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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Does the $\frac{4}{3}$ problem of classical electromagnetism remain in quantum mechanics?
In Volume II Chapter $28$ of the Feymann Lectures on Physics, Feynman discusses the infamous $\frac43$ problem of classical electromagnetism. Suppose you have a charged particle of radius $a$ and ...
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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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Why did Feynman's thesis almost work?
A bit of background helps frame this question. The question itself is in the last sentence.
For his PhD thesis, Richard Feynman and his thesis adviser John Archibald Wheeler devised an astonishingly ...
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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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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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Why don't electron-positron collisions release infinite energy?
Questions of the form:
An electron and a positron collide with E MeV of energy, what is the frequency of the photons released.
quite often come up in my A Level course (for often fairly arbitrary ...
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Physicists adding 3 decimals to the fine structure constant is a big accomplishment. Why?
Yesterday, a team of physicists from France announced a breakthrough in nailing down a "magic number" by adding three decimals to the the fine-structure constant (news article; technical ...
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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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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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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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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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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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Do Maxwell's equation describe a single photon or an infinite number of photons?
The paper Gloge, Marcuse 1969: Formal Quantum Theory of Light Rays starts with the sentence
Maxwell's theory can be considered as the quantum theory of a single
photon and geometrical optics as ...
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Why have our eyes not evolved to see "gluons"? [closed]
The photons are the propagators for QED, and we rely on photons to see the world around us.
The gluon is the propagator in QCD. Why have our eyes not evolved to see gluons (either on top of being ...
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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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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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Is light only emitted by atoms? i.e are they the only source of light in the universe?
This is a very trivial question I suppose.
But, I dont remember any other sources of light mentioned anywhere other than the atoms emitting radiation due to electrons changing energy levels.
So, are ...
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Feynman couldn't explain how the photon becomes instantaneously aware of the glass thickness. Do we have a better understanding now?
I remember reading in Richard Feynman's QED about this unknown physics mechanism which possibly involves information propagating instantly and it blew my mind:
The probability of photon to reflect or ...
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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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Why don't photons split up into multiple lower energy versions of themselves?
A photon could spontaneously split up into two or more versions of itself and all the conservation laws I'm aware of would not be violated by this process. (I think.) I've given this some thought, and ...
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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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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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The path integral and Feynman diagrams
This question is somewhat of a historical one, but it also contains some physics. I am curious to find how exactly the concept of Feynman diagrams arose (I assume from Feynman's path integral)?
The ...
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The divergence in QCD Series-- How many are they, and what do they mean?
I am referring to this question, and especially this answer.
In addition, QCD has - like all field theories - only an asymptotic
perturbation series, which means that the series itself will also
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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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Is there a strong force analog to magnetic fields?
In special relativity, magnetism can be re-interpreted as an aspect of how electric charges interact when viewed from different inertial frames.
Color charge is more complex than electric charge, but ...
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Why does light travel in a straight line if the uncertainty principle is true?
I've asked this on different websites and never gotten an answer that a layperson can understand. Most people just say that light does not have a trajectory and then they do some hand waving. If light ...
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What are the dimensions, width and length, of a photon?
Everyone is always talking about photon's wavelength. But what about its dimensions?
What is length and width of it?
And does it even have a point to think about such things? Or those dimensions are ...
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How can my window not scramble the image of my yard?
How can an image pass through a window if the atoms in the glass randomly emit photons in any direction?
I've read that glass is transparent because the atoms don't readily adsorb visible light, so it ...
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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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Stimulated Emission in QED
The explanations of stimulated emission which I have found all describe the phenomenon in terms of non-relativistic quantum mechanics. How might you describe it in a field theory such as QED? In ...
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Why do we use potential for quantizing the electromagnetic field?
For quantizing the electromagnetic field authors go to its potential and then find themselves facing to the problems of degree of freedom from gauge transformation.
Why we can't simply quantize ...
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Why don't very high order Feynman diagrams contribute significantly?
In a particle physics lecture I had today it was stated that the magnetic moment, $g$, is not quite equal to 2, and the difference is accounted for by QED.
Later it was stated that we can see this ...
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Are photons actually particles at all?
I just read this answer to "What exactly is a Photon?" which has me a bit confused. It seems to be arguing that "photon" is just a catch-all term for any sort of interaction with ...
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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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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 Feb ...
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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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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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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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Interpretation of Dirac Spinor components in Chiral Representation?
I failed to find any book or pdf that explains clearly how we can interpret the different components of a Dirac spinor in the chiral representation and I'm starting to get somewhat desperate. This is ...
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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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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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Faddeev-Popov Gauge-Fixing in Electromagnetism
Reading section 9.4 in Peskin, I am wondering about the following:
The functional integral on $A_{\mu}$ diverges for pure-gauge configurations, because for those configurations, the action is zero.
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Magnetic monopole and electromagnetic field quantization procedure [duplicate]
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 ...
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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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Can an observable be invariant under local $U(1)$ but not under global $U(1)$?
Consider a quantum field theory with two fields, a complex scalar field $\phi$ and a $U(1)$ gauge field $A$. Both fields are dynamic fields, not background fields. Suppose that spacetime is ...
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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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