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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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1answer
66 views

Feynman diagrams as topology

When we talk about Feynman diagrams we know they are tools to make calculations easier and more intuitive. Moreover, it's said that they are "topological" representations of the interactions. But, ...
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Why is the Coulomb repulsion between particles minimized if the orbital two-particle wave function is anti-symmetric?

In exchange coupling, we obtain the interaction term \begin{equation} \sum_{i\ne j} J_{ij} a_{i\sigma}^* a_{j\sigma'}^* a_{i\sigma'} a_{j\sigma} = --2 \sum_{i\ne j} J_{ij} (S_i\cdot S_j +\frac{1}{4} ...
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Does string theory really associate 6 dimensions to electromagnetism & the nuclear forces?

1) I understand string (superstring) theory often ends up with 10 dimensions, 9 space-like and 1 timelike. Typically I read that these are all associated to space-time. 2) So, I was interested when I ...
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Can I 'add potential together'?

I have the Lagrangian function for a particle in an electromagnetic field $$L = \frac{1}{2} m \dot{\mathbf{r}}^2 + q \dot{\mathbf{r}} \cdot \mathbf{A} (\mathbf{r}, t) - q \phi (\mathbf{r}, t)$$ If I ...
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2answers
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What is the strength of gravity compared to electromagnetism?

I would like to know the strength of gravity as compared to electromagnetism. Im not looking for their classical constant values. It can be compared by saying i.e. gravity is a million billion times ...
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1answer
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Mean position of particle in a Lennard-Jones potential

I want to compute the mean position $\langle x\rangle_\mathrm{eq}$ (let's say in $1$D) in thermal equilibrium (canonical ensemble) of a particle coupled to another particle via a $6$-$12$ Lennard-...
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1answer
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Confusion about interaction in 2nd quantization

Interaction terms in second quantization is written as $$ \sum_{ijkl}c_{i}^{\dagger}c_{j}^{\dagger}V_{ijkl}c_{k}c_{l} $$ Now, is spin is there then this term is written like $$ \sum_{ijkl\sigma\sigma'...
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Feynman rules for scalar field with second order derivatives in the interaction term

Given the interaction term with $N$ scalars $\phi_i$, each massless, what would be the Feynman rules for an interaction term in the action as $$ \int d^dx (\partial^2 \phi^i)\phi_i(\partial_\mu \phi^...
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Deriving the Lagrangian of a set of interacting particles only from symmetry

In section 5 of Landau and Lifshitz's Mechanics book, they show that the Lagrangian of a free particle must be proportional to its velocity squared, $\mathcal{L} = \alpha\mathbf{v}^2$ using only ...
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3answers
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Does general relativity account for the other 3 forces?

There are suppose to be 4 fundemental forces of nature, einstein described gravity not as a force but, put simply, the result of objects following curves in spacetime. However does it account for ...
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For which values of $n$ and $n'$ are $n \to n'$ particle scattering cross-sections well-defined?

When studying particle interaction events in QFT, we usually consider either (a) $2 \to 2$ particle "scattering" events, whose probabilities are quantified by scattering cross-sections, or (b) $1 \to ...
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1answer
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Casimir Effect for other interactions

I read about the Casimir and dynamical Casimir effect. However it is always mentioned in the context of electromagnetism and with two perfectly conducting plates. My question is whether there can be a ...
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How can cell phone towers transmit despite the rain?

I once read that waves of higher frequency (such as around 1 GHz range and higher) have difficulty passing through water (or other liquids/solids). Today, when I saw a cellphone tower, I wondered: "...
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Interaction between electrons obeying Pauli's exclusion principle

For fermions having half spin, obeying Pauli's exclusion principle, we know that for states with different spin states, the particles (say electrons) are distributed in such a way that if the first ...
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1answer
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is a photon interacting via photons?

