Now we think we have a good idea of what the world is made of: quarks and leptons. So...

What holds it together?

The universe, which we know and love, exists because the fundamental particles interact. These interactions include attractive and repulsive forces, decay, and annihilation.

There are four fundamental interactions between particles, and all forces in the world can be attributed to these four interactions!

That's right: Any force you can think of -- friction, magnetism, gravity, and so on -- is caused by one of these four fundamental interactions.

What's the difference between a force and an interaction?

This is a hard distinction to make. Strictly speaking, a force is the effect on a particle due to the presence of other particles. The interactions of a particle include all the forces that affect it, but also include decays and annihilations that the particle might go through. (We will spend the next chapter discussing these decays and annihilations in more depth.)

The reason this gets confusing is that most people, even most physicists, usually use "force" and "interaction" interchangeably, although "interaction" is more correct. For instance, we call the particles which carry the interactions force carrier particles. You will usually be okay using the terms interchangeably, but you should know that they are different.