Qft Definition and 956 Threads

From the view point of quantum field theory how does one describe the electromagnetic energy density between the plates of a charged capacitor? Thanks!

Homework Statement I'm working through Zee for some self study and I'm trying to do all the problems, which is understandably challenging. Problem 1.3.1 is where I'm currently stuck: Verify that D(x) decays exponentially for spacelike separation. Homework Equations The propagator in question...

Hi everyone. For anyone who has the book. I am going through Quantum Field Theory for the Gifted by Amateur Tom Lancaster and Stephen J. Blundell. Are the topics enough to prepare me for a course in QFT and then Advanced QFT? Of course I can look for other resources. But I just want to know how...

Hi all, is Fock space Poincaré invariant? As far as I can see, the scalar product in Fock space involves the scalar products in its N-particle subspaces, which, in turn, are the integrals of the properly (anti-)symmetrized wave functions over space. This works well in a Galilei-invariant...

My level is not sufficient enough to easily understand QFT yet there is some basic question I need to understand in it - what in QFT corresponds to a wave function in QM, for a single particle case and, say, for a more general case of multiparticle nonseparable state (suppose the particles are...

I'm interested in all discussions but particularly those concerning the areas of QFT and cosmology. I've done much studying on the concept of quantum gravity over the past couple years and I want to learn more :)

In field theory we most of deal with theories whose Lagrangian densities are of the form (sticking to scalar fields for simplicity) $$\mathcal{L}= -\frac{1}{2}\partial_{\mu}\phi\partial^{\mu}\phi - \frac{1}{2}m_{\phi}^{2}\phi^{2} + \cdots$$ where ##\partial := \frac{\partial}{\partial x^{\mu}}##...

Hi all, I'm little confused about the unitarity and perturbativity constrains which imposed on a potential's parameters, like 2HD potential. Look for example: [arXiv:1507.03618v3 [hep-ph]] First, I'd like to know what is most essential ? I mean if unitarity constraind ## \lambda## to say less...

I live in Texas, and I'm going on my senior year of high school. I've come to a problem, I don't know what college/university I should continue my education at. I've already decided what I hope to do when I'm older- qft and quantum gravity, but I don't know what place is good to further ones...

As far as I understand it, the non-zero vacuum energy attributed to a quantum field (at each point in space-time) is precisely due to the Heisenberg uncertainty principle (and the fact that the energy of the quantum field at each space-time point is quantised). Accordingly (in order to satisfy...

..where can be found: What in the whole wide world does Zee mean with ?? Thank you

Hello all, I hope you can give me a hand with a QFT homework I'm working on. We are to compute the beta equation of a Non-abelian SU(N) theory with: Complex scalars (massless), bosons, ghosts. My question is referring to the Boson self-energy scalar loop correction. 1. Homework Statement We...

I have been trying to improve my understanding of QFT via reading this forum, Zee's QFT in a Nutshell and Lancaster and Blundell's QFT for the Gifted Amateur. I thought I was making some progress but recent post have left me confused so any pointers would be welcome. As I currently understand...

Homework Statement As part of a problem, I need to derive the EOM for a generalized Lagrangian. Before I get there, I'm trying to refresh myself on exactly how these derivatives work because the notation is so bizarre. I am trying to follow a simple example I found online: Start with...

Hi. First off, I'm not sure if this is the right sub to talk about QFT. Apologies if t isn't. I'm halfway through an introductory QFT course and I still don't get what it is. What is different in its formalism that makes it able to tackle problems that quantum mehanics can't deal with? From...

H=p^2/2m+c What's c? It's of course a shift in energy, but can be thought also as a smoother and smoother real-space local potential that becomes a constant all over the space. On the other hand, why couldn't one think about it as a constant potential in reciprocal space? It's a shift in energy...

Hello everyone, First of all, I am a third year undergraduate student. I have just finished studying (on my own) Sakurai' s "Modern Quantum Mechanics" (and I have done almost all exercises). I have taken courses in Complex Analysis (contour integration, residues etc) and in PDE (unfortunately...

