What is Qft: Definition and 978 Discussions

In particle physics, the history of quantum field theory starts with its creation by Paul Dirac, when he attempted to quantize the electromagnetic field in the late 1920s. Major advances in the theory were made in the 1940s and 1950s, and led to the introduction of renormalized quantum electrodynamics (QED). QED was so successful and accurately predictive that efforts were made to apply the same basic concepts for the other forces of nature. By the late 1970s, these efforts successfully utilized gauge theory in the strong nuclear force and weak nuclear force, producing the modern standard model of particle physics.
Efforts to describe gravity using the same techniques have, to date, failed. The study of quantum field theory is still flourishing, as are applications of its methods to many physical problems. It remains one of the most vital areas of theoretical physics today, providing a common language to several different branches of physics.

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  1. D

    Quantum Learn Quantum Field Theory: Beginner Books & Resources

    Hi. I'm just starting to self-study QFT and need all the help I can get. I already have "QFT for the Gifted Amateur" and the books by Zee and Schwartz . I am looking for another book that takes things step by step from the very basics. Was looking at the book "Student friendly QFT" by Klauber. I...
  2. D

    Looking for online videos on QFT using path integral method

    Hi. I am just starting to study QFT using the path integral method and for which the main textbook is by Srednicki. Does anyone know of any good online videos which would be suitable Thanks
  3. D

    QFT Index Notation: A Beginner's Guide

    Hi. I'm just starting QFT for the first time. I've just finished a course in relativity but I'm confused about the index notation I've found in QFT. Here are 2 examples yi = Σ Mij xj and yj = δij yi . These examples don't seem right after what I have learned in relativity unless the index...
  4. stevendaryl

    Second Quantization vs Many-Particle QM

    Apparently, there are two different routes to get to quantum field theory from single-particle quantum mechanics: (I'm going to use nonrelativistic quantum mechanics for this discussion. I think the same issues apply in relativistic quantum mechanics.) Route 1: Many-particle quantum mechanics...
  5. Buzz Bloom

    Relationships between QM and QFT Particles

    What are the mathematical relationships (if any) between the particles as described by Quantum Mechanics and the particles described by Quantum Field Theory? A specific question related to the general question above arose in post #14 of the thread: How can a particle be a combination of other...
  6. M

    Qualitative explanation of scale dependence

    Hi all -- can anyone offer a qualitative explanation of why it is that couplings run with the energy in *relativistic* quantum theory, and not in non-relativistic? Some insight here would be much appreciated. Thanks.
  7. T

    Negative Probabilities and Macroscopic Realism in Quantum Field Theory

    Is there something wrong with negative probabilities per se? I don't what to hear that they are "unphysical" because it is not clear what reality is. Can negative probabilities (or probabilities > 1) be somehow consistent with macroscopic realism => not causing any macroscopic weirdness (no...
  8. A

    What is a fundamental particle according to QFT?

    In quantum field theory, a fundamental particle is an excitation in the underlying field, but what does that mean? Do fundamental particles have any physical existence according to QFT?
  9. Gvido_Anselmi

    New developments in QFT beyond the SM

    Hello everybody! I recently started to work in nonperturbative QFT (especially CFT and SUSY models). I love my work but wonder what is the recent development in this subject beyond the SM? It is not hard to see that at the dawn of XX - early 2000's the most exciting results in nonperturbative...
  10. H

    No problem, happy to help! Enjoy your vacation!

    I have been going through chapter 2 of Sakurai; the 1967 edition. Chapter 2 gets into the self energy of the electron, the concept of the bare mass of the electron, and vacuum fluctuations. Would these same concepts (self energy, bare mass, and vacuum fluctuations) apply to a scalar field (e g...
  11. H

    Scalar fields and the Higgs boson

    This is more of a QFT question, so the moderator may want to move it to another forum. The simplest example of a QFT that I learned was the scalar field; in Sakurai's 1967 textbook. I know the Higgs is a J=0 particle. Is it described by the simple scalar field discussed in Sakurai's text? I ask...
  12. M

