Quantum Field Theory (QFT) is the theoretical framework describing the quantisation of classical fields which allows a Lorentz-invariant formulation of quantum mechanics. QFT is used both in high energy physics as well as condensed matter physics and closely related to statistical field theory. Use ...

learn more… | top users | synonyms (1)

0
votes
0answers
15 views

Need help with Weinberg II (19.5.42)

In "The Quantum Theory of Fields", volume II, p. 202, I can't see how eq. (19.5.42) leads to the following equation. I can maybe see how $$\bar N N= tilde \bar N N$$ but that doesn't go very far... ...
0
votes
0answers
14 views

Repulsive potential for free fermions

My question -which is probably easy to answer for a physicist- stems from trying to understand the repulsive interaction between fermions. For instance the fact that states of multifermion systems are ...
1
vote
0answers
23 views

Electric field operator in 2D geomatry

In the free field (3D), transverse electric field operator is given by the below expression; $$e^{\bot}(\textbf{R}) =i \sum_{\textbf{p},\lambda}\Big( \frac{\hbar cp}{2V\epsilon_{0}}\Big)^{1/2} ...
3
votes
1answer
80 views

Why is a relativistic quantum theory of a finite number of particles impossible?

In Dyson's book Advanced Quantum Mechanics , he said "These two examples (the discovery of antimatter and meson) are special cases of the general principle, which is the basic success of the ...
-2
votes
0answers
24 views

time as consequence of hadronics [on hold]

it has occurred to me that time is solely consequence of non-electric fields, with latest work being reading about «anapole» cite: Simple theory may explain dark matter due to ...
1
vote
2answers
198 views

Is entropy a meaningful concept on a quantum level?

My naive assumptions, as I really am at a pretty basic stage in QM, are as follows: Classically, entropy gives us a practical measure of the direction of time, as opposed to our physical laws which, ...
0
votes
1answer
40 views

Are the particle-antiparticle pairs produced in vacuum virtual particles, and can they interact with normal particles?

If it is true that due to energy fluctuations of a vacuum being able to produce a particle-antiparticle pair that shortly annihilate with each other and disappear again, is the following circumstance ...
7
votes
1answer
87 views

Why is a vertex a derivative of the propagator?

Where can I find the proof to this nice trick: if the momentum $q$ is small, the vertex is the derivative with respect to the mass of a propagator times a factor $(-m/v)$ like in the picture:
0
votes
0answers
38 views

Can I Wick-contract terms with derivatives with terms without derivatives?

Consider for example the QCD three point vertex, can I contract a gluon field with the gluon field with a derivative in the vertex?
2
votes
2answers
107 views

Is there any $SU(\infty)$ gauge theory in quantum field theory?

The groups $U(N)$ and $SU(N)$ are the most important Lie groups in quantum field theory. The most popular are the $U(1),SU(2),SU(3)$ groups (these gauge groups form the Standard model). But is there ...
0
votes
0answers
42 views

QED+Classical Background Renormalization

I would like to ask a question related to quantum corrections and renormalization in QED. We have the QED vertex $\overline{\psi}[-ie \gamma^{\mu}(B_{\mu}+A_{\mu})]\psi,$ being $B_{\mu}$ a classical ...
0
votes
0answers
63 views

Klein-Gordon Field Angular Momentum Operator in Terms of Creation and Annihilation Operators

I am computing the angular momentum operator for real Klein-Gordon field (essentially question six of here (though please note this is not a homework question, I am following through Tong's course via ...
0
votes
0answers
17 views

If we considered chiral perturbation theory with coplex $\phi$-s, wold the next lo leading order renormalization $\gamma$-s change?

The Lagrangian of chiral perturbation theory (with two quark flavors) is written using the following matrix $U$ $$U=e^{i\sigma^i\phi_i/f}$$ where $\sigma^i$ are the Pauli matrices, $\phi_i$ are three ...
1
vote
1answer
55 views

Distinction of Dirac monopole and Polyakov-'t Hooft monopole

Can anybody explain the physical difference between Dirac monopole and Polyakov monopole? First, let me write down what I know briefly. Dirac monopole It comes from the symmetry of Maxwell ...
-6
votes
0answers
37 views

how could the sun photons be the source of light to our vision? [on hold]

if the atom has 99.99% empty space and the photon has no mass while our universe is 2dimensional flat so how could the sun photons be the source of our vision? how could photons be reflected by ...
1
vote
1answer
49 views

