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{{short description|Speculative fifth fundamental force}}
In physics, there are four observed [[fundamental interaction]]s (also known as fundamental forces) that form the basis of all known interactions in nature: [[gravity|gravitational]], [[electromagnetism|electromagnetic]], [[strong interaction|strong nuclear]], and [[weak interaction|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 [[strong nuclear force|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>{{cite journal |author=Chown, Marcus |title=Really dark matter: Is the universe made of holes? |url=https://www.newscientist.com/article/mg21128262-200-really-dark-matter-is-the-universe-made-of-holes/ |journal=[[New Scientist]] |date=17 August 2011 |quote=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 (physics)|quintessence]] could be a fifth force.<ref>{{cite web |title=Quintessence – a fifth force from variation of the fundamental scale |url=http://www.thphys.uni-heidelberg.de/~wetterich/DEBarcelona0706.pdf |publisher=Heidelberg University |first=C. |last=Wetterich}}</ref><ref>{{cite journal |last1=Cicoli |first1=Michele |last2=Pedro |first2=Francisco G. |last3=Tasinato |first3=Gianmassimo |year=2012 |title=Natural quintessence in string theory |journal=Journal of Cosmology and Astroparticle Physics |volume=2012 |issue=7 |page=044 |arxiv=1203.6655 |doi=10.1088/1475-7516/2012/07/044|bibcode=2012JCAP...07..044C |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>{{cite journal |last1=Fischbach |first1=Ephraim |last2=Sudarsky |first2=Daniel |last3=Szafer |first3=Aaron |last4=Talmadge |first4=Carrick |last5=Aronson |first5=S.H. |title=Reanalysis of the Eötvös experiment |journal=[[Physical Review Letters]] |date=6 January 1986 |volume=56 |issue=1 |pages=3–6 |doi=10.1103/PhysRevLett.56.3 |pmid=10032514 |bibcode=1986PhRvL..56....3F}}</ref><ref name=NewPhysicsAtoms>{{Cite journal |last1=Safronova |first1=M. S. |last2=Budker |first2=D. |last3=DeMille |first3=D. |last4=Kimball |first4=Derek F. Jackson |last5=Derevianko |first5=A. |last6=Clark |first6=Charles W. |date=2018-06-29 |title=Search for new physics with atoms and molecules |url=https://link.aps.org/doi/10.1103/RevModPhys.90.025008 |journal=Reviews of Modern Physics |language=en |volume=90 |issue=2 |page=025008 |doi=10.1103/RevModPhys.90.025008 |arxiv=1710.01833 |bibcode=2018RvMP...90b5008S |issn=0034-6861}}</ref>{{rp|57}} 
The reanalysis was sparked by theoretical work in 1971 by Fujii <ref>{{Cite journal |last=Fujii |first=Yasunori |date=November 1971 |title=Dilaton and Possible Non-Newtonian Gravity |url=https://www.nature.com/articles/physci234005a0 |journal=Nature Physical Science |language=en |volume=234 |issue=44 |pages=5–7 |doi=10.1038/physci234005a0 |bibcode=1971NPhS..234....5F |issn=0300-8746}}</ref><ref>{{Cite book |last1=Fischbach |first1=Ephraim |url=http://link.springer.com/10.1007/978-1-4612-1438-0 |title=The Search for Non-Newtonian Gravity |last2=Talmadge |first2=Carrick L. |date=1999 |publisher=Springer New York |isbn=978-1-4612-7144-4 |location=New York, NY |language=en |doi=10.1007/978-1-4612-1438-0}}</ref>{{rp|3}} 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>{{Cite journal |last=Will |first=Clifford M. |date=Dec 2014 |title=The Confrontation between General Relativity and Experiment |journal=Living Reviews in Relativity |language=en |volume=17 |issue=1 |doi=10.12942/lrr-2014-4 |issn=2367-3613 |pmc=5255900 |pmid=28179848|arxiv=1403.7377 |bibcode=2014LRR....17....4W }}</ref>{{rp|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.<ref name=NewPhysicsAtoms/>
However, many subsequent attempts to reproduce the deviations have failed.<ref>{{Cite book |last=Franklin |first=Allan |title=The rise and fall of the fifth force: discovery, pursuit, and justification in modern physics |date=2016 |publisher=Springer |isbn=978-3-319-28412-5 |editor-last=Fischbach |editor-first=Ephraim |edition=2|location=Cham Heidelberg New York Dordrecht London}}</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/>{{rp|58}}

