Quantum Field Theory (QFT) is the theoretical framework describing the quantisation of classical fields which allows a Lorentz-invariant formulation of quantum mechanics. QFT is used both in high energy physics as well as condensed matter physics and closely related to statistical field theory. Use ...

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What does the ordering of creation/annihilation operators mean?

When a system is expressed in terms of creation and annihilation operators for bosonic/fermionic modes, what exactly is the physical meaning of the order in which the operators act? For example, for ...
8
votes
1answer
769 views

Questions about the Dyson equation

I'm studying finite temperature many-body perturbation theory, and am trying to understand The Dyson equation. In particular, I'm using Mattuck - A guide to Feynman diagrams in the many body problem. ...
3
votes
1answer
247 views

Does kaon decay etc prove “CP violation” or just “CP or CPT violation”

Shlomo Sternberg (math professor at Harvard) wrote a book called "Group theory and physics". On p156 (link) there's a strange offhand comment: "Experiments done in 1964 by Fitch and Cronin seem to ...
8
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1answer
230 views

Is there literature on a continuous mass spectrum for the Higgs field?

Various masses for the Higgs field are compatible with experiment, but is it possible that the Higgs field is not observable because it has a continuous mass spectrum? Work in the 60s and 70s on free ...
8
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2answers
375 views

Quantizing EM field

Why when we quantize EM field, whe quantize the vector potential $A^\mu$ obtaining vectorial particles (photons) like the elastic field (phonons) and we can't quantize directly the EM-field tensor ...
9
votes
3answers
513 views

Spontaneous breaking of Lorentz invariance

Is it possible to spontaneously break Lorentz invariance, i.e., have a Lagrangian that respects LI but a vacuum which does not? If it is possible, why isn't there even the slightest hint of the ...
5
votes
3answers
362 views

bound states of massless fields?

Question: are they mathematically possible at all? physically? with finite mass systems, usually the binding energy contributes to the rest-mass of the system. It would seem that even if you could ...
7
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0answers
262 views

1-form formulation of quantized electromagnetism

In a perpetual round of reformulations, I've put quantized electromagnetism into a 1-form notation. I'm looking for references that do anything similar, both to avoid reinventing the wheel and perhaps ...
5
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0answers
217 views

When can the source term of a partition function be put in?

More specifically, in quantum field theory books, we usually have this: \begin{equation} Z = \int D(\bar{\psi}, \psi) e^{-S + \int_0^\beta d\tau \sum_l [\bar{\eta}_l (\tau) \psi_l (\tau) + ...
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5answers
7k views

Online QFT video lectures

I'm aware of Sidney Coleman's 1975/76 sequence of 54 lectures on Quantum Field Theory. Are there any other high-quality QFT lecture series available online?
9
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2answers
405 views

Is there any interacting quantum field theory of massless spin-1 fields expressed locally entirely in terms of F, with no vector potential?

Is there any interacting quantum field theory of massless fields with helicity $\pm 1$ which can be expressed entirely locally in terms of the field strength Fμν with no reference to vector ...
1
vote
1answer
483 views

What is the most natural definition of the weak hypercharge coupling constant if grand unification is wrong?

A tricky question. Here is the famous graph of the running of the three coupling constants in the standard model: http://www-ekp.physik.uni-karlsruhe.de/~deboer/html/Forschung/unification_eng.eps . ...
7
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0answers
281 views

Descent equation and anomaly polynomial

I am just reading Ryu, Moore and Ludwig's paper on classifications of topological insulators and quantum anomaly. They are trying to relate the quantum anomaly as a signal of the presence of a ...
6
votes
1answer
408 views

Does de Sitter space admit an asymptotic S-matrix?

From the Penrose diagram of de Sitter space, we see it has a future and past conformal boundary, and they are both spacelike. So, does de Sitter space admit an asymptotic S-matrix? Sure, in the usual ...
2
votes
1answer
636 views

What is the value of the fine structure constant at Planck energy?

At low energy, 511 keV, the value of the fine structure constant is 1/137.03599... At Planck energy $\sqrt{\frac{\hbar c^5}{G}}$, or 1.956 $\times$ 109 Joule, or 1.22 $\times$ 1028 eV, it has a ...
3
votes
2answers
361 views

How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?

