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Fifth force
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In physics, a fifth force is a hypothetical fundamental interaction (also known as fundamental force) beyond the four known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories. The specific characteristics of a putative fifth force depend on which hypothesis is being advanced. No evidence to support these models has been found.

The term is also used as "the Fifth force" when referring to a specific theory advanced by Ephraim Fischbach in 1971 to explain experimental deviations in the theory of gravity. Later analysis failed to reproduce those deviations.

History

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The term fifth force originates in a 1986 paper by Ephraim Fischbach et al. who reanalyzed the data from the Eötvös experiment of Loránd Eötvös from earlier in the century; the reanalysis found a distance dependence to gravity that deviates from the inverse square law.[1][2]: 57  The reanalysis was sparked by theoretical work in 1971 by Fujii [3][4]: 3  proposing a model that changes distance dependence with a Yukawa potential-like term: The parameter characterizes the strength and the range of the interaction.[2] Fischbach's paper found a strength around 1% of gravity and a range of a few hundred meters.[5]: 26  The effect of this potential can be described equivalently as exchange of vector and/or scalar bosons, that is a predicting as yet undetected new particles.[2] However, many subsequent attempts to reproduce the deviations have failed.[6]

Theory

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Theoretical proposals for a fifth force are driven by inconsistencies between the existing models of general relativity and quantum field theory, and also between the hierarchy problem and the cosmological constant problem. Both issues suggest the possibility of corrections to the gravitational potential around 100 μm.[2]: 58 

The accelerating expansion of the universe has been attributed to a form of energy called dark energy. Some physicists speculate that a form of dark energy called quintessence could be a fifth force.[7][8][9]

Experimental approaches

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There are at least three kinds of searches that can be undertaken, which depend on the kind of force being considered, and its range.

Equivalence principle

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One way to search for a fifth force is with tests of the strong equivalence principle, one of the most powerful tests of general relativity, also known as Einstein's theory of gravity. Alternative theories of gravity, such as Brans–Dicke theory, postulate a fifth force — possibly one with infinite range. This is because gravitational interactions, in theories other than general relativity, have degrees of freedom other than the "metric", which dictates the curvature of space, and different kinds of degrees of freedom produce different effects. For example, a scalar field cannot produce the bending of light rays.

The fifth force would manifest itself in an effect on solar system orbits, called the Nordtvedt effect. This is tested with Lunar Laser Ranging experiment[10] and very-long-baseline interferometry.

Extra dimensions

[edit]

Another kind of fifth force, which arises in Kaluza–Klein theory, where the universe has extra dimensions, or in supergravity or string theory is the Yukawa force, which is transmitted by a light scalar field (i.e. a scalar field with a long Compton wavelength, which determines the range). This has prompted much recent interest, as a theory of supersymmetric large extra dimensions — dimensions with size slightly less than a millimeter — has prompted an experimental effort to test gravity on very small scales. This requires extremely sensitive experiments which search for a deviation from the inverse-square law of gravity over a range of distances.[11] Essentially, they are looking for signs that the Yukawa interaction is engaging at a certain length.

Australian researchers, attempting to measure the gravitational constant deep in a mine shaft, found a discrepancy between the predicted and measured value, with the measured value being two percent too small. They concluded that the results may be explained by a repulsive fifth force with a range from a few centimetres to a kilometre. Similar experiments have been carried out on board a submarine, USS Dolphin (AGSS-555), while deeply submerged. A further experiment measuring the gravitational constant in a deep borehole in the Greenland ice sheet found discrepancies of a few percent, but it was not possible to eliminate a geological source for the observed signal.[12][13]

Earth's mantle

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Another experiment uses the Earth's mantle as a giant particle detector, focusing on geoelectrons.[14]

Cepheid variables

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Jain et al. (2012)[15] examined existing data on the rate of pulsation of over a thousand cepheid variable stars in 25 galaxies. Theory suggests that the rate of cepheid pulsation in galaxies screened from a hypothetical fifth force by neighbouring clusters, would follow a different pattern from cepheids that are not screened. They were unable to find any variation from Einstein's theory of gravity.

Other approaches

[edit]

Some experiments used a lake plus a tower that is 320 meters high.[16] A comprehensive review by Ephraim Fischbach and Carrick Talmadge suggested there is no compelling evidence for the fifth force,[17] though scientists still search for it. The Fischbach–Talmadge article was written in 1992, and since then, other evidence has come to light that may indicate a fifth force.[18]

The above experiments search for a fifth force that is, like gravity, independent of the composition of an object, so all objects experience the force in proportion to their masses. Forces that depend on the composition of an object can be very sensitively tested by torsion balance experiments of a type invented by Loránd Eötvös. Such forces may depend, for example, on the ratio of protons to neutrons in an atomic nucleus, nuclear spin,[19] or the relative amount of different kinds of binding energy in a nucleus (see the semi-empirical mass formula). Searches have been done from very short ranges, to municipal scales, to the scale of the Earth, the Sun, and dark matter at the center of the galaxy.

Claims of new particles

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Main article: X17 particle

In 2015, Attila Krasznahorkay at ATOMKI, the Hungarian Academy of Sciences's Institute for Nuclear Research in Debrecen, Hungary, and his colleagues posited the existence of a new, light boson only 34 times heavier than the electron (17 MeV).[20] In an effort to find a dark photon, the Hungarian team fired protons at thin targets of lithium-7, which created unstable beryllium-8 nuclei that then decayed and ejected pairs of electrons and positrons. Excess decays were observed at an opening angle of 140° between the e+ and e−, and a combined energy of 17 MeV, which indicated that a small fraction of beryllium-8 will shed excess energy in the form of a new particle.

