| Force is the engine of change. A shot from a hockey player sends a puck speeding off toward the net; friction between the puck and the ice slows it down. Perhaps surprisingly, all the myriad types of change known to occur in the universe can be explained in terms of just four fundamental forces. The first force physicists came to understand was the first one we all become aware of: gravity. Gravity pulls anything with mass or energy toward other objects with mass or energy. This is because, according to Albert Einstein's general theory of relativity, mass and energy distort space and time; any object will appear to follow a curved path as it travels along the distortions. The second force physicists demystified was electromagnetism, which acts on objects with positive or negative electric charge, such as protons and electrons (or anything with an unbalanced number of those particles). In addition to being responsible for staticky hair and fridge magnets, electromagnetism does most of the work of holding matter together. It keeps electrons in orbit around the atomic nucleus, gets atoms to clump into molecules, and sticks molecules together to make tables, chairs and humans. All electromagnetic attraction and repulsion can be described in terms of the exchange of photons, the massless particles that make up light. The two remaining forces are less familiar, because they are active only inside the heart of the atom. One is the strong force, which binds together the fundamental particles inside the nucleus — known as quarks. The strong force acts on objects that have a type of charge poetically dubbed "color," so it pulls quarks together but not larger, "color-neutral" structures like atoms, molecules or humans. The strong force is so strong that quarks can never escape from the nucleus to float freely though space, so unbalanced color charge — and the strong force itself — stays trapped inside the nucleus. The final force — as far as we know — is called the weak force. Its main effect is to change one type of particle (a quark, for instance) into another. This sort of transformation underlies radioactive events such as beta decay, in which a mutating quark inside a neutron causes that neutron to switch into a proton, emitting an electron and a neutrino along the way. A variation on this process makes it possible for the sun to fuse protons and shine. The weak force stays inside the nucleus because it's carried by particles — the W and Z bosons — that have large masses, limiting how far they can travel. What's New and Noteworthy The strong force, the weak force and electromagnetism, together with the particles they act on, form the "Standard Model" of particle physics, a theory of the quantum world developed in the 1970s that has been validated by countless experiments. Quanta has illustrated the Standard Model here and here. For decades, physicists have suspected that the Standard Model forces might all be shadows of the same über-force — one that reveals itself when particles collide with sufficient violence. Researchers already know that in extremely energetic collisions, the weak-force bosons lose their mass and act indistinguishably from photons, and the weak force merges with the electromagnetic force to form the "electroweak" interaction. At even higher energies, does the strong force join in? Many physicists once believed that it should, since at a certain very high energy, all three forces are expected to have a similar strength (the strong force weakens at high energies while the electroweak force intensifies). But experiments searching for the decay of protons, which would be a side effect of this "grand unification" of the forces, have so far come up short. The weak force also fascinates physicists because it is the only force that exhibits "chirality," treating particles that spin one way differently from those that spin the opposite way. This special treatment could explain why every species on Earth uses DNA that spirals in the same direction. (Subtle magnetic effects offer another potential explanation.) And even though physicists know the equation that governs the strong force, it can't be solved mathematically in everyday circumstances. So physicists must largely rely on experiments to peer inside the proton. Finally, there's gravity, the white whale of modern physics. Although general relativity works well in almost all situations, physicists ultimately hope to recast gravity in terms of the exchange of quantum particles. Gravity has features that make it completely unlike the other forces and therefore hard to fit into the same particle language. But intriguingly, certain gravitational calculations equal the square of certain particle calculations — a mysterious connection between gravity and the other forces that physicists are still trying to understand. | 
