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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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Another Quantum Eraser Question

What is mysterious about the so called quantum eraser? A typical double slit experiment produces an interference pattern on the screen. In a double slit quantum eraser experiment there are ...
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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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What holds together an electron when it leaves valence? [on hold]

Why doesn't it release all the photons in one shot? I am referring to both in the cathode ray tube and also in the vacuum of space.
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Is it possible to “override” the electromagnetic field from an MRI scan? And how do you do it? [on hold]

I know electro conductors lose their charge if you heat them. Is there a better way to negate it and how?
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What is the term for a particle spin's uncollapsed position? What is the orbiting “thing”?

I'm not sure if I have the correct visual model, but I imagine that a particle spin can be represented by a single point on the orbit, or by a superposition state (like a random plane through a corner ...
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What's the opposite of spin collapse? Superposition as a verb?

With regard to photon spin, I'm trying to figure out what the word is for being "more random" as opposed to collapsing and being "more determined" If I were to say "the spin collapsed", how would I ...
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What direction would this photon go?

Say a matter-antimatter pair of particles annihilates linearly (from a reference frame where the sum of the momenta is zero) to create a photon. Wouldn't momentum conservation prevent the photon from ...
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What is the principal cause of diffraction?

I have found another question similar to mine here. But I want to know why does diffraction of light happen in the first place. I have found other resources on google which explain the topic partially ...
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Energy of photon and charge [on hold]

What is the significance if any of the common energy value of the electron volt $(1.602 \times 10^{-19}$ joules), and the energy of a photon of wavelength $ \sim 1230$ nm (from the formula $E=h \nu$)?
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Can light radiation pressure change its own path

Well i have seen that light creates radiation pressure Can two light waves coming from two different direction change each others path?
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The chicken-egg nature of bending a photons pathway

I'm a chemist not a physicist, so I have a loose relationship with its laws. I read that light (specifically a photon) doesn't have mass, gravity doesn't act on a photon to alter its trajectory. ...
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Can a photon circle a black hole indefinitely?

Does a photon lose energy or redshift as it circles? Will it's wavelength be the rest wavelength with centripetal and gravitational forces exactly cancelling?
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Could photons decay into Dark Matter? [closed]

So, we can "see" 13.7 billion (intentionally not including expansion) light years in all directions and we "see" a red shift. What if, the reason we can't observe photons beyond this limit is that ...
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Does it give sense to speak about field distribution of a single photon?

A Point source of light has radial symmetry. If the source gets attenuated so that only a single photon is leaving each hour, can I still argue, that the field of the single photon is radial but the ...
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Can an electron pass through a gap smaller than an electron?

If an electron (photon) can pass through a gap smaller than an electron (photon) then it is a wave otherwise it it a particle. Is this a correct way of reasoning?
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If photons have no mass, why do they gain mass in photon entanglement?

Photons are massless particles. However, this article states that photons can gain mass when they become entangled. How can this happen? From the article: Physicists create new form of light ...
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How would Planck's formula be useful if there's a multiphoton interaction?

We all know Planck's formula is : $E=nhf$. But in deriving the Rydberg Equation and Einstein's Photoelectric Effect $n$ is assumed as $1$. https://en.m.wikipedia.org/wiki/Photoelectric_effect So ...
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How to experimentally measure spin of photon?

Given that we know the spin of photon should be +1 or - 1, is there any experiment to directly measure and confirm it? I looked up stern-gerlach experiment is used on charged particle not photon.
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Why can a particle decay into two photons but not one?

I recently read an old physics news about the Higgs boson where it was observed to decay into 2 photons and I was wondering why it wouldn't have decayed into a single photon with the combined energy ...
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Redshift of decoupled photons

How many years was passed after the photon decoupling when the wavelength of the caused by decoupling visible light redshifted out of visible one?
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How Does Gravitational force work on Light? [duplicate]

m'=$m_0/\sqrt{1 -v^2/c^2}$ How Will the gravitational force work on Light ?
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What is the interpretation of a photon in quantum mechanics? [duplicate]

Without considering the absorption and emission, can we define a wavefunction for a photon? If so, can we ever have monochromatic photon? Or is it equivalent to the free particle? In that case, would ...
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number density of photons and the total energy of the universe

I was reading over some assignment solutions that my lecturer put up and one of them said that instead of calculating the number of photons in the universe with the familiar method of $\int^{\infty}_0(...
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number of photons per area: after reflection != twice the distance

Say we have a light source emitting $N$ photons per unit area. In the first case, the photons are reflected at a distance $r$ from the source and measured again at the source. Divergence caused the ...
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2answers
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Reflective photon rocket

A space-craft, also of rest-mass $M_0$ and initially at rest, is propelled by reflecting a plane-parallel beam of photons, generated at a rate $αM_0$ from a stationary source mounted on the launch pad,...
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Photons exerting force

Since gravity affects photons, and forces always work in pairs. Does this mean that photons have a resultant force.? And would we be able to harness this resultant force to move objects using light?
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How does scattering of light happen in atmosphere?