    Quantum Advanced quantum mechanics book - toward QFT

    Hello everyone! I just finished studying basic quantum mechanics, using Liboff's "Introductory Quantum Mechanics", i.e. wavefunctions, uncertainty relations, basic 1D problems, dirac notation, angular momentum (orbital and spin, addition, eigenfunctions, Clebsch-Gordan coefficients etc)...
  13. R

    Contour Integral from Peskin & Schroeder Intro to QFT

    1. I'm having some trouble with some of the contour integrals covered in Chapter 2 of Peskin & Schroeder's Intro to QTF. I'm not so much as looking for answers to the integral (in fact, the answers are given in the textbook), but I was hoping someone could point me to some resources to use to...
  14. LarryS

    Is Probability conserved in RQM?

    Is probability (or probability current) conserved in RQM/QFT? I have never seen a simple answer to this question. As always, thanks in advance.
  15. FreeBiscuits

    Creation Operator is not a densely defined operator....

    Hi everyone, I am currently preparing myself for my Bachelor thesis in local quantum field theory. I was encouraged by my advisor to read the books of M. Reed and Simon because of my lag of functional analysis experience but I have quite often problems understand the “obvious” conclusions. For...
  16. Andre' Quanta

    The operator X for the position in QFT

    During a course of QFT my teacher said that in this theory is not possible to use the operator X for the position in order to construct with the momentum P and the spin S a set of irreducible operators that charachterize particles, and that we need a different point of wiev: the irreducible...
  17. D

    Why is QFT insensitive to absolute energies?

    In the canonical formulation of QFT (to which I've been exposed), it is always argued that only differences in energy are physically observable and so we can deal with the fact that the vacuum energy is infinite by redefining the vacuum such that its energy is zero and we subsequently measure...
  18. D

    Why is Gravity Sensitive to Absolute Energy?

    As far as I understand it gravity is sensitive to absolute energies, as seen directly through Einstein's equation G_{\mu\nu}=8\pi GT_{\mu\nu} Thus the local geometry of spacetime is directly affected by the local energy density (and not just differences in energy). So whenever gravity is taken...
  19. Gvido_Anselmi

    Solitons, instantons etc. -- Studying the general theory of solitons in QFT

    Hello everybody! I've recently decided that I'm interested in supersymmetric solitons and want to work in this area for future 2 undergrad years. I wonder what is the best place to study the general theory of solitons in QFT? Is Rubakov "Classical gauge fields" really the best book for those who...
  20. G

    Interacting Quantum Field Theory: Ground State

    Hi all, I have a question about the ground state of an interacting quantum field theory. The state space in non-interacting QFT is a space where each field mode (with specified momentum p) has some occupation number n. These modes are interpreted as n particles with momentum p. The vacuum is...
  21. G

    Literature for QM to QFT step-by-step

    Hi all, can anybody help me to find literature that takes the reader on a step-by-step path from non-relativistic quantum theory to relativistic quantum theory? I imagine something like that: it starts with a single harmonic oscillator, analyzes the non-harmonic oscillator (does the...
  22. D

    Interactions between field operators & locality in QFT

    Why is it required that interactions between fields must occur at single spacetime points in order for them to be local? For example, why must an interaction Lagrangian be of the form \mathcal{L}_{int}\sim (\phi(x))^{2} why can't one have a case where \mathcal{L}_{int}\sim\phi(x)\phi(y) where...
  23. Gvido_Anselmi

    Bound States in QFT: Learn Modern Formalism & Applications

    Hello everybody. I'm interested in some problems of bound states in external fields in QFT (especially QED). I wonder are there any lectures/books or reviews which provide modern treatment of this subject? I would like to learn more about general formalism and applications in QED (I allready...
  24. ddd123

    Sterman's QFT - 2.7b (on functional derivatives)

    I've been trying to fill in my mathematical blanks of things I just took as dogma before. Especially, not having a background in functional analysis, the functional derivatives often seem to me mumbo jumbo whenever things go beyond the "definition for physicists". In particular I tried looking...
  25. dsatkas

    I Introduction to relativistic quantum mechanics and maybe QFT

    I'm aware that most modern textbooks gloss over relativistic qm and jump to qft. Since I'm not that brilliant of a student I'm thinking that i should firstly familiarize myself with relativistic and then go to qft. So my first question: Is it worth it to study relativistic qm or should i jump...
  26. D