A question to gauge fixing in nonabelian gauge theories

In quantum gauge theories it is usual to fix the gauge with the equation $\partial^\mu A_\mu = 0$ where $A_\mu$ is the gauge connection. From this gauge fixing condition the remaining gauge degree of ...
1
vote
0answers
26 views

Covariant projection method - Meson bound states

I have seen many papers that discuss the production or decay of mesons ( quark bound states ) to make use of the covariant projection method where the product $\upsilon\bar{u}$ of the quark spinors ...
2
votes
2answers
61 views

Compact QED and Non-compact QED - Polyakov textbook

This question is related with Polyakov, "Gauge Fields and Strings" section 4.3 Firstly, Polyakov define a QED on a lattice Compact QED \begin{align} S = \frac{1}{2} \sum_{x, \alpha, \beta} ...
1
vote
1answer
36 views

Do contractions with Dirac matrices involve a metric?

When figuring out where the spacetime metric enters an equation it is often useful to write all vector indices as covariant indices and write out the inverse metrics that are needed to contract them, ...
4
votes
1answer
92 views

Strange use of complex analysis in Weinberg QFT 1?

In the beginning of chapter 3 on scattering theory in Weinberg's QFT book there is a use of the Cauchy residual theorem that I just cannot get. First some notation, we are looking at states that are ...
2
votes
0answers
37 views

Textbooks for nonequilibrium quantum field theory

What are good textbooks for nonequilibrium quantum field theory? Please answer by naming quality books and a short description and review about each one that you have personally read or benefited ...
1
vote
0answers
51 views

Negative interaction energies for relativistic bound states

I guess that bound states are treated as poles with infinite lifetime, i.e. precise determination for energy eigenvalues. But what's left with negative interaction energies, which are connected to ...
3
votes
1answer
94 views

Is the Dirac equation equivalent to the Klein-Gordon equation for its left handed component?

The Dirac equation $$(i\gamma^a\partial_a - m)\psi=0\tag{0}$$ is given by a first order operator acting on a Dirac spinor, which is the direct sum of a left handed spinor and a right handed spinor. ...
4
votes
0answers
120 views

Has Sen quantized superstring fields?

Today I saw a paper by Ashoke Sen titled "BV Master Action for Heterotic and Type II String Field Theories". Is it really the "quantization" of superstring fields for the first time? What can be its ...
0
votes
0answers
42 views

correlation functions [closed]

What is the physical meaning of following correlation functions in one-dimensional correlated electron systems: 1. density-density correlation function, 2. spin-spin correlation function, 3. ...
1
vote
0answers
41 views

How can the stress tensor components of a worldsheet CFT in general background be (anti)-holomorphic?

In all textbooks/lecture notes on string theory (e.g. Polchinski, page 43 at the bottom) it is proven that, as the stress tensor is traceless and conserved, $T^a_a=\partial^a T_{ab}=0$, we have ...
-3
votes
0answers
57 views

Singularity in the context of the Quantum mechanics [closed]

In the TV program , Professor Michio Kaku had told that singularity can be defined as the rotation of the neutron . I'm searching whether the insist was right or wrong . How does the quantum mechanics ...
4
votes
1answer
74 views

State space of interacting theories

Haag's theorem states that in general, an interacting quantum field and the corresponding free field have unitary-inequivalent state space representations. I would like to have an example of a state ...
4
votes
1answer
139 views
+50

Peskin eqn 7.2 contradiction

They state $\langle\Omega|\phi(x)|\lambda_p\rangle=\langle\Omega|e^{iP\cdot x}\phi(0)e^{-iP\cdot x}|\lambda_p\rangle$ where $|\lambda_p\rangle$ is a state of momentum $\textbf{p}$. They then rewrite ...
1
vote
0answers
39 views

Wightman axioms always imply triviality in 4D?

Someone mentioned to me in passing that it had been proven that the Wightman axioms are over-restrictive in four dimensions and provably always result in trivial correlators (or maybe a trivial ...
10
votes
2answers
254 views

Are there any inventions/applications in our world based on QFT?

Are there nowadays any actual devices or experimental applications which are based on the quantum field theory and if so, how are they related to QFT? I could not find any similar question besides ...
4
votes
0answers
68 views

Free probability in Physics

Recently I have started reading some materials on non-commutative probability. IN this area mathematicians sometimes consider quantum theory as a non-commutative version of classical probability, with ...
0
votes
3answers
83 views

How many fields that we know of permiate the universe?

The Higgs field, as I understand it reading layman's articles, permeated the entire universe only a fraction of a second after the big bang. Are there any other fields that they know about or ...
2
votes
2answers
54 views

What justifies the perturbative expansion in chiral perturbation theory?

