I was looking for "nightmare math" in the README and was confused because I didn't find any. I guess that's what a theoretical physics degree does to you: that formula looks very harmless to me.
Another example is standard model of particle physics. There’s a way to write down the Lagrangian of the standard model very compactly: https://visit.cern/content/standard_model_formula_t_shirt
But if you expanded all the implied sums and terms, it probably would be monstrous. After all it is supposed to contain all the terms that this figure contains for example: https://commons.wikimedia.org/wiki/File:Standard_Model_of_El...
Also, ask string theorists to show you calculations that they need to find the largest paper to perform. I’ve heard people doing calculations where a single line is the width of an A1 paper.
The language is important. The more you “understand” something, the simpler it is (over simplification here.) eg I would not consider the Einstein equation expanded out to be natural after understanding it. (Well Maxwell’s equation can be summarized in 1 single elegant equation as well.)
But the reason I’d consider that optics formula to be monstrous is that no matter how you group it into smaller pieces (ie refactoring it), there’s no way to hide the fact that it is not elegant at all. There’s no “understanding” there, it is just so happen the exact solution looks like that.
To put it that way then, often fundamental “master equation” are simple in some ways, but exact solutions to some particular manifestation of that master equation is often quite ugly and monstrous. In that sense then rather than quoting Einstein equation I’d quote its solution eg one with mass and spin and electric charge.
P.S. the solution of quartic equation is also a good example of this category