# What is Quantum electrodynamics: Definition and 53 Discussions

In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved. QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of photons and represents the quantum counterpart of classical electromagnetism giving a complete account of matter and light interaction.
In technical terms, QED can be described as a perturbation theory of the electromagnetic quantum vacuum. Richard Feynman called it "the jewel of physics" for its extremely accurate predictions of quantities like the anomalous magnetic moment of the electron and the Lamb shift of the energy levels of hydrogen.

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1. ### Equations of motion for Lagrangian of scalar QED

Well, I started with the first equation of motion for the scalar field, but I'm really not sure if I'm doing it the right way. \begin{split} \frac{\partial \mathcal{L}}{\partial \varphi} &= \frac{\partial}{\partial \varphi} [(\partial_\mu \varphi^* -...
2. ### I Confusion about Scattering in Quantum Electrodynamics

When it comes to scattering in QED it seems only scattering cross sections and decay rates are calculated. Why is that does anyone calculate the actual evolution of the field states or operators themselves like how the particles and fields evolve throughout a scattering process not just...
3. ### A Quadruple Integral in the Lamb Shift

The analytical computation of the shift in energy level of electrons in atoms due to quantum electrodynamics is carried out using perturbation theory. In particular, the fourth-order contribution is given in five different terms. One of them, usually called "Electron Self Energy", leads to seven...
4. ### I What is the probability of an electron emitting a photon?

I have seen many tutorials that provide steps how to transcribe a Feynman diagram into algebra, for instance [here]: However, I have never seen the final line of the calculation converted into a real number. What are the steps to get from the algebra equations transcribed using the Feynman...
5. ### A Orthogonality of variations in Faddev-Popov method for path integral

Hi there, I've been stuck on this issue for two days. I'm hoping someone knowledgeable can explain. I'm working through the construction of the quantum path integral for the free electrodynamic theory. I've been following a text by Fujikawa ("Path Integrals and Quantum Anomalies") and also...
6. ### A Are these papers on Quantum Electrodynamics true?

https://arxiv.org/abs/2010.10345 My question is, Has the Feynman Diagram IIc been published or not? Did Kroll & Karplus lie or not? https://vixra.org/abs/2002.0011 Are these critics valid?
7. ### I Calc QED Charge Particle Repulsion/Attraction

Maybe this will help.
8. ### B Question about QM and Neuroscience

I have one request, I hope you could answer it…. I saw this article that makes heavy assumptions based on Quantum Electrodynamics, like something like formation of an energy domain or sorts, and it connects it with brain activity which is super weird, I did ask a neuroscientist and he said he...
9. ### Quantum Intro to Quantum Electrodynamics

Hi folks, Electricity and magnetism have always been my favorite subjects, and I am just getting to learn the basics of quantum theory and it is absolutely terrific going. I was always fascinated by the story of Feynman, Schwinger, and Tomonaga tackling QED from the books Genius by James Gleick...
10. ### I Calculating 2nd Order Scattering Amplitude: Feynman Diagrams

In the following I will try to deduce the scattering amplitude for a specific interaction. My question is at the bottom, the entire rest is my reasoning to explain how I came to the results I present. My working Let's assume I would like to calculate the second order scattering amplitude in ##...
11. ### B The nature of virtual photons in quantum electrodynamics

In quantum electrodynamics, the energy exchange between two charged particles is called a virtual photon, allowing the exchange of quanta of energy between the two charged particles. But these are not the traditional photons of electromagnetic waves. They apparently just arise from the equations...
12. ### Quantum Introductory textbook for cavity quantum electrodynamics

Hi. I'm having a hard time learning the physical realization of quantum computers. I got stuck with the section of optical cavity quantum eletrodynamics. There are some concepts I am not familiar with. I think I should read some introductory textbooks which cover cavity quantum electrodynamics...
13. ### A Do QED effects make a huge change to the position of the electrons?

In https://en.wikipedia.org/wiki/Lamb_shift about the lamb shift, it's mentioned that the change in the electron's frequency due to QED effects (vacuum polarization and self-energy correction) is about 1 GHz, which would translate to an energy change of hf = 6.63E-25 J. This is 3E-7 times of the...
14. ### I Does the Compton wavelength put a limitation on position measurements?

