Tagged Questions
4
votes
1answer
102 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 $H$, ...
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 ...
2
votes
0answers
60 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 ...
4
votes
1answer
82 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 ...
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 ...
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 ...
8
votes
1answer
226 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."
...
2
votes
1answer
69 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 ...
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 ...
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, ...
13
votes
1answer
215 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 ...
5
votes
2answers
231 views
Geometrical significance of gauge invariance of the QED Lagrangian
The QED Lagrangian is invariant under
$\psi(x) \to e^{i\alpha(x)} \psi (x)$, $A_{\mu} \to A_{\mu}- \frac{1}{e}\partial_{\mu}\alpha(x)$. What is the geometric significance of this result? Also why is ...
1
vote
1answer
300 views
Feynman Rules for massive vector boson interactions
I am stuck at the beginning of a problem where I am given an interaction term that modifies the regular QED Lagrangian. It involves the interaction between a photon field and a massive vector boson: ...
3
votes
1answer
99 views
QED Commutation Relations Implications
In Brian Hatfield's book on QFT and Strings there is the following quote:
In particular $$ [A_i (x,t), E_j(y,t)] = -i \delta_{ij}\delta(x-y) $$
implies that $$ [A_i(x,t),\nabla \cdot E(y,t)] = ...
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?
...
2
votes
0answers
88 views
The state of Indefinite metric in Quantum Electrodynamics
I faced difficulties to grasp why indefinite metric is introduced from no where in QED, after searching internet I found that this is a problem in QED, because one needs it to preserve theory's ...
1
vote
2answers
106 views
A step in the derivation of the magnetic momentum of the electron in Zee's QFT book
In chapter III.6 of his Quantum Field Theory in a Nutshell, A. Zee sets out
to derive the magnetic moment of an electron in quantum electrodynamics.
He starts by replacing in the Dirac equation the ...
1
vote
2answers
100 views
3
votes
1answer
204 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 ?
4
votes
2answers
168 views
Why possibility for X-ray to excite inner electrons higher than outer electrons?
It seems X-ray absorption spectroscopy is usually ascribed to the interation between photons and inner electrons. Does it mean inner electrons are much preferred by X-ray photons to outer electrons? ...
6
votes
3answers
261 views
What is the massless limit of massive electromagnetism?
Consider electromagnetism, an abelian gauge theory, with a massive photon. Is the massless limit equal to electromagnetism? What does it happen at the quantum level with the extra degree of freedom? ...
3
votes
1answer
217 views
What's the role of field equation in QFT?
For free field theory, it seems the solutions of a field equation are used to give a representation of Poincare group, and the field equation is still satisfied after quantization. However for an ...
1
vote
0answers
81 views
Is there a simple explanation for Schwinger's relation g=2+alpha/pi for the g-factor of the electron?
Schwinger has on his grave (it seems) the relation between the g-factor of the electron and the fine structure constant:
$g = 2 \ + \ \alpha / \pi \ + \ ... $
Did Schwinger or somebody else ever ...
11
votes
2answers
263 views
What tree-level Feynman diagrams are added to QED if magnetic monopoles exist?
Are the added diagrams the same as for the $e-\gamma$ interaction, but with "$e$" replaced by "monopole"? If so, is the force between two magnetic monopoles described by the same virtual ...
3
votes
2answers
156 views
Radiative Corrections and Bremsstrahlung
I am having trouble understanding why it is consistent to include "Breamsstrahlung" diagrams in computations of scattering amplitudes.
For example, consider the scattering of two electrons to two ...
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 ...
3
votes
1answer
248 views
QED BRST Symmetry
This is a homework problem that I am confused about because I thought I knew how to solve the problem, but I'm not getting the result I should. I'll simply write the problem verbatim:
"Consider QED ...
2
votes
1answer
81 views
Is there a point interaction model of the electron?
Is there a point interaction model of the electron? Is there a point interaction model of the electron? I imagine something like $\propto(\bar \psi\psi)^2$ (edited). Is such a thing in use?
Since I ...
3
votes
3answers
90 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 ...
1
vote
3answers
200 views
EM field quantization
I'm trying to quantize the electromagnetic field by solving the vector potential wave equation, that is:
$$\nabla^{2} \mathbf{A} = \dfrac{1}{c^{2}} \dfrac{\partial ^{2} \mathbf{A}}{\partial t^{2}}, ...
0
votes
2answers
236 views
Anomalous magnetic moment of electron
It is known that the value of 2 of the electron g-factor arises from the Dirac equation. As far as I can see from the various sources, this value is obtained in non-relativistic limit, in particular ...
