Questions tagged [interactions]
Particle interactions are changes in the nature, number, or state of several particles, usually at a specific space-time point, underlying dynamics. They are represented by special "field interaction terms" in quantum field theory and normally entail interchanges of energy, momentum, and sundry quantum numbers. They include scattering, and particle creation and annihilation.
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What is the difference between interaction energy and self-energy?
From my understanding self-energy is the energy required to put charges in a certain charge distribution and interaction energy is the potential energy caused by the interactions between particles, ...
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Why Fock representation holds only in a free quantum field theory?
With a quantum system with $N$ degrees of freedom, all the representations are unitarily equivalent to Fock representation. However, if the number of degrees of freedom goes to infinity, there are ...
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Beta function of two copies of the same model with interactions
I have a particular model with two couplings, let's call it model A, for which I have the set of beta functions and fixed points. Now I am interested in a model where I have two copies of model A, ...
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Difference between Dicke superradiance and dipole-coupled emitter superradiance
This question is similar to "What causes the collective emission in Dicke Model?", but more specific. I wonder if the situation described in
[1] R. H. Dicke – Coherence in Spontaneous ...
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Interactions within constituents of dark matter
Inspired by Are the visible matter and dark matter separately segregated? I wonder if the constituents (I don't say particles, as we don't know) have actions and forces among themselves, that we ...
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Can any Hamiltonian be expressed as a sum of free and pair-interaction terms?
Consider a number of systems $\{S_i\}$ with a Hilbert space $\mathcal{H}=\otimes_i\mathcal{H}_{S_i}$. Consider an arbitrary Hamiltonian $H$ defined on this Hilbert space. Can this arbitrary ...
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When does a theory decouple?
The question is very broad, but it seems to me that the term 'to decouple' is also used in various contexts. For example, neutrinos decouple from the photons in the early Universe, when the ...
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How to evaluate interaction term in Hamiltonian?
If one has an interaction term in the Hamiltonian of a system as follows:$$
\sum_{\vec{q}}V(\vec{q}) \psi(\vec{k}-\vec{q})
$$
where $\psi(\vec{k}-\vec{q})$ and $V(\vec{q})$ is the wave function and ...
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Is the notion of particles dependent upon the Hamiltonian being quadratic?
I am studying interacting QFT in the context of quantum fields in curved backgrounds, and I am getting some confussion about the concept of particles. To study some gravitational phenomena involving ...
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Free higher spin fields and gravity
There are soft theorems that suggest that any massless boson with spin higher than 2 should be a free field theory and cannot have interactions. Does this mean that one cannot embed such fields into a ...
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What exactly are the categories of intermolecular forces
So I read the wikipedia and its catagorization is somewhat vague. (Link here: https://en.wikipedia.org/wiki/Intermolecular_force)
In wikipedia, the van der waals forces are quite clearly stated to ...
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${}$Higgs mechanism
Is is correct to say that, elementary particles have different masses, because they have different coupling strengths to Higgs field? And if yes. Does it make sense to question, why they have ...
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What constitutes a measurement?
In an intro quantum mechanics textbook theres some experimental/historical background, then the postulates are stated, then the rest of the text focuses mostly on the math. But there’s one issue I’ve ...
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Why can we ignore other charged particles when dealing with the Hydrogen atom?
Introductory treatments of the Hydrogen atom use the following Hamiltonian:
$$H=\frac{|\textbf{p}_p|^2}{2m_p} +\frac{|\textbf{p}_e|^2}{2m_e}-\frac{Ke^2}{|\textbf{r}_p-\textbf{r}_e|}$$
However, given ...
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The "observer effect": is this a basic law of every observation or measurement? [duplicate]
I would appreciate some clarification of the "observer effect":
Is it generally true that every observation (or measurement) disturbs the object being observed?
I understand the observer ...
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Under what (mathematical) condition can a quantum interaction potential be ignored?