==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 {{nowrap|force{{tsp}}{{mdash}}}}{{tsp}}possibly one with infinite range. This is because gravitational interactions, in theories other than general relativity, have [[degrees of freedom (physics and chemistry)|degrees of freedom]] other than the [[metric tensor|"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 starlight|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>{{cite web |title=Lunar laser ranging |url=http://funphysics.jpl.nasa.gov/technical/grp/lunar-laser.html |access-date=2005-05-07 |url-status=dead |archive-url=https://web.archive.org/web/20161128185551/http://funphysics.jpl.nasa.gov/technical/grp/lunar-laser.html |archive-date=2016-11-28 |df=dmy-all}}</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 potential|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 {{nowrap|dimensions{{tsp}}{{mdash}}}}{{tsp}}dimensions with size slightly less than a {{nowrap|millimeter{{hsp}}{{mdash}}}}{{tsp}}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>{{cite web |url=http://www.phys.utk.edu/see/ |title=Satellite Energy Exchange (SEE) |access-date=2005-05-07 |url-status=dead |archive-url=https://web.archive.org/web/20050507195406/http://www.phys.utk.edu/see/ |archive-date=2005-05-07 |df=dmy-all}}, 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)|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>{{cite journal |last1=Ander |first1=Mark E. |last2=Zumberge |first2=Mark A. |last3=Lautzenhiser |first3=Ted |last4=Parker |first4=Robert L. |last5=Aiken |first5=Carlos L. V. |last6=Gorman |first6=Michael R. |last7=Nieto |first7=Michael Martin |last8=Cooper |first8=A. Paul R. |last9=Ferguson |first9=John F. |last10=Fisher |first10=Elizabeth |last11=McMechan |first11=George A. |last12=Sasagawa |first12=Glenn |last13=Stevenson |first13=J. Mark |last14=Backus |first14=George |last15=Chave |first15=Alan D. |last16=Greer |first16=James |last17=Hammer |first17=Phil |last18=Hansen |first18=B. Lyle |last19=Hildebrand |first19=John A. |last20=Kelty |first20=John R. |last21=Sidles |first21=Cyndi |last22=Wirtz |first22=Jim |title=Test of Newton's inverse-square law in the Greenland ice cap |journal=Physical Review Letters |date=27 February 1989 |volume=62 |issue=9 |pages=985–988 |doi=10.1103/PhysRevLett.62.985 |pmid=10040395 |bibcode=1989PhRvL..62..985A }}</ref><ref>{{cite journal |last1=Zumberge |first1=Mark A. |last2=Ander |first2=Mark E. |last3=Lautzenhiser |first3=Ted V. |last4=Parker |first4=Robert L. |last5=Aiken |first5=Carlos L. V. |last6=Gorman |first6=Michael R. |last7=Nieto |first7=Michael Martin |last8=Cooper |first8=A. Paul R. |last9=Ferguson |first9=John F. |last10=Fisher |first10=Elizabeth |last11=Greer |first11=James |last12=Hammer |first12=Phil |last13=Hansen |first13=B. Lyle |last14=McMechan |first14=George A. |last15=Sasagawa |first15=Glenn S. |last16=Sidles |first16=Cyndi |last17=Stevenson |first17=J. Mark |last18=Wirtz |first18=Jim |title=The Greenland Gravitational Constant Experiment |journal=Journal of Geophysical Research |date=1990 |volume=95 |issue=B10 |pages=15483 |doi=10.1029/JB095iB10p15483 |bibcode=1990JGR....9515483Z |url=https://zenodo.org/record/1231434 }}</ref>

===Earth's mantle===
Another experiment uses the [[Earth's mantle]] as a giant particle detector, focusing on geoelectrons.<ref>{{cite magazine |last=Aron |first=Jacob |year=2013 |url=https://www.newscientist.com/article/dn23202-earths-mantle-helps-hunt-for-fifth-force-of-nature.html |title=Earth's mantle helps hunt for fifth force of nature |magazine=New Scientist}}</ref>

