Fifth force

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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.

Contents

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 [1] 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. [2] [3]

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. [4] [5] : 57  The reanalysis was sparked by theoretical work in 1971 by Fujii [6] [7] : 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. [5] Fischbach's paper found a strength around 1% of gravity and a range of a few hundred meters. [8] : 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. [5] However, many subsequent attempts to reproduce the deviations have failed. [9]

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 . [5] : 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 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

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. [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

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

Cepheid variables

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

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

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

Related Research Articles

In astronomy, dark matter is a hypothetical form of matter that appears not to interact with light or the electromagnetic field. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be seen. Such effects occur in the context of formation and evolution of galaxies, gravitational lensing, the observable universe's current structure, mass position in galactic collisions, the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.

In physics, the fundamental interactions or fundamental forces are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist:

<span class="mw-page-title-main">General relativity</span> Theory of gravitation as curved spacetime

General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalises special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of second order partial differential equations.

In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathematical problem with renormalization in general relativity. In string theory, believed by some to be a consistent theory of quantum gravity, the graviton is a massless state of a fundamental string.

<span class="mw-page-title-main">Quantum gravity</span> Description of gravity using discrete values

Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics. It deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars as well as in the early stages of the universe moments after the Big Bang.

<span class="mw-page-title-main">Theory of everything</span> Hypothetical physical concept

A theory of everything (TOE), final theory, ultimate theory, unified field theory or master theory is a hypothetical, singular, all-encompassing, coherent theoretical framework of physics that fully explains and links together all aspects of the universe. Finding a theory of everything is one of the major unsolved problems in physics.

<span class="mw-page-title-main">Standard Model</span> Theory of forces and subatomic particles

The Standard Model of particle physics is the theory describing three of the four known fundamental forces in the universe and classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists worldwide, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, proof of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.

<span class="mw-page-title-main">Scalar field</span> Assignment of numbers to points in space

In mathematics and physics, a scalar field is a function associating a single number to every point in a space – possibly physical space. The scalar may either be a pure mathematical number (dimensionless) or a scalar physical quantity.

<span class="mw-page-title-main">Anti-gravity</span> Idea of creating a place or object that is free from the force of gravity

Anti-gravity is a hypothetical phenomenon of creating a place or object that is free from the force of gravity. It does not refer to either the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism and aerodynamic lift. Anti-gravity is a recurring concept in science fiction. Examples are the gravity blocking substance "Cavorite" in H. G. Wells's The First Men in the Moon and the Spindizzy machines in James Blish's Cities in Flight.

<span class="mw-page-title-main">Effective field theory</span> Type of approximation to an underlying physical theory

In physics, an effective field theory is a type of approximation, or effective theory, for an underlying physical theory, such as a quantum field theory or a statistical mechanics model. An effective field theory includes the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale or energy scale, while ignoring substructure and degrees of freedom at shorter distances. Intuitively, one averages over the behavior of the underlying theory at shorter length scales to derive what is hoped to be a simplified model at longer length scales. Effective field theories typically work best when there is a large separation between length scale of interest and the length scale of the underlying dynamics. Effective field theories have found use in particle physics, statistical mechanics, condensed matter physics, general relativity, and hydrodynamics. They simplify calculations, and allow treatment of dissipation and radiation effects.

<span class="mw-page-title-main">Equivalence principle</span> The hypothesis that inertial and gravitational masses are equivalent

The equivalence principle is the hypothesis that the observed equivalence of gravitational and inertial mass is a consequence of nature. The weak form, known for centuries, relates to masses of any composition in free fall taking the same trajectories and landing at identical times. The extended form by Albert Einstein requires special relativity to also hold in free fall and requires the weak equivalence to be valid everywhere. This form was a critical input for the development of the theory of general relativity. The strong form requires Einstein's form to work for stellar objects. Highly precise experimental tests of the principle limit possible deviations from equivalence to very small.

<span class="mw-page-title-main">Introduction to general relativity</span> Theory of gravity by Albert Einstein

General relativity is a theory of gravitation developed by Albert Einstein between 1907 and 1915. The theory of general relativity says that the observed gravitational effect between masses results from their warping of spacetime.

Tests of general relativity serve to establish observational evidence for the theory of general relativity. The first three tests, proposed by Albert Einstein in 1915, concerned the "anomalous" precession of the perihelion of Mercury, the bending of light in gravitational fields, and the gravitational redshift. The precession of Mercury was already known; experiments showing light bending in accordance with the predictions of general relativity were performed in 1919, with increasingly precise measurements made in subsequent tests; and scientists claimed to have measured the gravitational redshift in 1925, although measurements sensitive enough to actually confirm the theory were not made until 1954. A more accurate program starting in 1959 tested general relativity in the weak gravitational field limit, severely limiting possible deviations from the theory.

