# Questions tagged [conventions]

A convention is a set of agreed, stipulated, or generally accepted norms. It typically helps common efficiency or understanding but is not required, as opposed to a strict standard or protocol.

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### What is the Planck scale magnetic field strength?

Using the constants $\mu_0$ (or $\varepsilon_0$), $c$, $\hbar$, $e$ and $G$, it is possible to define two quantities with units of magnetic field : \begin{align} B_1 &= \sqrt{\frac{\mu_0 c^7}{\...
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### How does GR reconcile its model of a smooth manifold against the limitations of the planck length?

To paraphrase the classic text, Gravitation by Misner, Thorne, and Wheeler, it is said that we are to accept the model of a continuous spacetime manifold, despite the calculated "violent fluctuations ...
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### Notation for vectors and covectors

This is probably a very simple question, and I think I know the answer, but I cannot find a place to solidly confirm this. So if I want to write a vector $\mathbf{V}$ in terms of its contravariant (...
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### How should we use time reversal operator?

In Peskin & Schroeder's QFT textbook, section 3.6. Time reversal operation on a operator, for example, Dirac field $\psi(t,x)$, is $$T\psi(t,x)T$$ Some other textbooks, mostly, like QFT written ...
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### Anticommutator BRST charge and $c$-ghost mode

My goal is to compute the anticommutator $\{Q_B, c_m\}$ where $Q_B$ is the BRST charge in string theory and $c_m$ is the $m$th mode of the $c$ ghost field $$c(z) = \sum_m \frac{c_m}{z^{m-1}}$$ (...
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### Wick rotation on Ward identities

I'm having trouble performing a Wick rotation back to Minkowski spacetime ($\eta_{\mu\nu}=(-1,1,1,\dots)$), following page 19 in the lecture notes here by C.P. Herzog. I have this expression (equation ...
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### Feynman rules for derivative couplings with identical particles

Consider the interaction Lagrangians given by \begin{align*} \mathcal{L}_1 = A_\mu \ \pi^{0} \ \partial^{\mu} \pi^{0}\\ \mathcal{L}_2 = B^{\mu\nu} \ \partial_\mu \pi^{0} \partial_{\nu} \pi^{0}\\ \...
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### Algebra of Noether's charges and algebra of symmetry transformations

I'm trying to understand the connection of algebra of transformations under a commutator and algebra of Noether's charges under Poisson bracket. I have a problem that results I infer from theoretical ...