===Cepheid variables===
Jain ''et al.'' (2012)<ref>{{cite journal |last1=Jain |first1=Bhuvnesh |last2=Vikram |first2=Vinu |last3=Sakstein |first3=Jeremy |date=25 November 2013 |title=Astrophysical tests of modified gravity: Constraints from distance indicators in the nearby universe |journal=The Astrophysical Journal |volume=779 |issue=1 |page=39 |id=39 |arxiv=1204.6044 |doi=10.1088/0004-637X/779/1/39 |bibcode=2013ApJ...779...39J|s2cid=119260435 }}</ref><!-- removed second ref to identical article (arXiv, bibcode, volume, journal page, etc.) listing these persons as authors: |last1=Cicoli |first1=Michele |last2=Pedro |first2=Francisco G. |last3=Tasinato |first3=Gianmassimo |year=1=2012 ; They are not listed in the indicated article -- lost ref from failure to fill in copied template? --> examined existing data on the rate of pulsation of over a thousand [[cepheid variable]] stars in 25&nbsp;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 {{val|320|u=m}}eters high.<ref>{{cite journal |last1=Liu |first1=Yi-Cheng |last2=Yang |first2=Xin-She |last3=Zhu |first3=Heng-Bin |last4=Zhou |first4=Wen-Hu |last5=Wang |first5=Qian-Shen |last6=Zhao |first6=Zhi-Qiang |last7=Jiang |first7=Wei-Wei |last8=Wu |first8=Chuan-Zhen |title=Testing non-Newtonian gravitation on a 320&nbsp;m tower |journal=Physics Letters&nbsp;A |date=September 1992 |volume=169 |issue=3 |pages=131–133 |doi=10.1016/0375-9601(92)90582-7 |bibcode=1992PhLA..169..131L}}</ref> A comprehensive review by [[Ephraim Fischbach]] and Carrick Talmadge suggested there is no compelling evidence for the fifth force,<ref>{{cite journal |last1=Fischbach |first1=Ephraim |last2=Talmadge |first2=Carrick |title=Six years of the fifth force |journal=Nature |date=19 March 1992 |volume=356 |issue=6366 |pages=207–215 |doi=10.1038/356207a0 |bibcode=1992Natur.356..207F|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>{{cite journal |last1=Jenkins |first1=Jere H. |last2=Fischbach |first2=Ephraim |last3=Buncher |first3=John B. |last4=Gruenwald |first4=John T. |last5=Krause |first5=Dennis E. |last6=Mattes |first6=Joshua J. |title=Evidence of correlations between nuclear decay rates and Earth–Sun distance |journal=Astroparticle Physics |date=August 2009 |volume=32 |issue=1 |pages=42–46 |arxiv=0808.3283 |doi=10.1016/j.astropartphys.2009.05.004 |bibcode=2009APh....32...42J|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 [[proton]]s to [[neutron]]s in an atomic nucleus, nuclear spin,<ref>{{cite book |last1=Hall |first1=A. M. |title=Progress in High Energy Physics |last2=Armbruster |first2=H. |last3=Fischbach |first3=E. |last4=Talmadge |first4=C. |publisher=Elsevier |year=1991 |editor=Hwang |editor-first=W.-Y. Pauchy |location=New York |pages=325–339 |chapter=Is the Eötvös experiment sensitive to spin? |display-editors=etal}}</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|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 [[X17 particle|a new, light boson]] only 34&nbsp;times heavier than the electron (17&nbsp;MeV).<ref name="Cartlidge2016">{{cite journal |last=Cartlidge |first=Edwin |year=2016 |title=Has a Hungarian physics lab found a fifth force of nature? |journal=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 {{math|e}}{{sup|+}} and {{math|e}}{{sup|−}}, and a combined energy of 17&nbsp;MeV, which indicated that a small fraction of beryllium-8 will shed excess energy in the form of a new particle.

In November&nbsp;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|X17]] particle's existence.<ref>{{cite news |title=Scientists may have discovered fifth force of nature, laboratory announces |newspaper=[[The Independent]] |place=London, UK |url=https://www.independent.co.uk/news/science/dark-matter-particle-hungary-atomki-nuclear-research-force-nature-a9210741.html |access-date=2019-11-26 |df=dmy-all}}</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&nbsp;MeV with suppressed couplings to protons relative to neutrons and electrons and [[femtometer]] range could explain the data.<ref>{{cite magazine |title=New boson claim faces scrutiny |magazine=Quanta Magazine |url=https://www.quantamagazine.org/new-boson-claim-faces-scrutiny-20160607/ |access-date=2019-11-24 |df=dmy-all}}</ref> The force may explain the [[Anomalous magnetic dipole moment#Muon|muon {{nowrap|{{mvar|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>{{cite journal |last1=Feng |first1=Jonathan L. |last2=Fornal |first2=Bartosz |last3=Galon |first3=Iftah |last4=Gardner |first4=Susan |last5=Smolinsky |first5=Jordan |last6=Tait |first6=Tim M. P. |last7=Tanedo |first7=Philip |title=Protophobic Fifth-Force Interpretation of the Observed Anomaly in {{sup|8}}Be Nuclear Transitions |journal=Physical Review Letters |date=11 August 2016 |volume=117 |issue=7 |page=071803 |doi=10.1103/PhysRevLett.117.071803 |arxiv=1604.07411 |pmid=27563952 |bibcode=2016PhRvL.117g1803F |s2cid=206279817 }}</ref>

==See also==
{{Columns-list|colwidth=25em|
* {{annotated link|Affine gauge theory}}
* {{annotated link|Complex system}}
* {{annotated link|Graviphoton}}
* {{annotated link|Modified Newtonian dynamics}}
* {{annotated link|Physics beyond the Standard Model}}
* {{annotated link|Self-organization}}
}}

==References==
{{reflist|25em}}

{{authority control}}

{{Fundamental interactions}}

{{DEFAULTSORT:Fifth Force}}
[[Category:Force]]