In November 2019, Krasznahorkay announced that he and his team at ATOMKI had successfully observed the same anomalies in the decay of stable helium atoms as had been observed in beryllium-8, strengthening the case for the X17 particle's existence.[21]

Feng et al. (2016)[22] proposed that a protophobic (i.e. "proton-ignoring") X-boson with a mass of 16.7 MeV with suppressed couplings to protons relative to neutrons and electrons and femtometer range could explain the data.[23] The force may explain the muon g − 2 anomaly and provide a dark matter candidate. Several research experiments are underway to attempt to validate or refute these results.[20][22]

See also

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References

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Grokipedia

Fifth force

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In physics, the fifth force is a hypothetical fundamental interaction proposed to exist beyond the four known forces of nature: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.[1] This concept suggests an additional force that could mediate interactions between particles, potentially explaining observed anomalies in experiments that deviate from predictions of the Standard Model of particle physics.[2] The hypothesis of a fifth force originated in the 1980s, when physicist Ephraim Fischbach reanalyzed historical torsion balance experiments by Loránd Eötvös and proposed that subtle deviations from Newton's law of universal gravitation might indicate a composition-dependent gravitational effect, possibly linked to a new force proportional to an object's baryon number.[3] Early enthusiasm for this idea waned as subsequent precision measurements, including those testing the weak equivalence principle, failed to confirm the anomalies, leading to stringent constraints on such a force's strength and range by the 1990s.[3] However, the concept has persisted and evolved, with modern proposals tying it to extensions of the Standard Model, such as the existence of new light bosons or mediators that could couple to electrons, neutrons, or dark matter particles.[4] Contemporary searches for a fifth force span multiple experimental domains, driven by discrepancies in particle behavior and cosmological observations. Initial results from Fermilab's Muon g-2 experiment in 2023 showed a deviation from Standard Model predictions at approximately 4.2 standard deviations, hinting at possible new physics, but the final results released in June 2025, based on the complete dataset, agree with theoretical predictions and do not support evidence for an additional force.[1] [5] Similarly, atomic physics experiments using calcium isotopes have placed new upper limits on a fifth force's strength, analyzing optical transition frequencies via King plots to detect nonlinear deviations that could signal a Yukawa-type interaction with mediator masses between 10 eV/c² and 10⁷ eV/c²; these June 2025 measurements constrain the force to be weaker than previously allowed, though small anomalies persist.[6] In cosmology, analyses of galaxy motions in gravitational wells, published in November 2025, indicate that dark matter follows standard gravity without evidence of an additional force stronger than 7% of gravitational strength, further limiting scenarios where a fifth force might dominate on large scales.[7] If confirmed, a fifth force could revolutionize our understanding of the universe, potentially bridging gaps in the Standard Model, explaining dark matter's nature, or revealing new physics at high energies.[2] Ongoing efforts, including investigations at the Large Hadron Collider (LHC) and data from the Vera C. Rubin Observatory (formerly LSST), aim to either detect or definitively rule out such a force, with sensitivities projected to probe effects as weak as 2% of gravity.[1][7]

Background

The Four Fundamental Forces

In physics, the four fundamental forces—or interactions—govern all observed phenomena, from the structure of atoms to the dynamics of the cosmos. These are gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Each force has distinct properties, including relative strength, range, mediating particles, and roles in nature, as summarized in the Standard Model of particle physics for the nongravitational forces and general relativity for gravity.[8][9]
ForceRelative Strength (to strong force)RangeMediating Particle(s)Key Role(s)
Strong nuclear1~10^{-15} mGluons (8 massless)Binds quarks into protons/neutrons; holds atomic nuclei together via residual effects.[8]
Electromagnetic~10^{-2}InfinitePhoton (massless)Governs interactions between charged particles; responsible for electricity, magnetism, light, and chemical bonds.[8]
Weak nuclear~10^{-6}~10^{-18} mW^{+}, W^{-}, Z^{0} bosons (massive, ~80-91 GeV)Mediates radioactive beta decay, neutrino absorption, and flavor changes in quarks/leptons; enables stellar fusion processes.[8]
Gravity~10^{-38}InfiniteGraviton (hypothesized, massless)Attracts masses over cosmic scales; shapes planetary orbits, galaxy formation, and spacetime curvature.[9]
The strong nuclear force, described by quantum chromodynamics (QCD) within the SU(3)_c gauge group, operates at the shortest distances among the forces and is the strongest, with a running coupling constant α_s(m_Z) ≈ 0.118. It acts on particles carrying "color charge" (quarks and gluons), confining quarks into hadrons like protons and neutrons through gluon exchange, which also exhibit self-interaction leading to asymptotic freedom at high energies. This force's residual effects bind nucleons in atomic nuclei, overpowering electromagnetic repulsion.[8] Electromagnetism, unified from electricity and magnetism in the 19th century and quantized in quantum electrodynamics (QED), is mediated by the U(1) gauge group with fine-structure constant α ≈ 1/137. It affects all charged particles, producing both attractive and repulsive effects, and dominates everyday phenomena like friction and light propagation. At low energies, it appears distinct from the weak force, but the two are unified in the electroweak theory under SU(2)_L × U(1)_Y, proposed by Glashow, Weinberg, and Salam in the 1960s, where symmetry breaking via the Higgs mechanism generates massive weak bosons while keeping the photon massless. This unification was experimentally confirmed with the discovery of W and Z bosons in 1983.[8] The weak nuclear force, with Fermi coupling constant G_F ≈ 1.166 × 10^{-5} GeV^{-2}, is responsible for processes violating parity and enabling neutrino interactions, such as those powering the Sun's fusion by converting protons to neutrons. Its short range stems from the large masses of its mediators, the charged W bosons and neutral Z boson.[8] Gravity, the universal attractive force between all matter and energy, is infinitely ranged but extraordinarily weak, making it negligible at subatomic scales yet dominant on astronomical ones. Unlike the quantum-described others, it is classically formulated in Einstein's 1915 general relativity as the geometry of spacetime warped by mass-energy, explaining phenomena like black holes and gravitational waves. Efforts to unify gravity with the quantum forces remain ongoing, but it stands apart as the only nonquantized fundamental interaction in current theory.[9]