Feynman diagrams are ordinarily the usual method of perturbative analysis for weakly interacting quantum field theories. However, over a de Sitter background, the total number of particles isn't even ...
5
votes
2answers
682 views

Why is there a minus sign in this wave equation derivation?

My book on quantum mechanics suggests a derivation of the wave equation $$\left(\Delta - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \right) \psi(\bar{r},t) = 0$$ from the photon energy-impulse ...
3
votes
1answer
369 views

Seiberg Witten theory

I'm currently reading the Seiberg-Witten paper on $N=2$ supersymmetric Yang Mills pure gauge theory (i.e. no hypermultiplets). I have the following question: How does one understand that the metric ...
3
votes
1answer
234 views

The vacuum as trigger

Do the apperance in the atomic nucleus of virtual matter-antimatter particle pairs play a role in the random nature of radioactive decay?
6
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2answers
490 views

Why is the spinor field anti-commutator not made gauge invariant?

When we introduce minimal coupling for the Dirac spinor field, we introduce terms into the Lagrangian, by the substitution $i\frac{\partial}{\partial x^\mu}\mapsto i\frac{\partial}{\partial ...
12
votes
3answers
583 views

Why is there a flux of radiation in the Hawking effect but not in the Unruh effect? (and other questions)

This question is slightly related to this one Do all massive bodies emit Hawking radiation?, which I think was poorly posed and so didn't get very useful answers. There are several questions in this ...
6
votes
1answer
462 views

Generalization of spin coherent states for an arbitrary group?

My question is inspired by the analogy of the Berry phase in the spin coherent state representation of a rotator and the Aharonov-Bohm phase of a magnetic monopole (see e.g., Section 1.8.3 in ...
13
votes
3answers
790 views

Does the Unruh effect violate Mach's principle?

Mach's principle says that it is impossible to tell if something is accelerating unless there is something else in the universe to compare that motion to, which seems reasonable. However, if you had ...
0
votes
1answer
451 views

Spin of an electron [closed]

I have a conceptual difficulty in understanding the electron spin. On the one hand, it is an experimental, observable feature of electrons. The problem is in understanding to what it belongs - to a ...
6
votes
1answer
644 views

Definition and difference between the R-symmetry and the $U(1)_R$ internal symmetry

For a general ${\cal N}$ the R-symmetry group is $U({\cal N})$ but for the ${\cal N}=2$ case why is it $SU(2)$ ? I guess it is again different for ${\cal N}=4$. How does one understand this? One ...
6
votes
3answers
502 views

What are fields?

I'm following my first course in field theory and the professor began, like many books do, by introducing the scalar field. However, I am a bit hesitant about the physical idea of fields. My question ...
2
votes
0answers
399 views

Correlators at large N and large N factorization

I am having this very basic problem. In e.g Maldacena's AdS/CFT review (0309246) (page 5), he has defined operators as $\mathcal{O}=N\,{\rm tr}[f(M)]$ for some matrices $M$ and got the connected ...
14
votes
3answers
742 views

Does renormalization make quantum fields into (slightly) nonlinear functionals of test functions?

Quantum fields are presented as operator-valued distributions, so that the operators in the theory are linear functionals of some test function space. This works well for free fields, giving us a ...
6
votes
6answers
2k views

Why is mass the quadratic term in a Lagrangian?

Why is mass the quadratic term in a Lagrangian?
7
votes
1answer
424 views

BPS sectors in $\cal{N}=4$ SYM

I am familiar with the idea of a BPS bound as in a lower limit on the mass of supermultiplets given by a certain function of the central charge and when I think of $\cal{N}=4$ SYM I see a complicated ...
6
votes
1answer
473 views

Is microcausality *necessary* for no-signaling?

There are proofs in the literature that QFT including microcausality is sufficient for it not to be possible to send signals by making quantum mechanical measurements associated with regions of ...
4
votes
3answers
554 views

Nomenclature: Yang-Mills theory vs Gauge theory

If you're writing about a theory with Yang-Mills/Gauge fields for an arbitrary reductive gauge group coupled to arbitrary matter fields in some representation, is it best to call it a Yang-Mills ...
6
votes
2answers
232 views

Does the lack of modular nuclearity in string theory mean anything?

