Qft Definition and 956 Threads

So I've been thinking about the axioms of quantum field theory. In particular the expression for the particle amplitudes: G(x1,x2,...,xn) = ∫Φ(x1)Φ(x2)...Φ(xn)ei S[Φ]/ħ D[Φ] / ∫Φ(xn)ei S[Φ]/ħ D[Φ] But I've been struggling to explain the existence of the 'i'. It seems like this is a...

Want to learn QFT but often lose courage when seeing such a huge book(M. Srednicki). The author also suggests learn with someone else. Is there any group for this?

Homework Statement Given that ##x_\mu x^\mu = y_\mu y^\mu## under a Lorentz transform (##x^\mu \rightarrow y^\mu##, ##x_\mu \rightarrow y_\mu##), and that ##x^\mu \rightarrow y^\mu = \Lambda^\mu{}_\nu x^\nu##, show that ##x_\mu \rightarrow y_\mu = \Lambda_\mu{}^\nu x_\nu##. Homework Equations...

Often I see QFT texts introduce dirac spinors by comparing them to the two component spin states (which I have come to accept are also spinors) in NRQM. And arguing that since the NRQM spinors transform via SU(2), our desired quantum fields for spin 1/2 particles should be some higher...

In most introductory QFT treatments, it's stated early on (and without proof) that particles with even integral spin are always attractive, while those with odd integral spin can be repulsive; sometimes this is even cited as evidence that the graviton must be spin 2 (I think Feynman's...

Is it true that if time stood still it would violate the uncertainty principle. therefor if you traveled at the speed of light time stands still and you would violate the uncertainty principle. if this is true does that sugest that QFT sets a lower max speed that matter can travel at than what...

I'm having problem in deriving 23.6.11 from Weinberg's-Quantum Theory of fields... We have: \psi_f \rightarrow \exp (i a_f \gamma_5) \psi_f, f denoting the flavor. Then for the mass term lagrangian he writes: L_m = - \frac{1}{2} \sum_f M_f \bar{\psi}_f (1+ \gamma_5) \psi_f - \frac{1}{2}...

Two (supposedly) trival questions in Schwartz's QFT notes. The notes can be found http://isites.harvard.edu/fs/docs/icb.topic521209.files/QFT-Schwartz.pdf. 1. page 155, equation 15.2, how does the integrand reduce to k dk? I would guess that there must be some logarithm, but k dk? 2. page 172...

I'm an experimentalist, so go easy on me... What does it mean for a particle to either couple or not couple to a field? I haven't taken a class in QFT yet, so please try to explain the general idea without any details or equations!

Homework Statement [/B] I'm trying to derive (14.25) in B&J QFT. This is ##U(\epsilon)A^\mu(x)U^{-1}(\epsilon) = A^\mu(x') - \epsilon^{\mu\nu}A_\nu(x') + \frac{\partial \lambda(x',\epsilon)}{\partial x'_\mu}##, where ##\lambda(x',\epsilon)## is an operator gauge function. This is all being...

This isn't a homework problem. I am preparing for a particle physics exam and although I understand the theoretical side of field theory, I have little idea how to approach practical scattering questions like these. THE PROBLEM: Dark matter might be observed at the LHC with monojet and...

I was reading Schwartz's qft book. I saw the proof of ward identity taking pair annihilation as an example. he claimed he didn't assume that photon is massless in this derivation. but i have confusion with this statement. gauge invariance is a fact related to massless particles. now he has...

hi guys, this is the first time i post a thread. I have an issue on proving the scalar products for arbitrary momenta. Can anyone help me ? I always end up with N(p)N*(p')(D(L^-1(p)L(p'))_rowrow'detla I have been spending hours on proving this..still i can't prove it...

Because of the Feynman path integral, QFT can be made into a statistical field theory. In rigourous relativistic field theories, this is formalized by the Osterwalder-Schrader conditions. At any rate, there are well established links between quantum field theory and statistical physics. A...

I'm reading through some lecture notes and there is a proof that the gamma matrices are traceless that I've never seen before (I've seen the "identity 0" on wikipedia proof) and I can't work out some of the steps: \begin{align*} 2\eta_{\mu\nu}Tr(\gamma_\lambda) &=...

