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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How do $\phi^4$ terms result in an interacting (classical) field?

It was my understanding that a scalar field, $\phi(t;\vec{x})$, varying in time and parametrised by position, was 'non-interacting' if the field evaluated at one point affected the field evaluated at ...
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Photon propagator counterterm in QED

The lagrangian for QED including counterterms is $$\mathcal{L} = -\frac{1}{4}F_{\mu \nu}^{2}+i\bar{\psi}\gamma^{\mu}{\partial_{\mu}} \psi-m_R \bar{\psi}\psi-e_R \bar{\psi}\gamma^{\mu}A_{\mu}\psi- \...
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Interaction Lagrangian with derivatives

I have a problem with derivatives in the interaction part of the Lagrangian. Considering the Lagrangian $${\cal L}=\frac{1}{2}\partial_{\mu}\phi\partial^{\mu}\phi+\frac{1}{2}\partial_{\mu}\psi\...
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Are all field interactions carried out through force-mediating particles?

To my knowledge, all field interactions are carried out through force-mediating particles. For example, electromagnetic interactions are carried out through exchanging photons. However, under the ...
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Definition of non-interacting particles

A system of non-interacting particles is equal to a system of free particles that is equal to an ideal gas?
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Long range forces?

A force/field which depends inversely on the square of the distance of the source from the point of interest (like electric field depends on $1/r²$, where $r$ is the distance between the the source ...
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Universal Law of Strong/Weak Force? [duplicate]

Okay, first things first, I'm very basic when it comes to the field of physics, but I'm very good at coding, and I was wondering if I could simulate an atom. I know that in order to make such a ...
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Perturbation theory in quantum mechanics: assumptions on eigenvectors [duplicate]

In a course that I follow, we use the perturbation method to find the eigenvectors and energies to an Hamiltonian written $ H_0 + V $ where $V$ is a weak perturbation. It is written that as $V$ is a ...
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Renormalization group in time and space, finite size systems

I have few questions about RG. For completeness and clearness I'll start more or less from the beginning. Initially I have bare two-point propagator: \begin{equation} G_0(r)=\frac{1}{r} \end{...
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S-matrix and time-evolution operator

On page 108 of Peskin Shroeder. If the formula $$ |\mathbf{P_\cal{A}}\mathbf{P_\cal{B}} \rangle \propto \lim_{T\to \infty(1-i\epsilon)} e^{-iHT}\, | \mathbf{P_\cal{A}}\mathbf{P_\cal{B}} \rangle_0 \...
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Can the MSW effect be modified by non-standard neutrino-neutrino interactions?

The MSW effect describes how propagation of neutrinos through matter can resonantly enhance the neutrino mixing. The reason for this enhancement is that the presence of electrons in matter changes the ...
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Energy-momentum relation for dressed particles and how interactions change mass

When a free real scalar field $\phi(x)$ described by the Lagrangian \begin{equation}\mathscr{L}=\frac{1}{2}\partial_\mu\phi\partial^\mu\phi-\frac{1}{2}m^2\phi^2\end{equation} is quantized, a quanta ...
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Einstein-Hilbert action as an effective field theory

I have read the following statement: "The Einstein-Hilbert action can be seen as an effective field theory: setting $g_{\mu\nu}= \eta_{\mu\nu}+h_{\mu\nu}$, one gets a free Lagrangian for the field ...
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Why do Neutrinos pass through us but photons can't pass through us? [duplicate]

Neutrinos have no mass and no charge. Therefore, they are not deflected by the other particles in our body and pass through us. Photons too have no mass and no charge, but why are they being deflected ...
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What is a hard core in inter particle potential?

I am reading the book A Modern Course in Statistical Physics by Reichl. In it, there is the following statement The theory of interacting semi-classical fluids is also easily described by the Grand ...
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Electron - phonon interaction and perturbation theory

It might be not so clear what the question actually is, so I'll start with that: why do we often use perturbation theory for non-degenerate spectrum in solid state physics, while the spectrum of $H_0$ ...
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Do gravitons interact with each other?

I always thought the non-linearity of Einsteins field equations implies that there should be direct graviton-graviton interactions. But I stumbled upon Wikipedia which argues: If gravitons exist, ...
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Dimensional analysis of the Lagrangian

In his 1995 paper, Kaplan explains what are relevant, irrelevant and marginal interactions. The idea is this: the action, S, has dimension $\hbar $. When taking $\hbar=c=1$, $[S]=0$. Besides that $[x]=...
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How can the Feynman rules be read off the Lagrangian?

I am reading Peskin. In his functional methods chapter he says that (i) "Once the quadratic terms in the Lagrangian are properly understood" and (ii) "The propagators of the theory are computed" ...
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Why is it that the interacting fields cannot be decomposed into Fourier modes like free fields?

In quantum field theory, a free scalar field can be decomposed into Fourier modes. But why is that, an interacting field cannot have Fourier decomposition? Can we not decompose any arbitrary ...
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Does spin state matters during interaction?

I'm asking whether the interaction between a pair of spin-up or spin-down electrons be any different from the interaction between a pair of electrons that comprises of opposite spin state? I think ...
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Higgs Coupling - Fifth Force [duplicate]

To which of the four forces of nature does the coupling of the Higgs scalar field to other quantum fields ultimately belong to, if any? If it doesn't fall into any of the four known categories of ...
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Is it a coincidence that there are 4 fundamental interactions and 4 quantum numbers? [closed]

I can imagine that a system where each component has four defining characteristics would also have four ways those components can interact; the characteristics define interaction for each group. I ...
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How neutrons interact if not through an electromagnetic interaction?

