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 ...
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59 views
is quantum fluctuations a completely uncaused event?
I read here http://universe-review.ca/R03-01-quantumflu.htm that particles/antiparticles appear and annihilate each other spontaneously in empty space.
since particles appear and disappear in empty ...
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0answers
33 views
Can cold low entropic “little bang” be described? [closed]
Please can anyone help? I have an idea for a low entropy process I considered while trying to model protein folding. I am not a mathematician, so I am having trouble describing the concept.
I need ...
1
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1answer
320 views
What is the meaning of the concepts of “operator mixing” (and anomalous dimensions) [closed]
I am looking for an explanation about the idea of "operator mixing" and its associated concept about when anomalous dimension has to be thought of as a matrix.
For example this idea is slightly ...
6
votes
1answer
123 views
Open strings from closed strings
This issue comes up in Shiraz's lecture here on 29th October 2008.
I understand that he is saying that one can think of closed string theory as having two minima and that the ground state in the ...
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0answers
58 views
Difference between oscillon and quasi breather
How do we differentiate between oscillon and quasi breather ?
Which equation can give me the property of these wave?
I know the solition equation and got it for different potential, need to know when ...
4
votes
1answer
173 views
Yang-Mills instanton
How can instanton solution to Yang-Mills theory with gauge group $SU(3)$ or $SU(N)$ be obtained? For $SU(2)$ it is explained in textbooks but what about more general color gauge groups?
EDIT: How ...
9
votes
1answer
123 views
When are the OPE relevant?
I've seen OPEs commonly used in 2d CFT, it's quite apparent to me that, in that case, it dresses a bridge between the algebraic and the operator formalism especially when combined with radial ordering ...
1
vote
1answer
91 views
Why no fundamental force from the Higgs? [duplicate]
I wish to ask whether I understand the following correctly. This universe seems to have six fundamental elementary bosons namely photon $(\gamma),\ W$-bosons$(W^+,W^-),$ gluon$(g),\ Z$-boson $(Z)$, ...
2
votes
2answers
98 views
Higher order covariant Lagrangian
I'm in search of examples of Lagrangian, which are at least second order in the derivatives and are covariant, preferable for field theories. Up to now I could only find first-order (such at ...
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4answers
343 views
Could all strings be one single string which weaves the fabric of the universe?
This question popped out of another discussion, about if the photon needs a receiver to exist. Can a photon get emitted without a receiver? A universe containing only one electron was hypothetically ...
4
votes
0answers
54 views
Fermion annihilation amplitude
Is there a physical reason why annihilation of 2 identical fermions with mass m to 2 scalars amplitude for $s=4m^2$ (fermions at rest) is zero? For example we can have 2 scalars annihilating in 2 ...
0
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1answer
112 views
QED photon propagator to one-loop order gets different answers
I'm a self-studying 14-year-old who has a passion for particle physics. I'm currently trying to calculate the QED photon propagator to one loop. However, in all the places I've looked, even with the ...
4
votes
0answers
99 views
What's wrong with this QFT thought experiment?
In quantum field theory, the propagator $D(x-y)$ doesn't vanish for space-like separation. In Zee's book, he claims that this means a particle can leak out of the light-cone. Feynman also gives this ...
13
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8answers
2k views
What equation describes the wavefunction of a single photon?
The Schrödinger equation describes the quantum mechanics of a single massive non-relativistic particle. The Dirac equation governs a single massive relativistic spin-½ particle. The photon is a ...
3
votes
1answer
280 views
From quantization under external classical gauge field to a fully quantized theory
Let me take QED for example to clarify my question: The textbook-approach(at least for Peskin&Schroeder) to quantize ED is to first quantize EM field and Dirac field as free fields respectively, ...
3
votes
1answer
39 views
Interaction of an electromagnetic wave with a two level system in the domain of quantum field theory
Suppose I shine an electromagnetic wave on a two-level system. I need to describe how the system evolves in context of quantum field theory i.e. using a quantized EM field in the problem. The first ...
1
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1answer
90 views
Integrating out high momentum modes in $\phi^4$ theory
I'm trying to follow section 12.1 of Peskin & Schroeder, which describes how integrating out the high momentum modes of the field in $\phi^4$ theory transforms the Lagrangian both by changing the ...
4
votes
0answers
74 views
Calculating the branching ratio of higgs for decay to two photons? [closed]
I need to use the three lowest order Feynman diagrams to first calculate the squared matrix element to put into Fermi's golden rule formula and then from there get the branching ratio of higgs decays ...
