Definition
The four fundamental forces are the four basic interactions known to physics: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. Each acts between specific kinds of matter, has a characteristic strength and range, and (in the quantum picture) is mediated by exchange of a force-carrying particle.
The long-running ambition of theoretical physics is to show that all four are facets of a single underlying interaction at sufficiently high energies.
Why it matters
How it works
Each force has a strength one can quote at, say, the proton scale: the strong force is about 1, electromagnetism about 10⁻², the weak force about 10⁻⁵, and gravity around 10⁻³⁸. Strength is one feature; range is another. Photons and gravitons are massless, so electromagnetism and gravity reach across the universe. The W, Z, and gluons are massive or confined, so the weak and strong forces operate only inside nuclei.
In quantum field theory, every force arises from a gauge symmetry. Electromagnetism comes from a U(1) symmetry, the weak force from SU(2), and the strong force from SU(3). The Standard Model packages all three into the gauge group U(1)×SU(2)×SU(3) — already a partial unification, since electromagnetism and the weak force share a single SU(2)×U(1) electroweak structure that splits below the Higgs scale.
Gravity is the holdout. General relativity describes it as curvature of spacetime, not as a force exchanged by particles. A quantum theory of gravity — the long-sought unified field theory — would need to fit gravity into the same framework as the other three. String theory and loop quantum gravity are the most-studied candidates.
Hawking's A Brief History of Time spends much of its middle topics on this picture: how a single hot interaction split into four as the early universe cooled, and how the project of running the film backward (toward unification) defines much of modern physics.