# The Strong Interaction

There must be some kind of force that glues together the quarks in the proton, just like the electromagnetic force attaches the electron to the atomic nucleus. And like for electromagnetism, there must be messenger particle of this force, and also a kind of “charge”, called color, that will determine which particles take part in this interaction and whether two particles will attract or repel each other. This force is very different from electromagnetism: no particle that has a color can exist freely, they must be in a bound system. The messenger particles of this interaction are called gluons, and they themselves carry color, so they cannot exist freely either.

Looking at the proton, we see that there are three quarks glued together - unlike in the hydrogen atom, where there are only two partners: the proton and the electron. Hence, something must be different about this strong, nuclear force, that glues together the quarks. There are three different kinds of "charge", as opposed to electromagnetism, where there are only two, which we call positive and negative. For the strong force, the "charge" is called color.