Qft Definition and 956 Threads

I have a question about the dimensions of quantum fields. In natural units, the dimensions of bosonic fields (both scalar and vector) is 1. The dimension of spin=1/2 fermion fields is 3/2. This is all very good, but I have never read any explanation anywhere why we cannot have other types of...

When quantizing boson fields, ghosts and gauge-fixing terms seem to break gauge invariance. The unitary gauge (where there are no ghosts or gauge-fixing terms) respects gauge invariance however. So which is correct - is the Standard Model a gauge theory or not? Sometimes I hear people speak...

Can anyone please comment on this? Any new progress in extending String theory to QFT & SM other than AdS/CFT?

I do a regular check through the new papers on arxiv and today I came across this http://arxiv.org/abs/0903.3176 which is a continuation of a paper which Peter Morgan published in the Journal of Mathematical Physics a couple of years ago. http://arXiv.org/abs/0704.3420 This is not something I...

Hello, as a new student of OFT and theoretical particle physics, I thought there might be good to create a thread where discussion on "Problems and solutions"-books in QFT are devoted. There is already discussion going on about "which book is best for QFT" and also thread where McMahons book...

If a Lagrangian has some symmetry leaving the action invariant, we can introduce the corrsponding currents and charges. Now, the transformation can also be implemented as a unitary transformation of the fields in the form (let's consider a scalar field) \phi \rightarrow \phi' = U \phi...

Homework Statement Verify that (2.16) follows from (2.14). Here \Lambda is a Lorentz transformation matrix, U is a unitary operator, M is a generator of the Lorentz group. Homework Equations 2.8: \delta\omega_{\rho\sigma}=-\delta\omega_{\sigma\rho} M^{\mu\nu}=-M^{\nu\mu} 2.14...

I need someone to point me towards an instance of a relatively simple QFT problem which illustrates what can be calculated from QFT, how it is calculated, and its physical significance. (Note the qualification "relative". I realize there are likely no simple problems in QFT.) If someone...

I don't have the time now to study QFT in depth but I need some basic knowledge about QED and quantum forces (like casimir). I want to study the role of casimir force to solid state devices (photonic and plasmonic devices). Which part of QFT is needed to study for now and which books are...

QFT Peskin p.30 eqn 2.54 Homework Statement i am perplexed with eqn 2.54 peskins introductory qft. just can't make out how to arrive at it from the previous step. i think that there are dirac delta funtions involved but simply can't make it out. can somebody help? provide some hint? thanks...

Homework Statement Hi I was trying to get the expression for H ,eqn 2.31 on page 21 peskins introductory QFT Homework Equations I get the final expression only if a^+_{-p}\ a_{-p} = a^+_{p}\ a_{p} so pls tell me if a^+_{-p}\ a_{-p} = a^+_{p}\ a_{p} (+ stands for dagger) thanks in advance

As suggested by Marcus, I have read the whole paper and decided to start a discussion of Kober's paper by asking a few questions. The paper certaintly presents a very interesting view. Part of it is an extension of Heisenberg's work on unified QFT, that the mass originates from self-interaction...

It is often stated that the transition amplitude between eigenstates of the free-field Hamiltonian H_0 is encoded by the S-matrix, defined by \langle \mathrm{f} | U{_\mathrm{I}} (\infty,-\infty)|\mathrm{i} \rangle. where U_{\mathrm{I}} is the time-evolution operator in the interaction...

...like in Photon-Photon-scattering. I know this doesn't make sense physically but the value for the diagram should still be computeable. If I want to put the expression for the matrix element together, I get a matrix, but it should be a scalar, right? Since the spinor bi-product is missing...

I'm not sure if this is the right place to post a graduate level course material, but I have a question about perturbative expansion of the 2n-point function of a scalar field theory. Homework Statement First, the question: In which space (position or momentum) is the topological distinctness...

