a theoretical force in nature in addition to the strong and weak forces, gravitation, and the electromagnetic force.
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British Dictionary definitions for 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
Scientific definitions for 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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.