The Longest Equation Ever Known to Man
 |
| Standard Model Lagrangian (density) |
The standard what?
The Standard Model is the current Theory which governs all known strong and electroweak interactions. This includes basically everything that has EVER been measured by humans (sans gravity). It really is the best Theory that we have. It has been fantastically successful in predicting and explaining data; there are no obvious or overt violations of this Theory. Nerds can go read (or add more information :/) about it here.
How long did it take to type?
In it's current form, the actual equation have took about four hours (spread over almost a week) to write into a file using LaTeX. And sadly, the typist think there is a sign error somewhere, but he just don't have the energy to hunt it down.