Quantum-ElectroDynamics (QED) is the quantum field theory believed to describe the electromagnetic interaction (and with some extension the weak nuclear force).
3
votes
1answer
182 views
Ward Identity makes QED logarithmic divergent?
quick question regarding superficial degrees of freedom and Ward identities.
For instance in Peskin and Schroeder it is stated that the photon-self energy is superficially quadratically UV divergent ...
4
votes
1answer
98 views
Mass gap for photons
I am puzzled by the answers to the question:
What is a mass gap?
There, Ron Maimon's answer gives a clear-cut definition, which I suppose applies to any quantum field theory with Hamiltonian $H$, ...
6
votes
2answers
621 views
EM wave function & photon wavefunction
According to this review
Photon wave function. Iwo Bialynicki-Birula. Progress in Optics 36 V (1996), pp. 245-294. arXiv:quant-ph/0508202,
a classical EM plane wavefunction is a wavefunction (in ...
2
votes
1answer
92 views
Photons, where do they come from? [closed]
Photons, where do they come from?
What exactly is a photon?
I've certainly heard how they get produced, but it doesn't seem to make sense that some sort of particle should need to be produced just ...
3
votes
1answer
61 views
What are the limits of applicability of Coulomb's Law?
Coulomb's law is formally parallel to Newton's Law of Universal Gravitation, which is known to give way to General Relativity for very large masses. Does Coulomb's Law have any similar limits of ...
1
vote
1answer
172 views
Which is this formula Feynman talks about in the QED book?
I am reading the fantastic QED Feynman book. He talks in chapter 3 about a formula he considers too complicated to be written in the book. I would like to know which formula he talks about, although I ...
0
votes
2answers
155 views
Why does Quantum Electrodynamics Allow a Photon to Exist Temporarily as a Positron and an Electron?
In this question...
Why does a photon colliding with an atomic nucleus cause pair production?
...I asked why a photon colliding with a atomic nucleus can become an electron and a positron. The ...
2
votes
2answers
170 views
what is the relationship between the dynamical casimir effect and virtual particles?
Since virtual particles are disturbances in a field, and not particles in any sense, as explained here, how is it that true photons arise from them when excited with kinetic energy via the dynamical ...
0
votes
1answer
79 views
QED photon propagator to one-loop order gets different answers
I'm a self-studying 14-year-old who has a passion for particle physics. I'm currently trying to calculate the QED photon propagator to one loop. However, in all the places I've looked, even with the ...
4
votes
3answers
258 views
How does one interpret the Dirac equation with a self-field potential?
EVERY QFT text I've ever examined states that if there is an external vector potential, $A_\mu$, then one writes the Dirac eq.(or Klein-Gordon eq.) using a covariant derivative to include this U(1) ...
0
votes
1answer
454 views
Neutral Pion Decay
While studying C-symmetry, a question about neutral pion decay came up.
The most probable channels in which neutral pion $\pi^0$ decays are:
$\pi^0\longrightarrow\gamma+\gamma$ (98%)
...
2
votes
1answer
441 views
Chiral anomaly and decay of the pion
I am told that if all classical symmetries were reflected as quantum symmetries, the decay of the neutral pion $$\pi^0 ~\longrightarrow~ \gamma\gamma$$ would not happen. Why would the conservation of ...
0
votes
0answers
51 views
How many particles are created in the strong electromagnetic field?
Consider a vacuum of charged massless scalar field.
Then the uniform and isotropic electric field $E$ is turned on for a time interval $\tau$.
The question is, how many scalar particles are created?
...
0
votes
0answers
49 views
How does QED deal with wavelength of quanta [duplicate]
Since QED treats photons as individual units (quanta) how does it treat the concept of the "wavelength" associated with the photon?
0
votes
2answers
272 views
Can electromagnetic fields be used to deconstruct and reconstruct atoms?
I was thinking one day and came up with a theory after reading about how scientists were studying anti-matter by using electro magnetic fields to separate matter from the anti-matter they made. It ...
5
votes
1answer
86 views
How can an asymptotic expansion give an extremely accurate predication, as in QED?
What is the meaning of "twenty digits accuracy" of certain QED calculations? If I take too little loops, or too many of them, the result won't be as accurate, so do people stop adding loops when the ...
6
votes
1answer
183 views
Can a photon exhibit multiple frequencies?
