Field theory Definition and 523 Threads

Wave functions and their exp(-ipx) are base of quantum mechanics and QFT. But how to derive them the most simply that we will understand why they are base of QM? It is interesting that Feynman in his book "QED: The Strange Theory of Light and Matter" very simplifies QED, but he did not give a...

Homework Statement I understand the premise of Noether's theorem, and I've read over it in as many online lectures as I can find as well as in An Introduction to Quantum Field Theory; Peskin, Schroeder but I can't seem to figure out how to actually calculate it. I feel like I'm missing a...

In quantum field theory do we "describe" static electric fields with proper combinations of transvers propagating photons? Is that a basis? Is there a basis using the longitudinal and timelike photons to describe static fields? Thanks for any help!

I am struggling with equation 1.5 in Tong's QFT course. I try to understand/explain it in strict calculus, i.e. without physics shortcuts like "small variations". I guess in the full blown explanation, \delta S is a total derivative. To be specific, with total derivative I mean the linear map...

I previously took part in two separate threads where I slowly went through various field theories from a rigorous perspective. I would like to restart the discussion. Here are the two previous threads for anybody who would like to take part this time: Thread 1 The pages from 12 on (and...

I have just read my first course on Quantum Field Theory (QFT) and have followed the book by Srednicki. I have peeked a bit in the books by Peskin & Schroeder and Ryder also but mostly Srednicki as this was the main course book. Now, I have to do a project in a topic not covered in the course...

1. I read that the picture of gauge bosons as mediators of interaction originates in and is valid in perturbation theory. But how do we know that picture is correct? We do perturbation theory only because we do not know how to study a system in a fully non-perturbative way. If someday we...

just looking at the standard model, which describes a host of fundamental particles and bosons, one could assume that the SM is a particle model. however, most of what i read appears to indicate that particles are merely "excitations" or manifestations of a field, and it has been stated on this...

The following statements are from the paper with the above title, recommended in another thread, are from here: http://fds.oup.com/www.oup.co.uk/pdf/0-19-922719-5.pdf An interpretion of these statements would be appreciated: 1. [first paragraph, page 3] What is 'conservation of...

I am taking a course in Electromagnetic Fields Theory, and the prescribed book for us is Engineering Electromagnetics by William Hayt. We are referring to its 7th edition, in one numerical Example 2.3 we have to find total charge contained in 2 cm length electron beam where as we are given the...

I'm an undergraduate with interest in theoretical particle physics. No course available in QFT. I have background in particle physics, quantum physics and relativity. Want to start learning it before going for M.Sc I've seen books like Mandl and Shaw, Landau and Lifgarbagez in university...

In some class notes I found those conditions interpreted as a "five requirements on Hamiltonian of physical system": 1. Unitarity -> Hamiltonian has to be hermitian operator 2. Claster decomposition (microcausality and locality) -> (? missing part) 3. Poincare invariance -> Hamiltonian...

when a continuous symmetry is broken, we say that the ground state is just one of the possible ground states, and there is no energy cost in moving from one to the other.. why doesn't the state keep changing with the slightest perturbation (production of goldstone boson). why don't we have a...

I'm a grad student in math, and I've been trying to learn some physics on the side by taking some classes and reading books. I took a class on quantum field theory last semester that was taught out of Srednicki; the class was very good, but I found myself at the end with a conceptual question...

Hello everyone! I want to study QFT and I want to know all of the required mathematics for it. I know most of quantum mechanics topics -wave function -schrodinger equation -state, operator -perturbation theory I know some of special relativity. And I almost familiar with einstein's...

My first question: QED has U(1) symmetry. The transformation is a function of x (psi -> psi*theta(x)). How does this depend on the spacetime? Do interactions therefore differ between curved space and flat space? Is this what leads on to QFT on curved spacetime research? My second question...

