I still consider Matrix theory (BFSS, 1996) one of the most conceptually original developments in theoretical physics of the last 20 years.

It is a relatively unusual way to describe physics in the 11-dimensional asymptotically flat vacuum of M-theory – and in other sectors of string theory. The physical phenomena are completely equivalent as in other descriptions; but the way how they're encoded in the mathematics looks very different.

There are several natural pedagogical ways to get to Matrix theory and I will try to sketch the following three of them:

- try to study some obviously beautiful quantum mechanical models in extreme limits (the infinite number of colors) and try to find a simplified description of this limit;
- start with M-theory whose explicit equations weren't known before BFSS 1996 and transform it via dualities and tricks into something that you may describe;
- try to invent a completely new framework (different from quantum field theory and second quantization) to describe multiparticle states and interactions between the particles, among other things, that may lead to the same kind of physics.