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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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Can QFT be a theory of Quantum Gravity? [duplicate]

Tom Banks argues that QFT cannot be a theory of gravity because of the vacuum state in QFT. https://www.youtube.com/watch?v=mHVmgNfA3pM Here is the first lecture on this argument. Does anyone see ...
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1answer
15 views

Real Lagrangian with complex variable

I have a general question concerning real valued Lagrangians that take complex arguments. I have seen in many works of physicists and lecture books where extremal problems are discussed in Lagrangians ...
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1answer
33 views

S Duality and Effective Couplings

I am brand new to this subject, so this will probably be a very stupid question, but I would appreciate any patient explanations. S-duality is typically described as a relationship between two QFTs (...
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43 views

Where does $\vert\cal{M}(k)\vert^2$ come from in the last equation, p235 in Peskin&Schroeder's book?

In the Optical Theorem section, in the middle paragraph of p235, it is said "What is left over in expression (7.52) is just the factor $\lambda^2$, the square of the leading-order scattering ...
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16 views

Why is “degeneracy pressure” not considered a force? [duplicate]

When you try to put two electrons or neutrons close together (e.g. in a white dwarf or a neutron star), these spin-$\frac{1}{2}$ particles, by the Fermi principle, tend to push each other apart. For ...
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45 views

What are Tom Banks's arguments against a QFT of QG cont..? [duplicate]

What are Tom Banks's arguments against a QFT of quantum gravity? I do not feel that the answer given here includes all of his arguments. The answer given in the other question only included his ...
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14 views

What are the equations used to define energy density and pressure in inflation driven by vector field?

I’ve read that there are inflationary models which use vector field instead of scalar field. What are the equations used to define energy density and pressure in inflation driven by vector field? I ...
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24 views

Canonical ensemble and Quantum field theory

As we all know in Canonical ensemble number is fixed. It is a closed system. But can one use canonical with quantum field theory? Particularly case like heavy ion collision, where the number is not ...
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54 views

Can a scalar field also has positive or zero pressure?

In inflation theory, the inflaton field produce negative pressure to drive inflation. But now I’m going to ask about scalar fields in general, not the inflaton field. Can a scalar field also has ...
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1answer
35 views

Don't the magnets slip and turn in Stern-Gerlach experiment, if their orientation is not aligned with the applied magnetic field?

I was watching two videos on YouTube: In this video, there is an attempt to explain what spin is, using analogy of magnets. But here it is shown that magnets which are not aligned with magnetic field ...
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50 views

Residual gauge invariance exercise

I'm doing an exercise about residual gauge invariance. It is exercise 9.3 in page 21 of this exercise sheet. In particular, I'm looking at question (iii) where I need to show that $$\left\langle \psi'...
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1answer
103 views

Can we get full non-perturbative information of interacting system by computing perturbation to all order?

As we know perturbative expansion of interacting QFT or QM is not a convergent series but an asymptotic series which generally is divergent. So we can't get arbitrary precision of an interacting ...
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20 views

Are internal string states analogues of internal particle states?

It is sometimes said that in string theory elementary particles correspond to modes of vibration of a string. Likewise in QFT, several particle fields take their values in a higher dimensional space,...
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19 views

Topological soliton objects in Minkowski v.s. Euclidean spacetime?

What makes the distinctions between the soliton objects in Minkowski or in Euclidean spacetime? It looks that usually, the Euclidean path integral is easier to be performed in many cases. In fact, ...
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37 views

Why does the SUSY vacuum energy vanish independently of the quantization scheme?

This question was inspired by the comments here. It is straightforward to show that the SUSY vacuum energy vanishes, $H|0 \rangle = 0$, using nothing but the SUSY algebra. For people who prefer a less ...
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3answers
131 views

QFT = QM + SR? for everyday people

I understand that originally QM was a successful theory that has been experimentally proven, and was non-relativistic. Now its successor was said to be QFT. Later on, it incorporated effects of SR, ...
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1answer
123 views

What are Tom Banks's arguments against a QFT of quantum gravity? [on hold]

https://arxiv.org/abs/1007.4001 Pages 1-9 What are his arguments here against a QFT of QG, can someone provide a simpler explanation of these? And to what specific theories of QG do these arguments ...
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15 views

