All Questions
Tagged with quantum-electrodynamics photons
339 questions
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What determines how wide are the absorption spectrum lines of atoms?
So let's assume we have atoms in their ground state and we measure what kind of wavelengths those atoms can absorb. These should occur at photon energies that correspond the the energy gaps between ...
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Can photons be seen in a superconductor?
In the video Electromagnetism as a Gauge Theory, Richard Behiel states that electromagnetism is a result of adding local phase symmetry to the Dirac equation. He also says (at 3h9m) that this phase ...
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Pole structure of the photon polarisation tensor
When it comes to the photon polarisation tensor in the book of Peskin & Schroeder on page 246, it is said:
Notice that as long as $\Pi(q^2)$ is regular a $q^2=0$, the exact propagator always has ...
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Atomic off-resonance absorption in a full quantum theory
When studying atomic absorption in time-dependent theory, we learn (see, for example, Griffiths, 1Ed, pg. 307), that the probability of absorption is given by,
$P_{a\rightarrow b}(t) \propto \frac{\...
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Wave-particle duality: interactions of like / different quantum fields
With my pop-sci level of understanding, it seems to me that quantum fields exhibit particle-like properties only when interacting with a different quantum field - i.e. electromagnetic field interacts ...
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What is the proper ansatz for describing an electron-photon many-particle System?
I am somewhat used to simplified non-relativistic quantum mechanics (both canonically and grand canonically), describing a system by a Hamiltonian containing a kinetic part, an external potential as ...
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Questions about QED Corrections to Coulomb's law and Electromagnetic Wave Equation
As a disclaimer, this is somewhat similar to this unanswered question, but not entirely.
In standard QED theory, it is frequently demonstrated that the derivation of the Coulomb Potential can be found ...
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Photon Mass Regulator in IR divergences
On Schwartz's QFT page 333, he metions that there is infrared divergence when we try to renormalized the two-point function of electron field in on-shell substraction scheme,
$$\frac{d}{d\,p_{\mu}\...
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Link between photon helicity and polarization of $A^\mu$ electromagnetic potential
From Wigner theorem we know that the irreducible unitary representation of the Poincarè group for massless and spin 1 particle is labelled by the momentum $p_\mu$ and the two possible helicity $+1,-1$ ...
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How is light interference explained with photons?
In the classical model of light as an EM wave, interference is a trivial consequence of the linearity of the wave equation. Now, if we model light as collections of photons, how is light interference ...
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Destructive interference pattern perpendicular to photon's propagation direction
I'm studying the interference pattern for light. For example, the following is the interference pattern for double slit experiment:
I'm curious to know if there is an existing interference pattern ...
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What happens if obstacles (walls) been put where destructive interference occurs in double slit experiment?
If photon doesn't have probability to be in dark (destructive interference) area, what will be the effect of adding obstacles (walls) in the dark (destructive interference) area for the double slit ...
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Spinor-helicity formalism: relationship between 1 and 2 reference vector setups
The spinor-helicity formalism is usually set up so that for a massless vector boson (photon or gluon) with momentum $k$ an arbitrary reference momentum $p$ is introduced and the corresponding ...
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Feynman trajectories of a photon from entangled pair
In a pair of Entangled photons does any of the two photons has the Feynman trajectories of the other one? Must the possible trajectories of photon 2 be taken in the Feynman's path integral for the ...
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How does a photon look like in QFT? [closed]
It is very well known that in QFT the particles are excitations of the field. But how exactly is a free photon looking like in spacetime? What is it shape in free space? And what is in fact in the ...
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Is Feynman's Compton scattering diagram the same as the one in most books?
Feynman used the following Compton scattering diagram:
Whereas most books use this diagram in their derivation:
Is there a way to relate the two? Feynman's diagram makes more sense to me ...
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Are EM waves telling us the probability of finding a photon?
I feel like I've been frequently presented with an interpretation of EM waves that goes something like this:
Light is an oscillating electromagnetic field. Because changes in the electric field ...
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Why do I find different polarization vector completeness relations?
I am currently working on computing the cross section of Compton scattering in QED and in the process need to evaluate an expression of the following form:
$$ \sum_{\lambda}^{} \epsilon^{\mu} (\...
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Pushing solid objects by photon exchange
As I understood it,
the reason I cannot stick my hand through a metal block is due to the repelling force between electrons in my hand and in the block. QED depicts two electrons repelling with a ...
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How to distinguish Coulomb and radiative parts of the electromagnetic field?
In classical electromagnetism, the EM field can be described by a 4-potential $A^\mu$. This potential describes two different phenomena.
Static fields: A static charge $q$ at fixed position $r_0$ ...
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Can there be an electron without photons?
Can the electron field be excited while the photon field is not? I'm guessing the answer is no, because electrons are supposed to interact with their own electric field. I don't know about ...
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Physical interpretation of photon propagator
Physically, propagator represents the probability amplitude of a particle to travel from one point to another. But the photon propagator $$D_{\mu\nu}(x,y) = \langle 0 | \mathcal{T}[A_\mu(x) A_\nu(y)] ...
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Connection between photons and EM waves
We learn about electric and magnetic fields and how they conform EM waves. Then we discover the photon and how there was a duality between this two ideas, sometimes radiation behaved like a wave and ...
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What causes the direction of a photon?
I wondered if protons or any charged particle could generate photons and I found this wonderful answer that says yes: Does shaking an atom produce photons?
The issue now is that the more I thought ...
