Fifth force

We know of four forces in our world. Some people think there is a fifth force. This force might be hidden from us.

Scientists look for this force in many ways. They look deep in mine shafts. They even look under the sea. They look at stars far away too.

One idea is that this force helps the universe grow. This could be a type of dark energy. It might be a new kind of energy.

Some think a tiny new particle makes the force. This particle would be very light. It might be very small.

No one has found proof yet. Scientists are still searching for it. It would be a big discovery!

Scientists know about four main forces in nature. These include gravity and forces inside atoms. Some thinkers believe a fifth force might exist. This would be a new way that things affect each other.

There is no proof for this force yet. Scientists use many ways to search for it. Some look at how gravity works. They check if gravity changes over certain distances. In one study, researchers looked deep in a mine shaft. They thought they saw a tiny change in gravity. They thought a fifth force might be pushing things away. Other tests were done in a deep part of the ocean.

Some people think a new particle makes this force. A particle is a tiny piece of matter. In 2015, a team in Hungary studied how atoms break apart. They saw something strange happen. They think a new, light particle called the X17 might be there. This particle could explain the strange results. Other tests look at far away stars. They also look at how the universe grows. This growth is linked to dark energy. Scientists are still working hard to find the truth.

Scientists currently know of four fundamental forces in nature. These include gravity, electromagnetism, and two forces that work inside atoms. Some thinkers believe a fifth force might exist. This would be a new way that things affect each other. This idea comes from seeing things that do not fit current rules. Some theories use this force to explain why the universe grows. This growth is linked to a mysterious energy called dark energy.

Researchers look for this force in many different ways. One way is to test how gravity works over distances. They check if gravity follows the same rules everywhere. Some tests look at how planets move in our solar system. Others look at how stars pulse in far away galaxies. Scientists even look at tiny particles inside atoms. They want to see if a new particle carries this force.

History shows how this idea began to grow. In 1971, a scientist named Ephraim Fischbach suggested a new model. Later, in 1986, he and others wrote a paper about it. They looked at old data from an experiment by Loránd Eötvös. They thought they saw gravity acting differently over certain distances. This new idea was called a fifth force. Many later tests failed to find these same changes.

Many specific tests have been done around the world. Australian researchers measured gravity deep inside a mine shaft. They found a value that was two percent too small. They thought a repulsive force might be pushing things away. Other tests happened on a submarine called the USS Dolphin. Scientists also looked at ice in Greenland to find clues. In 2015, a team in Hungary studied how atoms decay. They thought they saw a new particle called X17.

Finding a fifth force would change how we see everything. It might connect gravity to the tiny world of atoms. It could also explain the mystery of dark energy. If the X17 particle is real, it might be dark matter. This would help us understand the huge scale of space. Right now, scientists are still searching for more proof. They use sensitive tools to find these tiny signals.

In physics, a fifth force is a hypothetical fundamental interaction. Scientists currently recognize four known forces in nature. These are gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. A fifth force would be an entirely new way that objects affect one another. Some speculative theories propose this force to explain observations that do not fit existing models. Currently, no definitive evidence has been found to support these models. The characteristics of such a force depend on the specific hypothesis being tested.

Theoretical proposals for a fifth force are driven by several scientific inconsistencies. One major issue is the gap between general relativity and quantum field theory. Another is the conflict between the hierarchy problem and the cosmological constant problem. These problems suggest there might be corrections to the gravitational potential at very small scales, around 100 micrometers. Additionally, the accelerating expansion of the universe is linked to dark energy. Some physicists speculate that a form of dark energy called quintessence could actually be a fifth force.

One way to search for this force is by testing the strong equivalence principle. This principle is a central part of Einstein's theory of gravity, or general relativity. Some alternative theories, like Brans–Dicke theory, suggest a fifth force might have an infinite range. In these theories, gravity has extra degrees of freedom beyond the metric, which is the mathematical description of space curvature. For example, a scalar field cannot cause the bending of light rays. A fifth force might show up as the Nordtvedt effect, which influences solar system orbits. Scientists test this using Lunar Laser Ranging and very-long-baseline interferometry.

Another approach involves searching for a Yukawa force. This force might arise from Kaluza–Klein theory, supergravity, or string theory. These theories suggest the universe may have extra dimensions. The Yukawa force is transmitted by a light scalar field. The range of this field is determined by its Compton wavelength. Because of this, researchers are testing gravity on very small scales to see if it deviates from the inverse-square law. They are looking for signs that a Yukawa interaction begins to act at specific lengths.

History shows how the term "fifth force" became part of scientific discussion. In 1971, Fujii proposed a model that changed how gravity depends on distance. This model used a Yukawa potential-like term to describe the interaction. Later, in 1986, Ephraim Fischbach and his colleagues reanalyzed data from the Eötvös experiment. They found a distance dependence in gravity that seemed to deviate from the inverse-square law. Fischbach's analysis suggested a force with a strength around 1% of gravity and a range of a few hundred meters. However, many later attempts to reproduce these specific deviations have failed.

Many experiments have attempted to find these discrepancies in different environments. Australian researchers measured the gravitational constant deep in a mine shaft. They found a value that was 2% smaller than predicted. They concluded a repulsive fifth force with a range of centimeters to a kilometer might explain this. Similar experiments were conducted on the submarine USS Dolphin while it was deeply submerged. Scientists also measured gravity in a deep borehole in the Greenland ice sheet. They found discrepancies of a few percent, though they could not rule out geological causes.

In 2015, a team in Hungary led by Attila Krasznahorkay reported a potential discovery. They studied the decay of beryllium-8 nuclei created by firing protons at lithium-7 targets. They observed an excess of decays at a specific opening angle and energy of 17 MeV. This suggested the existence of a new, light boson called the X17 particle. This particle is only 34 times heavier than an electron. In 2019, the team found similar anomalies in the decay of stable helium atoms. Some researchers propose this X17 is a "protophobic" boson, meaning it has suppressed couplings to protons. This particle could potentially explain the muon anomaly or serve as a candidate for dark matter.