Fifth force

In physics, there are four observed fundamental interactions (also known as fundamental forces) that form the basis of all 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 characteristics of this fifth force depend on the hypothesis being advanced. Many postulate a force roughly the strength of gravity (i.e., it is much weaker than electromagnetism or the nuclear forces) with a range of anywhere from less than a millimeter to cosmological scales. Another proposal is a new weak force mediated by W′ and Z′ bosons.

The search for a fifth force has increased in recent decades due to two discoveries in cosmology which are not explained by current theories. It has been discovered that most of the mass of the universe is accounted for by an unknown form of matter called dark matter. Most physicists believe that dark matter consists of undiscovered subatomic particles<ref>Chown, Marcus (17 August 2011). "Really dark matter: Is the universe made of holes?". New Scientist. Pretty much everyone thinks that this so-called dark matter is made of hitherto undiscovered subatomic particles.</ref> that are difficult to detect, but some believe that it could be related to an unknown fundamental force. Second, it has also recently been discovered that the expansion of the universe is accelerating, which 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.<ref>Wetterich, C. "Quintessence – a fifth force from variation of the fundamental scale" (PDF). Heidelberg University.</ref><ref>Cicoli, Michele; Pedro, Francisco G.; Tasinato, Gianmassimo (2012). "Natural quintessence in string theory". Journal of Cosmology and Astroparticle Physics. 2012 (7): 044. arXiv:1203.6655. Bibcode:2012JCAP...07..044C. doi:10.1088/1475-7516/2012/07/044. S2CID 118461474.</ref>

History

The term fifth force originates in a 1986 paper by 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. <ref name=FischbachReanalysis>Fischbach, Ephraim; Sudarsky, Daniel; Szafer, Aaron; Talmadge, Carrick; Aronson, S.H. (6 January 1986). "Reanalysis of the Eötvös experiment". Physical Review Letters. 56 (1): 3–6. Bibcode:1986PhRvL..56....3F. doi:10.1103/PhysRevLett.56.3. PMID 10032514.</ref><ref name=NewPhysicsAtoms>Safronova, M. S.; Budker, D.; DeMille, D.; Kimball, Derek F. Jackson; Derevianko, A.; Clark, Charles W. (2018-06-29). "Search for new physics with atoms and molecules". Reviews of Modern Physics. 90 (2): 025008. arXiv:1710.01833. Bibcode:2018RvMP...90b5008S. doi:10.1103/RevModPhys.90.025008. ISSN 0034-6861.</ref> The reanalysis was sparked by theoretical work in 1971 by Fujii <ref>Fujii, Yasunori (November 1971). "Dilaton and Possible Non-Newtonian Gravity". Nature Physical Science. 234 (44): 5–7. Bibcode:1971NPhS..234....5F. doi:10.1038/physci234005a0. ISSN 0300-8746.</ref><ref>Fischbach, Ephraim; Talmadge, Carrick L. (1999). The Search for Non-Newtonian Gravity. New York, NY: Springer New York. doi:10.1007/978-1-4612-1438-0. ISBN 978-1-4612-7144-4.</ref> proposing a model that changes distance dependence with a Yukawa potential-like term:

<math>V(r) = -G_\infty \frac{m_im_j}{r_{ij}}(1+\alpha e^{-r/\lambda})</math>

The parameter <math>\alpha</math> characterizes the strength and <math>\lambda</math> the range of the interaction.<ref name=NewPhysicsAtoms/> Fischbach's paper found a strength around 1% of gravity and a range of a few hundred meters.<ref name=WillConfrontation>Will, Clifford M. (Dec 2014). "The Confrontation between General Relativity and Experiment". Living Reviews in Relativity. 17 (1). arXiv:1403.7377. Bibcode:2014LRR....17....4W. doi:10.12942/lrr-2014-4. ISSN 2367-3613. PMC 5255900. PMID 28179848.</ref> 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.<ref name=NewPhysicsAtoms/> However, many subsequent attempts to reproduce the deviations have failed.<ref>Franklin, Allan (2016). Fischbach, Ephraim (ed.). The rise and fall of the fifth force: discovery, pursuit, and justification in modern physics (2 ed.). Cham Heidelberg New York Dordrecht London: Springer. ISBN 978-3-319-28412-5.</ref>

Theory

Theoretical proposals in the fifth-force category are driven by two inconsistencies between the current models of general relativity and quantum field theory, the hierarchy problem and the cosmological constant problem. Both issues suggest the possibility of corrections to the gravitational potential around <math>100\mu\text{m}</math>.<ref name=NewPhysicsAtoms/>

Experimental approaches

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

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<ref>"Lunar laser ranging". Archived from the original on 28 November 2016. Retrieved 7 May 2005.</ref> and very-long-baseline interferometry.

