>>The "strong" force holds protons together - at a short distance, this force is stronger than the repulsive electrostatic force.
>>
>>At longer distances, it seems that this force has no influence.
>
>Oh? How so? Are you talking about magnetism?

No; if I understand correctly, in this model the electric and magnetic forces are considered together, as one single force.

When I say "at longer distances", I mean longer distances compared to the diameter of an atom. Both the electric/magnetic forces, and gravitation, do have the capacity to act over large distances.

"They don't know..." Perhaps the ultimate cause is unknown, but the way these forces themselves manifest themselves are known quite well.

The strong force, I remind you, is precisely the force that binds atoms together. It acts between protons and neutrons (that's the reason that for larger atoms, more neutrons than protons are required: otherwise, the electrostatic repulsion would blow the nucleus apart).

One more detail: it is generally believed that all of these 4 forces are transmitted through particles. The particle that transmits gravitation ("graviton") has yet to be found - it is believed to be difficult to find, because of the extreme (relative) weakness of gravitation, as compared to the other forces.

Hilmar.