# Questions tagged [notation]

This tag is for questions on the meaning, history, and usage of various symbols and notation that are used to denote different quantities in physics. Don't forget to mention the reference, where you encountered those notations.

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### What are $w,x,y,z$ electronic transitions? How are they classified?

This paper's abstract mentions transitions $(x,y,z)$ and mentions another paper which talk about $w$ transitions in He-like Ti ions. The papers have diagrams which clearly show what these transitions ...
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### Base change using bra-ket notation

If I have two orthonormal bases (of a vector space over $\mathbb{C}$) $A=\{|a_{1}\rangle, |a_{2}\rangle\}$ and $B=\{|b_{1}\rangle, |b_{2}\rangle\}$, the change of base matrix from $A$ to $B$ is given ...
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### Explanation for the $(a,b)$ Notation in Super Conformal Field Theories [duplicate]

In the literature, there are many theories and quantities which I usually seen in the context of conformal field theory that described via the notation of $(a,b);\quad a,b\in\mathbb{Z}$ for example: ...
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### Can unit prefixes be interpreted as algebraic stand-ins?

I am currently a little confused about whether unit prefixes can be interpreted algebraically For example: $$4km = 4\times 10^{3} m$$ Is it incorrect to say that $k$ is simply an algebraic stand in ...
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### Question about meaning of "bar"-ing in the context of Dirac fields

Following chapter 38 of Srednicki, "bar"-ing means (based on eq. 38.15) $$\bar{A} = \beta A^\dagger\beta$$ One can show for instance that $$\bar{\gamma^\mu} = \gamma^\mu$$ My question is, ...
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### Orthogonality of Lorentz transformation [duplicate]

I know this might be a general or well known topic, but please bear with me, since I have a very specific question regarding this and I want to see specifically whether there is a flaw in my reasoning ...
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### What is the effect of the operator $\hat{O} = |\phi \rangle \langle \psi|$ on any state $| \psi \rangle$? [closed]

I have seen this operator in some questions,including the book Liboff,Introductory Quantum Mechanics. I am not sure how it would act on a state.
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### Directional derivative $(\mathbf{A}\cdot\nabla)\mathbf{B}$ of the vector field $\mathbf{B}$

While reading Introduction to Electrodynamics by David J. Griffiths, I have encountered some issue with the notation of the directional derivative of the vector field and I was wondering if there are ...
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### Einstein Summation Convention Confusion

My textbook: The second bit confuses me. I asked a question on this site yesterday (Moment of Inertia tensor confusion) which involved the moment of inertia tensor and the term $$r_{i}r_{j}$$ The ...
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### Contracting the metric tensor with its inverse yields Kronecker delta

It's probably straightforward, but I would like to see the proof of the identity: $$g_{\mu\nu}g^{\nu\alpha}=\delta^\alpha_\mu.$$ In the book 'Spacetime and Geometry' by Carroll, this identity is the ...
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### Moment of Inertia tensor confusion

This is the moment of inertia tensor about the COM. $$I_{ij}'=\sum_{k=1}^Nm_k\left[\vert\mathbf{r}'^{(k)}\vert^2\delta_{ij}-r_i'^{(k)}r_j'^{(k)}\right]\neq I_{ij}$$ I don't understand how to compute ...
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### In diagrams, how can I indicate that a body is not moving or rotating?

I often create diagrams to illustrate the scenario of an exercise for my students. These are not necessarily free-body diagrams, as creating those may be part of the task or similar or it would be ...
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### Where does the negative sign disappear?

The defining equation for simple harmonic motion is such $$a=-ω^2x$$ When we find the centripetal acceleration of an object in orbit we use the formula $$a=ω^2r$$ As a consequence of the accleration ...
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### Showing the Variance of an observable in a determinate state is always zero

I am working through Introduction to Quantum Mechanics by David J. Griffiths, and part 3.2.2 shows that the standard deviation of an obervable, $Q$, is always $0$ but I do not understand the steps ...
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### Notation for intrinsic charge carrier

What exactly is $n_{\textrm{i}}$? It is said to be the intrinsic charge carrier, but for pure semiconductors, $n_{\textrm{h}} = n_{\textrm{e}} = n_{\textrm{i}}$, is $n_{\textrm{i}}$ not the sum of ...
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### What is the distinction between a ket and a state in quantum mechanics?

Sorry if this has been asked before in some manner, but I'm just a bit confused about the distinction between a state $\alpha$ and its ket $|\alpha\rangle$. I was recently told that a state $\alpha$ ...
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### How to draw topological defect line in LaTeX?

This is somewhat between TeX stackexchange and physics stackexchange. With Latex, it became much more easier to write equations, and with tikz, pictures can be drawn. However, with the introduction of ...
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