2 (two) is a number, numeral and digit. It is the natural number following 1 and preceding 3. It is the smallest and the only even prime number.
Because it forms the basis of a duality, it has religious and spiritual significance in many cultures.
In this thread Ifqm has asked what was the Lagrangian of an electron and a proton interacting together. He proposed a Lagrangian containing electron-proton interaction and the tiny QED field correction (electromagnetic tensor part). His first Lagrangian seems intuitive as a sum of 2 Dirac...