# Questions tagged [field-theory]

For questions where the dynamical variables are fields, that is, functions of several variables (typically, one time coordinate and several space coordinates). Comprises both classical field theory and quantum field theory. Use this tag when the question applies to both classical and quantum phenomena. Otherwise, use the specific tag instead.

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### Why do we use pole mass in LSZ formula?

I am reading "Quantum Field theory and the Standard Model" by Schwartz. It derives LSZ formula in chapter 6, \begin{equation} \langle f|S|i\rangle =\left[i\int d^4x_1e^{-ip_1x_1}(\Box + m^2)\...
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### Relativistic invariants of a classical field in 4D fashion: why the relation between the components of the current density holds?

I'm trying to understand how is justified the following relation between the first component of the current density integrated over the volume and the scalar product of the 4-vector current density ...
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### Relativistic derivation of energy for a photon inquiry

In Susskind's Special Relativity & Classical Field Theory, he presents the following argument for the energy of massless particles: We know there is a relationship between the components of the ...
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### Doubt in classical field theory/electromagnetism

What is the basic difference between electromagnetic fields, electromagnetic waves and constant electromagnetic fields?
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### Scale factor in conjugate scalar field inside conformally flat spacetime

Consider a lagrangian density of a scalar field $$\mathscr{L} = \frac{1}{2} \partial_\alpha \phi \partial^\alpha \phi - \frac{1}{2} m^2 \phi^2$$ inside a conformally flat spacetime with scale factor ...
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### Conservation law for quantum field theory with boundary

Consider a scalar field theory defined on 1+1 D space time where the space is limited to [-L,L]. The Lagrangian density of the theory is \begin{equation} \dot\Phi^2-(\partial_x\Phi)^2-h\partial_x\Phi(...
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### Similarity transformations in QFT

I am trying to understand the gaps in my knowledge that prevents me from completely understanding quantum field theory. Sometimes I ask pretty basic questions, but please excuse me if I make a blunder....
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### What's the difference between the conjugate momenta in the classical mechanics and in field theory?

In the classical mechanics the conjugate momenta was typically a derivative of the Lagrangian, i.e. \begin{equation} p_i=\frac{\partial L}{\partial \dot q_i}.\tag{1} \end{equation} However, in the QFT ...
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### How do we know that field produced by contact forces is time invariant?

I don't get the fact that action reaction forces act at the same time(Newton's third law). If this is so then it shouldn't be true for time invariant field since they would reach from source to sink ...
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### Deriving equation describing fermion-antifermion field

We know the Lagrangian of massless interacting Dirac field $\mathcal{L}=\bar{\psi}i\gamma^\mu(\partial_\mu-iA_\mu)\psi$ Now consider charge conjugation operator $C=i\gamma^2$ The Lagrangian for charge ...
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