Can a photon be a superposition of multiple frequency states? Kind of similar to how an electron can be a superposition of multiple spin states.
0
votes
0answers
21 views
What does it mean to erase the which-path information of something?
In this particular case, I am told that very fast measurements erase which-path frequency information of photons.
I'm not really sure what that means though. I do not entirely understand the concept ...
2
votes
0answers
59 views
How does this paper relate to standard QED?
This paper proposes a microscopic mechanism for generating the values of $c, \epsilon_0, \mu_0$. They state that their vacuum is assumed to contain ephemeral (meaning existing within the limits of ...
2
votes
1answer
68 views
Alternative methods to derive the static potential in the NR limit of QED
In QED, one can relate the two-particle scattering amplitude to a static potential in the non-relativistic limit using the Born approximation. E.g. in Peskin and Schroeder pg. 125, the tree-level ...
3
votes
1answer
195 views
Energy spectrum of a Dirac electron
How do you explain easily "The spectrum of an electron in a repulsive potential " and hence "bound state of charge conjugation" in Dirac hole theory ?
0
votes
3answers
88 views
Why doesn't a stationary electron lose energy by radiating electric field (as per coulomb's law)?
If an electron in a universe constantly generates an electric field why does it not get annihilated ? I am confused because I read that an accelerating charge radiates and loses energy. So, why won't ...
4
votes
1answer
81 views
Some questions about Ward-Takahashi Identity
I'm a learner of Peskin and Schroeder's textbook of quantum field theory.
I have proceeded to Ward-Takahashi identity and have one question when I look for Wikipedia for reference.
The following is ...
2
votes
0answers
34 views
Is it reasonable to interpret the Lamb shift as vacuum induced Stark shifts?
This is a pretty hand-wavy question about interpretation of the Lamb shift. I understand that one can calculate the Lamb shift diagrammatically to get an accurate result, but there exist ...
5
votes
0answers
234 views
Do EM waves transmit spin polarization?
Suppose you have a normal dipole antennae (transmitter and receiver) . Spin polarized current (as opposed to normal current) is sent into the transmitter, it emits an EM wave and the Receiver receives ...
3
votes
1answer
118 views
Photon as the carrier of the electromagnetic force
My physics background goes as "far" as reading popsci books on QM, Particle Physics, and Cosmology so pardon my ignorance in the below questions.
I've read that the photon is the particle (quanta in ...
1
vote
1answer
67 views
Does a quadrupole transition mean emission of one photon with spin 2?
If it's true and spin-2 photons do exist, could you please point to some literature that discusses spin-2 photons?
If not, then how exactly does a selection rule for quadrupole transition make sense ...
3
votes
0answers
100 views
Quantum Electrodynamics
I was wondering if anyone could give a simple explanation of how light interacts with matter. From what I have read in QED, electrons will repel each other because of their ability to emit and ...
4
votes
1answer
118 views
Are there 2 kinds of photons, one that mediate the electromagnetic interaction and the other the quanta of light?
It is usually said that photons are the force carriers or the mediators of the electromagnetic forces between electric charges. At the same time we know also that electromagnetic waves on the quantum ...
8
votes
1answer
224 views
Did the Feynman heuristic of “simple effects have simple causes” fail for spin statistics?
Someone here recently noted that "The spin-statistics thing isn't a problem, it is a theorem (a demonstrably valid proposition), and it shouldn't be addressed, it should be understood and celebrated."
...
3
votes
1answer
265 views
Effects of parallel superconducting plates
Assuming the existence of virtual particle field ( zero point energy field) Casimir force is produced by 2 parallel conducting plates excluding some of the frequencies between the plates, if these 2 ...
9
votes
4answers
322 views
The Schwinger model
The Schwinger model is the 2d QED with massless fermions. An important result about it (which I would like to understand) is that this is a gauge invariant theory which contains a free massive vector ...
4
votes
3answers
151 views
Does light really “travel”?
From what I've so far understood about light, a photon is emitted somewhere and after some time it's absorbed somewhere else.
Have we had experiments that confirm the path taken or something akin to ...
2
votes
3answers
137 views
What starts the movement of a photon
Although a photon has no (rest) mass, it does have a measurable speed. Its movement can be altered by gravity. A photon "travels". If I turn on a flashlight, seen by someone at a distance, the photons ...
