Questions tagged [photons]

The photon is the quantum of the electromagnetic four-potential, and therefore the massless bosonic particle associated with the electromagnetic force, commonly also called the "particle of light". Use this tag for questions about the quantum-mechanical understanding of light and/or electromagnetic interactions.

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Understanding the virtual states referenced in multiphoton absorption studies

The Heisenberg energy-time uncertainty tells us that we can have so-called virtual states between eigenstates as long as the lifetime of these states is at most: $\tau = (\frac{h}{4 \pi E_v})$ Where $...
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How many orders of magnitude in energy spans the Standard Model phenomenological spectrum?

I am wondering if it makes sense to state that the upper limit is roughly 1012 eV (up to know the physics probed by the LHC seems to be pretty consistent with the SM) and the lower one is ... the ...
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Photons and Electric Current

I am trying to understand how photons as the force carrier for the electromagnetic force (or field) manifest themselves in a flow of electric charge, i.e. an electric current. Both standard (...
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Photon pumping in Laser

Let's consider a ring laser where the laser must pass through the gain material before it is sent toward a partially reflective surface $\ R=1-T $. The other mirrors are perfect reflectors with $\ R_1=...
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Spatial wave-function of a single photon and its measurement

In the last decade there were several papers claiming that they've measured a "transverse quantum state" / "quantum wave-function" / "spatial Wigner function" of a single photon: Measurement of the ...
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Math behind photon stopwatch path integral in Feynman's QED book

I want to connect the math and the exact concepts hidden behind the simplified picture of photon propagation provided in Feynman's QED book. Background Feynman's photon stopwatches In his book QED - ...
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Does the Euler-Heisenberg Lagrangian lead to attractive or repulsive interaction between photons?

Photon - photon scattering in QED at low energies can be accurately described by the Euler-Heisenberg effective Lagrangian. This only involves the photons (naturally) because the electron has been ...
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What would happen to 2 separate photons on an uninterrupted path

So I am a middle schooler and from what i understand, photons can interact with surrounding particle via gravity. If this is true, would 2 photons on an uninterrupted and completely isolated path ...
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Electroweak to Electro/Weak Bosons?

I apologise in advance, this is something I just can't seem to get my head around. So it's my understanding that before the electroweak force split, there were four bosons - $W^1$, $W^2$, $W^3$ and $B$...
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Two photons or twice one photon?

Consider two photons emitted in two different modes $l$ and $l'$ (for instance by the annihilation of an electron and a positron), such that the initial state of the system is $\left|\psi\right\rangle ...
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Scalar photon states

I don't really understand what a scalar photon physically is. I know that in the Coulomb gauge the normal mode expansion of potential $A^{\mu}$ is given by the two transverse polarization, which ...
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Angular momentum of the Photon

I am reading the QED book by Landau and Lifshitz and I am having a confusion with the angular momentum of a photon. In the book, they show that the total angular momentum $j$ cannot be zero and takes ...
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Does the entropy of the universe change as expansion exceeds the speed of light?

The potential encoded information in a photon that is at the edge of the observable universe would seem to be lost as the universe expands. Does that loss of information contribute to the overall ...
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Smallest minimum feature size possible for transistors printed by photolithography

I'm doing a project about Moore's Law, one of the subtopics I've come to is photolithography. The way I understand it is that the MOSFET transistors are currently printed on a silicon wafer by ...
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Why is the speed of light in vacuum frequency-independent?

I recently came across this paper: Mainland & Mulligan, Foundations of Physics 50(5), 457–480, "Polarization of Vacuum Fluctuations: Source of the Vacuum Permittivity and Speed of Light" ...
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Which experiment can differentiate a statistical mixture from a superposition state?

I am in trouble with polarization and entanglement. Let's consider three cases : Case 1) : Statistical mixture of $|H\rangle$ and $|V\rangle$ polarized photons Case 2) : Photons in a superposition ...
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Do photons have "friction" passing through fluctuating gravitational waves?

In the 1998 biography, A Beautiful Mind, by Sylvia Nasar, John Nash is said to have entered Einstein's office to discuss his ideas that photons have friction. Nash had an idea about "gravity, ...
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Is the intrinsic parity the only contribution to parity for single-particle systems?

In the book A Introductory Course Of Particle Physics by Palash B. Pal, the author claims the following (on page 167, section 6.4.3) For single-particle systems, intrinsic parity is the only ...
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Doubt about the working principle of a silicon solar cell

I'm studying crystalline solar cell and their working principle, but I still don't completely understand the process. First of all, I understand that we dope a pure crystalline silicon with boron and ...
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Is it possible to prepare cat states of light over different frequencies?

Is it possible to form a superposition state of a single photon in multiple frequencies? As an archetypal example, consider the Schrödinger's-cat state $$ |\psi\rangle = |1;\omega\rangle + |1;2\omega\...
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What is the refractive index of an electron?

Consider a free electron or electron bunch, would it have a corresponding refractive index? At low or high energies, the effects are obviously much different. I am curious to know (I haven't found) ...
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On why in transparent media light speed is altered by elastic scattering of photons and their path lengths

I have read this: https://en.wikipedia.org/wiki/Higgs_boson Where it says: "Symmetry breaking in optics In a vacuum, the light of all colours (or photons of all wavelengths) travels at the ...
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Convergence of light by light scattering amplitude

Perhaps I'm too exhausted to see the answer of why the photon-photon scattering should contain no divergences. In Peskin and Schroeder page 320 we find that because of the Ward identity the photon-...
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A quantum mechanical description of a polarizing filter

When a single photon with polarization $\mathbf{a}$ arrives at a linear polarizing filter in the direction $\mathbf{p}$, the photon has a probability of $(\mathbf{a}\cdot\mathbf{p})^2$ to pass through ...
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Optical Bloch oscillation

I have a doubt about how the optical Bloch oscillations happen in a 1D photonic crystal. I try to explain: in a photonic crystal with discrete translational symmetry in one direction I superimpose a ...
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The nature of the probability distribution for the energy of a photon released via stimulated emission

The vanilla description of stimulated emission (e.g. in the context of an inverted population laser gain medium) says that a photon with some state vector specifying its energy / polarization / ...
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What is the Festina Lente Regime?