Hi Everyone. There is an equation which I have known for a long time but quite never used really. Now I have doubts I really understand it. Consider the unitary operator implementing a Lorentz transformation. Many books show the following equation for vector fields: U(\Lambda)^{-1}A^\mu...

Hey there I'm starting to work on my thesis and this is one possible subject. I'm reading a few papers (pretty much anything that appears on the first google search page) dealing with it from several different angles. From a completely general and mathematical stand to specific examples, like...

<< Moderator note: Split from https://www.physicsforums.com/threads/why-do-we-need-quantum-mechanics-so-much.859210/ >> What about particle physics? It bases on QFT therefore on QM.Is that right?

Hi, I am about to work on the problem of trying to find a renormalization program for bound states in QFT. Any suggestions/advice on where to start would be much appreciated.

Possibly very silly question in QFT. Consider the Lagrangian for a scalar field theory. A term like g/φ^2 should be renormalizable on power counting arguments. The mass dimension of g should be 2 (D-1) where D is the number of space-time dimensions.Does this make sense?

Given an inhomogeneous ODE of the form $$a_{n}(x)y^{(n)}(x)+a_{n-1}(x)y^{(n-1)}(x)+\cdots +a_{2}(x)y''(x)+a_{1}(x)y'(x)+a_{0}(x)y(x)=f(x)$$ where ##y^{(n)}(x)\equiv \frac{d^{n}y(x)}{dx^{n}}##, why is the function ##f(x)## on the right hand side referred to as a "source term" ? In what way does...

I need to clear up my (mis)understanding about QFT. Does QFT show how a particle propagates through spacetime? (Or maybe this is the realm of QM) Or does QFT only specify how a particle propagates as a particle through time without reference to where in space it is? But... if QFT specifies how...

I'm working with the signature ##(+,-,-,-)## and with a Minkowski space-stime Lagrangian ## \mathcal{L}_M = \Psi^\dagger\left(i\partial_0 + \frac{\nabla^2}{2m}\right)\Psi ## The Minkowski action is ## S_M = \int dt d^3x \mathcal{L}_M ## I should obtain the Euclidean action by Wick rotation. My...

Great youtube introduction video about Quantum Field Theory (QFT) from a couple of days ago by Dr Don Lincoln @fermilab. The video and description of a particle being a disturbance in a field and flying through the air at 3:25 is especially compelling.

Hello, It's been a long time I am trying to accept renormalization in QFT but still I cannot be satisfied. The usual pedagogical way one introduces renormalization is to cure infinities that arise from perturbative expansions. Now, I can accept the statement that we are doing the perturbation...

Is QFT on curved spacetime BtSM

Hey guys, just curious of your opinions on this matter. Which graduate schools are the best to earn a Ph.D. in physics theory from and research in concerning the theoretical sub-fields Quantum Field Theory, General Relativity, and Elementary Particle Physics (maybe String Theory)? Which has...

Why are particles in QFT assumed to be point-like? This assumption is the source of ultraviolet divergences. Does anyone know what is the source of this assumption, and what happens if you assume that particles are not point-like?

[Split off from https://www.physicsforums.com/threads/photon-energy-and-light-quantum-hypothesis.847848/#post-5329824] As we know, there is a serious contradiction between nonlocal indeterminacy of quantum theory and local reality of special relativity, specifically reflected in the superluminal...

Hello, I do not understand how to compute the infinitesimal variation of the field at fixed coordinates; under lorentz transformation . I am doing something wrong regarding the transformation of the ##x## coordinate. I am looking for: ##\Delta_a=\phi_a'(x)-\phi_a(x)##, variation appearing in...

I realize that Wiki is not the preferred reference source here, but I'll go ahead with this question anyway... In the latest iteration of the article on the Standard Model is the statement, "We see that the mass-generating interaction is achieved by constant flipping of particle chirality." Is...

