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The interaction term in the QED Lagrangian $$\mathcal{L}_{int}=e\bar\psi\gamma^\mu A_\mu\psi$$ changes under a gauge transformation $$A_\mu\rightarrow A_\mu+\partial_\mu\chi$$ Doesn’t it affect the QE …

However, the discussion here says that Lamb shift affects only S state. The statement that Lamb shift affects only $s$ states is only approximately correct i.e., true in first order radiative cor …

In the introduction part of this renowned paper by S. Coleman and E. Weinberg, the authors write The quartic self-coupling is required for renormalizability, to cancel the logarithmic divergence …

I'm lost amidst the calculation of regularization and renormalization process in QED. In addition to the divergent piece in the in the self-energy correction (similarly in vacuum polarization correcti …

To my understanding, unlike free fields, interacting fields cannot be expanded in terms of Fourier modes, with the Fourier coefficients representing creation and annihilation operators. Then is it pos …

A non-interacting quantum field $\hat{\phi}(x)$ can be decomposed into $a_{\textbf{k}}$ and $a_{\textbf{k}}^\dagger$. This enables us to calculate the variance of a free field. For example, the varian …

The term 'bremsstrahlung' is usually reserved for the emission of electromagnetic radiation caused by $(1)$ acceleration or deceleration of the charged particle by an electric field and $(2)$ in prese …

Consider a QED scattering process $e^-+e^-\rightarrow e^-+e^-$. The scattering crosssection at the tree-level depends on the square of the fine-structure constant $\alpha$ (apart from the electron mas …

Quantum electrodynamics (QED) is based on $U(1)$ symmetry. What happens to this symmetry in classical electrodynamics? Addendum The books on classical electrodynamics such as J. D. Jackson, does not …

A charged particle, decelerated by the electric field of another charged particle, typically an atomic nucleus, emits electromagnetic radiation, called Bremsstrahlung. This process typically happens i …

According to Wikipedia article on scale invarince, the equations for electric (and magnetic) fields : $$\nabla^2\vec{E}=\frac{1}{c^2}\frac{\partial^2\vec{E}}{\partial t^2}\hspace{0.3cm}\text{and}\hspa …

In a superconductor, $U(1)$ gauge symmetry is spontaneously broken. But $U(1)$ gauge symmetry is responsible for conservation of electric charge. Then it appears to me that the electric charge conserv …

In QED, one can calculate the correction to the form factor $F_2$. To the lowest order, $F_1=1$ and $F_2=0$. At one loop, it is found that $F_2(0)$ receives a non-zero finite correction which is relat …

Suppose I consider an interacting theory, say QED (with electrons and photons). Let, free electrons, I mean the quanta of the free Dirac Lagrangian. The dressed electron differs from the free electron …

It can be shown that the total angular momentum of a free electromagnetic field is given by (for example, in the book A Modern Introduction to quantum field theory by Maggiore, page 98, Eq. (4.82)) $$ …