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16 views

Superficial degree of divergence for scalar theories

I have a few questions regarding the derivation of the degree of divergence for feynman diagrams. The result is $$D = [g_E] - \sum_{n=3}^{\infty} V_n [g_n]$$ (following notation in Srednicki, $P118$) ...
5
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1answer
230 views

Lorentz invariance, energy-momentum conservation & the locality of interactions

I have been reading these notes ("Minkowski Spacetime: A Hundred Years Later", by Vesselin Petkov) 1, in which the author states (in the middle of the text on page 137) that "The only Lorentz ...
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1answer
70 views

Backing out of interactions: Does physics account for such a thing?

Does physics account for interactions between light and matter ever being "not completed" or backed out of? Here's what led me to the question. In learning about interference in light, I ended up ...
5
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1answer
100 views

Five-component field

Recently I was reading about 5-component field $(\varphi , \psi_{\mu})$, for which $$ \hat {p}^{\mu} \varphi = mc\psi^{\mu}, \quad \hat {p}_{\mu}\psi^{\mu} = mc\varphi . $$ This field refers to the ...
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0answers
35 views

Neutrino interaction probability [closed]

Just a quick question, if a single 1GeV neutrino (muon neutrino) were fired at a block of iron with a given density, $\rho$, and the neutrino-nucleon interaction cross section is $\sigma$, what would ...
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0answers
74 views

What is a soft photon?

I accidentally came across the words "soft photon" today after reading a few blogs. There was some discussion of special situations involving gauge redundancies and a theorem by Weinberg. What is a ...
5
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1answer
123 views

Fifth force in string theory [closed]

I can't seem to get a clear description of what the "fifth force" in string theory is. What is the fifth force in string theory? What does it do? What mediates it?
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1answer
77 views

What would happen if there was a fifth fundamental force? [closed]

Preceding this question, as a student of physics, my knowledge of theoretical physics is somewhat limited. So I came across this article, which posited what the universe would be like if three ...
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2answers
71 views

Why don't we call the fermions in the standard model force carriers?

Maybe this is a chicken-and-egg problem, but couldn't we call all the bosons fundamental and treat the fermions as force carriers between them? EDIT: After all we never see the asymptotic states of ...
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4answers
236 views

Is there some special case where a fermion can mediate a force?

Looking at the comments of this questions Does the gravitino contribute to the gravitational interaction? and even considering that the answers here in this other question Why are all force particles ...
2
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1answer
249 views

$G$-parity in an electromagnetic decay

I am looking at the decay $\eta\rightarrow\pi^+\pi^-\gamma$ and I would assume that the decay itself (ignoring the $\pi\pi$ final state interaction that is obviously strong) is electromagnetic since ...
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3answers
101 views

If neutrons aren't affected by electromagnetic force, what causes it to bounce off matter?

Say a neutron is heading for a mountain, what would cause it to bounce off said amalgamation of matter? Electromagnetism is ruled out, gravity is too weak, (if I'm not mistaken) nuclear strong force ...
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1answer
212 views

Why is there no fundamental force following from the $SU(4)$ symmetry?

I've understood that the three fundamental interactions described by the Standard Model (the electromagnetic, the weak and the strong force) are thought to correspond (roughly) to gauge invariances ...
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0answers
21 views

Trinification lepton Yukawa interactions

We have a trinification model of $SU(3)_c\otimes SU(3)_L\otimes SU(3)_R$, where the first is the usual colour group, the second a left $SU(3)$ and the third a right $SU(3)$. As usual, leptons and ...
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1answer
46 views

The charges of the forces

I am trying to establish a simplified understanding of the fundamental forces to explain them to a young audience. If we say that gravity has one charge (attractive), electromagnetism has two charges ...
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1answer
33 views

What is the relation between interaction range and the mass of gauge bosons?