I have read on Wikipedia (https://en.wikipedia.org/wiki/Compton_wavelength) that we cannot measure the position of a particle more precise than half of its Compton wavelength, since the photon we would need will be so energetic to produce electron-positron pairs. How does the creation of...
15. ### A Can we create a random variable using QED effects?

Quantum Electrodynamics (QED) has some observable effects such as the lamb shift, which is mainly caused by the vacuum polarization and the electron self-energy. These effects contribute to the "smearing" of the electron in an unpredictable manner, other than the uncertainty we already have...
16. ### A Is there any Bohmian approach to quantum electrodynamics?

I know that in some Bohmian papers (like https://arxiv.org/pdf/quant-ph/0303156.pdf), electron-positron pair creation and annihilation is modeled by different methods like stochastic jumps in the configuration space. My question is, is there any Bohmian approach to reproduce all of the...
17. ### A How, and in what atoms does the Lamb shift occur?

The Uehling potential due to vacuum polarization by virtual electron-positron pairs is said to be the dominant contribution — 205.0073 meV — to the Lamb shift between the 2P1/22P1/2 and 2S1/22S1/2 states of muonic hydrogen. In the Wikipedia page (https://en.wikipedia.org/wiki/Lamb_shift), it is...
18. ### I How does the collision between two atoms work?

Considering the quantum mechanical model for an atom, what exactly happens when two atoms (say, two Ca2+ ions in a Brownian motion) collide with each other? As I know, this collision is not like a regular elastic or inelastic collision between two macroscopic objects. Is it mainly due to the...
19. ### A Quantization of the electric field inside a box

Hello all, The second quantization of a general electromagnetic field assumes the energy density integration to be performed inside a box in 3D space. Someone mentioned to me recently that the physical significance of the actual volume used is that it should be chosen based on the detector used...
20. ### A Polarization vector sums in QED

I'm working through Lahiri & Pal's book A First Book of Quantum Field Theory, Second Edition and I'm stuck on their explanation of the polarization vector in quantum electrodynamics in Chapters 8 and 9. In section 8.8, they derive a formula for the sum over the transverse polarization modes of...
21. ### I Is Quantum Electrodynamics relevant to every electromagnetic field?

Is Quantum Electrodynamics relevant to every electromagnetic field?
22. ### A Quantum electrodynamics and the brain

Susan Pocket claims that part of the electromagnetic field of the brain is consciousness and that conscious qualities such as a red car are spatially patterned electromagnetic fields. She also claims that the quantum spatial scale is irrelevant. Intuitively it seems to me Quantum electrodynamics...

27. ### Insights A First Idea of Quantum Field Theory - 20 Part Series - Comments

Greg Bernhardt submitted a new PF Insights post A First Idea of Quantum Field Theory - 20 Part Series Continue reading the Original PF Insights Post.
28. C

### I (QED) The initial mass and the correction cancelling out

The prefix is a bit irrelevant This is on renormalisation. How do they cancel out? Isn't it adding? So the mass experimental = m + (c2correction) so how do you cancel out the m and correction? I'm new to this area (just finished watching lectures by Richard Feynman, specifically a 4 lecture...
29. ### A Description of imaging through lens using quantized EM field

Usually the imaging through lens is described using geometrical optics or wave optics. However, I wish to study some imaging processes through lens where the light intensity is at the single photon level, under which condition I think the correct description should use a quantized...
30. ### I What is the Equation on My T-Shirt?

I have a t-shirt with a next print: But I am not sure what equation is. I only know that is something related with light. But I haven't found it. I am not sure if it is one from quantum electrodynamics or some advanced course in physics. I would appreciate that somebody could tell me which...
31. ### A Issue in the electron’s infinite self-energy

Hello, Reading Richard Feynman’s book “Quantum Electrodynamics” (Edited by Advanced Book Classics), I read that the electron’s self-energy is infinite and that has been a trouble for QED during 20 years. Feynman proposed a solution based on a cut-off, but that’s not fully satisfactory and I...
32. ### Quantum Book on quantum electrodynamics

Can anyone suggest me a book on quantum electrodynamics for beginners? Thanks in advance
33. ### Total Internal Reflection explained with Quantum Mechanics

Is there an easy explanation of total internal reflection of light using Quantum mechanics(or QED)?
34. ### Can QFT adequately describe the elusive properties of photons?

What is the photon's equivalence of the electron's wavefunction? Can I measure qualities of the photon that will collapse into an eigenstate? What properties of the photon aren't fixed?
35. ### Why Is the Photon One-Point Function Zero in QED?