Concept and Motivations for a Fifth Force

In physics, the fifth force refers to a hypothetical fundamental interaction beyond the four established forces—gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—that could account for observed discrepancies in gravitational behavior at scales ranging from laboratory experiments to cosmological structures. This proposed force is typically mediated by new, undiscovered particles, such as light scalar or vector bosons from hidden sectors beyond the Standard Model, which would introduce additional attractive or repulsive interactions between ordinary matter particles or between ordinary and dark matter.[10] Several theoretical and observational motivations drive the search for a fifth force. In astrophysics, discrepancies in galaxy rotation curves, where stars orbit at unexpectedly high speeds without sufficient visible mass to explain the dynamics under Newtonian gravity, have prompted proposals for modified dynamics like MOND, which can be interpreted through an effective fifth force that alters gravitational attraction at low accelerations. Similarly, the nature of dark matter, inferred from its gravitational effects on galactic scales but otherwise undetected, could involve a fifth force mediating self-interactions among dark matter particles or couplings to ordinary matter, potentially resolving tensions in structure formation models. In particle physics, the hierarchy problem—the unnatural disparity between the electroweak scale (around 100 GeV) and the Planck scale (10^{19} GeV), which leads to fine-tuning issues in the Higgs mass—motivates light scalar mediators that could generate a fifth force to stabilize the Higgs vacuum through radiative corrections. Additionally, grand unified theories aiming to merge the non-gravitational forces at high energies often predict extra interactions that manifest as a fifth force at lower scales.[11][12][13][14] Fifth forces can vary in character: they may be attractive (like gravity) or repulsive, depending on the sign of the coupling constant, and exhibit short-range (screened at small distances) or long-range behavior, dictated by the mass of the mediating particle. The force might couple universally to all matter (baryons and leptons alike), preferentially to specific sectors like quarks or electrons, or exclusively to dark matter, influencing phenomena from equivalence principle tests to cosmic microwave background patterns. A generic parametrization of the fifth force potential between two masses m1m_1 and m2m_2 separated by distance rr takes the Yukawa form:
V(r)=−Gm1m2r(1+αe−μr), V(r) = -\frac{G m_1 m_2}{r} \left(1 + \alpha e^{-\mu r}\right),
where GG is the gravitational constant, α\alpha represents the relative strength of the fifth force compared to gravity (often constrained to ∣α∣≲10−3|\alpha| \lesssim 10^{-3}), and μ=mc/ℏ\mu = m c / \hbar is the inverse range parameter, with mm the mediator mass; for μ→0\mu \to 0, the force becomes long-range like gravity.[15]

Historical Development

Early Theoretical Proposals

The concept of a fifth force began to take shape in the mid-20th century through efforts to reconcile general relativity with quantum mechanics and Mach's principle, leading to modifications of gravity that implied additional interactions beyond the four known fundamental forces. A seminal early proposal came from the Brans-Dicke theory in 1961, which augments Einstein's general relativity with a scalar field φ that couples universally to matter, effectively introducing a long-range attractive force proportional to the gravitational interaction but screened over cosmological distances. This scalar-mediated effect arises from the theory's action, where the gravitational constant is replaced by 1/φ, allowing gravity to vary dynamically in response to the cosmic mass distribution. Motivated by philosophical considerations like Mach's principle, the theory predicts observable deviations from Newtonian gravity in weak-field regimes, such as slight violations of the equivalence principle. In the 1970s, the development of grand unified theories (GUTs) provided another avenue for fifth-force ideas, as these models unified the strong, weak, and electromagnetic interactions under a single gauge group, such as SU(5). Within GUTs, hypercharge—a conserved quantum number associated with the U(1)_Y subgroup—could couple to ordinary matter and mediate a new force if lightweight bosons or scalars carry hypercharge, potentially manifesting as a composition-dependent interaction at macroscopic scales. For instance, the original SU(5) model highlighted how hypercharge asymmetries in fermions might lead to long-range forces coupling differently to protons and neutrons, driven by the quest to explain baryon number conservation and unification at high energies around 10^15 GeV. Parallel to GUT developments, extensions of the Yukawa potential—originally formulated in 1935 for the strong nuclear force—were applied to gravity to probe short-range deviations from the inverse-square law. Yasunori Fujii proposed in 1971 a modified gravitational potential of the form V(r) = -G m_1 m_2 / r (1 + α exp(-r/λ)), where α parameterizes the strength of the additional Yukawa term and λ its range, motivated by theoretical explorations of compositeness in strong interactions and potential quantum corrections to gravity. This framework suggested testable predictions for laboratory experiments, such as enhanced attraction between masses at distances of 10 to 1000 meters, without relying on experimental anomalies. During the late 1970s, Ephraim Fischbach advanced the notion of composition-dependent gravity as a potential fifth force, positing that gravitational acceleration might vary with the baryon-to-mass ratio (B/μ) of materials, inspired by subtle discrepancies in atomic spectroscopy data hinting at non-universal couplings. This idea framed the fifth force as a hypercharge-like interaction, building on GUT concepts, and laid groundwork for reinterpreting classical tests of gravity. Although formalized in subsequent analyses, these 1970s conceptual seeds emphasized theoretical motivations from particle physics unification rather than direct empirical claims.

