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The Fundamental Forces

Updated on November 27, 2019

There are four fundamental interactions (commonly called fundamental forces) in nature. These forces are gravity, electromagnetic, weak interaction, and strong interaction. Gravity is attributed to the curvature of spacetime and is considered on a large scale, as described by Einstein's theory of relativity. The remaining three are quantum fields whose interactions are mediated by subatomic particles.


Gravity (or gravitation) is a phenomenon in which all things that have mass are pulled toward one another. Objects that have a larger mass have a larger gravitational pull, and the gravitational range is infinite, although it gets weaker as distance increases. It cannot be shielded against and is the only force that acts on all particles that have mass, energy, or momentum. Gravity is most accurately described by Einstein's general theory of relativity, which describes gravity as the curvature of spacetime. It can be well approximated by Newton's law of universal gravitation.

Gravity is the weakest of the four fundamental forces of physics. It has no significance on the level of subatomic particles, and is most effective on larger objects like planets.

Gravity as shown by the curvature of spacetime.
Gravity as shown by the curvature of spacetime. | Source

Gravity has allowed scientists to learn more about the solar system and space. We know the masses of the sun and other planets even without visiting them because we use the properties of gravity to calculate them. Gravity also allowed us to develop classical mechanics and perform calculations involving projectiles and falling objects.

A current area of research in this topic is quantum gravity. Quantum gravity aims to merge general relativity and quantum mechanics into one theory. It is hypothesized in this theory that gravity is dictated by a mass-less particle called the graviton.


Electromagnetism is the force that acts between electrically charged particles. Like gravity, electromagnetism has an infinite range, but it is much stronger than gravity. It explains common natural experiences like friction, rainbows, and lightning, and it explains everyday man-made electrical devices, like computers and televisions. Electromagnetism also determines many macroscopic and atomic-level properties of chemical bonding.


Originally, electricity and magnetism were considered two separate forces. James Clerk Maxwell's 1873 A Treatise on Electricity and Magnetism changed this idea by showing that positive and negative charges were dictated by one force instead of two. The four main effects of this argument, all of which have been proven, are:

1. Unlike charges attract one another and like charges repel one another. These charges do so with a force that is inversely proportional to the square of the distance between them.

2. Magnetic poles attract or repel one another like the way positive and negative charges attract or repel one another. Magnetic poles always exist as pairs, in which every north pole has a south pole.

3. An electric current inside a wire creates a corresponding circumferential magnetic field outside the wire. The direction depends on the current inside the wire.

4. An electrical current is created in a loop of wire when it is moved toward or away from a magnetic field.

Weak Interaction

The weak interaction is also called the weak force or the weak nuclear force. This is how subatomic particles interact to cause radioactive decay. It also plays roles in quantum flavordynamics, quantum chromodynamics, and quantum elecrtodynamics, but these are not as well known because the weak interaction is most well known in electroweak theory.

Radioactive beta decay is due to the weak force, which turns a neutron into a proton and an electron into an electron antineutrino.
Radioactive beta decay is due to the weak force, which turns a neutron into a proton and an electron into an electron antineutrino. | Source

The weak force has a limited subatomic range and reaches less than the diameter of a proton. The weak force has unique properties, though. For example, it is the only force that is capable of changing the flavor of a quark. The flavor of a quark is essentially its species, or what type of quark it is. Secondly, it is the only force that violates parity-symmetry. Lastly, the electrically charged and electrically neutral interactions are propagated by force carrier particles that have significant masses.

The first idea of the weak force was proposed in 1933 by Enrico Fermi. He suggested that beta decay could be caused by specific particles that involved a contact force with no range.

The weak force is better described as a non-contact force field with a small, finite range. In 1968, Sheldon Glashow, Abdus Salam, and Steven Weinberg showed that the weak force and the electromagnetic force were two aspects of the same force. This is called the electroweak force.

Electroweak Force

The electroweak force is a theory that explains the weak force and electromagnetic force as two aspects of the same (electroweak) force. The weak force and electromagnetic force are separate at low temperatures, but when the temperature exceeds 1015 K, the weak force and electromagnetic force merge into a single force. The only time this happened was shortly after the Big Bang.

Strong Interaction

As the name implies, the strong interaction is the strongest of all four forces. The strong force is responsible for holding together the nucleus of atoms, as it is the mechanism of the strong nuclear force. The strong force holds quarks together to form protons and neutrons, and it binds these protons and neutrons to create atomic nuclei.

A Hypothesized Fifth Force

Scientists at the Institute for Nuclear Research at the Hungarian Academy of Sciences published findings that could point to a fifth force. They were observing how an excited helium atom emitted light as it decayed, and the particles split at a 115 degree angle, which could not be explained by known physics. Attila Krasznahorkay, the lead scientist on the study, says that this was the second time that the X17 particle has been observed by his team. They call the new particle X17 because its mass is 17 megaelectronvolts.

In 2016, the researchers performed a similar experiment with beryllium-8 and saw similar results. Scientists in California published a paper after this study attempting to explain why this happened. They called this new force a protophobic force, meaning that it was afraid of protons.

No scientists could find flaws in the experiments or theories, so the only thing left to do was to repeat the experiment and yield the same results. This is what the 2019 results do because without experimental error, there was a one-in-trillion chance that these results were caused by anything except the X17 particle acting as the fifth force. If these results could be repeated with a third element, that would give scientists all the proof they need to validate a fifth force.



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