Suppose there are two quantum systems. Suppose that they are subject to an interaction potential which has a limited spatial range, as in, it is exactly zero outside of a certain spatial range. Thus ...
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What is a particle in the context of QFT with interactions?
This is a crossposting of the same question from mathoverflow: https://mathoverflow.net/q/454768/
It seems that this question was not received well there, claiming that this question is not ...
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Where does the motivation for discovering a 5th force come from? Has there been any evidence found yet? [duplicate]
Electromagnetism, nuclear strong, nuclear weak, and the weird notably weaker force of gravity. Now a force is something fundamental to reality, gravity described as the bending of spacetime causing ...
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Feynman rules for scalar QED [closed]
Can i have a derivation for the Feynman rules of scalar QED?
I don't understand the ones i find online such as the one in the link https://canvas.harvard.edu/files/936391/download?download_frd=1&...
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Why do gauge particles with odd spin cause opposite charges to attract, while those with even spin cause them to repel? What's the math behind it?
I have read that opposite charges attract when mediated by odd-spin gauge particles, like the photon, and that they repel when mediated by even-spin gauge particles, like the graviton.
Is there an ...
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Is it true that there are no known mathematically rigorous examples of interacting QFTs in 4 dimensions?
An answer to a question
(What physical processes other than scattering are accounted for by QFT? How do they fit into the general formalism?) about quantum field theory asserts
"we don't know ...
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Cold plasma approximation leads to infinite coupling
In plasma physics, there is an approximation called cold plasma approximation according to which, the temperature of a plasma species in an electron-ion plasma, say ions ( $T_i$ ) is assumed to be ...
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Ionization process
Is it possible that ions can be doubly or more charged in the ionization process? like if we have 22keV energy photon then can this much energy photon ionize the Ar atom twice ionize and more? because ...
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Coupling spin 3/2 fields with electromagnetism (causally)
I am trying to read literature regarding the coupling of massive spin-$\frac{3}{2}$ fields with electromagnetism. Apparently, an elementary (i.e. not composite) massive spin-$\frac{3}{2}$ field can't ...
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How to carry interaction over "ancilla" particles?
Let's say we have just two particles that interact with each other according to some quantum Hamiltonian $\hat{H}$. Is it possible to add a third "ancilla" particle between the first two and ...
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Why bound shell for photoelectric effect? [duplicate]
Why does the photoelectric effect occur in the inner shell, Is it possible in the outer shell?
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Interactions in nonlinear chiral theories
When discussing nonlinear realizations of $SU(3)_L \times S(3)_R$ in Chiral theories, it is usual to introduce the interactions between the baryon octet ($B$) and some meson matrix $M$ as
\begin{...
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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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Free and non-free Dirac's particles
In trying to understand evolution of Dirac particle in relativistic QM (not QFT) I have following guess that I don't know I'm correct or wrong? edited after
solution of akhmeteli:
It is well known ...
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Relation between matrix element of a process with its crossed processes
Consider a process in particle physics denoted by, $$(1) \quad a+b\to c+d$$ which is related to the reverse process $$(2)\quad c+d\to a+b.$$ By virtue of the hermiticity of the interaction Hamiltonian,...
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Up-to-date values of Yukawa couplings (2023)
Where can I find the most recent values for the Yukawa couplings in 2023? I currently have access to the values from 2011, but I guess precision has improved over the last decade in measurements.
Edit:...
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Physical meaning of Russell–Saunders and $jj$ coupling
In the case of Russell–Saunders coupling (LS coupling), the total $L$ and total $S$ are coupled to a total $J$. In the case of $jj$ coupling, first, all the individual $j = l + s$ values are formed, ...
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Resource Recommendation - Charge interaction with electric, magnetic and light fields
I am looking for a good book or review on the treatment of a charged particle interaction (an electron typically) with magnetic, electric and electromagnetic fields. Ideally with both classical and ...
