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fifth force

a theoretical force in nature in addition to the strong and weak forces, gravitation, and the electromagnetic force.
Origin of fifth force
1975-80 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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British Dictionary definitions for fifth-force

fifth force

a hypothetical non-Newtonian repulsive component of the force of gravity, postulated as an addition to the four known fundamental forces (gravitational, electromagnetic, strong, and weak)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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fifth-force in Science
fifth force
Any of various hypothetical, very weak forces thought to cause bodies to repel each other. Such forces are occasionally considered as a solution to unexpected deviations in the measured value of the gravitational constant or to explain the apparent acceleration of very distant galaxies away from earth.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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