1980s Searches and Initial Claims

In the mid-1980s, experimental interest in a possible fifth force surged following theoretical motivations for composition-dependent deviations from Newtonian gravity. A pivotal development came in 1986 when Ephraim Fischbach and colleagues reanalyzed data from the early 20th-century Eötvös experiment, which had compared the gravitational accelerations of various materials toward the Earth. Their analysis revealed a subtle pattern suggesting a 0.7% deviation in acceleration between materials like copper and water, correlated with differences in their baryon-to-mass ratios. This implied the existence of a new interaction with coupling strength α ≈ 10^{-3} relative to gravity and a characteristic range of approximately 10^{11} m, potentially mediated by a light particle. The Fischbach claim prompted immediate follow-up experiments to test for such a force. In 1987, Mark A. Zumberge and collaborators conducted a submersible-based measurement of gravitational differences between lead and copper test masses at varying depths and distances from the sea floor, aiming to detect intermediate-range interactions.[16] Their results showed no significant deviation, placing upper limits on the fifth force strength at α < 10^{-3} for ranges around 100 m, though the null result spurred refinements in experimental design. Similarly, in 1988, Eric Adelberger and the Eöt-Wash collaboration used a torsion balance with quartz and aluminum test bodies to probe equivalence principle violations, achieving sensitivities better than 10^{-10} and beginning to refute the proposed deviation while motivating even more precise torsion balance techniques. Other experimental efforts in 1987 hinted at long-range fifth force signatures. Peter Thieberger's experiment at the Palisades cliff reported a tentative 2σ detection of a composition-dependent force using a copper sphere in water, aligning with predictions for forces extending to planetary scales.[3] Additionally, analyses of glacial isostatic rebound data suggested anomalies in Earth's gravitational field recovery that could indicate a long-range force with strength comparable to 10^{-3} G, while planetary motion studies, including lunar laser ranging, provided hints of non-Newtonian effects over astronomical distances around 10^{11} m.[15] These claims, though not definitive, intensified global scrutiny and led to a proliferation of targeted tests throughout the decade.

Theoretical Models

Yukawa-Type Interactions

In phenomenological models of fifth forces, the interaction is often parameterized as an additive Yukawa-type potential to the Newtonian gravitational potential, given by
V5(r)=−αGm1m2re−r/λ, V_5(r) = -\frac{\alpha G m_1 m_2}{r} e^{-r/\lambda},
where $ G $ is the gravitational constant, $ m_1 $ and $ m_2 $ are the masses of the interacting bodies, $ \alpha $ is a dimensionless coupling strength relative to gravity, and $ \lambda $ represents the finite range of the force, corresponding to the Compton wavelength of the mediating particle ($ \lambda = \hbar / (m c) $, with $ m $ the mediator mass).[17] This form captures a short- to intermediate-range deviation from the inverse-square law of gravity, becoming negligible at distances much larger than $ \lambda $ due to the exponential suppression.[17] The Yukawa potential arises from the tree-level exchange of a massive scalar or vector boson in quantum field theory, analogous to the nuclear force mediated by pions but extended to macroscopic scales if the mediator is ultralight.[18] For a scalar mediator coupling proportionally to mass or baryon number, the low-energy effective potential between two sources takes the Yukawa form, with the coupling $ \alpha $ determined by the boson's self-interaction strength and the sources' quantum numbers.[17] Similarly, a massive vector exchange yields a comparable potential, though with possible tensor components that are typically constrained to be small.[18] These models provide a framework for testing deviations from general relativity at sub-millimeter scales, where the fifth force could mimic or enhance gravitational effects without altering long-range behavior.[17] Such Yukawa-type interactions were motivated by early 1980s analyses suggesting anomalies in Eötvös-type experiments, prompting searches for composition-dependent forces parameterized in this manner.[17] At short distances, these models predict measurable perturbations to the inverse-square law, accessible through precision force measurements between test masses. For instance, in the sub-millimeter regime ($ \lambda \sim 10^{-6} $ m to 1 mm), the exponential term allows the fifth force to dominate over Newtonian gravity if $ \alpha $ is sufficiently large, enabling tests via torsion balances or atomic interferometry.[18] Current experimental constraints on these parameters are stringent, particularly from measurements of the Casimir effect and neutron scattering. Casimir force experiments, which compare attractive forces between conducting plates or spheres at nanometer to micrometer separations, have placed stringent upper limits on $ \alpha $, typically below $ 10^{-3} $ to $ 10^{-4} $ for $ \lambda \sim 10^{-6} $ m, by detecting no significant deviations from quantum electrodynamic predictions.[18] Complementarily, neutron scattering experiments, analyzing total cross-sections and phase shifts in neutron-nucleus interactions at low energies, impose similar bounds in the same range, ruling out Yukawa forces strong enough to explain proposed anomalies while remaining sensitive to weaker couplings.[18] These limits highlight the Yukawa model's utility in bounding new physics without direct detection of the mediator.

