# 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 does the notation $(k \cdot \nabla ) v$ mean? [duplicate]

I am reading a paper and it uses a notation I am not too familiar about. Although I saw it used elsewhere, I don't remember the meaning of it and I don't want to misinterpret it and realize after ...
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### Help with strange notation involving fractions of tensors

I'm currently reading the paper Open strings in background gauge fields by Callan et.al. It is frequently used a notation that is not explained anywhere. If $F_{\mu\nu}$ is the electromagnetic field ...
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### Bra-Ket Notation vs Wavefunction Notation [duplicate]

We know that the rule for creating excited states for a Quantum Harmonic Oscillator is $|n\rangle=\frac{(a^\dagger)^n(|0\rangle)}{\sqrt{n!}}$. I wanted to derive from this the familiar rule in terms ...
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### Why don't we commonly use bra-ket notation in acoustics and vibration?

I have a background in acoustics and do more and more of quantum mechanics. I wonder why don't we use more often the bra-ket notation in acoustics and vibrations like in quantum mechanics? In modal ...
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### Tensor to vector notation [closed]

I have the expression $$\partial_{k} \left( a^{-1} b^{2} \eta^{j k} \partial_{j} G \right) = a \vec{\nabla} . \left( a^{-2} b^{2} \vec{\nabla} G \right)$$ Is the transition from tensor to vector ...
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### Why is the usage of just linear, unitary operator that isn't necessarily self-adjoint in the middle of a bra-ket a problem?

In the question Define antilinear or antiunitary operator $Θ$ acting on the ket state and on the bra state consistently? an answer mentions that the bra-ket $⟨β|A|α⟩$ in a proper way iff $A$ is ...
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### Can a collision excitation of an atom, in the collision event with an electron, be distinctly depicted in a Minkowski diagram? (And: How to annotate)

A Minkowski diagram is a two-dimensional graphical depiction of a portion of a (suitably selected) region of flat spacetime, and thus of a corresponding set of events, such that (among additional ...
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### Conceptual difference between ket and bra vectors in layman's terms

From the relation $\langle a|b \rangle = \langle b|a \rangle ^*$ it can be seen that we can get same inner product by "swapping vectors" in Hilbert space and taking complex conjugate of the ...
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### Expectation spatial representation on Dirac notation [closed]

A semester ago, I learned about Dirac notation. And I tried my best to ensure how it works by writing every equation down into dirac notation. I tried the same for the new semester class. but I came ...
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### About the notation of Lie algebra, upstairs and downstairs indices

In some textbooks, Lie algebra is written as $$[X_a, X_b]=if_{abc}X_c$$ where $X_a$'s are the generators of the Lie group. But sometimes, the Lie algebra is written as $$[X_a,X_b]=if_{ab}^{~~~ c}X_c$$ ...
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### Help me decipher ambiguities in notation from a textbook derivation of the QM momentum operator

I am familiar with the momentum operator in quantum mechanics, and have used it in problems, in at least one dimension. However, I am reading a new text in effort to absorb any details about the ...