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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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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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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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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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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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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What's the meaning of chemical potential of photons or phonons?

The chemical potential of photons or phonons vanishes due to their particle number not conserved, I don't understand it very well. So can you expain what's the meaning of chemical potential of them ...
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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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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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290 views

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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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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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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3answers
561 views

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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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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How are photons of the electrostatic and magnetostatic fields different from electromagnetic radiation?

I understand that a photon of electromagnetic energy (light or radio) has both wave and particle characteristics and is best modelled using quantum field theory. I also understand that static and ...
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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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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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In double slit experiment, It's the wave function who represent particles, travelling?

I saw some videos that tend to show traveling waves to represent the photon/electron in a double slit experiment. Is it right? Because the space between the electron gun and the screen where these ...
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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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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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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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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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Non-polarised light

We know the beam of light oscillates in electric field and magnetic field, both perpendicular to both the wave of propagation and each other. What does, however, a non-polarised beam of light look ...
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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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Would the nightsky be bright (filled with starlight) without accelerating space expansion?

As I currently understand, if a photon was emitted from a far away point in the Universe, beyond the event/particle horizon, so that the space inbetween the emitter and us is expanding faster then ...
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Conduction band energy level under illumination

I am new to the field and my apologies if its trivial question. Consider a semiconductor having valence and conduction band at certain energy level. When it is illuminated with photons having ...
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What happens to two singular photons when their pilot waves cancel out?

What happens to two singular photons when their pilot waves cancel out? Are those photons indeed particles and what happen to them in that case?
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Do sidebands corresponds to real photons at that frequency?

Say I have a carrier laser (optical) frequency $\omega_c$: $E=E_0 e^{i\omega_c t}$. I propagate it through an electro-optical modulator that modulates the phase by $\beta \sin\Omega t$: $E = E_0 e^{i\...
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Emissions from a Blackbody and the UV Catastrophe

Recently I found myself becoming confused over a topic that I thought I had previously understood. In a theoretical blackbody which reaches thermal equilibrium the energy absorbed is equal to the ...
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1answer
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contradiction between total photon energy in the universe and early matter antimatter ratio

When looking at the energy count in the Universe, on one hand we are told dark energy 68%, dark matter 27%, regular matter 5%, neutrinos and photons a very small fraction (trace like). https://en....
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Photoelectric effect - the reason for exponential graphs and a misconception about the concept of intensity

We have this basic PASCO Photoelectric Effect equipment in our lab and there are two things I would like to ask about the experiment. We keep either the area apertures or wavelength filters fixed and ...
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1answer
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Order of Magnitude for Photon Induction

What does magnetic induction look like when we imagine just one photon passing by? I'm imagining some sort of coil wrapped around the edge of a waveguide or something so that it picks up some of the ...
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What particle is created when a B- or E-field operator acts on the vacuum

I have some conceptual trouble understanding what the different field operators in QED do. According to Wikipedia, the field operators are given by $\mathbf A(\mathbf{r} )=\sum _{\mathbf {k} ,\mu }{\...
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Light Absorption by Doubly Ionized Helium

Doubly ionized Helium is known to absorb light in contracted phase of cepheid stars. What is the mechanism for photon absorption by an atom that has no electrons to raise to a higher energy level or ...
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Adding helicities

The background: I am looking at Compton scattering in its most general form with $p+\gamma^*\rightarrow p'+\gamma'^*$ in the Breit-frame (which implies that in my case $\vec{p} = -\vec{p}'$). The ...
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Are phase and particle (photon) number in QED conjugated variables?

I found in A. Zee's book "QFT in a nutshell" (1.edition) the interesting relation (8) respectively (9) in chapter III section 5 (p.173) which states that in a collective of non-relativistic bosons the ...
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Calcluating the photon propagator with gauge fixing parameter

I'm trying to calculate the photon propagator via the functional integral, with lagrangian (plus source) $L = -\frac{1}{4}F^{ab}F_{ab} - \frac{\lambda}{2}\left(\partial^aA_a\right)^2 + J^aA_a $ ...

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