Interactions §
Interactions are the fundamental forces through which matter and energy influence each other. This category is relevant whenever a force, field, or exchange particle mediates a physical effect: gravitational attraction, electromagnetic coupling, nuclear binding, or radioactive decay. It provides the classical (field-theoretic) description of each force; the quantum description lives in the Standard model.
Brief history §
- 1687: Newton’s law of universal gravitation unifies terrestrial and celestial attraction.
- 1865: Maxwell’s equations unify electricity, magnetism, and light.
- 1915: Einstein’s general relativity recasts gravity as spacetime curvature.
- 1960s–1970s: the electroweak theory and quantum chromodynamics complete the picture of the four fundamental interactions.
Categorization §
The four fundamental interactions are distinct in their source, range, and coupling strength. Each is described by a field (or geometry) and a mediator (in the quantum picture).
| Interaction | Source / charge | Mediator (quantum) | Range | Core idea | Page |
|---|---|---|---|---|---|
| Gravity | Mass-energy | Graviton (hypothetical) | Infinite | Spacetime curvature sourced by mass-energy; universally attractive, infinite range | Gravity |
| Electromagnetism | Electric charge | Photon | Infinite | Lorentz force on charge via E and B fields; attractive or repulsive, infinite range | Electromagnetism |
| Strong force | Color charge | Gluons | ~10⁻¹⁵ m (confinement) | Color-charge binding via gluon exchange; confines quarks into hadrons | — |
| Weak force | Flavor, weak isospin | W⁺, W⁻, Z⁰ bosons | ~10⁻¹⁸ m | Flavor change via massive W/Z exchange; mediates beta decay and neutrino interactions | — |
Details
Gravity is unique: in general relativity it is not a force in a fixed background but the curvature of spacetime itself. The other three interactions are described by gauge field theories with local symmetry groups (U(1) for electromagnetism, SU(3) for the strong force, SU(2)×U(1) for the electroweak theory). In the classical limit, the quantum field description reduces to continuous fields obeying differential equations (Maxwell’s equations, Einstein field equations, Yang-Mills equations).
Relativity is not a separate interaction but the kinematical framework within which all interactions are formulated: special relativity (Lorentz invariance) constrains the form of every field equation, and general relativity provides the dynamical spacetime geometry for gravity.
Authoritative sources