Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

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 ...

3
votes
1answer
46 views

Is Gravity still considered to be part of the “Superforce” which existed at the instant of the “Big Bang”

My limited understanding of the Big Bang is that all the Fundemental Forces (Electromagnetic, Weak Nuclear, Strong Nuclear and Gravity) were originally part of a "Superforce." As the universe cooled ...
-1
votes
1answer
28 views

Probability of Photoelectric and Campton decrease as photon energy increases, WHY? [on hold]

Why Probability of Photoelectric and Campton decrease as photon energy increases??
4
votes
1answer
110 views

Feynman rules out of the Lagrangian

Accordingly to chapter 10, section 10.6 Feynman Rules of 'Introduction to Elementary Particles' by David Griffiths, there is a way to extract the vertex and propagators just by inspection of the ...
3
votes
1answer
109 views

Path integral in interacting quantum field theory

From my understanding we do not yet know how to make much out of interacting QFT other than scattering amplitude at asymptotic infinity. (Correct me if I misunderstand.) But path integral, in ...
12
votes
5answers
2k views

Why do we say there are four fundamental forces instead of some other number? [duplicate]

In my physics textbook (and in popular science culture) it is stated that there are four fundamental forces: electromagnetism, strong, weak, and gravity. But Wikipedia tells me that there is a ...
0
votes
0answers
57 views

Interaction term in QFT

I've seen that the interaction term for scalar fields $\phi$ with mass $m$ is: $$\int d^3x\ d^3y\ \phi(\vec{y})\Omega^2_{\vec{x}, \vec{y}}\ \phi(\vec{x}) \equiv {\rm 2\ body\ interaction\ term}\tag1$...
1
vote
1answer
42 views

When exactly do identical fermions interact?

For the case of $N$ identical fermions in a three-dimensional box, the Pauli Exclusion Principle necessitates that the overall wavefunction of the system is antisymmetric. No two fermions can occupy ...
0
votes
0answers
20 views

What does it mean for something to interact with a mean field?

I think I just don't understand what a mean field is very well except that it's sometimes used as an approximative technique.
1
vote
0answers
45 views

Why isn't energy conserved in time-ordered diagrams?

I'm new to particle physics, and I'm reading chapter 5 of Prof. Mark A. Thompson's "Modern Particle Physics", which talks about Time-ordered perturbation theory vs QED. However, in page 119 he wrote: ...
3
votes
1answer
43 views

If a galaxy forms from a spherical stationary cloud, how much of the gas will escape?

Let's ignore the dark matter legend and stay with Keplerian physics. Assuming that there is a cloud with $N$ stationary particles with the same size uniformly distributed in a sphere and they ...
0
votes
2answers
47 views

Why is the kinetic energy of an electron in a hydrogen atom at $n=2$ level, the negative of its total energy?

My textbook says in order to determine the De Broglie wavelength of an e- in a hydrogen atom at energy level n=2, we can determine its kinetic energy first by finding its total energy from E=-13.6÷n2 ...
2
votes
1answer
70 views

What is the interaction of gravitational waves?

Please excuse my lack of knowledge about the subject but, If gravitational waves travel thru the fabric of space time do they interact with each other? Meaning do they create interference patterns ...
2
votes
1answer
64 views

Perturbation theory: justifying expansion in terms of eigenstates of the basis Hamiltonian

I have been wondering why anyone ever thought that we could find an expansion for eigenstates of some perturbed Hamiltonian in terms fo those for the basis Hamiltonian. My lecturer insisted that this ...
1
vote
1answer
86 views

Can the mass term be responsible for creation and destruction of particles?

In an interacting quantum field theory, for example, QED, the Dirac mass $m\bar{\psi}\psi$ is a piece of the free Dirac Lagrangian. On the other hand, the interaction term $j^\mu A_\mu=e\bar{\psi}\...
1
vote
0answers
52 views

Kallen-Lehmann representation derivation

I'm trying to understand the derivation of the Kallen-Lehmann representation given in Peskin & Schroeder (pages 211-214). I would really appreciate if anyone on here could answer a few questions I ...
2
votes
2answers
49 views

Are there any fundamental interactions behind centrifugal force?

