Fifth force

A hypothetical fundamental interaction beyond gravity, electromagnetism, and the strong and weak nuclear forces. Experiments search for it through deviations from established force laws or new particle effects.

Yukawa potential: A potential describing interactions whose strength falls exponentially with distance, often used to model massive-particle exchange. It parameterizes how a short-range fifth force could alter gravity.
Eöt-Wash experiments: University of Washington torsion-balance experiments testing gravity and the equivalence principle. Their measurements tightly constrain composition-dependent forces at short distances.
Irving Langmuir: An American chemist and physicist known for contributions to surface chemistry and the coining of the term 'pathological science.' He used the phrase 'fifth force' in discussing claims that later motivated renewed scrutiny.
General relativity: Albert Einstein's theory describing gravity as the curvature of spacetime caused by energy and momentum. A gravitational anomaly may indicate either new matter-coupled forces or a change to relativistic gravity.
Scalar field: A field assigning a single value to every point in spacetime. Many fifth-force models use a scalar field that couples to matter.
MICROSCOPE satellite: A French space mission that tested the universality of free fall using test masses of different composition. Its high-precision result limits long-range forces that couple differently to materials.
Fifth force (1986 proposal): A 1986 proposal of a new force prompted by reports of anomalous nuclear scattering measurements. This episode gave the modern fifth-force search its name and early impetus.
Dark matter: Matter inferred from gravitational effects that does not emit or absorb detectable light. Unexplained motions attributed to dark matter are sometimes investigated as possible modified-force effects.
Chameleon mechanism: A screening mechanism in which a scalar field's effective mass depends on its environment. It can hide a fifth force in dense environments while allowing effects in space.
Lunar Laser Ranging: A technique that measures the Earth–Moon distance by timing laser pulses reflected from lunar retroreflectors. Earth–Moon motion constrains anomalous forces acting over astronomical distances.

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