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1answer
102 views

Which interaction is the responsible for long-range magnetism?

I'm taking solid state physics, subject mostly based on Ashcroft-Mermin's "solid state physics". Yesterday I sat for an exam and there was a question I couldn' t even answer: "Which interaction is ...
1
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3answers
248 views

Relativity and photon interactions

A particle's interaction (with anything it can interact with) can be thought of as it making a measurement of the physical quantity associated with the interaction, (e.g. electric field in case of the ...
5
votes
1answer
97 views

Why can interactions be neglected for the Integer Quantum Hall effect?

Though the statement is made often, I've not seen any justification for neglecting electron-electron (Coulomb) interactions in the fully filled $\nu =1$ IQH state. I would highly appreciate if someone ...
2
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2answers
212 views

Potential in Quantum field theory

I studied free particle field like Dirac field and Klein Gordon field. My question is about interaction. How can I put a potential term in the Lagrangian density? $\mathcal{L} ...
2
votes
1answer
216 views

Why Does Knife Cuts?

Why is it that when we cut bread or anything else with a knife, the less effective way to cut it is just by pressing on it from above? And is it true that we can cut things with knife because of the ...
2
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0answers
74 views

In QFT, how can it be shown that the field out, ${\phi_{out}}$, is a free field if the field in, ${\phi_{in}}$, is a free field?

In the Dirac picture of QFT interacting fields, if the field in, ${\phi_{in}}$, is a free field, then I know that the field out, ${\phi_{out}}$ should also be a free field. How can this be shown? ...
0
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1answer
77 views

Definition: Coupling [closed]

What does it mean to say that 2 fields are coupled? More generally, what does "coupling" mean?
2
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0answers
79 views

Adiabatic theorem in the regime of quantum optics

I am wondering whether there is a version of adiabatic theorem in the regime of quantum optics. My understanding of quantum optics involves with the interaction between photon and atom. This ...
3
votes
1answer
524 views

Are fundamental forces conservative?

I wonder whether fundamental forces are conservative. First of all, I'm not sure if we can talk about conservative forces, since to study electromagnetism, weak and strong interactions we need QFT. ...
5
votes
2answers
431 views

How general relativity gets to an inverse-square law

I understand that a general interpretation of the $1/r^2$ interactions is that virtual particles are exchanged, and to conserve their flux through spheres of different radii, one must assume the ...
4
votes
2answers
212 views

Non-local Lagrangian contact interaction

Conside a contact interaction given by a delta function on their worldlines. Use a gauge fixed Lagrangian for two point particles in terms of their proper times $t$ and $t^{\prime}$. Is it possible to ...
3
votes
1answer
248 views

Deriving Feynman Rules (with the presence of a gluon field strength tensor)

If I have a Lagrangian of the form: $$ \mathcal{L} = k \bar{\psi} \varepsilon^{\mu \nu} \lambda^a \phi G^a_{\mu \nu} + h.c. $$ [where $\phi, \psi$ are fermions, $\lambda^a$ are Gellmann matrices, ...
0
votes
1answer
125 views

How fair is it to say that all chemistry arises from failures of the ideal gas law?

I was reading here about how the ideal gas law assumes point masses and non-interaction. Is it fair to say that all chemistry arises from failures of that? Of course, such a sweeping generalization ...
2
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3answers
200 views

What is the cause the light is affected by gravity? [duplicate]

I know that photons have no mass and that a photons exist only moving at the speed of light. So what is the cause that a massive astronomical object can bend a ray of light? I have two thoughts, but I ...
2
votes
1answer
114 views

Interacting particles

We are familiar with the grand partition function for the grand canonical ensemble. This makes me wonder: what kinds of modifications would be required if the particles interacted? Thanks.
3
votes
2answers
378 views

Gell-Mann Low Theorem and Vacuum Energy

I know that the sum of vacuum bubbles can be related to the Vacuum energy, but I'm trying to understand how this follows from the Gell-Mann Low theorem/equation. My question will use equations from ...
7
votes
1answer
1k views

How to measure a solid-solid surface energy?

Many techniques exist to measure the surface energy between a liquid and a liquid or a liquid and a gas (see e.g. the wiki page). Methods to measure the surface energy between a solid and a fluid are ...
0
votes
1answer
76 views

Strong interaction and the Lagrangian for electromagnetic interaction

The Lagrangian for electromagnetic field has the following expression: $$ L = -\frac{1}{c^{2}}A_{\alpha}j^{\alpha} - \frac{1}{8 \pi c}(\partial_{\alpha} A_{\beta})(\partial^{\alpha}A^{\beta}) $$ (I ...
1
vote
0answers
119 views

Range of forces from mass of force carrier?

Why is $\frac{\hbar}{mc}$ a good estimate of the range of the four forces, where $m$ is the mass of the carrier particle of the force? Inputting the pion mass gives $1.4\ \mathrm{fm}$ for the hadronic ...
1
vote
1answer
168 views

A strange particle, $X$, decays in the following way: $X → π^– + p$. State what interaction is involved in this decay

A strange particle, $X$, decays in the following way: $X → π^– + p$. State what interaction is involved in this decay. I know the answer to be weak interaction, but why is it weak interaction? What ...
4
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1answer
340 views

How are forces related to decays?

