## Definition The **four fundamental forces** are the four distinct interactions through which all matter and energy in the universe influence one another: the **electromagnetic force**, the **strong nuclear force**, the **weak nuclear force**, and **gravity**. Every physical process — from the binding of atoms to the collapse of stars — is ultimately the result of one or more of these interactions. Modern physics describes the first three as quantum fields whose disturbances are carried by specific force-mediating particles (gauge bosons); gravity is described geometrically by [[General Relativity]] and has not yet been successfully quantised. ## The Four Forces in Detail ### Electromagnetic Force The electromagnetic force acts between all electrically charged particles. It is responsible for holding electrons in orbit around atomic nuclei, for the chemical bonds that form molecules, for light, radio waves, and X-rays, and for the contact forces experienced in everyday life (the reason a hand cannot pass through a table is electromagnetic repulsion between electron clouds). Its mediating particle is the **photon** (see [[Photon]]). The electromagnetic force has infinite range and decreases as the inverse square of distance. ### Strong Nuclear Force The strong force is the most powerful of the four. It binds **quarks** together inside protons and neutrons via the exchange of **gluons**. A residual version of the same force (mediated by **mesons**) holds protons and neutrons together inside atomic nuclei despite the electrostatic repulsion between the positively charged protons. The strong force acts only over extremely short distances (on the order of $10^{-15}$ m, the size of a nucleus); beyond that range it falls to zero. Without it, no nucleus heavier than hydrogen could exist, and therefore no complex chemistry. ### Weak Nuclear Force The weak force is responsible for **radioactive decay** — the transmutation of one type of quark into another, which can convert a neutron into a proton (or vice versa) while emitting other particles. Its mediating particles are the heavy **W and Z bosons**. The weak force also acts only at extremely short range ($\sim 10^{-18}$ m). It is far less powerful than the electromagnetic or strong forces at nuclear distances, hence its name, but it plays an indispensable role: without it, certain fusion reactions in the Sun could not proceed, and the Sun would not shine. ### Gravity Gravity is the weakest of the four forces by an enormous margin — roughly $10^{36}$ times weaker than the electromagnetic force at the scale of a proton. Yet it dominates at cosmic scales because it is always attractive (never repulsive) and has infinite range, allowing it to accumulate over astronomical distances and masses. General Relativity describes gravity not as a force but as the curvature of spacetime produced by mass-energy; free-falling bodies follow the straightest possible paths (geodesics) in this curved geometry. A hypothetical quantum of the gravitational field — the **graviton** — has not been detected. ## Relative Strengths (Approximate) | Force | Relative strength | Range | Mediator(s) | |---|---|---|---| | Strong | $1$ | $\sim 10^{-15}$ m | gluons | | Electromagnetic | $\sim 10^{-2}$ | infinite | photon | | Weak | $\sim 10^{-6}$ | $\sim 10^{-18}$ m | W, Z bosons | | Gravity | $\sim 10^{-38}$ | infinite | graviton (hypothetical) | ## Unification: From Four Toward One A central goal of physics since Einstein has been to show that the four forces are different manifestations of a single underlying interaction. The first partial success came with Maxwell's unification of electricity and magnetism into electromagnetism in the 1860s. The next milestone was the **electroweak unification**: in 1979, Abdus Salam, Sheldon Lee Glashow, and Steven Weinberg won the Nobel Prize for showing that electromagnetism and the weak force are two aspects of a single *electroweak* field that existed as a unified interaction in the hot early universe. The unification was confirmed experimentally by the detection of the W and Z bosons at CERN. Unifying the electroweak force with the strong force (Grand Unified Theory) remains an open research programme. Including gravity — achieving a single unified description of all four forces — is the deepest unsolved problem in theoretical physics. See [[The Problem of Quantum Gravity]] and [[String Theory]]. ## Related - [[Standard Model of Particle Physics]] - [[General Relativity]] - [[Photon]] - [[The Problem of Quantum Gravity]] - [[String Theory]] - [[Black Holes and Hawking Radiation]] ## Sources - [[The Universe in Your Hand (Galfard 2015)]]