# Tag Info

## New answers tagged notation

4

From Wikipedia: The number 5 is a relic of old notation in which $\gamma^0$ was called "$\gamma^4$".

2

I don't know is there is a historical reason I just assumed that was just no to mistake it with $\gamma^3$ which would be called $\gamma^4$ if your Lorentz indices run from $\mu=1,2,3,4$ instead of the usual $\mu=0,1,2,3$.

2

I assume you're thinking about Minkowski space, i.e. the metric $\eta_{\mu\nu}=\text{diag}(c^2,-1,-1,-1)$. You should be aware that the dot notation is purely a notational shorthand, and has no other information contained in it. In particular, by definition we have $$\dot{A}\equiv\partial_0A=\frac{1}{c}\frac{\partial A}{\partial t}$$ Thus, there is no ...

1

The Delta symbol is often used to describe a "difference". Typically $$\Delta x=x_f-x_i$$ for some parameter $x$. That is, it is the final minus the initial. In this case, for potential energy, the final and initial states are explicitly mentioned as the limits of the integral. The way it is written here allows you to write the following: \Delta ...

1

Let the undotted index $a$ correspond to the irrep $(\frac12,0)$ of the Lorentz group. Then, $F_{ab}$ corresponds to the tensor product $(\frac12,0)\otimes (\frac12,0)$ which decomposes into the two irreps $(1,0)$ and $(0,0)$ of the Lorentz group. $(1,0)$ is given by the symmetric part of $F_{ab}$ while the antisymmetric part gives the scalar ...

11

It's c for constant or celeritas, which means speed in Latin. Everyone uses it because it's convention. You could use $\xi$ or $\zeta$ or $\gamma$ or any other symbol you wanted, but then you'd have to explain what it meant, and people would have to go through the trouble to remember this every time they read your papers. Better to go with convention and ...

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