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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28 views

$n^{th}$ power interaction vertex in QED

What is actually mean by power of interaction vertex or $n^{th}$ power interaction vertex in QED. My doubt come from 3rd point of modified feynman technique...
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What is the relative strength of the weak force?

When comparing the weak gauge coupling to the electromagnetic gauge coupling (fine structure constant alpha) textbooks give the value ("relative strength") 10^-5 or the value 10^-13. Why are ...
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Correspondence between Gaussian states and different photon statistics

What are the characteristics to identify a Gaussian state in quantum optics, and how to find its relation or overlap with classical and nonclassical states? I know that a quadratic Hamiltonian ...
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1answer
58 views

Vacuum polarization or electron with structure?

Is it possible to construct some charge density $ρ(r)$ to get the Uehling-Potential? $${\displaystyle V_{\text{Uehling}}(r)\approx -Z\alpha \hbar c{\frac {1}{r}}\left(1+{\frac {\alpha }{8\pi ^{2}{\...
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1answer
53 views

Photoelectric effect: Beginning of the complexities [duplicate]

We are taught that in photoelectric effect if the frequency of light is lower than the threshold, then no matter how long a metal is exposed to it there won't be any ejection of electrons. This made ...
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Path Integral on Feynman Hibbs: Interaction of EM field and matter, how can we get to equation (9.68) from (9.67)?

On Feynman Hibbs "Quantum Mechanics and Path Integrals", the equation (9.67) describes the transition amplitude of the matter (for example an atom) to go from the state $M$ to the state $M$ ...
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6 views

Spiral waveguide for single-photon generation

I would like to ask about the advantage of a spiral waveguide for single-photon generation(using SFWM) compared to the SPDC process using bulk optics.
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118 views

Energy loss of photons crossing a dielectric medium

How much energy do photons lose when crossing an "isotropic" dielectric medium? What would be possible mechanisms. I assume the straight forward answer would be that the photon loses zero energy and ...
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59 views

Suppression of $W$ boson propagator by its mass

In my experimental particle physics introductory class it was often said that quantum electrodynamics (QED) is very predictive for sufficiently small center of mass energys since the $W^\pm$-...
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237 views

How does a charged field's coupling to the electromagnetic field translate to a worldline coupling?

Suppose we're interested in (minimally) coupling a heavy scalar particle to the electromagnetic field. In quantum field theory, we describe the particle as an excitation of a complex scalar field $\...
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125 views

Do positively charged particles exchange photons? [duplicate]

I learned in this video and in this answer that electrons repel from each other by exchanging photons. This makes sense when it comes to electrons, but what happens when two protons or two positrons ...
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How does quantum electrodynamics explain the movement of nuclei in chemical reactions?

In Feynman's third Douglas Robb Memorial Lecture (which took place in 1976 at University of Auckland), he states that: It's fine to approximate nuclei as a point, although they really aren't. Quantum ...
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Proof for 1st wards identity [closed]

Can any one give a proof for 1st Ward identity of the form $$k_\mu \Delta_{\mu\nu}^{-1}(k) -\frac{k_\nu k^2}{\alpha} =0$$ where alpha is a gauge fixing parameter?
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Measuring qubit's state in a real experiment

In theory, it is fairly simple to measure a qubit's state. Let us consider a Ramsey experiment, where the qubit is in the ground state $|0\rangle$ initially. Then by applying a Hadamard gate the qubit ...
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278 views

How Ward Identity indicate vacuum polarization correction?

In Peskin & Schroeder Chapter 7.5 Renormalization of The Electric Charge, they mention that vacuum polarization correction is $$ iM= (-ie)^2(-1)\int_{}{}\frac{d^4k} {(2\pi)^4}Tr\bigg[\gamma^{\mu}\...
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A question about Feynman's 'Little Arrows' approach in “QED: The Strange Theory of Light and Matter”

I have a perplexity regarding Feynman's famous "little arrows" approach. To summarize and paraphrase Feynman as I understand him, he is claiming that we model the probability that an event ...
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66 views

How do you connect classical electromag, the photon, the wavefunction, and QFT to form a picture of light?