    Locality, unitarity & vacuum energy

    I've read in a set of lecture notes that the requirement of locality and unitarity in QFT imply that the vacuum must have a non-zero energy associated with it (http://arxiv.org/pdf/1502.05296v1.pdf , top of page 3 under heading "What is the problem?"). My question is, why does the locality and...
  27. D

    What is the definition of Locality (in QFT)

    I am slightly unsure as to whether I have understood the notion of locality correctly. As far as I understand it locality is the statement that if two events occur simultaneously (i.e. at the same time) then no information can be shared between them (they are causally disconnected). Thus a...
  28. D

    Simultaneous events, space-like separation & QFT

    First of all, sorry if this is in the wrong forum, wasn't quite sure which one to post it in given the question. My question is, given two space-time points ##x^{\mu}## and ##y^{\mu}##, if the events that occur at these points are simultaneous, i.e. ##x^{0}=y^{0}##, are the two events...
  29. C

    How Does Zee Simplify Integrals in QFT Derivations?

    On page 12 of Zee's QFT in a nutshell, he rewrites an integral from dirac notation to a mixture of schrodinger/dirac My question is, since , what happened to the integrals, and shouldn't there be four wave functions instead of two? I tried writing out the integrals explicitly, thinking maybe...
  30. JG1009

    Zero-point energy and QED vacuum state

    I'm relatively new to QFT and was wondering how the QED vacuum has a dormant zero average-field condition if there is a zero-point energy of the field as well? How is there a zero average and a zero point energy?
  31. M

    Properties of boson in $\phi^4$ theory

    I'm looking at the simplest example of an interacting theory, and this is the theory of a neutral scalar boson $\phi$ with $\lambda \phi^4$ interaction term. Can I ask: is there a physical interpretation of the `charge' through which this field interacts with itself? In particular, is \lambda...
  32. KylieVegas

    Programs Masters in physics without a different bachelors degree?

    Hello I am an incoming Biology student (college), i really wanted to take applied physics as my course but my parents told me that it is better to be a doctor, anyways, it's summer here and I started taking Calculus 1 in coursera.com (i have NO backgroud in calculus because we don't have...
  33. nikosbak

    Compute 3-Point Function QFT Homework with Fermions

    Homework Statement I'm working on path integrals for fermions and I came across an exercise that ask to compute the three point functions , one of that is the: $$<0|J^{\mu}(x_1)J^{\nu}(x_2)J^{\rho}(x_3)|0> $$ where $$J^{\mu}$$ is the current $$J^{\mu}=\bar{\psi}\gamma^{\mu}\psi$$. ***Can you...
  34. M

    How does one derive the Lagrangian densities used in QFT?

    I've been working through a qft book by Sadovskii (while I wait for my Peskin book to come in) and I've used some later chapters of Griffith's Into to Elementary Particles as an introduction to some qft. My issue with both of these is that, where in classical mechanics we have the Lagrangian...
  35. nikosbak

    Dimension of interaction in a QFT theory

    The problem statement. When an exercises say " the interaction in a QFT has dimensions Δ" , what does it mean?, it means the field or the Lagrangian has this mass dimension? In this exercise I'm trying to find the classical beta function (β-function) for the assciated couling.
  36. K

    Scattering amplitude, link between quantum mechanics and QFT

    In quantum mechanics, we can define the scattering amplitude f_k(\theta) for two particles as the coefficients of an outgoing spherical wave. More precisely, the asymptotic behaviour (when r\rightarrow\infty) of a wave function of two scattering particles, interacting with some short range...
  37. LarryS

    Are there separate quantum fields for spin 1/2 particles in modern QFT?

    The spin observable for spin 1/2 particles is represented by Pauli Matrices acting upon a 2-dimensional Hilbert Space. In RQM, forgetting about the matter-antimatter duality for the moment, that TWO-state Hilbert Space is directly related, through the Lorentz Group, to the TWO separate...
  38. F

    How can we build the spin 2 graviton representation of little group O(9) in QFT?

