Qft Definition and 956 Threads

Has anyone read Warren Siegel textbook on quantum field theory ? What's your opinion about this book?

Hi, Is there a non-perturbative version of Quantum Field Theory that doesn't require virtual particles? If there is. Why is it not taught or emphasized so we have to completely do away with virtual particles which many pop-sci books claimed to be real when many experts here state these are...

Could anyone give me a good reference for construction of asymptotic states in QFT. Usually one assumes that long before and long after collision particles almost don't interact because they're spatially separated. This statement is based on the assumption that particles might be represented as...

For example for quantized EM field the Hilbert space is made up of photon states which corrrespond to EM waves classically. However what corresponds to static EM field in QFT? It can't live in the Hilbert space of photons because any superposition of photon states will still correspond to a...

Richard Borcherds is a mathematician who won a Fields Medal for work involving conformal field theory. In recent years, he has been working on a rigorous formulation of perturbative QFT. http://arxiv.org/abs/1008.0129" , though somewhat technical, might interest the "foundations of QFT" people...

Hi all, I'm an amateur with some knowledge on the surface and actually am trying to make sense of the history of QM evolution and try to put things in place by trying to understand their 'raison d'être, at the time. So I'm back in: 1. 1926-28 with the question: how the introduction of...

I don't seem to be able to fully wrap my head around the equivalence of standard QM and 0+1 QFT. In particular, I am having difficulty with the relationship between the correlator in QFT, <T\phi(t_1) \phi(t_2)> and the propagator in QM, <x', t' |x,t>. First of all, is there any relationship...

I'm reading this book "Deep Down Things" by Bruce A. Schumm about Quantum Field Theory. It says that the charge of the bare electron is infinite. Since the virtual particles calculations produced infinities too. Renormalization means substracting the infinite bare electron by infinite virtual...

Quantum Entanglement allows spatially separated entangled particles to have impact on each other instantly (overcoming the allowed speed-limit of causal influence which is 'c'). How does Quantum Field Theory address this problem (protecting causality and chronology)?

Gravity when expressed as QFT suffers from renormalization problems. From the point of view of the techniques of (four-dimensional) quantum field theory, and as the numerous and heroic efforts to formulate a consistent quantum gravity theory by some very able minds attests, gravitational...

Suppose I have some sinusoidal electromagnetic waves, plane-shaped, moving in a vacuum with frequency f and wavelength \lambda . I want to consider the energy content of a certain volume of space through which they are passing. Let's say it is a cube with side lengths \Delta x where...

Notes: http://www.damtp.cam.ac.uk/user/tong/qft/three.pdf On page 59, in deriving 3.48, from the first step to the second step, why can we just insert |0><0| like that?

- a free field has a definite number of photons, which implies that the expectation value of the free field is zero - a static field has a indefinite number of photons (or none at all, rather), but its (classical) field strength can be measured precisely How are free fields and static...

Hi, Quantum Mechanics has so many Interpretations like Many Worlds, Bohmian, etc. How come we seldom hear about Interpretations in Quantum Field Theory? Is there something in the quantum vacuum ontology that cancel out all the Interpretations? Or is it also valid? For example, in Many Worlds...

In these notes: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf Why can we not have a u-channel diagram in Figure 13 on p64?

Let's consider self-interacting scalar field described by action: S = \int d^4 x \frac{1}{2} \left[ \partial_\mu \phi \partial^\mu \phi - m^2 \phi^2 - \frac{\lambda}{4!}\phi^4\right] Using relation between Poisson bracket in classical mechanics and comutators of quantum operators we...

Is there any value in taking a course in QFT for a nuclear physicist student? I'd like to take a course on QFT just out of interest, but if it's more useful to take an extra course on solid state physics (I'm not sure if I want to do research in nuclear or nuclear solid state physics) I'm doing...

Is there anyone on the forums that does research in QM or QFT? I ask because I find this area of physics very interesting at least to read about, and I enjoy challenging mathematics. While I am still only at the freshman/sophomore level in my physics education, I have always been interested in...

How do we go from the first line in 6.17 to the second in these notes? http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf Thank you!

Hello! I have a question conserning Srednicki´s book on QFT. On page 27 he introduces a quantum field a(x) and its h.c. . What does this field represent? What should I think of when he uses this field? He writes the hamiltionian on the regular form but multiplicates with a(x) and its h.c. and...

In Srednicki's book, he discusses quantizing a non-interacting spin-0 field \phi(x) by defining the KG Lagrangian, and then using it to derive the canonical conjugate momentum \pi(x) = \dot{\phi}(x). Then, he states that, by analogy with normal QM, the commutation relations between these fields...

Hi . In weinberg's QFT book 1st volume he states that any operator can be expressed as a sum of products of annihilation and creation operators but I can't understand the proof . can simeone simplify this please? ( page 175)

The Hamiltonian operator in quantum field theory (of Klein Gordon Lagrangian) is H=\frac{1}{2} \int \frac{d^3p}{(2 \pi)^3} \omega_{\vec{p}} a_{\vec{p}}^\dagger a_{\vec{p}} after normal ordering Now we construct energy eigenstates by acting on the vacuum |0 \rangle with a_{\vec{p}}^\dagger...

This is probably a holy-war sort of subject, but I'm curious if there is any sort of consensus on the board. I've been trying to teach myself some basic QFT, so far just by scrounging together what I can find online. I've primarily been reading Srednicki, since it's easily accessible for free...

I am reading Weinberg's Quantum theory of fields Vol I and have a question about the derivation on page 71. Right below eq. (2.5.37), it is written that A and B can be simultaneously diagonalized by \Psi_{k,a,b}. From the content, I inferred that \Psi_{k,a,b} is also the eigenstate of the...