In 'an introduction to quantum field theory' by peskin, he writes: To analyze the photon one-point function, note that the external photon must be attached to a QED vertex. Neglecting the external photon propagator, this amplitude is therefore: I really cannot justify this equation. Can...
36. ### Single electron wave packet in Fock space?

How might we construct a state most closely corresponding to the idea of a single electron wave packet as some superposition of Fock states?
37. ### Gauge Transformation Quantum Electrodynamics

It's well known when if we are working on problems related to particles in presence of an electromanetic field, the way we state the problem can be done using the next Hamiltonian: H=\dfrac{(p-\frac{e}{c}A)^2}{2m} +e \phi where the only condition for A is: \vec{\nabla } \times \vec{A} =\vec{B}...
38. ### Gauge Freedom Quantum Electrodynamics

It's well known when if we are working on problems related to particles in presence of an electromanetic field, the way we state the problem can be done using the next Hamiltonian: H=\dfrac{(p-\frac{e}{c}A)^2}{2m} +e \phi where the only condition for A is: \vec{\nabla } \times \vec{A} =\vec{B}...
39. ### Why the photoelectric absorption section finite at threshold

I mean the photoelectric effect of the hydrogen atom. It is weird. By the Fermi golden rule, the transition or absorption rate is proportional to the density of the final states. At threshold, the electron has zero momentum and thus zero density of state. Therefore, the absorption coefficient...
40. ### What is the polarization of a virtual photon

I read in few places that the virtual photons have longitudinal polarization while in some papers they say virtual photons have both transverse and longitudinal polarizations. Can anyone please help me to understand this
41. ### Cavity quantum electrodynamics

hi, i am studing cavity QED - but needs some comprehensive book on that - i will be very grateful if someone guide me in this regard.thanks
42. ### No drama quantum electrodynamics? (was: Local realism ruled out?)

As the only thread I've ever started before (https://www.physicsforums.com/showthread.php?t=369328 ) attracted a lot of interest, became one of the most viewed threads in the forum, and was closed :-), I'd like to make a short update here. I have published a new article on the topic: "No...
43. ### Quantum Quantum Electrodynamics by Walter Greiner

Author: Walter Greiner (Author), Joachim Reinhardt (Author) Title: Quantum Electrodynamics Amazon Link: https://www.amazon.com/dp/3540875603/?tag=pfamazon01-20 Prerequisities: Contents:
44. ### Understanding Electron Mass in Classical and Quantum Electrodynamics

Consider classical electron theory. Distinguish between the mechanical, electromagnetic, and observed electron mass. What is the degree of divergence of the classical electron self-energy? What are the roles of mass-renormalization, runaway solutions, and preacceleration in classical electron...
45. ### Scattering - quantum electrodynamics

hi, so i have just finished reading my first ever book on quantum electro-dynamics (the feynmann lectures). i am in year 13 of school, or High school some might call it. he draws a diagram in the book (a space time diagram) to represent an electron absorbing a photon and then re-emitting...
46. ### Quantum Electrodynamics in Curved Spacetime

Does anyone know if there exist a formulation of QED in curved spacetime. If yes could you give me some references to papers in which solution to these problem is described?
47. ### Quantum Electrodynamics - Achieving Graduate Level Understanding

Hello fellow scientists and engineers. I am a Physics-EE double major at the U of MN (twin cities). I was originally focused on doing pure physics and obtaining a research position in academia or government, but various pressures have turned me from this path, causing me to pick up a second...
48. ### Prerequisites for Quantum Electrodynamics

Although this is usually a graduate course, I want to know what all the preqrequisites for studying it seriously are. (I've been reading Feynman, Schweber and a bit of Griffiths which pumped my interest). I think mathematics should be calculus, group theory and all the stuff needed for QM. As...
49. ### Quantum Electrodynamics: A Basic Textbook for Self-Study in Undergrad Years

What is a good textbook that gives a basic understanding of quantum electrodynamics? My university doesn't have any courses in this subject so I am going to have to teach to myself. What do I need to know and understand before I dig down into quantum electrodynamics? So far I am at my...
50. ### Learning Quantum Electrodynamics: Resources and Advice

Hiya. I'm going to be taking a Quantum Electrodynamics course next semester. I was wondering if anyone could recommend : a) Good sites on the subject b) Good books on the subject Thanks in advance!