Metric-dependent operator and its variation

In the proof that the metric variation of correlation functions for certain operator ($\mathcal{O}$) vanishes, do we always need to assume that operator itself is metric independent? In other words, ...
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1answer
76 views

“The value of a quantum field at a point in spacetime is not precisely determined” explained in layman’s term

The value of a quantum field at a point in spacetime is not precisely determined. I haven’t studied quantum physics but came across this. Please explain it to me in simple term what it physically ...
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0answers
107 views

Simplify quantum electrodynamics expression

I have a quantum electrodynamics exercise on the one-loop electron self-energy correction in which I need to show that $$ ie^{2}\Sigma(p)=\frac{\left(-ie\right)^{2}}{\left(2\pi\right)^{4}}\int d^{4}...
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0answers
40 views

Can a solid have a different melting point depending on the orientation of its surface with respect to its crystal lattice?

If we assume the second law of thermodynamics, then it's easy to derive the zeroth law of thermodynamics and then show that since the second law of thermodynamics and zeroth law of thermodynamics are ...
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1answer
46 views

Why is it necessary to introduce different sets of creation and annihilation operators to quantize the complex K-G field?

I am reading Peskin & Schroeder and in chapter 2 (p.21) he quantizes the real K-G field such as: $$\phi=\int\dfrac{d^3p}{(2\pi)^3}\dfrac{1}{\sqrt{2E_p}}\left(a_pe^{ip·x}+ a^{\dagger}_pe^{-ip·x}\...
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0answers
34 views

Relativistic Von Neumann equation?

In non-relativistic quantum theory, Schrödinger's equation can be re-expressed using the density matrix $\rho=|\psi\rangle \langle\psi|$ as the Von Neumann equation: $$i\partial_t \rho = \frac{1}{\...
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67 views

Dark Matter Monojet signal

Dark matter can be observed at the LHC via a mono-jet signal i.e $pp\rightarrow XX + jet$. These signals can proceed via parton level processes like the mono gluon or mono photon ones. I was ...
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A book on quantum dots

I am looking for a good textbook in which theory would be formulated for quantum dots of different sorts both manufactured of a metal and a semiconductor, as well as its interaction with the ...
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1answer
71 views

Is there any proof that any result from perturbation theory is necessary an asymptotic series?

I know that almost all the series coming from perturbation theory are divergent, such as those from eigenvalue problems or the S-matrix in quantum field theory. The lore is that the series are ...
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36 views

What is the gravity equivalent of combining propagators?

In QFT we can combine two propagators like so: $$\Delta_{t_A,t_C}(A,C) = \int \Delta_{t_A,t_B}(A,B) \Delta_{t_B,t_C}(B,C) dB$$ i.e. the propagator from A to C (where A is a state - for example the ...
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26 views

How easy would be doing a bicycle wheelie on moon? [closed]

Ok! This kid was riding his brand new bicycle in the park. I asked the kid let me take the bicycle for of wheelie round around the park. He agreed hesitatingly. :P I did some wheelies 1 pedal ...
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2answers
65 views

Prerequisites for chiral Perturbation theory

I am Ms final year student. Could you pls guide me for the prerequisites of the chiral Perturbation theory? I have studied Quantum Field theory up to large $N$ gauge theory and renormalizations. Now I ...
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36 views

In the QED, is the polarized amplitude squared Lorentz invariant?

In the QED, if we sum over the polarization states of the initial photons and the initial leptons, the amplitude squared is Lorentz invariant. However, if we do not sum over the polarization states of ...
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1answer
71 views

What is the effect of including additional representations in the action of a lattice gauge theory?

I'm reading Introduction to Quantum Fields on a Lattice by Jan Smit. When introducing the lattice gauge-field action as a sum over plaquettes, Smit says that in general the action should include a sum ...
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1answer
78 views

Hydrogen and the neutron

I have a question about Hydrogen and the neutron. So a neutral hydrogen atom has structure ${}^1H\equiv[p^+ e^-]$, a bound proton and electron. From a quantum theoretical viewpoint, one would write ...
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1answer
53 views

What is the relationship between the JNR instantons and the BPST instanton?

