- 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.
- Dark matter consists of new, undiscovered subatomic particles, but some believe it could be related to an unknown fundamental force. Some physicists speculate that a form of dark energy called quintessence could be a fifth force.
- Strong evidence of a fifth fundamental force and a previously undiscovered subatomic particle has been found. The Standard Model says that muons are electrically charged, just like electrons, so they would be expected to spin when placed in a magnetic field.
- It has been discovered that most of the universe’s mass is accounted for by an unknown form of matter called dark matter. Most physicists believe that dark matter consists of new, undiscovered subatomic particles, but some think it could be related to an unknown fundamental force. Second, it has also recently been discovered that the universe’s expansion is accelerating, attributed to dark energy. Some physicists speculate that a form of dark energy called quintessence could be a fifth force.
- One way to search for a fifth force is with tests of the strong equivalence principle. This is one of the most powerful tests of Einstein’s theory of gravity, general relativity.
- The strong equivalence principle suggests the laws of gravitation are independent of velocity and location. In particular, the gravitational motion of a small test body depends only on its initial position in spacetime and velocity and not on its constitution. The viscous forces are due to the friction between the layers of any genuine fluid. Inertial force, as the name implies, is the force due to the momentum of the liquid. The denser the fluid is, the higher its velocity.
- Inertia is a passive property. It does not enable a body to do anything except oppose such active agents as forces and torques. Inertial force (in the direction of flow) and viscous force (in the reverse flow order) act in opposite directions.
- The Strong Equivalence Principle says that if we measure the acceleration of the Solar System relative to the Milky Way, the external gravitational fields from other galaxies should not influence the measurement.