The Lagrangian of chiral perturbation theory is ordered following a momenta power counting scheme, having terms at leading order (which is two 2 $O(p^2)$) next to leading order ($O(p^4)$) and so on. ...
1
vote
0answers
53 views

Quantizing Klein-Gordon via Lie Groups [closed]

I'm trying to understand second quantization of the Klein-Gordon equation, as explained in, say, standard books like Peskin and Schroder, but using the language of Lie (representation) theory. In a ...
0
votes
0answers
24 views

QFT Normalization of multi-particle states

Peskin 7.2 states that the identity operator for the entire Hilbert space is given by ...
1
vote
1answer
59 views

New “oscillator basis” of gamma matrices?

It was mentioned in http://kclpure.kcl.ac.uk/portal/files/12371620/Studentthesis-Mehmet_Akyol_2013.pdf page 28, a new concept "oscillator basis" or more precisely the author defines gamma matrices of ...
1
vote
1answer
79 views

How does one make sense of a delta function of a scalar field?

Disclaimer: Originally posted on math SE, but thought that it was better in physics SE, so deleted my post on math SE and posted here. In the classic review summary of stochastic quantization here, ...
0
votes
0answers
19 views

Do operator bases in Lagrangians have a vector space structure?

In effective field theories we deal with bases of operators. I wonder in which sense is this similar to bases of a vector space. We can change bases and write the Lagrangian in another basis, just as ...
8
votes
3answers
517 views

Two definitions of Green's function

In literature, usually two types of definition exist for Green's function. $\hat{L}G=\delta(x-x')$. This equation states that Green's function is a solution to an ODE assuming the source is a delta ...
5
votes
0answers
127 views
+50

Why are the quantum observables defined on opens sets a presheaf and not a sheaf?

In local quantum field theory or AQFT one can mathematically describe over each open set $U$ of a spacetime $M$ the quantum states or observables of the theory. This structure is commonly referred as ...
2
votes
0answers
45 views

QFT background needed for AdS/CFT integrability [closed]

Apologies if this type of question isn't permitted. I'm very interested in integrability in the context of AdS/CFT. I'm starting my Masters soon in a very GATIS-involved institute and would like to ...
1
vote
1answer
61 views

Does a charged particle propagating in free space have a 'self-energy' like term due to it’s interaction with the fluctuations of the quantum vacuum? [closed]

Does a charged particle propagating in free space have a 'self-energy' like term due to it’s interaction with the fluctuations of the quantum vacuum? (particle-antiparticle pairs popping into and out ...
2
votes
1answer
51 views

Ultraviolet behaviour in dimensional regularization

In dimensional regularization, we introduce an arbitrary energy scale $\mu$. Naively, it plays the role of another parameter of the theory that needs to be fixed experimentally, but actually it is not ...
0
votes
1answer
74 views

Derivation of eq 6.17, Peskin and Schroeder

I am having trouble following a derivation in Peskin and Schroeder's textbook, namely equation 6.17 on page 182. It seems benign at first, but I am completely stuck. Essentially, we have an expression ...
4
votes
0answers
36 views

How to handle the infrared divergence of massless $\phi^4$ in scattering

For massless $\phi^4$ theory, if exterior momentums are going to zero, then this diagram will be $$\int \frac{dk^4}{k^4}$$ will suffer from infrared divergence. Because the infrared divergence, ...
1
vote
0answers
23 views

Holography with wave functionals rather than partition functions

Roughly speaking Gubser-Klebanov-Polyakov Witten's (GKPW) prescription in the context of holography tells us partition function of CFT is "equal" to that of the gravity theory in one higher dimension ...
3
votes
1answer
65 views

Why is gravity sensitive to absolute energies?

In QFT absolute energies play no role in the physical set-up, only relative energies (i.e. energy differences) are important. However, in general relativity this doesn't appear to be the case, I've ...
0
votes
0answers
16 views

Bound states and corresponding elementary fields

Let's have some bound state, like positronium or meson. I need to calculate an amplitude of process which involves bound state in in- or out-state. Is it necessary to introduce corresponding ...
1
vote
2answers
89 views

Why Kink can not tunnel to vacuum, and is topologically stable?

Why the kink $$\phi(x)=v\tanh(\frac{x}{\xi}) ,$$ can not tunnel into vacuum $+v$ or $-v$ (Spontaneous symmetry breaking vacuum). From the boundary condition ($x\rightarrow \pm\infty, ...