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About the nature of gravity
Let's consider a corner of spacetime far enough of any other mass so that the spacetime would be nearly flat in this neighborhood, a kind of mass desert.
Let's consider a mass in the center of this ...
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What is the difference between the non-minimal coupling of GR to a scalar field and the coupling of the metric to the stress tensor?
In General Relativity we can non-minimally couple the action of the gravitational field to a scalar field, as seen here, where the lowest order gravitational action is coupled to the scalar fields ...
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Tensor product of quantum systems
If $H_A$ and $H_B$ are Hilbert spaces corresponding to two systems $A$ and $B$, then the Hilbert space for the combined system is given by the tensor product of $H_A$ and $H_B$. Does this hold ...
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Vertex in Feynman diagram
The Taylor expansion for $e^x$ to 2nd order is
$$e^x = 1 + x + x^2/2$$
If we look at the unitary operator for the $\phi^4$ theory
$$\exp\left[-i\frac{\lambda}{4!}\int{\phi^4}dt\right] , $$
we can also ...
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Nonequilibrium green function for interacting systems
In this book by Ryndyk on quantum transport, p. 89, the retarded single-particle nonequilibrium Green function for a non-interacting nanosystem coupled to semi-infinite reservoirs of non-interacting ...
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How inaccurate is the following mental picture of particle interaction in QFT?
Context: I ask this as a school teacher reaching past the boundaries of my expertise. A colleague was talking about the standard model with an advanced student, explaining how particles interact by ...
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What are the actual interactions between the Higgs field and fermions?
I have done a bunch of research regarding the Higgs field and Higgs boson but I keep running into issues when trying to understand how the interactions between the Higgs field and fermions occur. I ...
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Term in Lagrangian using a Single Field
If one tried to write a term in a Lagrangian which only used one field (for example, in QED if one tried to write a term which looked like $\gamma^{\mu} A_{\mu}$), would this term have any meaning in ...
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Specifics of orbiting self-interaction in GR?
As the orbiting body emits its own gravitational field, its front would curve around the host. In doing so, if the lensing caused by the host is sufficient, it could hypothetically intersect with the ...
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Radial distribution function of particles in water at room temperature
I am trying to figure out some experimental effects I'm seeing, and I hypothesize that what I'm seeing is due to particles interacting with each other (which I neglected, hoping that I didn't need to ...
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What's the correspondence between Feynman diagrams and field configurations?
If I understand correctly, a Feynman diagram represents a finite set of "interactions", such as the exchange of a photon between two electrons. You can think of it as a graph in which the ...
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Ground eigenstate of the quantum harmonic oscillator with the interacting vacuum $| \Omega \rangle$
According to this video (at the timestamp), the professor writes down the derivation of the ground state of the 1D quantum harmonic oscillator.
Here is the screenshot from the slides of where she does ...
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Are field theories necessary to make accurate predictions or do they just make calculations easier?
For example, if we really wanted to, could we, at least in principle, model electromagnetism just considering interactions between charged particles without using the EM field? That is, is it ...
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Most general Lorentz invariant Lagrangian
During lectures we've often written down 'the most general' Lagrangians in QFT with the reasoning 'Because why else?'.
Suppose we are looking at a theory with real scalar fields $\phi(x)$ in arbitrary ...
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Anomalous diffusion in a system of confined interacting particles
I have a system of charged particles confined parabolically. I want to analytically study anomalous diffusion in such a system using Langevin Dynamics. In a system of non-interacting particles there ...
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On definitions of interactions in thermodynamics—changing the energy of a closed system without work or heat?
On page 22 of his famous textbook on thermodynamics, Callen asks us to consider a particular gas (a simple $(N,V,U)$ system) enclosed in a cylinder. Callen then tells us that
a small paddle is ...
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How is the interacting vacuum defined in QFT?
I have seen this in a couple of textbooks (Schwartz and Zee), where the author would use the interacting vacuum $|\Omega \rangle$ in a calculation, but would never mention how the state is defined.
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