# qft

1. ### 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...
2. ### 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...
3. ### 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...
4. ### 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...
5. ### 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...
6. ### 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...
7. ### 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...
8. ### 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...
9. ### 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?

23. ### I Weak interaction between electrons

The two electrons will be repelled by electrostatic force, but they interact with weak force, means that in addition to the electrostatic force between the electrons there will be weak force?
24. ### A Goldstone's theorem proof

Hello everybody! I have a question regarding the first step of the quantistic proof of the Goldstone's theorem. Defining $$a(t) = \lim_{V \rightarrow +\infty} {\langle \Omega|[Q_v(\vec{x},t),A(\vec{y})]| \Omega \rangle}$$ where $|\Omega\rangle$ is the vacuum state of the Fock space, $Q_v$...
25. ### I Difference between spin repulsion and electrostatic repulsion of an electron?

What is the difference between spin repulsion and electrostatic repulsion of an electron? Is this the same mechanism?
26. ### I Is it possible to detect an electrostatic field?

If the electron creates an electric field around itself that can be detected,then the electrostatic field is real? So why was not the "virtual" photon found?
27. ### I Can an electron split?

What does it mean that an electron in a material can split into a spinon or an orbiton? Does QFT have a spinon or orbiton field?
28. ### I Electrons and the Pauli exclusion principle

electrons are repelled using the Pauli exclusion principle?
29. ### I Electrostatic repulsion at a distance

What does it mean ? "the virtual photon's plane wave is seemingly created everywhere in space at once, and destroyed all at once. Therefore, the interaction can happen no matter how far the interacting particles are from each other." As far as I know, the electrostatic force between two...
30. ### On-shell renormalization scheme

1. Homework Statement Show that, after considering all 1 particle irreducible diagrams, the bare scalar propagator becomes: $$D_F (p)=\frac{i}{p^2-m^2-\Sigma (p^2)}$$ And that the residue of the pole is shifted to a new value, and beomes...