Any particle can be exchanged in some context or another. So every kind of particle in one way or another produces a force.

In molecular physics, it’s the exchange of electrons which creates force [in covalent bond]. In electrodynamics, it’s the exchange of photons back and forth which creates force.

So [every] particle is connected with a force when that particle can be exchanged or jump back and forth between two something else.

There are people say that there are [only] four forces of nature.

No.

There is a force of every possible kind of particle.

— Lecture 1 | New Revolutions in Particle Physics: Standard Model 

— Leonard Susskind

2013.05.13 Monday ACHK