Qft Definition and 956 Threads

I'm taking up QFT for the first time. Basic question really. Given the free complex Klein-Gordon field \phi under the global gauge transformation exp(\imath\alpha) where \alpha is some real parameter, the Noether current is given by j^{\mu}=\imath \left( \phi \partial^{\mu} \phi\dagger -...

Any good references on the meaning of the vacuum in QM? What were you taught in school? What made sense? What did not? What did you discuss with the other graduate students? Any paradoxes regarding the vacuum? Any thoughts on why string theory is inundated with them? Bring em on. I...

(1) How does one obtain the density matrix formalism for quantum fields from the path integral? (2) Suppose I have a box containing interacting particles of different kinds. Is it possible to incorporate into the density matrix formalism both a non-zero temperature T as well as a time t...

How related are the concepts of the Higgs field and QFT in curved spacetime? As I understand it, The Higgs mechanism is derived from a false vacuum where no particles exist yet, but the Higgs field contains potential energy at an unstable equilibrium. This unstable potential is what drives...

Hi, This is regarding showing, in ch.7, around p.220, that the Pauli Vilars regularization technique is consistent with the ward takahashi identity. I cannot get the following to work: I add eq. 7.31 to eq. 7.32 and do not get zero. I get alpha over 4 pi. (I am left with integral ( 1...

Hi I've got a question about a term in a formula I've found in Mandl&Shaw's QFT book It's about equation 2.18 on page 31 L(t) = {\sum_i \delta \bf{x}_i {\cal L}_i \ ... Why is there a delta x_i when summing over all lagrangians for getting the lagrange-function for the whole...

Hey guys! A question: My QFT Lagrangian, fournishes through Noether's thm plus relativistic invariance a supposedly unitary and a supposed representation of the Lorentz group. These are the operators meant to act on my Hilbert space of possible states. What guarantees that this actually...

Hello, I am new in this forum. I studied physics up to the equivalent of a MSc. This included mainly QFT and GR. Then, instead of following a PhD, I went to finance. Now, after a few years, I have decided I want to go back and study QFT. Not full time, but part time (in my free time). I already...

I was reading some of the first chapters of Zwiebach's book. It is explained how quantized relativistic strings turn out to describe an infinite set of possible particles and, therefore, that it is possible to consistently describe the results obtained in the Fock representation in QFT on a flat...

Any hints for readings on non relativistic quantum field theory? I guess every NRQFT should be renormalisable, because vacuum polarisation is a relativistic effect. But I would like to read about.

If you inspect the QED lagrangian, you'll find that the electromagnetic vector potential must have the units of [1] energy. According to Peskin and Schroder's QFT text, the expansion of the electromagnetic field operator is...

I know that I have been asking that question before and that people are probably tired of it :redface:. I have done multiloop QFT calculations but when I get back to the fundamentals, something is really bugging me. There are several issues but let m start with a question that would...

In Quantum Field Theory and in particles physics, there's a postulate that is SIMMETRY. Why exists this postulate? Best reggards.

I have obtained a copy of this book and I am having a very difficult time understanding it. My problems start at the bottom of page 38 and continue through the middle of page 41. For instance: The last thing on page 38 is a definition of a scalar field in terms of an equation. The first thing...

After reading one of ZapperZ's posts in the Relativity subforum, I just thought I'd ask the following: What predictions of QED, QCD, Electroweak and the Standard Model still aren't confirmed? I'd be particularly interested in hearing unconfirmed predictions from QED.

I was wondering if anybody could recommend some books which deal with the development of QFT. I'm almost finished Schweber's "QED and the men who made it", which I found excellent, however it only deals with QED. So are there any other good histories of QFT? And is there a "History of QCD"...

Hi Can someone please explain to me Eq 2.30 in Schroeder's and Peskin's book? (page 21) how does he simplify the long equation of the commutator to this delta distribution? \imath\delta^(^3^)\ (x - x') Thanks

Hi I'm planning on studying quantum field theory, but I can't choose which book I should buy. I've heard great opinions of both, that's making my decision so hard Peskin's Introduction to Quantum Field Theory https://www.amazon.com/gp/product/0201503972/?tag=pfamazon01-20 or Mandl...

can anybody tell me how to induce the quantum field theory as long as you admit the quantum mechanics and the reletivity? it seems that in Weinberg's book ,he shows that,but i can not quite understand:confused: thank you :biggrin:

Hi, In Weinberg's QFT book(section 2.2), he said after proved the generator of the symmetry group is Hermitian and can be a candidate for an observable: My questions: 1. Does the observable here mean at least in principle, we can measure it in experment? 2. Does generator of any symmetry...

I read somewhere that quantum field theory does not allow calculations and predictions of bound states in a satisfactory way. Is that true and how much is that a problem given that qft claims to be so fundamental?

This seems to be the goal expressed most clearly at Loops '05. it is a slight redirection or change in emphasis in the common program. Matter should naturally occur in quantum gravity. It's explicit. two leading talks in this regard were by Rovelli and Freidel In the latter's talk, Feynman...

I've read on a few websites, as well as having heard from my professors that the relation between QM and QFT is that QM is the (0+1) approximation of QFT. Does mean that QM ignores the spatial degrees of freedom of field, or is there something else to it, or have I got it all completely...

Hi, I'm trying to do problem 3.5 of Peskin & Schroeder and I don't know where to start. First of all, I need to get the hermitian conjugate of the following expression \delta \chi = \epsilon F + \sigma^\mu \partial_\mu \phi \sigma^2 \epsilon^\ast where \epsilon is a 2...