Centrifugal force for the observer in a rotating frame is a force as real as the force of gravity. However I know that gravitational force is an interaction between a mass and another. And the agent ...
1
vote
1answer
61 views

Coupling strength $g$ in various contexts

I've come across the term coupling strength $g/2\pi$ in various contexts (transmon, quantum dots, optomechanics etc) Also, I've come across coupling term $J$ (exchange coupling) a lot. I think in ...
2
votes
1answer
61 views

Clarifications on the assumptions made for QFT interactions

I am reading about scattering and S-matrix in the context of quantum field theory and although I understand the math and the physical interpretation of the final results, I am confused about some ...
4
votes
2answers
68 views

Why does the weak nuclear interaction have a shorter range than strong nuclear interaction?

My textbook says: "Weak nuclear interaction acts on protons, neutrons, electrons and neutrinos in order to bring about beta decay. It has very short range (10-18m)" "Strong nuclear interaction ...
1
vote
0answers
35 views

Field Interaction [closed]

Can someone please explain (if possible) the fundamentals of the (for lack of a better word) physical interaction that governs the behavior between fields?
1
vote
1answer
41 views

Maximum velocity of interactions

In chapter 1, Section 1, para 7, of Landau & Lifshitz, Classical Theory of Fields, they argue that if a body moves faster than maximum velocity $V_m$ of interactions, that implies we can have an ...
0
votes
1answer
50 views

Unification of the strong and electromagnetic interaction

Is it somehow possible to theoretically unify the electric charge in electromagnetic interaction with the colour charge in the strong interaction and how these charges give rise to the respective ...
0
votes
2answers
76 views

A place of space-time where the only fundamental interaction will be the gravitational forces: does make sense?

We know that there are four (fundamental forces) fundamental interactions of nature, this Wikipedia. I'm curious about if we can speculate that there exists a place of the space-time in which, after ...
0
votes
0answers
59 views

Why is the energy required to create an electron-hole pair in semiconductors higher than their band gap energy?

I am studying semiconductor physics and in more than one book I have found that the energy needed to create an electron-hole pair in Si and Ge is a few times higher than the band gap energy. For ...
1
vote
2answers
41 views

Light interaction: interaction of light waves

First of all, I would like to apologise for my English and I hope my question will be clear (even if I guess it is a quite basic question). Let's assume a person is in a closed room lighted by a ...
0
votes
1answer
120 views

Electric force much stronger than gravitational force? [duplicate]

It is commonly said that the electric force is much stronger than the gravitational force. Indeed, Coulomb's constant $k_e$ is much larger than the gravitational constant $G$ but they are measured in ...
1
vote
0answers
16 views

Quantum Field interaction transferred via “exchanging fermions” [duplicate]

In Standard Model every fundamental interaction is described by means of exchanging gluons of particular kind. It is very natural as gluons has spin with values given by inteegers, and can share the ...
2
votes
1answer
60 views

Is partition function algorithm-dependent or configuration-dependent?

I am reading this resource to learn statistical mechanics: http://blancopeck.net/Statistics.pdf I am trying to learn about the partition function, which as I understand it, is equal to the number of ...
0
votes
0answers
29 views

Properties of Feynman diagrams in Fermi's effective interaction theory

What are the properties of the Feynman diagrams in Fermi's effective interaction theory and how can one draw a Feynman diagram in this theory in relation to the Feynman diagram in the standard model ...
2
votes
0answers
46 views

Feynman rules for anomalous vertex [closed]

We can read Feynman rules directly from the lagrangian in the simplest cases, but for the following lagrangian I am a few stuck $\mathcal{L}=4g\phi\epsilon^{\mu\nu\rho\sigma}\partial_{\mu}A_{\nu}\...
0
votes
1answer
84 views

Does electric and magnetic field interact?

Can electric field or magnetic field interact with an Electromagnetic radiation? And can 2 electromagnetic radiations interact with each other? Rather than the interference effect.
0
votes
1answer
40 views

Interaction between joint qubit quantum system [closed]

Consider the following interaction Hamiltonian $$H = \hbar \mu \sigma_{x} \otimes \sigma_x = \hbar \mu ( |01 \rangle \langle 1 0 | + |10\rangle\langle 01|)$$ acting on the joint states of qubits $\...
0
votes
0answers
29 views

How fundamental electromagnetic interaction occurs in electrostatic case (say, between two resting electrons )?