Electrically charged particles interact with each other via the exchange of a photon (as it is the exchange particle of the electromagnetic force). When considering, e.g., the Compton-effect, where a ...
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Compton effect in dependency on the atom number

I can't find information about the influence of the atom number on the compton effect. Often I read that the cross section is proportional to the atomic number but I also read that materials with a ...
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0answers
28 views

General force between two point particles, one of which has “spin”

Consider two points in the empty (isotropic and homogeneous) space: since the only vector that "makes sense" (the only vector that we can define) is the vector given by the difference of the two ...
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1answer
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How come subatomic particles interact with each other while not being in contact? [closed]

Although electrons stays away from protons an atom remains electrically neutral But two atoms one positively charged and another negatively charged remain so until they comes in contact. Can the ...
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2answers
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In QFT, are forces made out of multiple fields?

I’ve been reading about 1,5 books about quantum physics and I’ve also watched a few YouTube videos. In one book, I learnt that there are fields, such as the electromagnetic field, which carries forces ...
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1answer
50 views

How is the electromagnetic/gravitational force transmitted? [duplicate]

So I was thinking about how a positive and a negative charge (or positive/positive, negative/negative) interact. I have read previously about how photons carry the electromagnetic force. However, how ...
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1answer
28 views

Interaction energy of two brownian spherical particle in liquid [closed]

Let us consider two hard sphere in finite volume. Their motion is Brownian. What can we say about interaction energy? Is it less then $kT$? I know that we can describe this system by Langevin ...
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1answer
33 views

What kind of interaction phototelectric effect is?

All standard-elementary books which discuss modern physics quote the value of time of interaction for phototelectric effect to be less than $10^{-8}$ seconds. If the phenomenon takes about this time, ...
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1answer
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Questions about scattering matrix theory of non-free particles

Hi,I have a problem for scattering matrix theory. Currently, the book I've read is about collision between free particles. What if collision between non-free particles? For example, in lattice, only ...
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2answers
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Why do atoms repel when closer but attract when farther apart?

I was wondering why atoms, when pulled apart and then released, attract together, but I notice that there is a special distance at which this attractive force doesn't seem to act but rather those ...
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1answer
67 views

Decay rate of scalar Yukawa interaction

Considering the Lagrangian of two scalar fields in $d=4$: $$\mathcal{L}=\frac{1}{2}(\partial\phi)^2-\frac{1}{2}m^2\phi^2+\frac{1}{2}(\partial\chi)^2-\frac{1}{2}M^2\chi^2-g\phi^2\chi$$ Let's assume ...
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1answer
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Self-energy in two scalar Yukawa interaction

Considering the Lagrangian of two scalar fields in $d=4$: $$\mathcal{L}=\frac{1}{2}(\partial\phi)^2-\frac{1}{2}m^2\phi^2+\frac{1}{2}(\partial\chi)^2-\frac{1}{2}M^2\chi^2-g\phi^2\chi$$ What would be ...
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Is the Weak Force really a force?

My definition/understanding of a force still carries largely over from classical physics which is something that tends to change the motion of an object (a push, pull). Gravity and electromagnetism ...
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How are length and time scales for the different kinds of interactions (strong, weak, electroweak) determined?

I was recently asked what the length scale of the strong interaction is and found my self a bit lost at the question. A quick Google search revealed a result of $10^{-15}\,\text{m}\approx 1\,\text{GeV}...
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Particle interactions simulator

I'm an A-Level student trying to write a program that will take in two particles (like a proton and electron) and output the new particles. I'm planning to implement the conservation laws so that the ...
66
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4answers
6k views

How exactly is a normal force exerted, at the molecular level?

I've been surfing the web for quite a while, finding the answers I would need, but couldn't find a convincing one. First of all I need to remind you that this a very long/continuous question, so ...
2
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1answer
46 views

Why does a non-linear system lead to interaction and frequency mixing between input's?

When we have a system that is nonlinear and we apply a sum of two different frequency sine waves as an input, we see the output of this system has components that are at the sum frequency of the two ...
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1answer
218 views

Gravitons and self-interaction

In the book quantum field theory and standard model by Schwartz, there is a problem 9.4 that says by considering lorentz invariance of Compton scattering, you can prove that for spin 1 massless field ...
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1answer
51 views

How should two-photon transitions be modelled? Is second-order perturbation theory required? Or are sequential first-order processes sufficient?

