the four fundamental forces
Everything that ever happens — a ball falling, a magnet sticking, a star shining, a radioactive atom decaying — is, at bottom, particles affecting one another. Remarkably, all of that boundless variety of pushes, pulls, and transformations boils down to just four basic ways particles can influence each other. These are called the four fundamental forces (or interactions), and together they account for every event in the known universe.
The four are gravity, electromagnetism, the strong force, and the weak force. Gravity attracts everything with mass or energy and shapes planets, stars, and galaxies. Electromagnetism acts between electric charges and runs all of chemistry, light, and electronics. The strong force binds quarks into protons and neutrons and holds the atomic nucleus together. The weak force is the odd one out: it does not mainly hold things together but transforms particles, changing one type into another, and it is responsible for certain kinds of radioactive decay and for the reactions that power the Sun. Three of these four — all but gravity — are described by the Standard Model in terms of force-carrier particles being exchanged.
This four-way classification matters because it organizes all of physics: name a phenomenon, and physicists can tell you which force or forces are behind it. It also frames the field's grandest ambitions and its biggest gaps. Electromagnetism and the weak force have already been shown to be two faces of a single electroweak force, fueling the dream of unifying all four. But gravity stubbornly resists: we have no working quantum theory of it, so gravity sits outside the Standard Model. Whether the four are truly fundamental, or facets of something deeper, is one of the great open questions.
Inside the Sun, all four forces play a role: gravity squeezes the core, the strong force binds nuclei, electromagnetism lets the released energy radiate as light, and the weak force enables the key reaction that turns hydrogen into helium. The Sun shines because all four act together.
All four fundamental forces work together to make the Sun shine.
Gravity is a fundamental force but sits outside the Standard Model: there is no confirmed quantum theory of gravity, so it is not yet described in the same particle-exchange language as the other three.