How are decays related to forces, what is meant by particle X decays through the, say, strong force? The way I understand forces is by how they change the acceleration of particles with the right ...
12
votes
1answer
1k views

Interpretation of derivative interaction term in QFT

I am trying to understand what a term like $$ \mathcal{L}_{int} = (\partial^{\mu}A )^2 B^2 $$ with $A$ and $B$ being scalar fields for instance means. I understand how to draw an interaction term in ...
4
votes
2answers
159 views

Interpretation of an “interaction” term

In QFT a polynomial (of degree >2) in the fields is said to be an interaction term, Ex.: $\lambda\phi^4$. Question Is it possible to give an interpretation to terms like $\frac{1}{\phi^n}$? (for ...
1
vote
1answer
310 views

How does the dressed Klein-Gordon propagator look in position space?

The free Klein-Gordon propagator in momentum space $\sim (p^2-m^2+i\epsilon)^{-1}$ has just a single pole at $p^2=m^2$. The passage to Fourier space is difficult but possible. The result is very ...
1
vote
3answers
2k views

Long/short-range interaction

A potential of the form $r^{-n}$ is often considered long-range, while one that decays exponentially is considered short-range. Is this characterization simply relative/conventional, or is there a ...
2
votes
2answers
594 views

Why $\lambda\phi^4$ theory, where $\lambda>0$, is not bounded from below?

Why the following interaction, in QFT, $$\displaystyle{\cal L}_{\rm int} ~=~\frac{\lambda}{4!}\phi^4$$ where $\lambda$ is positive, represents a theory that is unstable (or unbounded from below as it ...
2
votes
2answers
227 views

Interaction speed between electric charges and magnetic materials

Einstein said that the speed of a matter in universe cannot exceed the speed of light. Is it correct for electric force transmission speed from one electric charge to other one? What is ...
3
votes
1answer
208 views

Interacting system and relaxation times

I got a question I'm not sure how to state precisely or is it even valid. Any help is most welcomed. I stripped the question of all details because I wanted to emphasize my problem, but should ...
14
votes
7answers
463 views

Macroscopic laws which haven't been derived from microscopic laws

Can you think of examples where a macroscopic law coexists with a fully known microscopic law, but the former hasn't been derived from the latter (yet)? Or maybe a rule of thumb, which works but ...
4
votes
1answer
121 views

Intuitive picture for spin-fluctuations contribution to specific heat of He3

Usually when discussing Fermi liquid theory, it is stated that due to the quasiparticles effectively behaving like a free electron gas with effective mass, the specific heat is linear in $T$ at small ...
1
vote
3answers
518 views

How do magnets work?

I've read a classbook on the field theory (including EM): it perfectly describes quantitive patterns in EM-theory, but I have no luck understanding how and why it works. I mean, magnetic substances ...
3
votes
2answers
138 views

Interacting classical strings?

May classical strings be interacting? I would guess no, I can not see any way to break a classical closed string in two of them (the "pants" diagram); but maybe I'm missing something.
2
votes
1answer
118 views

What's the meaning of the coupling change after a renormalization (in the 1-dim Ising Model)?

What does it mean that after the theory (1-dim Ising model here, but the question is general) is renormalized one time and $g_i\rightarrow g_i'$, that the couplings are weaker, even if the theory ...
7
votes
1answer
119 views

Expectation values of interacting fields

I was motivated to ask this question by the equality claimed in equation 10.3.3 of Weinberg's volume 1 of QFT books. My interpretation of that, If $O_s$ is a quantum field of spin $s$, $\psi_s$ is ...
5
votes
2answers
375 views

Lorentz transformation in light cone coordinates in string theory

What is the explicit form of the Lorentz transformation changing the light cone coordinates in the light cone gauge in string theory? The extended nature of the strings complicate matters, especially ...
3
votes
4answers
1k views

How can I explain why the weak nuclear interaction between individual nucleons is 'weak'?

By considering the energy-time uncertainty principle, estimate the range of the weak nuclear interaction at low energies. Compare this range to the size of a typical nucleon (for example, a proton) ...
6
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2answers
783 views

How do Leptons arise from Lambda decay?

I have a question for an assignment: Use your understanding of the quark model of hadrons and the boson model of the weak nuclear interaction to explain how leptons can arise from lambda decay, ...
2
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3answers
358 views

Fermionic interaction potentials

Are there any examples of fermionic particles or quasiparticles for which the interaction potential is a globally smooth function? i.e. no singularities or branch points. As an example, in Flügge's ...
-4
votes
3answers
870 views

What is physical in the principle of local gauge invariance? [closed]

Modern theories of interactions in particle physics are gauge ones. I know how the gauge fields are introduced in equations ($D = \partial + A$). I just do not see any physical motivation in it. I am ...