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Broken symmetry and three-photon vertex
I know that loop-level three-photon vertex in QED is zero since the contribution from fermion and antifermion cancel each other.
Also, from what I know this has something to do with gauge invariance ...
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The enigma of photon behavior, photon spatial co-inhabitance
The notion that multiple photons can occupy the same spatial coordinates seems perplexing. How is this experimentally validated, considering the intricate challenges and oddities it presents?
Imagine ...
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Probability amplitudes in Richard Feynman’s QED [duplicate]
So i’ve been reading Richard Feyman’s book, QED, and in it, he simplifies the idea of how physicists calculate the probability of a photon hitting a certain detector. He lets the magnitude of a vector ...
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Why does light interact with normal matter but not with other light?
Why does light interact with normal matter but not with other light?
Assumptions:
Light does not interact with other light at all.
Light does interact with other matter, i.e reflection/refraction.
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Why would energy conservation be violated during interference if a wave function represented multiple photons?
the below paragraph is from Paul Dirac's 'The principles of quantum mechanics'. He argues that representing multiple photons' probability distribution via a single wavefunction leads to energy ...
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QED photon path (direction of photon emission)
In QED we look at all possible path a photon could go from S to P, and I understand the most significant contributions to the final arrow are the few near straight paths connecting S and P while other ...
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Inverse square law of a photon in QED
So in Feynman's QED book strange theory of light and matter, he mentioned as a photon travels, it spreads a little, thus the "arrow" shrinks inversely with distance, and that is the inverse ...
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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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dB of squeezed light is equivalent to how much squeezing in different directions?
In many structures, the amount of squeezed light is expressed in dB. I wanted to know what is meant by squeezed light in dB.
For example, 10 dB of squeezed light is equivalent to how much squeezing in ...
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Electrostatics as the infrared limit of a "sea of photons"
Textbooks giving an intro to field theory (and sometimes advanced quantum mechanics) often casually remark (in chapter one or two) that ordinary Coulomb scattering, or that classical electrostatics, ...
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(How) Can one construct a Dirac equation analogue for photons?
So, the dirac equation exists
$$\hat{H} = -i\hbar c(\partial_x \gamma_1 + \partial_y \gamma_2 + \partial_z \gamma_3) + m c^2 \gamma_0$$
and describes spin 1/2 particles, and can be generalized to ...
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Do free electrons always interact with any photon they 'contact'?
Do free electrons (plenty of them in space and in stars) scatter every photon they come across?
Free electrons, unlike those in atoms and molecules, don't have specific energy levels after all...
Isn'...
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Is the total number of photons in a cavity meaningless?
The gran-canonical partition function for an ideal gas of bosons can be written as
$$\mathcal{Z}=\prod_{(\vec{r},\vec{p})\in\mathbb{R}^{6}}\sum_{N=0}^\infty e^{-\beta E(\|\vec{p}\|)N+\mu\beta N}=\...
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How can there be uncertainty reflecting the polarization of a photon?
I am aware that many quantum entanglement experiments are based on uncertainties related to the polarization of a single photon, which can be measured in the horizontal/vertical, clockwise/...
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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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How to interpret light and photon?
I've been trying to solidify my understanding on properties of light and this is what my understanding is so far:
Light is an EM wave that travels in a constant speed C.
Light has a quantized energy ...
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Why is it that, when light travels in a medium, we say it's made of "quasiparticles"?
I get why, in this model, light isn't really "made of" photons, because photons, by definition have zero mass and travel at $c$, whereas these quasiparticles, if I understand correctly, do ...
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How soft photons carry away energy
While studying the infrared traces, I came across the following Feynman diagram:
This diagram shows the braking radiation and the radiation correction of a soft photon. There are three vertices in ...
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Can low energy photons in equilibrium with a black body absorber create a high temperature in the absorber?
With enough flux can low energy photons warm a black body to any temperature?
eg a solar furnace can approach the surface temperature of the sun, but could it in theory go to any temperature? If all ...
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QED gauge invariance
On P & S page.297, in the second paragraph from bottom, the book discussed gauge invariance of Faddeev-Popov procedure, following
a QED example. Where the photon propagator is:
$$ \widetilde{D}_F^{...
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Why are gluons color charged but not photon? Could there be a charged EM force carriers like gluons or neutral color charge carrier like photon?
Gluons have a color charge why don't photons have an electric charge like gluons?
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What makes a photon a photon?
As i understand photons are excitation of the electromagnetic field. Therefore charged particles are affected by this excitation. But what if we have (highly theoretically) a particle that has the ...
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Feynman's view about light
Professor Feynman, in his “QED: The strange theory of light and matter”, states at page 15:
“I want to emphasize that light comes in this form - particles. It is very important to know that light ...
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If the photon hasn't an electric and magnetic field yet, then what does its polarization vector stand for?
In the context of quantum electrodynamics, The photon field is described by a operators connected to a Fock space of photon states $e^{-ip_{\mu}x^{\mu}}$.
The relativistic photon wavefunction can be ...
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Is the electric field the same as probability amplitude of a photon?
I am a novice in Quantum Mechanics and have seen many authors interchangeably using the two terms in the introductory textbooks. But I have never seen it written explicitly anywhere 'The probability ...
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Explanation for few things in this paper about photonic crystals
In this paper two weakly coupled cavities are excited with light. Their frequencies are modulated by a mechanical pulse. I have the following questions:
What is the Hamiltonian of this system ? I don'...
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