QFT it is possible to calculate the energy density of empty space. It is known to be around 10^50 orders to large. Does the calculated large cosmological constant in falsify QFT? Or does GR have to be re-done with a higher energy density

Can conservation of momentum be directly derived from quantum field theory (e.g. QED). My feeling is this should be true since the Dirac equation reduces to Schrodinger's wave equation in the nonrelativistic limit which is a reflection of Newton's second law, thereby implying conservation of...

I am starting to learn QFT and I am scanning my text (Mandl and Shaw) for the highlights. Would some please confirm whether these are the highlights. (1) The states are Fock states where the states are distinguished by occupation numbers for various single particle or single photon states...

I've been thinking about one of the postulates about one particle quantum mechanics, it says that whenever we measure an energy value, we get one of those eigenvalues. Firstly, pretty much 99% of the stuffs I know in nonrelativistic QM applies in the realm of electromagnetism. I just don't...

if we consider the propagators and other Fourier integrals in the sense of 'distribution' then are all the divergences that appear in QFT (quantum field theory) due to the divergent quantities \delta ^{k} (0) that is my idea, all the divergences appear because in the commutation...

Hello Forum Does anybody know of a good summary article or webpage that concisely and accurately outlines QFT (wikipedia's page is pretty bad)? I am not looking for a layman's guide but rather something suitable for a soon-to-be-student of QFT - something that can be read and understood in a...

It would be nice if someone commented a couple of propositions by Peskin and Schroeder in their QFT book in p.166. There they say that when the helicity is conserved in the high energy Compton scattering, one unit of spin angular momentum is converted to one unit of orbital angular momentum...

I am having some great difficulty getting intuition out of the standard quantization of the Klein-Gordon Lagrangian. consider the H operator. In QM, the expectation values for H in any eigenstates |n> is just <n|H|n> now, in QFT, suppose I have a state |p> in the universe, what do I get if I...

Hiya, just stumbled upon this forum searching out 'Peskin errata' when trying to figure out a simple QFT calculation in the textbook. Apparently, there is no mention of the simple derivation that I'm struggling with, so there must be something wrong with my own working. I would really...

Hi, I'm studying QFT (quantum field theory) again, with the help of Peskin&Schroeder. Altough the book is much better to read for a second time, there are some things that keep me wondering. It's about shifting physical quantities by a small imaginary constant in order to be sure that...

According to the https://www.amazon.com/review/product/0521864496/ref=dp_top_cm_cr_acr_img?_encoding=UTF8&showViewpoints=1"&tag=pfamazon01-20 on Amazon, it's pretty good. Download it http://www.physics.ucsb.edu/~mark/qft.html".

Hello: In quantum field theory, there is the Klein-Gordon Equation that describes particles with Spin 0, this equation reduces to the SchÖdinger equation when the non relativistic limit is taken, Does the Dirac equation that describes particles with spin ½, reduce to Pauli's...

What is a good introductory book on Quantum Chromodynamics that assumes QFT as a prerequisite?

Looking at most textbooks, I found Weinberg's Quantum Theory of Fields to be by far the most superior and I think that's how QFT should be taught as well. I also think the way Weinberg presenting the subject from first principle is the way to make progress in research in fields such as...

Let me be more rephrase what I wrote, to not sound rediculous as usualy I am. Loll's universe seems to mimick some futures of our universe, but I just can't see anywhere how a QFT can be defined over that space. To begin with, the space we observe in this universe is not a single one, but an...

In QM we have the postulate of measure giving rise to the notion of wave function collapse throught measurement. Is there an analog description of collapse in QFT (even an effective one) ? And what is then collapsing ? The field ? What would that mean ? I'm interested for references on the...

Homework Statement from Zee QFT in a nutshell the free propagator between two "sources" on the field is given by D(x_\mu) = -i \int \frac{d^3k}{(2\pi)^3 2 \omega_k}[e^{-i(\omega_kt-k\bullet x)} \Theta(x_0) + e^{i(\omega_k t-k\bullet x)} \Theta(-x_0) for a space like separation ( x_0 = 0 )...

Why is local gauge invariance needed in qft? I read that is allows interactions whereas global gauge invariance does not but was given no reason.

