# 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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### How to write completeness of wavefunctions without bra ket notation?

In the quantum textbook I'm currently working from, the completeness relation is written as: $$\sum_i |\psi_i \rangle \langle \psi_i| = \mathbb{1}.$$ But this seems to specifically require knowledge ...
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### Elliptical polarisation - unsure as to what quantity is on wikipedia page

quick question - https://en.wikipedia.org/wiki/Elliptical_polarization - what is $\alpha_{x}$ and $\alpha_{y}$? It's not defined on the page. I'm asking to answer part of a question, which requires me ...
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### Reference which explains Penrose Diagramatic notation in simple way

In both Penrose's Road to reality and Spinor's and space-time, the following notation is shown: With a lot other examples for doing calculation with Tensors. Could someone give another reference ...
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### Derivation of entropy, I don't understand the relation $\frac{\partial S_2}{\partial E_1} = -\frac{\partial S_2}{\partial E_2}$

My course guide gives the following derivation for change in entropy w.r.t. energy, where I don't understand a step: \begin{align} E & = E_1 + E_2 \\ S & = S_1 + S_2 \\ S(E,E_1 ) & = S_1 (...
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### Is $\bar{\psi} \psi$ its own complex (*)? transpose ($T$)? hermitian conjugate ($\dagger$)?

Related to this earlier A common standard model Lagrangian mistake? Here I am treating Dirac equation of Dirac field as QFT. You may want to consider the quantized version or the classical version. ...
### How to make notation like $Y_{l m_{l}}(\theta, \phi)\chi_{m_s}$ more rigorous as a tensor product?
Sometimes in quantum mechanics we come across notation like $Y_{l m_{l}}(\theta, \phi)\chi_{sm_s}$ where $Y_{lm_l}$ is a spherical harmonic representing the spatial part of some particle wavefunction ...