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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What is the relation between electromagnetic wave and photon?
At the end of this nice video (https://youtu.be/XiHVe8U5PhU?t=10m27s), she says that electromagnetic wave is a chain reaction of electric and magnetic fields creating each other so the chain of wave ...
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Deriving the Coulomb force equation from the idea of virtual photon exchange?
Since Newton's law of gravitation can be gotten out of Einstein's field equations as an approximation, I was wondering whether the same applies for the electromagnetic force being the exchange of ...
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Explain reflection laws at the atomic level
The "equal angles" law of refection on a flat mirror is a macroscopic phenomenon. To put it in anthropomorphic terms, how do individual photons know the orientation of the mirror so as to bounce off ...
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How are classical optics phenomena explained in QED (Snell's law)?
How is the following classical optics phenomenon explained in quantum electrodynamics?
Reflection and Refraction
Are they simply due to photons being absorbed and re-emitted? How do we get to Snell'...
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Why can't photons have a mass?
Why can't photons have a mass? Could you explain this to me in a short and mathematical way?
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The exchange of photons gives rise to the electromagnetic force [duplicate]
Pardon me for my stubborn classical/semiclassical brain. But I bet I am not the only one finding such description confusing.
If EM force is caused by the exchange of photons, does that mean only when ...
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Virtual photon description of $B$ and $E$ fields
I continue to find it amazing that something as “bulky” and macroscopic as a static magnetic or electric field is actually a manifestation of virtual photons.
So putting on your QFT spectacles, look ...
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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 ...
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Do electrons really perform instantaneous quantum leaps?
This is not a duplicate, non of the answers gives a clear answer and most of the answers contradict.
There are so many questions about this and so many answers, but none of them says clearly if the ...
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*Attractive* Coulomb interaction as virtual particles exchange?
I've been reading about virtual particle exchanges in physics books and in Physics SA posts, where a particle interpretation of gravity and Coulomb interaction is established. The Feynman Diagram ...
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What would the collision of two photons look like?
Could someone explain to me what the collision of two photons would look like? Will they behave like,
Electromagnetic waves: they will interfere with each other and keep their wave nature
Particles: ...
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Relation between radio waves and photons generated by a classical current
Several questions have been posted on Physics SE regarding the relationship between photons and electromagnetic waves, and several good answers have been given. Some of those questions are listed ...
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Spontaneous pair production?
So I've been looking into particle-antiparticle pair production from a gamma ray and don't understand one thing.
Let's say I have a 1,1 MeV photon and it hits a nucleus - electron-positron pair with ...
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What is the origin of the factor of $-1/4$ in the Maxwell Lagrangian?
I have seen numerous 'derivations' of the Maxwell Lagrangian,
$$\mathcal{L} ~=~ -\frac{1}{4}F_{\mu \nu}F^{\mu \nu},$$
but every one has sneakily inserted a factor of $-1/4$ without explaining why. ...
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What exactly is a quantum of light?
I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a photon as a quantum of light, which it further explains as some kind of a wave-packet.
What ...
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How do you visualize a quantized electromagnetic field?
Quantizing the EM field gives you the standard sum of all quantum harmonic oscillators as given by this hamiltonian:
$$
\hat{H} = \sum_k \hbar\omega_k( \hat{a}^\dagger_k \hat{a}_k + 1/2)
$$
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Why doesn't there exist a wave function for a photon whereas it exists for an electron?
A photon is an excitation or a particle created in the electromagnetic field whereas an electron is an excitation or a particle created in the "electron" field, according to second-quantization.
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$1/r^2$ force and "photon decay"
Background: My question is about the interpretation of the $1/r^2$ force in terms of the fundamental processes of the underlying QFT. We know that if the photon had mass $m$, then we may have a "...
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Why are there e.m. vacuüm fluctuations?
According to the rules of qft there are virtual photons in the vacuüm. But how can this be if for the production of photons you need an electric charge?
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How wrong are the classical Maxwell's equations (as compared to QED)?
Now, I don't really mean to say that Maxwell's equations are wrong. I know Maxwell's equations are very accurate when it comes to predicting physical phenomena, but going through high school and now ...
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Quantum description of radio antenna
I am trying to form a clearer picture of how a radio transmitter emits waves from a quantum point of view.
The classical description is quite easy: the electrons oscillate in the antenna and, as ...
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Is it really proper to say Ward identity is a consequence of gauge invariance?
Many (if not all) of the materials I've read claim Ward identity is a consequence of gauge invariance of the theory, while actually their derivations only make use of current conservation $\partial_\...
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Can an electron jump to a higher energy level if the energy is insufficient or exceeds the $\Delta E$?
Let's say we have an atom of hydrogen. It has one electron on $E_1 = -13.6~\mathrm{ eV} ~~(E_2 = -3.4~\mathrm{eV})$ energy level. I know that if we fire a photon with 10.2 eV energy the hydrogen atom ...