The JNR instantons are related to the t'Hooft ansatz, and take the form \begin{equation} A=\sigma_{\mu\nu}\frac{\partial_\nu \rho}{\rho}dx^\mu, \end{equation} where $\rho$ takes the form \begin{...
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1answer
137 views
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Why is technical naturalness enough?

There are two notions of "naturalness" used in particle physics today. Dirac naturalness: all dimensionless parameters $g$ in a theory should be order $1$. Technical naturalness: an observed coupling ...
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30 views

Are trans-Planckian field values a problem for the relaxion?

The relaxion is a new model-building gadget that solves the hierarchy problem by allowing the Higgs mass term to dynamically relax to zero. To describe it as simply as possible, leaving a lot out: we ...
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31 views

About ghost D-branes and wrong statistics

I'm currently reading this article, which is about Ghost D-branes in string theory. The authors define these objects as something that cancels the effects of an ordinary D-brane. However, there is ...
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158 views
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What motivates clockwork theory?

Clockwork is a new model-building gadget that produces very small couplings starting from a theory with no small numbers at all, in an attempt to solve the hierarchy problem. To describe it as ...
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1answer
70 views

What does soft symmetry breaking physically mean?

A symmetry can be explicitly broken by adding terms in the Lagrangian that aren't compatible with the symmetry, and we say the symmetry is softly broken if all these terms have positive mass dimension....
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52 views

Contact terms in derivation of Ward identity

(This post is a bit long; the key question is between the horizontal lines, and an example follows it.) I am following the derivation of the Ward identity in Schwartz's QFT book, and there is a key ...
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1answer
45 views

The observer effect, how is related to death of human brain or consciousness? [duplicate]

Are we just a measurement device that interacts with quantum wave function/photons or something more? if not then can we die/be destroyed as observer?
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1answer
45 views

Invariant terms of Chiral Lagrangian

Stupid question. Consider a global SU(N) theory spontaneously broken. I want to write the EFT of the Goldstone bosons in terms of the field $$ \Pi = e^{i\pi^a T^a} $$ where $T^a$ are the SU(N) ...
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1answer
68 views

Unitary Representations in Conformal Field Theory

So I am currently studying conformal field theory from the perspective of the representation theory of Lie algebras. I am trying to understand exactly why we care about unitarizable Verma modules. For ...
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37 views

Why can't QFT in curved spacetime be a valid hypothesis for quantum gravity? [duplicate]

Disclaimer: I've only learned basic QFT as part of an undergraduate course, if there's something I don't understand about this then please let me know. I've recently been learning about Dirac ...
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2answers
70 views

Why is slow-roll is prefered over non-slow-roll inflation?

Wikipedia says that in new inflation, the slow-roll conditions must be satisfied for inflation to occur. What is fast-roll or non-slow-roll inflation and why is slow-roll is prefered?
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1answer
53 views

Tracing over a Fock space?

Suppose you have a bosonic Fock space with a vacuum $|0\rangle$. A particular state is labeled by the parameter $N \in \mathbb{Z}$. You can construct states like $$ | n_{N} \rangle = \frac{ \left( \...
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1answer
69 views

What is the value of the mass gap in QCD?

Is there any estimation either from experiments or numerical simulations for the value of the mass gap in QCD?
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62 views

Which of the Wightman axioms are not incorporated by four dimensional quantum Yang-Mills?

I am trying to understand the quantum Yang-Mills existence problem but the best I have seen so far is the statement that there is no known interacting relativist field theory in four dimensions which ...
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0answers
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Diagonalizing the free Hamiltonian $\hat{H}$ in terms of smeared wave packet operators

Consider the free real scalar field in (1+1)-dimensions. If the (normal-ordered) Hamiltonian is $$ \hat{H}=\int_{-\infty}^{\infty} dk\ \sqrt{ k^2 + m^2 } \hat{a}_{k}^{\dagger} \hat{a}_{k} $$ where $\...
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1answer
41 views

Is Light Energy? [duplicate]

I have been thinking about different forms of energy and ways to define them, but I realized that most elementary forms of energy are waves, such as light (a wave in the electromagnetic field). Waves ...