Hola everybody, this is my first visit to PF. Can anyone help me please...my question is from QFT textbook phi(x)|0> is creation of particle at position x, being in superposition of momenum states - - - - correct? <0|phi(y) is birth of particle at position y. So if I consider both expressions...

Hi, Today in QFT class, we examined an infinitesimal Lorentz transformation. From this we showed that a LT is represented by an anti-symmetric matrix. But this means that the diagonal elements should all be zeros. My question is how this reduces to the standard LT with gamma, gamma...

Hi, I have a question regarding normal ordering in QFT. I know that it moves dagger operators to the right and that it gets rid of infinite constants turning up and that 'with observables defined in normal products their vacuum expectations vanish.' (Mandl and Shaw) I have a couple...

Is string theory the (more-or-less) only way of dealing with 'extended bodies' in a theory like QFT? Or does it only deal with very specific actions, or ones representing 'particles' with special dimensionalities (e.g 1+1 only)? If it is general, is all the commonly-stated phenomenology (11...

How much is this renormalization business a problem in QFT? Always read it’s complete ‘hand-waving’ and arbitrary, but also that QFT is the most precise theory ever. Also found this quote: "[Renormalization is] just a stop-gap procedure. There must be some fundamental change in our ideas...

For some reason, I skimmed the wikipedia article on QFT, and I feel like I kind of have an idea what the basic objects are... it would be nice if I can be told if I'm way off base, or something close, or whatever. I'm trying to figure out just what the objects are first, and I'll worry about...

I'm a grad student. I've done 2 semester of QFT using Peskin but I've didn't too well due to many reasons. I now want to go back and catch up on things I should have learned better, and then learn even more - I want to be a good field theorist. I'd like to see if there's anyone in a similar...

Here is a paper on the arxiv, preprint of an encyclopedia entry, which might be useful to many to introduce the ideas and techniques used in perturbative quantization of different fields. If you are content to accept for the moment brief definitions instead of leisurely motivations of the...

I just finished an introduction to quantum mechanics class, and I was wondering where I could learn QFT on my own (including any more QM I might need to learn before I could start QFT). I have Griffiths book, and in class we covered the first 7 chapters (which includes all the basics...

Hi, I have a question: The EPR experiment told us the world is nonlocal. And as we know, the QFT is a local theory, there is a principle that measurement do not affect each other between spacelike points. Do the two conflict? And can we say "we can not find an experiment for now which does not...

The electromagnetic neutral pion decay is a three-point interaction: it decays into two virtual and charged Kaons or Protons, of which one then radiates a photon and then annihalates with the other to produce a second photon. (Obviously, a neutral particle cannot radiate photons directly)...

I have three questions about QFT. I know you can answer this questions, sure you can. - How does a standing wave look like in 3 dimensions (x,y,z). I mean an electro magnetic oscillation between two walls or in free space. I can't really imagine that thing and looked for pictures (google) but...

I was wondering, is there a possibillity that QFT with curved spacetime is a the path to unification? Also, what advances are there in constructing QFTs with dynamic spacetime? If this way is feasible, who are the people spearheading it?

Hi, I'm stuck with problem 10.3 from Peskin's QFT book. I'm trying to evaluate the 2-loop sunset diagram in massless phi^4 theory however I keep running into trouble. I'm trying to use standard feynman parameter + dimensional regularisation techniques however i keep getting stuck when it...

First off let me start by stating that I am aware that you must get a great amount of "snake-oil theories" on this board and that I have no interests in "peddling". I am merely a high-school student, very interested in physics, who was lucky enough to stumble across these forums. I had an idea...

thanks for advices.

I want to read papers about the QCD and superstrings, which books are better to read firstly as an introduction.

Hi, I'm trying to work out the problems on peskin's book. Does anyone know that where I can find the solutions to them? It is really hard to do it all by myself. Thanks!

Hello, First I want to apologize for this FAQ-like question, but I couldn't find the answer anywhere here. so...I'm looking for a good book on quantum field theory and I'm new to this subject. I've had a introductory course on quantum mechanics. I found the following on the internet: -...

I just heard that the cosmological constant is a coupling constant in some perturbative expansion of some QFT and can be interpreted as a mass? Is this true? Wouldn't that be interesting? That would mean that the GR effect of expansion may be responsible (or may be an equivalent expression for)...

I am unsure if this is the proper forum for this, since it is not actually homework... but here goes anyway. I am trying to Prove Peskin and Schroeder equation 2.33( the second equal sign) P=-\int d^3 x \pi (x) \nabla \phi (x) = \int \frac{d^3 x}{(2 \pi)^3} p a^{\dagger}_p a_p so...

I've sometimes heard that there are string theory results that are quite similar to some of non-commutative qft. What are these?

Can anyone give me an explanation in layman-terms of the concept of a Polyakov-loop in QFT? What does it mean fysically. I know that this is often used in lattice QCD, but why ? I have the same questions for a socalled Ward-identity regards marlon

Hello all, I'm reading through this book now and I'd like to know if there are other people who are also doing so, have read it or are planning to read it in order to have a discussion about it. In fact, I've read before about the first half (a bit more) of Peskin and Schroeder and I think...

What is a Path in QM and QFT ? In classical mechanics, we are allowed to talk about path of a particle. But in quantum mechanics, the concept of path of a particle cannot be defined. This is due to the uncertainty principle. But Feynman talked about path integral in quantum mechanics. This...

Hello all, Just wondering if Peskin and Schroeder is a good first time QFT book? If so, what would be a good book to supplement it with? Cheers, Norm