A post for general readers:
This is the first of several posts celebrating the hugely successful Standard Model of particle physics, the concepts and equations that describe the basic bricks and mortar of the universe. In these posts, I’ll explain (without assuming readers have a science background) how we know some of its most striking features. We’ll look at simple facts that particle physicists have learned over the decades, and use them to infer basic features of the universe and to recognize deep questions that still trouble the experts.
The Elementary “Forces” of Physics: A Classification of Nature
Perhaps you’ve heard it said that “There are four fundamental forces in nature.” Whether you have or not, today I’ll show you how to verify this yourself. (Actually, there are five forces, though we’ll only see a hint of the fifth today.) The force everybody knows from daily life is gravity; ironically, this force has no measurable impact on particle physics, so it’s the only one we won’t be looking at in this post.
I’d better emphasize, though, that the word “force” is slippery. Normally, in everyday life, a force means something that will push or pull objects around. But when physicists say “force,” they often mean something much more general. Because of that they sometimes use the word “interaction” instead of “force”.
For example, static electricity that holds socks together when they come out of the dryer is an example of an honest electromagnetic force — the socks really are pulled together. So is the force that pulls a magnet to a refrigerator door. But when a light bulb glows, that doesn’t involve a force in the limited sense of a push or pull. Yet it still involves the “electromagnetic interaction”, i.e. the “electromagnetic force” in a generalized sense. That’s because, although it is far from obvious, the emission (or absorption) of light involves the same basic phenomena that govern the force between the socks.
[Physicists use “electromagnetic” rather than “electric” or “magnetic” separately because these two forces are so deeply intertwined that it is often impossible to distinguish them.]
So when physicists say there are “four forces” (or five), they are imposing a classification scheme on the world around us. They mean:
- All fundamental physical processes in nature can be divided up into five classes.
- Each class involves one of the following types of interactions:
- gravitational (holds the planet together and holds us to the ground),
- electromagnetic (creates light, controls chemistry and biology, and dominates daily life),
- weak-nuclear (essential in stars and in supernova explosions),
- strong-nuclear (forms protons, neutrons, and their agglomerations in atomic nuclei),
- Higgs-related (associated with the masses of all known elementary particles).
There are currently no verified exceptions to this classification scheme. And by examining basic facts about the various particles found in nature, we can see these classes (other than gravity) in operation.