# Tagged Questions

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 ...

596 views

### Degrees of freedom of the photon in $d=n$

It is well known that in ordinary $4$ dimension, the photon has on shell only two physical degrees of freedom. Physically this means its elicity is either $\lambda=+1$ or $\lambda=-1$ but cannot ...
308 views

### Gupta-Bleuler Formalism

In the Gupta-Bleuler formalism we have a problem with two states (scalar photons and longitudinal photons), because here $\langle \vec{k}_a|\vec{k}_b\rangle$ is negative or zero. However, I thought ...
108 views

### independence of the bare parameters on μ for beta function

So I know re-normalization has bean "beaten to death". I want to understand something a bit specific which might seem trivial. Independence of the bare parameters on $\mu$ and relevance to the beta ...
216 views

### Roadmap to the Renormalization Group Approach

I am an undergrad interested in HEP-Th. I have studied canonical quantization, and path integral approach for quantizing fields, and the EM field quantization, classical yang-mills theory. I want to ...
66 views

### $\mathcal{N}=4$ SUSY in $d=3$ versus $\mathcal{N}=2$ in $d=4$

Which is the field content of the hypermultiplet and the vector multiplet in $\mathcal{N}=4 \ d=3$ Supersymmmetry? Is it correct to state that $\mathcal{N}=4$ in $d=3$ has $8$ supercharges, (since ...
269 views

### How is the classical EM field modeled in quantum mechanics?

On the one hand, classical electromagnetism tells us that light is a propagating wave in the electromagnetic field, caused by accelerating charges. Then comes quantum mechanics and says that light ...
1k views

### Divergent Series

Why is it that divergent series make sense? Specifically, by basic calculus a sum such as $1 - 1 + 1 ...$ describes a divergent series (where divergent := non-convergent sequence of partial sums) ...
200 views

### Non-locality and quanta

Quantum mechanics is non-local in that long distance correlations are present, though there is no signalling possible. But QFT is Lorentz invariant and contains quantum mechanics as a special case. I ...
148 views

### Functionals of quantum states in QFT

Almost every book and article I can think of represents states of QFT using the Heisenberg picture of Hilbert space vectors, but Visser in "Lorentzian wormholes" does mention that you can also ...
76 views

### Compatibility conditions of spinors and Riemannian Metrics

I came across an interesting article by Montesinos (J. Geom. Phys. 2 (1985), no. 2, 145–153.). In it, he finds that spin structures (as lifts of $SO(4)$) are not compatible with all Riemannian metrics ...
92 views

### Born rule and Feynman propagators

Let us assume that we want to describe the full process of photon emission by electron A and absorption by electron B. Therefore electron B must be on the forward lightcone of electron A. In the ...
236 views

### Is $v(p)\exp(ipx)$ really the positron wave function?

In many textbooks the negative energy solution of the Dirac equation is quoted as describing the positron. Actually I don't understand this. For me $v(p)\exp(ipx)$ is the wave function of an electron ...
660 views

### What distinguishes time from space in Quantum Field Theory?

Consider the following expression for a general QFT action: $$S ~=~ \int_0^t\mathrm dt~L ~=~\int_0^t\mathrm dt\int_\mathbb {R^3}\mathrm d^3x~\mathcal L ~=~\int\mathrm d^4x~\mathcal L.$$ Here we ...
171 views

### Negative sign in the Dirac term from the SUSY Kahler potential

I want to calculate the Dirac term from the canonical Kahler potential, $$K = \Phi ^\ast \Phi \tag{1}$$ but I'm coming across a pesky negative sign in the final result. I ...
294 views

### Why is photon annihilation associated with the POSITIVE frequency component of the electric field?

I'm reading Glauber's paper "The quantum theory of optical coherence". In his work he does not introduce the annihilation and creation operators, but he refers instead to the positive and negative ...
134 views

### Exchange Particle between an Electron Neutrino and Neutron?

How can a neutrino turn a neutron into a proton? This is the equation, $$\nu_e + n \to p + e^- \,.$$ If you draw the Feynnmann Diagram which I attempted here, "Diagram" there isn't an exchange that ...
132 views

### How can W+ boson turn an electron to a electron neutrino?

If you look at the Feynmann Diagram of an electron capture: Whe W+ boson turns the electron into a neutrino. How is this possible? I thought the the boson carries the positive charge and converts ...
116 views

### Is it possible to Vectorialize Quantum Field Theories?

If I take the rules for classical electrodynamics in the covariant formulation (the closest to QFT), I have a tensor that describes the field, $F_{\mu\nu}$. Now we know that we can take some of the ...
177 views

414 views

### CP violation from the Electroweak SU(2)$_{weak,flavor}$ by $\int \theta F \wedge F$

Question: Why there is NO Charge-Parity (CP) violation from a potential Theta term in the electroweak SU(2)$_{weak,flavor}$ sector by $\theta_{electroweak} \int F \wedge F$? (ps. an explicit ...
925 views

### Is Conformal Symmetry Local or Global?

I'm just brushing up on a bit of CFT, and I'm trying to understand whether conformal symmetry is local or global in the physics sense. Obviously when the metric is viewed as dynamical then the ...
1k views

### What are mass eigenstates?

According to Wikipedia Neutrino oscillation arises from a mixture between the flavor and mass eigenstates of neutrinos. That is, the three neutrino states that interact with the charged leptons in ...
223 views

### Quantising the Electromagnetic Field in QED

How exactly do you derive this result?
944 views

### Is the Dirac Lagrangian Hermitian?

I'm wondering of the Dirac Lagrangian density $$\mathcal{L} =\overline{\psi}(-i\gamma^\mu \partial_\mu +m)\psi$$ is an hermitian operator, since upon complex conjugating one gets ...
216 views