fifth force

plural fifth forces

1. natural sciencesA hypothetical fundamental force, distinct from gravity, electromagnetism, and the strong force and weak force, proposed to explain various anomalous observations that do not fit existing theories.

   According to Ephraim Fischbach's 1986 theory, the more tightly packed these particles are, the more strongly the atom feels the repulsive effect of the fifth force.

   1989 May, John Boslough, “Searching for the Secrets of Gravity”, in National Geographic, volume 175, number 5, page 569

   1 more example

   The tantalising theory that a fifth force of nature could exist has been given a boost thanks to unexpected wobbling by a subatomic particle, physicists have revealed. […] Dr Mitesh Patel, from Imperial College London, said: “We’re talking about a fifth force because we can’t necessarily explain the behaviour [in these experiments] with the four we know about.”

   2023 August 11, Nicola Davis, “Scientists may be on brink of discovering fifth force of nature”, in The Guardian