Ryder, QFT, 1985, pg. 40, Eq. (2.69) K = sigma?? For the similar page in the 2nd edition, turn page 37 at this...

Let's say that we're trying to calculate J_z|0> for spinor field. Now, one would naturally expect that it vanishes as do all of the observables in vacuum state. On the other hand, J_z isn't diagonal in the number basis, so one has J_z|0> ~ |2>, i.e. particle + antiparticle state due to the two...

I have been struggling with this for a long time. I gave up to review GR and came back to Ryder. I started in Chapter 2 and the material was easier and more intuitive on this second pass. But the same topic, "SU(2) and the rotation group" has trapped me yet again. I am bogged down specifically...

Homework Statement Given the Lagrangian density L=-{1 \over 2}[\partial_\alpha\phi_\beta(x)][\partial^\alpha\phi^\beta(x)]+{1\over 2}[\partial_\alpha\phi^\alpha(x)][\partial_\beta\phi^\beta(x)]+{\mu^2\over 2}\phi_\alpha(x)\phi^\alpha(x) for the real vector field \phi^\alpha(x) with field...

Hi all, I am pretty sure this post is repetitive. But I am asking from a mathematician's point of view. I am currently a final-year grad student in a respectable university. I will get my phD next summer. My research interest is mainly in symplectic geometry and some operator algebras. I...

Hi, I haven't posted this in the homework section, as I don't really see it as homework as such. I'm trying to derive the Heisenberg equations of motion for the Klein Gordon field (exercise 2.2 of Mandl and Shaw). I'm trying to derive the commutator of the Hamiltonian and canonical momentum...

I'm still having trouble understanding the connection between a static electric field and harmonic oscillators. I understand that a static electric field can be expressed as a scalar (potential) field, and that through Fourier analysis this scalar field can in turn be expressed as the sum of...

Hi all, Does anyone know a decent set of notes/book dealing with issues re: quantum fluctuations around a (fixed) classical background? In particular say, scalar QED with a fixed background E&M field. Thanks, Dan

I am currently following a course of condensed matter physics and quite enjoying it. But after doig some research I found that many book deal with QFT applied to condensed matter. I wonder what all the calculus of QFT is needed to describe those phenomenas. I kinda feel that relativistic...

What does it mean for an electromagnetic field to be quantized? If I have a proton at point A, then classical physics tells me that at an electron at point B feels a constant electrical force described by Coulomb's law. If the field is quantized, does this mean that sometimes a force is felt...

In page 15 of the first edition of this textbook, in equations 2.6 and 2.7, he writes: (2.6)e^{i\alpha_a T^a_R} e^{i\beta_a T^a_R}=e^{i\delta_a T^a_R} where T^a_R is the generator of the group represented by R. Now in equation (2.7) he take the logarithm: (2.7) i\delta_a...

I got a e-book on QFT few months ago.but this one hasn't chapter 1 and title and index etc.i.e all the informations about author.but this book is really good,I want to see the full version,it's recommed to anyone who interests in QFT.It's style is a little mathematical but keep good physical...

i'm trying to learn qft. i have referred many books most of them have same pattern giving some intuitive idea followed by proof or formula derivation but i need solved examples/applications of this formulas or proofs for better understanding e.g. second quantization "momentum" and "position" are...

Homework Statement Derive the following commutation relations from the general commutation relation for the Lorentz generators: [J_i,J_j]=i\hbar\epsilon_{ijk}J_k [J_i,K_j]=i\hbar\epsilon_{ijk}K_k [K_i,K_j]=-i\hbar\epsilon_{ijk}J_kHomework Equations The commutator for the Lorentz...

In QFT given a Lagrangian L=\partial_a \phi^* \partial^a\phi, how do you take this derivative \frac{\partial L}{\partial_a \phi}?

I apologize for the vague title, I don't know the names for the objects I'm asking about, which also made it hard to search for more information on them. I'm reading Zee's QFT in a Nutshell and have the feeling I'm missing something. When introducing the path integral formalism, he defines a...

Can anybody help me out with equation 9.25 in Srednicki's QFT book? He says the in the eq. 9.25 an integration by parts has been carried out. I do not see how. I guess we integrate 9.9 and get then 9.25 somehow, but how? Any hints? thank you

I have a problem understanding the effect of source terms in qft. I am interested in understanding how a static point source will interact with a massless field. Let me consider a simple example of a linear chain of masses coupled by some springs. In the continuum limit, the Lagrangian...

I just picked up a book on QFT and have already found myself stuck on something that appears to be quite elementary. When disturbing the field, the potential gets an added term with a source function J, J(x, t) \phi(x). I'm not quite getting what J physically represents. It 'describes how...

The title is my question. What are the relative merits of the two books? I've only read part of Peskin & Schroder, and one of my complaints is that the book doesn't cover canonical quantization of QED which I need for my course. I don't know much about Weinberg's book, but it seems to be have...

I'm reading a QFT text right now and to fully understand the physical perturbation theory method I would like anyone to suggest a refrence or supply an example of a calculation using the physical perturbation theory: As an example to start a discussion consider. L = \frac{1}{2}((\partial...

Hello! Something about N-point Green's function in QFT really troubles me... In the path-integral formalism,why will we introduce the N-point Green's function? I mean is it enough because we have calculated the 2-point green's function. And in the canonical formalism, it seems we can finish...

What is the definition of thermal state of scalar field in QFT. Is it possible to express the condition in algebraic way (without referring to palticluar choice of representation).

Here are some cool video lectures on introductory QFT: QFT Video Lectures

Hello! I met some annoying problems on quantum field operators in QFT.They are as follows: (1)The quantum field operator( scalar field operator, for example),is often noted as φ（r,t). Can it be interpreted as like this: φ（r,t) is the coordinate represetation of a...