I am studying BEC formation without evaporative cooling, so realized only thrpugh optical means. One of the problems to face is "photon reabsorption": an atom absorbs a photon of the laser ...
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Do photons form bound states?

Since photons do not have electric charge, they do not interact via the electromagnetic force. They are also reported to be devoid of any colour-charge, so interaction via the strong force is ruled ...
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How is magnetic flux viewed in QED?

I know that QED is formulated in terms of Lagrangian density. So when we solve for a physical system we ultimately ends up finding $\psi$ and $A_{\mu}$. I'm not interested in $\psi$ for now. Since I ...
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Time-of-flight vs. which-path information for single photon interference

I wonder how different path length and therefore different times required for each path lead to interference. Wouldn't it be in principle be possible to tell which path was taken by looking carefully ...
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Which Particle would win the race?

I have read this question: Neutrinos vs. Photons: Who wins the race across the galaxy? https://en.wikipedia.org/wiki/Neutrino And it made me curious. The wiki article says that neutrinos travel at ...
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How can we calculate the number of photons in the Universe, yet a state of defined photon number has zero electric field?

The expectation value of the electric field of a state with defined photon number is zero, so any electromagnetic wave (with non-zero expectation value) must be in a superposition state of definite ...
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Inverse square law and the photon mass

I have seen many questions about this, but haven't really found the answer I was looking for. I was caught by an exercise that said: Consider two charges: $q$ (a test charge) and $Q$, at $\vec{R}...
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Photons gravitational interaction

Let's imagine this kind of experiment. We have ideal optical fiber circle (or ideal ring laser cavity without any losses, i.e. all the mirrors have 100% reflection rate) where light can go infinitely ...
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How frequently do accelerating electrons emit photons?

In the quantum 2-slit experiment with light, we know that as the source intensity is turned down, individual photons arrive at the detector. I would like to know if there is a formula which tells us ...
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Photons when passing through matter

I am wondering whether there is a simple—or complicated—way to explain from a purely quantum mechanics (or QFT) viewpoint what happens to the photons when they go through some material whose index of ...
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Photon Gas Stress-Energy Tensor

In standard texts it is typically discussed how one can obtain the stress energy tensor of a perfect fluid, in both coordinate-dependent and coordinate independent forms: \begin{equation} \mathbf{T}=(\...
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Rate of interaction of free electron with photons from sunlight

How many photons does a free electron (in sunlight say) interact with per second? I did a rough calculation assuming the electron interacts with any photon that enters through an area the size of ...
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Sound waves and chromism?

I am definitely not a physicist, but I have a question related to Physics so I thought I'd give it a try. Please excuse my ignorance. :) I am somewhat familiar with the changes in color caused by ...
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Is this "classical model of the photon" real?

I do not know how to phrase the question better but this article claims to have a classical model of the photon. Has this (rather extraordinary) claim been verified? Here is an extract from the link ...
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How do we show that photons generated by a constant electric current are distributed according to a Poisson distribution?

I saw the answer sometimes ago in a book "Quantum Electronics" or similar title. I don't remember the author since I lost the book. The book sets ( I believe so ) a constant electric current $I$ in a ...
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How does one calculate the quantum propagator for a massless photon

So I want to calculate the quantum massless photon propagator. To do this, I write $$ A_\mu(x) = \sum\limits_{i=1}^2 \int \frac{d^3p}{(2\pi)^3} \frac{1}{\sqrt{2\omega_p}} \left( \epsilon_\mu^i (p) a_{...
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Maxwell's Demon - laser cooling

There’s an interesting article on Scientific American that tells how to cool individual atoms to within a very tiny fraction of Absolute Zero. It uses two laser beams acting on a very cold rarified ...
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3 votes
1 answer
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An accelerating charge emits EM waves, but how can this be explained in terms of photons?

I was reading this response to a question involving EM radiation due to an accelerating charge. A charge's oscillations disturb its electric field, and this effect propogates at the speed of light. If ...
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Relationship between oscillator strength and cross-section

In the context of absorption of photons by atoms, I have come across two seemingly very related quantities, cross section and oscillator strength. In the book Physics of the Interstellar and ...
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Are the phases of entangled photons correlated?

I know the polarization of entangled photons are correlated, but are the phases of the wavelengths correlated as well?
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How is the direction of the electric field encoded in the QFT of photons?

Take a dipole receiving antenna(or visible light polarization filter), the receiving electrons are accelerated perpendicular to the incoming photons. How is this encoded in QFT? From wikipedia it ...
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Relativistic ship and base on Earth talking

Let say humans make it possible to travel so close to the speed of light that time dilation for the ship can achieve 100 times the time of a standing still object. So one of that ships starts its ...
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Numerical calculation of spherical photon orbits near Kerr black hole

I'm using RK4 method for solving differential equation $$\frac{d\theta}{d\phi}=\pm(2-2r)\frac{\sqrt{Q-\left(\frac{\Phi^2}{\sin^2\theta}-1\right)\cos^2}\theta}{2r+\left(r^2+\cos^2\theta-2r\right)\frac{\...
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Would a black hole act as a particle accelerator between the photon sphere and the event horizon?

I'm trying to understand the behavior of photons between the photon sphere and the event horizon. Here is what I think I understand so far : the radius of the PS is 1.5 times the Schwarzschild radius ...
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