Classically, if you wiggle an electron in a sinusoidal pattern up and down, you get a smooth electromagnetic wave that propagates, kinda like when you wiggle a jump rope. Do this fast enough and you ...
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Subtraction in bare perturbation theory on Peskin's book

On chapter 7 of their book on QFT, Peskin and Schroeder derive the vacuum polarization correction to the photon propagator in bare renormalization theory. On page 247, they state that to leading order ...
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Casimir Effect Analogue

The Casimir effect from what I understand is computed as the integral over a macroscopic object (e.g. a pair of conducting metal plates) of an essentially quantum phenomenon, namely the energy of the ...
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What is the mechanism by which an electron and a positron annihilate?

An electron and a positron annihilate in pure energy. What is the mechanism by which an electron and a positron annihilate each other, losing their mass and their electric and magnetic field? ...
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52 views

Macroscopic Limit of QED

How does one go about rigorously deriving special relativistic dynamics (both relativistic mechanics and electrodynamics) from quantum electrodynamics? Is this even possible from the mathematical ...
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1answer
127 views

Confused about the anapole (toroidal) moment

I am a bit confused about the existence of the anapole moment. As far as I understand, in order to fully describe a charge-current distribution, one needs, beside the normal electric and magnetic ...
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2answers
76 views

Is electric field operator in Schrödinger picture time-dependent?

We know that in the Schrödinger picture, operators are time-independent if they do not have explicit time-dependence. So do electric field and vector potential field operators have time dependence in ...
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2answers
72 views

How do photons create a uniform electric/magnetic field

Roughly speaking, Quantum Electrodynamics tells us that electromagnetic fields are really made up of photons (so I'm told). What does a uniform electric field in a specific direction look like in ...
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Infinitesimal variation of EM potential with two different approaches

Setup I'd like to show that the infinitesimal variation of electromagnetic potential $A_\mu$ for an translation by $a^\lambda$ and a Gauge transformation characterized by the gauge field $\alpha(x):=a^...
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Are antiparticles just antivibrations in their counter particle's quantum fields?

like particles are vibrations in their quantum fields are their corresponding anti particle a anti vibration of some sort? and if so does this diagram accurately represent it? (If I'm wrong please ...
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355 views

Finding the amplitude for the pair production $\gamma(p_1) + \gamma (p_2) \to e^- (p_3) + e^+ (p_4)$

I'm trying to find the amplitude for: $$\gamma(p_1) + \gamma (p_2) \to e^- (p_3) + e^+ (p_4)\tag{1}$$ (My questions are stated in the end) The possible answers are: My take on it: and so $$\tag{2} \...
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70 views

Calculating traces for triangle diagrams with massless fermions

I am following Schwarz Quantum Field Theory textbook. In particular, I am looking at triangle diagrams with massless fermions. On pg. 623 - 624 Schwarz attempts to calculate $q_\mu^1 M_{5}^{\alpha\mu\...
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Are residual gauge transformations related to large gauge transformations?

Is there any relation between residual gauge transformations (the remnant symmetry after gauge fixing) and large gauge transformations (gauge transformations that do not go to identity as $x \to \...
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1answer
49 views

Is the following Feynman diagram (given in Schwartz) wrong?

For the process $e^+e^-\rightarrow \gamma \gamma$ Schwartz gave following Feynman diagram in section 9.4 I think the diagram is wrong since it didn't conserve charge at both the vertices. The correct ...
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Does a $U(1)$ matrix, as in QED, have a single entry? A $1×1$ matrix ( degree 1)? [closed]

This is a very simple question, but somehow, I am getting confused.... Some examples of Q.E.D U(1) matrices I see gave four (2×2) elements, like the Weak SU(2) matrices....
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532 views

Doubt in Dyson's argument about the divergent nature of the perturbative expansion in QED

I am trying to understand Dyson's argument about the divergent nature of the perturbative expansion in QED. Quoting his own words [...] let $$F(e^2)=a_0+a_1e^2+a_2e^4+\ldots$$ be a physical ...
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331 views

Interpretation of the electromagnetic field strength tensor as a spin-1 field

If I understand correctly the electromagnetic field strength tensor $ F_{\mu\nu}$ could be considered as a spin-1 field. In that case, what can one say about the total spin and the $z$-component of ...
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35 views

Triplet state with symmetric wavefunction?