Homework Statement I'm starting QFT and many books I've started to read start with the introduction of a field in a classical string model with a Lagrange equation L(q,\dot{q}) = \sum[\frac{m}{2}\dot{q}_{j}^{2}-\frac{k}{2}(q_{j}-q_{j+1})^{2}] the equation of motion becomes m...

Hi I need to find the generators of the Poincare group in the representation of a clasical scalar field. Every textbook I found let them as P and M. But any buk does not what are they. I'm wondering if anybody help me to find this Uda

Dear all, While I was reading chap2 of Peskin, I got some questions. (1) The vanishment of the commutator of fields [\phi(x),\phi(y)]=0 means that the measurements at x and y do not interfere at all. Is this a postulate? Is this the so-called micro-causality? (2) How Peskin deform the...

Hi folks! A QFT question: you start from the lagrangian, compute the hamiltonian via Legendre transform and promote the the fields to operators with canonical equal-time commutation relations. Now you can compute the relation [H,F(x)]=-\mathrm{i}\partial_0 F(x) \ , where H is the hamiltonian...

My mathematical knowledge of QFT is nonexistant (I'm only just starting grad school) so I was wondering if anyone could clear up a questions I have: 1) Can the time it takes for an electron (which has just absorbed a photon of the correct frequency) to move to a higher orbital be said to be...

I have been working through Srednicki this summer to teach myself qft, and all too often I've gotten stuck on a small point and ended up spending a great deal of time clearing it up by myself. While this is probably an important part of the learning process, I am progressing a bit too slowly, so...

Coleman's QFT lectures on video! Everybody check out John Baez TWF 264 where he reports Coleman legendary QFT lecture notes are now online!

Hi all, I'm currently learning QFT out of Mandl and Shaw supplemented by Peskin and Schroeder. What are the best books for getting experience with QED calculations? Problems with worked solutions would be ideal.

Could someone suggest a good book for elementary QFT and particle physics?(I don't know which to read first). I have very little idea about QFT. Could someone also tell me what maths knowledge is required?

Sort of kidding with the title here. But seriously I am having trouble making progress in my study of QFT because I can't see where it is going. Please give me an example of something to calculate with QFT that corresponds to a measurable quantity, hopefully something which cannot be...

What are the important experimental verifications of QFT that someone studying QFT should be familiar with? The wikipedia QED article mentions the anomalous magnetic moment of the electron and the Lamb shift. The Higgs Mechanism article states "Although the evidence for the Higgs mechanism...

Homework Statement Compute the S-operator to first order in the coupling constant lambda. Homework Equations The given Lagrangian density is L = : \frac{1}{2} (\partial_{\mu} \phi)^2 - \frac{1}{2}m^2\phi^2 + \frac{1}{2}\frac{\lambda}{4!}\phi^4 : where phi is a scalar field. The...

I'm trying to study QFT using P&S book. Now I'm on the chapter 13 but I feel that I'm rather bad at solving practical problems on QFT and QED. Could anyone suggest a good source of problems (preferably with solutions) on QFT and QED (near the level of P&S ch. 1-13) to practise ? Thanks in advance !

In equation (2.5.47), I am getting U(R(\hat p) = e^{+i \phi J_3} e^{+i \theta J_2} Instead of the "-" signs in the exponential. This makes a phase difference under parity and time reversal of a massless particle. Is this one of the active rotation vs. passive rotation problem? If so...

When I learned QFT in school, I got lost among all the steps involved. There were the Feynman rules, wick's theorem, the interaction picture, the LSZ reduction formula, the S matrix, the T matrix, the transition amplitude matrix, and on and I lost track of the big picture pretty quickly. Later I...

A question I would like to get an answer is when is a QFT exactly solved? E.g. if I know the solution of the equation for the two-point function I have got all about the theory? This equation is classical in nature being the two-point function defined in the sense of distributions. I have read...

In Zee's QFT in a nutshell on page 236 between equations (1) and (2), the authors goes to polar coordinates and gets a new gauge derivative.Sure it 's simple, but I can't see how he gets it. thanks for any help