According to the question Do positively charged particles exchange photons? there was an answer Yes. Photons are the carriers for the electromagnetic force, regardless of the charges involved. ...
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Interaction in spin models

In the Heisenberg Spin model, the spin components of spins in the same direction at different lattice sites couple together. Is there a physical scenario where, say, the $x$ Spin component of a spin ...
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What is the “truth” of the electrostatic force?

I want to ask a question about the Coulomb force or the electrostatic force. As you can see, there are these statements: 1) The electric force is a "vector quantity". 2) The electric force between ...
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Can similar charges attract each other?

Yesterday my teacher was giving us an introduction to the fundamental forces of nature. She asked why opposite charges attract to each other and similar charges repel each other. This question gave me ...
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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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$\phi^{n}$ quantum scalar field theories where $n$ is not an integer

Consider a quantum scalar field theory with interaction terms of the form $\phi^{n}$, where $n$ is not an integer. Where are some examples of widely-studied quantum field theories which involve such ...
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Does the Higgs field correspond to a “fifth” fundamental force? [duplicate]

I am pretty much a layman regarding particle physics. Reading popular accounts on the Higgs particle and the Higgs field, which supposedly explains (all?) the masses of the elementary particles by ...
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How can Pauli's exclusion principle originate forces?

Recently I've seem two situation in which the Pauli exclusion principle is said to originate forces. The first was a derivation of the Lennard-Jones potential in the context of Solid State Physics ...
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Feynman diagram representation of variational derivative of S-matrix

For quite some time I am struggling to understand section 6.4 in Weinberg volume 1. He observes there that if interaction hamiltonian density is extended by coupling to c-number fields $\epsilon$, $$ \...
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About the notion of the self-interaction of a field

In QFT it's very common to hear (read) about the self-interactions of a field. e.g. there's the self-interaction terms of the Higgs field, that come with $\lambda^3$ and $\lambda^4$, right? But I ...
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Is it possible that there is a 5th force or is it proven that there is no other force?

I heard about in high energy the 4 fundamental forces can combine. They were not separated until $10^{-11}$ seconds after the big bang and as the univers gets cold, they begun to seperate. Is it ...
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Is virtual particle the same thing as force particle?

Is virtual particle the same thing as force particle? Which of the above categories do photon, graviton and gluon fall into? Virtual particle, force particle, or both?
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Do all four fundamental forces have effects on space time?

Since gravity, a fundamental force, takes effect through ripples in spacetime, do the other fundamental forces do the same? For example, since gravitational waves are ripples in space time, are ...
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Forces: In what manner do they act on objects?

Newton's laws explain how the application of a force affect other objects. But, in what manner is this force applied? I know about contact forces and action at a distance forces, so my question is not ...
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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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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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Sufficient conditions for a interaction to be classified as weak, strong, …?

Let us say I have been given the equation of a interaction/decay/etc. between particles: $$X+Y\rightarrow A+B$$ Are their any sufficient conditions that we can use to determine the type of interaction ...
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How do non-contact forces work?

One question has been bugging me ever since I started learning physics, the textbooks successfully describe the empirical observations but haven't given any reasoning behind it. What I want to ...
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How do force carrying particles “give” force? [duplicate]

So, I am not taking physics in school, but I do have an interest in it, and I was wondering, in the standard model, all of the force carrying particles (photons, Z Bosons, W Bosons, gluons, and (...
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How can fields interaction give rise to particles?

We say light a matter-wave, meaning along with its wave property it shows particle nature. But how can fields interaction (electric and magnetic) give rise to particles (photon)? I wish someone could ...
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The meaning of 'coupling'?

In quantum mechanics if two quantities $A$ and $B$ are said to be coupled what does this actually mean? I would guess that it means we have a term like $A\cdot B$ in the Hamiltonian but this is only ...
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Why can the 2p0 to 1s transition in hydrogen with dipole interaction not be solved without splitting into x,y,z polarisation?

The $2p_0$ and $1s$ wavefunctions for hydrogen; $ \psi_{2p_0} = \dfrac{1}{4\sqrt{2\pi}} \left(\dfrac{Z}{a_b}\right)^{\frac{3}{2}} \dfrac{Z r}{a_b} e^{\frac{-Zr}{2a_b}} \cos(\theta) $ $ \psi_{1s} = ...
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Angular momentum in annihilation $n\overline{n} \rightarrow \pi^0 \pi^0$

Consider the annihilation of a neutron by an anti-neutron $$ n\overline{n} \rightarrow \pi^0 \pi^0 $$ so that the initial relative angular momentum is zero. Because the spin of neutrons is $1/2$, $J_i$...
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Is it possible the weak force isn´t fundamental? [closed]

Once the strong force was thought to be a fundamental force mediated by the massive pion. Later colour and gluons were introduced. The old strong force was a residue force of the new fundamental force....
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How can I determine the interaction knowing the decay formula?

I know the kind of interaction occurring in very common reaction. For example, I know that the interaction: $e^- + e^+ \longrightarrow \mu^+ + \mu^-$ is driven by the electromagnetic force (there is ...
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Clarification of the concept of Boson Mediator and 'Mediation' in Physics? [closed]

I would like to have a clear concept of Higgs 'mediator' and that 'mediation' speak in physics, what you 'swap' a particle with ...
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Mystery $p^0$ particle [closed]

Some exercises in my physics book mention a particle denoted $p^0$, but I can't seem to find any information about this particle, neither in my book nor on the web. I've been able to deduce from the ...