- Fifth force
Occasionally, physicists have postulated the existence of a fifth force in addition to the four known
fundamental forces. The force is generally believed to have roughly the strength of gravity(i.e. it is much weaker than electromagnetismor the nuclear forces) and to have a range of anywhere from less than a millimeter to cosmological scales.
The idea is difficult to test, because gravity is such a weak force: the gravitational interaction between two objects is only significant when one has a great mass. Therefore, it takes very precise equipment to measure gravitational interactions between objects that are small compared to the
Earth. Nonetheless, in the late 1980s a fifth force, operating on municipal scales (i.e. with a range of about 100 meters), was reported by researchers (Fischbach "et al.") who were reanalyzing results of Loránd Eötvösfrom earlier in the century. The force was believed to be linked with hypercharge. Over a number of years, other experiments have failed to duplicate this result, and physicists now believe that there is no evidence for a fifth force.
Theory and experiment
There are at least three kinds of searches that can be undertaken, which depend on the kind of force being considered, and its range.
One way is to search for a fifth force with tests of the strong
equivalence principle: this is one of the most powerful tests of Einstein's theory of gravity, general relativity. Alternative theories of gravity, such as Brans-Dicke theory, have a fifth force—possibly 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 curvatureof space, and different kinds of degrees of freedom produce different effects. For example, a scalar fieldcannot 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 Experimentand very long baseline interferometry.
Another kind of fifth force, which arises in
Kaluza-Klein theory, where the universe has extra dimensions, or in supergravityor string theoryis 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 lot of recent interest, as a theory of supersymmetriclarge extra dimensions—dimensions with size slightly less than a millimeter—has prompted an experimental effort to test gravity on these very small scales. This requires extremely sensitive experiments which search for a deviation from the inverse square lawof gravity over a range of distances. Essentially, they are looking for signs that the Yukawa interaction is kicking in at a certain length.
Australian researchers, attempting to measure the
gravitational constantdeep 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 onboard a submarine (USS "Dolphin" (AGSS-555)) while deeply submerged.
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 balanceexperiments of a type invented by Loránd Eötvös. Such forces may depend, for example, on the ratio of protons to neutrons in an atomic nucleus, or the relative amount of different kinds of binding energyin 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 matterat the center of the galaxy.
A few physicists think that Einstein's theory of gravity will have to be modified, not at small scales, but at large distances, or, equivalently, small accelerations. They point out that
dark matter, dark energyand even the Pioneer anomalyare unexplained by the Standard Modelof particle physicsand suggest that some modification of gravity, possibly arising from Modified Newtonian Dynamicsor the holographic principle. This is fundamentally different from conventional ideas of a fifth force, as it grows stronger relative to gravity at longer distances. Most physicists, however, think that dark matter and dark energy are not "ad hoc", but are supported by a large number of complementary observations and described by a very simple model.
*Ephraim Fischbach, Daniel Sudarsky, Aaron Szafer, Carrick Talmadge, and S. H. Aronson, "Reanalysis of the Eötvös experiment", "Physical Review Letters" 56 3 (1986).
*University of Washington [http://www.npl.washington.edu/eotwash/ Eöt-Wash group] , the leading group searching for a fifth force.
*Lunar Laser Ranging [http://funphysics.jpl.nasa.gov/technical/grp/lunar-laser.html]
*Satellite Energy Exchange (SEE) [http://www.phys.utk.edu/see/] , which is set to test for a fifth force in space, where it is possible to achieve greater sensitivity.
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