Fundamental interaction

In physics, fundamental interactions, also called fundamental forces, are the basic ways that things in nature interact with each other. There are four known fundamental interactions: gravity, electromagnetism, weak interaction, and strong interaction. Gravity and electromagnetism are forces we can see and feel in everyday life. The strong and weak interactions work at very tiny scales, inside atoms, and are important for how atoms and nuclei behave.

Each of these interactions can be described using math as a kind of field. Gravity is described by how space and time curve, as explained by Einstein's general theory of relativity. The other three interactions are described by tiny particles called quantum fields. For example, the strong interaction is carried by particles called gluons and helps tiny parts called quarks stick together to form the building blocks of atoms, like protons and neutrons.

The weak interaction is carried by particles called W and Z bosons and helps atoms change over time in a process called radioactive decay. The electromagnetic force is carried by particles called photons and creates the forces that hold atoms together and make electric and magnetic fields possible. This force is also what makes visible light and many technologies work.

Scientists wonder if there might be a fifth force, but this is still just an idea. Because of how well these ideas about fundamental interactions work, scientists try to find ways to bring all four forces together into one big explanation called a theory of everything.

History

Classical theory

In 1687, Isaac Newton described space as a fixed structure where all objects move at a steady pace. He suggested that objects with mass attract each other, creating what we now call gravity. Later, Michael Faraday proposed that forces like magnetism work through an invisible field that fills space.

In 1873, James Clerk Maxwell showed that electricity and magnetism are linked through an electromagnetic field. This field also explained light, showing that light travels at a constant speed. His ideas later needed updating when new discoveries changed how we understand motion and space.

Standard Model

Main article: Standard Model

The Standard Model, developed in the last half of the 20th century, explains how tiny particles called elementary particles behave. These particles interact through forces carried by special particles. For example, up-quarks and down-quarks and electrons make up the atoms we see every day. These atoms interact through electromagnetic forces, sending and receiving particles called photons.

Other forces, like the weak interaction, have their own carriers, such as the W and Z bosons. The strong interaction, which holds atoms together, is carried by particles called gluons. All these ideas fit together in the Standard Model, which helps scientists understand how the universe works at its smallest levels.

Overview of the fundamental interactions

In physics, the basic ways that things in nature interact with each other are called fundamental interactions. These interactions cannot be broken down into simpler ones. There are four main interactions we know about: gravitation, electromagnetism, the weak interaction, and the strong interaction. These interactions can be seen in everyday life, like gravity keeping us on the ground or magnets sticking together.

These interactions happen when tiny particles, called fermions, exchange other particles, called bosons. This exchange changes how the fermions move and can even change what type of particle they are. Even though these interactions happen at very tiny scales, they explain bigger forces we feel, like pushing or pulling. Scientists use special numbers to compare how strong each interaction is.

Properties of fundamental interactions at low energy
Interaction	Current theory	Mediators	Strength	Long-distance behavior (potential)	Range (m)
Weak	Electroweak theory (EWT)	W and Z bosons	1.027×10⁻⁵	1 r e − m W , Z r {\displaystyle {\frac {1}{r}}\ e^{-m_{\rm {W,Z}}\ r}}	10⁻¹⁸
Strong	Quantum chromodynamics (QCD)	gluons	0.1 (short range), 1.0 (long range)	∼ r {\displaystyle {\sim r}} (Color confinement, see discussion below)	10⁻¹⁵
Electromagnetic	Quantum electrodynamics (QED)	photons	1/137	1 r 2 {\displaystyle {\frac {1}{r^{2}}}} (force)	∞
Gravitation	General relativity (GR)	gravitons (hypothetical)	5.9×10⁻³⁹	1 r 2 {\displaystyle {\frac {1}{r^{2}}}} (force)	∞

Interactions

Gravity

Main article: Gravity

Gravity is one of the four main forces in nature. It is the weakest at very small sizes, but it becomes very important for big objects like planets and stars. Unlike other forces, gravity only pulls and never pushes. This means that big objects like planets stay together because of gravity.

Gravity affects everything that has mass. It keeps us on the ground and makes things fall. It also helps shape the universe, from the way galaxies form to how stars move around each other.

Electroweak interaction

Main article: Electroweak interaction

Electromagnetism and a force called the weak interaction seem very different in everyday life. But when things get very hot and energetic, like right after the Big Bang, these two forces merge into one called the electroweak force.

Electromagnetism

Main article: Electromagnetism

Electromagnetism is the force between electrically charged particles. It includes forces between particles that are not moving and forces when they are moving, like with magnets. This force is very strong and is responsible for many things we see and use every day, like light, magnets, and electricity.

Electromagnetism holds atoms together and makes chemistry possible. It is so strong that it usually balances out on large scales, which is why gravity becomes the most important force for big objects like planets and stars.

Weak interaction

Main article: Weak interaction

The weak interaction is a force that helps certain types of atoms change into other types, a process important in the nuclei of atoms. It works over very short distances and is different from other forces because it treats left-handed and right-handed particles differently.

Strong interaction

Main article: Strong interaction

The strong interaction, also called the strong nuclear force, is the force that holds the nuclei of atoms together. It is very strong but only works over very short distances. Without this force, the positively charged parts of an atom would push each other apart, and atoms wouldn’t stay together.

This force is what allows the tiny nuclei of atoms to exist. It works hard to keep the pieces of an atom close together, even though those pieces naturally want to move apart because of another force called electromagnetism.

Higgs interaction

The Higgs interaction is not usually counted as one of the main four forces. However, it plays a role in giving particles their mass. This happens through a special field, and it creates a very weak attractive force between particles. But this force only works over extremely tiny distances and is much weaker than the other forces.

Unification

Scientists think that all the basic forces in nature might actually be one force when looked at very closely and with a lot of energy. Right now, we can’t create that much energy in our experiments. But we already know that two forces, the weak force and the electromagnetic force, can be described together with a special idea called the electroweak theory.

Some ideas try to show that three of the forces are really just one force that looks different depending on how you look at it. These ideas also try to explain why some numbers in nature are what they are. Other ideas try to understand how gravity works at the smallest levels, and some even try to bring all four forces together into one big idea. One popular idea is called string theory, but these theories are still just ideas and need more testing to see if they’re right.

Beyond the Standard Model

Main article: Physics beyond the Standard Model

Some ideas in physics talk about a possible fifth force that scientists are still looking for. These ideas also include special theories where certain particles get their weight in unusual ways. Scientists are also trying to understand why the universe seems to be growing faster, which might need new ideas about how space and time work. These fifth forces might also help explain some strange things we see in space, like how certain parts of the universe move together.