This article[1] by David Mermin does a good job explaining why the second particle couldn't have always been a ~k particle. [1] http://www.phy.duke.edu/undergr

This article[1] by David Mermin does a good job explaining why the second particle couldn't have always been a ~k particle. [1] http://www.phy.duke.edu/undergraduate/physics-articles/mermin-is-the-moon-there-when-nobody-looks-physics-today-1985.pdf http://www.phy.duke.edu/undergraduate/physics-articles/mermi...