5
votes
1answer
212 views
Can a photon see ghosts?
Does it make sense to introduce Faddeev–Popov ghost fields for abelian gauge field theories?
Wikipedia says the coupling term in the Lagrangian "doesn't have any effect", but I don't really know ...
1
vote
0answers
77 views
Can we use only the observables of Fermion fields?
There are legion ways to consider fermionic Dirac spinor fields, but is it possible to consider the asymptotic free field only in terms of observables, which in the case of the Dirac spinor field must ...
11
votes
3answers
630 views
Simple (but wrong) argument for the generality of positive beta-functions
In the introduction (page 5) of Supersymmetry and String Theory: Beyond the Standard Model by Michael Dine (Amazon, Google), he says
(Traditionally it was known that)
the interactions of ...
10
votes
4answers
728 views
QM and Renormalization (layman)
I was reading Michio Kaku's Beyond Einstein. In it, I think, he explains that when physicsts treat a particle as a geometric point they end up with infinity when calculating the strength of the ...
0
votes
0answers
266 views
Is there any relation between quantum mechanics and human subconscious mind? [closed]
I came across so many articles, books and blogs that there is connection between our subconscious mind and quantum mechanics. Quantum theory tell about relation between two electrons irrespective of ...
8
votes
1answer
54 views
Relativistic corrections to quantum mechanics of Coloumb potential
Systems of charged particles (such as atomic nuclei and electrons) can be described by nonrelativistic quantum mechanics with the Coloumb interaction potential. A fully relativistic description is ...
3
votes
0answers
151 views
Transition of electric charge to “magnetic charge” when $\alpha$ gets >> 1 in QED?
I`ve just learned that electrically charged particles and magnetically charged monopoles in QED are S-dual to each other such that it depends on the value of the fine structure constant which of the ...
9
votes
4answers
323 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
0answers
78 views
Can the Lamb shift be expressed in more-or-less closed form in terms of the renormalized 2-, 3-,…,n-point VEVs of QED?
I see here that there are three contributions to the Lamb shift, from vacuum polarization (-27 MHz), from electron mass renormalization(+1017 MHz), and from the anomalous magnetic moment (+68 MHz).
...
2
votes
1answer
444 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 ...
4
votes
0answers
231 views
What is the 2-point correlation function of the electron field in QED?
The Feynman propagator for the free electron field is the Fourier transform w.r.t. $y$ of the time-ordered 2-point VEV $\left<0\right|\mathcal{T}[\hat\psi(x)\hat\psi(x+y)]\left|0\right>$, taking ...
2
votes
1answer
440 views
Hyperfine structure vs Lamb shift in the hydrogen atom
The hyperfine structure of the energy levels of the hydrogen atom refers to the shifts in the evergy levels due to the magnetic moments of the nucleus and of the electron. This is an effect of ...
4
votes
2answers
596 views
What's the relation between virtual photons and electromagnetic potentials?
Given that:
1) virtual photons mediate the electric and magnetic force fields
2) the magnetic field is the curl of the magnetic vector potential
3) the electric field is the negative gradient of ...
6
votes
1answer
187 views
How to quantize the free electro-magnetic field in 2d?
I am wondering how one can quantize the free electro-magnetic field
in the two dimensional space-time. The standard method of fixing the
Coulomb gauge in 4d does not seem to generalize immediately to ...
8
votes
3answers
392 views
Why muonium is unstable?
This question is closely related to my previous question Bound states in QED.
Muonium is a system of electron and anti-muon. This article in wikipedia claims that muonium is unstable.
QUESTION: Why ...
8
votes
2answers
733 views
Bound states in QED
I am a beginner in QED and QFT.
What is known (or expected to be) about bound states in QED? As far as I understand,
in non-relativistic QM electron and positron can form a bound state. Should it be ...
8
votes
2answers
321 views
Quantizing EM field
Why when we quantize EM field, whe quantize the vector potential $A^\mu$ obtaining vectorial particles (photons) like the elastic field (phonons) and we can't quantize directly the EM-field tensor ...
1
vote
1answer
431 views
What is the value of the fine structure constant at Planck energy?
At low energy, 511 keV, the value of the fine structure constant is 1/137.03599...
At Planck energy $\sqrt{\frac{\hbar c^5}{G}}$, or 1.956 $\times$ 109 Joule, or 1.22 $\times$ 1028 eV, it has a ...