Extra Dimensions and Modified Gravity

In theories incorporating extra spatial dimensions, such as those arising in string theory and braneworld scenarios, gravity can propagate into these additional dimensions while standard model fields are confined to our familiar 4D spacetime, leading to modifications of the gravitational force observed in 4D. This "leakage" of gravity into the extra dimension results in an effective 4D potential that deviates from the standard Newtonian form at distances comparable to or shorter than the compactification radius $ R $ of the extra dimension. At distances $ r \gg R $, the potential recovers the standard 4D Newtonian $ 1/r $ form. For $ r \ll R $, gravity behaves as in higher dimensions, with the potential $ V(r) \propto 1/r^{1+n} $ (where $ n $ is the number of extra dimensions), corresponding to a force $ F \propto 1/r^{2+n} $, potentially manifesting as a long-range fifth force if $ R $ is sufficiently large.[19] The Arkani-Hamed-Dimopoulos-Dvali (ADD) model proposes large extra dimensions to address the hierarchy problem—the vast disparity between the electroweak scale (~TeV) and the Planck scale (~10^{19} GeV)—by allowing the fundamental Planck scale in higher dimensions to be as low as a few TeV. In this setup, the extra dimensions are compactified on scales of order microns to millimeters, with standard model particles localized on a 3-brane, while gravitons can propagate into the bulk. This leads to a fifth force mediated by Kaluza-Klein gravitons, which becomes significant at sub-millimeter distances, altering the gravitational law to a higher-dimensional form for $ r \ll R $, such as a $ 1/r^{3} $ potential in 5D ($ n=1 $), potentially detectable in precision gravity experiments. The model predicts that quantum gravity effects, including black hole production, could occur at TeV energies in particle colliders if the extra dimensions number two or more.[19] The Randall-Sundrum (RS) model introduces a warped geometry in a single extra dimension, compactified between two branes in anti-de Sitter (AdS_5) spacetime, to generate the hierarchy without requiring large extra dimensions. The visible brane, where standard model fields reside, is separated from the Planck brane by an exponential warp factor, exponentially suppressing the effective 4D Planck scale relative to the 5D fundamental scale, which can be near the TeV range. This warping modifies gravity at scales around the TeV, producing Kaluza-Klein modes of the graviton with masses in the TeV regime and couplings of order the inverse TeV, leading to deviations from general relativity that could appear as a fifth force in high-energy processes or short-range gravity tests. Unlike flat extra dimensions, the RS geometry confines gravity near the Planck brane, enhancing its strength on the visible brane and offering a solution to the hierarchy without fine-tuning.[20]

Supersymmetry and Dark Sector Forces

In supersymmetric extensions of the Standard Model, the incorporation of hidden sectors addresses the hierarchy problem while providing viable dark matter candidates, such as the neutralino, the lightest supersymmetric fermion. These sectors can introduce additional gauge interactions that couple weakly to the visible sector, potentially manifesting as a fifth force through mediators like the neutralino itself via mass mixing with hidden fermions or through a hidden U(1) gauge boson. Kinetic mixing between the Standard Model photon and this hidden gauge boson, with a mixing parameter χ on the order of 10^{-3}, enables feeble interactions between dark and visible matter, allowing the neutralino to contribute to long-range forces while remaining consistent with dark matter relic density requirements.[21] Dark photon models within supersymmetric frameworks extend this by positing an additional U(1)' gauge symmetry in the hidden sector, under which dark matter particles are charged. The associated dark photon acquires a small mass, typically around the meV scale, through mechanisms like the Stueckelberg or Higgs mechanism, and interacts with ordinary matter primarily via kinetic mixing with the photon, characterized by a coupling ε ≈ 10^{-3}. This mixing induces effective millicharges on dark sector particles, on the order of ε times the elementary charge, enabling the dark photon to mediate a fifth force that is long-range at sub-millimeter scales but screened at larger distances due to its mass.[22] Such models have significant implications for cosmology, particularly in alleviating discrepancies in small-scale structure formation observed in cold dark matter simulations. By introducing self-interactions among dark matter particles mediated by the dark photon or similar bosons, these frameworks yield a velocity-dependent cross section per unit mass σ/m ≈ 1 cm²/g, which promotes thermalization in dense regions like dwarf galaxy cores, resolving the cusp-core problem without conflicting with large-scale cosmic microwave background data. Recent extensions beyond supersymmetry include spin-dependent fifth forces mediated by axion-like particles or dark Z' bosons, and leptophilic axial-vector interactions, which could couple preferentially to electrons or neutrinos and explain anomalies in precision measurements as of 2025.[23][24]

Experimental Methods

Equivalence Principle Violations

The weak equivalence principle (WEP), which asserts the universality of free fall such that all bodies accelerate identically in a gravitational field regardless of their internal composition, serves as a key testbed for detecting fifth force effects that might depend on atomic or nuclear structure. Violations of the WEP would manifest as composition-dependent differential accelerations, Δa, between test masses, often parameterized by the Eötvös parameter η = 2|Δa|/a, where a is the nominal gravitational acceleration. Precision measurements using torsion balances and cold atom interferometers probe these potential deviations, focusing on couplings to electrons, protons, or neutrons that could differentiate materials like metals or atomic species. The MICROSCOPE satellite mission provided one of the most stringent space-based tests of the WEP in 2017 by comparing the free-fall accelerations of coaxial cylindrical test masses made of titanium and platinum alloys in Earth's orbit. The experiment achieved a precision of η = [-1 ± 9 (stat) ± 9 (syst)] × 10^{-15} at 1σ, corresponding to Δa/a < 10^{-14}, with no evidence of violation and thus tight constraints on long-range fifth forces independent of composition.[25] Laboratory torsion balance experiments by the Eöt-Wash group have similarly targeted WEP violations sensitive to material differences. Employing a rotating torsion balance with beryllium (Be) and titanium (Ti) test bodies suspended above a dense source mass, these setups measure torque signals from differential gravitational forces. Results yielded differential accelerations of Δa(Be-Ti) = (0.6 ± 3.1) × 10^{-15} m/s² toward the north and (-2.5 ± 3.5) × 10^{-15} m/s² toward the west, implying η < 2 × 10^{-13} at 1σ and limiting fifth force couplings for interaction ranges near 10 cm to α < 10^{-5} in Yukawa-type models.[26] Cold atom interferometry offers complementary sensitivity by launching ultracold atomic wave packets into free fall and interferometrically detecting phase shifts from acceleration differences. Dual-species setups, such as those using rubidium isotopes, have constrained WEP violations to Δa/a ≈ 5 × 10^{-13}, while interpretations in fifth force models sensitive to electron number differences (e.g., via varying atomic Z/A ratios) yield bounds on electron couplings as stringent as 10^{-14}.[27][28]