3
votes
1answer
70 views
Is conservation of statistics logically independent of spin?
If the number of fermions is $n$, we expect the quantity $(-1)^n$ to be conserved, i.e., $n$ never changes between even and odd. This is known as conservation of statistics. In the normal context of ...
2
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0answers
146 views
Proof of Spin-statistics theorem [closed]
Is this proof of spin-statistics theorem correct?
http://bolvan.ph.utexas.edu/~vadim/classes/2008f.homeworks/spinstat.pdf
This proof is probably a simplified version of Weinberg's proof.
What is ...
1
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0answers
112 views
A fundamental equation for solitary wave and dimension analysis [closed]
According to the scalar Field theory we write Lagrangian as $$\mathcal{L}=\frac{1}{2}\partial^\mu \phi \partial_\mu \phi -\frac{m^2}{2}\phi^2 -\frac{\lambda}{4!}\phi^4 \tag {1}$$
What I want to do is ...
1
vote
1answer
83 views
Usefulness of SUSY models when it cannot exist at any non-zero temperature
Unlike other symmetries (like electroweak symmetry), SUSY is spontaneously broken at any non-zero temperature due to some variation of the fact that the boundary conditions on bosons and fermions in ...
2
votes
1answer
232 views
Crazy Dirac Deltas
I'm not expecting any rigor in the following and the answers...since we're dealing with Dirac deltas in the context of QFT.
Consider the integral
$$
\int d^4q\ \Theta(q_0)\Theta(p_{3,0}+q_0)\ ...
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0answers
85 views
$\mathrm{i}\epsilon$ prescription makes a function analytical?
I've seen this everywhere where they say "Analytic continuation is obtained by the usual $\mathrm{i}\epsilon$ prescription..." but how is that?
How do you analytically continue (say) $\ln x$ with ...
3
votes
1answer
65 views
Emergence of $SU(2)\times SU(2)$ at the self-dual point in bosonic string theory
I want to understand the derivation of the equations 8.3.11 in Polchinski Vol 1.
I can understand that at the self-dual point the Kaluza-Klein momentum index $n$, the winding number $w$, and the ...
2
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1answer
147 views
For nonabelian Yang-Mills in the Coulomb phase, can soft gluons render the charge orientation of charged particles indefinite?
For nonabelian Yang-Mills in the Coulomb phase, can soft gluons render the charge orientation of charged particles indefinite? Let's say the gauge group is a nonabelian simple Lie group G. Suppose ...
1
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0answers
44 views
Anomalous dimension for bare actions with a standard kinetic term
In this paper on p42, it is explained that when starting with a bare action that contains a standard kinetic term, this kinetic term attains a correction in the course of the RG flow which can be ...
2
votes
1answer
88 views
Path integral representation of $\langle q_f t_f|p(t_1)|q_i t_i\rangle $
How do I calculate path integral representation of $\langle q_f t_f|p(t_1)|q_i t_i\rangle $ where $t_i<t_1<t_f$? I am doing this by discretizing, the time intervals and adding a complete set of ...
3
votes
0answers
38 views
Helicity dependence in loop diagrams
I am trying to evaluate a diagram that looks like
The middle of the diagram is a fermion loop. I know that the coupling between the $Z^0$ and fermions depends on the fermions' helicities, so it ...
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0answers
47 views
Intrinsic Physical Reality and observers [closed]
Intrinsic physical reality, is made of intrinsic relations between intrinsic physical objects. But this physical reality does not depend about which observer is seeing these physical objects and their ...
4
votes
0answers
43 views
Relations between diffeomorphism symmetry theories and invariant $SU(N), N \rightarrow \infty$ theories
Is it possible to have, an exhaustive panorama (as much as possible), about the relations between theories having a diffeomorphism symmetry, and theories having a $SU(N), N\rightarrow\infty$ ...
5
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1answer
146 views
Meaning of spin
I'm pretty astounded that I did not hear about this sooner, but in my course on QFT our professor told us that the concept of spin can be used to mean three things:
Mechanical spin (apparently a ...
5
votes
1answer
71 views
Gauge fields and strings: Loop equations
I am trying to derive Eq. (7.25) (p. 117) of Polyakov's book:
$$ \delta \Psi (C) = \int_{0}^{2\pi} {\rm P} \left(F_{\mu\nu}(x(s)) \exp \oint_C A_\mu dx^\mu \right)\dot{x}_\nu \delta x_\mu(x) \, {\rm ...