Extra dimensions

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 a 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.<ref>"Satellite Energy Exchange (SEE)". Archived from the original on 7 May 2005. Retrieved 7 May 2005., which is set to test for a fifth force in space, where it is possible to achieve greater sensitivity.</ref> 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.<ref>Ander, Mark E.; Zumberge, Mark A.; Lautzenhiser, Ted; Parker, Robert L.; Aiken, Carlos L. V.; Gorman, Michael R.; Nieto, Michael Martin; Cooper, A. Paul R.; Ferguson, John F.; Fisher, Elizabeth; McMechan, George A.; Sasagawa, Glenn; Stevenson, J. Mark; Backus, George; Chave, Alan D.; Greer, James; Hammer, Phil; Hansen, B. Lyle; Hildebrand, John A.; Kelty, John R.; Sidles, Cyndi; Wirtz, Jim (27 February 1989). "Test of Newton's inverse-square law in the Greenland ice cap". Physical Review Letters. 62 (9): 985–988. Bibcode:1989PhRvL..62..985A. doi:10.1103/PhysRevLett.62.985. PMID 10040395.</ref><ref>Zumberge, Mark A.; Ander, Mark E.; Lautzenhiser, Ted V.; Parker, Robert L.; Aiken, Carlos L. V.; Gorman, Michael R.; Nieto, Michael Martin; Cooper, A. Paul R.; Ferguson, John F.; Fisher, Elizabeth; Greer, James; Hammer, Phil; Hansen, B. Lyle; McMechan, George A.; Sasagawa, Glenn S.; Sidles, Cyndi; Stevenson, J. Mark; Wirtz, Jim (1990). "The Greenland Gravitational Constant Experiment". Journal of Geophysical Research. 95 (B10): 15483. Bibcode:1990JGR....9515483Z. doi:10.1029/JB095iB10p15483.</ref>

Earth's mantle

Another experiment uses the Earth's mantle as a giant particle detector, focusing on geoelectrons.<ref>Aron, Jacob (2013). "Earth's mantle helps hunt for fifth force of nature". New Scientist.</ref>

Cepheid variables

Jain et al. (2012)<ref>Jain, Bhuvnesh; Vikram, Vinu; Sakstein, Jeremy (25 November 2013). "Astrophysical tests of modified gravity: Constraints from distance indicators in the nearby universe". The Astrophysical Journal. 779 (1): 39. arXiv:1204.6044. Bibcode:2013ApJ...779...39J. doi:10.1088/0004-637X/779/1/39. S2CID 119260435. 39.</ref> 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

Some experiments used a lake plus a tower that is 320 meters high.<ref>Liu, Yi-Cheng; Yang, Xin-She; Zhu, Heng-Bin; Zhou, Wen-Hu; Wang, Qian-Shen; Zhao, Zhi-Qiang; Jiang, Wei-Wei; Wu, Chuan-Zhen (September 1992). "Testing non-Newtonian gravitation on a 320 m tower". Physics Letters A. 169 (3): 131–133. Bibcode:1992PhLA..169..131L. doi:10.1016/0375-9601(92)90582-7.</ref> A comprehensive review by Ephraim Fischbach and Carrick Talmadge suggested there is no compelling evidence for the fifth force,<ref>Fischbach, Ephraim; Talmadge, Carrick (19 March 1992). "Six years of the fifth force". Nature. 356 (6366): 207–215. Bibcode:1992Natur.356..207F. doi:10.1038/356207a0. S2CID 21255315.</ref> 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.<ref>Jenkins, Jere H.; Fischbach, Ephraim; Buncher, John B.; Gruenwald, John T.; Krause, Dennis E.; Mattes, Joshua J. (August 2009). "Evidence of correlations between nuclear decay rates and Earth–Sun distance". Astroparticle Physics. 32 (1): 42–46. arXiv:0808.3283. Bibcode:2009APh....32...42J. doi:10.1016/j.astropartphys.2009.05.004. S2CID 119113836.</ref>

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,<ref>Hall, A. M.; Armbruster, H.; Fischbach, E.; Talmadge, C. (1991). "Is the Eötvös experiment sensitive to spin?". In Hwang, W.-Y. Pauchy; et al. (eds.). Progress in High Energy Physics. New York: Elsevier. pp. 325–339.</ref> 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

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).<ref name="Cartlidge2016">Cartlidge, Edwin (2016). "Has a Hungarian physics lab found a fifth force of nature?". Nature. doi:10.1038/nature.2016.19957. S2CID 124347962.</ref> 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.<ref>"Scientists may have discovered fifth force of nature, laboratory announces". The Independent. London, UK. Retrieved 26 November 2019.</ref>

Feng et al. (2016)<ref name=Feng-etal-2016/> 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.<ref>"New boson claim faces scrutiny". Quanta Magazine. Retrieved 24 November 2019.</ref> 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.<ref name="Cartlidge2016"/><ref name=Feng-etal-2016>Feng, Jonathan L.; Fornal, Bartosz; Galon, Iftah; Gardner, Susan; Smolinsky, Jordan; Tait, Tim M. P.; Tanedo, Philip (11 August 2016). "Protophobic Fifth-Force Interpretation of the Observed Anomaly in ⁸Be Nuclear Transitions". Physical Review Letters. 117 (7): 071803. arXiv:1604.07411. Bibcode:2016PhRvL.117g1803F. doi:10.1103/PhysRevLett.117.071803. PMID 27563952. S2CID 206279817.</ref>

See also

• Affine gauge theory – Gauge theory with affine connections
• Complex system – System composed of many interacting components
• Graviphoton – Hypothetical particle in gravity theories
• Modified Newtonian dynamics – Hypothesis proposing a modification of Newton's laws
• Physics beyond the Standard Model – Theories trying to extend known physics
• Self-organization – Process of creating order by local interactions