For example, I want to consider the following situation: photon transit from $m$ energy level to $m+2$ after absorption of two phonons with frequency $\Omega$. I want to calculate a transition rate ...
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2answers
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Why can't two real photon, gluon, graviton, and $W$ and $Z$ fields interact by means of their virtual counterparts (the mediators of the process)?

It is a fact that two real (massless) photons, gluons, or gravitons can't react by means of their virtual counterparts (for example, two external photons that interact via one of these massless ...
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41 views

How do interaction terms appear in the Lagrangian?

How does forcing the Lagrangian to be invariant under $U(1)$ group give rise to the electromagnetic interaction term?
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How to determine which force mediates an interaction?

I have been given an assignement gives me a number of interactions, asks me to determine which, among these interactions are permitted under conservation laws (Charge, Baryon number, Leptonic numbers, ...
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1answer
99 views

Are force carrying particles always virtual particles?

Of course we have real (i.e. non-virtual) photons, but when photons play the role of "force carrier" are they virtual? Same thing for gluons. Real gluons have been detected, but when playing the role ...
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1answer
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What is meant by “Since the exchange of (mesons) occurs frequently, the nuclear force is stronger than the electromagnetic force”?

This is from a book written by Y. Nambu. What role does the mass of mesons (versus mass of photons) have in making the strong force strong? How does this relate to "frequent exchanges" of mesons? ...
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EM waves cancelling each other - do they interact? Does this mean photons interact?

Just watched a video explaining how oil film causes color patterns - first wave is reflected from the outer surface (air-oil), then another wave is reflected from the lower surface (oil-water). ...
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Feynman rules for a general Lagrangian

How do I find the Feynman rules for a general Lagrangian density? For example the Lagrangian $$L = \partial_\mu \psi \partial^\mu \psi +a \psi\partial_\mu \psi \partial^\mu \psi+b \psi^2 \partial_\mu ...
4
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1answer
77 views

Quantum field theory with only 3-point vertexes

Given an arbitrary quantum field theory, can I always write it in terms of another (different) quantum field theory containing only operators with 3 fields? (i.e. vertexes with 3 legs) I guess that ...
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1answer
127 views

Why coupling constants with negative mass dimensions lead to non-renormalizable theories?

can somebody explain or point to the relating mathematics showing Why coupling constants with negative mass dimensions lead to non-renormalizable theories?
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What if interactions such as gravity faded with respect to the cube of distance, rather than square? [duplicate]

It's generally understood that anything that "spreads out" in 3D space dissipates at a rate proportional to the square of distance. Light, sound, gravitational force, etc. I know this is a pretty ...
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1answer
64 views

Why do we go beyond two-body interaction?

Actually, my question is why do we study many-body interactions. I have just started working in Fractional quantum Hall systems. There we have Coulomb interactions between electrons, which we know is ...
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Why in the iron based superconductor interband interaction is dominating?

What is the mechanism of intraband and interband interaction in the iron based superconductor? and Why interband interaction is dominating?
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Why is it that forces like gravity and electricity (approximately) basically act between pairs of bodies only?

In classical mechanics, with the limit of little movement (so no relativity, waves, and/or "magnetic" effects), we can see that gravitation and electricity can both be described as "two-body" forces, ...
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Can all fields contribute to the potential energy that appears in QM Hamiltonian?

Most importantly: can that potential energy in the QM Hamiltonian able to describe the motion of a single particle in an external electromagnetic field? Background: In non-relativistic QM using the ...
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2answers
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Why do we observe the nuclear force only in scatterings and decays?

Why, at first glance, are the only forces we perceive to be gravity without quantizing, electromagnetism and nuclear forces only in disintegrations?
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Can someone explain the $ttH$ Coupling?

IS the $ttH$ an actual particle? If so, how can it form? I though the life of the top quark was way to short to form a particle. If it is not a particle, what is meant by coupling.
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1answer
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Is the direction of the Coulomb force at short distances indeterministic?

Suppose we have two electrons that apply a Coulomb force on each other. At large distances, we can consider the two electrons as point charges and the direction of the Coulomb force would be on the ...