I have just started to read spontaneous symmetry breaking, where it is mentioned that EM fields are infinite in range, so the gauge boson has to be massless, while for the strong and weak ...
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1answer
47 views

Can two photons form a bound state?

I've always wondered if it's possible to bind two photons, in particular by gravitational interaction. Photons don't have a rest mass but nevertheless have a gravitational mass, by which they can ...
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2answers
50 views

Questions about particles and their forces

Consider the hypothetical scenario where 2 particles are headed for each other in a collision course but neither interact with a common force. Do these particles simply pass through each other?
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4answers
94 views

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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0answers
36 views

Friction forces on car wheel

I know that frictions means a lot for car wheel. I've been looking all around a lot and trying to figure all out. I have found out about Coefficient of friction depends on slip ratio(which I am ...
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1answer
89 views

Newton's third law at the quantum level? [closed]

let's look at force at the atomic level to understand the newtons third law of motion. I'll use Helium atoms as an example. Now imagine we start with one atom HE2 stationary, and throw another atom ...
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0answers
39 views

Creation of momentum on vertex (quantum field theory)

For a an interaction term like $g(\overline{\psi} \gamma^\mu \psi) \partial_\mu \phi$ in which $\psi$ is a Dirac spinor and $\phi$ a scalar field (d=4), should we expect this vertex to have a momentum ...
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0answers
98 views

How do proteins perform their function? [closed]

Let's, for example, take a ribosome. It is an enzyme that is in turn just a molecule that must follow the laws of physics. Correct me if I'm wrong, but it can be looked upon as a molecular machine ...
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0answers
75 views

Question about interacting fields and feynman diagrams [closed]

The picture is taken from Chapter 4: 'Interacting Fields and Feynman Diagrams in An Introduction to Quantum Field Theory by Peskin and Schroeder. There is a two point correlation function ...
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0answers
25 views

How can a net force of 0 still lead to movement and how is movement possible with interaction pairs? [duplicate]

So I always thought of movement as interaction pairs. For example, I thought that a rocket moves by applying a force of hot exhaust gases, which then apply a reaction force back on the rocket ...
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6answers
9k views

Why is gravitation 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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0answers
74 views

Propagator with derivative interaction

I work with this interaction Lagrangian density $$\mathcal{L}_{int} = \mathcal{L}_{int}^{(1)} + \mathcal{L}_{int}^{(2)} + {\mathcal{L}_{int}^{(2)}}^\dagger = ia\bar{\Psi}\gamma^\mu\Psi Z_\mu ...
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0answers
41 views

Generating functional for free and interacting theories [closed]

I'm asking probably a stupid question. We define the generating functional for free theories as $$ Z_0[J] = \int D \psi e^{i\int d^4x \left[ L_0(x) + J_l(x)\psi^l(x) \right]} $$ with $L_0$ the free ...
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2answers
125 views

How is dark matter detected?

What methods do we use to detect Dark Matter? If I understand correctly, due to lack of electromagnetic interaction it should be able to phase through normal matter nearly like through void - since ...
0
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1answer
37 views

Interaction Hamiltonian and shifts

When we quantize a free field theory, we set $\phi(x)$ to be the operators and we take the Fourier transform to determine the creation and annihilation operators $a_\omega,a^\dagger_\omega$ such that ...
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2answers
219 views

Yukawa interaction between Dirac particles is universally attractive?

Can anyone provide me a specific reference to (or supply themselves) the derivation of the fact that the Yukawa interaction$$\mathcal{L}_{\text{int}} = -g\overline{\psi} \psi \phi$$between Dirac ...
0
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0answers
35 views

How can we get the interaction hamilton $H_\text{int}$ from the Lagrange $L$?

After we quantize the free field we continue on determining the form of $H$. We can impose, by example: $$H=H_0+\lambda V_\text{int}$$ My question is, can we determine $H_\text{int}$ by the ...
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0answers
52 views

Are there any universal forces which are cartesian in nature? [closed]

I was recently talking with someone about how I think the whole Cartesian xyz understanding of the universe evolved from animals thinking earth was flat. They could get along fine without having to ...
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1answer
60 views

What does Kaluza-Klein theory say about the attraction/repulsion of opposite/same charges?