<span class="mw-page-title-main">Gravitational interaction of antimatter</span> Theory of gravity on antimatter

The gravitational interaction of antimatter with matter or antimatter has been observed by physicists. As was the consensus among physicists previously, it was experimentally confirmed that gravity attracts both matter and antimatter at the same rate within experimental error.

In theoretical physics, a scalar–tensor theory is a field theory that includes both a scalar field and a tensor field to represent a certain interaction. For example, the Brans–Dicke theory of gravitation uses both a scalar field and a tensor field to mediate the gravitational interaction.

The Eötvös experiment was a famous physics experiment that measured the correlation between inertial mass and gravitational mass, demonstrating that the two were one and the same, something that had long been suspected but never demonstrated with the same accuracy. The earliest experiments were done by Isaac Newton (1642–1727) and improved upon by Friedrich Wilhelm Bessel (1784–1846). A much more accurate experiment using a torsion balance was carried out by Loránd Eötvös starting around 1885, with further improvements in a lengthy run between 1906 and 1909. Eötvös's team followed this with a series of similar but more accurate experiments, as well as experiments with different types of materials and in different locations around the Earth, all of which demonstrated the same equivalence in mass. In turn, these experiments led to the modern understanding of the equivalence principle encoded in general relativity, which states that the gravitational and inertial masses are the same.

<span class="mw-page-title-main">Entropic gravity</span> Theory in modern physics that describes gravity as an entropic force

Entropic gravity, also known as emergent gravity, is a theory in modern physics that describes gravity as an entropic force—a force with macro-scale homogeneity but which is subject to quantum-level disorder—and not a fundamental interaction. The theory, based on string theory, black hole physics, and quantum information theory, describes gravity as an emergent phenomenon that springs from the quantum entanglement of small bits of spacetime information. As such, entropic gravity is said to abide by the second law of thermodynamics under which the entropy of a physical system tends to increase over time.

Eric George Adelberger is an American experimental nuclear physicist and gravitational metrologist.

Blayne Ryan Heckel is an American experimental physicist, known for his research involving precision measurements in atomic physics and gravitational physics. He is now a professor emeritus at the University of Washington in Seattle.

Ephraim Fischbach is an American physicist and a professor at Purdue University. He is best known for his attempts to find a fifth force of nature and his research relating to the detection of neutrinos. He has also done work relating to the prediction of solar flares and the detection of radiation by cell phones. Fischbach studies variation in radioactive decay rates, suggesting that neutrino emission from the Sun reduces the rate of nuclear decay. He reanalysed the Eötvös experiment, which he saw as evidence for a fifth physical force. However, in 1992, he and Carrick Talmadge conducted an experiment which found no compelling evidence for a fifth force. Fischbach has been a fellow of the American Physical Society since 2001, and a professor at Purdue since 1979. He also was an associate professor at the Institute for Theoretical Physics in Stony Brook, New York from 1978 to 1979. He received a B.A. in physics in 1963 from Columbia University and a Ph.D. in 1967 from the University of Pennsylvania.

References

  1. ↑ 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.
  2. ↑ Wetterich, C. "Quintessence – a fifth force from variation of the fundamental scale" (PDF). Heidelberg University.
  3. ↑ 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.
  4. ↑ 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.
  5. 1 2 3 4 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.
  6. ↑ 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.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ "Lunar laser ranging". Archived from the original on 28 November 2016. Retrieved 7 May 2005.
  11. ↑ "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.
  12. ↑ 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.
  13. ↑ 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.
  14. ↑ Aron, Jacob (2013). "Earth's mantle helps hunt for fifth force of nature". New Scientist.
  15. ↑ 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.
  16. ↑ 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.
  17. ↑ 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.
  18. ↑ 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.
  19. ↑ 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.
  20. 1 2 Cartlidge, Edwin (2016). "Has a Hungarian physics lab found a fifth force of nature?". Nature. doi:10.1038/nature.2016.19957. S2CID   124347962.
  21. ↑ "Scientists may have discovered fifth force of nature, laboratory announces". The Independent . London, UK. Retrieved 26 November 2019.
  22. 1 2 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.
  23. ↑ "New boson claim faces scrutiny". Quanta Magazine. Retrieved 24 November 2019.
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