7. ### I QFT - Field operator commutation

Hi everyone, I'm taking a QFT course this semester and we're studying from the Otto Nachtman: Texts and Monographs in Physics textbook, today our teacher asked us to get to the equation: [Φ(x,t),∂/∂tΦ(y,t)]=iZ∂3(x-y) But I am unsure of how to get to this, does anyone have any advice or any...
8. ### A Bremsstrahlung single photon or spectrum

Non-relativistic Bremsstrahlung is discussed classically in Rybicki “Radiative Processes in Astrophysics” where Larmor’s formula is used to find the power radiated in a collision between an electron and a Coulomb field. The Fourier transform of the pulse allows for a description of the pulse in...
9. ### A Why does this amplitude not vanish by the Ward identity?

Consider the process e^-\rightarrow e^-\gamma depicted in the following Feynman diagram. The spin-averaged amplitude with linearly polarised photons is \overline{|M|^2}=8\pi\alpha\left(-g^{\mu\nu}+\epsilon^\mu_+\epsilon^\nu_-+\epsilon^\mu_-\epsilon^\nu_+\right)\left(p_\mu p^\prime_\nu+p_\nu...
10. ### Calculation of g-factor correction in Peskin p. 196

I'm reading peskin QFT textbook. In page 196 eq. (6.58) it says $$F_2(q^2=0)=\frac{\alpha}{2\pi}\int ^1_0 dx dy dz \delta (x+y+z-1) \frac{2m^2z(1-z)}{m^2(1-z)^2}\\=\frac{\alpha}{\pi}\int ^1_0 dz\int ^{1-z}_0 dy \frac{z}{1-z}=\frac{\alpha}{2\pi}$$ I confirmed the conversion from the first line...
11. ### Particle Schwartz and QFT Following Tong's Notes

Hello, I have been following Tong's notes on QFT and have found them to be a great introduction. I am almost at the end and am trying to figure out how to proceed. I have seen recommendations on David Skinner's notes, but I think I want to use a textbook either with Skinner's notes or maybe...
12. ### I Propagator of a Scalar Field via Path Integrals

I don't understand a step in the derivation of the propagator of a scalar field as presented in page 291 of Peskin and Schroeder. How do we go from: $$-\frac{\delta}{\delta J(x_1)} \frac{\delta}{\delta J(x_2)} \text{exp}[-\frac{1}{2} \int d^4 x \; d^4 y \; J(x) D_F (x-y) J(y)]|_{J=0}$$ To...

14. ### Quantum Intro to QFT notes/texts (minimal "external guidance")

Hi everyone, My university has completely scrapped an undergrad Intro to QFT course and I am essentially left with no choice but to do self-study with no (worst-case scenario) external guidance. Are there notes or texts that can be used for such a purpose? Would be a nice bonus if there's a...
15. ### Deriving commutator of operators in Lorentz algebra

Li=1/2*∈ijkJjk, Ki=J0i,where J satisfy the Lorentz commutation relation. [Li,Lj]=i/4*∈iab∈jcd(gbcJad-gacJbd-gbdJac+gadJbc) How can I obtain [Li,Lj]=i∈ijkLk from it?
16. ### I Why is position just a label in QFT?

At the beginning of every course in QFT we are told that, unlike in ordinary QM in which the position variable is a physical observable , the position variable in QFT is just a label. Yet there are areas within QFT where the position variable is treated like a real physical degree of freedom...
17. ### I Physical Meaning of a Quantum Field

Sorry in advance if this question doesn't make sense. Anyway, I am reading a paper about quantum field theory and the Whitman Axioms (http://users.ox.ac.uk/~mert2060/GeomQuant/Wightman-Axioms.pdf), and it describes a field (Ψ) as Ψ:𝑀→𝑉⊗End(𝐷) where 𝑀 is a spacetime manifold, 𝑉 is a vector...
18. ### A Gauge breaking and Faddeev-Popov ghost particles

Summary: In QFT, if we add a gauge breaking term to the Lagrangian, do we still need to introduce Faddeev-Popov ghost particles? Ghosts seems to be introduced to maintain gauge invariance. But suppose we have eliminated the gauge invariance, from the start, by explicitly introducing a gauge...
19. ### I Hermitian operators in QM and QFT

I have always learnt that a Hermitian operator in non-relativistic QM can be treated as an "experimental apparatus" ie unitary transformation, measurement, etc. However this makes less sense to me in QFT. A second-quantised EM field for instance, has field operators associated with each...
20. ### A Calculating the propagator of a Spin-2 field

Nieuwenhuizen uses a method for calculating the propagator by decomposing the field ## h_{\mu\nu}, ## first into symmetric part ## \varphi_{\mu\nu} ## and antisymmetric part ## \psi_{\mu\nu} ##, and then by a spin decomposition using projector operators. Using this he writes the dynamical...
21. ### I Position in QFT

A lecturer today told the class that relativistic QM for single particles is flawed by showing us that for a state centered at the origin, it was possible that ##Pr(\vec{x}>ct)>0##. He said that this was down to the fact that we should be considering multi-particle states in relativistic...
22. ### A Recent paper on QED using finite-dimensional Hilbert space - validity?

I've been struggling with a somewhat-recent paper by Charles Francis, "A construction of full QED using finite dimensional Hilbert space," available here: https://arxiv.org/pdf/gr-qc/0605127.pdf But also published in Electronic Journal of Theoretical Physics 10(28):27–80 · May 2006. Francis...
23. ### B From where did the ##ie\gamma## come into the picture? (QED)

While reading the electromagnetic vertex function at one loop, the authors of the book I am reading, wrote down the following vertex function: corresponding to this Feynman diagram: The superscript in ##\Gamma## is the number of loops being considered. My problem is with the equation. I...
24. ### I Touching and Zeno's paradox

How is the contact, or the interaction of electrons, if, according to Zeno's paradox, the distance between objects is divided into infinite points?
25. ### B How do you find the Lagrangians for different fields?

I am currently studying QFT from this book. I have progressed to the chapter of QED. In the course, the authors have been writing the Lagrangian for different fields as and when necessary. For example, the Lagrangian for the complex scalar field is \mathcal{L} \ = \ (\partial ^\mu...
26. ### A Observables when the symmetry is not broken?

Hi, Let be a scalar field φ that permeates all space. The quantum of the field has a mass m. The field is at the minimum of its potential. When this minimum is for φ≠0 (a broken symmetry), the quantum may be observed by exciting the field, as with the Higgs boson. But if the symmetry is not...
27. ### Where does energy come from in QFT?

When we say energy is applied to that field to create an excitation which is a particle, where does that energy come from and how is it applied? For example on beta decay where a new electron is formed, where does the energy come from the create an excitement in the electron field?
28. ### B How does the Higgs field “Give mass”?

Problem Statement: How does the Higgs field “give mass” Relevant Equations: The exact way that particles interact with the Higgs field and therefore create mass. I’m trying to figure out how the Higgs field works, one problem is that while I originally though of the Higgs field like a medium...