Does someone have solutions to Weinberg's books on QFT which he care to share with me?

Do the corrections after renormalization to the gyromagnetic ratio of the electrón, more precisely its g-factor, that slightly deviate from the relativistic Dirac equation prediction of g=2 as corresponds to a spin 1/2 fermion, mean that the fermions spin symmetry is only aproximate? i.e. that...

Hi all! I'm a beginner in QFT. I've read a lot of posts here about Haag's theorem, but I haven't found one which can answer simply and briefly to my question (if such an answer exists): Do UV divergencies appear because of the Haag's theorem? Thank you

why should Proca eqn be like ∂γ Fγμ + m2 Aμ = 0 but not ∂γ Fμγ + m2 Aμ = 0 ? another doubt is (λ-1 ω λ)μγ = λ-1 ρμ ωρσ λσγ ? why in λ-1 transformation got upper index in the second place but not in the first place? if someone clear my doubts...I would be thanful... regards.. Kau

Good day, I'm starting my master in physics, and it's time for me to choose my courses. I've decided that I probably want to pursue the direction of quantum information processing, and I'm trying to pick my courses with that in mind. For my first semester I'll be taking four courses: Quantum...

As Steven Weinberg put it, "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." At least, this is one way to look at QFT. The other approach is to imagine that these particles are...

In the page http://en.wikiped ia.org/wiki/Hamilton%E2%80%93Jacobi%E2%80%93Einstein_equation, under Shortcomings of four-dimensional spacetime", you can see the following line: Is it correct? If it is, how can it be? Because in GR, gravity is space-time curvature!

Hello. From Schwartz QFT BOOK, How could Eqn 5.26 can be Eqn 5.27? d \Pi_{LIPS}=(2 \pi) ^{4} \delta^{4}(\Sigma p) \frac{d^{3} p_{3}}{(2 \pi) ^{3}} \frac{1}{2 E_{3}} \frac{d^{3} p_{4}}{(2 \pi)^{3}} \frac{1}{2 E_{4}} Eqn(5.26) d \Pi_{LIPS}=\frac{1}{16 \pi ^{2}} dΩ ∫ d p_{f}...

Can anyone suggest me a good reference for path integrals (QFT), apart from peskin.

In chapter 2.2 of Feynman's book on QFT, he states that the probability amplitude of a particle going from a to b is the sum of contributions from all paths, and that each path contributes the same amplitude, but with a different phase. My question is, why does Feynman state that this is the...

Hi guys. I'm working on a model described by a non-local QFT. I think I got the Feynman rules right, but I get divergences from ##\delta(0)##-like factors.Homework Statement It's a QFT for a complex scalar field ##\psi(x)=\psi(\mathbf{x},t)## with action $$S= \int dx...

Hello Forum, The electromagnetic field EM must be treated relativistically because it travels at the speed of light in a vacuum. However, the idea of quantization forces us to treat the field as a quantum mechanical field. QFT is the answer to that. QFT is quantum mechanics with...

Sorry for my questions, (it does seem like QFT triggers quite a lot of questions :-D). Anyway, on page 103 (it has a preview in google books), I am not sure how did he get equation (14.40), obviously it should follow from (14.39), but I don't understand where did -ln(m^2) disappear ...

Homework Statement I am not sure about the problem set up. For (a), Using Equation of motion, need to express Lagrangian in terms of only J? I got, L=-\frac{1}{2 \Box^2 }(\partial_\mu J_\nu)^2 - \frac{{J_\mu}^2}{\Box}, using lorentz gauge (b) \partial_\mu J_\mu =0 means k_\mu J_\mu =0 ...

Hi all, I haven't been able to find an answer online but this seems like a pretty basic question to me. What are the commutator relations between the position/momentum operators and the field operator? I'm not even certain what the commutation relations between X/P and a single ladder operator...

Hi all Maybe you could help me understanding this bit from the beginning of the book (peskin - intro to QFT). Homework Statement In section 2.2, subsection "Noether's theorem" he first wants to show that continuous transformations on the fields that leave the equations of motion...

why are physisist having so much trouble quantising gravity and getting it to fit into QFT?