In the $1s,2s$ electronic configuration, I've found that the possible states are 1S0 and 3S1. $L=0$ for both of these terms, so would the parity of the spatial wave function not be $(-1)^L=1$, and ...
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1answer
28 views

Finding the effective drive on the emitter when driving the cavity in the jaynes-cummings model

I'm trying to figure out what the driving term in the Hamiltonian is when you drive an emitter (say a transmon qubit) through a coupled cavity (such as a CPW resonator). For this I am considering a ...
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1answer
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Is vacuum polarization the medium of propagation of EM waves?

Since the electric and magnetic fields are produced by charged particles, is it correct to think that light travels trough a medium consisting in this particle-antiparticle pairs from vacuum ...
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If electrons are identical and indistinguishable, how can we say current is the movement of electrons?

When we talk about current, we say electrons are "flowing" through a conductor. But if electrons are identical particles, how does it make sense to talk about them flowing? To expand on that:...
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Conservation of spin in Non-relativistic limit

Consider electron-positron interaction : $$e^-e^+\rightarrow\mu^+\mu^-$$ when peskin book come to compute Non relativistic limit of this process said that, because we are in Non relativistic limit we ...
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Most trivial neutral pion decay

Literature states neutral pion decay by QED cannot occur directly because the pion is uncharged. However, I cannot see why Photons are not allowed to interact with the Quarks directly. Please ...
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154 views

Does a photon attract or repel matter?

So, I know experimentally that matter and light interact. And I can calculate cross sections in Quantum Electrodynamics. Yet a fundamental intuition about how light and matter interact is completely ...
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1answer
39 views

How does one calculate the absolute value of a Feynman diagram's amplitude?

How do I obtain the absolute value of a Feynman diagram's amplitude if I do not have values for the components of this amplitude? If the amplitude of a process such as $e^+(p_1) + e^- (p_2) \to \phi (...
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Phase transition in 2+1D QED for $\theta =\pi$

QED in 1+1 dimensions with a massive fermion, called the massive Schwinger model, has a phase transition at $\theta=\pi$ for some finite critical mass $m_c$. Equivalently, one can say that the ...
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1answer
223 views

Difference between positron and electron scattering in Coulomb field

In first order of perturbation theory the S-matrix amplitude for electron scattering in the Coulomb field will be (up to normalization factors) $$ S_{fi} = \frac{iZ q^2}{\sqrt{2E_{f}2E_{i}}}\bar {u}(...
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514 views

Do virtual electron-positron pairs have mass?

When a photon produces an electron-positron pair, do both these particles have mass? Why or why not?
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Do photons annihilate each other? If yes, what do they produce? If no why not?

If photons are their own anti-particles, why don't they annihilate? Also on a side note, why do particles and anti-particles annihilate?
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1answer
221 views

Commutation of currents in QED

In an outline of a proof of the Ward identities in QED, the authors Green, Schwarz, and Witten in their book "Superstring theory", vol. I, Section 1.5.1, claim that in the QED the electromagnetic ...
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157 views

How molecules radiate heat as electromagnetic wave?

an object of higher temperature radiate infrared rays as a way to decrease the temperature. how a molecule produce a electromagnetic wave? in atoms electromagnetic radiation is caused by electrons. ...
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52 views

Quantization of free EM field, doubt about the field operators?

this is the first time that I study fields quantization and in particular I'm starting with the free em field. Previously I've just studied the non relativistic, Schrodinger equation for a non zero ...
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3answers
148 views

Schrodinger equation for free EM field

The question come from the fact that I've seen for the first time in my life the quantization of a field, in particular of the free em field. I've study how it is possible to write the energy of the ...
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1answer
39 views

Expansion of trace in photon self-energy

I am studying through the photon self-energy $$ i\Pi_{\mu\nu}(q) = \int\frac{d^4 k}{(2\pi)^4}Tr\left[(-ie\gamma_\mu)\frac{i(\require{cancel}\cancel k+m)}{k^2-m^2+i\epsilon}(-ie\gamma_\nu)\frac{i(\...

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