I know that the scattering of light decreases as inversely proportional to the 4th power of wavelength. But what happens at the atomic level? Does the photon get absorbed and re-emitted? Does the ...
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Energy change during electron transitions

I was studying Bohr's atomic model and came to know that when electrons make transitions in between the orbits they lose or gain energy in the form of electromagnetic radiations. I understand why they ...
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1answer
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Enthalpy of the photon gas

Find the enthalpy $H(S,p,N)$ of a the photon gas with state equations: $U = bVT^4$, where $b$ is a positive constant $p = \frac {U}{3V}$ Well, the wikipedia listed the enthalpy as $H = 4/3 ...
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Why can we use a global phase shifter to introduce a relative phase shift for only position?

I'm sure my question is fundamentally confused. I'm not at all a physicist (or even formally a physics student), so please go easy. I used to think of a mirror as introducing a global phase shift in ...
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Which phenomenon is related to the spin of photons? [duplicate]

The phenomenon of the deflection of a moving electron in a magnetic field is related to the electrons spin. From which phenomenon it is concluded, that photons have a spin?
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Photons, light and electricity

Light is ultimately composed of photons. Photons are also force carriers of the electrical force. When an electric motor is turning it is photons which are turning it. What is the relation between ...
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Can a free particle ever emit a particle?

I have found this question here Can a free particle absorb/emit photons?, along with other resources that show a free particle cannot emit a photon (in a vacuum). Now, I am 90% sure it does, but ...
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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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How are photons absorbed by electrons?

I study physics in high school and I was told about the Photoelectric Effect and Compton Effect, and there is something that seems strange to me: How does a photon physically absorbed an electron and ...
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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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Propagators for polarised photons

Photons can be thought to come in two types. Depending on which way they are spinning. The propagator for a photon (sum of both helices) in a certain gauge is $\frac{\eta^{\mu\nu}}{|k|^2}$. I read ...
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Spin conservation in spontaneous parametric down-conversion (SPDC)

If one pump photon "decays" through the process of spontaneous parametric down-conversion into two photons how can the spin be conserved?
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Photon energy comes in packets

From the HyperPhysics page on the Photoelectric Effect: According to the Planck hypothesis, all electromagnetic radiation is quantized and occurs in finite "bundles" of energy which we call photons....
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When atom absorb energy, how does the nucleon know? [duplicate]

Imagine a photon hits a hydrogen ion and is absorbed, how do the nucleon of the hydrogen knows and start to accelerate? Any difference between this and hydrogen ion in a electric field?
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Why is the photoionization cross section smaller at higher photon energies?

I read that the photoionization of a ground state hydrogen atom for photon energies $h\nu > I_H$ (where $I_H$ is the ionization energy of $H$ at $13.6 \, \mathrm{eV}$ has a smaller cross section ...
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I have a thought experiment that defies the uncertainty principle - what's wrong with it?

There's a particle somewhere on the x-axis (this whole thing is 1-dimensional) at x0, which we do not yet know. I fire a photon at it from the origin with a known speed (c) and a known wavelength λ1 ...
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Why can the pion decay into two photons?

The neutral pion belongs to the pseudoscalar meson octet, so it has, in the ground state ($L=0$): \begin{align} P_{\pi^0}&=-1 \\ C_{\pi^0}&=+1. \end{align} And the photon has \begin{align} ...
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Why light have angular momentum?

Light carries momentum which is an intrinsic property or ability to move something at least how I interpret it, I got no issue on how it is able to conserve momentum when it is absorbed by another ...
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When photons are moving at the speed of light, how come they change in position?

From what i know, photons are moving at the speed of light and that means the time that photons are experiencing is zero, but here is the thing... If the time is zero how can they travel astronomical ...
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Momentum of molecule after absorption of photon

I came across this question in text book and was not sure how to solve it. When momentum is conserved then $$ m_{H_2} \Delta v_{H_2} = -m_{Photon} \Delta v_{Photon} $$ But then photon has a zero ...
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Shot noise in optics

There are many explanations to be found about shot noise in optics, but the answers I find are incompatible. There are three ways shot noise in optics is explained. (Note that according to ...
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Will light be coherent while passing through circular polarisers?

The idea is to send light through a beam splitter and one part goes through a right circular polariser and the other goes through a left circular polariser. Will the emergent beams still be in phase?
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How to get electromagnetic field tensor of photon gas?

Momentum spectrum of ideal photon gas depends on its temperature.But sometimes I need these thermal photons to act like 'electromagnetic field'.I want to take them into charged particles' motion ...
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Can very very few photons form the EMWs?

One maybe interesting question please! In quantum point of view, the electromagnetic waves (EMWs) consist of photons. However, if there are only very very few photons, can they form a wave-like macro ...