Particle Collider and Precision Measurements

Particle colliders, particularly the Large Hadron Collider (LHC) operated by CERN, have been instrumental in searching for heavy mediators of potential fifth forces, such as Z' bosons, through high-energy proton-proton collisions. The ATLAS and CMS experiments analyze dilepton (electron-positron or muon-antimuon pair) events to detect resonances that could indicate a Z' particle, often modeled in extensions of the Standard Model motivated by dark sector forces. Recent analyses from Run 2 data (up to 139 fb⁻¹ at √s = 13 TeV) exclude Sequential Standard Model Z' bosons with masses below approximately 5 TeV at 95% confidence level, assuming couplings similar to the electroweak Z boson.[29] These limits extend to lower couplings, constraining g' < 0.1 for masses around 5 TeV in simplified models where the Z' mediates interactions between ordinary matter and a hidden sector.[30] Precision low-energy experiments provide complementary probes for lighter fifth force mediators by testing deviations from Standard Model predictions in quantum systems. The Muon g-2 experiment at Fermilab, in its 2021 result, measured the muon's anomalous magnetic moment with unprecedented precision, revealing a discrepancy with theory at 4.2σ significance when combined with prior Brookhaven data. This anomaly hints at new physics, including a dark photon mediator with mass m ≈ 20 MeV and kinetic mixing parameter ε² ≈ 2 × 10⁻⁷, which could represent a fifth force coupling the muon to a dark sector, though the interpretation remains tentative pending further theoretical and experimental confirmation.[31] Atomic parity violation (APV) experiments in cesium atoms offer sensitive tests for scalar mediators of fifth forces, as such particles could induce parity-violating interactions beyond the Standard Model weak force. High-precision measurements of the parity-nonconserving energy shift in the cesium ground state, achieved through enhanced atomic structure calculations and experimental techniques, constrain scalar exchanges coupling to electrons and nucleons. These results limit the effective fine-structure constant for such scalar-mediated fifth forces to α < 10⁻⁸ at 95% confidence level, assuming Yukawa-type interactions with ranges above atomic scales.[32]

Geophysical and Astrophysical Tests

Geophysical tests of fifth forces leverage large-scale natural laboratories such as Earth's interior to probe deviations from Newtonian gravity at intermediate ranges, typically hundreds of meters to kilometers. Early investigations in deep mines and boreholes, such as those conducted in Australian and Canadian sites, measured gravitational acceleration variations with depth to constrain Yukawa-type potentials. These experiments, which compared observed gravity profiles to models of Earth's density distribution, set limits on the fifth force strength α (the ratio of fifth force to gravitational coupling) of α < 10^{-3} for ranges λ > 100 m, consistent with no detectable deviation but highlighting the sensitivity of subsurface structures to composition-dependent forces.[33] Neutrino geophysics provides complementary constraints by probing Earth's mantle composition through geoneutrino fluxes detected at facilities like KamLAND and Borexino, which reveal uranium and thorium distributions influencing density models. If a fifth force couples differently to nuclear matter, it could alter effective gravitational binding in the mantle, affecting neutrino production rates; however, observed fluxes align with standard models, limiting such effects. Similarly, analysis of the ancient OKLO natural nuclear reactor in Gabon, which operated ~2 billion years ago, constrains time variations in fundamental constants that could signal a fifth force-mediated coupling. Reactor physics simulations of isotope ratios yield bounds on relative changes Δα/α < 10^{-6} over geological timescales, ruling out kilometer-range fifth forces with strengths α > 10^{-6} that would induce detectable shifts in nuclear resonances or decay rates.[34][35] Astrophysical tests extend these constraints to cosmic scales, using dynamics in galaxy clusters to probe long-range fifth forces, particularly those acting within the dark sector. The Bullet Cluster (1E 0657-56), a colliding galaxy system, separates baryonic gas from dark matter via weak lensing and X-ray observations, revealing dark matter's collisionless nature. Simulations incorporating dark matter self-interactions—potentially mediated by a fifth force—must match the observed offset and velocity (~4700 km/s) of the "bullet" subcluster, yielding upper limits on the self-interaction cross-section per unit mass of σ/m < 0.5 cm²/g at 95% confidence. Strong fifth forces exceeding this threshold would cause excessive dark matter scattering, disrupting the observed separation and contradicting the data.[36] Cepheid variable stars serve as standard candles in the cosmic distance ladder, with their period-luminosity relation (PLR) sensitive to fifth force modifications of stellar structure and pulsation dynamics. In the context of the Hubble tension—discrepancies between local (H₀ ≈ 73 km/s/Mpc) and CMB-inferred (H₀ ≈ 67 km/s/Mpc) expansion rates—fifth force models could alter Cepheid luminosities or periods via enhanced gravity in host galaxies, potentially resolving the ~5σ tension. However, precise Gaia and Hubble Space Telescope observations of PLR deviations in galaxies like NGC 4258 limit such effects to <1%, constraining fifth force strengths ΔG/G_N < 0.01 in low-acceleration environments without screening, as larger couplings would induce measurable scatter in distance estimates.[37]