Nuclearity is a postulate in algebraic quantum field theory (AQFT). Basically, it says thermal states at any temperature always have a thermodynamic limit with extensive quantities. This is violated ...
3
votes
1answer
316 views

The superconformal algebra

How does one derive the superconformal algebra? Especialy how to argue the existence of the operator $S$ which doesn't exist either in either the supersymmetric algebra or the conformal algebra? ...
5
votes
1answer
829 views

Some questions on Conformal Field Theory, Current algebras and the Sugawara construction

Since I don't know how to add another question to an already existing topic, I'm opening a new thread. However I'm referring to: Beginners questions concerning Conformal Field Theory As noted, a ...
6
votes
2answers
5k views

What is a complete book for quantum field theory?

I am searching for a complete and comprehensive book for QFT. What is, in your opinion, a good one?
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votes
4answers
908 views

Who works professionally on reformulation of QFT?

P. Dirac was worried with the infinities and their discarding in QED. He wanted us to reformulate the theory in order to eliminate infinities and renormalizations from the very beginning. Is there ...
12
votes
1answer
430 views

An unfamiliar way of writing supersymmetry transformations

This question is in relation to this recent paper. I would like to know how the so called supersymmetry transformations at the start of page 27 or at the end of page35 (equation 8.4) or at the end ...
9
votes
1answer
568 views

CFT and the Coleman-Mandula Theorem

The Coleman-Mandula theorem states that under certain seemingly-mild assumptions on the properties of the S matrix (roughly: one particle states are left invariant and the amplitudes are analytic in ...
3
votes
2answers
494 views

What is a “classical Schrodinger field”, really?

I have read through the wikipedia page and several lecture notes/arxiv papers from my google search (and several related P.SE questions), but I'm still hopelessly confused. Consider a 'classical ...
12
votes
3answers
2k views

Beginners questions concerning Conformal Field Theory

I started reading about Conformal Field Theory a few weeks ago. I'm from a more mathematical background. I do know Quantum Mechanics/Classical Mechanics, but I'm not really an expert when it comes ...
3
votes
1answer
495 views

Why is GR ghost-free?

I wonder how one can show that general relativity is ghost-free? By ghost I mean the negative norm state that breaks the unitarity. I think it is a well-known "fact" but I just couldn't find any ...
7
votes
2answers
952 views

What does “soft” in “soft symmetry breaking” mean?

For example it is stated that if supersymmetry breaking is soft then stability of gauge hierarchy can be still maintained.
3
votes
1answer
74 views

Experimental limits on anisotropies in the $e/m_{e}$ ratio

Currently the charge-to-mass ratio of the electron is known to 10 orders of magnitude. However, i'm curious if: Are there any experiments trying to bound the anisotropy of this ratio for different ...
8
votes
1answer
271 views

Weakly gauge a symmetry?

What does it mean to "weakly gauge" a global symmetry in a gauge theory? I have seen this term used in a number of papers, but have not seen it defined.
4
votes
1answer
421 views

Katz and Vafa's work on F-theory

I would like to know about the larger picture, current state and future prospects of the sequence of papers that were written by Sheldon Katz and Cumrun Vafa on F-theory. (Freddy Cachazo was also a ...
3
votes
3answers
476 views

Particle Lagrangians

As I learned in my string theory course, you can describe the quantum spin-0 particle by quantizing the arclength Lagrangian of a relativistic classical particle. My question is whether you can get a ...
7
votes
3answers
436 views

Horizon and Unruh radiation for a finite period of acceleration

It's a well known fact that an observer that accelerates at a constant rate from $-c$ at past infinity to $+c$ at future infinity sees a horizon in flat Minkowski spacetime. This is easy to see from a ...
6
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4answers
3k views

Chemical potential

This is something probably very basic but I was led back to this issue while listening to a recent seminar by Allan Adams on holographic superconductors. He seemed very worried to have a theory at ...
15
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3answers
1k views

No hair theorem for black holes and the baryon number

The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge. For me it is puzzling why ...