# Questions tagged [classical-field-theory]

For questions where the dynamical variables are classical fields, that is, functions of several variables (typically, one time coordinate and several space coordinates). If the question comprises both classical and quantum fields, use the tag [field-theory] instead.

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### What determines picking a gaussian surface?

I feel like I am missing something simple here, but how exactly are gaussian surface determined? Looking at the case of a charge outside a sphere, why don't we pick a gaussian surface not including ...
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### Why boundary terms make the variational principle ill-defined?

Let me start with the definitions I'm used to. Let $I[\Phi^i]$ be the action for some collection of fields. A variation of the fields about the field configuration $\Phi^i_0(x)$ is a one-parameter ...
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### What causes the evasion of the Goldstone theorem here?

For simplicity, I'll consider perhaps the simplest possible example of a gauge theory. Consider a spontaneously broken ${\rm U(1)}$ gauge theory of a charged scalar field coupled to the ...
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### Why a symmetry, in Lagrangian field theory, should allow a boundary term?

Following the discussion in this paper (discussion around Eq. (3) in Page 4) and these lecture notes (discussion in Section 1.2.1 in page 10) given a field theory in some spacetime $(M,g)$ described ...
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### Pertubational approach of a scalar field EOM

I am trying to understand a calculation in a QFT textbook. Given the equation of motion of a scalar field $h$ \begin{align} \Box h - \lambda h^2 - J = 0 \end{align} I now want to solve this equation ...
A Question in Classical Field Theory $\underline{\text{Assumption 1}}$: The definition of a transformation specifies how both the coordinates and the fields transform: They are namely $(1$-$1)$ and \$(...