I have a question concerning the notion of electric charge in QFT. What value of charge for electron should I use if I want to compute the force acting on electron in some external electric field. Of course in first approximation it is just elementary charge which value might be found in...

What does it mean if something "Transforms Covariantly"? (Quantum Field Theory) Referring to an isospinor, \psi which transforms as \psi(x)→ \psi'(x)=S(x) \psi(x) (S(x) being an n by n matrix) I'm told that it is clear that ∂_{μ}\psi does not transform covariantly. Now, correct me if I'm...

My question is, is it still widely believed that an all encompassing "theory of everything" exists for the universe? It seems to me that due to quantum mechanics it would be impossible to actually observe enough data to predict precisely the behavior of quarks and other subatomic particles, one...

What is the relationship between quantum field theory and string theory?

Hi, Can anyone explain why in field theory we require [f*(x),f(y)] = 0 for space-lkie intervals x,y; but not <0|f*(x)f(y)|0> = 0? Thanks!

Hi, My question is about why quantum fields are used. Are quantum fields just a reinvention of the ether? It seems like something superimposed on spacetime (from which theoretically spacetime should emerge in the case of gravity) from which particles can pop in and out of. How wrong is...

Hey everyone, I posted this a while back in General Physics without any reply, and it looks like this is actually the germane forum (despite the fact that I'm explicitly NOT looking for QFT) -- but I couldn't find the "move thread" option... Anyway: I'm looking for some books that...

Hey fellas am having some trouble understanding the concept behind a rotating magnetic field. Looked up the wiki entry and a couple of other webpages, but only ended up getting more confused. I am well versed with all EM fundamentals, but still...:confused: OK so in a basic alternator, we...

Hi there PF I just want to ask, what are the main ideas behind Group Field Theory? How does it work, and what is the formalism? And why is it called GROUP Field Theory? :) \Schreiber

Sorry this is largely chemistry based, but delves into electron theory. I'm a little confused about how the colour of transition metal compounds arises. I understand that say for [Cu(H2O)6]2+, you have an octahedral arrangement around the central atom, and the specific positioning of these...

Hi, I'd like to ask recommendation for a QFT book. I have seen several other recommendations on this forum, but hopefully I'll provide enough info so you guys can have more specific advice. Background: QM (Griffiths, some parts of Sakurai), GR (Carroll), some residue...

[FONT="Garamond"][SIZE="3"][SIZE="4"]Has anyone else found the solution to Einstein's Unified Field Theory equation? = MJA

Hi there PF. I have recently been working on the Classical Unified Field Theory, and i want to ask, why the Einstein-Maxwell equations does not candidate for a Classical Unified Field Theory, since it incorporates both general relativity and electromagnetism, into a single formalism? I...

"virtual particles" in rigorous quantum field theory If I am not mistaken "virtual particles" are just a name someone put to some integrals that we use to calculate different things, and those integrals depends on the perturbation scheme and on the gauge selected, and they don't even exist in...

Consider q3 in this exam: http://www.maths.cam.ac.uk/postgrad/mathiii/pastpapers/2006/Paper48.pdf I reckon I can manage a good part of the rest of the question. Unfortunately, I cannot manage the very first bit (drawing the Feynman diagram) and this is preventing me from continuing! I...

Can someone explain to me "Locality" in field theory? I know that two fields have to commute at space-like separations so that observations don't affect each other at these separation. what about the thing where the Lagrangian can't couple fields at different locations and time, is it so...

We have the effective action which obeys \frac{\delta \Gamma[\varphi]}{\delta \varphi(x)}=J(x) where and we are told the stationary point, \varphi_0, of this action, \frac{\delta \Gamma[\varphi_0]}{\delta \varphi(x)}=0, corresponds to the vacuum expectation value. (This is out of my notes -...

Homework Statement Could someone please explain what is meant by the term: \partial_{[ \mu}F_{\nu \rho ]} Homework Equations I have come across this in the context of Maxwells equations where F^{\mu \nu} is the field strength tensor and apparently: \partial_{[ \mu}F_{\nu \rho...