Reading through David Tong lecture notes on QFT.On pages 43-44, he recovers QM from QFT. See below link: [QFT notes by Tong][1] [1]: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdfFirst the momentum and position operators are defined in terms of "integrals" and after considering states that...

As I understand it, the 2-point fnuction is for 1 particle incoming, 1 particle outgoing. The 4-point function is for 2 particles incoming, 2 particles outgoing. Is this correct? So an N-point function describes N/2 incoming particles and N/2 outgoing particles? Thanks!

Hi, I'd like to ask you if you could write a list of QFT books in order of increasing difficulty. Thanks!

In, QFT, an elementary particles is an excitation of its quantum field. Quantum fields are just mathematical. For example an electron is excitation of the electron field. But is the excitation of the field physically real or just mathematical? What i mean is, is there something physically...

Hello everybody, I have three quite mathematical questions in modern QFT. 1) Why it's supposed that N=2 SUSY Yang-Mills probably cannot be put on a lattice? 2) What is the recent status of lattice approach to conformal quantum field theories? This question is motivated by the following...

Homework Statement Consider the following scalar theory formulated in two-dimensional Euclidean space-time; S=∫d2x ½(∂μφ∂μφ + m2φ2) , a) Determine the equations of motion for the field φ. b) Compute the propagator; G(x,y) = ∫d2k/(2π)2 eik(x-y)G(k). Homework Equations Euler-Lagrange equations...

I've been trying to teach myself the path integral formulation of quantum field theory and there's a point that's really bugging me: why is the integration measure ##\mathcal{D}\phi(x)## invariant under shifts in the field of the form $$\phi(x)\rightarrow\tilde{\phi}(x)=\phi(x)+\int...

Hi. I'm just starting to self-study QFT and need all the help I can get. I already have "QFT for the Gifted Amateur" and the books by Zee and Schwartz . I am looking for another book that takes things step by step from the very basics. Was looking at the book "Student friendly QFT" by Klauber. I...

Hi. I am just starting to study QFT using the path integral method and for which the main textbook is by Srednicki. Does anyone know of any good online videos which would be suitable Thanks

Hi. I'm just starting QFT for the first time. I've just finished a course in relativity but I'm confused about the index notation I've found in QFT. Here are 2 examples yi = Σ Mij xj and yj = δij yi . These examples don't seem right after what I have learned in relativity unless the index...

Apparently, there are two different routes to get to quantum field theory from single-particle quantum mechanics: (I'm going to use nonrelativistic quantum mechanics for this discussion. I think the same issues apply in relativistic quantum mechanics.) Route 1: Many-particle quantum mechanics...

What are the mathematical relationships (if any) between the particles as described by Quantum Mechanics and the particles described by Quantum Field Theory? A specific question related to the general question above arose in post #14 of the thread: How can a particle be a combination of other...

Hi all -- can anyone offer a qualitative explanation of why it is that couplings run with the energy in *relativistic* quantum theory, and not in non-relativistic? Some insight here would be much appreciated. Thanks.

Is there something wrong with negative probabilities per se? I don't what to hear that they are "unphysical" because it is not clear what reality is. Can negative probabilities (or probabilities > 1) be somehow consistent with macroscopic realism => not causing any macroscopic weirdness (no...

In quantum field theory, a fundamental particle is an excitation in the underlying field, but what does that mean? Do fundamental particles have any physical existence according to QFT?

Hello everybody! I recently started to work in nonperturbative QFT (especially CFT and SUSY models). I love my work but wonder what is the recent development in this subject beyond the SM? It is not hard to see that at the dawn of XX - early 2000's the most exciting results in nonperturbative...

I have been going through chapter 2 of Sakurai; the 1967 edition. Chapter 2 gets into the self energy of the electron, the concept of the bare mass of the electron, and vacuum fluctuations. Would these same concepts (self energy, bare mass, and vacuum fluctuations) apply to a scalar field (e g...