Laboratory Searches for New Interactions

Laboratory searches for new interactions focus on high-precision, controlled environments to detect short-range fifth forces that might mediate between fundamental particles, such as neutrons and electrons, or modify known forces like gravity at sub-micrometer scales. These experiments leverage techniques like neutron scattering, trapped ion spectroscopy, and measurements of the Casimir force to impose tight constraints on hypothetical Yukawa-type potentials, which could arise from extra dimensions or new bosons. By isolating test masses or particles and minimizing environmental noise, these setups achieve sensitivities far beyond geophysical or astrophysical tests, targeting ranges from nanometers to micrometers where fifth forces might dominate over standard interactions.[38] Neutron scattering experiments, particularly those employing crystal diffraction, have provided some of the strongest laboratory bounds on deviations from the gravitational inverse-square law at short distances. In a setup using a pulsed neutron beam incident on a silicon crystal, researchers measured the scattering cross-section to search for anomalous forces that would alter the neutron's trajectory or interaction profile. This approach tested the law down to approximately 1 μm, yielding limits on the relative deviation parameter of order 10^{-16}, effectively constraining the coupling strength α of any Yukawa-type fifth force to below detectable levels in this regime. These results rule out significant modifications to gravity at these scales and complement broader searches for non-standard interactions in neutron optics.[39] Trapped ion spectroscopy offers exquisite sensitivity to electron-neutron couplings, potentially revealing fifth forces mediated by light bosons in the dark sector. Prior work using ytterbium ions (Yb+) in a Paul trap performed high-precision isotope-shift spectroscopy on optical transitions, probing nonlinearities indicative of a new force and achieving an upper limit on the coupling α < 10^{-10} for mediators with masses around 1 eV (corresponding to interaction ranges of roughly 200 nm) as of 2023.[40] In 2025, a new experiment at ETH Zurich confines barium ions (Ba+)—selected for their nuclear octupole deformation enhancing sensitivity to new physics—in a Paul trap for similar isotope-shift measurements to search for fifth forces, continuing this line of research.[41] Casimir force experiments, involving parallel plates or spheres at nanometer separations, have constrained fifth force modifications to quantum vacuum fluctuations and short-range gravity. These setups measure attractive forces between metallic surfaces, isolating contributions from hypothetical Yukawa terms that could enhance or suppress the standard Casimir effect. Representative plate-based measurements, using atomic force microscopy to control separations down to 100 nm, have set upper limits of α < 10^{-3} for interaction strengths at these scales, excluding many models of extra-dimensional gravity or axion-like particles. Such constraints are derived from precise force gradients and roughness corrections, demonstrating Casimir physics as a cornerstone for sub-micron fifth force hunts.[42][43]

Recent Advances and Constraints

Dark Matter and Gravitational Behavior Studies

In November 2025, a study published in Nature Communications analyzed data from over 100 galaxy clusters, revealing that dark matter falls into gravitational wells in a manner consistent with ordinary matter, showing no significant deviations from standard gravitational expectations.[44] The research utilized weak gravitational lensing to infer dark matter distributions and velocity dispersions from galaxy motions to probe response to potentials, enabling precise tests for fifth-force signatures that could alter dark matter clustering or infall rates. These methods constrained any positive fifth force—enhancing attraction—to less than 7% of gravity's strength and any negative fifth force—repulsive—to less than 21%, based on combined cosmological datasets including DES and DESI surveys.[44] The implications of these findings are profound for self-interacting dark matter (SIDM) models, which hypothesize additional forces in the dark sector to address small-scale galactic structure anomalies like core-cusp problems. By demonstrating alignment between dark matter and baryonic matter responses on cluster scales, the study effectively rules out strong SIDM variants predicting deviations exceeding these limits, reinforcing general relativity's dominance while motivating refined weak-interaction theories.[44] Scientific reports from Space.com and Phys.org corroborated these results, emphasizing the confirmation of unaltered dark matter motion and its impact on SIDM frameworks, which now face heightened scrutiny without evidence for force-mediated self-interactions.[7][45] This work builds briefly on longstanding dark sector motivations, where fifth forces were proposed to reconcile observed gravitational behaviors across scales.[44]

Electron-Neutron Coupling Experiments

In June 2025, a study reported in Popular Mechanics detailed precision measurements of atomic isotope shifts in calcium, placing stringent upper limits on a protophobic fifth force that couples electrons to neutrons while avoiding interactions with protons. The research utilized high-precision spectroscopy to examine nonlinear deviations in King plots of optical transition frequencies across calcium isotopes (^{40}Ca, ^{42}Ca, ^{44}Ca, ^{46}Ca, and ^{48}Ca), revealing no evidence for such a force but improving previous constraints for mediator masses in the range of 10 to 10^7 eV/c².[46][4] These atomic-scale probes complement other searches for light mediators. The 2025 results also addressed potential connections to the long-standing beryllium-8 nuclear transition anomaly, where an excess of electron-positron pairs in excited ^{8}Be decays was initially interpreted as evidence for a protophobic force around 17 MeV. However, the new limits from isotope shifts provide no confirmation of the anomaly as arising from such a fifth force, instead tightening exclusions on the parameter space proposed in earlier interpretations.