    In QFT vol 3 of Weinberg write:''For d=11 the spin 2 graviton representation of little group O(9) is a symmetric traceless tensor with 9x10/2-1=44 independent components:there is one 2+(-)i3,2+(-)i3 component with J23=+(-)2, seven2+(-)i3,k components with J23=+(-)1; and twenty eight k,l...
  39. T

    Gauge invariance is not normal invariance?

    I recently learned that with (local) gauge invariance, functional quantization needs to factor out volume factor(Faddeev-Popov procedure). Why does this has to be done?Just to remove infinity? As far as I am concerned, ##\phi^4## theory contains invariance(for example ##\phi\to\phi\cdot e^{i...
  40. B

    What is the concept of scale invariance in quantum field theory?

    Hey guys! I was reading the following paper http://arxiv.org/abs/hep-ph/0703260 for Georgi and I have a conceptual question about it. Howard Georgi was talking about this Unparticle Physics theory and at the base of his analysis is the principle of scale invariance. So Georgi is saying what if...
  41. Quantioner

    The Wick rotation in position space

    The Feynman propagator is $$ G_{F}(x) = \int d^4p \, \frac{e^{-ip x}}{p^2 - m^2 + i\epsilon}. $$ I want to understand why the directions of Wick rotation in position space and momentum space are contrary. Every book I find says something like "we should keep ##xp## unchanged", but why? As we...
  42. S

    Hubbard-Stratonovich transformation

    Can one apply the Hubbard-Stratonovich transformation to the exponential of the Laplace–Beltrami operator?
  43. J

    QFT: Bogolyiubov transformations and KG inner product

    (I hope this post goes in this part of the forum) Hi, I was wondering if someone could help me with the following: I have a (1+1) scalar field decomposed into two different sets of modes. One set corresponds to a Minkowski frame in (t,x) coordinates, the other to a Rinder frame in conformal...
  44. P

    Gaussian Integral with Denominator in QFT

    Hi all, so I've come across the following Gaussian integral in QFT...but it has a denominator and I am completely stuck! \int_{0}^{\infty} \frac{dx}{(x+i \epsilon)^{a}}e^{-B(x-A)^{2}} where a is a power I need to leave arbitrary for now, but hope to take between 0 and 1, and \epsilon is...
  45. A

    Readings for a general understanding of Quantum Field Theor?

    Hi all, I have an undergraduate degree in Physics, but I've since specialized in more environmental applications of Physics. I've taken a few quantum Physics courses as part of my degree, but, if I'm being honest, what I took from those courses was more of a mathematical view of the field, such...
  46. W

    Schwartz QFT book, Problem 14.3

    I am working on Schwartz QFT book problem 14.3, particularly part (c). Basically, it asks us to evaluate the following integration. \int \cfrac{d^3 p}{2\pi^3} \omega_p e^{i \vec{p} \cdot (\vec{x}-\vec{y})} where \omega_p = \sqrt{p^2 + m^2} I could perform the angular integration, and the...
  47. S

    Prerequisites for Quantum Field Theory (QFT)

    Hi! I'm desperately trying to develop a list of prerequisites that will enable me to work on topics like quantum gravity, advanced QFT (on curved spacetime etc.) etc. I am currently in the second year of an undergrad theoretical physics degree in the UK, and am heavily unsatisfied with the way...
  48. S

    What are some interesting BS-MS project topics in QFT for a 1 year duration?

    I would like to know some of interesting topics in QFT for my major project (BS-MS); of 1 year duration... can anyone help me with topics? Thanks...
  49. L

    From observable to operators in QFT

    from the relativity forum https://www.physicsforums.com/threads/spacetime-in-qm-or-qft.802721/ Sonderval stated (transferred here so not off topic): http://scienceblogs.de/hier-wohnen-drachen/artikelserien/[/QUOTE'][/PLAIN] So the standard Schroedinger Equation can be used for both particles...
  50. L

    Understanding Spacetime in QM and QFT: Time Treatment and Particle Tracking

    spacetime is classical and continuous.. so when quantum system form superposition or entanglement, how is time treated esp in qft? how does spacetime keep track of the particles of fields? or how does the fields or particles able to jump in spacetime.. how is this handle in qm or qft?
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