Hi there! I'm attending an introduction to elementary particle physics and I came into this statement about the Dirac equation: "When an interaction is added (using the gauge principle) in a field theory, then some terms appear like: gBj where, g is the coupling constants, B is a bosonic...

Classical Field Theory without Force Has anyone seen how this has been approached?

Sometimes I find useful and more interesting to read a less rigorous textbook before tackling a reference textbook of a given subject. Having said that, I am looking for a book that introduces the ideas of quantum field theory in a relaxed way. This allows to grab the ideas and then formalize...

Pls. answer in the simplest and the most intuitive way. 1. What is the reason our quantum field theory needs perturbative approach. Is it because in the concept of fields, there is an infinite number of freedom in the oscillations of the virtual particles, or is it because the field is...

It's not a requirement at my school's Physics PhD program (it's a graduate-level pass/fail course), so I'm curious as to whether or not other schools have the requirement, and how many students ultimately opt to take it.

I'm trying to go through the Reissner-Nordstrom solution to the EFE's and since I'm trying to do this correctly, I find myself running into trouble about how to define everything. I set my coordinates up as x^a = x^a(r,\theta,\phi,ct) Now, I need to use the fact that \nabla^b F_{ab} = 0 as I...

Could I get a rough explanation of the following? I know I would need a whole course in field theory to understand a proper explanation of the following, but could someone give me rough answers to what certainly are naïve questions? First, the Higgs particles are supposed to produce the masses...

From a previous post, I now know that the 'qualities' of QM such as entanglement, superposition of states, decoherence, measurement problem, are still with us in QFT. 1. Is the Dirac equation the primary equation in QFT? 2. What equation governs the evolution of a macroscopic object in QFT...

Hi, I have a problem in classical field theory. I have a Lagrangian density \mathcal{L}=\frac{1}{2}\partial_\lambda \phi \partial^\lambda \phi + \frac{1}{3}\sigma\phi^3 . Upon solving the Euler-Lagrange equation for this density, I get an equation of motion for my scalar field \phi (x), where...

Please teach me this: Why we do not call the quantum field theory the quantum mechanics of a changeable number particles.Why we must use the term ''field''.I think that the indistinguish of identical particles,the dual particle-wave and changeable in number of particles mean the ''expansion''...

Steven Weinberg wrote: ''In its mature form, the idea of quantum field theory is that quantum fields are the basic ingredients of the universe, and particles are just bundles of energy and momentum of the fields.'' (see p.2 of his essay, ''What is Quantum Field Theory, and What Did We Think It...

The effective action Γ[ϕ] for a scalar field theory is a functional of an auxiliary field ϕ(x). Both Γ and ϕ are defined in terms of the generating functional for connected graphs W[J] as W[J] + \Gamma[\phi] = \int d^dx J \phi , \quad \frac{\delta}{\delta J(x)} W[J] = \phi(x) Show - \int...

The following must be false, since if it were true we'd have proof of a unified field, but can someone tell me why: Why doesn't the fact that a quantum field, which by definition is infinite (correct me if I'm wrong), prove that there is essentially only one underlying quantum field? How can...

Mark Srednicki's "Quantum Field Theory" I just wanted to know what level the book QFT by Mark Srednicki is, that is... Is it considered first year Masters, second year masters... Low PhD..?? I know it's not upper year undergrad, that's for sure. Just wondering. thanks!

Quantum Field Theory Purly in Momentum Space? Hello, I have a complicated nonlinear-nonlocal-nonrelativistic-effective action in momentum space and would like to do perturbation theory with that. I need to find propagator and Feynman rules. I can not go to x-space and follow the standard...

I am studying EE and want to know more about electric and magnetic field theory in more detail probably at the Quantum level.i realized that studying physics will suffice that but i am getting negative feedback from others that it will be not good for my career,they are saying that these things...