6
votes
1answer
131 views
Database of scattering amplitudes
I want to check whether my result for the invariant amplitude of the electron-electron scattering (to lowest order in $\alpha$; t+u channels) is correct or not.
I can't find any reference that has ...
10
votes
2answers
317 views
Using photons to explain electrostatic force
I am trying to understand the idea of a force carrier with the following example.
Let's say there are two charges $A$ and $B$ that are a fixed distance from each other. What is causing the force on ...
10
votes
3answers
266 views
Can the path of a charged particle under the influence of a magnetic field be considered piecewise linear?
Ordinarily we consider the path of a charged particle under the influence of a magnetic field to be curved. However, in order for the trajectory of the particle to change, it must emit a photon. ...
1
vote
1answer
138 views
Local $U(1)$ gauge invariance of QED
The Lagrangian density for QED is
$$ \mathcal{L}=-\frac{1}{4}F^{\mu\nu}F_{\mu\nu}+\bar{\psi}(i\gamma^{\mu}D_{\mu}-m)\psi $$
with
$$F_{\mu\nu}=\partial_{\mu}A_{\nu}-\partial_{\nu}A_{\mu} $$
$$ ...
1
vote
2answers
226 views
Ontology of the quantum field
I'll use QED as an example, but my question is relevant to any quantum field theory.
When we have a particle in QED, where is its charge contained in the field? Is the field itself charged? If so, ...
3
votes
1answer
60 views
Vanishing of photon one-point function in QED
I would like to know why the photon one-point function vanishes in QED. I am aware that any $n$-point functions vanishes for odd $n$ because of 'charge-conjugate" argument, this does not apply to ...
1
vote
1answer
101 views
Two-photon scattering: colours
Is there a particular conservation principle that necessitates that the outcoming photon pair has the same frequencies as the incoming photon pair?
I'm thinking in particular of these Feynman-like ...
1
vote
0answers
60 views
Photons interact with themselves
We know that photons are the antiparticles of themselves and if they interact with each other through higher order process do they annihilate and again produce photons?
Here is the Phys.SE question ...
13
votes
1answer
214 views
Spontaneous breaking of Lorentz invariance in gauge theories
I was browsing through the hep-th arXiv and came across this article:
Spontaneous Lorentz Violation in Gauge Theories. A. P. Balachandran, S. Vaidya. arXiv:1302.3406 [hep-th]. (Submitted on 14 ...
0
votes
2answers
95 views
Field energy of/from virtual Photons
I have a slightly out-of line question:
Consider a single electron (or it's current if you please)
The EM field surrounding it will (no doubt) have an EM field energy (T) to go with.
The standard ...
3
votes
3answers
89 views
QED as a Wightman theory of observable fields? With a collision theory?
[Note: I'm using QED as a simple example, despite having heard that it
is unlikely to exist. I'm happy to confine the question to
perturbation theory.]
The quantized Aᵘ and ψ fields are non-unique ...
0
votes
0answers
65 views
A question on charge renormalization in QED
Let us work with charge renormalization in QED. Consider 2-point photon correlation function $\Pi_2(q^2)$ at one loop level. We normalize the coupling constant at $q^2=0$ (point of normalization). ...
7
votes
0answers
132 views
Magnetic monopole and electromagnetic field quantization procedure
From the Maxwell's equations point of view, existence of magnetic monopole leads to unsuitability of the introduction of vector potential as $\vec B = \operatorname{rot}\vec A$. As a result, it was ...
2
votes
2answers
158 views
Where does the mass term come from in the Proca Lagrangian?
There are many good books describing how to construct the Lagrangian for an electromagnetic field in a medium.
$$
\mathcal{L}~=~-\frac{1}{16\pi}F^{\mu\nu}F_{\mu\nu}-\frac{1}{c}j^{\nu}A_{\nu}
$$
...
0
votes
1answer
80 views
Dichroism in uniaxial crystals
I need a same help with it. Some books where i can find a real math explanation of this effect will be good help!!
simple exp of this effect will be good too)
2
votes
2answers
153 views
proof of radius of convergence of perturbation series in quantum electrodynamics zero
Can anyone show detailed proof of why radius of convergence of perturbation series in quantum electrodynamics is zero? And how is perturbation series constructed?
So, as this argument requires ...
