# Tagged Questions

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### What is the Difference between the tensors, $T^a\ _{bc}, T_b\ ^a\ _c, T_{bc} \ ^a$?

The title completely expresses the question. I have come across for several times the mixed tensors where the lower index is written closer to the tensor symbol and the upper index is written a bit ...
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### Variation of a functional [closed]

in the paper I am reading now (https://arxiv.org/abs/1603.04824) I just stumbled over the following expression ${\delta}_{\eta_2} Q[\eta_1]$, where $\eta_1$ and $\eta_2$ are different parameters. I ...
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### Position of indices in QFT

I have recently started studying quantum field theory from the book Quantum Field Theory and the Standard Model by Schwartz. In chapter 2 it is said that, contrary to GR, one can ignore the index ...
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### Formatting of quark fields

I try to follow the ISO 80000-2 norm when typesetting physics. This is more a typesetting question but I did not want to ask this at the TeX SE site since this only applies to physics and I am not ...
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### What is W.u. as a unit in a nuclear physics context?

In a recent paper (doi/arXiv), of note for causing this recent tussle over its handling by science news media, there's some strange units I can't quite puzzle out. Specifically, the abstract contains ...
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### Notation to represent frequency and acceleration in preparation for algorithms

I hope this is the right place to ask. IMO it's the most appropriate SE site. I would have chosen mathematics; I didnt because I am using units such as force and Hz, and want equations to have ...
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### Basis/Projection Notation Question Quantum Mechanics

Lets say you have a inner product between two state vectors with an operator in between A|X|B. I can write this as a summation over I and j as A|i i|X|j j|B (sorry for notation). But I don't ...
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### Does $\partial_\mu =\frac{\partial }{\partial x^\mu}$ or $\partial_\mu =\frac{\partial }{\partial x_\mu}$? [migrated]

I am looking at the chain rule with covariant and contravariant vectors. I understand why we have: $$df=\frac{\partial f}{\partial x^\mu} dx^\mu$$ (Please correct me if I am wrong) since even though ...
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### How do I differentiate between units and variables on my notebook? [closed]

On computer systems, variables are written in italics, as $m$ or $s$, and the units as $\mathrm{m}$ or $\mathrm{s}$. It is very incovenient to do italics for variables and upright for units while ...
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### How can an orbital be recognised from the wavefunction notation?

I am a student and was working up the exercises in my book when I came across a problem that required me to identify the orbital given by $\psi_{3,2,1}\,.$ What I can work out is that the sub-shell ...
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### Why Does there Have to be Linearity in Ket and Skew Symmetry?

I'm reading Shankar's "Principles of Quantum Mechanics," and on page 8 he states that one axiom in Dirac notation is linearity in ket, and because they are also skew symmetric there is anti-linearity ...
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### What does $f(v)d^3v$ mean?

I am reading the derivation of Langmuir's Evaporation Equation. The author writes: That cylinder contains a volume $dA(vdt)cosθ$ and contains vapor molecules of the designated speed in the ...
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### Einstein Summation Convention: One as Upper, One as Lower?

My question refers to the often specified rule defining Einstein Summation Notation in that summation is implied when an index is repeated twice in a single term, once as upper index and once as lower ...
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### Different subscripts for $\nabla$ operators while deriving force on system of many particles

Consider a system of 4 particles in an external conservative field. So force acting on each particle is derived from potential energy $U(x,y,z)$of the particle+field system: Total (external) force on ...
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### Conventions for propagators in Feynman diagrams [closed]

So far, I picked up the following rules for the propagators: Scalars: Dashed Fermions: Solid Abelian gauge boson: Wavy Non-abelian boson: springy Ghost: Dotted This made much sense to me until I ...
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### What is $bfr$ in this expression?

I am reading 'Fundamentals of Quantum Mechanics' by Sakir Erokoc and came across this expression in relation to transition probabilities: $$\vec p=e \langle \psi_b |bfr|\psi_a \rangle$$ Which can be ...
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### Can a sum of Pauli matrices be a real value?

In the page 2 of Quantum Annealing for Constrained Optimization, the authors introduced a constraint term under the Constrained quantum annealing (CQA) section. The ultimate goal is to work out a ...
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