I think my weakest area is things like integration techniques, complex analysis, and integral transforms, but after a few google searches I can't really find what makes QFT so hard when people see it the first time. I really don't understand how to evaluate path integrals or those...

Hi, I was wondering if I could test my understanding on the S-matrix and its role in evolving initial states of systems to final states (after some scattering process has occurred). Would it be correct to say the following: Given a system in an initial state \vert i \rangle, the final...

Hi guys I need a good introductory Textbook with full solutions on Quantum Field Theory and General Relativity I am an upper level undergraduate. Please pick ones that have solution thanks.

Hello, Can anyone recommend me a book that develops QFT in Heisenberg picture? I have found Källen - Quantum Electrodynamics. Thanks in advance for the answers.

1. Why are there an unfixed number of particles? Texts usually present some hand-waving argument with bits and pieces of SR and NRQM thrown together. Are there more rigorous explanations? 2. How can the scalar fields suddenly be opeartors? I never understood this step mathematically, one...

Hi, please help me .. How can I derivative covariant and contravariant fields? as in the attached picture Thanks.. http://www.gulfup.com/?tNXcaN w.r.t alpha

Is it actually possible to calculate the probability of field states in QFT? For example the probability of some scalar field being found as some function f(x,t), i find this problem ignored in most texts.

Hallo! I know, I know - everywhere it says "NO HOMEWORK", but I am not entirely sure if writing a Historical Paper on Quantum Field Theory is classified as homework so will attempt to post it here - if it is classed, then I'm very sorry (please move this thread to the correct place). So yes...

Thanks for all the help on my QFT question I posted earlier. I have a new question I am very much interested in understanding QFT at a much higher level without going back to college and was wondering which mathematics' in particular are needed to really understand QFT. I did attend Ohio State...

Hello! I'm trying to understand QFT for the moment and have a question regarding the basic. So we have a vectorspace (Hilbertspace) of our states. The operator \phi(x) measures the amplitude at point x, whereas the operator \pi(x) measures the momentum density.. The ladder operator...

Hey guys. So I've spent some time with the following book: https://www.amazon.com/dp/0984513922/?tag=pfamazon01-20 (basically halfway through chapter 4 which is on spin 1/2 fields) and as far as pedagogy goes, this book is a godsend. I don't think I've seen a more lucid upper-division physics...

The propagator in 4-dimensions for a free scalar field has the form: Δ(x,0)=Θ(t)A(x,t)+Θ(-t)B(x,t) where Θ is the step function (eq 23 of Zee's QFT book, 2nd edition). He then makes the claim that for spacelike x, one can set t=0, and define Θ(0)=1/2. The going through all the math, he...

I have asked this question once, but no one seemed to notice it, so I'll try again. In my book the time ordering operator is used to rewrite an operator product: U(β,τ)A(τ)U(τ,τ')B(τ')U(τ',0) = T_τ(U(β,0)A(τ)B(τ')) To refresh your memories the time ordering operator T_τ orders operators...

So I'm currently studying QFT, and I got to the point where I realized that the S operator, initially assumed to be unitary, is not unitary anymore, since it is assumed to act between t0 = - infinity and tf = infinity. The author of the book I'm using says this is due to Haag's Theorem, so I...

Please demonstrate for me that: In any theory,the propagator \Delta_{f}(k) of a field of type f has asymptotic behavior: \Delta_{f}(k)~k^{-2+2sf} where sf is ''spin'' of the field.For massive fields of Lorentz type (A,B) then sf=A+B. (However,dropping terms that because of gauge...

Hi guys! So this question has been bugging me a bit and I can't seem to find any textbook (at least, restricted to physics textbooks) that talks about it. In QM, the overarching formalism is clear. We have ##L^2(\mathbb{R})## and states are given by ##|\psi\rangle \in L^2(\mathbb{R})## such...

hi, friends. i am 15 and have made a device using electric motors and generators which is able to produce more "electrical output" than the "electrical input". i have won the national science fair with this but the problem is that the judges say i am braking the 2nd law of thermodynamics and...

Nonlinearlity of QFT produces interactions, or so I was told today. Maxwell's eqns, though, are perfectly linear. Does that mean that Maxwell's eqns don't predict interactions? Thanks