form a standing wave in the cavity. Presumably the electron would not emit EM radiation unless it were ...
-1
votes
1answer
24 views

Do all kinds of accelerated fundamental particles show the quantum wave function collapse phenomenon? Is it also applicable to electromagnetic waves?

I had come to terms with the wave function collapsing on observations, but does this apply to every fundamental particles and electromagnetic waves too?
0
votes
2answers
122 views

If photons don't oscillate, how do they oscillate?

The internet has told me seemingly contradictory things, and I hope you folks might help me sort it out. This blog post states: "Electromagnetic waves are waves from electric and magnetic fields ...
0
votes
0answers
33 views

Longitudinal EM Waves in Vacuum

As far as I know longitudinal electromagnetic waves are possible in plasmas. I stumbled upon the notion that the QED vacuum is "made out of" virtual electron-positron pairs and effects like Vacuum-...
4
votes
1answer
116 views

Why is gauge invariance so important?

Quantizing the electromagnetic field (without ghosts or gauge fixing terms) using path integrals is not possible because the propagator is not well defined. Textbooks such as P&S or Ashok Das say ...
2
votes
1answer
142 views

Peskin QFT Contour Integral — Chapter 6

On page 178 of Peskin's QFT, they have the vector potential $$A^\mu(x)=\int\frac{d^4k}{(2\pi)^4}e^{-ik\cdot x}\frac{-ie}{k^2}\left(\frac{p'^\mu}{k\cdot p'+i\epsilon}-\frac{p^\mu}{k\cdot p-i\epsilon}\...
1
vote
1answer
65 views

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 ...
0
votes
2answers
43 views

Substituting field by its expectation value in path integral

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(...
1
vote
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 ...
0
votes
0answers
18 views

Vacuum Birefringence with alternating EM waves

I read [1] 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 ...
0
votes
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 ...
0
votes
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 ...
0
votes
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 ...
3
votes
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 ...
1
vote
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 ...
4
votes
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 ...
0
votes
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 ...
1
vote
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 ...
4
votes
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?
1
vote
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) ...
1
vote
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 ...
1
vote
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 ...
1
vote
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 ...
0
votes
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?
1
vote
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 ...
3
votes
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?
0
votes
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\!\!\!/\...
-5
votes
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 ...
12
votes
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 ...
1
vote
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\...
20
votes
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 ...
3
votes
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 ...
5
votes
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 ...
0
votes
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 ...
0
votes
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?
2
votes
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 ...
0
votes
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 ...
1
vote
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 ...
0
votes
2answers
65 views

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 ...
0
votes
0answers
69 views

Fermat's Principle of Least Time

I was going through the Feynman Lectures on optics when I came across an explanation for Fermat's Principle. I did not fully understand the explanation- can someone please break it down in a simpler ...
0
votes
1answer
67 views

The concept of force carrying photons

Are the force carrying photons really something physical? Do they really occur during electromagnetic interactions? Does this mean that in different mediums where the speed of light differ, the ...

1 2 3 4 5 28