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How many photons are needed to make a light wave?
What is the smallest number of photons needed to make a "light wave"?
In other words, how many (coherent?) photons start to exhibit classical behavior?
For example, how many photons are needed to get ...
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Can a light be bent by a magnetic field?
I'm struck with two competing ideas on the question in the title.
Listing #1:
How far can a magnetic field bend light?
A: Unfortunately, the path light takes is not affected by the presence of a ...
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Is there a "difference" between photons that act as virtual particles and photons that act as the quanta of EM radiation?
I) I know that virtual-photons are known to be the force-carriers for the Electromagnetic force, and that they are called "virtual" because the Energy-Time-inequality version of the Heisenberg ...
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Spin of photons and angular momentum of classical radiation field. Are they connected?
Photons, the quanta of the radiation field, carry spin $-$ an intrinsic angular momentum. Classical radiation also has angular momentum having the expression $$\vec{J}=\frac{1}{\mu_0c^2}\int dV \left(\...
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Do composite particles that are electrically neutral but have charged constituents radiate?
For example an electron radiates when accelerated. So does a positron. But is the radiation emitted by accelerated positronium the sum of the radiation emitted by each separately? If not, why not? If ...
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Principle of Reflection on atomic level
This well-observed phenomenon has, besides several others, always been a fascination to me. We are well aware of several theories, experiments, and practical applications of this well-known phenomenon,...
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Why electrons can't radiate in their atoms' orbits?
It's an old-new question (I found only one similar question with unsatisfactory (for me) answer: Where did Schrödinger solve the radiating problem of Bohr's model?)
It's strange for me how all books ...
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How do we know photons have spin 1?
Electrons have spin 1/2, and as they are charged, they also have an associated magnetic moment, which can be measured by an electron beam splitting up in an inhomogeneous magnetic field or through the ...
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How are bound states handled in QFT?
QFT seems very well suited to handle scattering amplitudes between particles represented by the fields in the Lagrangian. But what if you want to know something about a bound state without including ...
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How does light know which path is fastest?
We know from Fermat's principle of least time that light follows the fastest path. But how does light know which path is the fastest?
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Using photons to explain electrostatic force [duplicate]
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 $...
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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 ...
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How can an asymptotic expansion give an extremely accurate prediction, 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 ...
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Why can't a real scalar couple to the electromagnetic field?
If we have a complex scalar $\phi$ we know that the gauge-invariant interaction with $A$ is given by $A^\mu J_\mu$, where $J$ is the Noether current of the $U(1)$ symmetry of the Lagrangian
$$
J_\mu\...
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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 ...
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What are quantum fluctuations, really?
So, I have read a lot about quantum fluctuations, and how they are responsible for:
Lamb shift
Spontaneous emission of photons from excited atomic states
Casimir effect
...
and the explanation ...
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Are atoms unstable in $d\geq 4$ spatial dimensions when quantum mechanics is taken into account?
I understand that in 3+1 dimensions according to classical physics atoms should be unstable however atoms are stable in 3+1 dimensions because the behavior of atoms is governed by quantum physics ...
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What will happen if the photon has non-zero mass?
I want to know the theoretical implication if photons have a non-zero mass.
What happens to the Maxwell equations?
What happens to QFT?
If the photon have mass it can decade?
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Transparency of solids using bandgaps and relation to conduction and valence bands
I think I understand how a solid can appear transparent as long as the energy of the photons travelling through it are not absorbed in the material's bandgap. But how does this band gap relate to ...
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Conservation of BRST current in QED
I am trying to understand the conservation of the BRST current in QED but am having some trouble. This is what I have so far, QED lagrangian density in Lorenz gauge is,
$$L = \frac{1}{4}F_{\mu\nu}F^{\...
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If the electron is point like, then what is the significance of the classical radius of the electron?
What is the physical meaning/significance of the classical radius of the electron if we know from experiments that the electron is point like?
Is there similarly a classical radius of the photon? The ...
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Why does a magnetic field "curl" around a current carrying element? [duplicate]
From all the texts I've read, it's always stated that the magnetic field would either curl or warp around the the current flowing within a conductive element, yet, I never was clarified as to why that ...
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Virtual Photon in Electron Scattering Feynman diagram [duplicate]
If we know that the virtual photon emitted in an electron scattering Feynman diagram violates the energy and momentum conservation laws (though temporarily), why do we accept it as a feasible diagram ...
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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 ...
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Can a photon get emitted without a receiver?
It is generally agreed upon that electromagnetic waves from an emitter do not have to connect to a receiver, but how can we be sure this is a fact? The problem is that we can never observe non-...
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Is it possible to confine a photon in less than its* wavelength?
*(Its, or associated. That is somehow the question).
I can think of, at least in principle, a perfectly reflecting optical cavity with dimension comparable to the wavelength of the electromagnetic ...
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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 ...
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