# 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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### Gauss-Bonnet contact interaction

Can anyone please point me to a paper in which the tree level contact interaction has been systematically computed for the Gauss Bonnet Lagrangian, in D >= 5?
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### 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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### 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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### 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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### Inertia as a fundamental interaction

Why is Inertia not included as one of the fundamental interactions, yet gravity is included as one of the four known interactions, and there is an equivalence established between gravity and inertia.
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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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### 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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### 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 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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### 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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### 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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### Will anyone please explain this problem using 4 fundamental forces?

If there are only four fundamental forces (gravity,e.m.,strong,weak) in the universe and all other forces can be expressed as a function of them then consider this problem. Suppose I throw a particle '...
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### Why is the gravitational force always attractive?

Why is the gravitational force always attractive? Is there another way to explain this without the curvature of space time? PS: If the simple answer to this question is that mass makes space-time ...
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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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### 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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### What does a physical surface look like at a subatomic level?

At the macroscopic level, we are all quite familiar with the concept of a physical surface. From wikipedia: https://en.wikipedia.org/wiki/Surface_(topology) In mathematics, a surface is a ...
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### 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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### Why do irrelevant operators require infinitely many counterterms?

As far as I understand it, in the Wilsonian picture of renormalization, we view a theory as having some fixed cutoff and bare couplings, and integrate out high-momentum modes to understand what ...
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### Renormalizibility by power counting

When testing a theory for its renormalizability, in practice one always calculates the mass dimension of the coupling constants $g_i$. If $[g_i]<0$ for any $i$ the theory is not renormalizable. I ...
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### What's the significance of a dimensionless coupling constant?

In the preface to Mark Srednicki's QFT book (an online draft version can be found here http://web.physics.ucsb.edu/~mark/qft.html), Mark mentions that the $\phi^3$ theory in 6 dimensions would be a ...
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### What does it mean to say “Gravity is the weakest of the forces”?

I can understand that on small scales (within an atom/molecule), the other forces are much stronger, but on larger scales, it seems that gravity is a far stronger force; e.g. planets are held to the ...
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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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### 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 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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### 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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### 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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### What are “Force Carriers”?

The concept of "force carriers" is hard for me to understand. I can understand "energy carriers". I can understand mass x acceleration but I can't see how this applies. Does anyone have a ...
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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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### 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 ...
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### Deriving effective interactions, e.g. phonon-mediated electron-electron interaction

Upshot of the question: how can I derive the effective electron-electron interaction brought about by the electron-phonon interaction? I've read derivations of the electron-phonon interaction and ...
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### Are there anti virtual particles (mediator bosons)?

I have read these questions: Can bosons have anti-particles? Is there a possibility for discovery of anti-graviton, i.e. the graviton antiparticle? Antiparticle for Higgs boson? According to ...
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### Equation of state of Lennard-Jones spheres

One way of accounting for van der Waals interactions in fluids is to use the Lennard-Jones potential [*], which has a repulsive term that dominates at short distances which mimics the hard-core ...
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### Variance of an interacting quantum field in its vacuum state

A non-interacting quantum field $\hat{\phi}(x)$ can be decomposed into $a_{\textbf{k}}$ and $a_{\textbf{k}}^\dagger$. This enables us to calculate the variance of a free field. For example, the ...
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### Is there a way for light to be reflected out of the usual plane of incident?

Is it possible for use to apply a potential or magnetic field to the surface of the media, so that the light being reflected out of the plane of incident? i.e. Compare to an initial "vertical" plane ...
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### Why are there only four fundamental interactions of nature? [closed]

Is there an answer to the question why there are only four fundamental interactions of nature?
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### If the strong nuclear force is stronger than electrostatic repulsion, why don't nuclei collapse into a point?

Today in class we were discussing the strong nuclear force, and our teacher was explaining about how the strong nuclear force counteracts the repulsion force between protons in a nucleus. When asked ...
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### Does observation in quantum theories always imply interaction (affecting quantum system with photons, electromagnetic fields, etc.)?

The term observation is obscure. As I see so far, observation is always done by means of affecting (!) the quantum system by some means - often photons or electromagnetic waves or whatever else. ...