Since Kaluza-Klein theory is made out of general relativity - a gravitational theory in 4 dimensions which is only attractive, then how does it takes into account the attraction/repulsion of ...
1
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1answer
347 views

Interaction pictures of Quantum Mechanics

I want to understand the Schrödinger, Heisenberg and interaction picture and have a few questions about them: So in general you have a time-dependent Hamiltonian $H$, as for example the potential may ...
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0answers
50 views

Can quantum mechanics, general relativity and all physical theories be reduced to geometry? [duplicate]

I was told by my, perhaps ignorant (that is for you to decide), teacher that "all physical theories can be reduced to geometry in the manner of Newton's Principia/Euclid's Elements, however, due to ...
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6answers
28k views

How exactly does gravity work?

The electromagnetic force and strong and weak forces require particles like photons and gluons. But in case of gravity there is no such particle found. Every mass bearing object creates a ...
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2answers
151 views

Feynman diagram for attractive forces

I’m looking at Feynman diagrams for attractive forces and I'm thoroughly confused. Below are three diagrams from HyperPhysics: These all illustrate instances where the forces are attractive. ...
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0answers
39 views

Is this the correct way to obtain $<f|i>$ term in $\phi^4$ interaction theory? [closed]

Lets first write the expectation value of the fields in the interaction picture; $$ ...
0
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1answer
164 views

Propagator and probability amplitude that a particle propagates

My QFT knowledge has very much rusted and i got confused by these few lines from Peskin and Schroeder: p.27: " [..] the amplitude for a particle to propagate from $y$ to $x$ is $\langle 0| \phi(x) ...
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1answer
380 views

Källén–Lehmann spectral representation for massless particle?

Is it possible to write down a KL-like formula for massless particles (in particular, the photon)? The usual proof of the theorem assumes (see ...
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2answers
49 views

Do we actually understand what the forces are?

I'm not being cheeky. I'm not looking for a mathematical explanation of how to measure forces. I'm trying to figure out if humans have yet to understand what is actually happening in the space ...
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0answers
40 views

What is the current theory underlying the concept of fields? [duplicate]

When I went to school I was specifically told that fields are material (they occupy some region in space, and they "exist" there) and continuous. Recently, studying quantum physics I came across the ...
0
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1answer
75 views

Reason behind fundamental forces

Can anyone please explain the basic most fundamental reasons behind fundamental forces, i.e. what causes electromagnetic, nuclear and gravitational forces.
0
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2answers
111 views

Why bremsstrahlung occurs only with the nuclei? Why not with the electrons?

In many books I read that bremsstrahlung effect (for e+) only occur when the electron goes near the atomic nuclei. Why is not possible when cross near an atomic electron? Thanks,
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2answers
746 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 ...
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0answers
91 views

Is the phrase “coupling constant” interchangable with “ strength of interactions”?

Can I use the terms coupling constant and strength of interactions, interchangeably, or are there more subtleties to the term coupling constant that I am not aware of? Coupling Constants from ...
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2answers
323 views

Can all fundamental forces be repulsive?

If the electric force can be attractive (with opposite charges) or repulsive (same charges), and the magnetic force acts like this too, can all forces be repulsive in some cases? For example, could ...
0
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1answer
45 views

Interactions of light with the air

This is an interesting thought which I had when driving home today looking in my wing mirrors. If you are driving a car and looking in your, say, right wing mirror, you see an image of the car ...
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2answers
126 views

How force is transferred from one body to another

If there are 3 coins , namely 1 , 2 and 3 as in figure. When coin $1$ strike coin $2$ ,the coin $2$ passes the force to coin $3$ and the coin $3$ moves away. Case :1 How does this happen? What ...