Current Status and Future Prospects

Evaluation of Claims

In the 1980s, physicist Ephraim Fischbach and collaborators proposed the existence of a fifth force based on a reanalysis of historical data from the Eötvös torsion balance experiments conducted in the early 20th century, suggesting a composition-dependent deviation from the weak equivalence principle at the level of about 0.2% for certain material pairs.[47] This claim sparked widespread interest and prompted numerous follow-up experiments, but subsequent high-precision tests in the 1990s, including modern Eötvös-type torsion balances and lunar laser ranging, refuted the hypothesis by establishing equivalence principle violations below 10^{-12} at 95% confidence level.[48] More recent controversies include the 2016 observation of an anomalous excess in electron-positron pair production from excited beryllium-8 nuclei at the Atomki institute in Hungary, initially interpreted as evidence for a ~17 MeV boson (X17) that could mediate a protophobic fifth force. While some nuclear physics calculations have proposed explanations through conventional internal pair creation processes involving improved modeling of electromagnetic transitions, the anomaly has not been conclusively resolved. As of November 2025, subsequent observations at Atomki in helium-4 and carbon-12 nuclei provide additional support for the X17 interpretation, though with ongoing debates over nuclear models. Conflicting results from other experiments, including no excess observed by MEG II in July 2025 and a mild excess near 16.9 MeV reported by PADME in May 2025, maintain the controversy without definitive confirmation of a new particle or force.[49][50][51][52] Other notable claims under evaluation include the Muon g-2 experiment at Fermilab, where the measured muon magnetic moment deviates from Standard Model predictions by approximately 5σ as of 2025 updates, potentially indicating a new mediator particle consistent with fifth force models. Similarly, 2025 atomic physics experiments with calcium isotopes have imposed tighter upper limits on fifth force strengths via analysis of optical transitions, though small residual anomalies suggest possible subtle effects.[1][6] Particle collider searches, particularly at the Large Hadron Collider (LHC), have not yielded a 5σ discovery of fifth force mediators, with analyses of data up to 2025 constraining potential new bosons to masses and couplings incompatible with some earlier anomalies.[53] Overall, no confirmed evidence for a fifth force has emerged from these claims, rendering it a hypothetical extension of the Standard Model and general relativity, with experimental constraints squeezing allowable parameter space across coupling strengths and ranges, though select anomalies keep the possibility open.[54]

Ongoing and Planned Experiments

Ongoing efforts to detect or constrain fifth forces include upgrades to laboratory-based precision instruments designed to probe violations of the weak equivalence principle at sub-millimeter scales. The Eöt-Wash group at the University of Washington continues to refine its torsion balance apparatus, which has historically set stringent limits on composition-dependent interactions, with recent measurements constraining such forces to levels below 10^{-13} of gravity.[55] Planned enhancements aim to push sensitivities further by minimizing systematic errors, enabling searches for new interactions weaker than gravity over short ranges down to tens of microns.[56] Complementing torsion balances, atom interferometry experiments like MAGIS-100, a collaboration between Fermilab, Stanford University, and others, are under construction to achieve baselines of approximately 100 meters. This setup will use light-pulse atom interferometry with strontium atoms to search for ultra-weak forces, including fifth-force signatures, by measuring differential accelerations of strontium isotopes over interrogation times corresponding to paths up to 10 meters, targeting equivalence principle violations and new interactions sourced by Earth or test masses. Initial prototypes have demonstrated feasibility, with full deployment expected to probe forces in the 10^{-15} eV to 10^{-14} eV mass range for mediating particles.[57] At the particle physics frontier, the FASER experiment at the LHC, operational since 2022, targets long-lived particles such as dark photons that could mediate fifth forces within a dark sector. Positioned 480 meters downstream from the ATLAS interaction point, FASER is sensitive to dark photons with energies exceeding 100 GeV and couplings around 10^{-5}, potentially produced in proton-proton collisions and decaying into visible particles.[58] Early results from Run 3 data have excluded portions of the parameter space for light dark photons with masses between 10 and 100 MeV.[59] Similarly, the Belle II experiment at SuperKEKB investigates lepton flavor violations (LFV) and universality deviations, which could signal new forces coupling preferentially to specific lepton generations. By analyzing B meson and tau decays, such as τ → ℓ α where ℓ is an electron or muon and α is a photon, Belle II probes for beyond-Standard-Model interactions that might manifest as fifth forces, with data collected up to 2024 providing improved bounds on branching ratios below 10^{-8}.[60] These measurements complement recent constraints on lepton universality from LHCb, testing models where extra gauge bosons induce flavor-changing effects.[61] In cosmological contexts, the Euclid satellite, launched in 2023, is mapping the distribution of dark matter through weak lensing and galaxy clustering over vast sky areas, enabling tests of self-interacting dark matter (SIDM) models that incorporate fifth-force-like interactions between dark particles. By observing more than 1.5 billion galaxies, Euclid will constrain SIDM cross-sections and potential modifications to gravitational clustering, distinguishing them from cold dark matter predictions with precision better than 1% on structure growth parameters.[62] Initial data releases as of November 2025 have begun refining bounds on dark sector forces.[44] Future gravitational wave observatories, particularly the Laser Interferometer Space Antenna (LISA) planned for launch in the early 2030s, will detect modifications to waveform propagation and inspiral dynamics induced by fifth forces. LISA's sensitivity in the millihertz band will probe scalar-tensor theories where additional forces alter binary black hole or neutron star mergers, potentially revealing deviations in phase evolution or amplitude from general relativity predictions. Combined with electromagnetic counterparts, these observations could constrain fifth-force strengths to below 1% of gravity for mediator masses around 10^{-22} eV.[63]

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