Interaction between two resting charges (electrostatic case - in electrostatic field) is anyways electromagnetic (because we have only 4 fundamental interactions). Electromagnetic interaction means ...
0
votes
1answer
52 views

Decay and scattering terms in a field theory Lagrangian

Consider two genetic terms in a generic Poincare invariant quantum field theory: A trilinear term of the form $\phi_1\phi_2\phi_3$, and a quartic term of the form $\phi_1\phi_2\phi_3\phi_4$ where ...
4
votes
3answers
126 views

As the universe expands, do we have any reason to suspect further separation of the fundamental forces/interactions?

At some point, all four forces were one force. (another question: what exactly does that mean?). At some point gravity and the strong force separated out leaving the electroweak force. Then the ...
2
votes
0answers
48 views

How Forces are Represented from Quantum Computation or Entanglement Perspective?

In the fields of Quantum Computation and Entanglement there is content on the structure of quantum systems and artificial quantum systems like qubits and quantum cellular automata. I have even seen ...
39
votes
10answers
11k views

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?
0
votes
0answers
54 views

Lagrangian term for multiple particle interactions in curved spacetime

I have the following question. Suppose one wants to describe the dynamics of $N$ interacting classical particles in a metric background given by the line element: $$ds^{2}=g_{\mu\nu}dx^{\mu}dx^{\nu}.$$...
0
votes
0answers
45 views

Light-Matter interaction, full interaction Hamiltonian

I am interested of how to derive the (complete) interaction Hamiltonian for light-matter interaction. In every derivation I only find the result for the dipole and the rotating wave approximation, ...
16
votes
4answers
4k views

Other than magnetism, can any of the four fundamental forces be *repulsive*?

My son asks the above (if not in quite these words), and I am embarrassed to realize that I do not know. Can gravity, for example, or the strong or weak forces ever be repulsive? How/when?
1
vote
3answers
90 views

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.
0
votes
0answers
15 views

What makes a force fundamental, how does dark energy fit? [duplicate]

The 4 generally accepted fundamental forces are the gravitational, electromagnetic, strong nuclear, and weak nuclear forces. How did we come to define these 4 forces as fundamental? I am interested ...
4
votes
1answer
185 views

Feynman diagram of spin Hamiltonian

I am confused about the Feynman diagrams of spin Hamiltonian, for example, the Heisenberg model, the quartic terms like this $[1]$: \begin{align} V &= -\frac{z}{4}\frac{J}{N}\sum_{1,2,3,4}\delta_{...
1
vote
0answers
31 views

Eigenvalue equation for interacting Green's functions

Studying the articles "Topological Hamiltonian as an exact tool for topological invariants" (https://arxiv.org/abs/1207.7341) and "Simplified Topological Invariants for Interacting Insulators" (https:/...
3
votes
1answer
83 views

Understanding the behaviour of an interacting Bose gas

From the Bose-Einstein distribution it follows that a non-interacting Bose gas condenses into the Bose-Einstein condensate below a certain critical temperature. What happens when interactions are ...
0
votes
3answers
106 views

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 '...
0
votes
0answers
143 views

Is there a fifth fundamental force? [duplicate]

When particle physicists said interaction, I interpreted it as force. So, when I heard that elementary particles interact with the Higgs field, which confers them with mass, I thought this must be a ...
6
votes
3answers
202 views

Why there is no 3-body (or more generally $N$-body) fundamental force?

Fundamental forces are believed to be two body interactions. However, I found myself if there is no opportunity for a 3-body or more generally $N$-body "fundamental" force. Is there a proof that any ...
0
votes
2answers
178 views

Can all light-matter interactions be reduced to the photoelectric effect, the Compton scattering, and pair production?

My book says that all interaction of light and matter can be reduced to the photoelectric effect, the Compton scattering and pair production. How true is this? What about reflection and absorption ...
7
votes
2answers
397 views

Propagator Correction in $\phi^4$ theory - why doesn't this secular growth break perturbation theory?

The free propagator for a massive $m\neq0$ real scalar field is the following: $$ G_{0}(x,y) \ = \ \int \frac{d^{4}p}{(2\pi)^4} \frac{e^{i p \cdot (x-y)}}{p^2 +m^2 - i \epsilon} $$ It is a well-...