3
votes
1answer
65 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, ...
2
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0answers
39 views
Quantum field theory meson scattering calculation (scalar yukawa theory)
Please see this question for a clear background of the notation I use. My issue is that I want to use Wick's theorem to calculate the amplitude of meson ...
6
votes
0answers
53 views
What is energy in $z \neq 1 $ theories?
In a critical theory with dynamical critical exponent $z \neq 1 $, which amongst frequency, $\omega$, and dispersion, $E(\vec{k})$, may be referred to as ''energy''? I'm confused about this since in ...
1
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1answer
149 views
A question from weinberg QFT
I'm self-studying weinberg QFT , I'm confused by his treatment of scattering theory .
I have the following question :
He introduces the free particle states $\Phi_{\alpha}$ but I'm not sure what is ...
1
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1answer
45 views
Why are non-momentum DoFs of single-particle states discretely labeled?
Following the treatment of Weinberg, chapter 2, we consider $\psi_{p,\sigma}$ as single-particle eigenstates of the 4-momentum. Weinberg says that $\sigma$ labels all other degrees of freedom and we ...
1
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1answer
63 views
Physical Interpretation of Lorentz-transformed Single Particle states being linear
As in this question, let $\psi_{p,\sigma}$ be a single-particle 4-momentum eigenstate, with $\sigma$ being a discrete label of other degrees of freedom.
Weinberg discusses the effect of a homogenous ...
2
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0answers
33 views
Why isn't there a Heterotic string theory which tensors the fermionic state with the Type II state?
The Heterotic (HO and HE) string is found by tensoring the left movers of the bosonic string theory state and the right movers of the Type II string theory state:
...
2
votes
1answer
69 views
Why do single particle states furnish a rep. of the inhomogeneous Lorentz group?
Following up on this question: Weinberg says
In general, it may be possible by using suitable linear combinations of the $\psi_{p,\sigma}$ to choose the $\sigma$ labels in such a way that ...
4
votes
1answer
129 views
What if EM or QCD was spontaneously broken?
Suppose that Standard Model Higgs mechanism broke electromagnetism, by e.g. veving the charged component of the doublet, so that the photon was massive with $m_\gamma\sim v$. Could such a Universe ...
9
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0answers
144 views
Unitary quantum field theory
What do physicists mean when they refer to a quantum field theory being unitary? Does this mean that all the symmetry groups of the theory act via unitary representations? I would appreciate if one ...
5
votes
1answer
86 views
Quantum master equation in the Batalin-Vilkovisky formalism
I am reading the Section 15.9 of Weinberg's book "The Quantum Theory of Fields, vol. 2". Under a shift $\delta\Psi[\chi]$ in $\Psi[\chi]$, we have
$$
\begin{split}
\delta ...
8
votes
1answer
101 views
What is the reason why anyons escape spin-statistic theorem?
I'm wondering about the exact reason why anyons escape the spin-statistic theorem (SST), see e.g. http://en.wikipedia.org/wiki/Spin–statistics_theorem.
I've read somewhere (the wikipedia page is ...
5
votes
0answers
83 views
What goes wrong when one tries to quantize a scalar field with Fermi statistics?
At the end of section 9 on page 49 of Dirac's 1966 "Lectures on Quantum Field Theory" he says that if we quantize a real scalar field according to Fermi statistics, the quantum Hamiltonian is no ...
2
votes
1answer
87 views
Complex masses for Dirac and Weyl spinors
I'm trying understand how to rotate Dirac fields to absorb complex phases in masses. I have a few related questions:
With Weyl spinors, I understand, $$ \mathcal{L} = \text{kinetic} +
...
1
vote
1answer
65 views
Anti-particle problem for Dirac sea
According to the Dirac hole theory we know that Dirac sea is completely filled with negative energy, called vacuum. We will need $2mc^2$ or greater to get electron and a positron by incident photon.
...
7
votes
1answer
213 views
Introductory examples of AdS/CFT duality
I would like to know, what are the simplest/starting/basic examples that are typically used to introduce students to how AdS/CFT really works? (not the MAGOO paper, as I am not sure it has concrete ...
5
votes
1answer
235 views
Spin-Statistics Theorem (SST)
Please can you help me understand the Spin-Statistics Theorem (SST)? How can I prove it from a QFT point of view? How rigorous one can get? Pauli's proof is in the case of non-interacting fields, how ...
