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# 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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### Notation in the Liouville-von Neumann equation $i\hbar\frac{dI}{dt} = i\hbar\frac{\partial I}{\partial t} + [I, H]$ [duplicate]

The Liouville-von Neumannn equation is defined by $$i\hbar\frac{dI}{dt} = i\hbar\frac{\partial I}{\partial t} + [I, H]$$ where $I$ is any operator and $H$ is the Hamiltonian. I assume that the left-...
1 vote
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### Dirac Formalism/Notation Question

I'm reading the 3rd edition of Sakurai and Napolitano's Modern Quantum Mechanics, which (probably rightly) relegates wave mechanics to an appendix. Instead, it carefully develops Dirac's formalism, ...
1 vote
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### Why is Lorentz Transformation defined with one super and one sub index?

I came across the Lorentz transformation in tensor form, usually written as $$\Lambda ^\mu _{\nu}$$ I understand that the first index usually corresponds to rows and the second to columns, and while I ...
106 views

### Question on index notation

I am working my way through Carroll's text on GR and am having trouble understanding what it means when an index is inside/outside parentheses. For example, in his discussion of geodesic deviation, ...
105 views

### Inner Product of two 4-vectors

I have a question on the inner product of two 4-vectors. As per the definition the inner product of two 4-vectors is defined as, $$\vec A.\vec B = -A^0B^0+A^1B^1+A^2B^2+A^3B^3$$ From linear algebra, ...
75 views

### Elementary question about Einstein notation

I have encountered, in a physics textbook, the following Lagrangian: $$L=\dfrac{m}{2}g_{ij}(x^k)\dot{x}^i\dot{x}^j.$$ I understand that Einstein notation is being used, and therefore there is an ...
116 views

### Determinant of an operator in ket/bra/bracket form?

As is well known, two examples of basis invariant functions are the trace and determinant functions. These functions can therefore be thought of as a property of a linear operator, and not just a ...
162 views

### Time ordering of integral [closed]

Is $$T\int\mathrm{d}^4x\phi^4(x)$$ just notation for $$\int\mathrm{d}^4x~T\phi^4(x)$$ since after integrating we have no time dependence anymore?
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### Is this explanation of the bra-ket notation correct?

I would be very grateful for feedback, particularly pointing any factual mistakes, in the below explanation of the bra-ket notation: "Quantum mechanical expressions can be simplified using a bra-...
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### Error propagation formulation for intrinsic or implicit randomness

I have a question about how to formulate an equation with implicit uncertainties for error propagation, i.e. a quantity $y$ that is determined by input values $x$ and $t$ but also has its own ...
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### What does this means $| + ; \hat{x}\rangle$? [closed]

I know that the $+$ means spin up and that $\hat{x}$ is the position operator but I don't undestand the meaning of this two sign together in the same bra (like this $| + ; \hat{x}\rangle$ )?
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1 vote